  The electronic interaction between an optically active singlet state
($S_1S_0$) and a dark state of singlet multiplicity, known as correlated
triplet pair ($^1[TT]$), plays a crucial role in the effective transformation
from $S_1S_0$ to $^1[TT]$ during intramolecular singlet fission (iSF). This
process is understood through mechanisms such as direct exchange coupling and
incoherent processes that involve super-exchange coupling through
charge-transfer states. However, most insights into these mechanisms are
derived from theoretical studies due to the difficulties in obtaining
experimental evidence. In this study, we investigate the excited-state
interactions between $S_1S_0$ and $^1[TT]$ in spiro-conjugated iSF sensitizers
by employing transient two-dimensional electronic spectroscopy. This approach
allows us to focus on the early stages of the conversion from $S_1S_0$ to
$^1[TT]$. Upon optical excitation, a superposition of $S_1S_0$ and $^1[TT]$ is
created, which gradually transitions to favor $^1[TT]$ within the
characteristic time frames of iSF. The observed high-order signals indicate
circular repopulation dynamic that effectively reinitiates the iSF process from
higher energy electronic states. Our findings, supported by
semi-quantum-mechanical simulations of the experimental data, suggest the
presence of a direct iSF mechanism in the dimers, facilitated by weak
non-adiabatic coupling between $S_1S_0$ and $^1[TT]$. This experiment provides
new insights into the equilibrium between the two electronic states, a
phenomenon previously understood primarily through theoretical models.
