New fluorescent aryl- or ethynylaryl-boron-substituted indacenes as promising dyes

Article abstract of DOI:10.1039/b604830g

A generic design principle based on the use of organometallic (Li or Mg) reagents for the substitution of the fluoro ligands in difluoroboradiazaindacene is described. A library of compounds bearing various substituents (butyl, phenyl, naphthyl, pyrenyl, ethynylnaphthyl, ethynylfluorenyl, ethynylcarbazoyl, ethynylperylenyl, ethynylthiophenyl and ethynylterpyridinyl) has been prepared and characterized. Electrochemical and photochemical measurements show that these novel dyes remain redox-active and strongly luminescent. They offer an interesting perspective for applications in labelling and electroluminescence. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b604830g

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LETTER
www.rsc.org/njc | New Journal of Chemistry
New fluorescent aryl- or ethynylaryl-boron-substituted indacenes as
promising dyes
a
a
a
b
´
Gilles Ulrich,* Christine Goze, Sebastien Goeb, Pascal Retailleau and
Raymond Ziessel*a
Received (in Montpellier, France) 4th April 2006, Accepted 19th May 2006
First published as an Advance Article on the web 5th June 2006
DOI: 10.1039/b604830g
A generic design principle based on the use of organometallic
(Li or Mg) reagents for the substitution of the fluoro ligands in
difluoroboradiazaindacene is described. A library of compounds
bearing various substituents (butyl, phenyl, naphthyl, pyrenyl,
ethynylnaphthyl, ethynylfluorenyl, ethynylcarbazoyl, ethynyl-
perylenyl, ethynylthiophenyl and ethynylterpyridinyl) has been
prepared and characterized. Electrochemical and photochemical
measurements show that these novel dyes remain redox-active
and strongly luminescent. They offer an interesting perspective
Chart 1
for applications in labelling and electroluminescence.
polypyridine residues, although such derivatives might play
significant roles given recent advances in plastic electronics.17
Herein, we wish to report that the replacement of the fluoro
ligands in F-Bodipy can be readily achieved by reactions with
organometallic reagents under mild conditions. In some cases,
mono-aryl/mono-fluoro Bodipys were prepared, and in all
cases the optical and redox properties remain very intriguing.
The synthesis of alkyl- or aryl-boron-substituted Bodipys
(C-Bodipy, Chart 1) was effectively accomplished with lithio
derivatives, as shown in Scheme 1. These reactions can be
readily followed by thin-layer chromatography, useful because
this technique is essential for the successful purification of the
target fluorophores to make sure that polar side products and
unreacted starting materials are present at minimal levels.
The successful syntheses of boron-disubstituted Bodipys
using alkyl-lithium reagents prompted us to also examine the
use of Grignard reagents. As expected, the reactions were less
vigorous and could be conducted at higher temperatures.
Again, the C-Bodipy species, isolated in fair yields, were
chemically, thermally and photochemically stable.w The highly
fluorescent monosubstituted reaction intermediates could be
isolated more readily from the reactions involving Grignard
reagents (Scheme 2). The use of one equivalent of the
Grignard reagent at 0 1C enabled, for example, the isolation
of the monosubstituted compounds 6 and 7 in fair yield,
though both the disubstituted derivatives 3 and 4 and some
unreacted starting materials were also present in the product
mixture. Increasing the temperature dramatically decreased
the yield of the monosubstituted derivatives, while decreasing
the temperature to ꢀ78 1C inhibited the substitution process
completely.
Active current interests in light-emitting molecules encom-
pass both biological and materials sciences, as well as chem-
istry. Many highly efficient luminescent molecules have been
used as chemical probes, sensors and tracers,1 as well as in
biosensing2 and optoelectronics.3 Essential to the further
development of these fields is the design and synthesis of
chemically stable luminescent molecules emitting at selected
wavelengths. The ease and flexibility of synthesis, stability to
heat and light, redox activity and toxicity are other important
properties of such molecules. Useful platforms for their con-
struction include acridines,4 anthracene,5 phenanthrene,6 pyr-
ene,7 coumarins8 and indacene dyes.9,10
Recently, it has been shown that the intense fluorescence of
rod-shaped oligo-(para-phenylene ethynylene) systems can be
finely tuned by incorporating donor–acceptor groups.10 Very
convincing relationships between the fluorescence quantum
yield and the nature of electron-withdrawing11 or electron-
donating12 substituents have recently been reported, providing
valuable information for the molecular design of novel dyes.
