Probing the potential for anti-Kasha behavior through direct measurements of S2 lifetimes in xanthione and thioxanthone

Several examples of “anti-Kasha” emission from higher excited electronic states (Sn, n ≥ 2) have been reported recently for a variety of aromatic organic molecules. Even a modest suppression of the fast Sn → S1 internal conversion expected from the energy gap law, which underlies typical Kasha behavior, would have many advantages in photochemistry and in the development of photonic materials. Thus, it seems important to reassess the extent and reliability of the energy gap law. Here, we apply ultrafast spectroscopy and quantum chemical calculations to measure and interpret the S2 relaxation times of a pair of structurally similar molecules, thioxanthone and xanthione. Our measurements broadly agree with earlier literature data, and the calculations suggest very different S2 relaxation dynamics in these chromophores. The data are placed in the context of the energy gap law, which is found to work well for these and other planar aromatics regardless of the ππ* or nπ* character of the excited states involved. More flexible molecules placed on the energy gap law plot show enhanced rates of internal conversion. This reaffirmation of the broad applicability of the energy gap law suggests that where a molecule exhibits unexpectedly large “anti-Kasha” emission that result must be discussed in terms of particular features of vibronic coupling in that molecule.