Quantum chemical simulation of transient absorption spectroscopy in the CP29 light-harvesting complex

Transient absorption spectroscopy is among the most valuable methods for investigating the dynamics of excitons in light-harvesting complexes (LHCs). Even in the simplest LHCs, a physical model is needed to interpret transient spectra, as the number of excitation energy transfer (EET) processes occurring at the same time is too large to be disentangled from measurements alone. Physical EET models are commonly built by fittings of the microscopic exciton Hamiltonians and exciton-vibrational parameters, an approach that can lead to biases. Here we present a first-principles strategy to simulate EET and transient absorption spectra in LHCs, combining molecular dynamics and accurate multiscale quantum chemical calculations to obtain an independent estimate of the excitonic structure of the complex. The microscopic parameters thus obtained are then used in EET simulations to obtain the population dynamics and the related spectroscopic signature. We apply this approach to the CP29 minor antenna complex of plants, for which we follow the EET dynamics and transient spectra after excitation in the chlorophyll b region. Our calculations reproduce all the main features observed in the transient absorption spectra and provide independent insight on the excited-state dynamics of CP29. The approach presented here lays the groundwork for the unbiased interpretation of transient spectra in multichromophoric systems.