Spectroscopic and Thermodynamic Study on the Aggregation of Rhodamines in Solution Solvent Effect I. Urrecha Aguirresacona, F. L6pez Arbeloa, and I. Lopez Arbeloal Universidad del Pais Vasco-E.H.U.. Apartado 644, 48080-Bilbao. Spain
The students' interest in laboratory work can he stimulated by their performing experiments related to the topics covered in the lecture. The study of optical properties (absorption and emission) of rhodamines and their molecular aggregation is an important research area because these substances are often used as active media of dye lasers (1,Z). The molecular structure of rhodamiue dyes is characterized by an aromatic planar group with three rings, the xanthene chromonhore. res~onsihlefor their visible absorntion and visible e&issiodbaids. This chromophore is substituted a t the central 9-position carbon by a o-benzoic acid derivative that is sterically constrained to be roughly perpendicular to the xanthene skeleton. Figure 1shows the molecular structure for the specific case of rhodamine 6G. When two or more rhodamine molecules in solution are Figure 1. Molecular structure of rhDdamine 6G separated by a short enough intermolecular distance, their xanthene rings can interact electrostatically to form aggregates suchas dimers, trimers, etc. (3).The formation of these aggregates depends on many factors such as the molecular structure of the dve. . . concentration..OH of the medium., solvent nature, and temperature. Owing to the inter-xanthene interaction. the visible ahsorption spectrum of rhodamines in the monomeric form is -... ..-. G different to that ofthe anerenates -- - ( 3 . 4 ) . This interactioncan monomer d imer mmomer be treated by the exciton theory (5,6);a quantum mechanic method, giving for the dimer a splitting of the monomer excited states- he corresponding ener& diagram for the Figure 2. Energy dlagam of the groucd (GI and excited (E) states of the dimer excited states, shown in Figure 2, is based on the monomer and the dimer. molecular orbital theory, so well known to physical chemistry students. So, two absorption bands, one ar lower and the moderate concentration range used in this work (4,7),avoidanother one a t hieher enerw than the monomer one. are ~,~~~~ ing many complicated problems for the students. expected for the dimer absorption spectrum. Since the This undergraduate experimental proced&e also provides monomer and the dimer form of rhodamines present differthe students with agood way toresearch the thermodynamic ent absorption properties, the visible absorption technique parameters of the aggregation process, permitting the disis the standard one to follow the rhodamines dimerization cussion of the association forces. process. Evidently, the characteristic of the hands splitting depends stronglv on the geometric disposition of the monomdric units in the aggregate. ~dvancedstudentscould apply the exciton theory (5,6) to study the geometric structure of Materials the dimer from its absorption spectral characteristics. Rhodamine 6G Rhodamine 6G, R6G (Fig. I), has been chosen for this -Ethanol (dried) and water (hidistilled) laboratory experiment sinre an important dependence of its Visible absorption spectrophotometer dimer formation on thedyeconcentration, temperature,and Rectangular cells solvent ii nhaerved. On the other hand, the spectral rhararA 4 X lo-' M aqueous stock solution of R6G is recommended for this e~periment.~ By diluting the stock solution adequately the teristics of R6G are not affected by the pH of the medium working dye concentrations, i.e., 1 X 5X 2 X 10-5 1 x since the lack of molecular forms (abnenre of acidic erouos) .. and 2.5 X 10WM. are obtained. These concentrations (C)have implies the absence of more than one kind of monomeric been chosen in order to use optical-path rectangular cells (1) of 0.1, unit. Besides, the dimer is the only aggregate formed a t the 0.2,0.5,1.0, and 4.0 em, respectively. In this way, the absorbance of the samples are related to the molar absorptivity since the product Cl remains constant in the concentration range. Far the ethanolie system a diluted and a concentrated (-1 x lo-& M) solution are Author to whom corresoondence should be addressed. enough. R6G Merckx (pro-analysis)was used in this work. Carefulness A simple single-beam spectrophotometer that accommodatesdifshould be suggested to students because the dye can stain clothes. ferent rectangular cell sizes could he used to record the visible hands, glassware, and analytical balances. absorption spectrum,although adouble-beamspectrophotometer is
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866
Journal of Chemical Education
and the corresponding equilihrium constant, K d , is given by
where r is the monomer molar fraction a t a concentration C. Taking into account the Lamhert-Beer law and the expression of Kd in eq 1,the absorbance, A, and the molar ahsorptivitv. c. of the solution a t a waveleneth are eiven hv. eas 2 and 5; rkspectively.
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Figure 3. Visible absaption specbum of rhodamine 6G in water at diflerem concelmations: 1.0 X 10-'(a), 5.0 X 10-5(b).2.0 X to-$ (c). 1.0 X (d). and 2.5 X 10-6 M (e) at rwm temperature.
recommended. In order to determine the thermodynamic parameters of the dimerization the absorption spectrum of at least a concentrated aoueous solution.. i.e... 1 X M. as a function of the temperature, i.e., 20. 30, 40, and 50 T,ha& m be performed. A ~~~~~~~
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thermostatable cuvette holder is necessary for this purpose. A Shimah W - v i s speetrophommeter (model UV-2401 was used in the present work. The exact determination of the sample absorhances at 526 nm (the monomer absorption maximum) and at 500 nm (the dimer absorption maximum) is recommended to minimize the error in the calculation of the dimer equilihrium constant Results and Data Analysis Equilibrium Constant and Absorption Spectrum of the Dimer
Two effects are observed in the visible absorption spectrum of R6G in aqueous solution when the dye concentration up to 1X lo-' M (Fig. 3): first, is increased from 2.5 X the molar ahsorptivity a t the monomer absorption maximum (at -526 nm) decreases upon increasing the dye concentration, and second, the molar ahsorptivity a t -500 nm increases. This phenomenum should be attrihuted to the ahsorption of a new species formed a t concentrated solutions. The presence of an isohestic point in the absorption spectrum indicates that the new species is in equilibrium with the monomer and, as in other xanthene dyes (3), it is attrihuted to the dimer form of R6G (4). The dimer is formed to the detriment of the monomer concentration and the absorption spectrum of R6G in aqueous solution tends toward that of the dimer form upon increasing the dye concentration. Conseauentlv. the d m e r molar absorotivitv is smaller than that bf the monomer a t 526 nm, wkereas a t -500 nm the dimer shows a hieher molar absorotivitv. Contrary to these observations, the shape of the absorption soectrum of H6G in ethanolic solution does not depend on the dye concentration in the same concentration range as M)indicating that used in aqueous solutions (UD to 1 X the absence of the R6G dimer in ethanolic solution in the mentioned concentration range. Therefore the aggregation of R6G in solution depends strongly on the solvent used and it is discussed below. The dimerization process can he visualized as
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A
where cd is the molar ahsorptivity of the mononer in the dimer and 1is the optical pathlength of the cell. The ahsorpM) is the tion spectrum a t diluted solutions (-2.5 X monomer spectrum, and consequently c = c , a t this concentration. The knowledee of the monomer molar fraction is necessary for the calcktion of the equilihrium constant and the ahsorotionsoectrumof the dimer..eas . 1and 3. The monomer molar fraction can be approximately determined from the absorption spectrum by considering the ratio of the peak height a t the wavelength a t the most energetic dimer maxmum, A,, a t -500 nm (Fig. 3), over that a t the monomer maximum, Az, a t -526 nm:
For the diluted solution, the parameter R is reduced to Ro = %,,h, (5) An approximative evaluation of r can he simply accomplished by x
= RdR
(6)
where t,, = t d , and (d2 (1 - X )
