Photophysical Properties and Photobiological Activity of the Furanochromones Visnagin and Khellin

The larger photobiological activity of visnagin (VI) versus khellin (KH) toward several living organisms, including fungi, viruses, yeasts and bacteria, induced a detailed investigation of the photophysical properties of these naturally occurring furanochromones, using laser-flash-photolysis, photoacoustic calorimetry and fluorescence (steady-state and time-resolved) techniques in solvents with different polarity and content of water, including micelles and vesicles. The results have shown that the magnitude of all the three rate constants out of S₁ (radiative, k_f; internal conversion, k_ic and intersystem crossing, k_isc) for VI and KH strongly depend on the solvent, namely on its hydrogen bonding ability and polarity. The changes of k_f and k_isc are due to the solvent-assisted mixing and/or inversion of the two first singlet excited states (¹n,π* and ¹π,π*), while k_ic increases with a decrease of the S_o–S₁ energy gap. As a consequence, the quantum yield of triplet formation (φ_T) strongly decreases from values of ?0.8 in dioxane to < 0.05 in water for both compounds. The magnitude of solvent polarity/hydrogen bonding ability required, at which the state order is inverted and φ_T starts to decrease, is greater for VI than for KH and consequently φ_T (VI) > φ_T (KH) over a broad range of water content including that appropriate to the environment of the compounds in a living system. These facts account for the larger photobiological activity of VI with respect to KH, regarding both the fungus Fusarium culmorum L. and the wild strain of Escherichia coli, studied by us.