The torsional dynamics of phenyl rings of malachite green molecules in the excited state is studied in polymeric and monomeric glass matrices by measuring the fluorescence decay time as a function of temperature. It is shown that the phenyl rings rotate diffusively in solid polymers (polymethyl methacrylate and polyvinyl alcohol) quite rapidly even at low temperatures. To analyze the experimental results, we used the concept of microviscosity which controls the diffusive rotational motion of phenyl rings of malachite green molecules in solid matrices. By using the reaction-rate theory, we show that a horizontal excited-state potential surface rather than a downhill potential surface for the rotation of phenyl rings can more reasonably explain the rotational motion in polymers. If we assume that the potential is horizontal, the temperature dependence of the microviscosity can be described by Andrade equation with a definite activation energy which is known to be valid for many liquids over a wide range of temperatures. This implies that the microscopic dynamics of small molecular rotations in a solid polymer resembles the behavior in many liquids. By monitoring the fluorescence decay of malachite green molecules doped in ethanol monomeric glass during its phase transition, we show that the effects of phase transition are well represented in the fluorescence decay time. We then propose to use malachite green molecules as sensitive optical microprobes of local dynamics in various solid matrices and their phase transitions, etc.

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A dispersive Raman spectrometer was used with three different excitation sources (Argon-ion, He-Ne, and Diode lasers operating at 514.5 nm, 633 nm, and 782 nm, resp.). The system was employed to a variety of Raman active compounds. Many of the compounds exhibit very strong fluorescence while being excited with a laser emitting at UV-VIS region, hereby imposing severe limitation to the detection efficiency of the particular Raman system. The Raman system with variable excitation laser sources provided us with a desired flexibility toward the suppression of unwanted fluorescence signal. With this Raman system, we could detect and specify the different vibrational modes of various hazardous organic compounds and some typical dyes (both fluorescent and nonfluorescent). We then compared those results with the ones reported in literature and found the deviation within the range of ±2 cm−1, which indicates reasonable accuracy and usability of the Raman system. Then, the surface enhancement technique of Raman spectrum was employed to the present system. To this end, we used chemically prepared colloidal suspension of silver nanoparticles as substrate and Rhodamine 6G as probe. We could observe significant enhancement of Raman signal from Rhodamine 6G using the colloidal solution of silver nanoparticles the average magnitude of which is estimated to be 103.

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