Synthesis and fluorescence properties of thiazole-boron complexes bearing a β-ketoiminate ligand

Article abstract of DOI:10.1021/ol302179r

Novel fluorescent dyes, thiazole-boron complexes bearing β-ketoiminate ligands, have been synthesized, and their fluorescence properties were investigated. The BF₂ complexes showed a pronounced aggregation-induced emission enhancement effect because of the restriction of C-Ph intramolecular rotation. The BPh₂ complexes showed higher fluorescence quantum yields than the corresponding BF₂ complexes, both in solution and in the solid state.

Full text of DOI:10.1021/ol302179r

ORGANIC
LETTERS
2012
Vol. 14, No. 17
4682–4685
Synthesis and Fluorescence Properties of
ThiazoleꢀBoron Complexes Bearing a
β‑Ketoiminate Ligand
Yasuhiro Kubota,* Syunki Tanaka, Kazumasa Funabiki, and Masaki Matsui*
Department of Materials Science and Technology, Faculty of Engineering,
Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
kubota@gifu-u.ac.jp; matsuim@gifu-u.ac.jp
Received August 5, 2012
ABSTRACT
Novel fluorescent dyes, thiazoleꢀboron complexes bearing β-ketoiminate ligands, have been synthesized, and their fluorescence properties were
investigated. The BF2 complexes showed a pronounced aggregation-induced emission enhancement effect because of the restriction of CꢀPh
intramolecular rotation. The BPh2 complexes showed higher fluorescence quantum yields than the corresponding BF2 complexes, both in
solution and in the solid state.
Most fluorescent dyes that exhibit fluorescence in dilute
solutions quench or reduce the fluorescence intensity in the
solid state owing to aggregation-caused quenching
(ACQ).1 Recently, Tang et al. have reported unique fluo-
rescent dyes showing aggregation-induced emission en-
hancement (AIEE) effects which are opposite to the
ACQ effect.2 Owing to their unique fluorescent properties,
suchAIEE dyeshavea widerangeofpotentialapplications
such as organic electroluminescence devices,3 fluorescent
sensors,4 and photodynamic therapy.5 Thus, AIEE-active
molecules such as hydrocarbons6 and heterocycles7 have
been developed.
Organoboron complexes are one of the most important
types of fluorescent dyes. Notably, boron dipyrromethene
(BODIPY) dyes8 have attracted much interest in various
fields because of their advantageous properties such as
high quantum yield, sharp spectra, and high photostabil-
ity. However, BODIPY dyes often show a very small
Stokes shift and ACQ.9 The development of novel boron
complexes has been aggressively studied.10 However, little
has been published on organoboron complexes showing
(1) (a) Park, S.-Y.; Ebihara, M.; Kubota, Y.; Funabiki, K.; Matsui,
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r
10.1021/ol302179r
Published on Web 08/28/2012
2012 American Chemical Society

