Article pubs.acs.org/JPCA
Solvatochromic Fluorescence Properties of Pyrazine−Boron Complex Bearing a β‑Iminoenolate Ligand Yasuhiro Kubota,* Yusuke Sakuma, Kazumasa Funabiki, and Masaki Matsui* Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan S Supporting Information *
ABSTRACT: Pyrazine-based monoboron complexes bearing two fluorine atoms (BF2 complex) or two phenyl groups (BPh2 complex) on the boron atom were synthesized, and the fluorescence properties were investigated. The BPh2 complexes exhibited red-shifted maximum absorption and maximum fluorescence wavelengths and lower molar absorption coefficients than the corresponding BF2 complexes in n-hexane. The fluorescence quantum yields of the BPh2 complexes were higher than or comparable to the corresponding BF2 complexes owing to the relatively low nonradiative rate constants. Although the nonsubstituted and trifluoromethyl-substituted derivatives did not show solvatochromism, the dimethylamino-substituted BF2 and BPh2 complexes exhibited pronounced solvatochromism in the fluorescence spectra. Dual fluorescence was observed for the dimethylaminosubstituted BF2 complex in toluene, 1,4-dioxane, and chloroform, corresponding to locally excited (LE) and twisted intramolecular chargetransfer (TICT) states.
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INTRODUCTION Organoboron complexes have received increasing attention because of their unique fluorescence properties and rich structural diversity.1,2 Their fluorescence properties strongly depend on the type of ligand. Fluorescent boron complexes bearing monoanionic bidentate NN3−9 and OO10−14 chelating ligands have been thoroughly investigated. Boron dipyrromethene (BODIPY) dyes15−20 and difluoroboron dibenzoylmethane derivatives21−24 are typical NN- and OO-type boron complexes, respectively. Among boron complexes, BODIPY dyes, in particular, have excellent fluorescence properties, such as outstanding fluorescence quantum yields, sharp absorption and fluorescence spectra, and high photo- and chemical stability. Therefore, BODIPY dyes have widespread applications, such as in chemosensors,25,26 biomolecular labeling,27,28 photodynamic therapy,29,30 laser dyes,31,32 and solar cells.33−36 However, most BODIPY dyes exhibit a very small Stokes shift and hardly show solid-state fluorescence and solvatochromic fluorescence; these drawbacks limit their applications. Thus, novel boron complexes have been extensively developed.37−42 In contrast, fluorescence wavelengths of solvatochromic fluorescent dyes change with solvent polarity.43−45 Therefore, much attention has been paid to solvatochromic fluorescent dyes as valuable tools for chemical and biochemical research.46−49 Most solvatochromic dyes have donor−acceptor structures.50−53 Recently, boron complexes showing solvatochromic fluorescence have also been reported.54−58 Boron complexes containing monoanionic bidentate NO chelating ligands have also been a focus of active research in recent years.59−64 Among them, boron complexes bearing a β© 2014 American Chemical Society
iminoenolate ligand can be classified into seven types depending on the position of the annulated ring (type 1,65−69 2,70−76 3,77 4,78−85 5,86−104 6,105 and 7106) (Figure 1). Type 1 boron complexes showing an aggregation-induced emission enhancement (AIEE) effect have been reported most recently.65−67 Boron complexes classified as type 3, 6, and 7 have rarely been reported thus far. Type 4 and 5 boron complexes have been relatively well studied in the recent decade; the application of these complexes to organic lightemitting diodes,80−82,86−91 fluorescent labeling reagents,78,79 and nonlinear optical dyes83 has been examined. Recently, we have reported that type 2 boron complexes show interesting fluorescence properties, such as a large Stokes shift, solid-state fluorescence, and AIEE effect.70−72 The absorption and fluorescence properties of type 2 boron complexes depend on the type of the annulated ring, which is formed through the double bond between the N1 and C2 atoms, substituent group R′ on the C4 atom, and substituent group R on the boron atom. In this paper, we report the synthesis and fluorescence properties of type 2 pyrazine-boron complexes, which exhibit solvatochromic fluorescence.
