Does Twisted π-Conjugation Framework Always Induce Efficient Intersystem Crossing? A Case Study with Benzo[<i>b</i>]- and [<i>a</i>]Phenanthrene-Fused BODIPY Derivatives and Identification of a Dark State
Yuxin Yan, А. А. Суханов, Manon H. E. Bousquet, Qing‐Lin Guan, Jianzhang Zhao, Violeta K. Voronkova, Daniel Escudero, Antonio Barbon, Yong‐Heng Xing, Gagik G. Gurzadyan, Denis Jacquemin
Abstract
The photophysical properties, especially the intersystem crossing (ISC) of two heavy-atom-free BODIPY derivatives with twisted π-conjugated frameworks (benzo[b]-fused BODIPY, BDP-B; and [a]phenanthrene-fused BODIPY, BDP-P), 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, BDP-B has a planar π-conjugation framework, but it displays weaker UV–vis absorption (ε = 3.8 × 104 M–1 cm–1 at 569 nm) and fluorescence (ΦF < 0.1%), a short-lived singlet-excited state (fluorescence lifetime, τF = 0.2 ns), and a long-lived triplet state (τT = 132.3 μs). In comparison, the more twisted BDP-P shows stronger UV–vis absorption (ε = 9.8 × 104 M–1 cm–1 at 640 nm) and fluorescence (ΦF = 70%), longer singlet-excited-state lifetime (τF = 6.4 ns), and shorter triplet-state lifetime (τT = 18.9 μs). In contrast to helicenes (ΦT = ca. 90%), the ISC of BDP-P and BDP-B is nonefficient (ΦT < 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 BDP-B and BDP-P, respectively. The spatially confined T1 state wave function of the twisted molecule keeps the T1 state energy high (1.44–1.61 eV). A dark S1 state was identified for BDP-B. 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.