J. Phys. Chem. A 2007, 111, 1403-1409
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Photoisomerization and Thermal Isomerization of Arylazoimidazoles Joe Otsuki,*,† Kazuya Suwa,† Kamal Krishna Sarker,‡ and Chittaranjan Sinha*,‡ College of Science and Technology, Nihon UniVersity, 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan, and Department of Chemistry, JadaVpur UniVersity, Kolkata 700032, India ReceiVed: October 17, 2006; In Final Form: January 3, 2007
Photoisomerization and thermal isomerization behaviors of an extensive series of arylazoimidazoles are investigated. Absorption spectra are characterized by a structured ππ* absorption band around 330-400 nm with a tail on the lower energy side extending to 500 nm corresponding to an nπ* transition. The trans-to-cis photoisomerization occurs on excitation into these absorption bands. The quantum yields are dependent on the excitation wavelength, as observed for azobenzene derivatives, but are generally larger than those of azobenzene. The thermal cis-to-trans isomerization rates are also generally larger than that of azobenzene and are comparable to those of 4-N,N-dimethylaminoazobenzene and 4-nitroazobenzene. Arylazoimidazoles with no substituent on the imidazole nitrogen are unique in that the quantum yield for the trans-to-cis photoisomerization and the rate of thermal cis-to-trans isomerization are particularly large. It is proposed that the fast thermal isomerization is due to an involvement of self-catalyzed and protic molecule-assisted tautomerization to a hydrazone form.
Introduction
SCHEME 1: Isomerization of Phenylazoimidazole
Aromatic azo compounds are known for their ability to undergo light-induced and/or thermal cis/trans isomerization. Azobenzene and its derivatives constitute the major class of compounds that have attracted unabated research interest, from viewpoints of basic science and potential applications to molecular switches.1-4 Most of the derivatives have substituents on the parent azobenzene scaffold, whereas less attention has been paid to its analogues having a heterocyclic ring in place of the phenyl group. Arylazoimidazoles, among them, constitute an interesting class of heterocyclic azo compounds as a potential switching group in biological applications and in coordination chemistry, because imidazole is a ubiquitous and essential group in biology, especially as a metal coordination site. This family of compounds have been extensively used as ligands for metal ions by CS.5-9 and others.10-12 However, very few reports concerning the photochromic reactions, as shown in Scheme 1, of arylazoimidazole dyes can be found in the literature. Majima and co-workers prepared (deoxy)ribofuranosyl derivatives of phenylazoimidazole and observed that these compounds undertake reversible cis/trans isomerization.13 Very recently, Fukuda and co-workers prepared polymers containing arylazoimidazole dyes that exhibit photoinduced birefringence, although they did not directly observe the photochromic reactions.14 We have reported the photoisomerization and thermal isomerization behaviors of 2-(phenylazo)imidazole (Pai-H) and its methylated derivative, 1-N-methyl-2-(phenylazo)imidazole (Pai-Me).15 In these studies, we have found that Pai-Me undergoes reversible cis/trans photoisomerization with quantum yields higher than those for azobenzene. The quantum yield of photoisomerization exhibits a wavelength dependence just as for azobenzene derivatives. The thermal cis-to-trans isomerization rate for Pai-H is much larger than that of Pai-Me. It is still * Corresponding authors. E-mail:
[email protected] (J.O.);
[email protected] (C.S.). † Nihon University. ‡ Jadavpur University.
an open question whether these behaviors are specific to Pai-Me and Pai-H or general for arylazoimidazoles with and without a substituent on the imidazole nitrogen. Herein, we report photoisomerization and thermal isomerization behaviors of an extensive series of arylazoimidazoles, which are shown in Chart 1, to shed light on general substituent and structural effects on the isomerization phenomena. Experimental Methods Compounds. Known compounds among those listed in Chart 1 were prepared according to reported procedures.16-22 The following novel compounds were prepared in analogous methods. Cai-MeBn. Anal. Found: C, 65.68; H, 4.89; N, 18.04. Calcd for C17H15ClN4: C, 65.70; H, 4.87; N, 18.03. 4-Me-Pai-Me. Anal. Found: C, 65.31; H, 5.99; N, 27.72. Calcd for C17H15ClN4: C, 65.98; H, 6.04; N, 27.98. 4-Me-Pai-Et. Anal. Found: C, 67.26; H, 6.60; N, 26.18. Calcd for C17H15ClN4: C, 67.27; H, 6.59; N, 26.15. 4-Me-Pai-Bn. Anal. Found: C, 73.90; H, 5.85; N, 20.29. Calcd for C17H15ClN4: C, 73.89; H, 5.84; N, 20.27. 2-Me-Pai-Et. Gummy. 1H NMR (CDCl3): δ 1.52 (3H, t, J ) 8.0 Hz), 2.37 (3H, s), 4.38 (2H, quartet, J ) 8.0 Hz), 7.37 (1H, s), 7.53 (3H, m), 7.84 (2H, d, J ) 8.0 Hz). 2-Me-Pai-Bn. Gummy. 1H NMR (CDCl3): δ 2.38 (3H, s), 5.13 (2H, s), 7.25-7.35 (5H), 7.40 (1H, s), 7.59 (3H, m), 7.80 (2H, d, J ) 8.0 Hz).
10.1021/jp066816p CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007
1404 J. Phys. Chem. A, Vol. 111, No. 8, 2007
Otsuki et al.
