Using Structurally Well-Defined Norbornyl-Bridged Acene Dimers to Map a Mechanistic Landscape for Correlated Triplet Formation in Singlet Fission

TL;DR

This study investigates the mechanisms of singlet fission in well-defined norbornyl-bridged acene dimers using spectroscopic methods, revealing rapid triplet pair formation and the influence of molecular structure on efficiency.

Contribution

It provides detailed mechanistic insights into singlet fission in tetracene and pentacene dimers, highlighting the role of molecular structure and Marcus theory in triplet formation.

Findings

1TT forms rapidly within a few picoseconds.

Decay of 1TT to ground state occurs in about 100 ns.

Exchange interaction influences 1TT yield.

Abstract

Structurally well-defined TIPS-acetylene substituted tetracene (TIPS BT1') and pentacene (TIPS BP1') dimers utilizing a [2.2.1] bicyclic norbornyl bridge have been studied -primarily using time-resolved spectroscopic methods - to uncover mechanistic details about primary steps in singlet fission leading to formation of the biexcitonic 1TT state as well as decay pathways to the ground state. For TIPS BP1' in room temperature toluene, 1TT formation is rapid and complete, occurring in 4.4 ps. Decay to the ground state in 100 ns is the primary loss pathway for 1TT in this system. For TIPS BT1', the 1TT is also observed to form rapidly (with a time constant of 5 ps) but in this case it occurs in concert with establishment of an excited state equilibrium (K~1) with the singlet exciton state S1 at an energy of 2.3 eV above the ground state. The equilibrated states survive for 36 ns and are lost to ground state through both radiative and non-radiative pathways via the S1 and non-radiative pathways via the 1TT. The rapidity of 1TT formation in TIPS BT1' is at first glance surprising. However, our analysis suggests that the few-parameter rate constant expression of Marcus theory explains both individual and comparative findings in the set of systems, thus establishing benchmarks for diabatic coupling and reorganization energy needed for efficient 1TT formation. Finally, a comparison of TIPS BT1' with previous results obtained for a close constitutional isomer (TIPS-BT1) differing in the placement of TIPS-acetylene side groups suggests that the magnitude of exchange interaction in the correlated triplet manifold plays a critical role dictating 1TT yield in the tetracenic systems.