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

Abstract

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.

Triplet Photosensitizers and Their Applications in Triplet–Triplet Annihilation Upconversion

“…Furthermore, compound 53 exhibited a dark S 1 state, which is rare for a Bodipy chromophore. In another example, a diazabicyclo[5.4.0]undec-7-ene-fused NDI compound showed low ISC ability, although the geometry was significantly twisted . Thus, it is concluded that the ISC capability of twisted/fused molecules does not simply rely on the torsion of π-conjugated systems.…”

Driving Triplet State Population in Benzothioxanthene Imide Dyes: Let's twist!

“…[20] Sometimes called twisting-induced triplet state population in the literature, [21] this elegant strategy is currently attracting significant attention, particularly for preparing the next generation of heavy-atomfree photosensitizers. [22]…”

Enhanced Intersystem Crossing in a Thiohelicene

“…[22][23][24] However, whereas in some cases a strong increase in the efficiency of ISC by twisting has been observed, other twisted molecules showed just the opposite effect, and it was pointed out that other factors also play an important role. [25][26][27][28] Such a well-known factor is the effect of so-called heavy atoms, [29,30] i. e., atoms with a large nuclear charge. As the velocities of the inner electrons approach a substantial fraction of the velocity of light, relativistic effects and, thus, SOC becomes large.…”