Solvatochromic study of the effect of chain length, chain branching, and polymethylation of alkylbenzenes on solvent polarizability

The contribution of solvent polarizability to solvatochromic measures of solvent strength, such as the Kamlet-Taft scale of solvent dipolarity-polarizability, is well recognized. In this work, we measured the π* values of 23 nonpolar aromatic solvents, including 10 n-alkylbenzenes (benzene to pentadecylbenzene), 5 branched-chain alkylbenzenes, and 8 di-, tri-, and tetramethylated benzenes. The π* values of n-alkane solvents increase monotonically with chain length. In contrast, the π* values of the aromatic liquids systematically decrease with homologue number. The direction of these changes is consistent with the increase in the polarizability of the n-alkanes and the decrease in the polarizability of the n-alkylbenzenes as homologue number increases. For both series of liquids, π* increases linearly with solvent polarizability; however, the slopes and intercepts of the relationships are quite different. We hypothesize that this difference in behavior is due to concentration of the phenyl groups of the long-chain alkylbenzenes in the cybotactic region of the very polar solvatochromic probes. Chain branching decreases π* of the alkylbenzenes, as it does for the alkanes. Ring methylation increases π* relative to that of the n-alkylbenzene of the same carbon number. However, when compared on the basis of polarizability, ring-methylated aromatics have a lower π* than the hypothetical n-alkylbenzene of the same refractive index. The extent of this decrease is not related to the dipole moment of the polymethylated aromatic but does follow the extent of crowding of the methyl groups about the ring.