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The quenching of rhodamine 101 emission in methanol by triethylamine, N,N-dimethylaniline, diphenylamine and N-methyldiphenylamine was attributed to electron transfer. The values of the rate constants, which range from 108 1010 Ml sl , are in agreement with theoretical predictions of electron transfer. A comparison is made between the redox processes in methanol and on the surface of hydrocarbon latex particles suspended in 1 aqueous solution. The association constants for the equilibria of rhodamine 101 and diphenylamine between the latex and bulk solution are 3.0 x 104 and 1.6 x 104 Ml , respectively. The quenching efficiency of diphenylamine containing 17 to 98 ppm latex in solutions /is 22-72 folds higher than that in the methanolic solution. The fluorescence lifetime of rhodamine 101 is 4 ns in methanol. The fluorescence quantum yield in methanol (¢F = 0.99) and water (¢F = 0.71) were also determined. The implication of these results to photochemical storage reactions is discussed. 2 INTRODUCTION Efficient conversion of light quanta into chemical free energy requires that energy losses of the absorbing system due to luminescence and non-radiative heat production be m;'nimized. In homogeneous solutions, intermolecular electron transfer usually proceeds from the excited triplet state rather than the excited singlet state of a molecule because the lifetime of the latter is short compared with encounter times. Formation of the triplet state, however, entails the loss of the singlet-triplet splitting energy which may be as much as several tens of kcal/mo1e. l In turn, the decrease in excitation energy is associated with a diminution in the oxidizing and reducing power of the state. 2 The time between excitation and encounter can be an important factor in determining the nature and course of a reaction. By decreasing the distance between donor and acceptor molecules the efficiency of reaction may be markedly affected. In photosynthesis, for example, it is presumed that the proximity of the chlorophyll molecule to the primary ~ acceptor permits the excited singlet state rather than the triplet state of chlorophyll to drive .the redox reaction despite the short lifetime of the. former state. 3 Electron 2,4 and energy5 transfer efficiencies in micellar systems can be greatly enhanced relative to ordinary solution reactions. However, micellar assemblies are in constant dynamic equilibrium which is one of the problems in using them. We are thus trying a more stable system consisting of a cross-linked copolymer of styrene-butadiene acrylic acid. Ce11arius and Mauzeral1 6 in modeling the photosynthetic unit observed photosensitized dye reduction via pheophytin a adsorbed on neutral polystyrene colloids. Our current interest in visible light-induced electron-transfer processes has prompted us to examine some rhodamine compounds. The rhodamine dyes 6 have been extensively studied in relation to their lasing properties and the effect of molecular structure and environment have beeo discussed. 7 3 The redox behavior of the triplet state of several rhodamines has been ex~ amined by flash photolysis8 and the cation and anion radical of the dyes characterized. The rhodamines have been used as a spectral sensitizer of wide band gap semiconductors9; the process involves charge transfer from the excited state of the adsorbed dye to the conduction band of the substrate. Anions such as I-, SCN-, B~Cland OHcan reduce the excited t t f h d " 7,10 h"l E +3 "d"·t 11 s a e 0 r 0 amlne w 1 e u can OXl lze 1 • One of the newest members of this family of dyes is rhodamine 101 about which little photochemistry is known. The internal conversion and intersystem crossing efficiency of this dye are nearly zero in alcohol. In fact, rhodamine 101 has the highest fluorescence quantum yield of all the rhodamines. 7 This paper describes our effort tq utilize the excited singlet state of rhodamine 101 to initiate electron abstractions from triethylamine, N,N-dimethylaniline, diphenylamine and N-methyldiphenylamine. Fluorescence quenching rate constants of the rhodamine-amine" systems were measured in methanol and compared with theoretical predictions of Weller 12 , 13, 14 for electron transfer. A comparison is made between the quenching processes in methanol and those on the surface of anionic polymeric latex particles. Interpretation of the quenching data for the rhodamine-amine latex system requires knowledge of the concentration and location of the rhodamine and the quencher on the latex particles. The results of these measurements and the corresponding interpretations form the nucleus of this paper. EXPERIMENTAL Materials. Rhodamine 101 perchlorate (Exciton, laser grade) and diphenylamine (AR grade) were recrystallized from ethanol-water mixtures. N,N-dimethylaniline, N-methyldiphenylamine and triethylamine were AR grade and used without further purification. The water was fres~ly redistilled in Pyrex from an alkaline permanganate solution and the methanol was spectral grade. The latex material (Dow Chemical) was composed of a cross-linked 4 copolymer of 57% styrene, 41% butadiene and 2% acrylic acid. Emulsifier and ionic impurities were removed from an aqueous latex suspension by mixed-bed ion exchange consisting of Bio-Rad AG lX10 and AG 50 WX2 resins. The concentration of latex particles in solution was based on dry weight. A known volume of solution was dried at 60°C in a vacuum oven to a constant weight. Knowing the solution volume, the density of water and the weight of the dried latex, the density of the latex particles was determined to be 1.02 g/cc. At the concentration of' the rhodamine employed in methanol no aggregation takes place as indicated by the lack of deviation from .Beer1s law. The solutions used in fluorescence quenchin.9 studies were not deoxygenated since preliminary results indicated no difference between the emission intensity of argon and air-saturated dye solutions. The absence of the oxygen quenching effect is .cons i stent with the short lifetime of rhodami ne 101 and the concentration of oxygen in an air-saturated solution (~10-3M). Errors derived from self-absorption were insignificant at the concentration of dye employed. No photochemical deterioration of the rhodamine in quencher-free solution was observed spectrophotometrically during the course of the experiments. All fluorescence quenching measurements were carried out at 22°C. Absorption Spectra. Absorption spectra were obtained using 0.5, 1.0 and 10.0 cm path length quartz cuvettes on a Cary 118 spectrophotometer. Fluorescence Spectra and Quantum Yields. Fluorescence spectra for kinetic experiments were recorded on a Hitachi Perkin-Elmer MPF-2A spectrofluorimeter equipped with a red sensitive Hamamatsu R136 photomultiplier tube. Quantum yield measurements wer~ determined on a Perkin-Elmer MPF-3L spectrofluorimeter with a corrected spectrum attachment· Excitation and •

Quenching of Rhodamine 101 emission in methanol and in colloidal suspensions of latex particles