CHART 1
Methods. Spectroscopic grade solvents were purchased from Kanto Chemical (methanol, hexane, and acetonitrile) and Wako Pure Chemical Industries (toluene). Absorption spectra were taken with a Shimadzu UV-2400PC spectrometer or a Shimadzu Multispec-1500 photodiode array spectrometer in a 1 × 1 cm quartz optical cell maintained at 25 °C with a Peltier thermostat. The light source of a Shimadzu RF-5300PC fluorimeter was used as an excitation light for photoisomerization reactions. The full width at half-maximum was 4.5 nm. The wavelength values for excitation into the ππ* and nπ* bands were 355 and 454 nm, respectively, for all compounds except for R-Nai-X (X ) H, Me, Et, Bn), for which 380 and 500 nm were used. An optical filter was used to cut off overtones when necessary. The absorption spectra of pure cis-isomers were obtained by measuring the absorption spectrum and the 1H NMR for the same cis-rich mixture of representative compounds (see Supporting Information). Quantum yields (Φ) were obtained by measuring initial trans-to-cis isomerization rates (V) in a wellstirred solution within the above instrument using the equation
V ) (ΦI0/V)(1 - 10-Abs) where I0 is the photon flux at the front of the cell, V is the volume of the solution, and Abs is the initial absorbance at the irradiation wavelength. The value of I0 was obtained by using azobenzene (Φππ*,tfc ) 0.11 and Φnπ*,tfc ) 0.24)23 under the same irradiation conditions, except for R-Nai-X, for which Pai-Me (Φππ*,tfc ) 0.25 and Φnπ*,tfc ) 0.35)15 was used. The estimated errors in the values of Φ are (10%. The thermal cisto-trans isomerization rates were obtained by monitoring absorption changes intermittently for a cis-rich solution kept in the dark at a constant temperature of 25 °C. The estimated errors in the values of kcft are (3%. Density functional theory (DFT) calculations were carried out using a Gaussian 03w program pakage24 on a personal computer. We used the B3LYP functional25 and cc-pVDZ basis sets.26 Results and Discussion Absorption Spectra. Absorption spectra for the arylazoimidazole derivatives displayed in Chart 1 were measured in
methanol, toluene, and hexane. The results obtained in toluene are summarized in Table 1. The absorption spectrum of Pai-Me in toluene shown in Figure 1a illustrates the characteristics common to phenylazoimidazole derivatives: (1) a structured absorption band around 360 nm with a molar absorption coefficient on the order of 104 M-1 cm-1 and (2) a tail extending into 500 nm. From the analogy with the absorption spectra of azobenzene derivatives, it is likely that the large absorption band around 360 nm corresponds to ππ* transitions, and the tail corresponds to an nπ* transition. The assignment is also supported by theoretical calculations, as described later. The ππ* absorption peaks (λmax) for derivatives of (2-phenylazo)imidazole are in the range 361-375 nm, which is between the ππ* absorption bands of azobenzene (313 nm) and 4-N,N-dimethylaminoazobenzene (390 nm).27 Replacing the phenyl group by an R-naphthyl group results in bathochromic shifts of the λmax to around 400 nm, whereas the replacement by a β-naphthyl group results in bathochromic shifts to ca. 380 nm (Figure 1b,c). 4-(Phenylazo)imidazoles exhibit absorption spectra similar to those of 2-(phenylazo)imidazoles (Figure 1d). These spectra show little solvent dependence; the ππ* peak wavelengths differ only by 8 nm at most among the spectra in methanol, toluene, and hexane for most of the compounds investigated. In general, a slight blue shift is recognized on going from nonpolar toluene to polar methanol, suggesting that the ground state is more stabilized than the excited state in a polar solvent. DFT calculations were carried out to fully understand and identify the absorption spectra and the nature of optical transitions. We used B3LYP25 and cc-PVDZ26 as a functional and a basis set, respectively. First, the geometry was optimized using the standard procedure on the Gaussian program.24 The resulting stable conformations are all nearly planar. There are several nearly equivalent stable conformations for each compound related by a 180° flip about the N(azo)-C(ring) bonds. Then, the frontier orbitals were inspected for the most stable conformation among them. Figure 2 is a pictorial summary of the calculations, and the optimized coordinates are given in the Supporting Information.
Isomerization of Arylazoimidazoles
J. Phys. Chem. A, Vol. 111, No. 8, 2007 1405
TABLE 1: Absorption Spectra and Isomerization of Arylazoimidazolesa trans, ππ*
cis, ππ*
cis, nπ*
compounds
λmax/nm
/M-1 cm-1
λmax/nm
/M-1 cm-1
λmax/nm
/M-1 cm-1
φππ*,tfcb
φnπ*,tfcc
kcft/10-5 s-1
Pai-Hd Pai-Med Pai-Et Pai-Bn Tai-H Tai-Me Tai-Et Tai-Bn Cai-Me Cai-MeBn 4-Me-Pai-H 4-Me-Pai-Me 4-Me-Pai-Et 4-Me-Pai-Bn R-Nai-H R-Nai-Me R-Nai-Et R-Nai-Bn β-Nai-H β-Nai-Me β-Nai-Et β-Nai-Bn 2-Me-Pai-H 2-Me-Pai-Me 2-Me-Pai-Et 2-Me-Pai-Bn
362 363 364 365 364 365 374 372 369 370 371 361 375 373 395 396 397 398 376 377 377 378 355 340 352 351
18000 17000 12800 9860 9430 17500 11400 15400 21500 23100 19200 11030 15900 10700 7000 12070 14500 11700 19800 20500 19540 18300 14000 13620 7080 10900
306 329 321 318 311 320 342 316 333 333 297 298 343 333 321 326 328 328 314 322 330 329 e 298
