Boron-diindomethene (BDI) dyes and their tetrahydrobicyclo precursors - En route to a new class of highly emissive fluorophores for the red spectral range

FULL PAPER

Boron–Diindomethene (BDI) Dyes and Their Tetrahydrobicyclo

Precursors—en Route to a New Class of Highly Emissive Fluorophores for

the Red Spectral Range

[

a, d]

[b, e]

[b]

[c]

[b]

Zhen Shen,

Holger Röhr,

Knut Rurack,* Hidemitsu Uno, Monika Spieles,

[b] [a]

[

b]

Burkhard Schulz, Günter Reck, and Noboru Ono*

Abstract: The X-ray crystallographic,

optical spectroscopic, and electrochem-

ical properties of a newly synthesized

class of boron–diindomethene (BDI)

dyes and their tetrahydrobicyclo pre-

cursors (bc-BDP) are presented. The

BDI chromophore was designed to

show intensive absorption and strong

fluorescence in an applicationary ad-

vantageous spectral range. Its modular

architecture permits fusion of a second

subunit, for example, a receptor moiety

to the dyeꢀs core to yield directly

linked yet perpendicularly prearranged

composite systems. The synthesis was

developed to allow facile tuning of the

chromophore platform and to thus

adjust its redox properties. X-ray analy-

sis revealed a pronounced planarity of

the chromophore in the case of the

BDIs, which led to a remarkable close

packing in the crystal of the simplest

derivative. On the other hand, devia-

tion from planarity was found for the

diester-substituted bc-BDP benzocrown

that exhibits a “butterfly”-like confor-

mation in the crystal. Both families of

dyes show charge- or electron-transfer-

type fluorescence-quenching character-

istics in polar solvents when equipped

with a strong donor in the meso-posi-

tion of the core. These processes can

be utilized for signaling purposes if an

appropriate receptor is introduced.

Further modification of the chromo-

phore can invoke such a guest-respon-

sive intramolecular quenching process,

also for receptor groups of low electron

density, for example, benzocrowns. In

addition to the design of various proto-

type molecules, a promising fluoroiono-

+

phore for Na was obtained that ab-

sorbs and emits in the 650 nm region

and shows a strong fluorescence en-

hancement upon analyte binding. Fur-

thermore, investigation of the remark-

able solvatokinetic fluorescence prop-

erties of the “butterfly”-like bc-BDP

derivatives suggested that a second in-

trinsic nonradiative deactivation chan-

nel can play a role in the photophysics

of boron–dipyrromethene dyes.

Keywords: charge

transfer

·

dyes/pigments · electron transfer ·

fluorescence

Introduction

[

a] Dr. Z. Shen, Prof. N. Ono

Department of Chemistry, Faculty of Science

Ehime University, Matsuyama 790—8577 (Japan)

Fax : (+81)899279590

The search for new fluorophores that absorb and emit in the

red visible and/or near infrared (NIR) region of the spec-

trum is of continuing interest in many different fields of

E-mail: ononbr@dpc.ehime-u.ac.jp

[1]

chemistry, ranging from optical spectroscopy-based sensing

[

b] Dipl.-Phys. H. Röhr, Dr. K. Rurack, M. Spieles, B. Schulz,

Dr. G. Reck

[2]

through imaging applications to materials chemistry relat-

[3]

[4]

Div. I.3 Bundesanstalt für Materialforschung und -prüfung (BAM)

Richard-Willstätter-Strasse 11, 12489 Berlin (Germany)

Fax : (+49)308-104-5005

ed issues, such as molecular switches and devices, lasing

[5]

[6]

media, or electrooptical applications. In addition to the

wavelength range of operation, the key requirements for

fluorescent dyes to be suitably employable in such fields are

efficiency, in terms of a high extinction coefficient, a high

rate of conversion of absorbed photons into emitted pho-

tons, and versatility with respect to the dyeꢀs synthesis as

well as to its functionalization. A promising starting point

for the construction of dyes showing intense and well-de-

fined absorption bands in the red and/or NIR spectral range

E-mail: knut.rurack@bam.de

[

c] Prof. H. Uno

Integrated Center for Science, Faculty of Science

Ehime University, Matsuyama 790—8577 (Japan)

[

d] Dr. Z. Shen

Present address:

State Key Laboratory of Coordination Chemistry

Nanjing University, Nanjing 210093 (China)

[

e] Dipl.-Phys. H. Röhr

Associated with: Fachbereich Biologie, Chemie, Pharmazie

Freie Universität Berlin, Takustrasse 3, 14195 Berlin (Germany)

[7]

is the cyanine chromophore (Cy; Scheme 1). In these dyes,

according to the triad principle of colored organic com-

[8]

[9]

pounds and the concept of the (ideal) polymethine state,

the position of the absorption band depends on and can

Supporting information for this article is available on the WWW

under http://www.chemeurj.org/ or from the author.

Chem. Eur. J. 2004, 10, 4853 – 4871

DOI: 10.1002/chem.200400173

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4853

FULL PAPER

As an alternative, the internally zwitterionic boron–dipyr-

romethene (BDP) chromophore (Scheme 1) has gained con-

[

22]

siderable attention during the last three decades.

BDPs

are rigid cyanine-type dyes with a fixed planarity of the

chromophoric p-electron system. They combine the favor-

able properties of cyanines, such as their typical spectral

shapes in absorption and emission, high molar extinction co-

ꢀ

1

ꢀ1

efficients (usually e>80,000m cm ), and high fluorescence

quantum yields (commonly F >0.70), while absorbing and

f

emitting in the range of the simple heptamethine and rhoda-

[23]

mine dyes (Scheme 1), or fluorescein dyes. Additionally,

the BDP core has a relatively moderate redox potential,

which is a prerequisite when aiming at the construction of

fluorescent switches based on electron- (ET) or charge-

[22d,24]

transfer (CT) processes.

Owing to their specific chemi-

cal nature, BDPs are generally well soluble in a large variety

of common solvents of different polarity. Despite these ad-

vantageous features, the spectral properties of the classical

BDP chromophore are limited to the wavelength range be-

tween 470 and 530 nm. However, with regard to various ap-

plications it would be very interesting to have BDP dyes

available that absorb and emit at longer wavelengths. For in-

stance, in chemical sensing and imaging or for optical

switches, the matrix can spectroscopically interfere at wave-

Scheme 1. General structure of the cyanine (Cy), rhodamine (Rh),

boron–dipyrromethene (BDP; with the zwitterionic character indicated),

and boron–diindomethene (BDI) chromophores. In the majority of

actual examples, X and Y are heterocyclic or aromatic substitutents. For

Rh, R to R’’’ are mostly short alkyl substituents or hydrogens. For BDP

[1e]

[25,26]

lengths ꢁ600 nm.

Thus, other groups

and our

[27]

n

group have recently shifted the absorption and fluores-

and BDI, the substituents R can vary from H to alkyl and other small

cence maxima to longer wavelengths by chemical modifica-

tion of the BDP core with aryl or styryl substituents, or have

restricted bond rotation in rigid ring-fused systems. Here,

the constrained molecules generally exhibit more favorable

fluorescence characteristics than the unconstrained ones.

Based on this background, we have developed a new class

of aromatic ring-fused BDP derivatives, boron–di(iso)indo-

methene (BDI) dyes (for general structure, see Scheme 1;

the dyes prepared and investigated in this work are depicted

in Scheme 2), by adopting the strategy of benzoporphyrin

synthesis by means of a retro Diels–Alder reaction. In the

present paper, we elaborate the synthetic procedure and

transfer it to the design and tuning of chemically address-

able BDI derivatives that utilize charge-transfer processes

for communication. Besides synthetic details, the molecular

functional groups, or to larger aryl moieties. Three representative exam-

ples are included on the right.

thus be tuned by the number of vinylene groups n in the

[10]

chain (“vinylene shift”). However, whereas a vast number

of cyanine dyes have been synthesized so far and absorption

[

11]

maxima up to 1500 nm have been realized,

cence quantum yield of cyanines does not generally exceed

.50 (for certain di- or trimethines) and drops rapidly with a

further decrease towards monomethines or with an increase

the fluores-

[28]

0

[12]

in the chain length. This effect can be caused by various

factors that are predominantly connected to the (multiple)

flexible bonds of the molecules. For instance, twisting of

single and/or double bonds can readily dissipate the excita-

tion energy. Other researchers identified the many vibra-

tional degrees of freedom as a major nonradiative pathway

requirements, as can be deduced from X-ray structures, of

[13]

[29]

the bicyclo-appended BDP (bc-BDP)

precursors as well

as the BDI dyes are presented. The investigation of the pre-

cursor dyes, in particular, revealed some interesting mecha-

nistic features of general BDP photochemistry. Owing to

our interest in molecular switches and sensors and our expe-

[14]

for flexible and semiflexible cyanines. Even for largely

rigid or bridged cyanines, the remaining flexible bonds can

[15]

still be an effective funnel for nonemissive deactivation.

In analogy to rhodamine (Rh) chemistry (for the chemical

structure, see Scheme 1), only the complete fixation of the

chromophoric system seems to guarantee an intrinsically

high conversion of the absorbed light into fluorescence

rience with functionalized BDP dyes in this research

[16]

[27,30]

area,

specific spectroscopic and redox features of the

BDI derivatives have been introduced, by tuning the scaf-

fold, to obtain a sodium-ion-responsive fluorescent sensor

molecule.

[17]

emission. However, the disadvantages here are the time-

consuming and often complex synthesis of fully rigid cya-

nines and, in the case of the rhodamines, that the remaining

positions for functionalization at the chromophoric core are

limited. Moreover, as positively charged ions, the solubility

of rhodamines is generally restricted to highly polar solvents

and, for any application, the presence of the dyeꢀs counter-

Results and Discussion

Synthesis: The synthetic route for the preparation of com-

pounds 4a–f is depicted in Scheme 3. Ethyl 4,7-dihydro-4,7-

[18]

ion has to be taken into account.

4854

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

X-ray structure analysis: Single-

crystal X-ray structures were

obtained for compounds 3a,c–e,

4

a, and 4d and the basic fea-

tures are listed in Table 1. The

corresponding molecular con-

figurations and conformations

are shown in Figure 1. The

average BꢀN bond length

amounts to 1.549(10) , indi-

cating that all the investigated

compounds possess single BꢀN

bonds. The average BꢀF bond

is 1.380(10) and the average

N-B-N and F-B-F angles are

1

07.3(6) and 110.6(14)8, respec-

tively. These data indicate that

the bond lengths and angles of

the BF N tetrahedron of the

2

bc-BDP and BDI compounds

of the present work are in good

agreement with the correspond-

ing values in the simpler ana-

logues 4,4-difluoro-1,3,5,7,8-pen-

tamethyl-3a.4a-diaza-4-bora-5-

Scheme 2. Structures of the BDI compounds investigated (4a–f), the bicyclo-type precursors (3a–f), and some

BDP (5a–c, 6a,b, 7a,b), as well as cyanine (8a–c) reference compounds.

[32]

indacene, 2,2-difluoro-1,3,4,6-

tetramethyl-3-aza-1-azonia-2-

[33]

borata-4,6-cyclohexadiene,

and 2,2-difluoro-4,6-dimethyl-

1

2

,3-di-p-toluyl-3-aza-1-azonia-

-boracyclohexa-4,6-dione.

With regard to the prerequi-

[34]

sites for the general spectro-

scopic features as well as the

Scheme 3. Synthetic route to the preparation of compounds 4a–f. Reagents and conditions: i) LiAlH

THF, reflux; ii) TFA, DDQ, EDIPA, BF ·Et O in CH Cl ; iii) 2208C, 30 min.

4

, dry charge-transfer characteristics

of such highly internally twisted

composite chromophores, the

conjugation and the planarity

3

2

ethano-2H-isoindole carboxylate (1) was prepared according

within the molecular fragment are important. The average

[31]

to a method reported earlier by us

and was converted

bond length of N1ꢀC4/N2ꢀC5 is 1.353(10) , which indi-

into 2-methyl-4,7-dihydro-4,7-ethano-2H-isoindole (2) by re-

cates a pronounced double-bond character in contrast, for

duction with excess LiAlH in dry THF under reflux for 2 h.

instance, to the average bond length of N1ꢀC1/N2ꢀC8,

4

Acid-catalyzed condensation of pyrroles 1 or 2 with various

aldehydes followed by treatment with N-ethyl-N,N-diisopro-

pylamine and BF ·Et O yielded the bicyclo[2.2.2]octadiene-

which is significantly longer (1.405(8) ). Nonetheless, in all

the compounds a strong p-electron delocalization is ob-

served in 3a,c–e within the central six-membered and both

adjacent five-membered rings, and in 4a,d within the whole

ring system. This p-electron delocalization is interrupted be-

tween both BꢀN bonds.

3

2

fused precursors 3a–f. The latter were then quantitatively

transferred into the aromatic ring-fused systems upon heat-

ing to 2208C under reduced pressure (10 mmHg) for 30 min.

We favored this strategy for several reasons:

For symmetry reasons, all ring atoms of 3a and 4a are ex-

actly positioned within a plane. The atoms are on a mirror

plane which coincides with that of the space group Pnnm. In

3c,d and 4d, the root-mean-square (rms) deviations of the

ring atoms are 0.022 , 0.037 , and 0.012 , respectively,

which also reveals a high planarity of the ring systems. In

contrast, we found a certain distortion of the chromophoreꢀs

planarity in 3e, as manifested in a corresponding mean rms

deviation of 0.195 . In this case, the ring system displays a

“butterfly” conformation; the single “wings” consisting of

two planes B1-N1-C1-C2-C3-C4-C9 and B1-N2-C5-C6-C7-

1

2

3

) Pyrroles fused with aromatic rings, namely isoindoles,

are too unstable to be used directly for the preparation

of such compounds.

