Color-Tunable Solid-State Fluorescence Emission from Carbazole-Based BODIPYs

Article abstract of DOI:10.1002/chem.201505150

Several carbazole-based boron dipyrromethene (BODIPY) dyes were synthesized by organometallic approaches. Thiazole, benzothiazole, imidazole, benzimidazole, triazole, and indolone substituents were introduced at the 1-position of the carbazole moiety, and boron complexation of each dipyrrin generated the corresponding compounds 1, 2 a, and 3-6. The properties of these products were investigated by UV/Vis and fluorescence spectroscopy, cyclic voltammetry, X-ray crystallography, and DFT calculations. These compounds exhibited large Stokes shifts, and compounds 1, 2 a, and 3-5 fluoresced both in solution and in the solid state. Complex 2 a showed the highest fluorescence quantum yield (Φ_F) in the solid state, therefore boron complexes of the carbazole-benzothiazole hybrids 2 b-f, which had several different substituents, were prepared and the effects of the substituents on the photophysical properties of the compounds were examined. The fluorescence properties showed good correlation with the results of crystal-packing analyses, and the dyes exhibited color-tunable solid-state fluorescence.

Full text of DOI:10.1002/chem.201505150

DOI: 10.1002/chem.201505150
Full Paper
&
Dyes/Pigments
Color-Tunable Solid-State Fluorescence Emission from Carbazole-
Based BODIPYs**
Chihiro Maeda,* Takumi Todaka, Tomomi Ueda, and Tadashi Ema*[a]
Abstract: Several carbazole-based boron dipyrromethene
(BODIPY) dyes were synthesized by organometallic ap-
proaches. Thiazole, benzothiazole, imidazole, benzimidazole,
triazole, and indolone substituents were introduced at the 1-
position of the carbazole moiety, and boron complexation of
each dipyrrin generated the corresponding compounds 1,
2a, and 3–6. The properties of these products were investi-
gated by UV/Vis and fluorescence spectroscopy, cyclic vol-
tammetry, X-ray crystallography, and DFT calculations. These
compounds exhibited large Stokes shifts, and compounds 1,
2a, and 3–5 fluoresced both in solution and in the solid
state. Complex 2a showed the highest fluorescence quan-
tum yield (FF) in the solid state, therefore boron complexes
of the carbazole–benzothiazole hybrids 2b–f, which had sev-
eral different substituents, were prepared and the effects of
the substituents on the photophysical properties of the
compounds were examined. The fluorescence properties
showed good correlation with the results of crystal-packing
analyses, and the dyes exhibited color-tunable solid-state
fluorescence.
Introduction
Boron dipyrromethene (BODIPY) and its related BF2 complexes
have received considerable attention because of their high
molecular coefficients and FF values, good thermal- and
photo-stability, and versatile functionality.[1] Such properties
allow these dyes to be applied as fluorescent probes and sen-
sors, as well as in biolabeling. To date, various BODIPY deriva-
tives and congeners have been synthesized, including periph-
erally functionalized and core-modified compounds. For exam-
ple, the fusion of a benzene, furan, or thiophene unit with the
BODIPY skeleton results in expanded p conjugation.[2] Aza-
BODIPYs, in which the meso-bridged atom is replaced with a ni-
trogen atom, exhibit redshifted absorption and emission spec-
tra relative to the spectra of regular BODIPYs.[3] Despite their
intense fluorescence in solution, most BODIPYs exhibit fluores-
cence quenching in the solid state.[4]
Scheme 1. Synthesis of carbazole-based BODIPYs.
