Understanding remarkably high triplet quantum yield in thione analogs of perylenediimide: A detailed theoretical study

The diverse and tunable electronic structures of perylenediimide (PDI), together with its high thermal and chemical stability, make the compound suitable for applications in bioimaging, electrical, and optical devices. However, a large singlet-triplet gap (ΔES-T) and almost zero spin-orbit coupling (SOC) between the lowest excited singlet (S1) and triplet (T1) restrict intersystem crossing (ISC) in highly fluorescent pristine PDI, yielding a near zero triplet quantum yield (ΦT). Interestingly, a thione analogs of PDI with varied S content (mS-PDIs, m = 1-4) was experimentally shown to yield ΦT ∼ 1.0 through efficient ISC. Time-dependent optimally-tuned range-separated hybrid calculations are performed to rationalize the experimentally observed red-shifted optical absorption and also the remarkably high ISC with almost zero radiative fluorescence reported for these mS-PDIs. To this end, the relative energies of low-lying excited singlets Sn (n = 1, 2) and a few triplets Tn(n = 1-3), along with their nature (nπ* or ππ*), are assessed for each of the mS-PDIs studied in chloroform. To our surprise and contrary to the earlier reports, both S1 and T1 are found to be of the same ππ* character, originating from the highest occupied to lowest unoccupied orbital transition, which, therefore, leads to a still large ΔES-T and vanishingly small SOC, as expected from the identical wavefunction symmetry. Increasing S content lowers S1(ππ*) due to a greater extent of π-delocalization, which well complements and supports the observed red-shift. More importantly, the T2 (or T3) closely lying to the S1 is of nπ* and, therefore, produces a relatively smaller ΔES-T and larger SOC. Detailed kinetics analysis suggests S1(ππ*) → T2(nπ*) is the primary ISC channel for all mS-PDIs, which is responsible for the remarkably high ΦT observed. In addition, comparable SOC and ΔES-T produce similar ISC rates for all mS-PDIs.