Photophysical Processes in Coumarin Sensitizers
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PHOTOPHYSICAL PROCESSES IN COUMARIN SENSITIZERS O. N. Tchaikovskaya, E. N. Bocharnikova, N. G. Dmitrieva, and I. V. Sokolova
UDC 539.194:535.37
The quantum-chemical and spectroscopic methods make it possible to record the features of photoactivated processes occurring within and between molecules, as well as the structural rearrangements of molecules. To solve the fundamental problem of establishing a relationship between the photophysical and spectralluminescent properties of organic molecules and their structure, a comparative analysis of the nature of electronically excited states and photoprocesses in coumarin and its related compounds (furocoumarin and 8-methoxypsoralen) has been carried out. To understand the mechanism of forming the absorption and fluorescence spectra of compounds, the schemes of electronically excited states of the molecules under study and the charge localization on the molecular orbitals have been calculated and analyzed. The investigated compounds are effective sensitizers, and the presence of the OCH3 group in the molecular structure has the strongest effect on the spectral-luminescent characteristics. This paper presents the results of studying the induced absorption spectra of 8-methoxypsoralen by the pumb-probe method with a delay of 30 ns in the range of 350–700 nm. The experimentally recorded induced absorption of 8-methoxypsoralen belongs to long-living photoproducts of a radical nature. Keywords: coumarin, psoralen, absorption, fluorescence, photophysical processes, quantum chemical calculation, 8-methoxypsoralen.
INTRODUCTION Coumarin, also called coumarou, is a representative of the group of polyphenolic compounds first extracted from tonka beans (Dipteryx Odoranta Wild; Fabaceae family) by Vogel [1] in 1820. Since then the extraction of coumarin compounds from plants, bacteria, and mushrooms [2–4] is repeatedly reported together with the structural characteristic and biological activity of thousands of natural coumarins and their chemical synthesis [5–7]. Coumarins are secondary metabolites of plants whose biological activity varies depending on the character of their substitution. The attention of the modern scientific community is focused on a study of photophysical properties of this compound and its derivatives, because the compounds of this class with various photophysical and photochemical properties are used in optics, green chemistry, radiation chemistry, photobiology, medicine, quantum electronics, and other branches of science and technology [8–10]. For example, many coumarins possess intensive fluorescence used to develop fluorescent labels, sensors, optical brighteners, and laser-active media [11]. Coumarins possessing weak fluorescence are used for water purification or as antioxidants [12]. Furocoumarins (or psoralens) are used in medicine for photochemotherapy of some diseases (psoriasis, vitiligo, mycosis, etc.). The most carefully studied is PUVA therapy used for treatment of dermatoses [13–17]; it allows the frequency of treatment procedures to be reduced
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