Synthesis of carbazole-based BODIPY dimers showing red fluorescence in the solid state

Chihiro Maeda *, Takumi Todaka , Tomomi Ueda and Tadashi 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

Received 28th September 2017 , Accepted 10th October 2017

First published on 10th October 2017


Carbazole-based BODIPY dimers 2a–g were synthesized via direct arylation. They showed red-shifted solid-state fluorescence spectra as compared with the corresponding monomer. In addition, unsymmetrical dimers 2d, 2f, and 2g with two different substituents showed red fluorescence with improved quantum yields in the solid state.


Boron dipyrromethene (BODIPY) derivatives have been studied because of strong photoabsorption and fluorescence, thermal and photostabilities, facile preparations, and versatile functionalizations.1 In the last decade, various BODIPY oligomers have been synthesized to investigate the electronic communication within the molecules and to extend the π-conjugation.2,3 Despite strong fluorescence in solution, general BODIPYs often show fluorescence quenching in the solid state. Recently, BODIPY derivatives and congeners showing solid-state fluorescence have attracted much attention. The introduction of bulky substituents and/or the enlargement of Stokes shifts has been reported to be effective for overcoming this problem.4,5 The former and the latter decrease the intermolecular interaction and the self-absorption (self-quenching), respectively, in the solid state. However, solid-state near-infrared (NIR) fluorescent dyes are still rare because a nonradiative decay process easily occurs as the HOMO–LUMO gap becomes small.6

We have investigated carbazole-based porphyrinoids,7 and recently reported the synthesis of carbazole-based BODIPYs via an organometallic approach.8 Various heterocycles were introduced into the 1-position of carbazole, and the boron complexation of the resulting dipyrrins produced a series of carbazole-based BODIPYs. These dyes showed large Stokes shifts in solution, and carbazole–benzothiazole hybrid 1 exhibited relatively strong fluorescence both in solution and in the solid state (Fig. 1). Because of the unique properties of 1, we came up with biscarbazole–benzobisthiazole triad 2, a class of BODIPY dimers that have the dimeric structure of 1. Here, we report the synthesis and photophysical properties of the carbazole-based BODIPY dimers 2. These newly synthesized dyes had extended π-conjugation and showed red solid-state fluorescence.


image file: c7ob02419c-f1.tif
Fig. 1 Structures of carbazole-based BODIPYs 1 and 2.

Scheme 1 shows the synthesis of carbazole-based BODIPY dimers 2a–c. The direct arylation of 3,6-di-tert-butyl-1-bromocarbazole (3a) with benzobisthiazole 4 provided the bis-coupling product 5a in an 18% yield along with the mono-coupling product 6a in a 29% yield. The boron complexation of the dipyrrins 5a and 6a afforded 2a and 7 in 95% and 76% yields, respectively. The HR-MS spectra of 2a and 7 exhibited parent ion peaks at m/z values of 842.3450 (calcd for C48H48N4S2B2F4 842.3464 [M]) and of 517.1628 (calcd for C28H26N3S2BF2 517.1629 [M]+), respectively. Fortunately, single crystals of 2a and 7 suitable for X-ray diffraction analysis were obtained. X-ray crystal structures revealed the structures of 2a and 7 (Fig. 2). Similarly, BODIPY dimers 2b and 2c with aryl groups were obtained from the corresponding 1-bromocarbazoles 3b and 3c, respectively.


image file: c7ob02419c-s1.tif
Scheme 1 Synthesis of carbazole-based BODIPYs 2a–c and 7.

image file: c7ob02419c-f2.tif
Fig. 2 X-ray crystal structures of (a) 2a and (b) 7. Hydrogen atoms are omitted for clarity. The thermal ellipsoids are drawn at the 50% probability level. Intermolecular distances are shown between the mean planes of the molecules, excluding the peripheral substituents.

The UV/vis absorption and fluorescence spectra are shown in Fig. 3, and the data are summarized in Table 1. The absorption and fluorescence maxima of 7 were red-shifted as compared with that of 1 due to the expanded π-conjugation. In addition, the absorption bands of dimers 2a–c were intensified and further red-shifted. However, the fluorescence maxima were slightly shifted, and the Stokes shifts of 2a–c were smaller than those of 1 or 7. The fluorescence quantum yields of 2a and 2b in solution were high, while that of 2c was low probably because of the rotation of the aryl groups which enhances the nonradiative decay relaxation. On the other hand, the fluorescence maxima of 2a–c in the solid state were red-shifted as compared with that of those in solution, and 2a–c showed red fluorescence (Fig. 3c). Unfortunately, the fluorescence quantum yields of 2a–c were very low in the solid state. These results are not inconsistent with the crystal packing analysis of 2a. In the crystal packing, 2a forms a one-dimensional column with an interplanar distance of 3.62 Å. The large overlap of the π-planes between the neighboring molecules as well as the smaller Stokes shift enhance the self-absorption in the solid state. Thus the solid-state fluorescence of 2a was found to be weak. 7 recorded a lower fluorescence quantum yield than 1 in the solid state, which is also considered to be due to the packing manner as well as the smaller Stokes shift.


