Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Incrementing Stokes Shifts through the Formation of 2,2′Biimidazoldiium Salts Shoji Matsumoto,* Mei Watanabe, and Motohiro Akazome Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoicho, Inageku, Chiba 263-8522, Japan S Supporting Information *
ABSTRACT: The formation of biimidazoldiium structures by the introduction of methyl substituents on the N atoms at the 3 and 3′ positions of 2,2′-biimidazoles led to increments in the Stokes shift of these structures. Based on time-dependent density functional theory (TDDFT) calculations, the imidazolium rings become distorted and the N atoms of the imidazolium rings underwent structural changes through sp2 to sp3 rehybridization in the excited states.
M
substituting the phenyl rings for methyl groups at the 1 and 1′ positions. In contrast, the twisted conformation dominates through the introduction of a substituent at the sp2 N atoms at the 3- and 3′-positions of the azole rings. 1,1′,3,3′-Tetraalkyl2,2′-biimidazoldium and bibenzimidazoldium dications have been examined as reducing reagents12 and were analyzed as (co)crystalline structures.13,14 However, to the best of our knowledge, the absorption spectra of 2,2′-bibenzimidazoldiium ions have only been reported once;15 these compounds were rotationally restricted about the 2,2′-bond by their N,N′bridged structures. Herein, we report the optical properties of 1,1′,3,3′-tetramethyl-5,5′-diaryl-2,2′-biimidazoldium ions (2) (see Scheme 1). We found that large Stokes shifts are achieved through the introduction of methyl groups at the 3- and 3′positions of the azoles, with the largest Stokes shifts (∼200 nm) being observed for compounds bearing electron-donating substituents at the 5- and 5′-positions. We synthesized the 1,1′,3,3′-tetramethyl-5,5′-diaryl-2,2′biimidazoldiiums (2) from the corresponding 1,1′-dimethyl5,5′-diaryl-2,2′-biimidazoles (1 (R = Me, G = N)) (see Scheme 2), by the coupling reactions of 1,1′-dimethylbiimidazole (3)16 and aryl halides.17 The counterion was exchanged for PF6− for purification by recrystallization via a procedure that has been reported in the literature. 16 For comparison, we also synthesized 1,1′,3,3′-tetramethyl-2,2′-biimidazoldiium bishexafluorophosphate (4). Ultraviolet−visible light (UV-vis) absorption and fluorescence spectra of 1a, 2a, 3, and 4 were acquired in toluene, THF, and CH3CN (see Table 1 and Figure 1a). 1a exhibited absorption peaks (λmax) of ∼300 nm (Table 1, entries 1−3), which are ∼20 nm longer than the λmax values of 1,1′,5,5′-
ethods for the control of light are important, because of their applications to display devices, sensors, memory mediums, and quantum-computer processors, among others. Recent interest has focused on materials that exhibit large Stokes shifts for use in solar cells,1 lasers,2 and probes,3 among other applications.4,5 Consequently, a variety of compounds with large Stokes shift have been reported.6 An X-shaped structure was one of the unique compounds to give large Stokes shifts.7 Lin and co-workers reported a fluorescent probe that consisted of pyridinium cations that exhibited a large Stokes shift (over 200 nm in dimethylsulfoxide (DMSO)).8 We reported that, regarding the physical properties of biimidazo[1,2-a:2′,1′-c]quinoxalinium salts, they are fluorescent under a variety of conditions.9 Bielawski and co-workers also reported the optical properties of imidazolium compounds.10 These materials are cationic through alkylation of the sp2 nitrogen in the chromophore. Such a procedure is a straightforward method for altering the physical properties of a material, including its optical behavior. We have investigated 1,1′,5,5′tetraaryl-2,2′-bipyrroles and biimidazoles (1 (R = Ar)) (see Scheme 1). We found that these compounds exhibit large Stokes shifts that are affected by the planarity of the excited state.11 The planarity of the ground state will be increased by Scheme 1. Depicting the Structural Refinements Introduced in This Study
Received: May 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01376 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Preparation of 1−4
Figure 1. UV-vis absorption (bold traces; 3.0 × 10−5 M) and fluorescence (narrow traces; 3.0 × 10−7 M, excited at λmax) spectra of (a) 1a (blue solid line), 1d (green solid line), 2a (blue dashed line), 3 (red solid line), 4 (red dashed line), (b) 1a (blue solid line), 1b (orange solid line), 1c (purple solid line), 2a (blue dashed line), and 2c (purple dashed line) in CH3CN.
