Solvatochromism in Pure Solvents: Effects of the Molecular Structure of the Probe

The following solvatochromic probes were synthesized: 1‐methyl‐8‐oxyquinolinium betaine (QB), sodium 1‐methyl‐8‐oxyquinolinium betaine‐5‐sulphonate (QBS), and 1‐methyl‐3‐oxypyridinium betaine (PB). Their solvatochromic behavior in 28 protic and aprotic solvents was investigated, and the data compared to those of 2,6‐diphenyl‐4‐(2,4,6‐triphenyl‐1‐pyridinio)‐1‐phenolate, (RB). Solvent polarity scales based on QB, QBS, and PB correlate linearly with the ET(30) scale of RB. The Taft‐Kamlet‐Abboud equation satisfactorily applies to the solvatochromic data of RB, QB, QBS and PB. The multiparameter regression coefficients (a) and (s) of the above mentioned equation show that the sensitivity of the probe to solvent dipolarity/polarizability, and hydrogen bond donation is clearly dependent on its molecular structure. This dependence was tested for a total of 26 probes of widely different structures. It is shown that this is a general behavior, i.e., log |a| and |s| are linearly dependent on the pKa and the dipole moment of the probe, respectively.