Ultrafast Spectroscopy And Computational Study Of The Photochemistry Of Diphenylphosphoryl Azide: Direct Spectroscopic Observation Of A Singlet Phosphorylnitrene
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Date
2010-05
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Abstract
In order to characterize the active site of human paraoxonase 1 (HuPON1), photochemistry of a possible photoaffinity label diphenylphosphoryl azide was studied by femtosecond transient absorption spectroscopy, chemical analysis of light-induced reaction products as well as by RI-CC2/TZVP and TD-B3LYP/TZVP computational methods. Theoretical methods predicted two possible mechanisms for singlet diphenylphosphorylnitrene formation from the photoexcited phosphoryl azide. (i) Energy transfer from the (π, π*) singlet excited state, localized on a phenyl ring, to the azide moiety, thereby leading to the formation of the singlet excited azide, which subsequently loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. (ii) Direct irradiation of the azide moiety to form an excited singlet state of the azide, which in turn loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. Two transient species were observed upon ultrafast photolysis (260 nm) of diphenylphosphoryl azide. The first transient absorption, centered at 475 nm (lifetime (τ) ~ 28 ps), was assigned to a (π, π*) singlet S1 excited state localized on a phenyl ring, and the second transient observed at 525 nm (τ ~ 480 ps) was assigned to singlet diphenylphosphorylnitrene. Experimental and computational results obtained from the study of diphenylphosphoramidate, along with the results obtained with diphenylphosphoryl azide, supported the mechanism of energy transfer from the singlet excited phenyl ring to the azide moiety, followed by nitrogen extrusion to form the singlet phosphorylnitrene. Ultrafast time-resolved studies performed on diphenylphosphoryl azide with the singlet nitrene quencher, tris(trimethylsilyl)silane, confirmed the spectroscopic assignment of singlet diphenylphosphorylnitrene to the 525 nm absorption band.
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Mathematical and Physical Sciences: 2nd Place (The Ohio State University Edward F. Hayes Graduate Research Forum)
Keywords
Phosphorylnitrene, ultrafast spectroscopy, RI-CC2, TD-B3LYP, energy transfer