Why vertically π-expanded imidazo[1,2-a]pyridines are weak fluorescence emitters: experimental and computational...

Area of Science:

• Photochemistry and Photophysics
• Organic Heterocyclic Chemistry
• Computational Chemistry

Background:

• Imidazo[1,2-a]pyridine is a significant heterocyclic scaffold.
• π-expanded analogs are explored for tunable photophysical properties.
• Understanding structure-property relationships is crucial for material design.

Purpose of the Study:

• To investigate the photophysical properties of π-expanded imidazo[1,2-a]pyridine regioisomers.
• To elucidate the factors governing fluorescence quantum yields and decay dynamics.
• To predict molecular designs with improved fluorescence.

Main Methods:

• Experimental spectroscopic studies (UV-Vis absorption, fluorescence emission, lifetime measurements).
• Density Functional Theory (DFT) calculations for electronic structure and excited states.
• Analysis of intersystem crossing pathways.

Main Results:

• Two regioisomeric systems displayed distinct optical properties, including transition energies and fluorescence characteristics.
• Experimental results were corroborated by DFT calculations.
• Low fluorescence quantum yields were attributed to efficient S1→ T2 intersystem crossing.

Conclusions:

• The arrangement of the benzene ring significantly impacts the photophysics of these π-expanded systems.
• Intersystem crossing is a key deactivation pathway limiting fluorescence.
• Computational modeling can guide the design of novel imidazo[1,2-a]pyridine derivatives with enhanced fluorescence quantum yields.