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

Abstract

The photophysical properties, especially the intersystem crossing (ISC) of two heavy-atom-free BODIPY derivatives with twisted π-conjugated frameworks (benzo[<i>b</i>]-fused BODIPY, <b>BDP-B</b>; and [<i>a</i>]phenanthrene-fused BODIPY, <b>BDP-P</b>), 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, <b>BDP-B</b> has a planar π-conjugation framework, but it displays <i>weaker</i> UV-vis absorption (ε = 3.8 × 10⁴ M^-1 cm^-1 at 569 nm) and fluorescence (Φ_F < 0.1%), a short-lived singlet-excited state (fluorescence lifetime, τ_F = 0.2 ns), and a long-lived triplet state (τ_T = 132.3 μs). In comparison, the more twisted <b>BDP-P</b> shows <i>stronger</i> UV-vis absorption (ε = 9.8 × 10⁴ M^-1 cm^-1 at 640 nm) and fluorescence (Φ_F = 70%), <i>longer</i> singlet-excited-state lifetime (τ_F = 6.4 ns), and <i>shorter</i> triplet-state lifetime (τ_T = 18.9 μs). In contrast to helicenes (Φ_T = ca. 90%), the ISC of <b>BDP-P</b> and <b>BDP-B</b> is nonefficient (Φ_T < 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 <b>BDP-B</b> and <b>BDP-P</b>, 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 <b>BDP-B</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.