Singlet fission is a process in which a singlet excites state converts into two triplet excitons. We have investigated this phenomenon between acenes in amorphous thin films, in aqueous nanoparticles and in crystallites. To further unravel the mechanism for this phenomenon, we studied two covalently linked 5-ethynyl-tetracene (ET) dimers with differing degrees of inter-tetracene overlap: BET-X, with large, cofacial overlap of tetracene π-orbitals, and BET-B, with twisted arrangement between tetracenes exhibit less overlap between the tetracene π-orbitals. The two compounds were crystallographically characterized and studied by femtosecond transient absorption and emission spectroscopy in solution, in PMMA and neat thin films. The results show that singlet fission occurs within 1 ps in an amorphous thin film of BET-B with high efficiency (triplet yield: 154%). In liquid solution and in a PMMA matrix the S1 of BET-B relaxes to a correlated triplet pair 1(T1T1) on a time scale of 2 ps. This state has a unique spectroscopic signature, and its decays to the ground state without forming separated triplets. Subsequent triplet energy transfer from the dimer 1(T1T1) to a nearby chromophore is an essential part of the singlet fission process to produce free triplets. Selective excitation of BET-B doped into a thin film of diphenyltetracene (DPT) demonstrates this step. We can follow the formation first of the 1(T1T1) state of BET-B, followed by generation of both DPT and BET-B triplets. For the structurally cofacial BET-X, an intermediate characterized by a similar transient absorption signature seen for BET-B forms in &lt; 180 fs but it returns to the ground state much more rapidly than BET-B. First-principles calculations are used to support the assigned kinetics and electronic structure.

Singlet Fission in isolated molecular dimers and in amorphous thin films

252nd National Meeting (2016)

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