New molecular design for blue BODIPYs

Article abstract of DOI:10.1039/c9nj01114e

Diverse dihydrodipyrrins are available as precursors in the de novo synthesis of bacteriochlorins and chlorins. Each dihydrodipyrrin contains one pyrrole and one pyrroline (3,4-dihydropyrrole) ring joined at the respective α-positions via a methylene unit as well as a geminal-dimethyl group at one of the pyrroline β-positions. Complexation of the dihydrodipyrrin ligands occurs smoothly upon treatment with Bu₂B-OTf or BF₃·OEt₂ in dichloromethane containing triethylamine at room temperature. Six such dihydrodipyrrinatoboron complexes have been prepared and are examined here. The complexes with -BF₂ or -BBu₂ absorb in the blue region (λ_abs ～ 400 nm) and fluoresce (λ_em ～ 500 nm) with large Stokes shift (～100-150 nm), almost no absorption-fluorescence spectral overlap, and high fluorescence quantum yield (Φ_f ～ 0.4-0.9). The spectral features are rather insensitive to substituents in the pyrrole nucleus (carboethoxy, bromo, and p-tolyl) and the presence of a 1-naphthalenyl group at the meso-position. In one case examined, the spectral properties including Φ_f value were almost identical in toluene and acetonitrile. The blue BODIPYs may be useful as broadband photosensitizers upon violet-laser excitation.

Full text of DOI:10.1039/c9nj01114e

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New molecular design for blue BODIPYs†
a
a
b
c
Zhiyuan Wu,
Don Hood,
Hikaru Fujita,
Nikki Cecil M. Magdaong,
James R. Diers,
Cite this:DOI: 10.1039/c9nj01114e
b
a
d
Srinivasarao Allu,
Christine Kirmaier,
Jonathan S. Lindsey
Dariusz M. Niedzwiedzki,
David F. Bocian, *c Dewey Holten *b and
b
a
*
Diverse dihydrodipyrrins are available as precursors in the de novo synthesis of bacteriochlorins and
chlorins. Each dihydrodipyrrin contains one pyrrole and one pyrroline (3,4-dihydropyrrole) ring joined at
the respective a-positions via a methylene unit as well as a geminal-dimethyl group at one of the
pyrroline b-positions. Complexation of the dihydrodipyrrin ligands occurs smoothly upon treatment with
Bu2B–OTf or BF3ÁOEt2 in dichloromethane containing triethylamine at room temperature. Six such dihydro-
dipyrrinatoboron complexes have been prepared and are examined here. The complexes with –BF2 or
–BBu2 absorb in the blue region (labs B 400 nm) and fluoresce (lem B 500 nm) with large Stokes shift
(B100–150 nm), almost no absorption-fluorescence spectral overlap, and high fluorescence quantum yield
(Ff B 0.4–0.9). The spectral features are rather insensitive to substituents in the pyrrole nucleus (carboethoxy,
bromo, and p-tolyl) and the presence of a 1-naphthalenyl group at the meso-position. In one case examined,
the spectral properties including Ff value were almost identical in toluene and acetonitrile. The blue BODIPYs
may be useful as broadband photosensitizers upon violet-laser excitation.
Received 2nd March 2019,
Accepted 7th April 2019
DOI: 10.1039/c9nj01114e
rsc.li/njc
The parent BODIPY (I, Chart 1) absorbs and fluoresces near
500 nm. While addition of conjugated groups has given rise to
Introduction
The discovery of dipyrrinatoboron difluoride complexes half a BODIPYs with spectral features shifted to the red and even
century ago ushered in a rich new era of research.1 BODIPYs near-infrared region,3,8,11 designs for hypsochromic shifting
afford many attractive features: (1) strong absorption and have been less forthcoming. The chief molecular design for
fluorescence (e B 5 Â 104 MÀ1 cmÀ1; Ff up to nearly unity) in hypsochromic shifting entails installation of an amino group at
the visible region; (2) neutral chromophore rather than charged the meso-position of the dipyrrin ligand. With the amino group
as is the case with many dyes, thereby facilitating handing; (3) alone (II), the absorption shifts to B400 nm and a high Ff value is
facile synthetic access from the corresponding dipyrrin or retained. But a mere change to an alkylamino group (III) causes
dipyrromethane (upon oxidation and complexation in a one- significant diminution of the fluorescence yield. A further limitation
flask process); and (4) malleable molecular design with regards of the meso-amino strategy, at least in some cases, is that the meso-
to bathochromic tuning of the key spectral features.2–11 Interest position is attractive for installation of synthetic handles (via the
shows no sign of abating; 44600 papers in the past decade are corresponding and readily accessible meso-substituted dipyrro-
elicited upon searching ‘‘BODIPY’’ in Web of Science, more methanes). Here, we describe a new molecular design for BODIPYs
than 5 times that in the preceding decade.
that absorb in the blue spectral region. The design was arrived at
a Department of Chemistry, North Carolina State University, Raleigh, North
Carolina 27695-8204, USA. E-mail: jlindsey@ncsu.edu; Tel: +1-919-515-6406
b Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889,
USA. E-mail: holten@wustl.edu; Tel: +1-314-935-6502
c Department of Chemistry, University of California, Riverside, California 92521-0403,
USA. E-mail: david.bocian@ucr.edu; Tel: +1-951-827-3660
d Department of Energy, Environmental & Chemical Engineering, and Center for
Solar Energy and Energy Storage, Washington University, St. Louis,
Missouri 63130-4889, USA
† Electronic supplementary information (ESI) available: Photophysical data; char-
acterization data including NMR spectra for all new compounds; and single-crystal
X-ray data. CCDC 1900105 (1-BBu2), 1900106 (2-BBu2) and 1900107 (2-BF2). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c9nj01114e
Chart 1 Parent BODIPY and prior ‘‘blue’’ analogues.