) Precursors 3a–f can be prepared conveniently with satis-

factory yields; they are also more stable in acidic or

basic solutions and on silica gel than compounds 4a–f.

) Compounds 4a–f are obtained via a retro Diels–Alder

reaction and do not require further purification.

Chem. Eur. J. 2004, 10, 4853 – 4871

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ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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K. Rurack, N. Ono et al.

FULL PAPER

Table 1. Crystallographic data and details of the structure determinations of the compounds 3a,c–e and 4a,d.

3

a

3c

3d

3e

4a

4d

formula

C

29

H

27BF

2

N

2

C

39

H

46BF

2

N

3

O

4

·CH

2

Cl

2

C

37

H

41BF

2

N

2

O

5

·2CHCl

3

C

41

H

45BF

2

N

2

O

9

C

25

H19BF

2

N

2

33 2 2 5

C H33BF N O

M

T[K]

crystal system

space group

a[]

b[]

c[]

a[ ]

b[ ]

r

452.34

293(2)

754.55

298(2)

triclinic

881.26

293(2)

orthorhombic

Pbca

19.307(3)

18.032(3)

23.990(4)

90

8352(3)

8

1.402

758.62

298(2)

monoclinic

Cc

22.653(7)

11.952(3)

16.342(5)

90

124.563(5)

90

396.23

293(2)

586.42

293(2)

triclinic

orthorhombic

Pnnm

13.214(2)

18.342(3)

9.732(2)

90

orthorhombic

Pnnm

14.302(4)

20.467(5)

6.909(2)

90

¯

P1

11.938(2)

12.384(1)

14.872(2)

74.083(9)

69.963(10)

70.100(10)

1911(4)

2

8.2399(13)

12.537(2)

14.634(2)

70.993(2)

89.259(4)

85.554(3)

1424.9(4)

2

o

90

o

g[ ]

V[ ]

Z

3

2358.9(6)

4

1.274

3643(1)

4

1.383

2022.5(9)

4

1.301

ꢀ

3

1

calcd [gcm

]

1.311

0.224

1.367

0.100

ꢀ

1

m[mm

]

0.085

0.465

0.103

0.089

F(000)

crystal size[mm]

952

796

3648

0.320.380.42

27.5

1600

824

616

0.200.220.28 0.200.400.60

28.5

0.100.150.30 0.050.100.25 0.040.110.23

27.5

o

q

max [ ]

27.5

25.0

index ranges

ꢀ18ꢁhꢁ16

ꢀ15ꢁhꢁ14

ꢀ16ꢁkꢁ0

ꢀ19ꢁlꢁ18

9185

ꢀ25ꢁhꢁ24

ꢀ24ꢁkꢁ11

ꢀ34ꢁlꢁ31

9579

ꢀ29ꢁhꢁ25

ꢀ15ꢁkꢁ15

ꢀ18ꢁlꢁ21

6785

ꢀ18ꢁhꢁ19

ꢀ29ꢁkꢁ24

ꢀ9ꢁlꢁ7

1937

ꢀ11ꢁhꢁ10

ꢀ17ꢁkꢁ16

ꢀ12ꢁlꢁ20

4942

ꢀ

26ꢁkꢁ23

13ꢁlꢁ7

unique reflections

3136

reflections observed [I>2s(I)] 1948

4369

5175

5194

1295

2370

parameters

250

486

649

493

222

389

R1(on F) [I>2s(I)]

wR2(on F )

largest diff. peak/hole[e

0.0452

0.1311

0.234/ꢀ0.156

0.0650

0.1650

0.47/ꢀ0.62

0.0672

0.2002

0.637/ꢀ0.5010.78/

0.0772

0.1760

0.0448

0.1090

0.174/ꢀ0.235

0.0591

0.1565

0.434/ꢀ0.162

2

ꢀ

3

]

ꢀ0.38

C9. The dihedral angle between both planes is 14.3(9)8. The

significant deviation from coplanarity is most probably

caused by steric hindrances between both fluorine atoms

and the oxygen atoms O7 and O8 of the ester groups

the planes of the chromophore E1 (B1, N1, C1–C9), the

methene-appended phenyl ring E2, and the crownꢀs non-hy-

drogen atoms E3. Interestingly, as can be seen from Table 2,

the expected virtually orthogonal conformation is only

found for the BDI derivative 4d. However, 3c–e exhibit an

internal twist of the E1–E2 planes that is rather close to

biaryl derivatives, such as 9,10-diphenylanthracene (q=

(

Figure 1).

In 3c–e, the double bonds C11=C12 and C15=C16 are on

opposite sides of the molecular plane; that is, the ring sys-

tems contain approximately a twofold axis passing through

B1and C9. This in turn leads to the conclusion that in the

synthesis, molecular parts with the same absolute configura-

tion were fused. This should also be true for compound 3a.

However, the crystal structure of 3a is disordered as the

mirror plane of the space group Pnnm passing through the

atoms of the fused rings is fulfilled only statistically.

[35]

678)

rate (q=578),

or 10-methyl-9-p-A15C5-phenylacridinium perchlo-

[36]

which do not carry sterically demanding

substituents at positions close to the interannular bond.

Figure 2 shows the molecular packing in the crystal struc-

tures of 4a and 4d. In 4a, strong p–p electron interactions

exist between n-glide-plane-related molecules to form infi-

nite layers perpendicular to the b axis in the crystal. The

plane-to-plane distance is 3.45 . In the crystal of 4d

(Figure 2), strong p–p electron interactions are found be-

tween the molecular ring systems of molecules related by a

With respect to the methene-appended crown-substituted

phenyl moieties in 3c–e and 4d, the important structural

features of the heteroatom fused to the phenylꢀs para (and

meta, in the case of the benzocrowns) position can be ratio-

translation in the x direction by a to form infinite stacks.

o

2

nalized as follows: in 3c, the nitrogen atom N3 has a sp hy-

The plane-to-plane distance here is 3.49 . In addition to

these interactions, relatively strong van der Waals contacts

(shorter than 3.5 ) between adjacent crown moieties relat-

ed by a center of symmetry are found for this molecule. For

sterical reasons, comparable stacking interactions are not

possible in the dyes of the 3 series.

bridization state, because the sum of the bond angles equals

3

59.78. Moreover, the bond length N3ꢀC21=1.387(3) is

substantially shortened with respect to the alkyl amino

bonds N3ꢀC26=1.450(5) and N3ꢀC35=1.452(4) . In

3

d,e and 4d, the distances between the crown O atoms and

the p/m-phenyl C atoms are also significantly shorter than

the other ether-type OꢀC bonds within the crown. The aver-

Absorption and fluorescence spectroscopy of 3a–d and 4a–

d: The absorption and emission spectra of the bc-BDP and

BDI dyes carrying only alkyl substituents on the (p-extend-

ed) core are of similar shape to those of the classical BDP

dyes. In absorption, both series show narrow spectra with

two maxima, the global maximum appearing at l=527ꢂ

3 nm for 3a–d and at 598ꢂ3 nm in the case of 4a–d, irre-

age value for the first type of bond is 1.365(5) , and the

ether bonds show an average length of 1.419(12) .

For symmetry reasons, the phenyl rings C18-C19-C20-

C21-C20’-C19’ are positioned exactly perpendicularly to the

ring system in 3a and 4a. For the other molecules, the con-

formation may be described by the dihedral angles between

4856

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Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

2 2 3

Figure 1. ORTEP plots of 3a, 3c–3e, 4a, and 4d. For better clarity, only ordered molecules are shown; in the case of 3c and 3d, the CH Cl and CHCl

solvent molecules, respectively, were omitted.

Chem. Eur. J. 2004, 10, 4853 – 4871

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K. Rurack, N. Ono et al.

FULL PAPER

Table 2. Dihedral angles [8] of the molecular planes as defined in the

text.

q

E1–E2

q

E2–E3

3

4

c

65.0(1)

67.6(1)

67.9(1)

80.9(1)

60.3(1)

16.2(1)

19.9(2)

4.9(1)

d

e

d

spective of the solvent employed. The second maxima, or

shoulders on the high-energy side, are centered at lꢃ496

and 555 nm, respectively (Figure 3). In addition, a consider-

ably weaker, broad absorption band is found at ꢃ400 nm

for the bicyclo compounds (Figure 3, upper part), and its po-

sition is also not significantly affected by solvent polarity.

Analysis of the molecular orbitals (MOs) obtained in 1SCF

calculations for the AM1-optimized ground state geometries

of 3a and 4a reveals that the HOMO–LUMO transition

shows typical cyanine features (Figure S1, Supporting Infor-

mation). Both MOs are delocalized along the polymethinic

chain of the BDP and BDI core with alternating patterns

and show a large orbital overlap, and are thus strongly al-

lowed in both cases. Besides this oscillator-strong transition,

a second lowest transition in the 350–400 nm range with a

ꢃ5 times reduced oscillator strength is only found for 3a.

The molecular orbitals predominantly involved here are the

HOMOꢀ1/ꢀ2 and the LUMO, the two former are now ap-

proximately equally localized on the polymethinic BDP core

and the bicyclo fragments. For 4a, the second lowest transi-

tion is even less intensive and is only found at higher ener-

gies. Again, it is located on the entire BDI core. We thus at-

Figure 3. Absorption (c) and emission (a) spectra of 3a (upper

part, excited at 495 nm) and 4a (lower part, excited at 553 nm) in aceto-

nitrile.

these two transitions. The typical weak aniline-type absorp-

tion band is found at ꢃ270 nm only in the case of the 14-al-

kylanilino derivatives. In relation to the twist angles q

E1–E2

between the two biarylic molecular fragments, as found in

the X-ray analyses, the subunits seem to be entirely elec-

tronically decoupled in the dyes in liquid solutions. Packing

effects most probably account for q_E₁_–_E₂~708 of some of the

bc-BDP derivatives in the crystal.

To obtain further information on the influence of the sol-

!

vent on the lowest energy transition, the intense S₁

S

0

bands were converted to the energy scale and spectrally de-

convoluted by means of a progression of Gaussian functions

(Section S2 in the Supporting Information). For both series

of dyes, the full width at half maximum (fwhm) of the ab-

sorption bands only increase slightly as a function of the po-

tribute the broader and weaker absorption band in the 3

!

series to the S₂S transition, which partly involves the bi-

0

cyclo skeleton. Within the 3 and the 4 series, the variation

of the substituent in the 14-position has no influence on

Figure 2. Molecular packing in the crystal structure of 4a (left) and 4d (right).

4858

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

larity of the solvent (Table 3, and Tables S1and S2 in the

Supporting Information), which is presumably related to the

high rigidity and weak dipolar nature of the chromo-

As can be deduced from Figure 3, excitation of the bc-

BDPs and BDIs yields fluorescence spectra with a mirror-

image shape. The Stokes shifts are generally larger for the 3

series than those of dyes 4a–d (Table 3), and the fluores-

cence excitation spectra match the absorption spectra in all

the cases. While the BDP dyes with the core not fully alky-

[37]

phores. At the same time, it is evident that the fwhm of

a–d are distinctly smaller than those of 3a–d. In both

4

cases, the nature of the substituent in the 14-position has

again virtually no effect on these properties (Table 3, and

Tables S1and S2 in the Supporting Information; note that

the 14-position of the tetrahydrobis(ethano)diisoindoles or

bc-BDPs corresponds to the 8-position of the classical

BDPs). A similar reduction in bandwidth is found for the

[30a–c]

lated show emission maxima at l=500–510 nm,

the

fluorescence bands of 3a–d are found at 542ꢂ2 nm, which

is comparable to those of the tetramethyldiethyl-(8-phenyl)-

[30d]

substituted derivatives.

The spectra of 4a–d are further

red-shifted, and are centered at 606ꢂ2 nm. Again, the band

positions do not show any particular trend as a function of

solvent polarity. These features suggest that emission occurs

from the weakly polar, relaxed Franck–Condon excited state

of the dyes. The use of a similar fitting procedure as for the

absorption spectra showed the same trends of slightly nar-

rowing bands with reduced solvent polarity. In general, the

fwhm and the spacings D n˜ of the absorption and emission

bands agree rather well, and support the classification of the

[38]

cyanine reference compounds 8a–c with increasing degree

of bridging in the polymethine chain; that is, fwhm=970,

ꢀ

1

7

80, and 710 cm for 8a–c, respectively. In both cases, the

reduction in the degree of flexibility—by bridging the bonds

in the 8 series and by increasing the planar aromatic skele-

ton for 4 versus 3—diminishes the degrees of vibrational

freedom. Moreover, the results of the spectral analysis sup-

port these assumptions. The fits yield equidistant spacings

for the maxima of the single subbands; this freely fitted pa-

[39]

bc-BDP and BDI chromophores as polymethines. Howev-

er, the agreement is better for the highly conjugated dyes

ꢀ

1

rameter was calculated as 710ꢂ40 cm for 3a–d and 460ꢂ

ꢀ

1

ꢀ1

3

0 cm for 4a–d. As can be deduced from Figure S4 (in the

(e.g., D n˜ =460 vs 480 cm for absorption vs emission for

ꢀ1

Supporting Information), the molecular CꢀC frame vibra-

the 4 series, but 710 vs 810 cm for the 3 series), which

stresses the influence of the rigidity of the BDI chromo-

phore. In addition to the decrease in fwhm, the Stokes shift

gradually decreases for a certain dye on going from an ace-

tonitrile to a hexane environment, reflecting the influence

of the solvation capability of the solvents on such types of

highly delocalized and weakly dipolar chromophores. Dual

fluorescence as has been observed for several 8-p-amino-

phenyl-substituted classical BDP dyes (e.g. 5b; as well as

ꢀ

1

tion of ꢃ1300 cm , which is typical for cyanine dyes, is also

found for the present compound families. On the other

hand, the more expanded and more rigid BDI chromophoric

system is also reflected by the considerably higher extinction

coefficients of the dyes of the 4 series relative to the bc-

BDPs. A representative couple here is 4d with e

=

98

5

ꢀ

1

ꢀ1

1

12760ꢂ2250 cm

m

and 3d with e₅₂₆=46160ꢂ

ꢀ

1

ꢀ1

5

00 cm in acetonitrile, while the oscillator strengths of

m

[30a,b]

the entire transition differ only by a factor of 1.5 (see also

Figure 3 for comparison).

other dyes

) could be reliably detected in the present

case only for 3b and 3c in THF; the maximum of the broad

Table 3. Selected spectroscopic data of 3a–d and 4a–d in hexane and acetonitrile. For comprehensive solvent-dependent data, see Tables S1and S2 in

the Supporting Information.

