Solvent modulation of ground and excited state properties of 5-dimethyl amino 5'-nitro-2,2'-bithiophene (DNBT): a case study within the framework of a scaled self consistent reaction field method

Abstract

The ground state structural and electronic properties of DNBT (5-dimethylamino 5'-nitro-2,2'-bithiophene) have been calculated by the AM1 method. The excited state electronic parameters have been obtained by using the CNDO/S-CI method. The CNDO/S self consistent (SC) reaction field (RF) method in the image-charge representation has been employed to study the influence of polar solvents on various electronic parameters. The trans form of DNBT has been found to be the most stable conformation in the ground state, with the cis form being very close in energy. The calculated structural parameters show a satisfactory agreement with the available experimental values. A scaled SC-RF model which takes into account the differences in cavity sizes in the ground and excited states of DNBT has been proposed to obtain reliable estimates of the solvent modulated transition energies. The predicted intramolecular charge transfer (π→π^∗) transitions, S₀→S₂ and S₀→S₄ of DNBT show a strong positive solvatochromism, i.e. bathochromic shift in polar solvents. Computed transition energies in a number of polar solvents show excellent agreement with the available experimental values. The possible role of specific interactions in the particular case of DNBT-water system is also investigated.