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Chart 2 Dihydrodipyrrinatoboron complexes.
Scheme 1 Retrosynthesis of dihydrodipyrrin ligands and boron complexes.
Dihydrodipyrrin 1 is known12 but was prepared here in a
streamlined manner, which illustrates the simplicity of the
synthesis. The Pd-mediated coupling of 2-iodopyrrole 512 and
pentynoic acid 612 afforded the pyrrole–lactone 7 (80% yield
versus 55% previously), which upon reaction with the Petasis
reagent gave pyrrole–ene–lactone 8 (Scheme 2). Reaction of 8 via a
process similar to a Paal–Knorr reaction gave dihydrodipyrrin 1
along with putative isomer 1E. Such isomers have been inferred
serendipitously during the course of fundamental studies con-
cerning bacteriochlorin synthesis methodology.
The de novo synthesis of bacteriochlorins relies on the self-
condensation of dihydrodipyrrin species.12,13 Two complementary
routes (Scheme 1) readily afford diverse dihydrodipyrrins that vary
in substituents at the pyrrolic positions (Rpyr), the meso-position
(Rm), and the location of the gem-dimethyl group in the pyrroline
ring. The gem-dimethyl group is essential to block inadvertent
oxidation leading to the corresponding dipyrrin. We earlier had
employed complexation with dialkylboron reagents to facilitate
purification of acyldipyrromethanes14,15 and alter substitution
reactions of tetrahydrodipyrrins.16 Here we report analogous
studies with a collection of dihydrodipyrrins and characterize
the resulting absorption and fluorescence features of the dihydro-
dipyrrinatoboron species. The spectroscopic features including
broad absorption at B400 nm and broad yet strong fluorescence
centered at B500 nm are more reminiscent of aminocoumarins17
than the parent dipyrrinatoboron complexes. Such features may
support broad-band photosensitization upon violet-laser excitation
(405 nm) and microscopy applications where a large Stokes
shift is desirable.
1
previously19 on the basis of H NMR spectroscopy. Treatment of
the crude mixture containing 1 and 1E with triethylamine and
dibutylboron triflate gave the desired complex in 52% yield
(from 8). Crystals of 1-BBu2 suitable for X-ray analysis were
obtained by slow evaporation of a solution of hexanes/chloro-
form at room temperature.
As shown for the preparation of 1-BBu2, the complexation
procedure is simple – treatment of the dihydrodipyrrin ligand
with Bu2B–OTf16 or BF3ÁOEt2 in dichloromethane containing
2
triethylamine at room temperature readily affords the corresponding
–BF2 or –BBu2 complex, respectively. In this manner, complexes
2-BBu2 (1 h, 85% yield), 2-BF2 (2 h, 93%), 3-BF2 (overnight, 86%),
and 3-BBu2 (overnight, 71%) were obtained as yellowish solids
with bright yellow-green fluorescence in solution. Complex
4-BBu2 was prepared previously.16 The complexes are stable
under routine handling to air and moisture, and were readily
purified by chromatography and/or crystallization, although
2-BF2 slowly decomposed upon TLC analysis (silica or Si-diol)
at room temperature. On the limited comparison of two pairs of
compounds, the –BBu2 complexes were more stable than the
Results and discussion
Synthesis
We prepared a handful of boron complexes from available –BF2 complexes. Crystals of 2-BBu2 and 2-BF2 suitable for X-ray
dihydrodipyrrin ligands 1,12 2,18 3,18 and 416 (Chart 2). Each analysis were obtained from vapor diffusion of hexanes to ethyl
dihydrodipyrrin contains a gem-dimethyl group in the 2- or acetate solution at room temperature. While attempts at synthetic
3-position depending on the synthetic method of preparation, manipulations of the dihydrodipyrrin–boron complexes were
a 1-methyl group, and one or two (carboethoxy, bromo, very limited, attempts to debrominate 2-BBu2 to give 1-BBu2 with
1-naphthenyl, p-tolyl) substituents that together enable an initial n-butyllithium or the isopropylmagnesium chloride lithium
assessment of substituent effects on spectroscopic features.
chloride complex were unsuccessful.
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Fig. 1 ORTEP diagrams (contoured at the 50% level with omission of H
atoms) for the single-crystal X-ray data of (A) 2-BF2, (B) 2-BBu2, and (C)
1-BBu2. The larger display for 1-BBu2 reflects disorder, which may in part
reflect the temperature of collection (180 K) versus that of 2-BF2 and
2-BBu2 (100 K). Atom coloration: N, blue; O, red; B, pink; F, lime green.
high-angle intensity of the data are only suitable for establishing
connectivity within the molecule.
Spectroscopic features
The complexes were characterized by static and time-resolved
absorption and fluorescence spectroscopy. Fig. 2 shows the static
absorption and fluorescence emission spectra of the boron–
dihydrodipyrrins in toluene. Fig. 3 shows the spectra for a
representative compound, 1-BBu2 in toluene, acetonitrile and
dimethylsulfoxide. The peak wavelengths are listed in Table 1.