F ^[

c]

t

k

nr

[1

Solvent

l

abs

fwhmabs

l

em

fwhmem

D n˜ abs-em

ꢀ1 [b]

f

r

ꢀ

1

[a]

ꢀ1 [a]

8

ꢀ1

8

ꢀ1

[nm]

[cm

]

[nm]

[cm

]

[cm

]

[ns]

[10 s

]

s

]

0

3

a

hexane

MeCN

hexane

529

526

527

524

527

524

529

526

599

597

599

597

599

598

n.d.

598

754

812

718

796

824

742

841

746

784

440

582

450

532

487

516

n.d.

537

544

543

540

544

542

539

543

542

541827

605

606

605

606

605

607

848

484

563

427

624

687

416

728

434

0.90

0.81

0.93

0.16

<0.001

0.80

5.96

1.5

1.3

0.2

0.3

907

808

984

980

791

1024

777

6.18

5.51

0.03

<0.003

5.53

0.01

6.59

1

b

1.7

0.1

[

d]

[e]

THF

MeCN

[

f]

[g]

[h]

n.d.

1.4

1.5

1.4

0.2

1.6

n.d.

0.4

1660

0.1

3

4

c

hexane

[

MeCN

0.0009

0.90

d

a

b

c

hexane

MeCN

hexane

MeCN

hexane

MeCN

hexane

MeCN

hexane

MeCN

503

0.84

6.45.3

530

593

527

577

509

560

511

605

176

2410.87

155

222

143

210

n.d.

252

0.91

.55 .63

0.90

0.022

0.74

0.093

0.78

0.86

5.71

0.2

1

5.65

0.19

5.52

0.70

5.45

5.66

1.6

1.2

1.3

1.4

1.5

0.2

51

0.5

13

0.4

0.2

[i]

d

ꢀ

1

[

5

a] Obtained from the data treatment and fitting procedure described in the text and Supporting Information, fwhm ꢂ1 0 cm (for 3 series) and ꢂ

ꢀ

1

cm (for 4 series). [b] Obtained as described in the Experimental Section. [c] Fluorescence quantum yield in the region of LE emission; after deconvo-

lution in case of dual emission. [d] Features of the dominant species in the region of the typical BDP or LE emission; CT emission band characteristics

ꢀ

1

are l=674 nm, fwhm=6900 cm , F

f

=0.022. The decays are biexponential over the entire emission spectrum with a second decay time of t

f

=3.39 ns.

[

e] See discussion and data in the text for more details. [f] No CT fluorescence could be detected. [g] Could not be reliably determined. [h] Not deter-

mined. [i] Not determined due to too low solubility.

Chem. Eur. J. 2004, 10, 4853 – 4871

www.chemeurj.org

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4859

K. Rurack, N. Ono et al.

FULL PAPER

and red-shifted charge-transfer emission appearing at l=

674 and 641nm, respectively. The time-resolved fluores-

cence measurements gave further evidence for this observa-

tion, vide infra.

The similarity of the nature of the absorbing and the emit-

ting state can be deduced from an analysis of the transition

dipole moments of absorption and emission, M_a_b_sand M_e_m,

by way of the radiative rate constants observed experimen-

tally [k , Eq. (1)] and calculated from the absorption spectra

r

calcd

[k_r, Eq. (2)]. Here, n is the refractive index of the solvent,

^c0 the speed of light in vacuum, and h and N are the

Planck and the Avogadro constants, respectively.

A

40]

Figure 4. Plot of the nonradiative rate constant vs solvent polarity for 4b

(data from Table S2 in the Supporting Information, EA=ethyl acetate,

CHone=cyclohexanone, PrCN=propionitrile). The two different regions

are indicated by a fit (c) and a trendline (g), the latter as a guide to

the eye.

[

4

^Ff

64p

3h

3

em

2

k_r¼_t_f

¼

n ~n jM_e_mj

ð1Þ

ð2Þ

4

6

3

4p

h

3

8

calcd

3

em

2

length dyes of the 4 series, which are equipped with a strong

anilino-type donor in the 14-position, do not show any

dual fluorescence. Apparently, as has been found for the

distyryl-substituted BDP chromophore that also emits at

k_r

¼

n ~n jM_a_b_sj

Z

hc ln 10

p N n

eð ~n_a_b_s^Þ^d^~ⁿ abs

2

0

3

^j^Mabsj ¼

A

^~ⁿ abs

band

[27b]

>

600 nm,

the excited-state charge-transfer process leads

to the formation of a nonemissive CT state in the BDI case,

too. Furthermore, because the flexibility in 4b,c is reduced

to a minimum, it seems reasonable to assume that an ener-

getic limit exists for the charge-recombination fluorescence

from strongly forbidden CT transitions, which is located

somewhere between 2.07 and 2.48 eV for the present type of

dye. At energies below this threshold, deactivation of such

highly twisted excited-state species seems to be entirely ra-

diationless. On the other hand, 3d as well as 4d, which car-

ries the less electron-donating benzocrown unit, show a high

fluorescence emission that is independent of solvent polari-

ty.

calcd

The ratios k /k_r=1.00ꢂ0.05, obtained for 3d and 4d in

r

acetonitrile and dibutyl ether, stress the previous interpreta-

tion of the data that the differences in conformation in-

volved in the absorption and emission processes seem to be

rather small and solvent-independent.

The fluorescence quantum yields (F ) are always high in

f

solvents of rather low polarity, such as diethyl ether or

hexane (Table 3 and Tables S1and S2 in the Supporting In-

formation). For the 14-phenyl-substituted derivatives 3a and

4

a, these features are unchanged upon increasing the polari-

ty of the solvent. However, an inspection of the respective

data of the 14-dimethylanilino-substituted dyes 3b and 4b

reveals that the fluorescence is quenched with increasing

solvent polarity. A similar trend is registered for the fluores-

cence lifetimes. Except for 3b in THF (and 3c, see Table S1

in the Supporting Information), monoexponential decays

are found for all dyes (Table 3 and Tables S1and S2 in the

Supporting Information). Accordingly, the rate constants of

Analysis of the dual fluorescence features of 3b in THF,

according to a procedure described in more detail in refer-

ence [30a], revealed the following characteristics: the ratio

of charge transfer to BDP-localized (or locally excited, LE)

fluorescence F (CT)/F (LE)=0.14. This value is distinctly

f

lower than F (CT)/F (LE)=2 of the all core-alkylated

f

[42]

model dye 7a in THF.

In combination with the overall

[

41]

fluorescence k_r

show a minor solvatokinetic modulation

stronger fluorescence of 3b in this solvent, F =0.18 versus

f

[42]

for 3a–d and 4a–d. The concomitant, distinctly more pro-

nounced increase in the nonradiative rate constant k_n_rthus

suggests that not only does 3b behave similarly to its classi-

cal BDP analogue with less sterically demanding substi-

0.03 for 7a, this suggests that CT state formation is less

efficient in the sterically more hindered dye (for the respec-

tive, virtually identical redox data and electron-transfer driv-

ing forces, see below). Following the procedure in refer-

ence [30a], the excited-state reaction dynamics of 3b in

[30a]

tuents,

but also an addressable and analytically potential-

9

ꢀ1

9

ꢀ1

ly valuable deactivation process can be installed in the BDI

derivatives. Here, two different regions are noticed in a plot

of k_n_ras a function of the solvent polarity; this suggests that

the quenching process is activated in solvents more polar

than THF (Figure 4). Consequently, when equipping the bc-

BDP and BDI core with a phenyl-A15C5 substituent, these

features are preserved and dyes 3c and 4c also exhibit

charge- or electron-transfer-type quenching of the emission.

As has been discussed above, the electron-donating nitrogen

THF are determined to k =31.910 s , k =3.610 s ,

LC

ꢀ1

CL

CT ꢀ1

9

and (t₀

)

=0.3110 s (for parameters, see Scheme 4

CT/ LE

and reference [43]). The ratio of k_fk_f=0.02 indicates

2

atom in the 14-anilino moiety of 3c is sp hybridized and

Scheme 4. Generalized kinetic scheme for the charge-transfer process in

thus readily feeds its electrons into the 14-phenyl ring. The

distance between these “virtually decoupled” ET partners is

at a minimum. It is interesting to note that the long wave-

3

0

b in THF. Where k are the respective rate constants and t the intrinsic

lifetimes of the LE and CT states. The CT-type ground state is highly un-

stable and relaxes immediately to the initial ground state.

4860

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

that fluorescence from the perpendicular state is forbidden,

and the ratio of the excited-state reaction rate constants k_L_C

/

k_C_L=8.9 stresses the observation that the CT state is still ef-

ficiently populated, yet the reaction is less strong than in the

simpler alkylated BDP analogues.

To obtain a better insight into the nature of the quenching

process in the alkylanilino-substituted derivatives and its de-

pendence on molecular motions, absorption as well as

steady-state and time-resolved fluorescence measurements

were carried out with 3b in ethanol as a function of temper-

ature, as well as with 3a and 4a,b in ethanol at 299 and

77 K. In all cases, the typical spectroscopic features of the

bc-BDP and BDI dyes are observed; that is, the narrow ab-

sorption and emission bands of mirror-image shape and

[

44,45]

monoexponential decay kinetics.

Upon reducing the

Figure 5. Fluorescence excitation (*) and emission (*) spectra of 3a

(

upper left), 3b (upper right), 4a (lower left), and 4b (lower right) in

temperature, the width of the spectral bands is further re-

duced, the vibronic structure more pronounced, and the

maxima show slight red-shifts below the freezing point of

the solvent. Concomitantly, the fluorescence quantum yields

and lifetimes of dyes 4b and 3b increase significantly with

decreasing temperature, while those of 3a and 4a show a

much less pronounced increase. For instance, t of 3a and

a change from 6.44 and 5.53 to 7.37 and 8.52 ns, respective-

ly, upon cooling a solution of the dye in ethanol from 299 K

to 77 K. The corresponding data of the 14-donor-substituted

derivatives were determined to 0.09 and 6.06 ns for 3b and

propylene glycol at 223 K. The corresponding anisotropies are included.

The reduction in r to ꢃ0 around 440 nm for 3b and 470 nm

for 4b is probably related to oscillator-weak transitions in-

volving the meso-substituent. Moreover, in the case of 3b,

the decrease of r on the low-energy side of the BDP emis-

sion and at ꢃ500 nm on the high-energy side of the excita-

tion bands suggests some admixing of other transitions and

remains unclear at the present stage of investigation. This

fact along with the peculiar CT features of 3b indicate that

further experiments are necessary to obtain a more detailed

picture of the deactivation processes in the meso-donor-sub-

stituted precursors of the BDI dyes.

f

4

2

.48 and 5.09 ns for 4b at 299 and 77 K, respectively. Follow-

ing the arrest of the CT process in 3b by gradually reducing

the temperature revealed that the photophysical parameters

are largely constant at temperatures below the glass temper-

EtOH

ature of ethanol (T_g=158 K). This suggests that the

quenching process is blocked below T . Furthermore, analy-

Cation-induced chemical switchingfeatures of 3c and 4c in

g

sis of the temperature-dependent emission data above T in

MeCN and of 3d and 4d in MeOH: Whereas the absence of

g

terms of the Arrhenius equation [Eq. (3)] yielded a linear

a quenching process in 3d might be rationalized in terms of

ꢀ

1

[30c]

correlation for a plot of lnk_n_rversus T and allowed the cal-

the similar behavior of 5c,

the immunity of 4d against

ꢀ

1

culation of an activation barrier of 7.9 kJmol for the non-

the activation of an electron-transfer process has a strong

impact on the design of efficient BDP-type fluorescent

switches for the red spectral range. Before we attempt to

access the underlying energetics and thus a means of how to

improve the BDI chromophore, exemplary cation-induced

switching features of the crowned dyes 3c,d and 4c,d will be

briefly presented.

[46]

radiative processes in 3b in ethanol. On the other hand,

k does not show a distinct temperature dependence with

r

8

ꢀ1

values of 1.5(ꢂ0.3)10 s over the entire temperature

range studied. This behavior points to a predominantly vis-

cosity-controlled quenching process that is active in the 14-

alkylaminophenyl derivatives.

The cation selectivities of the A15C5-substituted dyes are

similar to those found previously for many other such fluo-

ln k ¼ln AꢀE =RT

ð3Þ

A

[48]

roionophores,

and thus we will concentrate on protons

An analysis of the fluorescence anisotropy data of 3a,b

and 4a,b in propylene glycol at 223 K provided further in-

formation on the orientation of absorption and emission di-

poles within such molecules. Figure 5 reveals that r values

close to 0.4 are found for 3a, 4a, and 4b in the region of the

and particular Group I and Group II metal ions here. The

quenched fluorescence of 3c and 4c can be entirely revived

by protonation. This yields F =0.72 and t =6.01ns for 3c-

f

+

[49]

H

and F =0.91and t =5.58 ns for 4c-H in MeCN. Ac-

f f

cordingly, upon binding of alkali and alkaline-earth metal

ions to the A15C5 unit, the latterꢀs electron-donating ability

is also reduced and thus the CT process is interrupted. For

[47]

BDP and BDI bands. Thus, the dipoles of both transitions

are highly colinear, and oriented along the long axis of the

+

respective core. For 3a,b, r decreases in the region of the

instance, 3c-Na shows a high fluorescence enhancement

!

