Core-Twist Modulated Intersystem Crossing in a π-Fused Single-Molecule

Area of Science:

• Photochemistry
• Single-molecule spectroscopy
• Organic electronics

Background:

• Intersystem crossing (ISC) is a critical photophysical process influencing molecular photochemistry and optoelectronics.
• Understanding ISC mechanisms in complex organic molecules is essential for designing advanced materials.

Purpose of the Study:

• To provide direct, single-molecule evidence of intersystem crossing (ISC) in a core-twisted naphthalenemonoimide-fused perylenediimide (NP).
• To elucidate the mechanism of twist-induced ISC in polyaromatic hydrocarbons.

Main Methods:

• Single-molecule fluorescence measurements to observe quantum jumps.
• Ultrafast transient absorption spectroscopy to corroborate ISC.
• Theoretical investigations to explore excited state dynamics.

Main Results:

• Direct observation of frequent quantum jumps (short off states) indicative of ISC at the single-molecule level.
• Quantification of ISC rate constant (k_ISC = 6.5 × 10^7 s^-1) and triplet quantum yield (Φ_T = 25.8 ± 2.4%).
• Theoretical insights into the S1-T3 state mixing and relaxation pathways.

Conclusions:

• The study provides definitive evidence of ISC in NP at the single-molecule level.
• The findings offer critical insights into the twist-induced ISC mechanism in polyaromatic hydrocarbons.
• This work bridges single-molecule and ensemble-level understanding of photophysical processes.