Distortion-Controlled Thermal Stability in Phenanthrene-Bridged Diarylethene Photoswitches

Abstract

Chiral dithienylethene (DTE) provide a distinctive platform for elucidating how subtle geometric and electronic perturbations govern photochromic switching behavior. Herein, we report four fused phenanthrene-based DTE with site-selective fluorine substitution. All compounds display reversible photochromism, high fatigue resistance, and photostationary-state conversion yields above 95% in solution. Fluorination at the fjord region of the phenanthrene bridge substantially extends the thermal half-life of the closed-ring isomer, with DTE-1 exhibiting a lifetime nearly two orders of magnitude greater than those of the other derivatives. Eyring analysis reveals an increased activation barrier for thermal ring opening, while DFT calculations show that fjord-region fluorination induces phenanthrene distortion and reduces π-conjugation across the central ethylene bridge, thereby reshaping the free-energy landscape and slowing thermal cycloreversion. Together, these results identify substitution-controlled bridge distortion as an effective strategy for modulating the thermal stability and switching kinetics of fused-ring DTE photoswitches.