Furthermore, the structural changes in the dibenzotropylium unit in the relaxed S1 state reflect the concomitant population of LUMO and depopulation of the HOMO (Fig. ) upon the excitation. On the other hand, only negligible changes in the bond lengths of the anilinium unit are observed in the S1 state when compared to the S0 state (Fig. ). Interestingly, the (photo)acidic proton is attached to the unit that is not involved in the electronic transition, i.e., the anilinium group, which opens up a series of questions as to which mechanism is driving the photoacidity in the excited state.