Electronic absorption spectra and fluorescent properties of non-associated 16,17-bis(alkoxy)violanthrone dyes and their dependence on the nature of substituent and solvent's parameters

This article is devoted to spectral properties of non-associated 16,17-dihydroxy- and dialkyloxy violanthrones, in order to study the relationship between the nature of alkoxy substituents and the UV/Vis absorption/emission characteristics of these dyes. Solvatochromic and solvatofluorochromic effects of the violanthrone derivatives are investigated as well. It is found that lengthening of the substituent's alkyl chain in the dye molecules results in a hypsochromic shift of the long-wavelength absorption band and does not influence the position of the emission band. This spectral behavior is explained by the interplay of the electron-releasing ability of the alkoxy substituents and the varying flatness of the dialkoxy substituted violanthrone molecules in their electronic ground and excited states. It is also shown that the position of the absorption and emission bands depends on the intensity of non-specific interactions between dyes and solvents, and linearly correlate with values of the Kirkwood and Lippert parameters. The results can be useful in process development of new light concentrators and photovoltaic films. Violanthrone derivatives can be used as various mixtures of non-associated and dimer forms with different UV/Vis spectral behavior, depending on dye concentration and medium polarity.