A thermodynamic analysis of the pi* and E(T)(30) polarity scales

The solvent-induced UV-vis spectral shifts in 4-nitroanisole and pyridinium N-phenoxide betaine-30 dyes utilized in the famous pi* and E(T)(30) polarity scales, respectively, are analyzed by molecular theories in terms of long-range solute-solvent interactions due to induction, dispersion, and dipole-dipole forces. The solvent-induced shift is represented as a sum of the differential solute-solvent internal energy and the differential energy of binding the solvent molecules in the solute vicinity. The aim of the study is 3-fold: (i) to clarify and quantify the relative effects of the three types of interactions, (ii) to elicit the magnitude of the effect of specific forces, and (iii) to evaluate the contribution of the differential solvent binding to the spectral shift. For (i), the dye properties directing the weighting are the size and the differences in both polarizability and dipole moment between ground and excited states. Accordingly, the distinctions pi* vs E(T)(30) derive from the different sizes (4.5 vs 6.4 Angstrom), dramatically different polarizability enhancement upon excitation (6 vs 61 Angstrom(3)), and opposite changes in the dipole moment (+8.2 vs -8.6 D) of the two dyes. As a key result, the importance of dispersion forces to the spectral shift even in highly polar liquids is emphasized. While the contributions of dispersions and inductions are comparable in the pi* scale, inductions are clearly overshadowed by dispersions in the E(T)(30) values. Both effects reinforce each other in pi*, producing the well-known red shift. For the ET(30) scale, the effects due to dispersion and dipolar solvation have opposite signs making the red shift for nonpolar solvents switch to the blue for polar solvents. For (ii), there is overall reasonable agreement between theory and experiment for both dyes, as far as the nonpolar and select solvents are concerned, but there are also discrepant solvent classes. Thus, the predicted E(T)(30) values for protic solvents are uniformly too low, revealing a decrease in H-bonding interactions of the excited state with lowered dipole moment. Further, the calculated pi* values of aromatic and chlorinated solvents are throughout too high, and this is explained by an increase in charge-transfer interactions of the more delocalized excited state. For (iii), the differential solvent binding energies have been extracted from experimental thermochromic data. For strongly polar fluids, the solute-solvent component of the shift overshadows that from the solvent binding energy variation. In nonpolar and weakly polar liquids the two parts are comparable for 4-nitroanisole, but the latter is still small for betaine-30. Experimental and calculated values in the present work parameters for betaine-30 are applied to calculating solvent reorganization energies lambda(s) of intramolecular electron transfer. lambda(s), is separated into polar activation by the solvent permanent dipoles and nonpolar activation due to induction and dispersion forces. Experimental reorganization energies due to the classical solvent and solute modes are throughout higher than the calculated lambda(s) values. The difference depends on solvent polarity and was attributed to the solute donor-acceptor vibrational mode coupled to the solvent polarization.