Photoisomerization and thermal isomerization behaviors of an extensive series of arylazoimidazoles are investigated. Absorption spectra are characterized by a structured pipi* absorption band around 330-400 nm with a tail on the lower energy side extending to 500 nm corresponding to an npi* 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.

The isomerization behaviors of 2-(phenylazo)imidazole (Pai-H) and 1-N-methyl-2-(phenylazo)imidazole (Pai-Me) have been investigated. The crystal structure of trans-Pai-Me was determined, revealing that key structures around the azo group are nearly identical among azobenzene, Pai-H, and Pai-Me. Pai-Me undergoes reversible cis/trans photoisomerization, whereas Pai-H responds poorly to irradiation. The quantum yields of trans-to-cis isomerization of Pai-Me on 454 and 355 nm excitation are 0.35 +/- 0.03 and 0.25 +/- 0.03, respectively, in toluene. The wavelength-dependent isomerization quantum yield is well-known for azobenzene, but these values are substantially higher than those of azobenzene. The activation energy of thermal cis-to-trans isomerization of Pai-Me in toluene is 79.0 +/- 3.5 kJ mol(-1), which is lower than that of azobenzene by 15 kJ mol(-1). The thermal cis-to-trans isomerization of Pai-H is even faster. Density functional theory calculations were performed, revealing that the energy gaps between the azo n-orbital and the highest pi-orbital of azoimidazoles are much narrower than that of azobenzene. Finally, a preliminary study suggested that metal ions can modulate the absorption spectrum of Pai-Me without a loss of the gross photochromic behavior.

Reaction between CdX2 and 1-alkyl-2-(phenylazo)imidazole (RaaiR') has isolated complexes of composition Cd(RaaiR')2X2 in MeOH or MeCN. Crystallization of Cd(RaaiR')2I2 from N,N-dimethylformamide (DMF) has separated [Cd(RaaiR')I2.DMF], while Cd(RaaiR')2X2 (X = Cl and Br) remains unchanged in its composition upon crystallization under identical conditions. The structure has been established by spectral (UV-vis and 1H NMR) data and confirmation in the latter case by a single-crystal X-ray diffraction study of [Cd(TaiMe)I2.DMF] [where TaiMe = 1-methyl-2-(p-tolylazo)imidazole]. UV-light irradiation in a MeCN solution of Cd(RaaiR')2I2 and [Cd(RaaiR')I2.DMF] shows trans-to-cis isomerization of coordinated azoimidazole. The reverse transformation, cis-to-trans, is very slow with visible light irradiation. Quantum yields (phit-->c) of trans-to-cis isomerization are calculated, and the free ligand shows higher phi values than their cadmium(II) iodo complexes. The cis-to-trans isomerization is a thermally induced process. The activation energy (Ea) of cis-to-trans isomerization is calculated by a controlled-temperature experiment. The effects of the anions (Cl-, Br-, I-, and ClO4-) and the number of coordinated azoimidazoles (RaaiR') [Cd(RaaiR') or Cd(RaaiR')2] on the rate and quantum yields of photochromism are established in this work. A slow rate of photoisomerization of [Cd(RaaiR')4](ClO4)2 compared to Cd(RaaiR')I2 or Cd(RaaiR')2X2 may be associated with the increased mass and rotor volume of the complexes. The rate of isomerization is also dependent on the nature of X and follows the sequence Cd(RaaiR')2Cl2 < Cd(RaaiR')2Br2 < Cd(RaaiR')2I2. It may be related to the size and electronegativity of halide, which reduces the effective molar association in the order of I < Br < Cl and hence the rate. Gaussian 03 calculations of representative complexes and free ligands are used to explain the difference in the rates and quantum yields of photoisomerization.