Figure 1: Singlet exciton fission in polyacenes.
From: The entangled triplet pair state in acene and heteroacene materials
(a) Molecules investigated in this study. (b) Schematic potential energy surface denoting key photophysical processes (dashed arrows) in singlet fission materials. Photoexcitation (Exc.) is followed by singlet (black) relaxation along inter- and intramolecular coordinates, resulting in the observed Stokes shift (PLS1). Further relaxation into the bound, spin-coherent triplet pair state 1(TT) (red) constitutes singlet exciton fission (SEF). Three detectable decay processes are possible from 1(TT). (i) Thermally activated dissociation into free triplets (purple), which is aided by disorder/grain boundaries and exhibits a typical activation energy of 20–40 meV. (ii) Direct ‘delayed’ emission from 1(TT) (PLTT) through Herzberg–Teller intensity borrowing. As a consequence of this mechanism, the 0-0 transition is suppressed. (iii) Thermally activated back-transfer into the singlet manifold. This process results in delayed fluorescence (PLS1), and is suppressed in tetracene and F2-TES ADT at ≤ 200 K, indicating the presence of a slight energy barrier. (c) Temperature-dependent absorption and PL spectra of F2-TES ADT films.