The molar absorption coefficients of all six dihydrodipyrrinato-
boron complexes were measured (Table S2, ESI†). In general, the
long-wavelength absorption band exhibits a molar absorption
coefficient value of B104 MÀ1 cmÀ1 in toluene at room temperature.
Several compounds also were examined in acetonitrile, where values
quite similar to those in toluene also were obtained.
Scheme 2 The synthesis of dihydrodipyrrin complex 1-BBu2.
Chemical characterization
Each boron complex was examined by 11B NMR spectroscopy
using B(OH)3 (19.8 ppm in DMF20) as a standard. Each boron
complex exhibited a broad peak in the range 0.95–3.70 ppm,
to be compared with that of similar compounds such as N-(9-
borabicyclo[3.3.1]non-9-yl)pyrrole (59.9 ppm)21 and the 9-BBN
complex of 1-acyldipyrromethanes (B13 ppm).14 The relative
upfield shift of the complex is characteristic for species wherein
boron is coordinated with an Nimino nitrogen.22
X-ray structural analysis was performed on the 1-BBu2,
2-BBu2, and 2-BF2 complexes. The ORTEP diagrams are shown
in Fig. 1, and the crystallographic data are listed in Table S1
(ESI†). For 1-BBu2, with the lack of a bromine substituent, there
is a significant amount of positional disorder in the ring system
and the n-butyl groups. The crystal was well formed, but shattered
when placed in a cold stream of nitrogen on the instrument. The
data collection was performed at a higher temperature (180 K)
Fig. 2 Absorption spectra (blue solid) and fluorescence emission spectra
(red dashed) of boron–dihydrodipyrrins in toluene. Arrows indicate the
excitation wavelength used to obtain each emission spectrum. The same
than typical (100 K). The overall effect is that the resolution and emission spectra were obtained using other excitation wavelengths.
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Fluorescence yields were determined with respect to two
standards using several excitation wavelengths, with good agreement
among the results for each compound. The results of the individual
measurements are given in Table S3 (ESI†) and the average values in
Table 1. The Ff values range from 0.87 for the simplest of the
compounds (1-BBu2) to 0.30 for the analogue that bears 7-p-tolyl and
3-gem-dimethyl substituents (4-BBu2). The emission yield is also
quite high for 3-BBu2 (0.88) and 2-BBu2 (0.81), but is roughly half as
large for counterparts 3-BF2 (0.38) and 2-BF2 (0.42). Our prior
studies on boron–dipyrrins (not boron–dihydrodipyrrins) bearing
a 5-mesityl group and no other substituents show that two
fluorines on boron affords a greater Ff (0.93) than two methyl
groups (0.33).24 The interplay between substituents on the boron
and dihydrodipyrrin (or dipyrrin) framework on the excited-
state decays will be discussed further below.
The excited-state decay routes and dynamics were investigated
by transient absorption (TA) and time-resolved fluorescence
spectroscopies. The S1 lifetimes determined by the various
methods are listed in Table S3 (ESI†) and the average values
in Table 1. Representative TA difference spectra for 3-BBu2 are
shown in Fig. 4A. The negative-going features in the TA
difference spectra (excited minus ground state) are bleaching
of the S0 - S1 absorption at B400 nm and S1 - S0 stimulated
emission at B540 nm, positions that are consistent with
Fig. 3 Absorption spectra (blue solid) and fluorescence emission spectra
(red dashed) of boron–dihydrodipyrrin 1-BBu2 in toluene, acetonitrile and
dimethylsulfoxide. Arrows indicate the excitation wavelength used to
obtain each emission spectrum. The same emission spectra were obtained
using other excitation wavelengths.
Comparing pairs 2-BF2 versus 2-BBu2 and 3-BF2 versus features in the static absorption and fluorescence spectra
3-BBu2, the absorption maximum of the difluoro construct (Fig. 4B). The TA difference spectra also show a prominent
differs by r8 nm compared to the dibutyl analogue. More excited-state absorption (e.g. S1 - S3) at B460 nm that has an
dramatically, the fluorescence peak is bathochromically shifted extinction coefficient substantially greater than that for the
by 37 nm in the fluoro- versus butyl-containing compounds. S0 - S1 transition in the ground-state absorption spectrum.
Thus, the Stokes shift between the fluorescence and absorption The spectral evolution displayed in Fig. 4A and the representative
maxima of 98–110 nm for the dibutyl-bearing constructs is kinetic traces in Fig. 4C show only minor changes over the first few
increased to 143–145 nm for the difluoro analogues (Fig. 2 and hundred picoseconds with time constants of B5 and B60 ps that
Table 1). Increasing the solvent dielectric constant [toluene likely represent some combination of vibrational, solvent or
(2.38) o acetonitrile (37.5) o dimethylsulfoxide (46.7)] for conformational relaxations in the S1 excited state accompanied
1-BBu2 results in a substantial hypsochromic shift of the by little ground-state recovery. The S1 excited-state decay occurs
absorption maximum (400 nm o 376 nm B 377 nm) without for 3-BBu2 in toluene with a time constant of 7 ns. The TA data
much change in the fluorescence peak position (507 nm B show that during this time S1 decays essentially completely to
506 nm B 507 nm). Thus, the Stokes shift for 1-BBu2 increases the ground state, with virtually no formation of the lowest
from 107 nm in toluene to 130 nm in acetonitrile and dimethyl- triplet excited state by S1 - T1 intersystem crossing. Thus, for
sulfoxide (Fig. 3 and Table 1).
