Ultrafast nonradiative decay from higher-lying excited states in azulene-dimers

The anti-Kasha fluorescence nature of azulene has been explained by the large energy gap between the second excited state and the lowest excited singlet state. While such property is expected to be perturbed in the presence of electronic interchromophore interactions without a change in the S_n–S₁ energy gap, the study of the excited-state dynamics of azulene assemblies or oligomers remains scarce. In this study, we designed dimers with diverse rigid bridge structures with controlled intermolecular interactions. Based on the ultrafast spectroscopy, we reveal that nonradiative deactivation from S₂ states was markedly accelerated upon conjugation, indicating that anti-Kasha behavior is highly sensitive to interchromophoric coupling. The results strongly suggest that careful optimization of interchromophore interactions is crucial for utilizing anti-Kasha-type molecules for energy or charge transport.