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4205systems, one should use azethoxyl, rather than doxyl, nitroxides.Acknowledgment. We are grateful to Professor Z. Luz for helpful discussions. This study was made possible in part by funds granted by the Charles H. Revson Foundation (to E.M.). The statements made and views expressed, however, are solely the responsibility of the author (E.M.).The effect of solvent viscosity and temperature on the fluorescence quantum yield of p-(dimethylamino)benzylidenemalononitrile (1) and julolidinemalononitrile (3) has been studied in ethyl acetate, dimethyl phthalate, and glycerol. In the low-viscosity solvent, ethyl acetate, when temperature and viscosity variations are studied, a hydrodynamic model with a stick boundary condition gives an accurate and consistent description of the dynamics of the torsional motion of the probes. However, in medium-and high-viscosity solvents deviations from the hydrodynamic model are seen. In these solvents the free-volume concept was found to provide an accurate description of the solvent viscosity-temperature behavior and the probe torsional relaxation dynamics.

Langmuir-Blodgett (LB) films of arachidic acid have been used as spacer layers to control the separation distance between evaporated In and Ag metal island films and LB monolayers of a lightly substituted phthalocyanine. From relative intensity measurements, the distance dependence of the surface-enhanced Raman scattering (SERS) enhancement factor for the phthalocyanine was determined and found to be long range. In addition the magnitude of the enhancement was determined by measurement of the normal Raman intensity of phthalocyanine monolayers deposited directly onto unmetalized substrates. The results are compared with calculations of the enhancement factor based on an electromagnetic field enhancement mechanism. Good quantitative agreement is found between calculation and experiment for both the distance dependence and magnitude of the enhancement. We conclude that our SERS from island films is due to the electromagnetic mechanism and rule out short-range chemical mechanisms.

The spectroscopic and electrochemical properties of a series of p-iV^ZV-dialkylaminobenzylidenemalononitriles derivatives (1, 2, and 3) have been studies. These materials exhibit a strong intramolecular charge-transfer absorption. In this series the conformational freedom of the electron-donating group (R2N) was progressively restricted by structurally anchoring the nitrogen atom to the aromatic ring by alkyl chains. Molecular restriction of rotation of the NR2 group had a profound effect on the nonradiative decay rate and the excited state dipole moment. However, it had little effect on the ground state dipole moment. The most pronounced change in dipole moment ( µ) on electronic excitation was observed when the NR2 group was free to rotate. The rate of internal rotation of the dialkylamino group was estimated to be 2 X 10n s'1.

Abstract

The effect of polymerization of methyl methacrylate on the fluorescence intensities of a series of donor-acceptor molecules ([p-()VJV-dialkylamino)benzylidene]malononitriles) has been investigated. The fluorescence of the probes increases gradually (1.5 times) as conversion increases to 60%. Further increase in conversion causes a sharp increase in fluorescence (20-40 times), reaching a limiting value at the limiting conversion. This sudden rise in the fluorescence occurs when the polymerization approaches the glassy state. In this region, mobility becomes restricted and internal molecular relaxation of the probes becomes controlled by the microscopic free volume provided by the polymer. This leads to a decrease in the nonradiative rate and consequently to an increase in fluorescence. Bulk polymerization of methyl methacrylate was carried at 50, 70, and 90 °C in the presence of the fluorescent probe. A correlation between the limiting fluorescence yield and the limiting conversion was found. It is assumed that these results reflect the influence of free volume on the molecular relaxation of the probes.

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