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with a xenon flash transmitting light of X >a40 nm as long as the Hg lamp was on. The absence of a signal using the 0-52 filter with the lamp on is the basis for this conclusion. (See curve 1 of Figure 2.) Other conceivable species could be impurities, H-, and excited water. Impurities are difficult to exclude since their copcentration is increased with the addition of NaC104. At this stage H- is also a candidate, but this ion may be easily tested by increasing the pH to a point where the steady-state concentration of H atoms is vanishingly small. Since our optical system transmits light well into the first fundamental band of water it is conceivable that a small concentration of a dissociable “excited” water may contribute to this effect. However, the large enhancement by iYa+ indicates that this source of eaq- is unlikely. At the present time we know only that this species absorbs around 300 nm. It is tempting to associate it with the absorbing “monovalent” species produced by eaq- reaction with bivalent ions such as Zn2+, Cd2+,A h 2 + , and Pb2+.4 Each of these [‘monovalent” ions has an intense absorption band centered at 310 nm. We plan to investigate the photodissociation of these ions by our apparatus. If ea,- forms a complex with Na+ a new area of photochemical research opens up encompassing the alkali and alkaline earth ions as well as other unstable reduced complexes.
sibilities of the pure liquids [(Ks‘)i”]. Jacobson2 used an expression similar to eq 2, but with volume fractions instead of mole fractions. Neither of these expressions is correct. The isentropic compressibility of any solution is related to the isothermal compressibility [K,]by the ratio of the heat capacities at constant volume and constant pressure, as in eq 5 . The isothermal compressibility of an ideal solution can easily be shown to equal the volume-fraction (vi) average of the isothermal compressibilities of the pure liquids, and the heat capacity to equal the mole-fraction average oE the heat capacities of the pure liquids. For an ideal binary solution, the isentropic compressibility is correctly represented by eq 6. Values of Cy”for the pure liquids can be calculated from C,”, Kt”, the molar volume, and the coeffi-
cient of thermal expansion [a = ( b In V/bT),], using eq 7, as suggested by Lewis and Randall.3 Since accurate values of Kto are not available for all liquids, and since there is likely to be some discrepancy between values of K,’ calculated from sound velocity experiments and those calculated from eq 5 and 7, we suggest (4) G. E. Adams, J. H . Baxendale, and J. W. Boag, Proc. Chem. SOC., that the value of Kto to be used for calculations of the 241 (1963). excess isentropic compressibilities based on sound velocBHABHA ATOMIC RESEARCH CENTRE C. GOPINATHAN ity measurements might best be calculated from K,’ for TROMBAY, BOMBAY, INDIA the pure liquid. ARGONNENATIONAL LABORATORY E. J. HART I n Figure 1, we show the excess isentropic compressiARGONNE,ILLINOIS K. H. SCHMIDT bilities calculated from eq 1 and 6 for the systems methanol-water, ethanol-water, n-propanol-water, tertJuly 20, 1970 RECEIVED butanol-water, and acetone-water. On each of these plots we indicate the composition at which the ultrasonic absorption is a maximum (PSAC).4 Blandamer and Waddington attempted to show a correlation beIsentropic Compressibility of Ideal Solutions tween the PSAC and the position of zero excess isentropic compressibility in terms of the real and imagiSir: I n a recent publication,’ Blandamer and Waddnary components of the total isentropic compressibility. ington defined the excess isentropic compressibility A much more substantial correlation between the PSAC [(KB’)E]of a liquid mixture as the difference between and the position of the minimum excess isentropic comthe observed isentropic compressibility [K,’ ] and that of pressibility can be seen in our plots using the correct an ideal solution [(K,’)id] as in eq 1. The isentropic expression for the ideal solution term. A striking compressibility of an ideal solution was represented as failure of this correlation is the absence of a PSAC for the mole fraction (Xi) average of the isentropic compresthe system methanol-water. (1) M. J. Blandamer and D. Waddington, J . Phys. Chem., 74, 2569 (1970). (2) B . Jacobson, A r k . Kemi,2, 177 (1950).
K,’ = - ( b In V/bP),
(3)
(3) G. N. Lewis and M. Randall (revised by K . S. Pitzer and L . Brewer), “Thermodynamics,”2nd ed, McGraw-Hill, New York, N. Y ., 1961, p 107. (4) C. J. Burton, J. Acoust. SOC.Amer., 20, 186 (1948).
