Photochemistry of Biphenyl Occluded within X Faujasite Type Zeolites

The UV laser flash photolysis (248 nm, 10 ns) of biphenyl (BP) occluded in void space of aluminum rich faujasitic X zeolites Na<INF>n</INF><INF></INF>(AlO<INF>2</INF>)<INF>n</INF><INF></INF>(SiO<INF>2</INF>)<INF>192</INF><INF>-</INF><INF>n</INF><INF></INF> 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(T<INF>1</INF>), 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(T<INF>1</INF>), BP^•+, BP^•-, and trapped electron as Na<INF>4</INF>³⁺. 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(T<INF>1</INF>) was found to be the major transient species for all the loading values. Lifetime mean values of BP(T<INF>1</INF>) 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, Na<INF>4</INF>³⁺ 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(S<INF>0</INF>). 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.