Photophysical and photochemical properties of 5,7-dimethoxycoumarin under one- and two-photon excitation

The spectroscopic and photochemical properties of 5,7-dimethoxycoumarin (DMC) in acetonitrile (ACN) were investigated at room temperature under one- and two-photon excitation. Two-photon induced [2 + 2] photocycloaddition of DMC was investigated with 650 run laser excitation (120 A pulse width). GC and GC-MS analysis confirmed dimer formation, while HPLC analysis allowed specific photodimer identification through comparison with samples prepared via one photon irradiation. Photodimer products formed via two-photon excitation correlated with those formed via UV irradiation. Syn head-to-head and syn head-to-tail photodimers were formed in 1:3.0 and 1:2.8 ratios under one-photon (broadband UV) and two-photon (650 nm, 120 fs) irradiation in anisole, respectively. The quantum yield for the photoreaction of DMC solution (10(-4) M) under broadband UV irradiation was of the order of I X 10(-3) and increased dramatically to I X 10(-2) under two-photon excitation. The spectral investigation of the photochemical products of DMC revealed efficient fluorescence emission in the spectral region 290-340 nm, attributed to the syn-head-to-tail dimers of DMC. Evidence for an additional manifold for the photochemical reaction of DMC was found, resulting in the production of photochemical products that exhibited low fluorescence emission intensity along with dependences on excitation wavelength. Copyright (C) 2002 John Wiley Sons, Ltd.