Effects of donor and acceptor substituents on the photophysics of 4-ethynyl-2,1,3-benzothiadiazole derivatives

The present work explores the photophysical, electrochemical, and fluorescence polarization properties of a group of π-conjugated phenylethynyl-2,1,3-benzothiadiazole derivatives ( BTD s) bearing different electron-donating (ED) or electron-withdrawing (EW) substituents at the para position of the phenylethynyl moiety. The BTD s were synthesized through the Sonogashira cross-coupling reaction between 4-bromo-2,1,3-benzothiadiazole and the respective para -substituted phenylethynyl derivatives. The BTD s with the EW-substituents show relatively weak solvatochromic behavior, while the BTD s with the strong ED-substituents like methoxy and N , N -dimethylamino-based substituents ( BTDPhOMe and BTDPhNMe 2 ) exhibit a pronounced solvatochromic behavior. The change in dipole moments in the excited states of the derivatives was calculated using Lippert-Mataga plots. The conclusions drawn on the spectral behavior of the molecules could be rationalized by TD-DFT calculations involving electron density difference (EDD) maps that correlate with the ICT characteristics of the molecules. The experimental and theoretical calculations reveal that the BTD s with the strong ED-substituents (strong push-pull type BTD s) have a strong ICT character in the excited state. These strong push-pull type BTD s show high fluorescence quantum yield ( Φ F ) in apolar solvents and low Φ F in polar solvents. In contrast, the BTD s with the weak ED-substituents (weak push-pull type BTD s) and EW-substituents (pull-pull type BTD s) have a weaker ICT character with low Φ F in apolar and high Φ F in polar solvent media. There is good a agreement among the HOMO-LUMO band gaps obtained from absorption spectroscopy and electrochemical studies and theoretical calculations. The fluorescence anisotropy measurement in the glycerol medium shows that the studied BTD s generally exhibit higher sensitivity towards microviscosity than the traditional DPH fluorescence anisotropy probe. Photophysical and fluorescence polarization properties of a set of pull-pull and push-pull type π-conjugated phenylethynyl-2,1,3-benzothiadiazole derivatives.