The quantum yields of adduct formation and isomerisation of (E)-and (Z)-1-phenylpropene by 3-phenylcyclopentenone 1 and 3-phenylcyclohexenone 2 have been measured by 1 H-NMR spectroscopy in solutions of d 6 -benzene. The adducts formed for each photoreaction have been shown to be independent of the starting 1-phenylpropene isomer. For 3-phenylcyclopentenone 1, a photostationary state of 32 : 68 (E)-to (Z)-1-phenylpropene is achieved during the course of the reaction. This isomerisation reaction may occur through reversion of triplet 1,4-biradical intermediates to ground state molecules or by an energy transfer process. However for 3-phenylcyclohexenone 2 isomerisation occurs only through reversion of triplet 1,4-biradical intermediates. The mechanism of photoadduct formation and that of the isomerisation reaction are discussed for both phenylenones.Rate constants for quenching of the 3-phenylcyclopentenone 1 and 3-phenylcyclohexenone 2 triplet states by (E)-and (Z)-1-phenylpropene, 3 and 4, in cyclohexane and benzene solutions have been recorded using laser flash photolysis. In cyclohexane at high quencher concentration, complex decay kinetics are observed consistent with observation of both the enone triplet state and either a triplet exciplex or 1,4-biradical intermediates.

The photophysical properties of 3-phenyl-5-[2-(1-naphthyl)ethyl]cyclopent-2-enone and 5-[2-(1-naphthyl)methyl]-3-phenylcyclopent-2-enone have been compared with those of their constituent alkylated chromophores 1-ethylnaphthalene, 3-phenyl-5-(2-phenylethyl)cyclopent-2-enone and 3-phenyl-5-benzylcyclopent-2-enone by means of UV absorption spectroscopy, steady state and time resolved low temperature emission spectroscopy and laser flash photolysis. In mixed solvent low temperature glasses such as ethanol-isopentane-diethyl ether (2 : 5 : 5, v/v) (EPA) phosphorescence decay of all the enones shows multiexponential behaviour consistent with different lifetimes at different solvent sites. The naphthalene derivatives also show a longer lived emission consistent with slow processes involving a naphthalene localised triplet excited state. By contrast in fluid solution flash photolysis experiments reveal that there is rapid equilibration between the enone-and naphthalene-localised states.

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