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 1 1 B u + ) state. To account for the experimental and theoretical uncertainty in the relative energetic ordering of the nominal 1 1 B u + and 2 1 A g - 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 g " family of states. For one parameter set, internal conversion from the 1 1 B u + to 2 1 A g - states occurs via the dark, intermediate 1 1 B u - state. In this case, there is a cross over of the 1 1 B u + and 1 1 B u - diabatic energies within 5 fs and an associated avoided crossing of the S 2 and S 3 adiabatic energies. After the adiabatic evolution of the S 2 state from predominately 1 1 B u + character to predominately 1 1 B u - character, there is a slower nonadiabatic transition from S 2 to S 1 , accompanied by an increase in the population of the 2 1 A g - state. For the other parameter set, the 2 1 A g - energy lies higher than the 1 1 B u + energy at the Franck-Condon point. In this case, there is cross over of the 2 1 A g - and 1 1 B u + energies and an avoided crossing of the S 1 and S 2 energies, as the S 1 state evolves adiabatically from being of 1 1 B u + character to 2 1 A g - 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 1 1 B u + to 2 1 A g - internal conversion, we show that the dominant transition at ca. 2 eV, being close to but lower in energy than the T 1 to T 1 * transition, can be attributed to the 2 1 A g - component of S 1 . Moreover, we show that it is the charge-transfer exciton component of the 2 1 A g - 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.