Picosecond Transient Absorption Spectra and the Twisted Intramolecular Charge Transfer Phenomenon

An improved model of a computer-controlled picosecond absorption spectrometer was designed and constructed. Its operation is described, and applied to the measurement of transient spectra of p-N, N-dioctylamino-acetophenone. The spectroscopy and photophysics of this compound are compared to the other compounds exhibiting the dual fluorescence, assigned usually to the adiabatic formation of the "twisted" intramolecular charge transfer excited state. The data are discussed in relation to alternative suggestion of an inversion of the pyramidal amino nitrogen atom. The whole evidence supports the decoupling of the π-electronic systems of D+ and A- in the D+-A- polar excited state, in full accord with the "twisted" intramolecular charge transfer model. The peculiar behaviour of the long alkyl -NR2 derivatives (favouring the "twisted" intramolecular charge transfer state emission) seems to be primarily due to the decreasing oxidation potential of such electron donor groups.