all practical purposes the yields of S1 - S0 fluorescence and
Table 1 Photophysical propertiesa
b
labs calc.
labs
(nm)
lem
(nm)
Stokes shift
(nm)
Stokes shift
(cmÀ1
)
Ffc
tS (ns)
kfÀ1 e (ns)
kic
(ns)
d
À1 e
Compound
Solvent
(nm)
1-BBu2
1-BBu2
1-BBu2
2-BBu2
2-BF2
3-BBu2
3-BF2
Toluene
MeCN
DMSO
Toluene
Toluene
Toluene
Toluene
Toluene
399
373
373
397
399
400
404
431
400
376
377
397
399
409
401
434
507
506
507
507
544
507
544
557
107
130
130
110
145
98
528
683
680
547
668
473
656
509
0.87 Æ 0.04
0.88 Æ 0.04
0.90 Æ 0.04
0.81 Æ 0.01
0.42 Æ 0.01
0.88 Æ 0.03
0.38 Æ 0.04
0.30 Æ 0.04
7.5 Æ 0.8
8.8 Æ 0.3
7.9 Æ 0.2
7.0 Æ 0.5
4.9 Æ 0.2
7.0 Æ 0.7
4.9 Æ 0.1
3.9 Æ 0.1
8.7
10
8.7
8.6
12
8.0
13
13
56
75
77
37
8.4
59
143
123
7.8
5.5
4-BBu2
a
b
All data were acquired at room temperature. MeCN is acetonitrile and DMSO is dimethylsulfoxide. The S0 - S1 transition wavelength
calculated by TDDFT was shifted to lower energy in all cases by 2500 cmÀ1 to obtain better overall agreement with the measured spectra.
c
Fluorescence yields were determined for deoxygenated samples relative to the standards pyranine (Ff = 1.0 in 0.10 M NaOH23) and
5-mesityldipyrrinatoboron difluoride (Ff = 0.93 in toluene24) and averaged (Table S3, ESI). Singlet excited-state lifetimes are the average of
d
e
results from three methods; see Table S3 (ESI). S1 decay rate constants were derived as described in the text.
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conversion (kic) for 2-BF2 and 3-BF2 are 4–8 fold greater than
those for 1-BBu2, 2-BBu2, and 3-BBu2 but comparable to that for
4-BBu2. Prior work24,26 on boron–dipyrrins (not dihydrodipyr-
rins) that vary in the presence and type of aryl ring at the
5 (meso) position, two fluorines or two alkyl groups on boron,
along with the presence, number and types of alkyl groups at
the 2, 3, 7, and 8 positions suggests that the combination of
substituents can impact the rate constant and yield for non-
radiative deactivation via internal conversion by allowing or
suppressing motions that alter the planarity of the dipyrrin
framework and the rotation of the aryl ring. It is reasonable that
such motions are influenced by interplay of the substituents on the
dihydrodipyrrin and boron (Chart 2) and impact internal conversion
and thus Ff and tS for the compounds studied herein.
Calculations using density functional theory (DFT) and the time-
dependent extension TDDFT were undertaken to gain further
insights into the relationships between chemical composition,
electronic structure and photophysical properties of the boron–
dihydrodipyrrins. The electron-density distribution of the highest
occupied molecular orbital (HOMO) and of the lowest unoccupied
molecular orbital (LUMO) for each compound are shown in Fig. 5.
Below each orbital is the calculated energy in toluene (upper value)
and in acetonitrile (lower value). Below the structure of each
compound is the corresponding S0 - S1 transition energy, wave-
length and oscillator strength calculated by TDDFT in the two
solvents. The calculated transition wavelengths generally reproduce
several key trends (Fig. 5 and Table 1), which include (1) the
bathochromic shift of 4-BBu2 relative to the other analogues, and
(2) the hypsochromic shift for 2-BBu2 in polar versus nonpolar media.
The MOs for 2-BBu2 also show that there is considerable
electron density on the bromine and adjacent positions on the
dihydrodipyrrin. Thus, the lack of an effect of the bromine on
the Ff and tS values of 2-BBu2 relative to the other dihydrodipyrrin
complexes studied cannot be attributed to a lack of communication
Fig. 4 TA difference spectra (A), static absorption (blue solid) and fluores-
cence (red dashed) spectra (B), and kinetic trace (C) of 3-BBu2 in toluene.
The TA studies used 100 fs excitation flashes at 400 nm.