Our interests have been focused on the planar difluorobor-
adiazaindacene (F-Bodipy, Chart 1) framework because its
optical properties (absorption, emission and quantum yield)
can be tuned at will by modifying the pyrrole substituents,13
the central meso position,14 and the boron substituents.15
Nevertheless, knowledge of the structures and properties of
F-Bodipy chemically modified at the boron center16 is very
limited, and surprisingly little is known about the replacement
of fluoride by aryl, ethynylaryl, ethynylthienyl or ethynyl-
Two conformers of 4, displaying minor differences, were
identified in the crystal state (Fig. 1),z confirming both the
stoichiometry and the structure anticipated on the basis of the
synthetic procedure.y The boron center is a distorted tetrahe-
dron, with B–N1, B–N2 and B–C100 equivalent bonds of
1.580(8), 1.579(7) and 1.608(6) A, respectively, and an
a
´
Laboratoire de Chimie Mole´culaire, Ecole de Chimie, Polyme`res,
´
Materiaux (ECPM), 25 rue Becquerel, 67087 Strasbourg Cedex 02,
France. E-mail: gulrich@chimie.u-strasbg.fr. E-mail:
ziessel@chimie.u-strasbg.fr; Fax: þ33 3 90 24 26 35; Tel: þ33 3 90
24 26 89
b Laboratoire de Cristallochimie, ICSN-CNRS, Baˆt 27-1 avenue de la
Terrasse, 91198 Gif-sur-Yvette Cedex, France
ꢁc
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Scheme 1 a: RLi, THF, 0–20 1C.
elongated B–C113 bond of o1.641(9) A, with bond angles
N1–B–N2, C100–B–C1100, N1–B–C100 and N2–B–C1100 of
o104.3(3), 115.3(4), 108.9(3) and 110.9(3)1, respectively. In
the B–C1100 equivalent direction, the boron is displaced from
the mean plane of the dipyrromethene by 0.39 A, a deforma-
tion not observed in the related F-Bodipy.14,18
Subsequent to the syntheses of the alkyl- and aryl-Bodipy
compounds, we examined the reactions of various lithiated
arylacetylenes with F-Bodipy 1, leading to E-Bodipy (Chart 1).
Here, the reaction is very fast, with the intermediate mono-
substituted compounds being undetectable and a temperature
higher than 20 1C being detrimental. Under optimal condi-
tions, the naphthalene, fluorene and perylene compounds are
readily prepared, but the isolated yields decrease with increas-
ing size and electron density of the aryl residues (Scheme 3).
Interestingly, this reaction tolerates heteroatoms (S and N),
but the yield dramatically decreases with the terpyridine
fragment, possibly due to the lithiation of the terpyridine unit
in the ortho-position of the pyridine ring.
Fig. 1 ORTEP views of 4 with ellipsoids at the 50% probability level.
Fig. 2. In all cases, quasi-reversible oxidation and reduction
processes were found around Eþ0.9 and Eꢀ1.4 V, which can
be straightforwardly assigned to the formation of the ꢀBodipy
radical cation (Bodipy1ꢂ) and radical anion (Bodipy ꢂ), re-
spectively. Remarkably, replacing the fluoro groups by butyl
fragments in 2 has no influence on either redox process.
Switching from butyl to ethynylthienyl facilitates the oxida-
tion by 100 mV and shifts the reduction by 210 mV to more
anodic potentials (Fig. 2a). In the ethynylthienyl series of
To demonstrate the generality of this protocol, we have
replaced the methyl group on the dipyrromethene unit by a
para-tolyl group (14) or a proton (15). No significant loss of
reactivity was found when similar conditions were applied
(Chart 2). Also, the successful introduction of ethynylpyrene
fragments in compound 16, carrying an empty terpyridine
fragment in the meso position, is noteworthy.
Interestingly, the 1H NMR chemical shifts of the methyl
groups ortho to the nitrogen in the indacene core depend upon
the nature of the boron substituents (Table 1). A strong
shielding of these methyl groups is observed, and is more
pronounced when two bulky aromatic groups are grafted onto
the boron center: 2.49 ppm for 1 (phenyl) and 1.38 ppm for 5
(pyrene). In contrast, the substitution of the fluorine by
ethynyl groups gives a small downfield shift of the methyl
residues by up to ca. 3 ppm.
compounds (11, 14 and 15), the Bodipy1 species are formed
ꢂ
at nearly the same potential (þ0.87 V), whereas the formation
of the Bodipyꢀ species is shifted by 50 to 60 mV from 16
ꢂ
(R = H) to 15 (R = para-tolyl) and from the latter to 11
(R = CH3) (Fig. 2b). This reflects the better electron-donating
ability of the central methyl group with respect to para-
tolyl and proton units.