(Bpin)2 =bis(pinacolato)diboron, cod=1,5-cyclooctadiene, dtbpy=4,4’-di-
tert-butyl-2,2’-dipyridyl, XPhos=2-dicyclohexylphosphino-2’,4’,6’-triisopropyl-
biphenyl.
erties of carbazole-based porphyrinoids and recently reported
the facile synthesis of carbazole-based BODIPYs (Scheme 1).[8]
Various substituents could be introduced onto the carbazole
moiety and efficient substituent effects were confirmed. How-
ever, these carbazole-based BODIPY dyes showed extensive
fluorescence quenching due to intramolecular charge transfer
(ICT). To develop fluorescent dyes, new carbazole-based BODI-
PYs were designed in the present work (Figure 1). Here, a thia-
zole, benzothiazole, imidazole, benzimidazole, 1,2,3-triazole, or
indolone moiety was attached at the 1-position of the carba-
zole and the BF2 complexes 1, 2a, and 3–6 were subsequently
synthesized. These unsymmetrical BODIPYs exhibited large
Stokes shifts and several were fluorescent even in the solid
state. Herein, we report the synthesis and photophysical prop-
erties of these carbazole-based BODIPYs.
Carbazole derivatives have also been extensively studied as
potential electron conductors, catalysts, sensors, and biologi-
cally active alkaloids.[5] A carbazole is simply a benzene-fused
pyrrole, therefore incorporation of a carbazole unit into fused
porphyrinoids presents interesting possibilities.[6–9] We have
previously investigated the synthesis and photophysical prop-
[a] Dr. C. Maeda, T. Todaka, T. Ueda, Prof. T. Ema
Division of Applied Chemistry
Graduate School of Natural Science and Technology
Okayama University, Tsushima
Okayama 700-8530 (Japan)
E-mail: cmaeda@okayama-u.ac.jp
ema@cc.okayama-u.ac.jp
Results and Discussion
[**] BODIPY=boron dipyrromethene.
Scheme 2 outlines the synthesis of the new BODIPY derivatives
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201505150.
1–6. Thiazole substituents were introduced into the carbazole
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Figure 1. Structures of carbazole-based BODIPYs 1, 2a, and 3–6. Bn=benzyl.
skeleton by Ir-catalyzed borylation of 3,6-di-tert-butylcarbazole
(7a) then subsequent Suzuki–Miyaura coupling of the product
with 2-bromothiazole or 2-bromobenzothiazole. Boron com-
plexation of the resulting dipyrrins 8 and 9a with BF3·OEt2 pro-
vided the corresponding BODIPYs 1 and 2a, respectively. In
contrast, imidazole substituents were attached by Pd-catalyzed
direct arylation of 1-bromo-3,6-di-tert-butylcarbazole (10),
which was prepared by bromination of 7a with N-bromosucci-
nimide (NBS). The reaction of 10 with 1-methylimidazole or 1-
methylbenzimidazole gave 11 and 12, which were subsequent-
ly converted into complexes 3 and 4, respectively. We intro-
duced 1,2,3-triazole and indolone substituents by a cyclization
strategy. 3,6-Di-tert-butyl-1-ethynylcarbazole (13) was prepared
by Stille coupling of 10 and subsequent TMS deprotection.
The click reaction of 13 with benzylazide and subsequent
boron complexation of triazolylcarbazole 14 provided 5. An in-
dolone moiety was introduced by Sonogashira coupling of 13
with 2-iodoaniline to generate 15, followed by ZnI2-catalyzed
cyclization of 15 to provide indolylcarbazole 16. Oxidation of
16 afforded 17,[10] and finally boron complexation of 17 gave
6. In this manner, BODIPYs 1, 2a, and 3–6 were prepared and
then fully characterized by both NMR and mass spectroscopy.