S1 - S0 internal conversion sum to unity (Ff + Fic B 1). The between the halogen and the p-system of the dihydrodipyrrin. This
corresponding rate constants can be calculated from the expressions finding tends to reinforce the view noted above that these molecules
kf = Ff/tS and kic = (1 À Ff)/tS. The values for 3-BBu2 in toluene are appear to have very small spin–orbit coupling and thus small rate
given in Table 1.
constants for S1 - T1 intersystem crossing. Accordingly, any
Similar TA and fluorescence decay results were obtained for heavy-atom enhancement does not increase kisc sufficiently to
3-BBu2 in acetonitrile and dimethylsulfoxide. The analogous give effective competition with kf and kic, which dominate the
measurements for all the other boron–dihydrodipyrrins in excited-state dynamics of the boron–dihydrodipyrrins.
toluene also showed no significant triplet yield, and S1 lifetimes
The spectral properties of the blue BODIPYs described herein
roughly in the 4–7 ns range (Table 1). The low T1 yields indicate that can be compared with other blue-absorbing fluorophores. The
the rate constant for intersystem crossing is very small relative absorption and fluorescence spectra of 1-BBu2, the 5-amino-
to those for fluorescence and internal conversion (Table 1), likely dipyrrinatoboron difluoride II, and coumarin 151 are shown in
o(1 ms)À1. Thus, even if the bromine substituent in 1-BBu2 and Fig. 6 and summarized in Table 2. Compared with II, 1-BBu2
3-BBu2 gives a heavy-atom enhancement of the S1 - T1 rate absorbs at nearly the same wavelength but with a substantially
constant, the magnitude is still apparently not great enough to give broader absorption band (85 versus 48 nm), larger Stokes shift
a measurable triplet yield and commensurate reduction in Ff and tS (107 versus 39 nm), and comparable (and strong) Ff value. Compared
compared to analogues that lack the halogen atom.
with coumarin 151,27,28 1-BBu2 exhibits similar absorption features
Examination of Table 1 shows that the derived rate constant and Stokes shift although 1-BBu2 exhibits a broader emission
for S1 - S0 fluorescence (kf) varies little among the compounds. band and larger Ff value. In this regard, the Ff value of
This is consistent with the comparable extinction coefficients coumarin 151 increases substantially with increasing solvent
for S0 - S1 absorption (Table S2, ESI†), the two quantities polarity (e.g., 0.17 in 3-methylpentane; 0.57 in acetonitrile)27
being connected by the relationships between the Einstein whereas that of 1-BBu2 is 0.87–0.90 in solvents having a wide
coefficients.25 The derived rate constants for S1 - S0 internal range of polarity (toluene, acetonitrile and dimethylsulfoxide).
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Fig. 5 Results of DFT and TDDFT calculations for the boron–dihydrodipyrrins. Below each MO, the upper value is the calculated energy in toluene and
the lower value is for acetonitrile. Below each structure are the calculated S0 - S1 absorption wavelength (l) and oscillator strength (f), with the top
values being for toluene and the lower ones for acetonitrile. The transition energies are all arbitrarily shifted to lower energy by 2500 cmÀ1 to give better
agreement with the measured spectra (Table 1); this systematic shift does not affect the calculated trends with compound or medium.
We also note that the molar absorption coefficient of II in The following points appear noteworthy. First, the sizable Stokes
methanol was reported as 2.6 Â 104 MÀ1 cmÀ1 in 201129 (see shift with almost no overlap32 of the absorption and fluorescence
Fig. 6 and Table 2) and as 1.12 Â 105 MÀ1 cmÀ1 in 2013.30
bands suggests applications for broad-band photosensitization
upon violet-laser (405 nm) excitation or use in stimulated emission
depletion (STED) microscopy33 where the large Stokes shift is
Outlook
The results reported herein indicate a new molecular design essential. Second, the spectral features resemble those of members
for achieving blue absorption in the general BODIPY family. of the aminocoumarin family,17,27,28,31 although more extensive
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was used for column chromatography. Si-diol (functionalized
silica) TLC plates were purchased from SiliCycle. Commercial
compounds were used as received unless noted otherwise.
16
Compounds 2,18 3,12 5,12 6,12 and 4-BBu2 were prepared as
described in the literature.
8-Carbethoxy-10-(dibutylboryl)-2,3-dihydro-1,2,2-trimethyldipyrrin
(1-BBu2)
Following reported methods12,16 with some modification, a
solution of 8 (194 mg, 0.742 mmol) in DMF (7.1 mL) was
treated with 6 M HCl (185 mL). After 15 min, NH4OAc (1.2 g,
15 mmol) and triethylamine (2.1 mL, 15 mmol) were added.
The resulting mixture was stirred overnight at room temperature
and then quenched by the addition of saturated aqueous KH2PO4
solution. CH2Cl2 was added, and the organic layer was washed
with water and brine, dried (Na2SO4) and concentrated. 1H NMR
analysis of the crude product indicated the presence of free base
dihydrodipyrrins 1 and 1E (estimated 4 : 1 ratio). TLC analysis
[silica, hexanes/ethyl acetate (2 : 1)] showed one mobile component
and material remaining at the origin, which were attributed to 1
and 1E, respectively. Chromatographic separation proved difficult
because of extensive streaking. Prior evidence indicates that the
E and Z isomers of dihydrodipyrrins can interconvert under some
reaction conditions.34 Accordingly, the crude mixture was then
dissolved in CH2Cl2 (6.0 mL) and treated with triethylamine
(0.36 mL, 2.6 mmol) and Bu2B–OTf (1.5 mL of 1 M solution in
CH2Cl2, 1.5 mmol). The reaction mixture was stirred at room
temperature for 1 h then quenched by the addition of saturated
aqueous NaHCO3 solution. The organic layer was washed twice
more with saturated aqueous NaHCO3 solution, dried (Na2SO4)
and concentrated. Column chromatography [silica, hexanes/
ethyl acetate (8 : 1 to 4 : 1)] afforded a yellow solid (149 mg, 52%):
mp 118–120 1C; 11B NMR d 3.21; 1H NMR (CDCl3, 400 MHz) d 7.40
(d, J = 1.5 Hz, 1H), 6.46 (s, 1H), 6.14 (s, 1H), 4.26 (q, J = 7.1 Hz, 2H),
2.67 (d, J = 1.8 Hz, 2H), 2.38 (s, 3H), 1.34 (t, J = 7.1 Hz, 3H), 1.27
(s, 6H), 1.20–1.07 (m, 4H), 0.92–0.57 (m, 15H); 13C NMR (CDCl3,
100 MHz) d 186.2, 165.9, 137.1, 130.8, 129.2, 116.3, 111.2, 108.4,
59.3, 48.5, 40.1, 27.7, 26.1, 25.9, 14.7, 14.4, 14.3; ESI-MS obsd
385.3017, calcd 385.3021 [(M + H)+, M = C23H37BN2O2]; labs (toluene)
398 nm; lem (toluene) 506 nm.
