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

Photophysics of π-expanded analogs of imidazo[1,2-a]pyridine, a well-known heterocyclic compound, has been experimentally and theoretically studied. Two regioisomeric systems differing only in the arrangement of the benzene ring have shown different optical properties (electronic transition energies, fluorescence quantum yields and decay times). DFT calculations have been in agreement with experimental results. Low fluorescence quantum yields have been attributed to the S1→ T2 intersystem crossing channel. Additional computations predict architectures which should possess higher fluorescence quantum yields.