Single crystals of 2a, 3, and 4 suitable for X-ray diffraction anal-
ysis were also prepared: the X-ray crystal structures of these
products unambiguously confirmed the BODIPY skeletons
(Figure 2).[11]
The UV/Vis absorption and fluorescence spectra of the
target compounds are shown in Figure 3 and the associated
data are summarized in Table 1. The absorption maxima were
redshifted in the following order: 5<3<4<1<2a<6. Thus,
the absorption of thiazole-appended compound 1 was red-
shifted relative to the absorptions of 3–5, whereas benzothia-
zole-appended complex 2a was even further redshifted and
was also intensified absorption. Time-dependent DFT calcula-
tions suggested that the main absorption bands at lꢀ400
and 300 nm may be assigned to the HOMO–LUMO and
HOMOÀ2–LUMO transitions, respectively, whereas the
HOMOÀ1–LUMO transition generates a somewhat weaker
band (Figure S26 in the Supporting Information). Unlike 1, 2a,
and 3–5, 6 generated a broad absorption spectrum that
stretched to the NIR region, which suggests the presence of
ICT. Complexes 1, 2a, 3, and 4 exhibited fluorescence with
moderate-to-high FF values, whereas the fluorescence of 5
was weak and that of 6 was significantly quenched (Fig-
Scheme 2. Synthesis of 1, 2a, and 3–6. NBS=N-bromosuccinimide, TBAF=-
tetrabutylammonium fluoride.
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Table 1. Selected photophysical properties of the carbazole-based BODIPYs
in solution in CH2Cl2 or in the solid state.
[a]
[b]
[c]
Compound
labs
lem
DnSt
[cmÀ1
FF
lpowder
FF
[nm]
[nm]
]
(CH2Cl2)
[nm]
(powder)
1
2a
3
4
5
436
458
394
419
382
663
513[d]
534[e]
427[f]
477[g]
477[g]
793[h]
3440
3110
1960
2900
5210
2470
0.334
0.320
0.424
0.547
0.074
530[d]
542[e]
428[f]
478[g]
431[g]
0.165
0.214
0.130
0.141
0.150
[i]
[i]
6
<0.001
–
–
[a] Absorption wavelength. [b] Emission wavelength. [c] Stokes shift. [d] Ex-
cited at l=420 nm. [e] Excited at l=450 nm. [f] Excited at l=360 nm.
[g] Excited at l=380 nm. [h] Excited at l=660 nm. [i] Not detected.
had the highest value in the solid state (FF (powder)=0.214).
The fluorescence maxima of 1 and 2a in the solid state were
redshifted compared to those in solution, which suggested J-
type packing of these two products.[13] Indeed, the crystal
packing of 2a was confirmed to have a J-type arrangement,
with an interplanar distance of 3.67 Š and a zigzag pattern
(Figure 2a). This type of packing in conjunction with a large
Stokes shift tends to minimize self-absorption (self-quen-
ching),[3d–e,4a,14–15] which leads to the relatively strong fluores-
cence of 2a in the solid state. In contrast, the fluorescence
maxima of 3 and 4 were not shifted, which indicated that J-
type interactions were not dominant for these compounds.
The FF values of 3 and 4 in the solid state were much lower
than those in solution. Interestingly, the FF value of 5 in the
solid state was 0.150, which is higher than the value measured
in solution (FF(CH2Cl2)=0.074). This effect most likely results
from the flexible benzyl moiety, which accelerates nonradiative
deactivation in solution but not in the solid state. In addition,
the large Stokes shift of 5 reduces self-absorption, and there-
fore aggregation-induced emission enhancement is ob-
served.[16]
Figure 2. X-ray crystal structures and crystal packing of a) 2a, b) 3, and c) 4.
Hydrogen atoms are omitted for clarity. The thermal ellipsoids are drawn at
the 50% probability level. Interplanar distances are shown between the
mean planes of the molecules, excluding the peripheral substituents and
the two fluorine atoms.