Fig. 6 Absorption and fluorescence emission spectra of 1-BBu2, II, and
coumarin 151. The absorption spectra are shown (solid lines) according to
their molar absorption coefficient. For each compound, the emission
intensity is adjusted commensurably (dashed lines).
Table 2 Photophysical properties of 1-BBu2, II and coumarin 151
labs
eabs
fwhm lem
Stokes
Compound
(nm) (MÀ1 cmÀ1
)
(nm)
(nm) (nm)
Ff
a
1-BBu2
400
399
1.1 Â 104
2.6 Â 104
1.7 Â 104
85
48
67
507
438
484
107
39
100
0.87
0.92
0.53
IIb
Coumarin 151c 384
a
b
In toluene. In methanol.29 c In ethanol.31
studies (e.g., photostability) are required for in-depth comparisons.
Third, use of the dihydrodipyrrin (versus the 5-aminodipyrrin) to
achieve short-wavelength absorption leaves open the 5-position for
synthetic manipulation, such as for incorporation into arrays or
attachment of bioconjugation handles. Fourth, the dihydrodipyr-
rinatoboron complexes are neutral fluorophores,2 like BODIPYS,
and hence can be tailored for use in diverse (organic or aqueous)
media. Fifth, owing to two complementary routes to dihydrodi-
pyrrins (Scheme 1), synthetic capabilities are available for facile
molecular tailoring of the three pyrrole positions, the gem-
disubstitution site (enabling swallowtail architectures34), the
5-position, and the pyrroline 1-position. Finally, numerous dihydro-
dipyrrins are known,12,13 indicating the untapped potential for
development of an in-depth structure–activity relationship con-
cerning tailored blue-absorbing/emitting complexes.
7-Bromo-8-carbethoxy-10-(dibutylboryl)-2,3-dihydro-1,2,2-
trimethyldipyrrin (2-BBu2)
Following a reported method16 with some modification, a
solution of 2 (76 mg, 0.22 mmol) in CH2Cl2 (1.5 mL) was treated
with triethylamine (0.11 mL, 0.79 mmol) and Bu2B–OTf (0.45 mL of
1 M solution in CH2Cl2, 0.45 mmol). The reaction mixture was
stirred at room temperature for 1 h and then quenched by the
Experimental section
General methods
1H and 13C NMR spectra were recorded in CDCl3 at room addition of saturated aqueous NaHCO3 solution. The organic layer
temperature unless noted otherwise. 11B NMR spectroscopy was washed twice more with saturated aqueous NaHCO3 solution,
(160 MHz) was performed at room temperature using a boron- dried (Na2SO4) and concentrated. Column chromatography [silica,
free NMR tube, CDCl3 as solvent, and B(OH)3 in DMF as the external hexanes/ethyl acetate (5 : 1)] afforded a yellow solid (88 mg, 85%):
standard (referenced to 19.8 ppm).20 All solvents (anhydrous or mp 4117 1C (dec.); 11B NMR d 3.70; 1H NMR (CDCl3, 400 MHz) d
reagent-grade) were employed as received from commercial 7.39 (s, 1H), 6.30 (s, 1H), 4.29 (q, J = 7.1 Hz, 2H), 2.70 (d, J = 1.6 Hz,
suppliers. Absorption and emission spectra were collected in 2H), 2.38 (s, 3H), 1.36 (t, J = 7.1 Hz, 3H), 1.29 (s, 6H), 1.20–1.08 (m,
toluene at room temperature. Silica (40 mm average particle size) 4H), 0.89–0.54 (m, 14H); 13C NMR (CDCl3, 100 MHz) d 187.5, 164.4,
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138.4, 130.9, 128.4, 114.5, 108.8, 95.7, 59.6, 48.7, 40.2, 27.6, 26.1, 2.38 (ABq, DdAB = 0.08, J = 17.4 Hz, 2H), 1.29–1.21 (m, 9H); 13C
25.9, 14.7, 14.6, 14.3; ESI-MS obsd 463.2131, calcd 463.2126 NMR d 194.4, 165.1, 134.7, 133.9, 132.2, 131.1, 129.9, 128.9,
[(M + H)+, M = C23H36BBrN2O2]; labs (toluene) 398 nm; lem 128.7, 127.2, 126.8, 126.3, 125.6, 125.1, 122.8, 118.4, 111.0, 59.6,
(toluene) 506 nm.
