Photochromic Diarylethenes with Heterocyclic Aromatic Rings: Correlation between Thermal Bistability and Geometrical Characters of Transition States

We present a density functional theory study on the thermal bistability of a number of photochromic diarylethenes, with emphasis on the free energy barrier of the ground-state ring-opening process. We found that the free energy barrier is correlated with the geometrical and vibrational character of the transition state, in particular the distance between the two reactive carbon atoms, the out-of-plane angles of the methyl groups at the reactive carbon atoms, and the imaginary vibrational frequency. Based on these relationships we propose a linear fitting expression for the free energy barrier in terms of the three aforementioned transition-state quantities and obtained a correlation coefficient of R² = 0.971. In this way quantum chemical calculations may provide insight and structure-property relationships, which can be applied in the development of novel photochromic materials. (Chemical Equation Presented).