Time-resolved absorption and resonance Raman investigations of short-livedintermediates in solution or occluded in zeolites

Two examples of structural investigation of photogenerated short-lived tran sient species of aromatic compounds, by using time-resolved electronic abso rption and resonance Raman spectroscopy are provided. The picosecond Raman spectra of the lowest excited singlet state (S-1) of 4,4'-bipyridine and 2, 2'-bipyridine in solution are discussed first. The results provide strong e vidence for a surprising asymmetric S-1 structure in which the electronic e xcitation and the consequent structural distortion are localized in one of the pyridyl rings, the other ring being almost unaffected. The laser photol ysis of biphenyl occluded in the pores and cavities of ZSM-5 and faujasitic zeolites have been investigated in the nanosecond time-scale as functions of aluminium content, the extraframework cations, sample :Loading and inten sity of photolysis. At low pump power and low aluminium content the triplet state of biphenyl is observed. At higher pump power and in aluminated zeol ites, biphotonic ionization was observed to generate the cation radical. As the loading increased, the anion radical yield increased. This is characte ristic of faujasitic zeolites in which scavenger molecules with restricted diffusional motions are confined in the vicinity of photoionized molecules. The lifetimes of cation radical were particularly long in aluminated ZSM-5 zeolites with efficient trapping sites of the photoejected electron. (C) 1 999 Elsevier Science B.V. All rights reserved.