Driving forces for the mutual conversions between phenothiazines and their various reaction intermediates in acetonitrile.

The thermodynamic driving forces (defined as the enthalpy changes or redox potentials in this work) of the 18 phenothiazines and their analogues, phenoxazine, N-methyl-dihydrophenazine, 9H-thioxanthene, 9H-xanthene and 9,10-dihydro- N-methylacridine, to release hydride, hydrogen atom, proton, and electron in acetonitrile, the thermodynamic driving forces of the radical cations of the phenothiazines and the analogues to release hydrogen atom, proton, and electron in acetonitrile, and the thermodynamic driving forces of the cations of the phenothiazines with two positive charges and their analogues to release proton in acetonitrile were estimated by using experimental methods. The effect of the remote substituents on the 11 determined thermodynamic driving forces were examined according to Brown's substituent parameters; the results show that the values of the 11 thermodynamic driving forces all are linearly dependent on the sum of Brown substituent parameters (sigma +) with very good correlation coefficients, which indicates that for any one- or multisubstituted at para- and/or meta-position phenothiazines and their various reaction intermediates, the 11 thermodynamic driving forces all can be easily and safely estimated from the corresponding Brown substituent parameters (sigma +). The relative effective charges on the center nitrogen atom in phenothiazines and their various reaction intermediates were estimated from the related Hammett-type linear free-energy relationships, which can be used to efficiently measure the electrophilicity, nucleophilicity, and dimerizing ability of the corresponding reaction intermediates of phenothiazines and their analogues. All the information disclosed in this work could not only supply a gap of the chemical thermodynamics on the mutual conversions between phenothiazines and their various reaction intermediates in solution but also strongly promote the fast development of the chemistry and application of phenothiazines and their analogues.