The redox potentials (Ereduction and Eoxidation), of 1, 2a, and 3–6
were determined by cyclic voltammetry (Table 2, and Fig-
ure S28 in the Supporting Information). The thiazole-appended
compound 1 showed higher potentials than 3–5, whereas 2a
and 6 generated even higher potentials than 1. The associated
electrochemical HOMO–LUMO gaps (DECV) are in good agree-
Table 2. Electrochemical properties.[a]
Compound
Ered [V]
Eox [V]
DECV [V]
DEopt [V]
DEDFT [V][b]
1
2a
3
4
5
À2.087
À1.859
0.831
0.932
0.770
0.808
0.716
1.073
2.914
2.791
–
2.84
2.71
3.15
2.96
3.25
1.87
3.12
3.07
3.64
3.37
3.60
2.12
[c]
[d]
–
Figure 3. a) UV/Vis absorption spectra of 1–6 in CH2Cl2 and b) fluorescence
spectra in CH2Cl2 (dotted line) or in the solid state (solid line). c) Photograph-
ic images of the powdered compounds under black light (l=365 nm).
À2.341
3.149
[c]
[d]
–
–
6
À0.665
1.738
[a] Determined by cyclic voltammetry; solvent: CH2Cl2, supporting elec-
trolyte: Bu4NPF6 (0.10m), reference electrode: Ag/Ag+, scan rate: 0.1 VsÀ1
[b] Calculated at the B3LYP/6-31G* level. [c] Not detected. [d] Not calcu-
lated.
ure 3b).[12] These dyes were also observed to fluoresce in the
solid state, and the FF of 1, 2a, and 3–5 for solid-state fluores-
cence were all greater than 0.13. Among these compounds, 2a
.
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ment with the absorption spectroscopy results (DEopt) and with
the calculated values (DEDFT). The HOMO–LUMO gap for 6 is
notably narrow because 6 showed an exceptionally redshifted
absorption.
DFT calculations were performed to examine the electronic
states of the HOMOs and LUMOs of 1, 2a, and 3–6
(Figure 4).[17] The HOMOs exhibit large electronic coefficients
Figure 5. a) UV/Vis absorption spectra of 2a–f in CH2Cl2 and b) fluorescence
spectra in CH2Cl2 (dotted line) or in the solid state (solid line). c) Photograph-
ic images of the various powdered samples under irradiation by
a l=365 nm lamp.
Figure 4. HOMO (bottom) and LUMO (top) of a) 1, b) 2a, c) 3, d) 4, e) 5, and
f) 6, calculated at the B3LYP/6-31G* level.
on the carbazole moieties, whereas the LUMOs show large
electronic coefficients on each heterocycle. In fact, these latter
coefficients are remarkably high in the case of 5 and 6 (Fig-
ure 4e and f). This result, as well as the broad absorption spec-
trum of 6, suggests that the fluorescence quenching observed
for 6 is due to ICT from the carbazole moiety to the indolone
moiety.[7b,18]
Table 3. Selected photophysical properties of the carbazole-based BODI-
PYs 2a–f in solution in CH2Cl2 or in the solid state.
Compound
labs
lem
DnSt
[cmÀ1
FF
lpowder
FF
[nm]
[nm]
]
(CH2Cl2)
[nm]
(powder)
2a
2b
2c
2d
2e
2 f
458
495
440
420
469
457
534[a]
579[b]
510[c]
483[d]
540[a]
529[a]
3110
3010
3120
3110
2800
2980
0.320
0.008
0.380
0.296
0.205
0.358
542[a]
651[b]
575[c]
485[d]
573[a]
560[a]
0.214
0.027
0.158
0.119
0.106
0.248
Because 2a showed both a redshifted absorption and rela-
tively high FF values both in solution and in the solid state,
we decided to synthesize the BF2 complexes of carbazole–ben-
zothiazole hybrids to generate 2b–f and examine the substitu-
ent effects (Scheme 3). Compounds 2b–f were synthesized
from 7b–f, respectively, by sequential Ir-catalyzed borylation,
Suzuki–Miyaura coupling, and boron complexation.
[a] Excited at l=450 nm. [b] Excited at l=490 nm. [c] Excited at l=
430 nm. [d] Excited at l=380 nm.