AIEE effects. To the best of our knowledge, only two
types of AIEE active boron complexes, BODIPY11 and
BF2ꢀhydrazone (BODIHY),12 have been reported thus
far. Recently, we reported that pyrazineꢀboron complexes
having a β-ketoiminate ligand show a large Stokes shift
and solid-state fluorescence.13 In this paper, we report the
synthesis and fluorescence properties of thiazoleꢀboron
complexes bearing β-ketoiminate ligands, which show the
AIEE effect.
The reaction of 2-methylbenzothiazole with methyl
benzoate gave benzothiazole-based β-ketoiminate ligand
1 (Scheme S1). In solution, ligand 1 exists in two tauto-
meric forms, iminoketone 1a and iminoenol 1b, through
ketoꢀenol tautomerization. The structure of 1a was con-
firmed by the methylene signal at δ 4.84 (s, 2H) in its 1H
NMR spectrum. Tautomer 1b was also confirmed by the
1H NMR signals at δ 6.38 (s, 1H, CdCH) and 13.9 (brs,
1H, OH). The ratio of 1a and 1b was 1:1.5 in CDCl3. The
tautomeric mixture 1 was then allowed to react with the
boron trifluorideꢀdiethyl ether complex in the pres-
ence of triethylamine in dichloromethane to afford the
corresponding BF2 complex, 2 (Scheme S2). The reaction
of 1 with triphenylborane also gave corresponding BPh2
complex 3. The structures of 2 and 3 were confirmed by
X-ray crystallography (Figures S1 and S2).
The absorption and fluorescence spectra of 1ꢀ3 in
hexane are shown in Figure 1. The BF2 complex 2 showed
a sharp absorption peak at 380 nm along with a vibrational
peak. In contrast, BPh2 complex 3 exhibited a structureless
absorption spectrum. The maximum absorption wave-
length (λmax) of 3 (402 nm) was more bathochromic than
that of 2 (380 nm), and the molar absorption coefficient (ε)
of 3 (25,800) was lower than that of 2 (43,700) (Table 1).
This red shift of λmax and decrease in ε of 3 may be due to
the molecular bending of the BPh2 complex caused by the
introduction of bulky phenyl groups at the boron
atom.13,14
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Commun. 2008, 4777. (b) Hepp, A.; Ulrich, G.; Schmechel, R.; Von
Seggern, H.; Ziessel, R. Synth. Met. 2004, 146, 11. (c) Kubota, Y.;
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Figure 1. UVꢀvis absorption and fluorescence spectra of 1ꢀ3 in
hexane.
Table 1. UVꢀvis Absorption and Fluorescence Properties
~
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a Measured at a concentration of 1.0 ꢁ 10ꢀ5 mol dmꢀ3
.
b The
excitation wavelengths (λex) were as follows: 2 (380 nm), 3 (400 nm), 5
(394 nm), and 6 (412 nm). c Measured using a Quantaurus-QY. d Mea-
sured using a Quantaurus-τ. e Radiative rate constant (kf = Φf/τf).
f Nonradiative rate constant (knr = (1 ꢀ Φf)/τf). g The λex were as
follows: 2 (429 nm), 3 (440 nm), 5 (425 nm), and 6 (492 nm). h Too short
to be measured by the Quantaurus-τ (τf < 0.1 ns).
(11) Hu, R.; Lager, E.; Aguilar-Aguilar, A.; Liu, J.; Lam, J. W. Y.;
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Org. Lett., Vol. 14, No. 17, 2012
4683