The Journal of Physical Chemistry, Vol. 743N o . 28,1970
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tissues. Dye aggregation, rather than interaction between the ionic site and the dye, has been shown to be the major driving force in such interactions.2 Changes in the fluorescence properties of the dye have also been noted, but this aspect has received little detailed study. A limitation of the dye-binding technique in quantitative study is the tendency of the emission to fade rapidlyra I n emission studies attention has been mainly devoted to DNA-acridine orange complexes, in an attempt to understand the powerful mutagenic activity of acridine dyes.4 Little is known about the mechanism of fluorescence quenching of cationic dyes by polyanions, and this is the objective of our investigation. When acridine orange is added to dilute aqueous solutions of dextran sulfate or heparin the absorption and emission spectra depend on the ratio P / D where P is the number of available anionic sites and D is the number of dye molecules in solution. When P / D 1 the dye has an absorption spectrum very similar to the aggregated dye in polymer free solution. When P I D >> 1 the absorption spectrum is similar to that of the free monomeric dye. Unlike the spectral shift t o the blue observed in absorption, increase of acridine orange concentration results in the buildup of a new emission band at longer wavelengths arising from excited dimer molecules. There is no detectable emission from excited states of larger aggregates. The main features of both the emission and absorption spectra can be explained by the theory of exciton splitting.6v6 An upper singlet state of a monomer splits on N-fold aggregation into an N-fold band of levels with a band width depending on the intensity of monomer absorption as well as the relative orientations and separation of the dye molecules. The behavior of acridine orange molecules on interaction with the polyanions mentioned above is consistent with this theory and with the dye molecules forming laminar or card-pack aggregates. Apart from the strong exciton coupling which results in the blue shift in absorption, weak coupling is to be expected and should lead to rapid excitation energy transfer effects. Experiments have been carried out on the quenching of fluorescence from acridine orange bound to polyanions by very low concentrations (lo-* t o 10-71!f) of methylene blue or thionine. I n Figure 1 the SternVolmer plots for the quenching of acridine orange on dextran sulfate fluorescence are shown. F is the
, -
- 1
- 2
- 3
Figure 1. Excess isentropic compressibilities for the systems: (a) tert-butyl alcohol-water a t 27”. Data are scaled from graph in ref 4. Density data are extrapolated from the data of A. Doroshevski, J . Russ. Phys. Chem. Soe., 43,66 (1911); listed in J. Timmermans, “Physico-chemical Constants of Binary Systems in Concentrated Solutions,” Vol. 4,Interscience, New York, N. Y . 1960; (b) acetone-water a t 20’ (ref 2); (c) n-propyl alcohol-water a t 20’ (ref 2); (d) ethanol-water at 20” (ref 2); (e) methanol-water at 20” (ref 2). Thevertical hash marks on the curves indicate the composition at which the maximum ultrasonic absorption occurs (PSAC, ref 4).
(Dr. Blandamer has indicated to the editors that he concurs with the authors’ comments on ref 1 of this communication.)
* To whom correspondenceshould be addressed. DEPARTMENT O F CHEMISTRY
UNIVERSITY OF MISSOURI, ROLLA, MISSOURI65401 ROLLA,
GARY L. BERTRAND* LARRY E. SMITH
RECEIVED JUNE 29, 1970
Electronic Excitation Energy Transfer in Dye-Polyanion Complexes
Sir: The shift in the absorption spectra of cationic dyes upon interaction with polyanions (metachromasia) is the basis of a wide range of cytological and histochemical methods for identifying these materials in cells and The Journal of Physical Chemistry, Vol. 74, No. 29, 1970
(1) For an evaluation see J . W. Kelly, Acta Histochcm., Suppl., IS, 66 (1958). (2) J. S. Moore, G. 0. Phillips, D. M. Power, and J. V . Davies, J . Chem. SOC.A , 1155 (1970). (3) P. J. Stoward, “Luminescence in Chemistry,” E. J. Bowen, Ed, Van Nostrand, London, 1968, pp 222. (4) 1. Isenberg, It. B. Leslie, S. L. Baird, R. Rosenbluth, and R. Bersohn, Proc. Nat. Acad. Sci. U.S., 52,374 (1964). (5) A. S. Davydov, “Theory of Molecular Excitons (trans. by M . Kasha and M. Oppenheimer),” MoGraw-Hill, New YorB, N. Y., 1962. (6) M. Kash:b, Radiat. Res., 20,55, (1963).
