Photoexcited State Dynamics and Singlet Fission in Carotenoids.

We describe our simulations of the excited state dynamics of the carotenoid neurosporene, following its photoexcitation into the "bright" (nominally 1B) state. To account for the experimental and theoretical uncertainty in the relative energetic ordering of the nominal 1B and 2A states at the Franck-Condon point, we consider two parameter sets. In both cases, there is ultrafast internal conversion from the "bright" state to a "dark" singlet triplet-pair state, i.e., to one member of the "2A" family of states. For one parameter set, internal conversion from the 1B to 2A states occurs via the dark, intermediate 1B state. In this case, there is a cross over of the 1B and 1B diabatic energies within 5 fs and an associated avoided crossing of the S and S adiabatic energies. After the adiabatic evolution of the S state from predominately 1B character to predominately 1B character, there is a slower nonadiabatic transition from S to S, accompanied by an increase in the population of the 2A state. For the other parameter set, the 2A energy lies higher than the 1B energy at the Franck-Condon point. In this case, there is cross over of the 2A and 1B energies and an avoided crossing of the S and S energies, as the S state evolves adiabatically from being of 1B character to 2A character. We make a direct connection from our predictions to experimental observables by calculating the time-resolved excited state absorption. For the case of direct 1B to 2A internal conversion, we show that the dominant transition at ca. 2 eV, being close to but lower in energy than the T to T transition, can be attributed to the 2A component of S. Moreover, we show that it is the charge-transfer exciton component of the 2A state that is responsible for this transition (to a higher-lying exciton state), and not its triplet-pair component. These simulations are performed using the adaptive tDMRG method on the extended Hubbard model of π-conjugated electrons. The Ehrenfest equations of motion are used to simulate the coupled nuclei dynamics. We next discuss the microscopic mechanism of "bright" to "dark" state internal conversion and emphasize that this occurs via the exciton components of both states. Finally, we describe a mechanism relying on torsional relaxation whereby the strongly bound intrachain triplet-pairs of the "dark" state may undergo interchain exothermic dissociation.