Pyrazole and Triazole Fused Perylene Bisimides: Synthesis and Photophysical Properties

We studied the intersystem crossing (ISC) of neutral, monoanion, and dianion of a series of phenyl, pyrazole, and triazole fused perylene bisimides (PBI) derivatives. The neutral compounds exhibit high fluorescence quantum yields (Φ_F = 66.5%-91.9%) and low singlet oxygen quantum yields (Φ_Δ < 7.8%), with no triplet state signals detected via femtosecond transient absorption (fs-TA) or nanosecond transient absorption (ns-TA) spectral measurements. Time-resolved electron paramagnetic resonance (TREPR) revealed an e,e,e,a,a,a electron spin polarization (ESP) for the T₁ state of these PBIs. Calculations of spin-orbit coupling matrix elements (SOCMEs) and g-tensor anisotropy both confirm the weak spin-orbit coupling (SOC) of the molecules, accounting for their low ISC efficiency. Pulse electron paramagnetic resonance (pulse EPR) measurements of radical anions showed that PBI derivatives lacking pyrazole/triazole moieties have much longer transverse relaxation times (T_m = 2.5-2.6 µs) than the other compounds. Additionally, ns-TA spectra confirmed ISC in PBI dianions, which possess long-lived triplet states (213-558 µs). These species can be used as super strong reductants in photocatalytic redox organic reactions.