image file: c7ob02419c-f3.tif
Fig. 3 (a) UV/vis absorption spectra of 1, 2a–c, and 7 in CH2Cl2. (b) Fluorescence spectra in CH2Cl2 (dotted line) or in the solid state (solid line). (c) Photographic images of the powdered compounds under black light (λ = 365 nm).
Table 1 Photophysical data for 1–6 in CH2Cl2 and in the solid state
Compd CH2Cl2 Powder
λ A (nm) λ F (nm) ΔνSt[thin space (1/6-em)]a (cm−1) Φ F τ F (ns) k f[thin space (1/6-em)]b (108 s−1) k nr[thin space (1/6-em)]c (108 s−1) λ F (nm) Φ F τ F (ns) k f[thin space (1/6-em)]b (108 s−1) k nr[thin space (1/6-em)]c (108 s−1)
a Stokes shift. b Rate constant for radiative decay: kf = ΦF/τF. c Rate constant for nonradiative decay: knr = (1 − ΦF)/τF. d Excited at 450 nm. e Excited at 480 nm. f Excited at 490 nm. g Excited at 470 nm.
1a 458 534d 3110 0.320 4.4(97%), 39(3%) 0.6 1.3 542d 0.214 3.8(90%), 9.1(10%) 0.5 1.8
7 474 541d 2610 0.306 3.8(98%), 17(2%) 0.8 1.7 564d 0.133 1.5(92%), 4.8(8%) 0.7 4.9
2a 517 548e 1090 0.445 3.2(99%), 27(1%) 1.3 1.6 588e 0.064 0.7(70%), 1.9(30%) 0.6 8.9
2b 516 547e 1100 0.441 3.0(97%), 9.3(3%) 1.4 1.8 640e 0.031 0.4(76%), 2.1(24%) 0.4 11.5
2c 531 573f 1440 0.153 1.5(90%), 3.7(10%) 0.9 4.8 642f 0.036 0.9(67%), 2.8(33%) 0.2 6.2
2d 516 550g 1200 0.425 2.5(22%), 3.9(78%) 1.2 1.6 609g 0.110 1.7(61%), 3.3(39%) 0.5 3.8
2e 523 555e 1100 0.190 1.5(65%), 3.0(35%) 0.9 4.0 630e 0.029 1.1(78%), 3.2(22%) 0.2 6.4
2f 523 569e 1550 0.155 1.1(37%), 2.9(63%) 0.7 3.7 639e 0.098 1.5(66%), 3.1(34%) 0.5 4.5
2g 515 554g 1370 0.395 2.3(14%), 3.3(86%) 1.2 1.9 630g 0.089 0.9(40%), 3.1(60%) 0.4 4.1


Recently, unsymmetrical BODIPYs have been reported as a good fluorophore in the solid state.5 Therefore we then tried to synthesize unsymmetrical dimers 2d–f with two kinds of substituents via the direct arylation reaction of 6a or 6b with 1-bromocarbazoles having different substituents followed by boron complexation (Scheme 2). 2d–f showed similar absorption and fluorescence spectra to symmetrical dimers 2a–c (Fig. 4). However, 2d and 2f recorded improved fluorescence quantum yields of 0.110 and 0.098, respectively, in the solid state. These results suggested the importance of the mesityl groups which might prevent the intermolecular approach. Thus, we additionally synthesized unsymmetrical BODIPY 2g with mesityl and different aryl groups. 2g also showed red solid-state fluorescence with a similar quantum yield.


image file: c7ob02419c-s2.tif
Scheme 2 Synthesis of unsymmetrical BODIPYs 2d–g.

image file: c7ob02419c-f4.tif
Fig. 4 (a) UV/vis absorption spectra of 2a–g in CH2Cl2. (b) Fluorescence spectra in CH2Cl2 (dotted line) or in the solid state (solid line). (c) Photographic images of the powdered compounds under black light (λ = 365 nm).

