Photochromism of 2-(Phenylazo)imidazoles

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 π-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.