tetraphenyl-2,2′-biimidazole (1d; 1 (Ar = Ph, G = N, W = H)) (see Table 1, entries 5−7).11b The fluorescence peaks (λem) of 1a were shorter than those of 1d, leading to smaller Stokes
shifts (Table 1, entries 1−3 vs 5−7). On the other hand, 2a exhibited absorption maxima at ∼275 nm, which are shorter
Table 1. Optical Properties of 1a−1d, 2a−c, 3, and 4 Absorptiona
Fluorescenceb −1
−1
Stokes Shift Δλ (nm)
Δνe (cm−1)
0.65i 0.39 0.046
75.5 84.5 78.0 64.6g 106.0 110.0 112.0 95.28g 132.5 134.0 116.5g 80.0 95.0 156.5
6227 7261 6485 4823g 9175 9654 10 080 6835g 11 716 11 914 6356g 4975 5800 8722
NDf 0.34 0.43 0.10 0.13
82.5 84.0 87.0 195.5 196.0
6800 6998 7362 14 079 14 139
entry
compound
solvent
λmax (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
1a
toluene THF CH3CN
312.5 301.5 310.0
30 800 33 200 33 900
388.0 386.0 388.0
ND 0.59 0.49
1dh
toluene THF CH3CN
291.0 287.0 282.0
21 700 24 800 22 800
397.0 397.0 394.0
NDf 0.37 0.57
2a
THF CH3CN
276.5 275.0
43 300 25 000
409.0 409.0
0.25 0.38
1b
toluene THF CH3CN THF CH3CN toluene THF CH3CN THF CH3CN
363.0 360.0 352.5 285.0 283.5 309.5 307.0 303.0 287.5 287.0
34 500 37 500 35 700 23 400 36 900 32 700 37 100 31 700 51 900 28 600
443.0 455.0 509.0 j j 392.0 391.0 390.0 483.0 483.0
3
THF CH3CN
272.5 270.0
14 200 12 300
326.0 326.0
0.43 0.57
53.5 56.0 38.1g
6022 6362 4567g
4
THF CH3CN
k 244.0
13 600
335.0
0.30
91.0 92.9g
11 133 11 870g
22 23 24 25 26 27
2b 1c
2c
εmax (M
cm )
λem (nm)
ΦFc f
d
a Concentration = 3.0 × 10−5 M. bConcentration = 3.0 × 10−7 M. Excited at λmax. cDetermined using p-terphenyl in cyclohexane as a standard (ΦF = 0.87, excited at 265 nm). dΔλ = λem − λmax. eΔν = 1/λmax − 1/λem. fNot determined. gEstimated from calculation data. See the Supporting Information. h1,1′,5,5′-Tetraphenyl-2,2′-biimidazole (1d) in ref 11b. iDetermined using quinine sulfate in 0.1 M aq. H2SO4 as a standard (ΦF = 0.55, excited at 366 nm). jNo fluorescence. kInsoluble in THF.
