Excited-State Dynamics in the Covalently Linked Systems: Pyrene−(CH2)n−Aryl Azide

Abstract

The primary physical and chemical processes which follow photoexcitation of the covalently linked systems, 1-[(4-azido-2,3,5,6-tetrafluorobenzoyloxy)methyl]pyrene (2) and 1-[3-(4-azido-2,3,5,6-tetrafluorobenzoyloxy)propyl]pyrene (3), have been studied using femto- and nanosecond transient absorption spectroscopy and computational chemistry. Excitation of 2 and 3 at 336 nm results in the population of the second excited singlet state of the pyrene moiety (S2). Internal conversion to the lowest excited singlet state of the pyrene moiety (S1) occurs with a time constant of ∼140 fs, a value which is similar to that of unsubstituted pyrene. The S1 local pyrene state, initially formed with excess vibrational energy, undergoes vibrational cooling with a time constant ∼2 ps. The decay of the local pyrene S1 state measured in the pump−probe experiments was described by a two exponential function with time constants which agree well with the values obtained previously from the fluorescence decay kinetics (Barabanov, I. I.; Pritchina, E. A.; Takaya, T.; Gritsan, N. P. Mendeleev Commun. 2008, 18, 273). In both systems decay of the local pyrene S1 state on the time scale of tens of picoseconds is accompanied by formation of a product with a narrow band at ∼460 nm. This transient absorption was assigned to the radical cation of the pyrene moiety. Calculations predict very fast dissociation of the counterpart aryl azide radical anion with formation of the corresponding arylnitrene radical anion. Decay of the local pyrene S1 state on the longer time scale (hundreds of ps) is not accompanied by formation of a noticeable amount of pyrene radical cation. Most likely, both electron transfer and energy transfer from pyrene to aryl azide moiety are responsible for the photosensitization of the perfluorinated aryl azide decomposition.