Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene Dimer

Abstract

Singlet fission is a process in which a singlet exciton converts into two triplet excitons. To investigate this phenomenon, we synthesized two covalently linked 5-ethynyl-tetracene (ET) dimers with differing degrees of intertetracene overlap: BET-X, with large, cofacial overlap of tetracene π-orbitals, and BET-B, with twisted arrangement between tetracenes exhibits less overlap between the tetracene π-orbitals. The two compounds were crystallographically characterized and studied by 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 solution and the PMMA matrix the S1 of BET-B relaxes to a correlated triplet pair (1)(T1T1) on a time scale of 2 ps, which decays to the ground state without forming separated triplets, suggesting that triplet energy transfer from (1)(T1T1) to a nearby chromophore is essential for producing free triplets. In support of this hypothesis, selective excitation of BET-B doped into a thin film of diphenyltetracene (DPT) leads to formation 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 forms in <180 fs and returns to the ground state more rapidly than BET-B. First-principles calculations predict a 2 orders of magnitude faster rate of singlet fission to the (1)(T1T1) state in BET-B relative to that of crystalline tetracene, attributing the rate increase to greater coupling between the S1 and (1)(T1T1) states and favorable energetics for formation of the separated triplets.

Triplet transport in thin films: fundamentals and applications

“…Current investigations into the intermediates in singlet fission, e.g. bound triplet pair excimers 99 or strongly interacting triplet pairs, 136 etc., enabled by various monomer or dimer configurations 97,[137][138][139][140][141][142] will shed light on the dielectric environment needed to stabilize triplet excitons. Such interdisciplinary approaches will reveal the fundamental structure-property relationships affecting triplet diffusion in thin films, and ultimately enable the engineering of optoelectronic devices making use of multi-excitonic processes.…”

Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers

“…Spectroscopic corroboration for two photoexcited pentacenes came from a 2-3 nm blue-shift not only relative to the singlet excited state bleaching but also relative to the triplet excited state bleaching seen in direct excitation and triplettriplet sensitization experiments, respectively. The product of the triplet-triplet annihilations is the singlet ground state of XC2 via radiationless relaxation, 70 and independent on concentration or laser power. But, a spectroscopically invisible excited state cannot be ruled out.…”

Synthesis and Exciton Dynamics of Donor-Orthogonal Acceptor Conjugated Polymers: Reducing the Singlet–Triplet Energy Gap

“…We consider that the smaller ΔEST here results in some dynamic equilibrium or mixing between 3 (π-π*) and emissive CT states, analogous to signatures of singlet-mixed triplet-pair states observed in certain small molecule systems. [25][26][27][28] Nonetheless, on the longest timescales eventual conformational relaxation in the 3 (π-π*) makes back-transfer to CT thermally inaccessible and emission is quenched. Similar results are obtained in the solid state, with only slight shifts in the characteristic PIA bands of each excited state.…”