Ultrafast Intersystem Crossing in Selenium-Annulated Perylene Bisimide

An intersystem crossing (ISC), the non-radiative transition between two electronic states with different spin multiplicities, is ubiquitous and imperative in molecular photochemistry. The manifestation of a triplet manifold in conjugated chromophoric materials has a crucial role in pi-enhancing the efficiency of photofunctional devices. Herein, we explore the triplet-state population in a series of chalcogen-annulated perylene bisimides (O-PBI, S-PBI, and Se-PBI), where the selenium-annulated PBI (Se-PBI) exhibits a near-quantitative triplet quantum yield (empty set(T) = 94 +/- 1%). Annulation of Se in the PBI core results in a drastic decrease in the fluorescence quantum yield (empty set(f) = 1.5 +/- 0.2%) compared to the bare PBI (empty set(f) = 97.0 +/- 1%), indicating the possibility of an efficient non-radiative decay pathway in the Se-PBI motif. Femtosecond and nanosecond transient absorption measurements unambiguously confirmed the ultrafast triplet population in Se-PBI with an ISC rate constant of k(ISC) = 2.39 x 10(10) s(-1) and the triplet-state decay to the ground state with a time constant of 3.78 mu s. A theoretically calculated spin-orbit coupling constant (V-SOC) of 122.4 cm(-1) employing the SA-CASSCF/NEVPT2 method has rationalized the excited-state dynamics of Se-PBI. By virtue of the poor SOC between the singlet and triplet states, we observed a partial triplet population in S-PBI, whereas ISC is negligible in O-PBI. We demonstrate an increase in the spin-orbit coupling constant(V-SOC(O-PBI) << V-SOC(S-PBI) < V-SOC(Se-PBI)) and rate constant of ISC (k(ISC)(O-PBI) << k(ISC)(Sc-PBI) < k(ISC)(Se-PBI)) across the series of chalcogen-annulated PBIs (O-PBI, S-PBI, and Se-PBI). The heavier chalcogenide PBI (Se-PBI) thus adds to the array of potential organic photoactive materials for the design of efficient solar energy conversion devices.