Does Twisted Molecular Structure Always Induce Intersystem Crossing? A Case Study with Near-IR Absorbing...

Heavy atom-free organic chromophores showing absorption in the near-IR region with intersystem crossing (ISC) ability are important for applications in various fields, e.g., photocatalysis and photodynamic therapy. Herein, we studied the photophysical property of a naphthalenediimide (NDI) derivative, in which the NDI chromophore is fused with pentacyclic 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which shows a strong charge-transfer (CT) absorption band (S₀ → ¹CT transition) in the near-IR region of 600-740 nm. The effect of extended π-conjugation framework in NDI-DBU compared with the derivative of mono-amino substitution (NDI-NH-Br) was studied by steady-state and nanosecond transient absorption (ns-TA) spectra, electron paramagnetic resonance (EPR) spectroscopy, and theoretical computations. The fluorescence is almost completely quenched for NDI-DBU (Φ_F = 1.0%) as compared with NDI-NH-Br (Φ_F = 24% in toluene). However, the ISC of NDI-DBU is poor, and the singlet oxygen quantum yield was determined as Φ_Δ = 9% versus Φ_Δ = 57% for NDI-NH-Br, although the compound has significantly twisted molecular structure. The ns-TA spectral study showed a long-lived triplet excited state (τ_T = 132 μs) in NDI-DBU, with T₁ energy of 1.20-1.44 eV, and the ISC is via the S₂ → T₃ path, which is verified by theoretical calculations. This study displayed that the twisting of molecular geometry does not always assure efficient ISC.