# Twist and Shine: The Impact of Halogen Substitution on Thiele Hydrocarbon's Optical Properties

## TL;DR

This study shows how halogen substitution affects the structure and light-emitting properties of Thiele hydrocarbons, enabling the design of materials with unique optical behaviors.

## Contribution

The discovery of a folded boat-like conformation and zwitterionic excited states in halogenated Thiele hydrocarbons is novel.

## Key findings

- Folded derivatives exhibit large Stokes shifts and mechanofluorochromic responses.
- Planar derivatives show near-unity photoluminescence yields and solvatochromism.
- Halogen substitution enables precise tuning of charge separation and material responsiveness.

## Abstract

In this work, two new mixed‐halide Thiele hydrocarbons were synthesized and characterized to elucidate the influence of halogenation patterns on their photophysical properties, addressing the role exerted by steric constraints and electronic effects. Interestingly, their interplay governed a unique spectroscopic behavior that is driven by pronounced geometric rearrangements upon photoexcitation. Quantum‐chemical calculations revealed that derivatives with limited diradical character, and correspondingly shorter exocyclic C═C bonds, are likely to adopt a folded boat‐like conformation in the ground state, a structural motif not previously observed in Thiele hydrocarbons. Upon photo‐excitation, these exocyclic bonds were subject to a significant elongation, facilitating mixing between the bright singly excited (SE) state and the dark, doubly excited (DE) state. These interactions enabled the formation of a zwitterionic excited state, a finding that is consistent with recent observations for halogenated derivatives. Transient absorption spectroscopy confirmed this mechanism, which provides a promising strategy for designing fluorophores with exceptionally large Stokes shifts (more than 2 eV) and tailored photophysical and electronic properties. In addition, a pronounced mechanofluorochromic response has been observed for the first time in the case of folded derivative, opening the way to the use of these species as multi‐stimuli‐responsive molecular materials.

Halogenation in Thiele hydrocarbons strongly influences structure and excited‐state dynamics. Depending on substitution, “boat” folded forms show large Stokes shifts and strong solvatochromism, while planar “butterfly” derivatives exhibit near‐unity photoluminescence yields. Halogen substitution thus enables precise tuning of charge separation and responsiveness in functional materials.