B
DOI: 10.1021/acs.orglett.8b01376 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
compounds, compared with their biimidazoles. It would be caused by the vibrational nonradiative decay, because the 2,2′biimidazoldiium conjugation had small restriction (vide infra), which gave the ease of the rotation. To reveal the reasons behind their large Stokes shifts, we examined 1−4 via density functional theory (DFT) calculations at the B3LYP/6-31+G** level of theory using the Gaussian09 suite of programs;18 the excited-state structures were optimized by time-dependent DFT (TDDFT) calculations. Smaller values for the dihedral angles between the two imidazole rings in the ground-state structures of 1a and 3 were found (17.2° and 0.03°, respectively), compared with that of 1d (33.5°)11b (see Figure S2 in the Supporting Information). Focusing on the differences between the dihedral angles of the biimidazole moieties in the ground and excited states, little change was observed for 1a and 3 (6.2° and 0.03°, respectively), whereas a change (23.1°) was calculated for 1d. Higher HOMO and lower LUMO energies were found for 1a, 3, and 1d in the excited states, relative to the corresponding ground states. As a result, smaller HOMO − LUMO energy difference (ΔE) values exist in the excited states, leading to large Stokes shifts. Those estimations were in accordance with the similarity of the experimental and calculated values (entries 4, 8, and 24 in Table 1). From these investigations, conformational as well as energetic changes are responsible for the observed Stokes shifts. More-twisted structures were obtained for both 2a and 4 than 1a and 3 (see Figure 2). The twisted angles between the two imidazole rings were observed to relax in the excited states. However, in the case of the excited-state structure of 2a, a more-twisted conformation was obtained at the 5-position of one of the imidazole rings (from 51.6° in the ground state to 85.0° in the excited state) (see Figure 2a). Through such torsion, 2a exhibited a localized HOMO, while its LUMO was located on the biimidazoldiium moiety in the excited states. The HOMO of the ground state of 2a, and of the ground and excited states of 4, were spread over the biimidazoldiium structures. In addition, the HOMO − 1 of the excited state of 2a resembled the HOMO of the ground state of 2a. Given that the solvent did not affect the fluorescence of 2a, we conclude that fluorescence is derived from π−π* transitions. Therefore, the orbitals involved in the fluorescence of 2a are its HOMO − 1 and LUMO. The LUMO energies of 2a and 4 were observed to be dramatically lower (over 1 eV) in their excited states. In addition, a higher HOMO energy was also found for 4, and for the HOMO − 1 of the excited state of 2a. As a result, smaller ΔE values in the excited state were obtained from 2a and 4, and larger differences in ΔE between the ground and excited states (ΔΔE) were obtained for 2a and 4, compared with 1a and 3 (0.7744, 0.7349, 0.9382, and 2.1582 eV for 1a, 3, 2d, and 4, respectively; also see Figure S2 in the Supporting Information), which is in agreement with the larger Stokes shifts observed for the biimidazoldiium compounds. TDDFT calculations also predicted dramatic changes in the biimidazoldiium structures. In their ground states, the imidazole rings possess planar structures in which the constituent atoms are sp2 hybridized, as evidenced by the sums of their three bond angles being close to 360° (see Figures 3a and 3c). However, those values around the N atoms deviate from 360° in the excited states (see Figures 3b and 3d), which indicates that the hybridizations of these N atoms have sp3 character. As a consequence, the methyl groups are displaced from the planes of the imidazolium rings by ∼11°−25°. We did not find any correlation between such structural changes and differences in
than the corresponding wavelengths of 1a and 1d (Table 1, entries 9 and 10), although the spectrum of 2a in toluene was not obtained, because of its low solubility. This is ascribable to the twisted geometry of the 2,2′-biimidazole moiety resulting from the introduction of the additional substituents on nitrogen. The torsion angle between the two imidazole rings of the 1,1′,3,3′-tetramethyl-2,2′-biimidazoldiium ion in the crystalline state was reported to be 62°−87°.13 In the case of 1d, density functional theory (DFT) calculations suggested a structure that tended toward planarity with a dihedral angle of 33.5°.11b 2a fluoresced at a longer wavelength than structures depicted by 1; it also exhibited a large Stokes shift, in excess of 130 nm (Δλ), with an associated energy value (Δν) of ∼12 000 cm−1. The possibility of alternating transitions between a π−π* transition and charge transfer can be excluded by the negligible effect of solvent on both the absorption and fluorescence wavelengths (see entries 9 and 10 in Table 1, and Figure S1a in the Supporting Information). To gain more information, we