Spin-dependent recombination mechanisms for quintet bi-excitons generated through singlet fission

TL;DR

This study explores the spin-dependent recombination mechanisms of quintet bi-excitons generated via singlet fission, introducing models that align with experimental magnetic resonance data to elucidate the underlying processes.

Contribution

The paper presents two theoretical models for quintet spin-recombination and demonstrates that one model accurately reproduces experimental spectra, advancing understanding of spin dynamics in bi-excitons.

Findings

The spin-recombination behavior can be modeled to match experimental spectra.

The model inspired by triplet intersystem crossing fits the data well.

Quantitative analysis reveals dominant recombination pathways.

Abstract

We investigate the physical mechanisms for spin-dependent recombination of a strongly bound pair of triplet excitons generated by singlet fission and forming a spin quintet (total spin of two) bi-exciton. For triplet excitons the spin-dependent recombination pathways can involve intersystem crossing or triplet-triplet annihilation back to the singlet ground state. However the modeling of spin-dependent recombination for quintets is still an open question. Here we introduce two theoretical models and compare their predictions with the broadband optically detected magnetic resonance spectrum of a long lived quintet bi-exciton with known molecular structure. This spectrum measures the change in the fluorescence signal induced by microwave excitation of each of the ten possible spin transitions within the quintet manifold as function of the magnetic field. While most of the experimental features can be reproduced for both models, the behavior of some of the transitions is only consistent with the quintet spin-recombination model inspired by triplet intersystem crossing which can reproduce accurately the experimental two-dimensional spectrum with a small number of kinetic parameters. Thus quantitative analysis of the broadband optically detected magnetic resonance signal enables quantitative understanding of the dominant spin-recombination processes and estimation of the out-of equilibrium spin populations.