# Tunable Dynamic Excimer Formation in Bisphenalenyl Derivatives through Molecular Packing

## TL;DR

This paper shows how the structure of bisphenalenyl derivatives can control excimer formation, enabling tunable and stimulus-responsive fluorescent materials.

## Contribution

The study introduces a structure–environment framework for modulating excimer formation in π-conjugated systems through side-chain engineering.

## Key findings

- Phenyl-substituted bisphenalenyl derivatives show concentration-dependent, reversible excimer emission.
- Aliphatic substitution suppresses excimer formation, favoring monomeric emission.
- HBF4 induces excimer emission at low concentrations, enabling stimulus-responsive control.

## Abstract

Dynamic excimer formation in solution-phase π-conjugated
systems presents a promising route toward tunable photophysical properties,
yet precise control over these transient species remains limited.
Herein, a series of bisphenalenyl derivatives is shown to exhibit
excimer emission that is modulated through strategic tailoring of
side chains (ethylphenyl, n-butylphenyl, and n-hexyl). Two phenyl-substituted derivatives exhibit reversible,
concentration-dependent excimer emission consistent with excited-state
dimerization. In contrast, an aliphatically substituted bisphenalenyl
moiety displays exclusively monomeric emission. Steady-state and time-resolved
spectroscopy, time-dependent density functional theory, and diffusion-ordered
NMR spectroscopy are employed to confirm that excimer formation arises
due to excited-state encounters, with no evidence of ground-state
aggregation in acetonitrile. However, diffusion-ordered NMR spectroscopy
data reveal dimer formation in tetrachloroethane. Notably, the introduction
of substoichiometric molar ratios of HBF4 induces excimer
emission at even lower concentrations of the bisphenalenyl moiety,
demonstrating a route to stimulus-responsive control. These results
provide a structure–environment framework for modulating dynamic
excimer formation in charged π-systems and inform the rational
design of responsive fluorescent materials.