S₂

S

transition between 350–400 nm. This is consistent

factor (FEF) of 41. Very similar to the characteristics of 5b-

0

+

[30a]

with the involvement of the bicyclo rings, which induce a

Na ,

this pair also forms two different, spectrally undis-

symmetry distortion. The higher transitions (<400 nm) in

tinguishable complex species, as manifested in a biexponen-

4

a,b are partly mixed and involve various molecular orbitals

tial decay of the typical BDP fluorescence (t =0.35 and

f

localized asymmetrically on the BDI fragment, entailing r

values between 0.1–0.2 with dipole differences up to ꢃ458.

2.72 ns, amplitude ratio=1.02; see reference [30a] for a

more detailed discussion). In contrast, all the alkali and al-

Chem. Eur. J. 2004, 10, 4853 – 4871

www.chemeurj.org

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4861

K. Rurack, N. Ono et al.

FULL PAPER

kaline-earth metal ion complexes of 4c display only single

Table 4. Selected complex stability data of the sensor molecules investi-

gated.

exponential decays, ranging for example, from t =4.46 ns

f

+

2+

Complex

Solvent

logK

for 4c-K to 5.75 ns for 4c-Ca . The corresponding fluores-

+

cence quantum yields are generally high with an average of

3c-Na

MeCN

MeOH

1.95

3.95

2.53

4.56

2.20

4.40

2.50

2.35

2

+

3

4

c-Ca

c-Na

c-Ca

0.7 for the monovalent and 0.85 for the divalent cation com-

+

plexes. This simultaneous increase in fluorescence quantum

2

+

yield and lifetime signifies that the k of the complexes are

+

r

5b-Na

2+

rather similar to those of the unbound dyes and only the k_n_r

are reduced; the quenching process is switched off. Owing

to the higher quantum yield of 4c in the unbound state, the

FEF between 7.5 and 9.5 are lower for the red-emitting dye.

Moreover, the bulkiness of the BDI core with its annelated

phenyl rings, which point in the direction of the receptor

unit, seems to shift the equilibrium of different complex

conformers to one form only, thus yielding monoexponential

decays.

5b-Ca

3e-Na

+

4

e-Na

Evidently, 3c and 4c with an A15C5 unit suffer from a

low selectivity and this nitrogen-atom-containing receptor is

not very suitable for selectively targeting a biologically im-

portant alkali metal ion, such as sodium. The use of a ben-

zocrown instead would improve the chances of obtaining se-

The spectral properties of the complexes are rather simi-

+

n+

+

lar to those of the free dyes. For 3b-H as well as 3c-M

,

lective Na fluoroionophores. However, as already men-

the absorption and fluorescence spectra are slightly red-

tioned above, 3d and 4d show a strong fluorescence in sol-

+

shifted (e.g. to 528/545 and 531/551 nm for 3b-H and 3c-

vents from hexane to methanol and, thus, these molecules

2

+

Ca , respectively), and the widths are slightly larger. The

mirror-image relationship is also found. Since upon excita-

tion, the electron density is increased at the 14-position of

the bc-BDP core in the 3 series (see Section S1in the Sup-

porting Information), the red-shift upon protonation or

cation binding is conceivable with the change from a donor

to an acceptor moiety in this position. All these effects are

less pronounced in the case of 4b,c (spectral red-shifts of 1

or 2 nm, Figure 6), in which the change in electron density

at the 14-position, obtained from the quantum chemical cal-

culations, is less pronounced. The complex stoichiometries

determined from titrations are 1:1. However, whereas 4c

remained spectroscopically silent toward Na in methanol.

Therefore, further modification of the BDI chromophore

became necessary.

Electrochemical properties of 3a–d and 4a–d: Before set-

ting out to enhance the BDI core, a detailed analysis of the

electron-transfer features of the dyes presented so far is in-

dispensable. Thus, the redox properties of the compounds

were studied by cyclic voltammetry in 0.1m TBAP in aceto-

nitrile. The electrochemical data and electron-transfer pa-

rameters of 3a–d, 4a–d, and the reference compounds 5a–c,

and 7a are given in Table 5. Compound 3a displays a rever-

sible one-electron reduction wave at the same potential

+

2+

and Na or Ca

show complex stability constants that

red

=

2

+

agree well with those of 5b, complex formation is evidently

hampered by the bulky bicyclo groups in 3c, resulting in

(E

1

=ꢀ1570 mV vs Fc /Fc) and a reversible oxidation

ox

=

2

wave at a slightly less positive potential (E

1

=670 mV) than

+

2+

lower logK for both Na and Ca in MeCN (see Table 4).

the conventional BDP model dye 5a. The reduction poten-

tials of 3b–d are slightly negatively shifted into the range

from ꢀ1595 to ꢀ1640 mV, and a second oxidation wave ap-

Table 5. Redox potentials and electron-transfer data of compounds 3a–d,

ꢀ

3

4

a–d and various model compounds (in CH CN, c=10 m, 0.1m TBAP,

3

ꢀ

1 [a]

u=250 mVs ).

red

=2

ox

1=2

ox

1=2

E₁

[

E

[eV]

00

DGET

[eV]

[

b]

[c]

mV]

[mV]

3

4

5

7

a

b

c

d

a

b

c

ꢀ1570

ꢀ1640

ꢀ1595

ꢀ1530

ꢀ1590

ꢀ1600

ꢀ1565

ꢀ1560

ꢀ1600

ꢀ1575

ꢀ1686

670

740

730

665

330

290

310

325

760

840

750

700

+

–

540

570

1150

–

630

700

1150

–

570

1150

512

2.36

2.37

2.36

2.07

2.08

2.07

2.49

2.50

2.49

2.39

–

ꢀ0.3

ꢀ0.1

0.3

-

0.0

0.1

0.5

–

[

d]

e]

[

e]

[e]

d]

[

d

[

f]

a

[f]

b

ꢀ0.4

0.1

ꢀ0.3

[

g]

[d]

c

[

f]

a

+

Figure 6. A) Absorption and emission spectra of 4c (c) and 4c-Na

(

[a] Referenced against Fc /Fc. [b] BDI or bc-BDP chromophore.

[c] Group in the 14- or 8-position. [d] Value of dimethoxybenzene.

[e] Quasireversible. [f] Redox data taken from reference [42]. [g] Redox

data taken from reference [30c].

g) in acetonitrile. The absorption band of the complex overlaps

strongly and is omitted for clarity. B) Corresponding, representative titra-

ꢀ

6

tion curve (c4c =1.210 m).

4862

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

peared that is attributed to the oxidation of the 4-nitrogen

substituents on the meso aromatic ring in compounds 3b

and 3c. The oxidation potential for the benzocrown moiety

in 3d is much higher and cannot be detected. To calculate

the driving force DG_E_Tof the excited-state electron-transfer

reaction according to Equation (4), we thus take the tabulat-

Tuningthe BDI chromophore toward 4e ,f: On the basis of

[30c]

previous design considerations,

the introduction of elec-

tron-withdrawing substitutents, such as ester groups, to the

pyrro- or indomethene core seems to provide a means to ac-

tivate a redox-based quenching process in otherwise silent

ensembles. Accordingly, we synthesized 4e,f via the precur-

sors 3e,f, vide supra. As is evident from Table 6, the intro-

ox

+

ed value of dimethoxybenzene (E =1150 mV vs Fc /

1

=

2

[30c]

Fc).

A comparison of this value with those given for the

amino substituents in Table 5 indicates that the benzocrown

group is a considerably weaker donor than the anilino or

azacrown units.

Table 6. Redox potentials and electron-transfer data of compounds 3e,f,

ꢀ

3

4

3

e,f and reference compounds 6a,b (in CH CN, c=10 m, 0.1m TBAP,

1 [a]

ꢀ

u=250 mVs ).

red

=2

ox

1=2

ox

1=2

E₁

[

E

[eV]

00

DGET

[eV]

[b]

[c]

mV]

[mV]

ꢀ

DG_E_T¼DE ꢀIP þEA þE

ð4Þ

00

D

A

C

[

d]

[e]

3

4

4 f

6a

e

f

e

ꢀ970

ꢀ940

ꢀ870

ꢀ840

n.m.

1210

750

820

1040

1090

+

1150

–

1150

–

1150

–

2.30

1.96

2.49

ꢀ0.3

–

0.0

–

[

f]

In Equation (4) DE₀₀is the zero–zero transition energy of

[f]

[e]

the excited donor–acceptor pair, IP is the ionization poten-

D

[

g]

h]

[e]

tial of the donor, EA_Ais the electron affinity of the accep-

ꢀ1250

ꢀ0.2

[

6

b

ꢀ1250

–

tor, and E is the Coulomb stabilization energy of the radi-

C

+

ꢀ

[30c]

cal ion pair DC ꢀAC , taken to 0.1eV.

As can be deduced

[a] Referenced against Fc /Fc. [b] BDI or bc-BDP chromophore.

c] Group in the 14- or 8-position. [d] Not measurable. [e] Value of dime-

thoxybenzene. [f] Irreversible peak (E ). [g] Redox data taken from refer-

[

from the data of 3b,c, 5b, and 7a in Table 5, the driving

forces for these anilino-substituted BDPs are rather similar

and largely correlate with the fluorescence deactivation pa-

rameters for a single series.

p

ence [30c]. [h] Redox data taken from reference [42].

Compared with 3a–d, the reduction potentials of 4a–d

are only slightly shifted to more positive values, for example,

duction of the ester groups indeed causes the redox poten-

tials to dramatically shift to the positive. Moreover, in com-

parison to our previous example, the 300 mV shift of the re-

duction and oxidation potentials of 5c with respect to 6a,

the shifts in the bc-BDP system are almost twice as large.

For instance, the reduction potential of 3 f is 630 mV less

negative, and the oxidation potential is 540 mV more posi-

tive than that of 3a. Even more favorably, such dramatic ef-

fects are also observed in the more conjugated 4 f (690 mV

positive shift for reduction and 490 mV positive shift for oxi-

dation) and 4e (695 mV positive shift for reduction and

425 mV positive shift for oxidation) with respect to 4a and

4d. Accordingly, the HOMO–LUMO energy gaps of the

present dyes are narrowed even further. With regard to the

electron-transfer driving force, both 3e and 4e should now

show the favorable quenched fluorescence in the unbound

state.

4

a shows a reversible one-electron reduction at=ꢀ1530 mV.

However, their oxidation potentials are considerably nega-

tively shifted (e.g. to 330 mV for 4a), approximately ten

times more than the reduction potentials. These findings in-

dicate that fusion of the benzene rings strongly increases the

HOMO energy level, while the LUMO energy levels are vir-

[50]

tually unaffected. As a consequence, the HOMO–LUMO

separation in 4a–d is narrowed. In addition, a second oxida-

tion wave appeared for 4b,c that is attributed to the oxida-

tion of the nitrogen donor atom. The oxidation of the recep-

tor group in the benzocrown derivative 4d is again much

higher and could not be detected under the experimental

conditions used. From the DG_E_Tdata in Table 5, it is appa-

rent that the values for 4b,c are distinctly higher than those

of the corresponding bc-BDPs; this reflects the generally

higher fluorescence emission of the 14-anilino BDIs in

highly polar solvents.

The validity of this assumption can be directly seen from

Table 7 and Table S4 in the Supporting Information. Where-

Table 7. Selected spectroscopic data of 3e,f and 4e,f in various solvents.

Solvent

l

abs

fwhmabs

l

em

fwhmem

D n˜ abs-em

ꢀ1 [b]

F

f

t

f

k

r

k

nr

[1

ꢀ

1

[a]

ꢀ1 [a]

8

ꢀ1

8

ꢀ1

[nm]

[cm

]

[nm]

[cm

]

[cm

]

[ns]

[10 s

]

s

]

0

3

4

e

f

hexane

MeCN

MeOH

hexane

MeCN

MeOH

hexane

MeCN

MeOH

hexane

MeCN

MeOH

547

538

541

548

538

542

638

634

636

637

633

634

907

1071

996

568

569

571

570

571

572

655

662

944

500

894

833

654

914

834

386

592

0.32

4.15

0.02

0.03

3.95

0.15

0.19

6.53

0.22

0.8

0.3

0.8

0.4

0.5

1.6

588

312

1.7

66

52

1090

1159

902

1095

1031

683

0.005

0.001

0.32

0.006

0.008

0.35

914

1029

1001

848

867

87166 18 55

702

e

f

1.0

44

834

51 0.046

0.018

0.8

[c]

7

0.37

654

662

664

678

857

867

382

639

652

0.44

0.43

0.34

6.09

6.52

5.84

0.7

0.6

0.9

1.1

784

918

ꢀ

1

[

a] Obtained from the data treatment and fitting procedure described in the text and Supporting Information, fwhmꢂ1 0 cm . [b] Obtained as described

in the Experimental Section. [c] Biexponential decay with a second t =3.47 ns, see ref. [52]; k and knr data not determined.

f

r

Chem. Eur. J. 2004, 10, 4853 – 4871

www.chemeurj.org

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4863

K. Rurack, N. Ono et al.

FULL PAPER

as 4 f, for instance, shows strong emission in solvents of high

and low polarity, the fluorescence of 4e is progressively

quenched upon going from hexane to methanol. The

quenching characteristics are again similar to those reported

for 4b,c above; that is, significant spectral shifts are not no-

ticed and the fluorescence lifetime concomitantly decreases.