All the new C-Bodipy and E-Bodipy compounds exhibit an
intense absorption around 520 nm, with extinction coefficients
of B70 000 Mꢀ1 cmꢀ1 assigned to a spin-allowed S0 - S1
transition centered on the indacene core (Table 1). A second
weak and broad S0 - S2 (p–p*) transition located at ca.
Replacement of the fluorine of F-Bodipy causes the loss of
the triplet at 3.8 ppm in the 11B NMR spectrum and the
appearance of a singlet, even for the monofluoro compounds.
The chemical shift of this singlet is sensitive to the boron
substituent, with a small downfield shift for the aryl-substi-
tuted compounds (just above 3.8 ppm) but a very strong
upfield shift to ca. ꢀ9 ppm for the ethynyl derivatives. This
effect probably reflects the strong s-donor character of the
ethynyl carbon. In the E-Bodipy compounds, the nCRC
stretching vibration is found in the 2100 to 2200 cmꢀ1 range
(Table 1).
All the newly synthesized molecules were also characterized
by cyclic voltammetry, and some of the traces are given in
Chart 2
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Scheme 2 a: 2 RMgBr, Et2O, 20 1C. b: RMgBr, Et2O, 0 1C.
380 nm was also clearly evident for most of the new species.
The peak appearing in the high energy region around 250–300
nm may be assigned to spin-allowed p–p* transitions centered
on the aromatic rings grafted either onto the boron or onto the
indacene core (Fig. 3). Very strong fluorescence, with quantum
yields reaching 98% in some cases, was measured using
Rhodamine 6G, as in ref. 19, and is in keeping with that of
F-Bodipy analogues.9,18 The excitation spectra, performed
under similar conditions, perfectly match the absorption spec-
tra, indicating that the emitted light originates from a single
excited state, with almost no contribution from alternative
radiative deactivation pathways. The absence of any signifi-
cant dynamic quenching of the luminescence by molecular
oxygen excludes the presence of an emissive triplet excited
state.
Fig. 2 (a) Cyclic voltammograms of compounds 2 and 11. (b) Cyclic
voltammograms of compounds 11, 14 and 15 in CH2Cl2, at rt, using
0.1 M nBu4PF6 as the supporting electrolyte, at a scan rate of 200 mV
s
ꢀ1. Potentials were standardized using ferrocene (Fc) as an internal
1
1
2
reference and converted to SCE, assuming that E (Fc/Fc ) = þ0.38 V
1
2
(DEp = 60 mV) vs. SCE. E accounts for half peak potential and DEp
accounts for peak difference, SCE for saturated calomel electrode.
yields are retained. This design strategy paves the way for the
construction of optical tags suitable for labelling biological
materials. Further work will explore the design of dendritic-
like frameworks for biphotonic studies.
In summary, we have established a synthetic route for
substitution of the fluoro ligands in difluoroindacene deriva-
tives by alkyl, aryl and ethynylaryl fragments. Fifteen new
dyes have been successfully synthesized and characterized by
various methods, including, in the case of naphthyl, X-ray
crystallography. The redox properties of these molecules differ
significantly from those of their F-Bodipy parent compounds,
although both the photochemical stabilityw and high quantum
Experimental
A typical example of the synthesis of 16 is described: Butyl-
lithium (0.15 mL, 1.55 M in hexane) was added to a solution
of 1-ethynylpyrene (0.041 g, 0.179 mmol) in 5 mL of THF at
Table 1 Selected spectroscopic data of the novel dyes
IRc
CRC/cmꢀ1
l
max/nmd
1H NMR/ppma
11B{1H}/ppmb
n
(e/Mꢀ1 cmꢀ1
)
l
em/nmd
F (%)e
Compound
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2.49
2.41
1.70
1.73
1.38
2.16
2.17
2.93
2.96
3.00
2.79
2.84
2.98
2.79
3.02
3.15
3.82
3.83
2.70
2.87
4.97
5.73
5.75
ꢀ9.63
ꢀ9.60
ꢀ9.40
ꢀ9.60
ꢀ9.88
ꢀ9.48
ꢀ9.60
ꢀ9.24
ꢀ8.92
–
–
–
–
–
–
–
2172
2122
2125
517 (64 500)
527 (60 000)
514 (64 500)
516 (74 500)
524 (46 300)
519 (75 000)
516 (57 000)
517 (77 700)
517 (70 000)
517 (53 500)
516 (67 000)
517 (87 000)
515 (81 000)
519 (60 000)
526 (76 800)
526 (70 000)
538
538
548
552
577
543
543
537
535
535
536
536
536
533
537
590
d
83
95
91
50
25
90
92
90
94
95
98
80
97
89
95
40
2168
f
2165
2167
2167
2164
b
a
c
Chemical shift of the methyl groups in the position ortho to the nitrogen atoms. Determined in CDCl3. KBr pellets. Measured in CH2Cl2
e
at 23 1C, lmax maximum absorption and lem maximum emission. Relative quantum yields determined using Rhodamine 6G as in ref. 19
(F = 0.78%, lexc = 488 nm). Excitation wavelength is lmax of the boradiazaindacene entity. Not observed.
f
ꢁc
984 | New J. Chem., 2006, 30, 982–986 This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006

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100), 722.2 ([M–pyr–R–]1, 20); anal. calc. for C68H50BN5: C,
86.16; H, 5.32; N, 7.39. Found: C, 85.95; H, 5.12; N, 7.27%.
