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 (ΦF = 1.0%) as compared with NDI-NH-Br (ΦF = 24% in toluene). However,
the ISC of NDI-DBU is poor, and the singlet oxygen quantum
yield was determined as ΦΔ = 9% versus ΦΔ = 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 (τT = 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.