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 p hases have been studied by means of their static absorption and fluore scence spectra and by picosecond time-domain emission measurements. Mo lecules of I contain a chromophore which is capable of forming intramo lecular charge-transfer (ICT) excited states. Dilute solutions of I in polar solvents exhibit dual emission similar to that of p-(N,N-dimeth ylamino)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 solution s as well as in the neat liquid-crystalline and melt phases of I. In t he 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 e xcitation at the red edge of the lowest energy absorption band rises ' 'instantaneously'' and the decay can be fit to a triple exponential fu nction. The corresponding time-resolved emission spectra exhibit a tim e-dependent Stokes shift due to relaxation about a charge-transfer dom inated 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 em ission band and an ''instantaneous'' rise when monitored at the short wavelength side of the band. A kinetic model consistent with these obs ervations is proposed. It is shown that energy hopping from the excite d singlet of I or its complexes is not an important contributor to the population of the charge-transfer species.