A plot of the nonradiative rate constant versus the solvent

Temperature-dependent studies carried out as described

above revealed the typical electron-transfer-type quenching

features. Upon reducing the temperature, the absorption

and emission bands become narrower and more structured,

and shift by ꢃ8 nm to the red in the glass. The fluorescence

decays are singly exponential, and the increase in lifetime is

pronounced for 4e, from 1.75 to 7.36 ns, yet less dramatic

for 4 f, from 6.12 to 6.33 ns at 299 and 77 K, respectively.

A remarkable effect, however, is found for 3e,f. For these

two dyes, not only does 3e show a reduced fluorescence in

polar solvents relative to 3 f, but the fluorescence of the

model dye 3 f itself is quenched upon increasing the polarity

of its environment, a behavior that is evidently independent

of the CT process described so far (Figure 7, Table 7, and

Table S3 in the Supporting Information). The effect of ester

substitution on the spectral band positions is yet again com-

parable to their BDI relatives, that is, they are bathochromi-

cally shifted by lꢃ25 nm (Figure 8). Also in agreement with

polarity function shows

a

very similar development

(

Figure 7). Apparently, the acceptor-modified BDI core is

Figure 7. Plot of the nonradiative rate constant vs solvent polarity for 3e

) and 3 f (*), both in the upper part, as well as 4e ( ) and 4 f (*) in

(

&

the lower part (data from Tables S3 and S4 in the Supporting Informa-

tion; for solvent key, see Figure 4).

Figure 8. Absorption and fluorescence spectra of 3d (c) and 3e (a)

in acetonitrile.

now strong enough to activate an electron-transfer process

from the considerably weak benzocrown donor. Further-

more, an advantageous side-effect of the substitution of the

BDI core is the fact that both the absorption and emission

band maxima shift by ꢃ40 nm to the red and are now

found in a wavelength range for which a variety of low-cost

laser diodes (e.g., of AlGaInP- or GaInP-type) are commer-

cially available. Spectral analysis by means of the deconvo-

lution and fitting procedure introduced previously reveals

very similar behavior as for the nonacceptor-substituted

BDIs; that is, narrowing bands and Stokes shifts on decreas-

ing the solvent polarity (Table 7 and Table S4 in the Sup-

porting Information). The ester groups themselves do not

seem to have a pronounced influence on the polymethinic

conjugation within the chromophore other than introducing

a higher degree of flexibility. While the spectra are broad-

the general optical excitation features of the BDI dyes are

!

the relationships of the integral S₁S transitions and the

0

molar extinction coefficients between 3d and 3e, the quo-

tient of the integrals amounting to 0.99 and the respective

ꢀ

1

ꢀ1 [51]

value of 3e being e₅₃₈=34540ꢂ580m cm .

Besides the fact that the two lowest transitions are not

well-separated anymore for 3e,f (see, for example, Figure 8),

the mirror-image relationship between absorption and fluo-

rescence spectra is fulfilled. Thus, apparently, the accelerat-

ed radiationless deactivation of 3 f is not caused by a differ-

ent emitting species, accelerated internal conversion, or any

pronounced conformational changes between the Franck–

Condon and the fluorescing excited state, but to the activa-

tion of another “dark” deexcitation channel. Keeping in

mind that the most stable conformation of 3e in the crystal-

line state is a “butterfly”-like kinked geometry (see the sec-

tion on X-ray diffraction), we tentatively assume that we ob-

served here, for the first time, the solvatokinetically control-

led activation of a nonfluorescent competitor state postulat-

ꢀ

1

ened, as manifested in a fwhm=784 versus 537 cm of the

subbands of 4e versus 4d in acetonitrile, the molar extinc-

tion coefficients are reduced, for example, e =68600ꢂ

6

34

ꢀ

1

ꢀ1

4

00m cm versus e₅₉₈=112760m cm for this pair of

[22f]

dyes, with the integral strength of the transition remaining

ed recently by Lindsey et al.

Inspection of the theoretical

osz

very similar with f_S₁_S₀(4e)/f_S₁_S₀(4d)=1.05. Moreover, a

comparison of the observed and the calculated radiative

rate constants performed in the same way as outlined above

results in that work suggested that this “butterfly”-type spe-

cies has a certain charge-transfer character and should pos-

sess a higher dipole moment than the planar conformer.

Thus, its formation might be facilitated with increasing sol-

vent polarity and might lead to the quenching observed here

calcd

r

also yields ratios of k /k =1.00ꢂ0.05 for 3e and 4e in

r

acetonitrile and dibutyl ether; this supports the composite

dye model and the polymethinic nature of the ester-substi-

tuted chromophore.

[53]

for 3 f. In the highly rigid and conjugated BDI chromo-

phore, such a bending motion should be much less favored

4864

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

and interference by this quenching channel is not expected.

The data in Table 7 support these considerations.

probability of the backreaction seems to be a determining

factor here. The latter is expected to be lower in 7b. At the

present stage, we cannot provide a more detailed picture;

however, further studies involving a more sophisticated the-

oretical treatment as well as femtosecond transient absorp-

tion spectroscopy and other model compounds are currently

being performed to access the underlying mechanism.

A study of the effect of temperature variation on the

spectroscopic properties of the ester-substituted dyes 3e,f

promised a better picture of the quenching process. As

would be expected, the spectroscopic changes upon steadily

reducing the temperature of solutions of 3 f in ethanol as

well as going from 299 K to 77 K for 3e are small and in

agreement with the previous results. Moreover, the increase

in fluorescence yield is in accordance with the other low-

temperature data as well, resulting in lifetime changes from

With respect to the dye molecule of predominant analyti-

cal interest here, namely 4e, Figure 10 reveals its favorable

0.39 to 5.13 ns and from 0.09 to 5.15 ns at 299 and 77 K for

3

f and 3e, respectively. An Arrhenius-type analysis of the

temperature-dependent fluorescence behavior of 3 f in etha-

ꢀ

1

nol (Figure 9) gave an activation barrier of 5.5 kJmol for

+

Figure 10. A) Absorption and emission spectra of 4e (c) and (4e)Na

ꢀ

1

Figure 9. Plot of lnknr versus T for 3 f in ethanol. The temperature de-

pendence above (c) and below (virtually constant, g) the glass tem-

perature of ethanol are indicated.

(

g) in methanol. The absorption band of the complex largely overlaps

l

max =636 nm) and is omitted for clarity. B) Corresponding, representa-

ꢀ6

tive titration curve (c4e =210 m).

the quenching reaction. Comparison with literature data on

other polyaromatic compounds in which such a structural re-

laxation pathway in the excited state plays an important role

for nonradiative deactivation, for example, 9-tert-butyl- or 9-

sensing features. Without any pronounced shifts in absorp-

tion and emission, the formation of a 1:1 complex of log K=

2.35 stability leads to a ꢃ8-fold enhanced fluorescence in

+

methanol, F (4e-Na )=0.35, with the typical ET-type char-

f

[54a]

[55]

2+

tert-pentyl-anthracenes,

er in the present case is rather small. For instance, Jahn and

suggests that the activation barri-

acteristics as described above for 4c. Ca and Mg do

not induce any modulation of the fluorescence, and the

ꢀ

1

+

Dreeskamp found an activation energy of 17.5 kJmol for

effect of K is much less pronounced. In principle, these ad-

[54a]

such a process in 9-tert-butyl-anthracene in ethanol.

vantageous features are preserved in water. Unfortunately,

Moreover, as in the case of the anthracenes, a certain “but-

terfly”-like asymmetry in the ground state of the molecule

the solubility of 4e in water is not high enough to prevent

aggregation. Thus, when attempting a detection of Na with

+

[54b]

seems to facilitate the reaction.

However, recent solvatokinetic studies on the fluores-

4e in highly aqueous media, complicated band shifts are al-

[56]

ready observed in absorption. We are thus presently fol-

lowing two different routes with this system: modification of

the BDI skeleton to improve the actual performance of 4e,

and development of a sensing protocol that can profit from

aggregation phenomena.

[22e]

cence behavior of model compound 7b

show that the in-

tramolecular quenching reactions of the anthracene and the

BDP chromophore are not identical. This closely related

BDP derivative deviates from 3 f only by the bulkiness of

the alkyl substituents in the 1-, 2-, 6-, and 7-positions—the

bicyclo skeleton versus methyl and ethyl groups—and also

possesses a kinked conformation in the crystalline state (ꢃ

Conclusion

13.38 as derived from the X-ray structure given in refer-

ence [22e]). In contrast to 3 f, however, 7b exhibits the

common BDP features of uniform and considerably high

The present contribution introduced and characterized a

new class of strongly absorbing and highly emissive fluores-

cent dyes that can be used at advantageously long wave-

lengths. On the basis of preparative pyrrole chemistry, which

constitutes an active and intensively studied area of organic

chemistry, the facile synthesis in comparatively high yields

of a variety of BDI derivatives shown here should stimulate

fluorescence yields F =0.26ꢂ0.03 in solvents of any polari-

f

[

42]

ty.

Apparently, in the BDP case, not only a prekinked

ground-state conformation seems to be the driving force to-

wards this “dark” channel, but most probably the barrier

height between the two excited conformers, and thus the

Chem. Eur. J. 2004, 10, 4853 – 4871

www.chemeurj.org

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4865

K. Rurack, N. Ono et al.

FULL PAPER

further research in this area. X-ray structure analyses pro-

vided detailed insight into the molecular specifications of

these dyes. An important outcome of the work reported

here is the interdependence of redox and spectral data. In

the same way as the extension of the p system does not dra-

matically change the energetics that are important for the

CT process, it is not expected that a further step to the red

by increasing the size and conjugation of the BDI chromo-

phore would have a different impact on such an intramolec-

ular process. However, by simple synthetic means and

design considerations it is possible to tune a “silent switch”,

such as 4d, to an active one, such as 4e. Based on this im-

proved understanding of the energetics and photophysics of

the bc-BDP and BDI chromophores, we have now a tool in

hand that should allow us to custom-design new fluorescent

signaling ensembles. Moreover, in addition to supramolec-

ular sensor and switch chemistry as well as electron- or

energy-transfer cascades or arrays, such functional dyes are

promising candidates for new active materials for dye lasers,

organic light-emitting diodes, or other redox-controlled opti-

cal applications; they are also expected to give new impulses

to labeling chemistry for imaging applications.

32.22, 33.33, 105.57, 117.18, 125.24, 129.47, 136.07, 136.40 ppm; MS (EI):

+

m/z: 159 [M] , 131 [MꢀC

2 4

H ] ; elemental analysis calcd (%) for

C H13N: C 82.97, H 8.23, N 8.80; found: C 82.82, H 8.05, N 8.83.

11

General procedure for the preparation of 3a–d: Compound 2 (2 mmol)

[58]

and an aldehyde [benzaldehyde (3a), 4-(dimethylamino)benzaldehyde

(

[59]

3b), N-(4’-formylphenyl)aza[15]crown-5 (3c) or 15-formylbenzo[15]c-

rown-5 (3d); 1mmol] were dissolved in dry CH Cl (100 mL) under ni-

2

trogen. One drop of trifluoroacetic acid (TFA) was added, and the solu-

tion was stirred at room temperature in the dark for 2–4 h (until TLC in-

dicated complete consumption of the aldehyde). A solution of dichlorodi-

2 2

cyanobenzoquinone (DDQ, 1mmol) in dry CH Cl (20 mL) was added,

and the mixture was stirred for additional 15 min. The reaction mixture

was then treated with N-ethyl-N,N-diisopropylamine (3 mL) and boron

trifluoride etherate (3 mL). After stirring for another 30 min, the dark

brown solution was washed with water (320 mL) and brine (30 mL),

dried over Na SO , and concentrated at reduced pressure. The crude

2 4

product was purified by silica-gel flash column chromatography and re-

crystallization from CHCl /hexane.

3

7,7-Difluoro-5,9-dimethyl-14-phenyl-1,4,10,13-tetrahydro-1,4;10,13-bis-

ethano-7-bora-7H-benzo[1,2-a;4,5-a’]diisoindole (3a): Diisoindole 3a was

prepared from the reaction of 2 (318 mg, 2 mmol) and benzaldehyde

(

2 2

106 mg, 1 mmol) in dry CH Cl (100 mL) under nitrogen. The crude

product was purified by chromatography (silica gel, 10% EtOAc/

hexane). Recrystallization from CHCl /hexane yielded orange-red, metal-

3

1

like shining crystals (190 mg, yield 42%). M.p. 1968C (decomp);

NMR (400 MHz, CDCl ): d=7.57–7.39 (m, 5H), 6.40–6.36 (m, 2H),

.08–6.03 (m, 2H), 3.80–3.82 (m, 2H), 2.61 (m, 2H), 2.53 (s, 6H), 1.15–

H

3

6

1

3

.42 ppm (m, 8H); C NMR (100 MHz, CDCl ): d=12.52 (both), 25.84

(

one diastereomer), 25.88 (other diastereomer), 26.42 (one diastereo-

mer), 26.48 (other diastereomer), 32.87 (both), 35.16 (both), 110.40

Experimental Section

(

both), 121.74 (one diastereomer), 121.77 (other diastereomer), 127.75

both), 130.5 (both), 130.90 (both), 134.20 (one diastereomer), 134.29

Materials: Metal perchlorates purchased from Aldrich were of highest

(other diastereomer), 135.82 (one diastereomer), 135.84 (other diaster-

eomer), 140.62 (one diastereomer), 140.65 (other diastereomer), 146.41

(both), 148.57 (both), 150.11 ppm (both); IR (KBr): n˜ =1562, 1509 (C=C,

[

57]

purity available and were dried in a vacuum oven before use. HClO

4

,

7

0% Suprapur from Merck, was used for the protonation experiments.

