Re-evaluation of the tert-Butyl Method in Crystal Engineering of Salicylideneanilines by Simultaneous Observation of Photochromism and Thermochromism in Single Crystals

Single-crystal microscopic absorption spectroscopy was used to clarify the effect of a bulky substituent on the photochromic properties of salicylideneanilines. We discovered that 3,5-di-tert-butylsalicylideneaniline (1), exhibiting debatable chromic properties, had at least three crystalline polymorphs, α1, α1, and β. α1-1 is thermochromic from −75 to 45 °C and also photochromic, particularly from −75 to 0 °C; α2-1 is photochromic but considerably unstable and transforms into the β form; β-1 is thermochromic from −75 to 90 °C but not photochromic. The 4′-methyl analogue of 1 (2) is simultaneously thermochromic and photochromic, especially around 75 °C, and the 4′-chloro analogue (3) is thermochromic from −75 to 90 °C and is more strongly photochromic at lower temperatures. Comparison of the crystal structures facilitated the identification of some common features among the photochromic species. We demonstrated a quantitative temperature-dependent photochromic response profile for the first time. The observed temperature dependence of photoinduced spectral changes presents an intrinsic challenge to the conventional dualistic classification of compounds as photo- or thermochromic.