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

The ground state structural and electronic properties of DNBT (5-dimethylam ino 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 th e image-charge representation has been employed to study the influence of p olar 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 sca led 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 intr amolecular charge transfer (pi --> pi*) transitions, S-0 --> S-2 and S-0 -- > S-4 of DNBT show a strong positive solvatochromism, i.e. bathochromic shi ft in polar solvents. Computed transition energies in a number of polar sol vents 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. (C) 1999 Elsevier Science B.V. All rights res erved.