49.3, 39.6, 25.4, 14.6; ESI-MS obsd 435.2059, calcd 435.2050
[(M + H)+, M = C25H25BF2N2O2]; labs (toluene) 401 nm; lem
(toluene) 550 nm; labs 388 nm (CH2Cl2).
7-Bromo-8-carbethoxy-10-(difluoroboryl)-2,3-dihydro-1,2,2-
trimethyldipyrrin (2-BF2)
Following a reported method2 with some modification, a solution of
2 (17 mg, 50 mmol) in anhydrous CH2Cl2 (1.0 mL) was treated with
4-Ethoxycarbonyl-(E)-2-[(4,4-dimethyl-5-oxodihydrofuran-2(3H)-
ylidene)methyl]pyrrole (7)12
triethylamine (110 mL, 750 mmol) and BF3ÁOEt2 (170 mL, 1.3 mmol). Following a reported procedure12 with slight modification, a
The reaction mixture was stirred at room temperature for 2 h and solution of pyrrole 5 (11.8 g, 44.5 mmol), pentynoic acid 6
then loaded onto a silica column. Chromatography (silica, CH2Cl2) (11.23 g, 89.01 mmol), and BnEt3NCl (11.15 g, 48.95 mmol) in
afforded a yellow solid (18 mg, 93%): mp 4128 1C (dec.); 11B NMR d anhydrous MeCN (191 mL) and triethylamine (105 mL) was
0.95; 1H NMR (CDCl3, 400 MHz) d 7.68 (s, 1H), 6.38 (s, 1H), 4.29 (q, deaerated by two freeze–pump–thaw cycles. Pd(PPh3)4 (1.54 g,
J = 7.1 Hz, 2H), 2.78 (d, J = 2.0 Hz, 2H), 2.59 (s, 3H), 1.35 (t, J = 7.1 Hz, 1.34 mmol) was then added. The resulting mixture was heated
3H), 1.35 (s, 6H); 13C NMR (CDCl3, 100 MHz) d 195.7, 163.6, 137.5, to 60 1C for 2 h and then allowed to cool to room temperature.
130.0, 128.8, 116.3, 107.8, 97.8, 59.8, 49.7, 40.1, 25.5, 14.6; ESI-MS The reaction mixture was diluted with CH2Cl2 (200 mL) and
obsd 387.0691, calcd 387.0686 [(M + H)+, M = C23H36BrN2O2]; labs washed with 1 M HCl (710 mL) and brine (200 mL). The organic
(toluene) 400 nm; lem (toluene) 546 nm.
layer was dried (Na2SO4) and concentrated under reduced
pressure. Column chromatography [silica, CH2Cl2/acetone (30 : 1
to 8 : 1)] afforded a white solid (9.43 g, 80%): mp 132–134 1C;
1H NMR (400 MHz, CDCl3) d 1.35 (s, 6H), 1.35 (t, J = 7.1 Hz, 3H),
8-Carbethoxy-10-(dibutylboryl)-2,3-dihydro-5-(1-naphthyl)-
1,2,2-trimethyldipyrrin (3-BBu2)
Following a reported procedure,16 a solution of 3 (50 mg, 4.29 (q, J = 7.1 Hz, 2H), 6.15 (dd, J = 2.0, 1.6 Hz, 1H), 6.39 (s, 1H),
0.13 mmol) and triethylamine (73 mL, 0.52 mmol) in CH2Cl2 7.39 (dd, J = 3.0, 1.6 Hz, 1H), 9.00 (br s, 1H); 13C NMR (100 MHz,
(1.8 mL) was treated with Bu2B–OTf (0.26 mL, 1 M in CH2Cl2, CDCl3) d 14.6, 25.4, 40.1, 40.6, 60.1, 97.6, 107.0, 117.8, 123.5,
0.26 mmol) under argon at room temperature. The reaction 127.5, 148.3, 165.2, 180.1; ESI-MS obsd 264.1231, calcd 264.1230
mixture was stirred for 3 h under argon. The reaction mixture [(M + H)+, M = C14H17NO4].
was quenched with water and then extracted with CH2Cl2 (2 Â
4-Carbethoxy-(E)-2-[(4,4-dimethyl-5-methylenedihydrofuran-
2(3H)-ylidene)methyl]pyrrole (8)
30 mL). The organic phase was dried (Na2SO4), concentrated,
and chromatographed [silica, hexanes/ethyl acetate (4 : 1)] to
afford a light yellow oil (48 mg, 71%): 11B NMR d 3.45; 1H NMR Following a reported method12 with some modification, a
d 7.90 (d, J = 8.1 Hz, 2H), 7.77 (d, J = 8.2 Hz, 1H), 7.59–7.36 (m, solution of TiCp2Cl2 (1.77 g, 7.11 mmol) in toluene (19.0 mL)
5H), 5.89 (d, J = 1.5 Hz, 1H), 4.20–4.10 (m, 2H), 2.45 (s, 3H), 2.26 was treated dropwise with MeLi (9.7 mL of 1.6 M solution in
(ABq, DdAB = 0.12, J = 17.4 Hz, 2H), 1.37–1.08 (m, 15H), 0.99– Et2O, 16 mmol) over 5 min at 0 1C under an argon atmosphere.