examined simple analogues (3 and 4) devoid of phenyl rings at the 5- and 5′-positions of the imidazole ring. As a result, an increment in the Stokes shift was observed upon formation of the biimidazoldiium ion 4 (Table 1, entry 23 vs entry 26), and similar energy differences (Δν) were observed for both the biimidazoles (Table 1, entry 3 vs entry 23) and the biimidazoldiium ions (Table 1, entry 10 vs entry 26). Hence, we conclude that a large Stokes shift can be achieved by converting the biimidazole into the biimidazoldiium ion. We investigated the effects of substituents on the phenyl rings at the 5- and 5′-positions of 1 and 2 (Table 1 and Figure 1b). When the formyl group, as an electron-withdrawing substituent, was introduced onto the biimidazole, as in 1b, longer absorption and fluorescence wavelengths were observed, compared to those of 1a (Table 1, entries 1−3 vs entries 12− 14). In particular, the fluorescence emission in CH3CN was bathochromically shifted by >100 nm. The λem of 1b in the polar CH3CN was longer than that in toluene and THF (Table 1, entries 12−14, as well as Figure S1b in the Supporting Information), which means that the excited state of 1b possesses polar characteristics. The main 5,5′-diphenyl-2,2′biimidazole structure acts as the electron-donor component; when a formyl group is introduced, a donor−π-acceptor structure is created. Therefore, a partially charge-separated condition is achieved in the excited state. As a result, a Stokes shift (Δλ) of >150 nm was observed for 1b in CH3CN (Table 1, entry 14), in addition to the 80 and 95 nm Stokes shifts observed in toluene and THF, respectively (Table 1, entries 12 and 13). However, the energy value of the Stokes shift (Δν) was decreased in toluene and THF, compared with 1a (Table 1, entries 1 and 2 vs entries 12 and 13), although the increment of Δν was observed in CH3CN (Table 1, entry 3 vs entry 14). Unfortunately, 2b was not fluorescent in either THF or CH3CN (Table 1, entries 15 and 16). The methoxy-bearing biimidazole structure 1c, with electron-donating substituents, exhibited little change in its absorption and fluorescence spectra, compared with those of 1a (Table 1, entries 1−3 vs entries 17−19). However, a large Stokes shift of ∼200 nm (15 000 cm−1) was observed for 2c (Table 1, entries 20 and 21). On the basis of the small solvent effect, we conclude that there was no transition-based charge transfer during either absorption or fluorescence (see Table 1, entries 20 and 21, as well as Figure S1c in the Supporting Information). The lower quantum yields (ΦF) were observed in the biimidazoldiium C
DOI: 10.1021/acs.orglett.8b01376 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Figure 3. Three-angle sums around selected atoms and model views depicting bonds connected by two imidazolium rings in (a, b) 2a and (c, d) 4, calculated by DFT and TDDFT for structures optimization at the B3LYP/6-31+G** level of theory. The ball-and-stick style is used for the methyl groups, and the capped-stick style is used for the imidazolium and phenyl rings.
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Figure 2. Selected dihedral angles, orbitals, HOMO and LUMO energies, and HOMO − LUMO energy differences (ΔE) of (a) 2a and (b) 4 calculated by DFT (left) and TDDFT (right) for structures optimized at the B3LYP/6-31+G** level of theory.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01376. Experimental procedures and characterization data for all compounds, UV−vis absorption and fluorescence spectra in CH3CN and THF, calculated conformations, orbitals and energies of 1a, 3, and 1d, and detail information on the calculation (PDF)
orbital energies; however, we suggest that the dramatic lowering of the LUMO energy in the excited state is a consequence of the additional s character of the orthogonal orbitals on these sp2 hybridized atoms as they transition toward sp3 hybridization.19 In conclusion, we demonstrated that Stokes shifts were incremented by the introduction of methyl substituents on the N atoms at the 3- and 3′-positions of 2,2′-biimidazoles. The largest Stokes shift was observed following the construction of donor−π-acceptor character through the addition of electrondonating substituents at the 5- and 5′-positions. TDDFT calculation revealed that the LUMO energies were lowered and the imidazolium rings became distorted in the excited states. Such dramatic changes are possible in a variety of imidazole compounds. Further investigations with monoimidazole and imidazolium compounds are currently underway.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] ORCID
Shoji Matsumoto: 0000-0003-0417-8319 Author Contributions
The manuscript was written through the contributions of all authors. Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.orglett.8b01376 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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DOI: 10.1021/acs.orglett.8b01376 Org. Lett. XXXX, XXX, XXX−XXX
