Photochromic switching in semiconducting films of zinc dithizonate

A photochromic transition between the red (R) and blue (B) forms of zinc dithizonate has been followed experimentally by measuring the photovoltage of this semiconducting film. The wavelength for maximum photovoltage signal is 620 nm, which corresponds to the maximum optical absorption of the photoexcited blue form. This is consistent with the following reactions: Rk1(hν)Bk2(hν)B++e− (observed photovoltage). The reactions dynamics of this photochromic switching were probed with a pulsed tunable dye laser and (k1)−1 and (k2−1) are both faster than 10 ns, the experimental resolution limit. The decay of the sample back to its dark equilibrium has been examined and is consistent with the reactions B+k3R+ and e−+R+k4R, where k3 = 23.66 s−1 and k4 = 4.558 s−1. The validity of this reaction mechanism was checked by repeating the experiments with additional incident background light, and the photovoltaic decay differed as expected theoretically.