The photophysical properties of 2a–f are presented in
Figure 5 and summarized in Table 3. Compound 2d, which
contains an electron-withdrawing group, had a blueshifted ab-
sorption band, whereas the compounds with electron-donat-
ing or aryl groups exhibited redshifted absorption and fluores-
cence bands in solution. The methoxy-substituted product 2b
was redshifted the furthest, although its fluorescence intensity
was very weak, most likely due to ICT. For the other complexes
FF (CH2Cl2)=0.2–0.4 FF (powder)=0.1–0.25. Both 2a and 2 f
showed stronger fluorescence in the solid state than the other
complexes studied, presumably because they contain larger
substituents that reduce intermolecular interactions.[19] The
solid-state fluorescence bands of 2 f and 2a were redshifted
relative to the bands observed in solution, which suggests J-
Scheme 3. Synthesis of 2a–f. Mes=mesityl.
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type packing. The fluorescence maximum of 2d was not shift-
ed and its solid-state FF was low, similar to the results ob-
tained for 3 and 4. It is noteworthy that 2c generated the
most highly redshifted fluorescence band in the solid state,
which suggests the formation of an excimer.
cence properties were consistent with the results of crystal-
packing analyses, and these compounds exhibited color-tuna-
ble solid-state fluorescence. The further development of carba-
zole-based fluorescent dyes is currently underway in our labo-
ratory.
Single crystals of 2c and 2e were obtained and their struc-
tures were elucidated by X-ray diffraction (Figure 6).[11] In their
Acknowledgements
This work was supported by the JSPS KAKENHI Grant Number
15K05427. We thank Prof. A. Osuka and Dr. T. Tanaka (Kyoto
University) for performing the X-ray diffraction analyses and
fluorescence spectroscopy, Prof. H. Yorimitsu (Kyoto University)
for performing mass spectroscopy measurements, and Prof. S.
Suga and Prof. K. Mitsudo (Okayama University) for performing
the cyclic voltammetry measurements.
Keywords: boron dipyrromethene (BODIPY) · carbazoles ·
dyes/pigments · fluorescence · solid-state emission
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Conclusion
We successfully synthesized a series of carbazole-based BODI-
PYs by using organometallic reactions. Thiazole substituents
were introduced at the 1-position of the carbazole by Suzuki–
Miyaura coupling; imidazole substituents were introduced by
Pd-catalyzed direct arylation. In addition, 1,2,3-triazole and in-
dolone moieties were attached by cyclization reactions. These
dipyrrins smoothly formed the boron complexes 1, 2a, and 3–
6. The structures of these compounds were characterized by
NMR and mass spectroscopy and by X-ray diffraction. Each
compound showed characteristic properties; 6 exhibited a red-
shifted absorption in the NIR region, whereas 1, 2a, 3, and 4
fluoresced with relatively high FF values both in solution and
in the solid state. Product 2a had the highest FF value in the
solid state (FF (powder)=0.214). The carbazole–benzothiazole
hybrids 2b–f were synthesized and the substituent effects on
the photophysical properties were examined. With the excep-
tion of 2b, these compounds showed large Stokes shifts and
exhibited solid-state fluorescence with FF (powder)=0.11–
0.25. Complex 2c formed an excimer and showed the greatest
extent of redshifted emission in the solid state. These fluores-
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the carbazole and heterocycle moieties, which is supported by the al-
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the S1 excited state of the dyes.
[8] C. Maeda, T. Todaka, T. Ema, Org. Lett. 2015, 17, 3090.
