Photoexcited state dynamics and singlet fission in carotenoids

We describe our dynamical simulations of the excited states of the carotenoid, neurosporene, following its photoexcitation into the 'bright' (nominally $1^1B_u^+$) state. We employ the adaptive tDMRG method on the UV model of $\pi$-conjugated electrons and use the Ehrenfest equations of motion to simulate the coupled nuclei dynamics. To account for the experimental and theoretical uncertainty in the relative energetic ordering of the nominal $1^1B_u^+$ and $2^1A_g^-$ 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. We make a direct connection from our predictions to experimental observables by calculating the transient absorption. For the case of direct $1^1B_u^+$ to $2^1A_g^-$ 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 $2^1A_g^-$ component of $S_1$. Moreover, we show that it is the charge-transfer exciton component of the $2^1A_g^-$ state that is responsible for this transition, and not its triplet-pair component. We next discuss the microscopic mechanism of 'bright' to 'dark' state internal conversion, emphasising that this occurs via the exciton components of both states. Finally, we describe a mechanism whereby the strongly bound intrachain triplet-pairs of the 'dark' state may undergo interchain exothermic dissociation. We predict that this is only possible if the molecules are twisted in their ground states. The computational methodology underlying the calculations described here is explained in our companion paper, $\textit{Dynamical simulations of carotenoid photoexcited states using density matrix renormalization group techniques}$, D. Manawadu, D. J. Valentine, and W. Barford, $\textit{J. Chem. Theo. Comp.}$ (2023).