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RESULTS AND DISCUSSION Synthesis. The reaction of 2,3-dichloropyrazine with acetophenone in the presence of sodium hydride afforded pyrazine-based bidentate NO ligand 1, which consists of a Received: July 4, 2014 Revised: August 29, 2014 Published: August 29, 2014 8717
dx.doi.org/10.1021/jp506680g | J. Phys. Chem. A 2014, 118, 8717−8729
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Figure 1. Classification of boron complexes bearing a β-iminoenolate ligand.
Scheme 1. Synthesis of Pyrazine-Based Monoboron Complexes
Figure 2. (a) UV−visible absorption and (b) fluorescence spectra of 4−9 in n-hexane (1.0 × 10−5 M). Solid and dotted lines represent the BF2 and BPh2 complexes, respectively.
afforded the corresponding BF 2 complexes, 5 and 6, respectively. The reaction of 2 and 3 with BPh3 gave the corresponding BPh2 complexes, 8 and 9, respectively. The structures of 4−9 were confirmed by their 1H and 13C NMR spectral analyses. UV−visible Absorption Properties. The absorption spectra of 4−9 in n-hexane are shown in Figure 2. In the BF2 complexes, the maximum absorption wavelengths (λmax) of trifluoromethyl derivative 5 (404 nm) and dimethylamino derivative 6 (462 nm) blue- and red-shifted, respectively, compared to nonsubstituted derivative 4 (410 nm) (Table 1). Similar trend was observed in the BPh2 complexes (7: 438 nm, 8: 435 nm, and 9: 476 nm). The λmax of the BPh2 complexes were more bathochromic than those of the corresponding BF2 complexes. The molar absorption coefficient (ε) of the BPh2 complexes (7: 15,300, 8: 13,200, and 9: 33,100) were lower than those of the corresponding BF2 complexes (4: 25,300, 5: 24,400, and 6: 37,800). Since the degree of double-bond character is known to be higher in ground states than in its
tautomeric mixture of iminoketone 1a and iminoenol 1b (Scheme 1). The iminoketone and iminoenol structures were confirmed by their 1H NMR spectral analyses. The methylene signal of 1a appeared at δ 4.71 ppm (s, 2H), whereas the olefinic and hydroxyl signals of 1b appeared at δ 6.57 ppm (s, 1H) and δ 14.1 ppm (brs, 1H), respectively. The ratio of 1a to 1b was 2.8:1 in CDCl3. The tautomeric mixture of 1a and 1b was allowed to react with boron trifluoride-diethyl ether (BF3· OEt2) complex to afford BF2 complex 4. The reaction of the tautomeric mixture with triphenylborane (BPh3) gave BPh2 complex 7. Similar reactions of 2,3-dichloropyrazine with 4′(trifluoromethyl)acetophenone and 4′-(dimethylamino)acetophenone produced bidentate NO ligands 2 and 3, respectively (Scheme 1). Trifluoromethyl derivative 2 existed in tautomeric equilibrium with iminoketone 2a and iminoenol 2b in CDCl3; the ratio of 2a to 2b was 1:1.4. Dimethylamino derivative 3 existed almost exclusively in the iminoketone form, 3a, in CDCl3. The reaction of 2 and 3 with BF3·OEt2 complex 8718
dx.doi.org/10.1021/jp506680g | J. Phys. Chem. A 2014, 118, 8717−8729
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Table 1. UV−visible and Fluorescence Properties of Pyrazine Boron Complexes in n-Hexanea compound
λmax [nm] (ε)
7
410 (25,300), 428 (17,400) 404 (24,400), 423 (17,000) 462 (37,800), 482 (32,000) 438 (15,300)
8
435 (13,200)
9
476 (33,100), 499 (28,700)
4 5 6
k f = 2.88 × 10−9n2
[nm]
Φfb,c
[ns]
kfe [109 s−1]
446, 472 438, 464 506, 534 505, 531 502, 526 529, 549
0.29
2.1
0.14
0.34
0.13
0.7
0.19
1.24
0.80
1.6
0.50
0.13
0.62
9.0
0.07
0.04
0.32
4.4
0.07
0.15
0.71
5.9
0.12
0.05
λemb
τsd
knrf [109 s−1]
∫ f (υ)̃ dυ ̃ ∫ f (υ)̃ υ−̃ 3 dυ ̃
∫ ε(υυ)̃ ̃ dυ ̃
(1)
