Stable High-Energy Excited States Observed in a Conjugated Molecule with Hindered Internal Conversion Processes

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When the excited states of a molecule above its lowest excited state (S n or T n, n > 1) have a sufficient lifetime, photophysical and photochemical processes may occur directly in these high-energy levels, bypassing the Kasha’s rule and significantly influencing the system properties. Investigation of the relationship between molecular structure and intramolecular electronic relaxation processes targets molecular design to achieve a long-lifetime higher excited states. Here, we report stable high-energy excited states with lifetimes approaching several nanoseconds in our newly designed and synthesized compound PPI–AnCN. Through experimental and theoretical investigation, we reveal that both large energy gap and poor electronic coupling between the high excited state and the S 1 state are responsible for this stability. More importantly, we find that the emission from the higher exited states can be promoted when the lowest excited state is quenched. This study provides new insights into understanding long-lifetime higher excited states and intramolecular electronic relaxation processes.