Effect of solvent and encapsulation in β-cyclodextrin on the photophysical properties of 4-[5-(thiophen-2-yl)furan-2-yl]benzamidine

•TFB shows dual fluorescence in aprotic solvents.•Excited state properties depend mainly on the non-specific interactions.•Dipole moment changes upon excitation by about 15 D.•1:1 complex formation between TFB and β-cyclodextrin is obtained.•Two dimensional NOESY 1H NMR was used to clarify the mode of inclusion.

Photophysical characteristics of the ground and excited states of the bichalcophene derivative 4-[5-(thiophen-2-yl)furan-2-yl]benzamidine (TFB) were investigated in different solvents and in β-cyclodextrin (β-CD). The photophysical properties of TFB were correlated with the two commonly used solvent polarity parameters viz., ETN and reaction field factor Δf. In addition, the photophysical properties of TFB were also found to be well correlated with Kamlet–Taft relationship. Multiple linear regression analysis of solvent dependent photophysical parameters indicated that both non-specific dipolar interaction and specific hydrogen bonding interactions play important roles in determining the photophysical properties of TFB. It has been found that the non-specific dipolar interaction is the dominating parameter. Fluorescence decay curves are well fitted to double exponential decay function, with shorter component having the higher relative integrated intensity. Molecular encapsulation of TFB by β-CD in aqueous solution has been studied by steady state and time resolved fluorescence techniques. Steady state measurements show that the fluorescence quantum yield is enhanced strongly upon the addition of β-CD and increases from 0.03 ± 0.01 in aqueous solution to 0.11 ± 0.02 at 7.5 mM concentration of β-CD. Steady state fluorescence measurements show 1:1 inclusion of TFB in the β-cyclodextrin cavity with an association constant of 2.3 ± 0.2 × 103 M−1, while data derived from time resolved measurements gave very close value for the association constant of 2.6 ± 0.3 × 103 M−1. Time resolved measurements and global analysis of the results indicated that the excited state lifetime is strongly affected by the presence of β-CD. The changes observed for the chemical shifts of TFB and β-CD 1H NMR spectra and corresponding 1H NMR spectra of their mixture confirmed the formation of the inclusion complex and showed that TFB is partially encapsulated in β-CD cavity.

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