Rational Control of Photochromic Colors in Aza-Diarylethenes

Photochromic molecules capable of reversible structural transformation upon light irradiation have attracted considerable attention for applications in molecular switches, optical memories, and sensing systems. In such applications, controlling the color of the photogenerated isomer is a key challenge. Here we report the systematic control of photochromic colors in aza-diarylethenes through molecular design of nitrogen-containing aryl units. A series of aza-diarylethenes incorporating pyrimidine-based aryl groups was synthesized, and their photochromic properties were investigated in solution. Upon UV irradiation, the compounds undergo reversible electrocyclic reactions to generate closed-ring isomers exhibiting distinct visible absorption bands. TD-DFT calculations revealed that the visible absorption originates predominantly from the HOMO→LUMO and HOMO→LUMO+1 transitions. Extension of π-conjugation through phenyl-substituted pyrimidine units stabilizes the LUMO levels, resulting in a redshift of the absorption bands and enabling systematic color tuning. Furthermore, the perceived colors predicted from the calculated absorption spectra capture the overall trends of the experimentally observed colors. These results demonstrate that the combination of rational molecular design and computational prediction provides an effective strategy for controlling photochromic colors in aza-diarylethenes. The present work establishes design guidelines for color-tunable photochromic molecules and provides a framework for predictive color design in molecular photoswitches.