0.71 (m, 12H); 13C NMR d 186.3, 165.8, 135.6, 133.9, 133.7, After 1 h at 0 1C, the reaction was quenched by the addition of
132.0, 131.4, 131.0, 128.55, 128.46, 127.2, 126.6, 126.1, 125.6, 6% aqueous NH4Cl solution. The organic layer was washed with
125.3, 123.8, 116.3, 109.3, 59.3, 48.3, 39.5, 28.3, 27.8, 26.2, water and brine, dried (Na2SO4) and filtered. The filtrate was
26.09, 26.08, 25.9, 14.7, 14.6, 14.5, 14.4; ESI-MS obsd treated with lactone–pyrrole 7 (394 mg, 1.50 mmol) and additional
511.3494, calcd 511.3490 [(M + H)+, M = C33H43BN2O2]; labs TiCp2Cl2 (22 mg, 88 mmol). The mixture was heated to 80 1C in the
(toluene) 411 nm; lem (toluene) 503 nm; labs (CH2Cl2) 388 nm. dark for 4 h and then allowed to cool to room temperature,
whereupon NaHCO3 (75 mg), MeOH (1.8 mL) and H2O (1.8 mL,
8-Carbethoxy-10-(difluoroboryl)-2,3-dihydro-5-(1-naphthyl)-
1,2,2-trimethyldipyrrin (3-BF2)
0.1% v/v relative to MeOH) were added. The mixture was then
stirred overnight at 40 1C. The reaction mixture was filtered
Following a reported method2 with some modification, a solution through Celite. The filtrate was concentrated and chromato-
of 3 (0.180 g, 0.466 mmol) and triethylamine (0.33 mL, 2.3 mmol) graphed (silica, CH2Cl2 with 0–5% ethyl acetate) to afford a
in CH2Cl2 (6.0 mL) was treated with BF3ÁOEt2 (0.58 mL, yellow-brown solid (271 mg, 69%): mp 99–100 1C; 1H NMR
4.66 mmol) under argon at room temperature. The reaction (CDCl3, 300 MHz) d 8.24 (b, 1H), 7.33 (dd, J = 3.0, 1.5 Hz, 1H),
mixture was stirred overnight under argon. The reaction mixture 6.35–6.29 (m, 1H), 5.83 (dt, J = 2.0, 1.0 Hz, 1H), 4.39 (d, J =
was quenched by addition of saturated aqueous NaHCO3 (20 mL) 2.4 Hz, 1H), 4.28 (q, J = 7.1 Hz, 2H), 4.01 (d, J = 2.4 Hz, 1H), 2.71
and then extracted with CH2Cl2 (2 Â 50 mL). The organic phase (d, J = 1.9 Hz, 2H), 1.34 (t, J = 7.1 Hz, 3H), 1.26 (s, 6H); 13C NMR
was dried (Na2SO4), concentrated, and chromatographed [silica, (CDCl3, 75 MHz) d 169.6, 165.1, 155.0, 129.6, 122.4, 117.8,
hexanes/ethyl acetate (2 : 1)] to afford a light yellow solid (0.180 g, 105.7, 92.2, 80.9, 59.9, 42.7, 40.2, 28.1, 14.7.
86%): mp 222–224 1C; 11B NMR d 1.28 (a peak between 0 and
Photophysical measurements
À1 ppm was unassigned and may stem from slight decomposition);
1H NMR d 7.91 (d, J = 8.1 Hz, 2H), 7.79–7.73 (m, 2H), 7.57–7.38 Photophysical studies in toluene (and for one compound in
(m, 4H), 6.00 (s, 1H), 4.17 (q, J = 7.1 Hz, 2H), 2.66 (s, 3H), acetonitrile and dimethylsulfoxide) were carried out on dilute (mM)
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argon-purged solutions. Fluorescence quantum yields were References
determined for deoxygenated samples relative to the standards
pyranine (Ff = 1.0 in 0.10 M NaOH23) and 5-mesityldipyrrinato-
boron difluoride (BDPY1) (Ff = 0.93 in toluene24). S1 lifetimes
were determined by time correlated single photon counting
(TCSPC) fluorescence spectroscopy and by transient absorption
(TA) spectroscopy, both employing B100 fs excitation flashes
from an ultrafast laser system (Spectra Physics). TCSPC studies
utilized a simple-Tau 130 system (Becker & Hickl) with an
instrument response function of o200 ps using B100 fs
visible-region excitation pulses (at 8 MHz) attenuated to avoid
exciton annihilation. Acquisition of TA difference spectra
(400–900 nm) from B100 fs to B7.5 ns utilized a spectrometer
that employed B100 fs white-light probe pulses (Ultrafast Systems,
Helios) and on the time scale from B100 ps to B0.5 ms using a
white-light pulsed laser (B1 ns rise time) in 100-ps time bins
with pump–probe delay to 0.5 ms (Ultrafast Systems, EOS). TA
studies also afforded the yield of S1 - T1 intersystem crossing by
comparing the extent of bleaching of the ground-state absorp-
tion bands due to T1 at the asymptote of the S1 decay versus the
extent due to S1 immediately after excitation. TA data sets were
analyzed at individual wavelengths and globally using Surface
Explorer (Ultrafast Systems), CarpetView (Light Conversions) and
OriginPro (Origin Labs). Time profiles were fit to the convolu-
tion of the instrument response with a series of exponentials plus a
constant.
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This work was funded by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences
of the U.S. Department of the Energy (FG02-05ER15661). Mass
spectrometry data were obtained in the Molecular Education,
Technology, and Research Innovation Center (METRIC) at
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Antenna Research Center, an Energy Frontier Research Center
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