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 1¹B_u⁺) state. To account for the experimental and theoretical uncertainty in the relative energetic ordering of the nominal 1¹B_u⁺ and 2¹A_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 1¹B_u⁺ to 2¹A_g^- states occurs via the dark, intermediate 1¹B_u^- state. In this case, there is a cross over of the 1¹B_u⁺ and 1¹B_u^- diabatic energies within 5 fs and an associated avoided crossing of the S₂ and S₃ adiabatic energies. After the adiabatic evolution of the S₂ state from predominately 1¹B_u⁺ character to predominately 1¹B_u^- character, there is a slower nonadiabatic transition from S₂ to S₁, accompanied by an increase in the population of the 2¹A_g^- state. For the other parameter set, the 2¹A_g^- energy lies higher than the 1¹B_u⁺ energy at the Franck-Condon point. In this case, there is cross over of the 2¹A_g^- and 1¹B_u⁺ energies and an avoided crossing of the S₁ and S₂ energies, as the S₁ state evolves adiabatically from being of 1¹B_u⁺ character to 2¹A_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 1¹B_u⁺ to 2¹A_g^- internal conversion, we show that the dominant transition at ca. 2 eV, being close to but lower in energy than the T₁ to T₁^* transition, can be attributed to the 2¹A_g^- component of S₁. Moreover, we show that it is the charge-transfer exciton component of the 2¹A_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.