ꢀ

1

+

The dyes used for the determination of the relative fluorescence quantum

yields and the calibration of the laser impulse fluorometer were obtained

from Lambda Physik GmbH (Göttingen, Germany). All the solvents em-

ployed were of UV spectroscopic grade (Aldrich, Merck, and Fluka).

C=N), 1197 cm (BꢀF); MS (EI): m/z: 452 [M] , 396 [Mꢀ2C

2 4

H ] ; ele-

O: C 75.50, H 6.12, N

1

mental analysis calcd (%) for C29

H

27BF

2

N

2

· =

2

H

2

6.07; found: C 75.23, H 6.05, N 6.03.

,7-Difluoro-5,9-dimethyl-14-[4-(dimethylamino)phenyl]-1,4,10,13-tetrahy-

dro-1,4;10,13-bisethano-7-bora-7H-benzo[1,2-a;4,5-a’]diisoindole (3b):

Acid-catalyzed condensation of 2 (318 mg, 2 mmol) and 4-(dimethylami-

no) benzaldehyde (149 mg, 1 mmol) in dry CH Cl (100 mL) at room

temperature, followed by flash chromatography (silica gel, 10% EtOAc/

hexane) and recrystallization from CHCl /hexane yielded red crystals

(258 mg, yield 52%). M.p. 2008C (decomp); H NMR (400 MHz,

CDCl ): d=7.23–7.25 (m, 2H), 6.79–6.81(m, 2H), 6.38–6.41(m, 2H),

6.1–6.15 (m, 2H), 3.8–3.81 (m, 2H), 3.10 (s, 6H), 2.93–3.07 (m, 2H), 2.52

7

General synthesis methods: All syntheses were carried out under a nitro-

gen atmosphere. Unless otherwise noted, all reagents or solvents were of

commercial reagent grade and were used without further purification.

Dry dichloromethane and N-ethyl-N,N-diisopropylamine were obtained

2

by refluxing and distilling over CaH

2

under inert atmospheres. Dry THF

3

1

was distilled from sodium/benzophenone under an inert atmosphere.

Column chromatography and TLC were carried out on C-200 (Wakogel)

and Kieselgel 60F254 (Merck), respectively. Melting points were mea-

3

1

13

sured with a Yanagimoto BY-1apparatus. H NMR and C NMR spectra

were recorded in CDCl on a JEOL EX400 spectrometer at ambient

3

(s, 6H), 1.18–1.44 ppm (m, 8H); C NMR (100 MHz, CDCl ): d=12.48

(both), 25.94 (one diastereomer), 25.98 (other diastereomer), 26.52 (one

diastereomer), 26.60 (other diastereomer), 32.89 (both), 35.28 (one dia-

stereomer), 35.31 (other diastereomer), 40.37 (both), 111.09 (both),

122.19 (one diastereomer), 122.21 (other diastereomer), 127.81 (both),

129.92 (one diastereomer), 130.25 (other diastereomer), 130.65 (both),

134.20 (one diastereomer), 134.28 (other diastereomer), 135.87 (both),

137.67 (both), 140.63 (one diastereomer), 140.65 (other diastereomer),

146.55 (both), 150.16 (one diastereomer), 150.87 ppm (other diastereo-

3

temperature. NMR chemical shifts are expressed relative to TMS as the

internal standard. IR and UV/Vis spectra were obtained with Hitachi

2

70–30 and Shimadzu UV-2200 spectrometers, respectively. Mass spectra

were measured with a JEOL JMS-700 spectrometer (20 eV for EI and m-

nitrobenzyl alcohol as matrix for FAB). Elemental analyses were per-

formed with a Yanako MT-5 recorder. DSC and TG experiments were

performed on a Seiko Instruments EXSTAR6000 apparatus.

ꢀ

1

mer); IR (neat): n˜ =1566, 1504 (C=C, C=N), 1188 cm (BꢀF); MS (EI):

2

-Methyl-4,7-dihydro-4,7-ethano-2H-isoindole (2): LiAlH

4

(759 mg,

+

m/z: 495 [M] , 439 [Mꢀ2C

2 4

H ] ; elemental analysis calcd (%) for

2

0 mmol) powder was added in small portions over a period of 30 min to

31 2 3

C H32BF N : C 75.16, H 6.51, N 8.48; found: C 75.12, H 6.50, N 8.45.

a stirred solution of ethyl 4,7-dihydro-4,7-ethano-2H-isoindole-1-carboxy-

[

31]

late (869 mg, 4 mmol) in dry THF (100 mL) under argon, and the reac-

tion mixture was refluxed for 2 h. After cooling to room temperature,

water (50 mL) was added dropwise, and the resulting mixture was filtered

through a Celite pad, which was washed with EtOAc (350 mL). The or-

ganic layer was separated, and the aqueous layer was extracted with

EtOAc (220 mL). The combined organic layers were washed with

7,7-Difluoro-5,9-dimethyl-14-[4-(1,4,7,10,13-azatetraoxaheptadecanyl)-

phenyl]-1,4,10,13-tetrahydro-1,4;10,13-bisethano-7-bora-7H-benzo[1,2-

a;4,5-a’]diisoindole (3c): Diisoindole 3c was prepared from acid-cata-

lyzed condensation of 2 (318 mg, 2 mmol) and N-(4’-formylphenyl)-

aza[15]crown-5 (323 mg, 1 mmol) in dry CH

flash chromatography (silica gel, 50% EtOAc/CHCl

tallization from CHCl /hexane to yield orange-red crystals (482 mg, yield

72%). M.p. 1928C (decomp); H NMR (400 MHz, CDCl

2

Cl

2

(100 mL) and purified by

3

) followed by recrys-

water (550 mL) and brine (30 mL), dried over Na

ed at reduced pressure. The residue was purified by chromatography on

silica gel (50% CHCl /hexane) to give 573 mg (yield 90%) of 2 as color-

less plates. M.p. 129–1308C; H NMR (400 MHz, CDCl

2

SO

4

, and concentrat-

3

1

3

): d=7.20–7.22

(m, 2H), 6.73–6.75 (m, 2H), 6.38–6.4 (m, 2H), 6.14–6.15 (m, 2H), 3.85–

3.87 (m, 2H), 3.82 (m, 2H), 3.67–3.71(m, 18 H), 3.0 (m, 2H), 2.51(s,

3

1

3

): d=8.40 (s,

H), 7.22 (s, 1H), 6.50–6.45 (m, 2H), 3.76 (m, 2H), 2.19 (s, 3H), 1.57–

.43 ppm (m, 4H); C NMR (100 MHz, CDCl ): d=11.09, 27.42, 27.50,

3

1

3

1

6H), 1.21–1.45 ppm (m, 8H); C NMR (100 MHz, CDCl

3

): d=12.45

1

3

(both), 25.93 (one diastereomer), 25.98 (other diastereomer), 26.51(one

4866

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

+

diastereomer), 26.60 (other diastereomer), 32.88 (both), 35.31(one dia-

stereomer), 35.34 (other diastereomer), 52.79 (both), 68.55 (both) , 69.84

(FAB): m/z: 759 [M+H] ; elemental analysis calcd (%) for

: C 64.91, H 5.98, N 3.69; found: C 64.62, H 6.01, N 3.65.

41 2 2 9

C H45BF N O

(

both), 70.26 (both), 71.27 (both), 110.40 (both) , 121.74 (one diastereo-

Diethyl-7,7-difluoro-14-phenyl-1,4,10,13-tetrahydro-1,4;10,13-bisethano-7-

bora-7H-benzo[1,2-a;4,5-a’]diisoindole-5,9-dicarboxylate (3 f): Dicarboxy-

late 3 f was prepared from the acid-catalyzed condensation of 1 (434 mg,

mer), 121.77 (other diastereomer), 127.75 (both), 130.17 (one diastereo-

mer), 130.5 (other diastereomer), 130.90 (both), 134.20 (one diastereom-

er), 134.29 (other diastereomer), 135.82 (one diastereomer), 135.84

2

mmol) and benzaldehyde (106 mg, 1 mmol) in dry CH

under nitrogen. It was purified by chromatography (silica gel, 50%

EtOAc/CHCl ) and recrystallization from CHCl /hexane to yield red

crystals (210 mg, yield 37%). M.p. 1308C (decomp); H NMR (400 MHz,

CDCl ): d=7.57–7.68 (m, 3H), 7.40–7.42 (m, 2H), 6.38–6.42 (m, 2H),

.03–6.06 (m, 2H), 4.46 (q, J=7.3 Hz, 4H), 4.31(m, 2H), 2.57–2.59 (m,

2 2

Cl (100 mL)

(

other diastereomer), 140.62 (one diastereomer), 140.65 (other diaster-

eomer), 146.41 (both), 148.57 (both), 150.11 ppm (both); IR (neat): n˜ =

1

elemental analysis calcd (%) for C39

N 6.19; found: C 68.92, H 6.83, N 6.11.

3

ꢀ

1

+

558, 1508 (C=C, C=N), 1195 cm (BꢀF); MS (FAB): m/z: 670 [M+H] ;

1

H

46BF

2

N

3

O

4

· =

2

H

2

O: C 69.02, H 6.98,

3

6

1

3

7

,7-Difluoro-5,9-dimethyl-14-(1,4,7,10,13-pentaoxabenzoheptadecan-15-

2H), 1.46 (t, J=7.3 Hz, 6H), 1.13–1.42 ppm (m, 8H);

C NMR

yl)-1,4,10,13-tetrahydro-1,4;10,13-bisethano-7-bora-7H-benzo[1,2-a;4,5-

a’]diisoindole (3d): Diisoindole 3d was prepared from the acid-catalyzed

condensation of 2 (318 mg, 2 mmol) and 15-formylbenzo[15]crown-5

(100 MHz, CDCl ): d=14.12 (both), 25.57 (one diastereomer), 25.85

(other diastereomer), 26.12 (both), 34.09 (both), 35.53 (both), 61.55

3

(both), 65.83 (both), 110.37 (both), 121.69 (one diastereomer), 121.73

(other diastereomer), 127.73 (both), 130.3 (both), 130.92 (both), 134.19

(one diastereomer), 134.27 (other diastereomer), 135.77 (one diastereo-

mer), 135.81 (other diastereomer), 140.61 (one diastereomer), 140.63

(other diastereomer), 146.39 (both), 148.53 (both), 150.13 ppm (both); IR

(

296 mg, 1mmol) in dry CH

tography (silica gel, 50% EtOAc/CHCl

from CHCl /hexane to yield orange-red crystals (372 mg, yield 58%).

M.p. 1928C (decomp); H NMR (400 MHz, CDCl

2

Cl

2

(100 mL), and purified by flash chroma-

3

) followed by recrystallization

3

1

3

): d=6.93–6.99 (m,

H), 6.38–6.41(m, 2H), 6.09–6.1(m, 2H), 4.27 (m, 2H), 4.1(m, 2H),

.01–4.02 (m, 2H), 3.92–3.94 (m, 2H), 3.81–3.83 (m, 10H), 2.82 (m, 2H),

ꢀ

1

3

4

2

1

(neat): n˜ =1558, 1510 (C=C, C=N), 1190 cm (BꢀF); MS (EI): m/z: 568

+

[M] , 540 [MꢀC H ] ; elemental analysis calcd (%) for

2

4

1

3

1

.52 (s, 6H), 1.15–1.44 ppm (m, 8H); C NMR (100 MHz, CDCl

3

): d=

33 31 2 2 4 2 2

C H BF N O · = H O: C 68.64, H 5.58, N 4.85; found: C 68.77, H 5.46,

2.52 (both), 25.88 (one diastereomer), 25.92 (other diastereomer), 26.51

N 4.83.

General procedure for the preparation of 4a–f: Compounds 3a–f were

heated at 2208C under N at reduced pressure (10 mmHg) to afford

quantitative yields of the corresponding pure 4a–f.

,7-Difluoro-5,9-dimethyl-14-phenyl-7-bora-7H-benzo[1,2-a;4,5-a’]diisoin-

(

one diastereomer), 26.57 (other diastereomer), 32.90 (one diastereo-

mer), 32.92 (other diastereomer), 35.26 (one diastereomer), 35.28 (other

diastereomer), 68.39 (both), 68.72 (both), 69.21(both), 69.23 (both),

2

6

(

9.36 (both), 70.14 (both), 70.98 (both), 71.06 (both), 112.3 (both), 114.3

one diastereomer), 114.4 (other diastereomer), 122.0 (one diastereom-

er), 122.44 (other diastereomer), 127.33 (one diastereomer), 127.38

other diastereomer), 127.41 (one diastereomer), 127.5 (other diaster-

eomer), 134 (both), 135.84 (both), 135.90 (one diastereomer), 135.93

other diastereomer), 138.0 (one diastereomer), 138.82 (other diaster-

7

1

dole (4a): Copper-colored crystals; m.p.>2508C; H NMR (400 MHz,

CDCl ): d=7.64–7.68 (m, 5H), 7.51–7.53 (m, 2H), 7.08–7.11 (m, 2H),

.98–7.02 (m, 2H), 6.16–6.18 (m, 2H), 2.97 ppm (s, 6H); C NMR

100 MHz, CDCl ): d=12.45, 121.18, 121.94, 123.75, 124.76, 128.72,

29.01, 129.20, 129.38, 130.50, 130.53, 133.89, 135.05, 150.41 ppm; IR

3

(

1

3

6

(

3

(

1

eomer), 147.4 (both), 148.35 (one diastereomer), 148.4 (other diastereom-

ꢀ

1

(

[

neat): n˜ =1542, 1510 (C=C, C=N), 1191 cm (BꢀF); MS (EI): m/z: 396

er), 149.6 (both), 150.2 ppm (both); IR (neat): n˜ =1562, 1504 (C=C, C=

+

M] ; elemental analysis calcd (%) for C25

H

19BF

2

N

2

·H

2

O: C 72.49, H

ꢀ

1

+

N), 1187 cm (BꢀF); MS (EI): m/z: 642 [M] , 623 [MꢀF] , 586

5

.11, N 6.76; found: C 72.73, H 4.82, N 6.77.

