Excited-state polarizability in crystalline sexithiophene: Charge-transfer and vibronic effects

► Non-Born-Oppenheimer electric field effect in excited sexithiophene crystal. ► Charge transfer strongly enhances Frenkel exciton response to electric field. ► Field-induced shift of 0-0 line of the lowest Frenkel state is amplified seven times. ► For 0-1 replica the effect is even greater (amplification factor of 11). ► Experimental evidence found in electroabsorption spectra of sexithiophene crystal.