The solvent effect on the absorption and fluorescence
properties of 2 and 3 was examined (Figures S3ꢀS6; Tables 2
and S1). The λmax and ε values of 2 and 3 were hardly
affected by solvent polarity, suggesting that the dipole
moments of the molecules in the ground and excited states
were almost the same.3a Contrary to our expectation, BF2
complex 2 hardly exhibited fluorescence in solution. The
absolute fluorescence quantum yields (Φf) of 2 were only as
high as 0.01 in low-viscosity solvents (Table 2). On the other
hand, BPh2 complex 3 showed relatively strong fluorescence
along with two vibrational peaks in solution (Table S1). The
maximum fluorescence wavelengths (Fmax) showed almost
no variation with changing solvent polarity (497ꢀ502 nm),
and the values of Φf in polar solvents (MeOH, 0.17; MeCN,
0.25) were lower compared with those in other solvents
(hexane, 0.41; toluene, 0.46; CHCl3, 0.46; THF, 0.46;
CH2Cl2, 0.42). Complex 3 exhibited a relatively large Stokes
shift (4770ꢀ5070 cmꢀ1) than fluorescent boron complexes
such as BODIPY dyes12 (400ꢀ600 cmꢀ1) and pyrido-
metheneꢀBF2 complexes12 (250ꢀ400 cmꢀ1).
properties, density functional theory (DFT) calculations
were performed with the Gaussian 09 package.15 The ge-
ometries of 2, 3, 5, and 6 were optimized at the B3LYP/
6-31G(d,p) level. Time-dependent DFT (TDDFT) calcula-
tions were also performed at the B3LYP/6-311þþG(d,p)
level. The calculated λmax, the main orbital transition, and
oscillator strength f are shown in Table S2. These calcu-
lated values of λmax and f are in good agreement with the
experimental values. For all complexes, the first absorp-
tions are mainly attributed to the transitions from HOMO
to LUMO. For each complex, the HOMO and LUMO
orbitals are delocalized over the entire molecule (Figure S9).
To determine whether thiazoleꢀboron complexes have
AIEE character, the fluorescence properties of 2 were
investigated in THFꢀwater mixtures of various ratios,
since 2 is soluble in THF but not in water. The fluorescence
properties were almost the same until the addition of 70%
water by volume, because 2 was soluble in the THFꢀwater
mixtures with a lower water content. However, upon
addition of 80% water in THF, the fluorescence color
and Fmax changed dramatically from blue to yellow-green
and from 445 to 495 nm, respectively (Figure S10). The
fluorescence intensity also increased with the increasing
water content of the solvent (Figure S11). Notably, the
intensity was significantly enhanced when the water frac-
tion exceeded 80%. These changes result from the forma-
tion of aggregates in the THFꢀwater mixtures. When the
water content was increased to 80%, the absorption
spectra exhibited leveled-off tails extending into the long-
wavelength region (Figure S12). This may be due to Mie
light scattering caused by nanoparticles.6j,7c The Tyndall
phenomenon was observed when the 80% THF/water
solution was irradiated by laser light (Figure S13). This
also suggests the formation of nanoparticles. Similar re-
sults were observed for naphthothiazole derivative 5
(Figures S14 and 15). Therefore, BF2 complexes are con-
sidered to have AIEE character.
Table 2. UVꢀvis Absorption and Fluorescence Properties of 2
in Various Solventsa
To determine the reason for the AIEE effect in the
complexes, time-resolved fluorescence spectroscopy was
performed. The fluorescence decays were fitted to a mono-
exponential function. The fluorescence lifetimes (τf) of 3, 5,
and 6 in hexane were measured to be 2.9, 0.13, and 3.0 ns,
respectively; however, the lifetime of 2 was too short to
measure (<0.1 ns). The radiative (kf = Φfτf) and non-
radiative (knr = (1 ꢀ Φf)/τf) rate constants were also
a Measured at a concentration of 1.0 ꢁ 10ꢀ5 mol dmꢀ3
.
b The
excitation wavelengths (λex) were as follows: hexane (380 nm), toluene
(382 nm), chloroform (381 nm), THF (381 nm), dichloromethane
(382 nm), methanol (379 nm), acetonitrile (378 nm), ethylene glycol
(382 nm), and glycerol (389 nm). c Measured using a Quantaurus-QY.
d Measured using a Quantaurus-τ. e Radiative rate constant (kf = Φf/τf).
f Nonradiative rate constant (knr = (1 ꢀ Φf)/τf). g Too short to be
measured by the Quantaurus-τ (<0.1 ns).
A similar reaction of 2-methylnaphtho[2,1-d]thiazole
with acetophenone gave naphthothiazole-based β-keto-
iminate ligand 4 (Scheme S3). CorrespondingBF2 complex
5 and BPh2 complex 6 were obtained by the reaction of 4
with the trifluorideꢀdiethyl ether complex and triphenyl-
borane, respectively (Scheme S4). The absorption and
fluorescence spectra in hexane are shown in Figures S7
and S8. Theλmax and Fmax valuesof naphthothiazolederiv-
(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian 09, revision A. 02; Gaussian, Inc.: Wallingford, CT,
2009.
atives 5 (λmax = 396 nm, Fmax = 459 nm) and 6 (λmax
406 nm, Fmax = 510 nm) were slightly red-shifted com-
=
pared with those of the corresponding benzothiazole deri-
vatives 2 (λmax = 380 nm, Fmax = 440 nm) and 3 (λmax
=
402 nm, Fmax = 499 nm), owing to the extension of
π-conjugation (Table 1). To understand the absorption
4684
Org. Lett., Vol. 14, No. 17, 2012

