Tunable Dynamic Excimer Formation in Bisphenalenyl Derivatives through Molecular Packing

Area of Science:

• Materials Science
• Photochemistry
• Organic Chemistry

Background:

• Dynamic excimer formation in π-conjugated systems offers tunable photophysical properties.
• Precise control over transient excimer species in solution remains a challenge.

Purpose of the Study:

• To investigate the influence of side-chain engineering on excimer formation in bisphenalenyl derivatives.
• To establish a structure-environment framework for controlling dynamic excimer formation and designing responsive fluorescent materials.

Main Methods:

• Synthesis of bisphenalenyl derivatives with varying side chains (ethylphenyl, n-butylphenyl, n-hexyl).
• Steady-state and time-resolved spectroscopy.
• Time-dependent density functional theory (TD-DFT) calculations.
• Diffusion-ordered NMR spectroscopy (DOSY).

Main Results:

• Phenyl-substituted bisphenalenyls showed reversible, concentration-dependent excimer emission attributed to excited-state dimerization.
• Aliphatically substituted derivatives exhibited only monomeric emission.
• Excimer formation was confirmed to arise from excited-state encounters in acetonitrile, with no ground-state aggregation.
• DOSY revealed dimer formation in tetrachloroethane.
• Substoichiometric addition of HBF4 induced excimer emission at lower concentrations, demonstrating stimulus-responsive control.

Conclusions:

• Side-chain structure and solvent environment significantly modulate dynamic excimer formation in bisphenalenyl systems.
• Excited-state dimerization is the primary mechanism for excimer formation in these derivatives.
• The findings provide a rational design strategy for responsive fluorescent materials based on charged π-systems.