Near‐IR‐Absorbing Bis‐Donor Functionalized Aza‐BODIPY Derivatives: Synthesis and Photophysical Study by Using Transient Optical Spectroscopy

A series of near-IR absorbing 2,6-diarylated BF₂-chelated aza-boron-dipyrromethenes (aza-BDPs) derivatives bearing different electron donors (benzene, naphthalene, phenanthrene, phenothiazine and carbazole) were designed and synthesized. The effect of different electron donor substitutions on the photophysical properties was studied by steady-state UV-vis absorption and fluorescence spectra, electrochemical, time-resolved nanosecond transient absorption (ns-TA) spectroscopy and theoretical computations. The UV-vis absorption spectra of AzaBDP-PTZ and AzaBDP-CAR (λ_abs=710 nm in toluene) showed a bathochromic absorption profile compared with the reference AzaBDP-Ph (λ_abs=685 nm in toluene), indicating the non-negligible electronic interaction at the ground state between donor and acceptor moieties. Moreover, the fluorescence is almost completely quenched for AzaBDP-PTZ/AzaBDP-CAR (fluorescence quantum yield, Φ_F=0.2-0.7 % in toluene) as compared with the AzaBDP-Ph (Φ_F=27 % in toluene). However, the apparent intersystem crossing ability of these compounds is poor, based on the singlet oxygen quantum yield (Φ_Δ=0.3-1.5 %). The ns-TA spectral study showed typical Bodipy localized triplet state transient features, short-lived excited triplet state for AzaBDP-Ph (τ_T=53.2 μs) versus significantly long-lived triplet state for AzaBDP-CAR (τ_T=114 μs) was observed under deaerated experimental conditions. These triplet state lifetimes are much longer than that obtained with diiodoAzaBDP (intramolecular heavy atom effect, τ_T=1.5~7.2 μs). These information are useful for molecular structure design of triplet photosensitizers, for which longer triplet state lifetimes are usually desired. Theoretical computations displayed that the triplet state is mainly localized on the AzaBDP core, moreover, it was found that the HOMO/LUMO energy gap decreased after introducing donor moieties to the skeleton as compared with the reference.