Does Twisted π-Conjugation Framework Always Induce Efficient Intersystem Crossing? A Case Study with Benzo[]- and []Phenanthrene-Fused BODIPY Derivatives and Identification of a Dark State.

The photophysical properties, especially the intersystem crossing (ISC) of two heavy-atom-free BODIPY derivatives with twisted π-conjugated frameworks (benzo[]-fused BODIPY, ; and []phenanthrene-fused BODIPY, ), are studied with steady-state and time-resolved optical and electron paramagnetic resonance (TREPR) spectroscopic methods as well as with ADC(2) theoretical investigations. Interestingly, has a planar π-conjugation framework, but it displays UV-vis absorption (ε = 3.8 × 10 M cm at 569 nm) and fluorescence (Φ < 0.1%), a short-lived singlet-excited state (fluorescence lifetime, τ = 0.2 ns), and a long-lived triplet state (τ = 132.3 μs). In comparison, the more twisted shows UV-vis absorption (ε = 9.8 × 10 M cm at 640 nm) and fluorescence (Φ = 70%), singlet-excited-state lifetime (τ = 6.4 ns), and triplet-state lifetime (τ = 18.9 μs). In contrast to helicenes (Φ = ca. 90%), the ISC of and is nonefficient (Φ < 23%). The electron spin selectivity of the ISC of the derivatives is different, manifested by the phase pattern of the TREPR spectra as AAEAEE and EEEAAA for and , respectively. The spatially confined T state wave function of the twisted molecule keeps the T state energy high (1.44-1.61 eV). A dark S state was identified for . This work demonstrated that the twisted π-conjugated framework does not necessarily induce efficient ISC and we found a dark singlet state for BODIPY, which is rare.