Pyrazole and Triazole Fused Perylene Bisimides: Synthesis and Photophysical Properties

Area of Science:

• Photochemistry
• Organic Electronics
• Spectroscopy

Background:

• Perylene bisimides (PBIs) are versatile organic dyes with tunable photophysical properties.
• Intersystem crossing (ISC) is crucial for understanding photophysical pathways and applications like organic photovoltaics and photodynamic therapy.
• The influence of fused heterocyclic moieties on ISC in PBI derivatives remains an active area of research.

Purpose of the Study:

• To investigate the intersystem crossing (ISC) efficiency in neutral, monoanion, and dianion states of phenyl, pyrazole, and triazole fused PBI derivatives.
• To elucidate the factors governing ISC, including spin-orbit coupling (SOC).
• To explore the potential applications of PBI dianions as photocatalysts.

Main Methods:

• Femtosecond and nanosecond transient absorption (fs-TA and ns-TA) spectroscopy.
• Time-resolved electron paramagnetic resonance (TREPR) and pulse electron paramagnetic resonance (pulse EPR).
• Computational calculations of spin-orbit coupling matrix elements (SOCMEs) and g-tensor anisotropy.

Main Results:

• Neutral PBIs exhibit high fluorescence quantum yields and low singlet oxygen generation, with no detectable triplet states via TA spectroscopy.
• TREPR revealed specific electron spin polarization (ESP) patterns for the triplet state (T1) of PBIs.
• Calculations confirmed weak SOC in neutral PBIs, explaining their low ISC efficiency.
• PBI dianions display efficient ISC and possess long-lived triplet states (213–558 µs).

Conclusions:

• Fused pyrazole/triazole moieties significantly impact the photophysical properties of PBIs, particularly ISC efficiency.
• PBI dianions with long-lived triplet states are promising candidates for super-strong reductants in photocatalytic redox reactions.
• The study provides fundamental insights into the photophysics of PBI derivatives and their potential in advanced applications.