Effect of the π-bridge structure on the intramolecular charge transfer of push–pull 2-phenylthiophene and 2-(furan-2-yl)pyridine derivatives

Abstract

Effect of the bridge structure on the photophysical properties of D-π-A of structurally similar 2,5-diphenylthiophene (cpd. I) and 2-(5-phenylfuran-2-yl)pyridine (cpd. 2) derivatives having the same electron donating and electron withdrawing groups were studied in different solvents. Solvent effect on the absorption and fluorescence emission spectra as well as their excited state decay were studied in detail. Introduction of 2,5-diphenylthiophene and 2-(5-phenylfuran-2-yl)pyridine between methoxy and carbonitrile groups lead to a shift of about 20 nm in both the absorption and fluorescence emission maxima of the former. Stokes shift of about 90 nm was observed for both compounds in protic solvents and polar aprotic solvents and decreases as the solvent polarity decreases. The solvatochromism in the fluorescence emission spectra was higher than the observed in absorption spectra, indicating higher dipole moment in the excited state than in the ground state for both compounds. DFT and TD-DFT optimized geometry calculations show that (I) is planar in the excited state while (II) is planar in both the ground and excited states reflecting the importance of the bridge structure on the geometry in the ground and excited states. Theoretical calculations of the dipole moment change from the ground to the excited states were consistent with the values obtained from Lippert-Mataga relationship and higher than values obtained from Reichardt-Ravi relationship. It has been found that the fluorescence lifetime of both compounds was found to decrease as the emission energy increases and being higher for (II) than for (I), and also was found to increase as the solvent’s dielectric constant increases. All photophysical properties were found to correlate well with Reichardt’s solvent polarity parameter, ETN. We have employed Kamlet-Taft, Catalán and Laurence et al. multi-parametric relationships to assign the contribution of the specific and non-specific solute–solvent interactions. Non-specific interactions were found to be mostly dominating in all cases.