# Quenching mechanism in rotaxane mechanophores: insights from acene-based luminophores

## TL;DR

This study investigates how rotaxane-based mechanophores lose fluorescence, finding that electron transfer is the main cause.

## Contribution

The paper identifies photoinduced electron transfer as the primary quenching mechanism in acene-based rotaxane mechanophores.

## Key findings

- Quenching efficiency decreases with increasing π-conjugation in acene-based luminophores.
- Photoinduced electron transfer is the main quenching mechanism for anthracene-based systems.
- Pentacene-based mechanophores show minimal quenching due to low electron transfer efficiency.

## Abstract

Rotaxane-based mechanophores that exploit spatial separation between a luminophore and a quencher are attractive due to their high structural design flexibility, enabling high-contrast changes in fluorescence intensity. However, it remains unclear whether their quenching mechanism is predominantly governed by photoinduced electron transfer (PET) or ground-state charge-transfer (CT) complex formation. This study unveils the quenching mechanism using rotaxane mechanophores incorporating π-extended anthracene, tetracene, or pentacene. In toluene, the quenching efficiency decreases with increasing π-conjugation of the fluorophore. Steady-state and transient absorption spectroscopy clarify that the fluorescence quenching of the anthracene-containing rotaxane is primarily due to PET, with a minor contribution from CT complex formation. In contrast, no clear CT complex formation is observed for the tetracene- and pentacene-containing mechanophores. PET moderately quenches the fluorescence for the tetracene-based system, while the low PET efficiency in the pentacene-containing mechanophore results in minimal quenching. Polyurethane elastomer films containing the anthracene-based mechanophore exhibit a significant increase in fluorescence intensity upon mechanical deformation. In contrast, almost no activation is observed for the pentacene-based mechanophore embedded in polyurethane. These findings clarify that PET is the primary quenching mechanism in rotaxane-based mechanochromic mechanophores, offering valuable insights for the future design of supramolecular mechanophores.

The quenching mechanism of the rotaxane mechanochromic mechanophore is investigated with a series of π-extended acene groups. Photoinduced electron transfer is found to be primarily responsible for the quenching.