# Conformational switching modulates excited-state pathways in a cofacial perylene dimer

## TL;DR

This study shows how changing the shape of a perylene dimer can control its light-related reactions, offering a new way to design advanced materials for optoelectronics.

## Contribution

The paper introduces a non-covalent strategy for modulating excimer and multiexciton dynamics through conformational switching in a flexible PDI dimer.

## Key findings

- In chloroform, the dimer forms a slow, partial excimer due to structural inhomogeneity.
- In polar DMSO/water, the dimer forms a barrierless excimer within 200 femtoseconds.
- Vibrational coherences promote coherent excimer formation in the closed conformer.

## Abstract

Controlling excited-state pathways in supramolecular chromophore assemblies is key to designing next-generation optoelectronic and photonic materials. Here we elucidate the conformation-dependent photophysics of a flexible perylene diimide (PDI) dimer, valPDI2, which undergoes reversible solvent-driven switching between two distinct dimer geometries, within the same structure. Exciton-coupling calculations and ultrafast spectroscopy show that in chloroform the dimer adopts an open, weakly coupled geometry that supports slow, partial excimer formation due to structural inhomogeneity within the excited state potential. In contrast, in polar DMSO/water the dimer collapses into a cofacial stacked conformer that enables barrierless, sub-200 femtosecond excimer formation, a subset of which forms a multiexciton state over tens of picoseconds. Half-broadband 2D electronic spectroscopy reveals conformation-dependent vibrational coherences, with nuclear wavepacket motion along the π-stacking coordinate promoting vibrationally coherent excimer formation in the closed conformer. These findings demonstrate that environmentally driven conformational control offers a powerful non-covalent strategy to modulate excimer and multiexciton dynamics in PDI assemblies. More broadly, they establish supramolecular switching as a general design principle for tuning excited-state behaviour in flexible organic chromophore arrays, with implications for the development of responsive optoelectronic and energy-conversion materials.

Ultrafast coherent spectroscopies uncover conformation-dependent excimer dynamics in a switchable bis-perylene diimide macrocycle, behaviour that is key to modulating excited state pathways in optoelectronic materials.