Modulation of the photophysics of 2-(4 '-N,N-dimethylaminophenyl)imidazo-[4,5-b]pyridine by long chain N-alkylations

The N-alkylated products of 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b) were synthesized to study the effect of long alkyl chains on the photophysics of the fluorophore. Two different compounds were synthesized: in molecule 1, the alkyl chain was attached to the pyridine nitrogen of DMAPIP-b and in molecule 2 the alkyl chain was substituted on the imidazole nitrogen of DMAPIP-b. Absorption, steady state and time-resolved fluorescence techniques were employed to investigate the spectral properties in different solvents. Density functional theory (DFT) and restricted configuration by singles (CIS) combined with time-dependent DFT (TDDFT) calculations were performed on the ground state and the excited state respectively to calculate the geometrical and the electronic properties of the molecules. Unlike DMAPIP-b, where dual emission was observed in protic solvents, which originates from both a locally excited state and an intramolecular charge transfer (ICT) state, the pyridine nitrogen alkylated product (1) emits a single emission from the ICT state and the imizadole nitrogen alkylated molecule (2) emits dual emission only in methanol and water.