350
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The Journal of Physical Chemistry, Vol. 83, No. 3, 7979
discussed in detail previously,' these compressibility deviations are believed to arise because the adiabatic pressure fluctuations of the ultrasound are associated with small entropy changes under the experimental conditions. This means that the thermodynamic constraint of isentropic conditions for eq 1 does not hold. In other words, the maintenance of equilibrium is not achieved completely in the ultrasonic measurements. The relationship of @@-, , to the natural entropy fluctuations of water (section IV) further supports this conclusion. In addition, the closer correspondence of 0,to the hypersonic compressibilities (Figure 3) rather than to the ultrasonic ones follows naturally, if it can be assumed that isentropic conditions are more likely to be achieved for the scattering measurements. This seems probable as ultrasonic velocity measurements involve driven sound waves so that gradients in pressure are imposed on the system. In contrast, no gradients are imposed on the system in light-scattering experiments. This makes it easier to maintain thermal equilibrium during the experiment. The assumption that equilibrium is maintained during the ultrasonic measurements is, after all, only an assumption and has not been verified directly by experiment. On the other hand, it might be argued that no direct experimental evidence has yet disproved this assumption. Nevertheless, when all of the indirect evidence is considered (that presented here and that presented in ref 11, the probability that equilibrium is not maintained during the ultrasonic measurements seems the most plausible conclusion. If this were not so, some peculiarity related to the structure of water must introduce an error when the compressibilities are measured directly, even though the techniques differ widely. This does not seem likely and would be hard to reconcile with the thermal energy results mentioned above. In addition, the close correspondence of the zero and hyperfrequency velocities surely cannot be coincidental. Further measurements via light-scattering techniques over the temperature range 0-100 " C may help clarify the true situation, especially if the experimental errors can be reduced to 1ms-l or less and it can be shown t h a t the experimental conditions ensure isentropic propagations.
References and Notes J. V. Leyendekkers, "Thermodynamics of Seawater as a Multicomponent Electrolyte Solution", Part 1, Marcel Dekker, New York, N.Y., 1976. 6.R. Lentz, A. T. Hagler, and H. A. Scheraga, J . Phys. Chem., 78,
1531 (1974).
I. L. Fabelinski, "Molecular Scattering of Light", Plenum Press, New York, N.Y., 1968. B. J. Berne and R. Pecora, "Dynamic Light Scattering", Wiley, New York, N.Y. 1976. P. Lallemand, "Photon Correlation and Light Beating Spectroscopy", H. Z. Cummins and E. R. Pike, Eds., Plenum Press, New York, N.Y.,
1974. J. Frenkel, "Kinetic Theory of Liquids", Oxford Unbersity Press, London,
1946. R. D. Mountain, Rev. Mod. Phys., 38, 205 (1966). R. D. Mountain and J. M. Deutch, J. Chem. Phys., 50, 1103 (1969). L. Landau and 0. Placzek, Phys. 2. Sowletunion, 5, 172 (1934). G. A. Miller, J . Phys. Chem., 71, 2305 (1967). J. Rouch, C. C. Lai, and S.H. Chen, J. Chem. Phys., 65,4016(1976). C. L. O'Connor and J. P. Schlupf, J . Chem. Phys., 47, 31 (1967). L. W. TiRon and J. K. Taylor, J. Res. Natl. Bur. Stand., 20, 419 (1938). H. Eisenberg, J . Chem. Phys., 43, 3887 (1965). E. Reisler and H. Eisenberg, J . Chem. Phys., 43, 3875 (1965). "Handbook of Chemistry and Physics," Chemical Rubber Publishing Co., Cleveland, Ohio, 1971-72. R. Vedam and G. Holton, J. Acoust. SOC.Am., 43, 108 (1968). N. E. Dorsey, "Properties of Ordinary Water-Substance", Reinhold, New York, N.Y., 1940. V. W. Ekman, Pub/. Circonst. Perm. Expior. Mer., 43, l(1908). E. W. Amagat, Ann. Chim. Phys., 68 (1893). N. Low, J. Phys. (Paris) Colloq. C7, 33, 1 (1972). W. L. Ginzburg, Dokl. Akad. Nauk SSSR, 36, 8 (1942). S.H. Chen, C. C. Lai, and J. Rouch, J. Chem. Phys., 67, 5080 (1977). R. D. Mountain and T. A. Litovitz, J . Acoust. Sac. Am., 42, 516
(1967). J. J. Markham, T. T. Beyer, and R. 6. Lindsay, Rev. Mod. Phys., 23, 353 (1951). A. 6.Bhatia and E. Tong, Can. J. Phys., 47, 361 (1969). J. Rouch, C. C. Lai, and S.H. Chen, J. Chem. Phys., 66, 5031 (1977). S. H. Chen has kindly supplied the original experimental data which are only presented in graphical form in ref 1 1 . V. A. Del Grosso and C. W. Mader, J . Acoust. SOC.Am. 52, 1442
(1972). M. Diaz Pena and M. L. McGlashan, Trans. Faraday SOC.55,2018
(1959). R. J. Speedy and C. A. Angell, J . Chem. Phys., 65, 851 (1976). A. Bradshaw and K. Schleicher, Deep-sea Res. Oceanogr. Abstr., 23. 583 (1976). G . 8 . Kell and E. Whalley, Philos. Trans. R. SOC.London Ser. A , 258, 565 (1965). Y.-H. Li, J. Geophys. Res., 72, 2665 (1967).
