The branching ratio of the excited-state population at the conical intersection between the S1 and S0 energy surfaces (ΦCI) of a protonated Schiff base of all-trans retinal in protic and aprotic solvents was studied by multipulse ultrafast transient absorption spectroscopy. In particular, pump–dump–probe experiments allowed to isolate the S1 reactive state and to measure the photoisomerization time constant with unprecedented precision. Starting from these results, we demonstrate that the polarity of the solvent is the key factor influencing the ΦCI and the photoisomerization yield.
