Molecular-Scale Design of Azulene-Based Triplet Photosensitizers: Insights from Time-Dependent Optimally Tuned Range-Separated Hybrid.

Metal-free triplet photosensitizers are ubiquitous in photocatalysis, photodynamic therapy, photovoltaics, and so forth. Their photosensitization efficiency strongly depends on the ability of the low-lying excited spin-triplet to be populated through intersystem crossing. Small singlet-triplet gaps and considerable spin-orbit coupling between the excited spin-singlet and spin-triplet facilitate efficient intersystem crossing. Azulene (Az), a classic example of Anti-Kasha's blue emitter with considerable fluorescence quantum yield, holds great promise because of its chemical stability, rich electronic properties, and high structural rigidity. Here, we provide computationally modeled Az-derived photosensitizers, namely, Az-CHO and Az-CHS, implementing polarization consistent time-dependent optimally tuned range-separated hybrid. Calculations reveal energetic reordering of low-lying ππ* and π* singlet states between Az-CHO and Az-CHS and, thereby, rendering the latter to a nonfluorescent one. Importantly, a small singlet-triplet gap and large spin-orbit coupling for Az-CHX with X = O and S produce remarkably high intersystem crossing rates. Furthermore, strong nonadiabatic coupling between the (π*) and (ππ*) in Az-CHS due to substantially smaller energy gap causes enhanced population via fast internal conversion. These research findings provide new insights into the development of functional Az and or related heavy-atom-free small organic molecule-based triplet photosensitizers.