Synthesis and Kinetics of Sterically Altered Photochromic Dithizonatomercury Complexes

Following a previous study where 12 electronically altered dithizones were synthesized, here we report on attempts to synthesize 26 dithizones. The purpose was to explore the boundaries within which dithizones may be synthesized, explore spectral tuning possibilities, and investigate steric effects on the photochromic reaction of its mercury complexes. Contrary to expectation, large substituents like phenoxy groups increased the rate of the photochromic back-reaction. In the series H-, 2-CH₃-, 4-CH₃-, 3,4-(CH₃)₂-, 2-OC₆H₅-, and 4-OC₆H₅-di­thizon­ato­phenyl­mercury­(II), the lowest rate of 0.0004 s^–1 was measured for the 2-CH₃ complex, while the rate for the 2-OC₆H₅ derivative was 20 times higher. A solvent study revealed a direct relationship between dipole moment and the rate of the back-reaction, while the relationship between temperature and rate is exponential, with t_1/2 = 2 min 8 s for the 4-phenoxy complex. The crystal structures of two dithizone precursors, 2-phenoxy- and 4-phenoxy­nitro­formazan, are reported.