To understand the fluorescence properties in more detail, fluorescence lifetimes in CH2Cl2 and in the solid state were measured, and the decay curves were fitted by the second-order exponential decay function (Table 1 and Fig. S1). The fluorescence lifetime (τF), rate constant for radiative decay (kf), and rate constant for nonradiative decay (knr) of 7 are similar to those of 1 in CH2Cl2. In the solid state, however, the knr of 7 increases, which may lead to weaker fluorescence than that of 1. For symmetrical dimers 2a–c, the decays are mainly composed of a single fluorescence lifetime in CH2Cl2. In contrast, two-component lifetimes were observed for unsymmetrical dimers 2d–g, and the τF values seem to be related to those of 2a–c: e.g. the two fluorescence lifetimes of 2d seem to correspond to those of 2a and 2b. In the solid state, 2a–g all show two-component lifetimes, which suggests considerable intermolecular interactions.9 Indeed, the knr values of 2a and 2b significantly increase in the solid state. 2c, 2e, and 2f with 3,5-di-tert-butylphenyl groups show larger knr values than the other dimers in CH2Cl2 probably due to the rotation of the substituent. The knr values of 2c, 2e, and 2f increase slightly in the solid state, and the rotation seems to be suppressed. Collectively, the low solid-state fluorescence quantum yields of 2c and 2e are considered to be mainly due to the small kf values. On the other hand, the moderate kf and relatively low knr values of 2d, 2f, and 2g might lead to acceptable fluorescence quantum yields in the solid state.10

In summary, carbazole-based BODIPY dimers 2a–g were synthesized via the direct arylation reaction of 1-bromocarbazoles and benzobisthiazole, followed by boron complexation. These dyes showed solid-state fluorescence in the visible to NIR region. Although the fluorescence quantum yields of symmetrical BODIPYs 2a–c were very low, unsymmetrical BODIPYs 2d, 2f, and 2g showed red solid-state fluorescence with improved quantum yields. Further development of carbazole-based BODIPY oligomers is currently underway.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number 15K05427. We thank Prof. A. Osuka and Dr T. Tanaka (Kyoto University) for X-ray diffraction analysis and solid-state fluorescence measurements, Prof. K. Matsuda and Dr T. Hirose (Kyoto University) for fluorescence lifetime measurements, and Prof. H. Yorimitsu (Kyoto University) for mass measurements.

Notes and references

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  9. We consider that shorter fluorescence lifetimes are ascribed to intermolecular interactions. A high percentage of the longer fluorescence lifetime of 2g suggests weak intermolecular interactions.
  10. The 3,5-di-tert-butylphenyl groups can take nearly coplanar arrangement with the BODIPY planes as the fluorescence maxima of 2c, 2e, and 2f are red-shifted in the solid state. The larger π-plane might cause strong intermolecular interactions and fluorescence quenching. On the other hand, the bulky mesityl groups can hinder the intermolecular access and lead to a reduction of the intermolecular interactions. We consider that these substituent effects induce the weak fluorescence of 2c and 2e, and the moderate fluorescence of 2f.

Footnotes

Electronic supplementary information (ESI) available: Experimental procedures and compound data. CCDC 1569437 and 1569438. For ESI and crystallographic data in CIF or other electronic format, see DOI: 10.1039/c7ob02419c
Crystallographic data for 2a: C48H46N4S2B2F4·3(C6H5Cl), Mw = 1078.28, tricrinic, space group P[1 with combining macron](2), a = 8.2634(10), b = 12.174(2), c = 15.232(3) Å, α = 88.77(3), β = 79.422(18), γ = 76.323(18), V = 1463.2(4) Å3, Z = 1, ρcalcd = 1.337, T = −180 °C, 18[thin space (1/6-em)]884 measured reflection, 5066 unique reflections (Rint = 0.0426), R1 = 0.0487, wR2 = 0.1387 (all data), GOF = 1.004. Crystallographic data for 7: 2(C28H26N3S2BF2)·0.615(CCl2), Mw = 1085.71, tricrinic, space group P[1 with combining macron](2), a = 9.6199(15), b = 11.2244(19), c = 24.169(4) Å, α = 88.899(15), β = 85.634(13), γ = 83.446(12), V = 2585.0(7) Å3, Z = 2, ρcalcd = 1.395, T = −180 °C, 32[thin space (1/6-em)]248 measured reflection, 8710 unique reflections (Rint = 0.0258), R1 = 0.0520, wR2 = 0.1517 (all data), GOF = 1.024.

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