+

1

[

Mꢀ2C

2

H

4

] ; elemental analysis calcd (%) for C37

2 2 5 3 2

H41BF N O · = H O: C

7

,7-Difluoro-5,9-dimethyl-14-[4-(dimethylamino)phenyl]-7-bora-7H-

6

8.52, H 6.48, N 4.32; found: C 68.54, H 6.43, N 4.33.

benzo[1,2-a;4,5-a’]diisoindole (4b): Copper-colored crystals; m.p.

General procedure for the preparation of 3e and 3 f: Compound 1

1

>

2508C; H NMR (400 MHz, CDCl

3

) d=7.63–7.65 (m, 2H), 7.30–7.32

(

2 mmol) and 15-formylbenzo[15]crown-5 (3e, 1mmol) or benzaldehyde

3 f, 1mmol) were dissolved in dry CH Cl (100 mL) under nitrogen. One

(

m, 2H), 7.01–7.11 (m, 4H), 6.91–6.93 (m, 2H), 6.43–6.45(m, 2H), 3.13

2

1

3

s, 6H), 2.95 ppm (s, 6H); C NMR (100 MHz, CDCl

3

): d=12.41, 40.44,

12.55, 121.61, 121.74, 122.19, 123.55, 125.51, 128.54, 129.75, 130.38,

34.06, 135.96, 149.68, 150.85 ppm; IR (neat): n˜ =1531, 1508 (C=C, C=N),

drop of TFA was added, and the solution was refluxed in the dark for

2 h (until TLC indicated complete consumption of the aldehyde). After

cooling to room temperature, a solution of DDQ (1mmol) in dry CH Cl

20 mL) was added, and the mixture was stirred for additional 15 min.

1

2

ꢀ

1

+

195 cm (BꢀF); MS (EI): m/z: 439 [M] ; elemental analysis calcd (%)

: C 73.82, H 5.51, N 9.56; found: C 73.77, H 5.58, N 9.44.

,7-Difluoro-5,9-dimethyl-14-[4-(1,4,7,10,13-azatetraoxaheptadecanyl)-

phenyl]-7-bora-7H-benzo[1,2-a;4,5-a’]diisoindole (4c): Copper-colored

(

2 3

for C27H24BF N

The reaction mixture was then treated with N-ethyl-N,N-diisopropyla-

mine (3 mL) and boron trifluoride etherate (3 mL). After stirring for an-

other 30 min, the dark brown solution obtained was washed with water

7

1

crystals; m.p.>2508C; H NMR (400 MHz, CDCl

3

): d=7.63–7.65 (m,

(

320 mL) and brine (30 mL), dried over Na

under reduced pressure. The crude product was purified by silica gel

flash column chromatography and recrystallization from CHCl /hexane.

Diethyl-7,7-difluoro-14-(1,4,7,10,13-pentaoxabenzoheptadecan-15-yl)-

,4,10,13-tetrahydro-1,4;10,13-bisethano-7-bora-7H-benzo[1,2-a;4,5-a’]di-

2 4

SO , and concentrated

2

H), 7.27–7.29 (m, 2H), 7.07–7.11 (m, 4H), 6.86–6.88 (m, 2H), 6.48–6.50

1

3

(

m, 2H), 3.89–3.91(m, 2H), 3.63–3.74 (m, 18 H), 2.96 ppm (s, 6H);

C

3

NMR (100 MHz, CDCl ): d=14.12, 52.81, 68.57, 69.86, 70.27, 71.33,

1

12.40, 121.67, 121.75, 127.95, 130.67, 131.43, 134.07, 135.37, 136.63,

1

41.67, 146.61, 149.57, 150.91 ppm; IR (neat): n˜ =1538, 1508 (C=C, C=N),

isoindole-5,9-dicarboxylate (3e): Dicarboxylate 3e was prepared from the

acid-catalyzed condensation of 1 (434 mg, 2 mmol) and 15-formylben-

zo[15]crown-5 (296 mg, 1 mmol) in dry dichloromethane (100 mL), and

ꢀ

1

+

195 cm (BꢀF); MS (EI): m/z: 61 3 M[ ] ; elemental analysis calcd (%)

: C 68.52, H 6.24, N 6.85; found: C 68.45, H 6.32, N

for C35

2 3 4

H38BF N O

6

.78.

was purified by chromatography (silica gel, 50% EtOAc/CHCl

crystallization from CHCl /hexane to yield red crystals (258 mg, yield

4%). M.p. 1408C (decomp); H NMR (400 MHz, CDCl

m, 3H), 6.39–6.44 (m, 2H), 6.09–6.11 (m, 2H), 4.45 (q, J=7.3 Hz, 4H),

3

) and re-

7

,7-Difluoro-5,9-dimethyl-14-(1,4,7,10,13-pentaoxabenzoheptadecan-15-

3

1

yl)-7-bora-7H-benzo[1,2-a;4,5-a’]diisoindole (4d): Copper-colored crys-

3

): d=6.92–7.05

1

tals; m.p. >2508C; H NMR (400 MHz, CDCl

3

): d=7.64–7.66 (m, 2H),

(

7

4

.0–7.13 ( m, 7H), 6.34–6.36 (m, 2H), 4.31–4.33 (m, 2H), 4.05–4.09 (m,

H), 3.81–3.87 (m, 10H), 2.95 ppm (s, 6H); C NMR (100 MHz, CDCl ):

3

4

3

.30 (m, 4H), 4.10–4.12 (m, 2H), 4.02–4.04 (m, 2H), 3.94–3.96 (m, 2H),

1

3

.81–3.83 (m, 8H), 2.81 (m, 2H), 1.45 (t, J=7.3 Hz, 6H), 1.14–1.29 ppm

1

3

d=12.43, 68.48, 68.50, 69.16, 69.41, 70.16, 70.17, 71.00, 71.11, 113.66,

(

m, 8H); C NMR (100 MHz, CDCl

3

): d=14.10 (both), 25.67 (one dia-

1

13.81, 121.50, 121.65, 121.85, 123.80, 125.00, 127.52, 128.92, 130.42,

stereomer), 25.87 (other diastereomer), 26.11 (both), 34.09 (both), 35.51

33.84, 134.48, 149.55, 149.60, 150.28 ppm; IR (neat): n˜ =1538, 1509 (C=

(

both), 61.56 (both), 65.82 (both), 68.32 (both), 68.84 (both), 69.07

both), 69.18 (both), 70.02 (both), 70.08 (both), 70.9 (both), 70.96 (both),

ꢀ

1

+

C, C=N), 1192 cm (BꢀF); MS (EI): m/z: 586 [M] ; elemental analysis

calcd (%) for C33 ·H O: C 65.57, H 5.84, N 4.63; found: C

5.55, H 5.74, N 4.37.

Diethyl-7,7-difluoro-14-(1,4,7,10,13-pentaoxabenzoheptadecan-15-yl)-7-

bora-7H-benzo[1,2-a;4,5-a’]diisoindole-5,9-dicarboxylate (4e): Blue

): d=8.10–8.12 (m,

2H), 7.26–7.30 (m, 4H), 6.99–7.19 (m, 3H), 6.36–6.38 (m, 2H), 4.61 (q,

H

33BF

2

N

2

O

5

2

1

12.25 (both), 113.9 (one diastereomer), 114.16 (other diastereomer),

21.84 (one diastereomer), 122.33 (other diastereomer), 126.43 (both),

30.46 (both), 133.60 (both), 135.92 (one diastereomer), 135.98 (other di-

6

astereomer), 138.26 (both), 142.44 (both), 147.72 (one diastereomer),

1

48.72 (other diastereomer), 150.58 (both), 153.17 (both), 161.02 ppm

powder; m.p. 143–1458C; H NMR (400 MHz, CDCl

3

ꢀ

1

(

both); IR (neat): n˜ =1546, 1512 (C=C, C=N), 1192 cm (BꢀF); MS

Chem. Eur. J. 2004, 10, 4853 – 4871 www.chemeurj.org

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4867

K. Rurack, N. Ono et al.

FULL PAPER

J=7.3 Hz, 4H), 4.33 (m, 2H), 4.05–4.08 (m, 4H), 3.81–3.86 (m, 10H),

sion spectra (cf. Figure S4 in the Supporting Information), and not from

the global band maxima (in nm) as given in the tables. It should further

be noted that in addition to deconvolution, conversion of the fluores-

1

3

1

6

1

3

.55 ppm (t, J=7.3 Hz, 6H); C NMR (100 MHz, CDCl ): d=14.14,

2.21, 62.30, 68.36, 68.52, 68.99, 69.22, 70.04, 70.04, 70.90, 71.00, 113.06,

21.24, 122.07, 124.04, 126.51, 126.57, 129.42, 129.92, 131.14, 134.64,

40.03, 141.06, 149.88, 150.17, 160.63 ppm; IR (neat): n˜ =1546, 1511 (C=

[

66]

cence spectra to the energy scale requires another correction step.

Time-resolved fluorescence spectroscopy: Fluorescence lifetimes (t

f

)

ꢀ

1

+

C, C=N), 1192 cm (BꢀF); MS (FAB): m/z: 703 [M+H] ; elemental

analysis calcd (%) for C37 : C 63.26, H 5.31, N 3.99; found: C

2.98, H 5.09, N 3.90.

Diethyl-7,7-difluoro-14-phenyl-7-bora-7H-benzo[1,2-a;4,5-a’]diisoindole-

were measured with a unique customized laser impulse fluorometer with

picosecond time resolution. To be able to excite the BDI dyes in their

longest-wavelength absorption band, the setup described by us in an ear-

H

2 2 9

37BF N O

6

[

67]

lier publication was modified with respect to the excitation source. The

80 fs pulses provided at 800 nm with a repetition rate of 82 MHz by a fre-

1

5

(

,9-dicarboxylate (4 f): Copper-colored crystals; m.p. >2508C; H NMR

400 MHz, CDCl ): d=8.18–8.20 (m, 2H), 7.52–7.71 (m, 5H), 7.21–7.25

m, 2H), 6.95–6.99 (m, 2H), 6.11–6.13 (m, 2H), 4.58 (q, J=7.3 Hz, 4H),

3

4

quency-doubled Nd:YVO laser (Spectra Physics Millennia Xs) pumped

Ti:sapphire laser (Spectra Physics Tsunami) were used to seed a regener-

ative amplifier (Spectra Physics Spitfire P-5K), whereby pulse amplifica-

tion was obtained by pumping with a frequency-doubled Nd:YLF laser

(Spectra Physics Evolution X) operated at 5 kHz. The typically 90 fs long

output pulses were fed into an OPA (Spectra Physics OPA-800-F-HGII)

and used to generate the respective excitation light by frequency quadru-

pling the idler of the OPA. The fluorescence was collected at right angles

1

3

1

6

1

3

.56 ppm (t, J=7.3 Hz, 6H); C NMR (100 MHz, CDCl ): d=14.14,

2.21, 68.36, 121.17, 121.97, 123.75, 124.77, 128.75, 129.02, 129.23, 129.43,

30.51, 130.57, 133.92, 135.15, 150.50 ppm; IR (neat): n˜ =1546, 1509 (C=

ꢀ

1

+

C, C=N), 1191 cm (BꢀF); MS (EI): m/z: 51 2 [M ] , 464 [MꢀBF

] ; ele-

2 2 4

H23BF N O : C 67.99, H 4.53, N 5.47;

2

mental analysis calcd (%) for C29

found: C 67.76, H 4.62, N 5.51.

(

polarizer set at 54.78; monochromator with spectral bandwidths of 4, 8,

X-ray crystallography: A summary of the crystallographic data and de-

tails of the structure determinations is given in Table 1. The data were

collected on a Bruker SMART CCD diffractometer (3a,d and 4a,d;

MoKa radiation, l=0.71073 , w scans), a RigakuAFC5R diffractometer

and 16 nm) and the fluorescence decays were recorded with a modular

single-photon timing unit described in reference [67]. With typical instru-

mental response functions of 25–30 ps (full width at half maximum), the

time division was 5.2 ps per channel and the experimental accuracy was

ꢂ3 ps. The laser beam was attenuated with a double prism attenuator

from LTB and typical excitation energies were in the nanowatt-to-micro-

watt range (average laser power). Other details on the characterization

and calibration of the instrument can be found in references [57,67]. The

fluorescence lifetime profiles were analyzed with a PC and the software

package Global Unlimited V2.2 (Laboratory for Fluorescence Dynamics,

University of Illinois). The goodness-of-fit of the single decays, as judged

by reduced chi-squared (c ) and the autocorrelation function C(j) of the

residuals, was always below c <1.2. For all dyes showing a single fluores-

cence band, decays were recorded at three different emission wave-

lengths and analyzed globally. For the dual emissive dyes, decays were re-

corded at ten different wavelengths over the entire emission spectrum

and also analyzed by global analysis. Such a global analysis of decays re-

corded at different emission wavelengths implies that the decay times of

the species are linked while the program varies the preexponential fac-

(

3c; MoKa radiation, 2Vꢀw scans), and a Rigaku/MSC Mercury CCD

diffractometer (3e; MoKa radiation, w scans). The data were corrected

for Lorentz and polarization effects, absorption, and secondary extinc-

tion. The structures were solved by direct methods and refined by full-

matrix least-squares methods based on F (SHELX97

4

2

[60]

for 3a,d and

[

61]

a,d, teXsan for 3c,e). In 4a and 3d, hydrogen atoms were located by

difference Fourier syntheses. For all other compounds, hydrogen posi-

tions were calculated in correspondence to their geometrical conditions.