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[10] During preparation of this manuscript, Shinokubo reported a facile syn-
thesis of 17 from carbazole 7a and isatin.[18b]
[11] Crystallographic data for 2a: C54H54N4S2B2F4; Mr =920.75; monoclinic;
space group P21/n; a=15.413(2), b=13.1802(14), c=23.360(3) Š; b=
100.591(4)8; V=4664.7(10) Š3; Z=4, 1calcd =1.311 gcmÀ3; T=À1808C;
30555 measured reflections; 8239 unique reflections (Rint =0.0205); R1 =
0.0300 (I>2s(I)); wR2 =0.0818 (all data); GOF=1.056. Crystallographic
data for 2c: C19H11N2SBF2; Mr =348.17; monoclinic; space group P21/c;
a=8.2201(19), b=12.158(3), c=14.854(4) Š; b=93.027(9)8; V=
1482.4(6) Š3; Z=4; 1calcd =1.560 gcmÀ3; T=À1808C; 9911 measured re-
flections; 2678 unique reflections (Rint =0.0204); R1 =0.0293 (I>2s(I));
wR2 =0.0832 (all data); GOF=1.011. Crystallographic data for 2e:
4(C31H19N2SBF2)·5(CH2Cl2); Mr =2426.03; monoclinic; space group P21/c;
a=9.6032(12), b=24.752(4), c=46.617(6) Š; b=93.092(3)8; V=
11065(3) Š3; Z=4; 1calcd =1.456 gcmÀ3; T=À1808C; 65315 measured
reflections; 18774 unique reflections (Rint =0.0698); R1 =0.0873 (I>
2s(I)); wR2 =0.2789 (all data); GOF=1.068. Crystallographic data for 3:
[16] a) M. Shimizu, T. Hiyama, Chem. Asian J. 2010, 5, 1516; b) Y. Hong,
J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40, 5361; c) J. Mei, Y. Hong,
J. W. Y. Lam, A. Qin, Y. Tang, B. Z. Tang, Adv. Mater. 2014, 26, 5429.
[17] Gaussian 09, Revision C.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Men-
nucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,
A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr. , J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Ko-
bayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyen-
gar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg,
S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cio-
slowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
[18] a) S. Nakamura, S. Hiroto, H. Shinokubo, Chem. Sci. 2012, 3, 524; b) A.
Shigeno, T. Matsuno, S. Hiroto, H. Shinokubo, Chem. Lett. 2015, 44,
1703.
[19] a) T. Ozdemir, S. Atilgan, I. Kutuk, L. T. Yildirim, A. Tulek, M. Bayindir, E. U.
Akkaya, Org. Lett. 2009, 11, 2105; b) Y. Kubota, J. Uehara, K. Funabiki, M.
Ebihara, M. Matsui, Tetrahedron Lett. 2010, 51, 6195; c) H. Lu, Q. Wang,
L. Gai, Z. Li, Y. Deng, X. Xiao, G. Lai, Z. Shen, Chem. Eur. J. 2012, 18,
7852; d) Y. Ooyama, Y. Hagiwara, Y. Oda, H. Fukuoka, J. Ohshita, RSC
Adv. 2014, 4, 1163.
¯
4(C24H28N3BF2)·CH3OH; Mr =1661.26; triclinic; space group P1; a=
14.279(6), b=15.589(5), c=21.281(7) Š; a=101.867(3), b=98.133(15),
g=100.4090(10)8; V=4480(3) Š3; Z=2; 1calcd =1.231 gcmÀ3
;
T=
À1808C; 54094 measured reflections; 14196 unique reflections (Rint
=
0.1083); R1 =0.0675 (I>2s(I)); wR2 =0.1884 (all data); GOF=1.003. Crys-
tallographic data for 4: C28H30N3BF2·CH2Cl2; Mr =542.29; triclinic; space
¯
group P1; a=8.849(2), b=12.152(3), c=12.830(4) Š; a=92.7805(10),
b=101.133(10), g=98.924(5)8; V=1332.9(6) Š3; Z=2; 1calcd
=
1.351 gcmÀ3, T=À1808C; 17039 measured reflections; 4535 unique re-
flections (Rint =0.0575); R1 =0.0441 (I>2s(I)); wR2 =0.1207 (all data);
GOF=1.013. CCDC 1437585 (2a), 1437586 (2c), 1437587 (2e), 1437588
(3), and 1437589 (4) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystal Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
Received: December 24, 2015
Published online on April 13, 2016
Chem. Eur. J. 2016, 22, 7508 – 7513
www.chemeurj.org
7513
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim