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We describe our dynamical simulations of the excited states of thecarotenoid, neurosporene, following its photoexcitation into the 'bright'(nominally $1^1B_u^+$) state. We employ the adaptive tDMRG method on the UVmodel of $\pi$-conjugated electrons and use the Ehrenfest equations of motionto simulate the coupled nuclei dynamics. To account for the experimental andtheoretical uncertainty in the relative energetic ordering of the nominal$1^1B_u^+$ and $2^1A_g^-$ states at the Franck-Condon point, we consider twoparameter sets. In both cases there is ultrafast internal conversion from the'bright' state to a 'dark' singlet triplet-pair state. We make a directconnection from our predictions to experimental observables by calculating thetransient absorption. For the case of direct $1^1B_u^+$ to $2^1A_g^-$ internalconversion, we show that the dominant transition at ca. 2 eV, being close tobut lower in energy than the $T_1$ to $T_1^*$ transition, can be attributed tothe $2^1A_g^-$ component of $S_1$. Moreover, we show that it is thecharge-transfer exciton component of the $2^1A_g^-$ state that is responsiblefor this transition, and not its triplet-pair component. We next discuss themicroscopic 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 intrachaintriplet-pairs of the 'dark' state may undergo interchain exothermicdissociation. We predict that this is only possible if the molecules aretwisted in their ground states. The computational methodology underlying thecalculations described here is explained in our companion paper,$\textit{Dynamical simulations of carotenoid photoexcited states using densitymatrix renormalization group techniques}$, D. Manawadu, D. J. Valentine, and W.Barford, $\textit{J. Chem. Theo. Comp.}$ (2023).

Photoexcited state dynamics and singlet fission in carotenoids