Two‐Photon Excitation of 4'‐Hydroxymethyl‐4,5',8‐Trimethylpsoralen

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Abstract
Abstract— Psoralens are a class of pharmaceutical agents commonly used to treat several cutaneous disorders. When irradiated with a mode‐locked titanium: sapphire (Ti: sapphire) laser tuned to 730 nm, an aqueous solution of 4'‐hydroxymethyl‐4,5',8‐trimethylpsoralen (HMT) emits blue light. The emission spectrum is centered at 452 nm and is identical to that obtained by one‐photon excitation with UVA excitation, and its magnitude depends quad‐ratically on the intensity of laser excitation. These results suggest that two‐photon excitation occurs to a potentially photochemically active state. To estimate the two‐photon absorption cross section, it was first necessary to measure the emission quantum yield of HMT using 365 nm excitation at room temperature that resulted in a value of 0.045 ± 0.007. The two‐photon absorption cross section of HMT at 730 nm is therefore estimated to be 20 ± 10−50 cm4 s (20 Göppert‐Mayer). The excited‐state photophysics and photochemistry of psoralens suggest potential applications to cutaneous phototherapy in diseases such as psoriasis and dystrophic epidermolysis bullosa.