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

Journal of the American Chemical Society

4 years ago

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

Michael T. Quick, Ilya N. Ioffe, Sergey A. Kovalenko, Alexander L. Dobryakov, Falko Berndt, Alex A. Granovsky, Celin Richter, Rainer Mahrwald, Martin Quick, Nikolaus P. Ernsting

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 S₁^FC and S₂^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 S₁ state. After optical excitation of tF2356, one observes an ultrafast (∼0.06 ps) evolution that reflects relaxation from initial nonpolar S₃^FC to long-lived (1.3 ns in n-hexane and 3.4 ns in acetonitrile) polar S₁. The polarity of S₁ 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.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b09611

DOI: 10.1021/jacs.7b09611