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

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.