Photochemistry of Biphenyl Occluded within X Faujasite Type Zeolites

Abstract

The UV laser flash photolysis (248 nm, 10 ns) of biphenyl (BP) occluded in void space of aluminum rich faujasitic X zeolites Nan(AlO2)n(SiO2)192-n with n = 85, 96 has been investigated by diffuse reflectance transient UV−visible absorption (DRTUV) and time-resolved resonance Raman (TR3) measurements on the nano- and microsecond time scales. TR3 measurements were carried out using an 8 ns probe pulse at 370 nm. Delays between pump and probe pulses (50 ns to 100 μs) provide evidence of triplet state BP(T1), radical cation BP•+, and radical anion BP•- with planar quinoidal structures. Self-modeling mixture analysis (SIMPLISMA) of DRTUV data sets resolved pure absorption spectra of BP(T1), BP•+, BP•-, and trapped electron as Na43+. Data processing determines also the decays of the corresponding concentrations between 0.5 and 340 μs for loading values corresponding to 1, 2, 4, and 8 BP per unit cell. All the decays of specific concentration were found to fit a model of dispersed heterogeneous kinetics. At lower laser fluence, BP(T1) was found to be the major transient species for all the loading values. Lifetime mean values of BP(T1) were found to be 40 and 110 μs at low and high loading, respectively. Photoionization was found to be dominant at higher pump laser fluence. At low loading, Na43+ cluster and BP•+ are produced in high yield with relatively different lifetimes, 10 μs and ∼500 μs, respectively. The recombination process includes probably supplementary electron transfers with the zeolite framework. At high loading, BP•- (20 μs) concomitantly forms BP•+ (12 μs) as a result of photoejected electron capture by BP(S0). The most important role of porous solids with large pores such as X faujasite with respect to photophysical and photochemical BP behaviors in solution appears to be the efficient trapping of photoejected electron in the solid network.