Fast intersystem crossing in the tetracene dimers as a source of "dark" state

Tetracene (Tc) is a prototype material undergoing singlet fission (SF), the formation of a pair of triplet excitons from a singlet exciton. The tetracene dimer Tc-2 is supposed to be a structural unit providing SF behavior. This work is devoted to the study of the mechanism of singlet exciton decay in van der Waals dimers of Tc-2. A nanosecond pump-probe approach is used, tuning both pumping and probing wavelengths. It is shown that the photoexcitation of both the Tc monomer and dimer gives rise to a triplet Tc(T-1) with very similar photoionization spectra, indicating an intersystem crossing (ISC) as the source of Tc(T-1) in both cases. This finding, together with the very short lifetime of the singlet exciton in van der Waals Tc-2 dimers as reported earlier in the literature, indicates that the ISC process is much faster in the dimer than in bare Tc. The factors that increase the rate of ISC in donor-acceptor complexes are the low-lying charge-transfer state in the Tc-2 dimer and the proximity in the energy between the singlet S-1 and triplet T-2 states in tetracene. This fast ISC process is assigned to the temperature-independent process reported earlier in the literature, leading to a "dark " state in tetracene. The results obtained indicate that the dimer of tetracene can be considered to be a structural unit responsible for both fast ISC and SF processes.