Anti-Kasha emission in DCM-IFC: Computational evaluation of the Type III separated wavefunction hypothesis

Abstract

Kasha's rule, which states that the emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity, is central to the understanding of photochemistry, and the exception due to a large (S1-S2 ) energy gap, exemplified by azulene, is well understood. Over the last few years, examples of large flexible molecules with modest (S1-S2 ) energy gaps have been reported to show anti-Kasha (AK) emission, and have been rationalised on the basis of highly spatially separated wavefunctions of the two states. Recently, a fluorophore, having such characteristics, was constructed from dicyanomethylene-4H-pyran (DCM) and integrated fluorescein -chromene (IFC), where a spirolactane open/closed switch was shown to regulate AK behaviour. The open form of the molecule illustrated dual emission which was interpreted as emission from both S1 and S2 states. Here, we investigate theoretically both the FC region and the interpolated pathways towards S2/S1 -intersection region using implicit solvation. We calculate excited state energies and oscillator strengths employing both time-dependent density functional theory (with a range of functionals) and high-level wavefunction theories, characterising that the S1 state is the brighter of the valence excited states, dominated by locally excited character, whereas the S2 state is of charge-transfer character and is darker in comparison. Our ab-initio calculations indicate that the spectral absorption profiles for the two states overlap, and that there are barrierless internal conversion pathways from the FC region on the S2 surface towards the S2/S1-intersection region, which is consistent with efficient and rapid S2 population decay in this molecule making AK emission less likely. Moreover, energy difference between S1 and S2 states at each of the S1 and S2 excited state minima is unlikely to favour thermal equilibration of population at timescales relevant to emission. Taken together, the present calculations suggest that the experimentally observed anomalous emission requires an alternative interpretation.