Thermo‐solvatochromism of zwitterionic probes in aqueous aliphatic alcohols and in aqueous 2‐alkoxyethanols: relevance to the enthalpies of activation of chemical reactions

The thermo‐solvatochromic behavior of 2,6‐dichloro‐4‐(2,4,6‐triphenylpyridinium‐1‐yl)phenolate (WB), 1‐methylquinolinium‐8‐olate (QB) and 4‐[2‐(1‐methylpyridinium‐4‐yl)ethenyl]phenolate (MC) was investigated in binary mixtures of water (W) and 2‐alkoxyethanols (ROEtOH) in the temperature ranges from 10 to 60°C (2‐ethoxyethanol and 2‐n‐propoxyethanol) and 10 to 40°C (2‐n‐butoxyethanol). Thermo‐solvatochromic data were treated according to a model that is based on the presence in bulk solution of three solvents, W, ROEtOH and a 1:1 H‐bonded species, ROEtOH–W. Solvation by ROEtOH–W is favored over solvation by either of the two precursor solvents. The present data, and those recently published on thermo‐solvatochromism of the same probes in five alcohols (methanol, ethanol, 1‐propanol, 2‐propanol and 2‐methyl‐2‐propanol) and one ROEtOH (2‐methoxyethanol), were submitted to regression analysis. The results indicate that solvation is more sensitive to solute–solvent hydrophobic interactions than H‐bonding between the probe phenoxy oxygen and the hydroxyl group of the H‐bond donating solvent (HBD). Temperature increase results in gradual desolvation of the probes, due to the concomitant decrease of the structure of all components of the binary solvent mixture. For pure solvents, the temperature‐induced desolvation depends on the structure of the probe (order: WB>MC>QB) and the HBD solvent (order: 2‐ethoxyethanols>aliphatic alcohols, for the same alkyl group; organic solvent>water). The probe solvatochromic response is due to the electronic transition zwitterion→di‐radical; it serves as a model for reactions that are associated with a large polarity difference between the reagents and activated complexes. For WB, for ΔT = 50°C, the desolvation energies range from 2.1 to 3.7 kcal mol−1. The contribution of temperature‐induced desolvation to the activation enthalpies of these reactions is, therefore, important.