Nature and energetics of low-lying excited singlets/triplets and intersystem crossing rates in selone analogs of perylenediimide: A theoretical perspective

The structural rigidity and chemical diversity of the highly fluorescent perylenediimide (PDI) provide wide opportunities for developing triplet photosensitizers with sufficiently increased energy efficiency. Remarkably high intersystem crossing (ISC) rates with a complete fluorescence turn-off reported recently for several thione analogs of PDI due to substantially large spin-orbit coupling garners huge attention to develop other potential analogs. Here, several selone analogs of PDI, denoted as mSe-PDIs (m = 1-4) with varied Se content and positions, are investigated to provide a comprehensive and comparative picture down the group-16 using density functional theory (DFT) and time-dependent DFT implementing optimally tuned range-separated hybrid in toluene dielectric. All mSe-PDIs are confirmed to be dynamically stable and also thermodynamically feasible to synthesize from their oxygen and thione congeners. The first excited-state singlet (S1) of mSe-PDI with relatively low Se-content (m = 1, 2) is of n pi* character with an expected fluorescence turn-off. Whereas, the pi pi* nature of the S1 for 3Se-PDI and 4Se-PDI suggests a possible fluorescence turn-on in the absence of any other active nonradiative deactivation pathways. However, similar to 4-6 orders greater ISC rates (similar to 10(12)-10(14 )s(-1)) than the fluorescence ones (similar to 10(8) s(-1)) for all mSe-PDIs signify highly efficient triplet harvest. Importantly, significantly higher ISC rates for these mSe-PDIs than their thione congeners render them efficient triplet photosensitizers.