Does Twisted Molecular Structure Always Induce Intersystem Crossing? A Case Study with Near-IR Absorbing 1,8-Diazabicyclo[5.4.0]undec-7-ene-fused Naphthaldiimide

Heavy atom-free organic chromophores showing absorptionin thenear-IR region with intersystem crossing (ISC) ability are importantfor applications in various fields, e.g., photocatalysis and photodynamictherapy. Herein, we studied the photophysical property of a naphthalenediimide(NDI) derivative, in which the NDI chromophore is fused with pentacyclic1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU), which shows a strong charge-transfer(CT) absorption band (S-0 -> (CT)-C-1 transition)in the near-IR region of 600-740 nm. The effect of extended pi-conjugation framework in NDI-DBU compared withthe 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 theoreticalcomputations. The fluorescence is almost completely quenched for NDI-DBU (phi(F) = 1.0%) as compared with NDI-NH-Br (phi(F) = 24% in toluene). However,the ISC of NDI-DBU is poor, and the singlet oxygen quantumyield was determined as phi(Delta) = 9% versus phi(Delta) = 57% for NDI-NH-Br, although the compound has significantly twisted molecular structure. The ns-TA spectralstudy showed a long-lived triplet excited state (tau(T) = 132 mu s) in NDI-DBU, with T-1 energyof 1.20-1.44 eV, and the ISC is via the S-2 -> T-3 path, which is verified by theoretical calculations.This study displayed that the twisting of molecular geometry doesnot always assure efficient ISC.