# Exciton-Exciton Annihilation Is Coherently Suppressed in H-Aggregates, but Not in J-Aggregates

## TL;DR

This paper theoretically shows that exciton-exciton annihilation rates are coherently suppressed in H-aggregates due to destructive interference, unlike in J-aggregates, impacting device performance at high excitation densities.

## Contribution

It reveals how the sign of dipole-dipole couplings determines annihilation suppression, providing a fundamental principle explaining experimental observations.

## Key findings

- H-aggregates exhibit a ~3 times lower annihilation rate than J-aggregates at room temperature.
- Low-temperature aggregates show up to an order of magnitude difference in annihilation rates.
- Destructive interference in H-aggregates causes coherent suppression of exciton-exciton annihilation.

## Abstract

We theoretically demonstrate a strong dependence of the annihilation rate between (singlet) excitons on the sign of dipole-dipole couplings between molecules. For molecular H-aggregates, where this sign is positive, the phase relation of the delocalized two-exciton wavefunctions causes a destructive interference in the annihilation probability. For J-aggregates, where this sign is negative, the interference is constructive instead, as a result of which no such coherent suppression of the annihilation rate occurs. As a consequence, room temperature annihilation rates of typical H- and J-aggregates differ by a factor of ~3, while an order of magnitude difference is found for low-temperature aggregates with a low degree of disorder. These findings, which explain experimental observations, reveal a fundamental principle underlying exciton-exciton annihilation, with major implications for technological devices and experimental studies involving high excitation densities.