where n is the refractive index of the solvent, and f(υ̃) and ε(υ̃) are the fluorescence intensity and molar extinction coefficient curves in wavenumbers (cm−1), respectively.111−113 The timedependent density functional theory (TDDFT) calculations indicate that the f values of the BPh2 complexes (7: 0.18, 8: 0.19, and 9: 0.44) are smaller than those of the BF2 complexes (4: 0.37, 5: 0.43, and 6: 0.47) (Table 2). Therefore, the smaller Table 2. TDDFT Calculations at the B3LYP/6-311++G(d,p) Level and Experimentally Obtained Absorption Properties
Measured at a concentration of 1.0 × 10−5 mol dm−3. bThe excitation wavelengths (λex) were as follows: 4 (410 nm), 5 (404 nm), 6 (463 nm) 7 (440 nm), 8 (437 nm), and 9 (474 nm). cMeasured by Hamamatsu Photonics Quantaurus-QY. dMeasured by Hamamatsu Photonics Quantaurus-τ. eRadiative rate constant (kf = Φf/τs). f Nonradiative rate constant (knr = (1 − Φf)/τs). a
compd
transition
λmaxa [nm]
4
S0 → S1
387
S0 → S2
319
S0 → S1
383
S0 → S2
319
S0 → S1
450
S0 → S2
359
S0 → S1
421
S0 → S2
333
S0 → S1
421
S0 → S2
337
S0 → S1
455
S0 → S2
359
5
6
excited states, the bending of double-bonds leads to larger energy increase in the ground state than in the excited state. The bending of double-bonds also causes less orbital overlap between the ground state and the excited state. Therefore, in many cases, the bending of a chromophore results in the red shift of λmax and decrease of ε value.107−110 Thus, the longer λmax and lower ε of the BPh2 complexes are probably because of the molecular bending of the BPh2 complexes caused by the introduction of bulky phenyl groups at the boron atom.70 Fluorescence Properties. The fluorescence spectra of 4−9 are shown in Figure 2. In accordance with the absorption spectra, dimethylamino (6: 506 nm and 9: 529 nm) and trifluoromethyl derivatives (5: 464 nm and 8: 502 nm) exhibited red-shifted and blue-shifted maximum fluorescence wavelengths (λem) compared to the corresponding nonsubstituted derivatives (4: 472 nm and 7: 505 nm), respectively (Table 1). The fluorescence quantum yields (Φf) of the dimethylamino (6: 0.80 and 9: 0.71) and trifluoromethyl derivatives (5: 0.13 and 8: 0.32) were higher and lower than those of the corresponding nonsubstituted derivatives (4: 0.29 and 7: 0.62), respectively. The λem values of the BPh2 complexes 7−9 (502−529 nm) were more bathochromic than those of the corresponding BF2 complexes (464−506 nm). The Φf values of the BPh2 complexes (0.32−0.71) were higher or comparable to the corresponding BF2 complexes (Φf: 0.13−0.80). To obtain the fluorescence lifetimes (τs), time-resolved fluorescence spectroscopy was performed. The fluorescence decays were fitted to a monoexponential function. The radiative (kf = Φf/τs) and nonradiative (knr = (1 − Φf)/τs) rate constants were also calculated (Table 1). The BPh2 complexes (7: 9.0 ns, 8: 4.4 ns, and 9: 5.9 ns) have longer τs values compared to the corresponding BF2 complexes (4: 2.1 ns, 5: 0.7 ns, and 6: 1.6 ns). The kf values of the BPh2 complexes (7: 0.07 × 109 s−1, 8: 0.07 × 109 s−1, and 9: 0.12 × 109 s−1) are smaller than those of the BF2 complexes (4: 0.14 × 109 s−1, 5: 0.19 × 109 s−1, and 6: 0.50 × 109 s−1). According to the Strickler−Berg equation, kf is proportional to the integral of molar extinction coefficient curve (∝ oscillator strength f) (eq 1):
7
8
9
f
λmaxb [nm]
LUMO
0.37
410
LUMO + 1
0.40
313
LUMO
0.43
403
LUMO + 1
0.33
304
LUMO
0.47
462
LUMO + 1
0.51
367
LUMO
0.18
438
LUMO + 1
0.16
325
LUMO
0.19
435
LUMO + 1
0.10
325
LUMO
0.44
476
LUMO + 1
0.35
370
main orbital transition HOMO → (0.69) HOMO → (0.68) HOMO → (0.69) HOMO → (0.69) HOMO → (0.70) HOMO → (0.68) HOMO → (0.65) HOMO → (0.68) HOMO → (0.56) HOMO → (0.67) HOMO → (0.69) HOMO → (0.63)
a Calculated absorption maximum. bObserved absorption maximum in n-hexane.
