화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.116, No.14, 6111-6120, 1994
Liquid-Crystalline Solvents as Mechanistic Probes .53. Effects of Aggregation and Mesomorphic Order on the Photophysical Properties of 4-Alkyl-N-(P-Cyanophenyl)Piperidines - Molecules Capable of Forming Intramolecular Charge-Transfer States
The photophysical properties of 4-alkyl-N-(p-cyanophenyl)piperidines (I) in dilute and concentrated solutions and in neat melt and nematic phases have been studied by means of their static absorption and fluorescence spectra and by picosecond time-domain emission measurements. Molecules of I contain a chromophore which is capable of forming intramolecular charge-transfer (ICT) excited states. Dilute solutions of I in polar solvents exhibit dual emission similar to that of p-(N,N-dimethylamino)benzonitrile (DMABN). Evidence for several ground- and excited-state species, including two distinct complexes and a charge-transfer excited state of I, has been obtained in concentrated hexane solutions as well as in the neat liquid-crystalline and melt phases of I. In the time domain experiments-on nematic and melt phases of a 1/1 (wt/wt) mixture of the pentyl and heptyl homologues of I, fluorescence from excitation at the red edge of the lowest energy absorption band rises "instantaneously" and the decay can be fit to a triple exponential function. The corresponding time-resolved emission spectra exhibit a time-dependent Stokes shift due to relaxation about a charge-transfer dominated excited-state species. In contrast, excitation near the lowest energy absorption maximum leads to fluorescence with a measurable rise time when emission is monitored at the long wavelength side of the emission band and an "instantaneous" rise when monitored at the short wavelength side of the band. A kinetic model consistent with these observations is proposed. It is shown that energy hopping from the excited singlet of I or its complexes is not an important contributor to the population of the charge-transfer species.