How Dispersion Interactions at the Excited State Can Tune Photochromism of Embedded Chromophores

We present QM/MMPol-cLR³, a polarizable embedding quantum mechanics/molecular mechanics (QM/MM) framework that includes explicit, state-specific dispersion terms. This method enables a rigorous treatment of dispersion on top of electrostatic and induction effects in ground- and excited-state calculations. Using QM/MMPol-cLR³, we show that dispersion interactions control excited-state solvatochromism through two distinct mechanisms. In azulene, opposite shifts of the L_a and L_b states arise from state-specific dispersion linked to changes in excited-state polarizability. In bacteriochlorophyll a, dispersion instead stems from the interplay between polarizability changes and transition-dipole-driven response, governing the Q_y and Q_x shifts. Finally, application to the LH2 complex reveals pigment-dependent dispersion shifts between the B800 and B850 rings, impacting the excitation-energy transfer. These results establish dispersion as an essential, nonempirical component for predictive excited-state simulations in complex environments.