Tuning Stilbene Photochemistry by Fluorination: State Reordering Leads to Sudden Polarization near the Franck–Condon Region

High Resolution Image Download MS PowerPoint Slide Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization effect) earlier discussed for 90°-twisted alkenes is observed and calculated for planar ring-fluorinated stilbenes, trans -2,3,5,6,2′,3′,5′,6′-octofluorostilbene (tF2356) and trans -2,3,4,5,6,2′,3′,4′,5′,6′-decafluorostilbene (tF23456). Due to the fluorination, Franck–Condon states S 1 FC and S 2 FC are dominated by the quasi-degenerate HOMO–1 → LUMO and HOMO–2 → LUMO excitations, while their interaction gives rise to a symmetry-broken zwitterionic S 1 state. After optical excitation of tF2356, one observes an ultrafast (∼0.06 ps) evolution that reflects relaxation from initial nonpolar S 3 FC to long-lived (1.3 ns in n -hexane and 3.4 ns in acetonitrile) polar S 1 . The polarity of S 1 is evidenced by a solvatochromic shift of its fluorescence band. The experimental results provide a sensitive test for quantum-chemical calculations. In particular, our calculations agree with the experiment, and raise concerns about the applicability of the common TDDFT approach to relatively simple stilbenic systems.