calculated (Table 1). Although there was little difference
between the kf valuesofBF2 complex 5 (0.31ꢁ 109 sꢀ1) and
the BPh2 complexes (3, 0.14 ꢁ 109; 6, 0.16 ꢁ 109 sꢀ1), the
bathochromic than those in hexane (440ꢀ510 nm). Similar
to the results obtained in solution, BPh2 complexes 3 and 6
showed higher Φf values (0.60 and 0.50, respectively) than
the corresponding BF2 complexes 2 (0.26) and 5 (0.23) in
the solid state (Table 1). To consider the difference, we
investigated the crystal packing. Two independent crystal-
lographic conformers, 2A and 2B, were observed in the
crystal of 2 (Z0 = 2) (FigureS1). Inthe crystal packing of 2,
2A and 2B formed independent stacking columns (Figures 2
and S19). Each stacking column formed consecutive
k
nr of 5 (7.4 ꢁ 109 sꢀ1) was obviously larger than those of 3
(0.20 ꢁ 109 sꢀ1) and 6 (0.17 ꢁ 109 sꢀ1). This suggests that
the lower fluorescence quantum yields of the BF2 com-
plexes are attributed to an increase in nonradiative pro-
cesses. To obtain further insight, we investigated the effect
of solvent viscosity and temperature on fluorescence.
Regardless of the solvent polarity, BF2 complex 2 hardly
showed fluorescence (Φf e 0.01) in low-viscosity solvents
(0.31ꢀ0.59cP) (Figure S4 and Table 2). On the other hand,
2exhibited fluorescence in ethylene glycol (Φf = 0.05, 23.5 cP)
and glycerol (Φf = 0.12, 1412 cP). This suggests that the
viscous medium inhibits intramolecular rotation, thereby
suppressing the nonradiative process which leads to
increased Φf.16 The fluorescence intensity of 2 in hexane
increased gradually as the temperature decreased (Figure S16).
This also suggests that the restriction of intramolecular
rotation (RIR) associated with decreasing temperature
leads to enhanced fluorescence. Therefore, the main reason
for AIEE of 2 is considered to be the RIR associated with
the phenyl group.
˚
˚
intermolecular interactions by C S (2A, 3.64 A; 2B,
˚
3 3 3
3.68 A) and CH/F interactions (2A, 2.41ꢀ2.66 A; 2B,
˚
2.47ꢀ2.66 A). Furthermore, CH/F interactions were also
˚
observed between 2A and 2B (2.41ꢀ2.56 A). As a result,
BF2 complex 2 formed consecutive intermolecular inter-
actions throughout the molecule. In the case of BPh2
complex 3, similar to BF2 complex 2, stacking columns
having consecutive intermolecular interactions were
˚
formed through C S (3.70 A) and CH/π interactions
3 3 3
˚
(2.85ꢀ2.89 A) (Figure S20). Since the CH/π interaction is
weaker than CH/F interactions, the intrermolecular in-
teractions of stacking columns of 3 are weaker than those of
2. Additionally, unlike BF2 complex 2, interactions between
stacking columns were not observed in 3. Because CH/F
interactions were inhibited, BPh2 complex 3 exhibited a
higher Φf than did BF2 complex 2 in the solid state.
In summary, thiazoleꢀboron complexes bearing β-
ketoiminate ligands have been synthesized. Although the
BF2 complexes hardly showed fluorescence in hexane (Fmax:
440ꢀ459 nm, Φf e 0.04), the BPh2 complexes exhibited
relatively strong fluorescence (Fmax, 499ꢀ510 nm; Φf,
0.41ꢀ0.49). Time-resolved fluorescence spectroscopy re-
vealed that the lower values of Φf for the BF2 complexes are
attributed to the promotion of nonradiative processes. The
BPh2 complexes also showed higher Φf values in the solid
state (Fmax, 503ꢀ527 nm; Φf, 0.50ꢀ0.60) compared with
those of the BF2 complexes (Fmax, 495ꢀ508 nm; Φf,
0.23ꢀ0.26). X-ray crystallographic analysis suggested that
the lower Φf of the BF2 complexes is due to the formation of
consecutive intermolecular CH/F interactions. These com-
plexes exhibited higher Φf values in the solid state than in
solution; in other words, thiazoleꢀboron complexes have
AIEE character. The Φf in solution became higher and the
nonradiative process was suppressed as the solvent viscosity
increased, indicating that the main reason for AIEE in the
complexes is the restriction of intramolecular CꢀPh rota-
tion. Since intramolecular rotations are inhibited in the solid
state, the thiazoleꢀboron complexes are considered to show
higher Φf values in the solid state than in solution.
Figure 2. (a) Top view of the crystal structure of 2 (Z = 4, Z0 = 2).
(b) Side view of the crystal structure of 2. Red and blue molecules
show 2A. Pink and green molecules show 2B. The black, light blue,
and orange dotted lines show intermolecular C S, CH/F inter-
actions of 2A or 2B, and CH/F interactions between 2A and 2B,
respectively.
3 3 3
All thiazoleꢀboron complexes, 2, 3, 5, and 6, showed
fluorescence in the solid state (Figure S17). The solid-state
absorption spectra of2, 3, 5, and 6areshowninFigureS18.
The absorption spectra in the solid state were broader and
red-shifted compared to those in the solution. As a result,
the Fmax values in the solid state (495ꢀ527 nm) were more
Acknowledgment. This work was supported by Grant-in-
Aid for Young Scientists (B) (23750228).
Supporting Information Available. Details of experi-
mental procedures and copies of 1H, 13C NMR, and 2D
spectra for new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
(16) (a) Bloor, D.; Kagawa, Y.; Szablewski, M.; Ravi, M.; Clark,
S. J.; Cross, G. H.; Palsson, L.-O.; Beeby, A.; Parmerc, C.; Rumbles, G.
J. Mater. Chem. 2001, 11, 3053. (b) Barbara, P. F.; Rand, S. D.;
Rentzepis, P. M. J. Am. Chem. Soc. 1981, 103, 2156.
The authors declare no competing financial interest.
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