Acknowledgements
The Centre National de la Recherche Scientifique and the
French tax payer are gratefully acknowledged for financial
support.
References
w In our hands, no obvious degradation of the novel dyes has been
observed by treatment with water, protic solvents, acids or strong
bases in daylight. For instance, these dyes are stable in hot piperidine
overnight. No particular precautions have to be taken during the
work-up of these compounds, or during chromatography, crystal-
lization or spectroscopic studies. Compounds could be stored at rt in
air and in daylight. However, some decomposition might occur in
excess Fꢀ anions or strong oxidants. This statement of stability is
based on current observations.
Scheme 3 a: ArCRCLi, THF, 0–20 1C.
z X-ray crystal data for 4 at 293 K. Formula sum: C38H39BN2,
formula weight: 534.52, crystal system: triclinic, space group: P-1
(no. 2). Unit cell dimensions: a = 13.236(5), b = 13.934(5), c =
18.545(5) A, a = 102.109(5), b = 90.215(5), g = 112.416(5)1, unit cell
ꢀ78 1C. The dark green anion was allowed to slowly warm up
0 1C and was then transferred via cannula to a solution of 2,6-
diethyl-4,4-difluoro-1,3,5,7-tetramethyl-8-(2:20:60:200-terpyridin-
40-yl)-4-bora-3a,4a-diaza-s-indacene14 (0.048 g, 0.089 mmol)
in 10 mL of THF. Complete consumption of the starting
material was observed after 10 min. The solution was then
readily quenched with water and the compound extracted with
dichloromethane. Column chromatography (alumina,
CH2Cl2/cyclohexane 20 : 80) followed by recrystallization
(CH2Cl2/cyclohexane) gave the desired compound 16 (0.025
g, 30% yield). 1H NMR (CDCl3, 400 MHz) (d/ppm): 8.82 (d, 2
H, 3J = 9.0 Hz), 8.75–8.69 (m, 6 H), 8.22–7.98 (m, 16 H), 7.91
volume: 3075.05(180) A3, Z = 4, density (calculated): 1.153 g cmꢀ3
.
Reflections collected/unique 15816/6224 (Rint = 0.0304), data/re-
straints/parameters 6224/4/768. Final R indices [I 4 2s(I)]: R1 =
0.0587, wR2 = 0.1565; R indices (all data): R1 = 0.0881, wR2 =
0.1809. CCDC 606381. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b604830g.
y Indeed, there are two independent molecules in the asymmetric unit
of the triclinic cell (Z = 4). If the rms coordinate difference between
boradiazaindacene moieties is only 0.136 A, the main differences
between them mainly reside in the relative orientations of the naphtyl
groups.
3
4
(dt, 2H, J = 8.0, J = 2.0 Hz), 7.37 (m, 2 H), 3.15 (s, 6 H),
2.45 (q, 4 H, 3J = 7.6 Hz), 1.57 (s, 6 H) and 1.09 (t, 6 H, 3J =
7.5 Hz); 13C{1H} NMR (CDCl3, 75 MHz) (d/ppm): 156.4,
155.7, 154.6, 149.5, 147.1, 137.0, 136.3, 133.6, 132.3, 131.5,
131.4, 130.5, 129.9, 128.6, 128.0, 127.6, 127.5, 126.5, 126.1,
125.3, 125.2, 124.75, 124.70, 124.6, 124.3, 121.5, 121.3, 120.8,
17.2, 15.0, 14.7 and 12.9; 11B{1H} NMR (CDCl3, 128 MHz)
(d/ppm): ꢀ8.92 (s); UV-vis (CH2Cl2) (l/nm (e/Mꢀ1 cmꢀ1)):
526 (70 000), 370 (103 000), 358 (78 000), 285 (111 400), 275
(74 400) and 248 (106 000); IR (KBr) (n/cmꢀ1): 2961 (s), 2164
(m), 1582 (s), 1402 (s), 1178 (s), 978 (s) and 845 (s); FAB1 (m/z
(nature of peak, relative intensity (%))): 948.2 ([M þ H]1,
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