Figure 5.
Triplet-pair model for low-temperature TSPS-PDT films. (a) Dynamical model showing triplet-pair formation (SF), diffusion, and deactivation via singlet-channel and triplet-channel annihilation (SCA and TCA), T1 deactivation through intersystem crossing (ISC), and 1TT ⇌ 5TT equilibration (energy gaps are not to scale). Due to a large antiferromagnetic JTT,105TT forms in low yields. (b) 1TT dissociation yields either TX + TX, TY + TY, or TZ + TZ. X, Y, and Z correspond to TX, TY, and TZ. RF absorption drives one partner into a different sublevel, preventing reassociation to 1TT. (c) An example pathway in which geminate triplet pairs produce a PADMR signal. Molecular stacks are extended along the crystal’s a-axis. 1TT formation is followed by a hop. Spatial separation electronically decouples the triplets (JTT → 0), but they remain spin-correlated: here, as TZ + TZ. Decoupled triplets can absorb RF photons: here, RF with frequency DT – ET drives a TZ → TY transition. With reassociation, strong electronic coupling returns; the new pair configuration has net-triplet or net-quintet character. Both 3TT and 5TT may dissociate again, but 3TT can also deactivate via TCA.