Rationalization and Design of Enhanced Photoinduced Cycloreversion in Photochromic Dimethyldihydropyrenes by Theoretical Calculations

This study presents a computational investigation of the initial step of the dimethyldihydropyrene (DHP) to cyclophanediene (CPD) photoinduced ring-opening reaction using time-dependent density functional theory (TD-DFT). In particular, the photochemical path corresponding to the formation of the CPD precursor (CPD*) on the zwitterionic state is scrutinized. The TD-DFT approach was first validated on the parent compound against accurate ab initio calculations. It confirms that CPD* formation is efficiently quenched in this system by an easily accessible S₂/S₁ conical intersection located in the vicinity of the CPD* minimum and leading to a locally excited state minimum responsible for DHP luminescence. Increased ring-opening quantum yields were observed in benzo­[e]-fused-DHP (DHP-<b>1</b>), isobutenyl-DHP (DHP-<b>2</b>), and naphthoyl-DHP (DHP-<b>3</b>). The calculations show that CPD* formation is much more favorable in these systems, either due to an inversion of electronic states in DHP-<b>1</b>, suppressing the formation of the locally excited state, or due to efficient stabilization of CPD* on the S₁ potential energy surface in DHP-<b>2</b> and DHP-<b>3</b>. Both effects can be combined in a rationally designed benzo­[e]-fused-naphthoyl-DHP (DHP-<b>4</b>) for which we anticipate an unprecedented efficiency