Modeling linear absorption spectra of solvated chromophores is highly challenging, as both coupling of the electronic states to nuclear vibrations and solute-solvent interactions have to be explicitly accounted for. A successful, albeit computationally expensive approach to model linear absorption spectra in the condensed phase is the cumulant approach, where the effective system-bath coupling of the electronic excitation to nuclear vibration is computed from the fluctuations of the excitation energy in equilibrium sampled along molecular dynamics trajectories. The approach has the advantage of explicitly including the coupling of the chromophore to its complex environment, but fails in systems that exhibit nonadiabatic couplings between electronic excited states. These couplings can have a significant effect on the absorption lineshapes in systems where several excited states become close in energy in the Condon region.  A way to account for nonadiabatic effects is through the linear vibronic coupling (LVC) Hamiltonian, where the system is described through a set of diabatic electronic states that are explicitly coupled. Here, using the example of the Methylene blue (MB) molecule, we demonstrate that an LVC Hamiltonian can be parameterized directly from correlation functions of energy gap fluctuations, fully accounting for environmental effects on the coupling parameters. The linear absorption spectrum is then computed by calculating the time-evolution of the density matrix under the LVC Hamiltonian using a powerful, formally exact tensor network approach. We show that the absorption lineshape of MB is strongly influenced by nonadiabatic coupling between the bright S1 and the dark S2 state. The approach presented here is expected to be widely applicable to the simulation of linear spectra of chromophores in complex environments.

Including both nonadiabatic and solvent effects in linear absorption spectra

ACS Spring 2022

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