  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).
