Wavelength-dependent intramolecular singlet fission through an excimer-like intermediate

Area of Science:

• * Photochemistry and Photophysics
• * Materials Science
• * Quantum Information Science

Background:

• * Singlet fission (SF) converts one high-energy singlet exciton into two lower-energy triplet excitons.
• * SF is a promising mechanism for high-efficiency photovoltaics and optically initialized qubits.
• * Understanding the electronic-nuclear dynamics of SF is crucial for designing efficient SF chromophores.

Purpose of the Study:

• * Investigate the intramolecular SF (iSF) pathway in contorted naphthalenediimide dimers.
• * Elucidate the electronic-nuclear motions and kinetics accompanying iSF.
• * Guide the synthetic design of SF chromophores to optimize triplet yield.

Main Methods:

• * Polarization-controlled white-light two-dimensional (2D) and pump-probe spectroscopies.
• * Analysis of 2D cross-peaks to identify Davydov components.
• * Polarization anisotropy measurements to track electronic motion.

Main Results:

• * Excitation of either Davydov component leads to a rapid intermediate state.
• * Intermediate relaxation to the TT1 state shows kinetics dependent on the excited component.
• * Vibronic coherence and quantum beats observed in TT1 formation, indicating displaced nuclear coordinates.
• * Minimal electronic reorientation during TT1 formation suggests persistent singlet-triplet mixing.

Conclusions:

• * The observed dynamics in contorted naphthalenediimide dimers provide mechanistic insights into iSF.
• * Persistent singlet-triplet mixing may limit the formation of long-lived high-spin triplets.
• * Synthetic strategies should focus on minimizing this electronic mixing for improved SF efficiency.