kf values of the BPh2 complexes are probably because of the smaller f values. Interestingly, the BPh2 complexes (7: 0.04 × 109 s−1, 8: 0.15 × 109 s−1, and 9: 0.05 × 109 s−1) have smaller knr values compared to the BF2 complexes (4: 0.34 × 109 s−1, 5: 1.24 × 109 s−1, and 6: 0.13 × 109 s−1). Regardless of the lower kf values, the relatively high Φf values of the BPh2 complexes are because of the smaller knr values. Theoretical Calculation. To understand the UV−vis absorption properties, the DFT calculations were performed by using the Gaussian 09 package.114 The geometries of 4−9 were optimized at the B3LYP/6-31G(d,p) level. The DFT calculation results indicate the following: (1) the introduction of trifluoromethyl and dimethylamino groups decrease and increase the HOMO and LUMO energy levels, respectively; (2) the substitution of fluorine atoms to the phenyl groups on the boron atom increase the HOMO and LUMO energy levels. The TDDFT calculations were also performed at the B3LYP/6311++G(d,p) level. The calculated λmax, the main orbital transition, and oscillator strength f values are shown in Table 2. The calculated values of the λmax are in good agreement with the experimental values. For all the complexes, the S0 → S1 and S0 → S2 transitions are mainly attributed to the transitions from the HOMO to LUMO and from the HOMO to LUMO+1, 8719
dx.doi.org/10.1021/jp506680g | J. Phys. Chem. A 2014, 118, 8717−8729
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Figure 3. Molecular orbital energy diagram and isodensity surface plots of the HOMO−1, HOMO, LUMO, and LUMO+1 of 4−9.
Figure 4. (a) UV−visible absorption and (b) normalized fluorescence spectra of 6 in various solvents (1.0 × 10−5 M).
Figure 5. (a) UV−visible absorption and (b) normalized fluorescence spectra of 9 in various solvents (1.0 × 10−5 M).
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dx.doi.org/10.1021/jp506680g | J. Phys. Chem. A 2014, 118, 8717−8729
The Journal of Physical Chemistry A
Article
Table 3. UV−visible Absorption and Fluorescence Properties of 6 and 9 in Various Solventsa 6
9
no.
solvent
Δfb
λmax [nm]
λemc [nm]
1 2 3 4 5 6 7 8 9 10
n-Hexane Cyclohexane Toluene 1,4-Dioxane Chloroform Ethyl acetate THF Dichloromethane Acetone Acetonitrile
0.00 0.00 0.01 0.02 0.15 0.20 0.21 0.22 0.28 0.31
462, 482 465, 485 481 475 489 477 478 489 478 478
506e, 509e, 507e, 514e, 530e, 513e 514e 539e 531e 546e
534f 539f 565g 584g 592g
Φfd
λmax [nm]
λemh [nm]
Φfd
0.80 0.72 0.43 0.14 0.09 0.04 0.02 0.03
