Photophysics of the Intramolecular Exciplex Formation in ω-(1-pyrenyl)-α-N,N-dimethylaminoalkanes

Article abstract of DOI:10.1021/ja00236a005

The photophysical properties of ω-(1-pyrenyl)-α-N,N-dimethylaminoethane, -propane, -butane, and -octane are investigated.Excitation of the pyrene chromophore leads, in solvents of medium polarity, to the formation of an intramolecular exciplex, but the compounds with four and eight methylene groups in the alkyl chain do not show exciplex emission.The dipole moment and the emission energy of the exciplex decrease with increasing chain length.The kinetics of exciplex formation are analyzed in solvents of medium polarity using time-correlated fluorescence measurements.These results indicate that intramolecular exciplex formation in these compounds cannot be considered as the monomolecular analogue of intermolecular exciplex formation, but must be interpreted in the framework of rotational isomerism.The influence of the chain length on the exciplex formation process is evaluated.In polar solvents, such as acetonitrile, no exciplex emission is observed except for the compound with an ethyl chain.The standard free enthalpy of the radical ion pair is in this solvent some 10 kJ mol^-1 lower than the standard free enthalpy of the intramolecular exciplex.The formation of the ion-pair state has been confirmed by picosecond transient absorption spectroscopy.