Temperature dependence of phenanthrene cavity radius in apolar solvents and in water

Phenanthrene, a three ring aromatic hydrocarbon, is used as a model substance for study of intermolecular interactions in dilute solutions of organic solvents and water by spectroscopic method. Temperature dependencies of shift of the S$_2\leftarrow$S$_0$ spectrum of phenanthrene dissolved in apolar solvents and in liquid water are studied in this work. The spectroscopic data are used for analysis of the cavity radius $R$ for phenanthrene molecule in solvents. It is shown that the value of $R$ increases with temperature in organic solvents. In contrast, for water solution, the value of $R$ initially grows with temperature increase (in the interval of 273.3-296 K), but becomes constant at higher temperatures. The changes of water structure in the neighbourhood of a phenanthrene molecule and probable hypothesis about causes of $R$ value constancy in high-temperature region are discussed. Conclusion about strengthening of the water structure in an aqueous cage with phenanthrene molecule inside is done.