Intramolecular charge transfer emission of trans-2-[4′-(dimethylamino)styryl]benzimidazole: Effect of solvent and pH

• Heteroatom replacement effect in 2-[4′-(dimethylamino)styryl]benzazoles is studied.
• t-DMASBI emits from LE state in nonpolar media, but from ICT state in polar media.
• Photoisomerization competes with both emissions and it follows non-adiabatic path.
• Unlike other azole analogs, only azole nitrogen is protonated to form monocation.

The intramolecular charge transfer (ICT) exhibiting fluorophore trans-2-[4′-(dimethylamino)styryl]benzimidazole (t-DMASBI) is synthesized and its spectral characteristics are investigated using absorption and fluorescence techniques. The solvatochromic studies reveal that the molecule is emitting from the locally excited state in non-polar and from an ICT state in polar solvents. Photoisomerization competes with fluorescence, and in viscous solvents restriction on twisting of olefinic bond increases the fluorescence. Theoretical calculations predict that t-DMASBI exists in two confomeric forms. trans-B conformer is more stable than trans-A. The cis isomer also exists as two conformers. However, the relative stability of conformer B further increased in cis isomer. The spectral characteristics of both the conformers of trans isomer are nearly same. The longest wavelength transition of trans isomer has ππ* character and is due to HOMO-LUMO single excitation. The theoretical calculated excitation and emission energies of trans conformer differ very little from each other and are in excellent agreement with the absorption and fluorescence spectral maximum in cyclohexane. The prototropic study indicates that unlike other benzazole derivatives t-DMASBI forms only one kind of monocation upon protonation at benzimidazole nitrogen in water. But at lower pH the second protonation occurs at dimethylamino nitrogen to form dication.