Solvent polarity indicators based on bithiophene carboxamidine hydrochloride salt derivatives

Abstract

The photophysical properties of water soluble 5′-(4-methoxyphenyl)-2,2′-bithiophene-5-carboxamidine hydrochloride salt (MBTCH) and 5′-(3,5-dimethoxyphenyl)-2,2′-bithiophene-5-carboxamidine hydrochloride salt (DMBTCH) were studied in different solvents. The absorption spectra of MBTCH and DMBTCH show very weak solvent dependence, while the fluorescence emission spectra show bathochromic shift of about 35 nm and 48 nm on changing the solvent from cyclohexane to DMSO for DMBTCH and MBTCH, respectively. In protic solvents further bathochromic shift is observed in case of DMBTCH. The excited state time resolved decay of MBTCH and DMBTCH were measured over the entire fluorescence spectra and fitted very well with bi-exponential function in all solvents. Correlation of MBTCH and DMBTCH photophysical properties with ETN (Reichardt's normalized polarity parameter) was successful when the solvents are classified according to their hydrogen bonding properties, namely, protic and aprotic solvent categories. Reichardt-Ravi equation was used to determine dipole moment changes, Δμ, in both categories of solvents. Several multiple linear regression solvation energy relationships were used to assess the dependence of the photophysical properties on specific and non-specific properties of the solvents, such as Kamlet-Taft, Catalán and Laurence et al. relationships. These relationships have shown that hydrogen bonding interactions (specific interactions) and dipolar interaction (non-specific) competes strongly in their contribution to the photophysical properties of the studied compounds with slight domination of the latter.