Time resolved resonance Raman, transient diffuse reflectance and kinetic studies of species generated by UV laser photolysis of biphenyl occluded within dehydrated Y-faujasite zeolites
Transient diffuse reflectance UV–vis absorption (TDRUV) and time resolved resonance Raman (TR3) spectroscopy (370 nm exciting laser line, 8 ns) were performed to investigate the species generated by UV pulsed laser photolysis (248 nm, 20 ns) of biphenyl (BP) occluded in cavities of dehydrated silica-rich Y-faujasitic (MnFAU) zeolites with Mn(AlO2)n(SiO2)192−n formulae per unit cell. The TDRUV spectra were recorded in the time range 0.5–340 μs and the TR3 spectra were recorded in the 0.05–100 μs as functions of aluminum content (n = 0, 56), extra framework cations (M = Na+, K+, Cs+), BP loading (1, 2, 4, 8, 16 BP/UC) and laser energy (0.15–1.5 mJ). Specific spectra of BP(T1) triplet state, BP+ radical cation, trapped electron as Na43+ cluster and BP− radical anion were resolved by Multivariate curve resolution (MCR) of TDRUV and TR3 data sets. The RR spectra of BP+ and BP(T1) correspond to nearly planar structures. The concentration decays fit a model of dispersed heterogeneous kinetics. At lower laser energy, BP(T1) was generated as major transient species. The energy transfer via T1 occurs with rates not greater than 10−6 s−1. Photoionization is found to be the dominant phenomenon at higher pump laser energy. BP+, Na43+ and BP− exhibit distinct decay behaviors with rates not lower than 10−3 s−1. In polar environment of Na56FAU supercage, the ejected electron is trapped as Na43+ cluster at low loading and as BP− at high loading. In the non-polar environment of FAU supercage, the ejected electron is captured by BP(S0) even at low loading. The effects of Si/Al ratio, extraframework cation and BP loading upon the energy and electron transfer rates were discussed according to BP photochemical behavior in solvents.
Journal: Journal of Photochemistry and Photobiology A: Chemistry - Volume 195, Issues 2–3, 15 April 2008, Pages 156–166