It should be noted that 4a contains a mirror plane which coincides with

that of the space group Pnnm. For 3a, the Pnnm symmetry is fulfilled

only statistically. This leads to an overlap of the double bonds C11=C12

and C15=C16 with the single bonds C11AꢀC12A and C15AꢀC16A, re-

2

R

2

R

spectively. Therefore, four corresponding restraints for bond lengths were

used in the structure refinement. In the structure of 3e, a conformational

disorder of the benzocrown ring was observed. This disorder leads to a

splitting of one oxygen and three carbon atoms.

2

tors and lifetimes until the changes in the error surface (c surface) are

minimal, that is, convergence is reached. The fitting results are judged for

every single decay (local c ) and for all the decays (global c ). The

errors for all the global analytical results presented here were below a

CCDC-215231 (3a), CCDC-210669 (3c), CCDC-214765 (3d), CCDC-

2

10668 (3e), CCDC-212601 (4a), and CCDC-212600 (4d) contain the

2

R

2

R

supplementary crystallographic data for this paper. These data can be ob-

tained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or

from the Cambridge Crystallographic Data Centre, 12 Union Road, Cam-

bridge CB21EZ, UK; fax: (+44)1223-336033; or deposit@ccdc.cam.uk)

2

R

global c =1.2.

Low-temperature and temperature-dependent measurements: Absorp-

tion and fluorescence measurements as a function of temperature or at

7

Steady-state absorption and fluorescence spectroscopy: UV/Vis spectra

were recorded on a Bruins Instruments Omega10 spectrophotometer and

for the steady-state fluorescence spectra, a Spectronics Instruments 8100

spectrofluorometer was employed. Titrations and low-temperature phos-

phorescence checks were performed on a Perkin–Elmer LS50B spectro-

fluorometer. All measurements were carried out at 298ꢂ1K. Molar ex-

tinction coefficients were determined from N=8 individual samples. For

the fluorescence experiments, only dilute solutions with an optical density

7 K were conducted with a continuous flow cryostat CF1204 (Oxford

Instruments), modified for us with liquid nitrogen. Liquid was

N

2

pumped from a storage container through a transfer tube (GFS300,

Oxford Instruments) and flow control (VC30, Oxford Instruments) by a

gas flow pump (GF2, Oxford Instruments) through the cryostat. The

temperature was externally controlled by heating and flow adjustment

with the temperature controller ITC4 (Oxford Instruments). The temper-

ature in the sample rod was monitored by means of the temperature-de-

pendent resistance of a sensor, which was calibrated with a Peltier ele-

ment. In the cooling regime, the temperature was gradually decreased in

steps of 30 K with equilibration times of 20 min at every point of mea-

surement.

(

OD) below 0.1at the absorption maximum were used and the fluores-

cence measurements were performed with a 908 standard geometry and

polarizers set at 54.78 (emission) and 08 (excitation). The fluorescence

quantum yields (F

NaOH (F

=0.90ꢂ0.03)

thylaminostyryl)-4H-pyran in methanol (F

series as well as rhodamine 101 in ethanol (F

compounds of the 4 series, except 4e,f. For the latter, cryptocyanine in

f

) were determined relative to fluorescein 27 in 0.1n

[

62]

f

and 4-dicyanomethylene-2-methyl-6-(p-dime-

[

63]

Cyclic voltammetry: Cyclic voltammograms were obtained in acetoni-

trile/0.1m TBAP (tetra-n-butylammonium perchlorate) on a BAS electro-

chemical analyzer model BS-1with a platinum disk as the working elec-

trode, Ag/AgNO₃as the quasi-reference electrode, and a platinum wire

as the counterelectrode. Redox potentials were referenced internally

against ferrocenium/ferrocene (Fc /Fc). Measurements were performed

under an inert atmosphere with a scan rate of 250 mVs at room temper-

ature.

f

=0.43ꢂ0.08)

for the 3

[

64]

f

=1.00ꢂ0.02) for all the

[

65]

ethanol (F

f

=0.007ꢂ0.001) was used, which was additionally checked

against rhodamine 101 for comparability. All the fluorescence spectra

presented here were corrected for the spectral response of the detection

system (calibrated quartz halogen lamp placed inside an integrating

sphere; Gigahertz-Optik) and for the spectral irradiance of the excitation

channel (calibrated silicon diode mounted at a sphere port; Gigahertz-

Optik). The uncertainties of the fluorescence quantum yields were deter-

+

ꢀ

1

Complex stability constants: The complex stability constants reported

here were determined from fluorescence measurements. The complex

stability constants K were measured by titrating a dilute solution (typical-

ly 10 m) of ligand by adding aliquots of metal ion solution (cM0 titra-

tion). Methods detailed in reference [57] based on models introduced by

mined to ꢂ5% (for F

f

>0.2), ꢂ10% (for 0.2>F

f

>0.02), ꢂ20% (for

ꢀ

3

ꢀ3

ꢀ6

0

.02>F

f

>510 ), and ꢂ30% (for 510 >F

f

). The Stokes shifts were

calculated from the differences between the maxima of the lowest/highest

energy subband of the corresponding deconvoluted absorption and emis-

[

68]

Valeur et al. were employed to fit the complexometric titration data

4868

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

(

with Origin V7.0, Originlab Corp.). The reported values are mean

[12] N. Tyutyulkov, J. Fabian, A. Mehlhorn, F. Dietz, A. Tadjer, Polyme-

thine Dyes, St. Kliment Ohridski University Press, Sofia, 1991,

pp. 128–137.

values of at least two measurements with correlation coefficients >0.99.

Quantum chemical calculations: Geometry optimizations were performed

employing the semiempirical AM1method (gradient <0.01) and transi-

tion energies were calculated on the basis of these ground state geome-

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[

69]

(

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[

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[12]

cycanine with X=Y=N(CH

3 2

) and n=4 is found at 625 nm. For

[

representative fluorescein dyes and their spectroscopic features, see

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[44] In accordance with the room-temperature behavior of 3b in acetoni-

trile, no long wavelength CT emission band could be detected for 3b

in ethanol at 299 K or at any lower temperature.

[45] Phosphorescence could not be detected for any of the dyes at 77 K

in the wavelength range up to 850 nm and at various delay times in

the range of 0.05–100 ms with a Perkin–ElmerLS50B spectrometer

with a low-temperature luminescence accessory, flash lamp excita-

tion (fwhm<10 ms), and gated PMT.

[

26] K. Yamada, T. Toyota, K. Takakura, M. Ishimaru, T. Sugawara, New

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[46] This value lies in between the activation energies determined for the

charge-transfer reactions connected with an intramolecular rotation

coordinate of a less sterically hindered molecule such as N,N-dime-

1

13, 396–399; Angew. Chem. Int. Ed. 2001, 40, 385–387; b) K.

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92.

ꢀ

1

thylaminobenzonitrile (4 kJmol

;

see K. A. Zachariasse, M.

Grobys, T. von der Haar, A. Hebecker, Y. V. Ilꢀichev, Y.-B. Jiang, O.

Morawski, W. Kühnle, J. Photochem. Photobiol. A 1996, 102, 59–

2

[

28] A preliminary account of the synthetic procedure was published in

M. Wada, S. Ito, H. Uno, T. Murashima, N. Ono, T. Urano, Y.

Urano, Tetrahedron Lett. 2001, 42, 6711–6713.

29] To better distinguish the chromophoric properties and owing to the

fact that, from the spectroscopic point of view, the tetrahydrobis-

ethano boron–diindomethenes are more closely related to the classi-

cal BDPs than to the fully aromatic BDIs, the tetrahydrobisethano

derivatives are referred to and abbreviated as bicyclo-appended

BDPs or bc-BDPs throughout the paper.

7

4

0) and a more sterically hindered molecule such as 4-(2,6-dimethyl-

ꢀ1

-N,N-dimethylaminophenyl)benzonitrile (14.3 kJmol

;

see M.

Maus, W. Rettig, J. Phys. Chem. A 2002, 106, 2104–2111).

[

47] r=(FVVꢀFVH)/(FVV +2FVH) with r=anisotropy, F=fluorescence in-

tensity and subscripts V and H=vertically and horizontally aligned

polarizers. Measurements above and below the glass point of etha-

nol at 170 and 140 K yielded very similar results in the case of 3b.

48] A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M.

Huxley, C. P. McCoy, J. T. Rademacher, T. E. Rice, Chem. Rev. 1997,

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30] a) M. Kollmannsberger, K. Rurack, U. Resch-Genger, J. Daub, J.

Phys. Chem. A 1998, 102, 10211–10220; b) K. Rurack, M. Koll-

mannsberger, U. Resch-Genger, J. Daub, J. Am. Chem. Soc. 2000,

9

7, 1515–1566.

49] The dimethylamino derivatives show a similar behavior, resulting in

+

F

f

=0.67 and t

f

=5.76 ns for 3b-H and F

f f

=0.88 and t =5.47 ns for

1

22, 968–969; c) M. Kollmannsberger, K. Rurack, U. Resch-Genger,

+

4

b-H , respectively.

W. Rettig, J. Daub, Chem. Phys. Lett. 2000, 329, 363–369; d) G.

Beer, K. Rurack, J. Daub, Chem. Commun. 2001, 1138–1139.

[

50] Note that these experimental data are well reproduced by the theo-

retical data given in Section S1of the Supporting Information.

51] It is important to note that a theoretical analysis of the respective

transitions in the ester-substituted bc-BDP chromophore revealed

[

31] S. Ito, T. Murashima, N. Ono, J. Chem. Soc. Perkin Trans. 1 1997,

3

161–3165.

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an even closer energetic spacing of the two lowest transitions as re-

1

!

ported for the bc-BDP core above. Again, the S

2

S

0

transition in-

volves the HOMOꢀ1/ꢀ2 and LUMO with the former MOs being

again largely localized on the bicyclo fragments (see Supporting In-

formation). We thus tentatively assign the broad band on the high-

energy side which overlaps considerably with the cyanine-type band

to this transition and took this into account during spectral analysis.

52] The biexponential decay of 4e in MeOH is presumably caused by

hydrogen bonding of solvent molecules to the benzocrown donor

atoms.

53] Note that the electrochemical data listed for 3 f in Table 6 do not

show any peculiarities or hint at any such excited-state (quenching)

reaction mechanism.

54] a) B. Jahn, H. Dreeskamp, Ber. Bunsen-Ges. 1984, 88, 42–47; b) A

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1

2

[

35] V. Langer, H.-D. Becker, Z. Kristallogr. 1992, 199, 313–315.

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[

1

989, 45, 1356–1358.

[

37] Representative ground-state dipole moments as obtained from the

quantum chemical calculations are for example, 3.8 D for 3a and

2

.1D for 4a.

[

38] We have already studied some features of these dyes (J. L. Bricks,

J. L. Slominskii, M. A. Kudinova, A. I. Tolmachev, K. Rurack, U.

Resch-Genger, W. Rettig, J. Photochem. Photobiol. A 2000, 132,

1

93–208, compounds 8a–c are equivalent to compounds 1a–c in

that paper), and they were reinvestigated here only as polymethine

model systems. In the fits of the respective spectra of these cyanines

55] Although the model 3 f exhibits a much weaker fluorescence in

methanol (0.008) than in apolar solvents, complexation of 3e with

8

a–c, performed here for comparison, four species also had to be

+

Na also leads to the expected increase in emission from a very low

employed to describe the bands satisfactorily.

(

0.001) to a low fluorescence yield (0.005).

[

39] The BDP (and the bc-BDP) chromophore can be considered as a

[

56] Unbound 4e shows an additional narrow absorption band at 785 nm

[

23]

cross-conjugated polymethine,

consisting of a trimethine and a

in aqueous solution, which is shifted to 825 nm in the presence of

nonamethine chain. On formal consideration, the BDI core consti-

tutes a doubly cross-conjugated polymethine with an additional hep-

tadecamethine chain.

+

Na . The fluorescence of the latter species, occurring at l=830 nm,

is narrow and rather resonant. As this behavior is not straightfor-

ward and thus cannot be readily exploited for analytical sensing ap-

plications, we will not discuss the aggregation phenomena of 4e in

detail here.

[

40] J. B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience,

London, 1970.

ꢀ

1

ꢀ1

[

41] k

r

=F

f

t

and knr =(1ꢀF

f

)t

.

f

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lished results.

[

43] The parameters derived from a global analysis of the fluorescence

decay data and used in the present formalism are t

1

=3.39 ns, t

=decay species and Aij =am-

For further assumptions, the reader is also referred to

2

=

2

8 ps, A11 =2.68, and A12 =23.4 with t

i

[

30a]

plitudes.

reference [30a]. The photostationary state is reached after 188 ps.

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[

30a]

Similar to the findings for the p-dimethylamino derivative of 5a,

9

ꢀ1

irr

the process is not irreversible (Y=4.4310 s

1

vs

0 s ). As pointed out by an attentive referee, there is a misprint

in the denominator of Equation (16) of reference [30a] that should

contain kCL instead of kLC

Y =2.95

8

ꢀ1

.

4870

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 4853 – 4871

Boron–Diindomethene Dyes

4853 – 4871

[

66] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 2nd ed.,

Plenum, New York, 1999, p. 52.

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[69] M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, J. J. P. Stewart, J. Am.

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1

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S. Fery-Forgues, M. T. Le Bris, J.-P. Guettꢃ, B. Valeur, J. Phys.

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Received: February 20, 2004

Revised: May 26, 2004

Published online: August 17, 2004

Chem. Eur. J. 2004, 10, 4853 – 4871

www.chemeurj.org

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4871

Article Doi

Boron-diindomethene (BDI) dyes and their tetrahydrobicyclo precursors - En route to a new class of highly emissive fluorophores for the red spectral range

DOI: 10.1002/chem.200400173

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Article abstract of DOI:10.1002/chem.200400173

Full text of DOI:10.1002/chem.200400173

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