Electron Spin Dynamics of the Photoexcited Triplet States of Bromoperylenebisimide Compounds: Computation and Analysis of the Electron Spin Polarization Evolution in TREPR Spectra

The dynamics of the photogenerated triplet electron spin polarization (ESP) of two perylenebisimide (PBI) derivatives, PBI-2Br and PBI-Br-NH, is studied by the time-resolved electron paramagnetic resonance (TREPR). The ESP inversion in the triplet TREPR spectra of two compounds is detected at delays after the laser pulse (DAF) of ~ 500–900 ns. A fairly efficient algorithm for the exact numerical solution of the equation of motion of the spin density matrix of an ensemble of photoexcited triplets taking into account the anisotropy of the decay rates of sublevels of the triplet state, spin–lattice and spin–spin relaxations is elaborated in order to analyze the ESP inversion of TREPR spectra. The calculated time evolution of the TREPR spectra and its fitting experimental ones shows that even a slight difference in decay rates for the triplet sublevels of PBI-2Br results in the ESP inversion in the TREPR spectra already at DAF of ~ 500 ns. At the same time, decay rates ( τ_X⁻¹ = 4 µs⁻¹; τ_Y⁻¹ = 4.4 µs⁻¹; τ_Z⁻¹ = 3.3 µs⁻¹) are higher than those for the available examples of the ESP inversion of the triplet TREPR spectra. The replacement of a Br atom by NH leads to noticeable changes in the photoinduced properties of the triplet including decay rates (τ_X⁻¹ = 1.25 µs⁻¹; τ_Y⁻¹ = 0.125 µs⁻¹; τ_Z⁻¹ = 0.125 µs⁻¹). PBI-Br-NH is characterized by an axial decay rate anisotropy with τ_x⁻¹, which is ten times higher than τ_Y⁻¹ and τ_Z⁻¹. The trend in the triplet lifetime change obtained from the TREPR spectra is consistent with the data of the nanosecond transient absorption spectra.