Thermochromic and Hyperchromic Effects in Rhodamine B Solutions I. Rosenthal,+* P. Peretz, Israel Atomic Energy Commission, Nuclear Research Center-Negev, Beer-Sheva, Israel
and K. A. Muszkat" Department of Structural Chemistry, The Weizmann Institute of Science, Rehovot, Israel (Received March IO, 7978; Revised Manuscript Received July 25, 1978)
The equilibrium between the colored zwit,terion and the colorless lactone forms of rhodamine B depends strongly on solvent, temperature, and concentration. Protic solvents, high concentrations, and low temperatures shift the zwitterion-lactone equilibrium toward the zwitterion form. hydrochloride) as a lasing dye1 has promoted a vivid inThe widespread use of rhodamine B.HC1 (3-diethylimino-6-diethylamino-9-a-carboxyphenyl-3-isoxanthene terest in the photochemical properties of this compound. The existence of two chemical isomers of rhodamine B (RBI was already mentioned in the early reports on this 'Also at the Department of Organic Chemistry, The Weizmann c ~ m p o u n d . Thus ~ , ~ the intensely colored form observed Institute of Science, Rehovot, Israel. 0022-3654/79/2083-0350$01 .OO/O
0 1979 American
Chemical Society
Zwitterion-Lactone Rhodamine B Equilibrium
in water or alcohol was described as a zwitterion (A), while
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979 351
TABLE I: Position of A 2 B Equilibrium in RB (10-5M) as Function of Solvent (at 293 K )
-
solute RB.HC1
4
the inner lactone (Is) structure has been attributed to the colorless form obtained in nonpolar solvent^.^-^ In the present study we have reexamined the conditions which control the existence of a certain isomer in solutions of RB, and we wish to draw attention to the labile equilibrium A $ B which gives rise to striking thermochromic and hyperchromic effects. Under certain circumstances such effects might influence the efficiency of RB in some of its better known applications such as in dye lasers and in fluorescence studies as a quantum counter.
Experimental Section Rhodamine B-HC1 (Eastman-Kodak Co.) was purified by a procedure similar to that described for rhodamine 6G.7 The Pactone of rhodamine B was prepared as described.6 Spectrophotometric measurements were performed on a Cary 14 spectrometer equipped for low temperature measurements.* The optical densities at low temperatures were corrected to account for solvent contraction. 'This correction amounts to a 10% volume contraction per 100 "C cooling. Results and Discussion B Equilibrium. (a) Influence of Solvent on the A Using straightforward spectrometric absorption measurements we examined the role played by the solvent in determining the position of the equilibrium A e B, starting with either pure RB-HC1or pure RB lactone. The results obtained are summarized in Table I. These show clearly that at room temperature the aprotic character of the solvent and not its polarity is the decisive factor determining the position of the A B equilibrium. Thus dilute solutions M) of RB.HC1 in highly polar solvents such as dimethyl sulfoxide, dimethylformamide, or hexamet hylphosphoric triamide are entirely colorless, indicating complete conversion to the lactone. Other less polar but still aprotic solvents, such as acetonitrile or methylene chloride, exhibit hypochromic effects as compared to aqueous or alcoholic solutions of equal concentrations of RB-HCl. That is, in these solvents only a partial conversion to the lactone occurred, and both forms A and B are in equilibrium. Assuming that in a highly protic solvent such as glacial acetic acid in the zwitterion A was present exclusively, the difference between the optical density a t 550 nm (where only A absorbs) of solutions of identical concentrations in acetic acid and any other solvent reflects the proportion of the colorless form B present. The conversion of B to A is sensitive to the presence of hydroxyl groups to such an extent that a white piece of cellulose turns red by contact with a colorless solution of lactone. (b) Effect of Temperature on the A + B Equilibrium. Temperature was found to be another factor determining the position of equilibrium A F! B. Thus a hyperchromic effect is easily observed by cooling solutions of RB.HC1 or lactone, indicating a shift of the equilibrium toward the zwitterionic form. In methylene chloride the hyperchromic M solution effect is especially pronounced. Cooling a of the lactone from +20 to -110 "C (methylene chloride can be readily overcooled below its melting point of -95.1
RB.1acton.e
-
%A
OD,,,a
%A
0.94 0.83 0.77 0.84
89.2 81 89
0.93 0.74 0.74 0.83
0.59 0.66
63.5 70.5
0.03
protic solveints acetic acid methanol ethanol water aprotic solvents* acetonitrile methylene chloride
100
100 79 79 90
0
0 3
In dimethyl sulfoxide, dimethyla 1-cm opticaH path. formamide, dioxan, pyridine, hexamethylphosphoramide, and methyltetrahydrofuran OD,,, = 0 (e.g., equilibrium is shifted completely t o lactone side). TABLE 11: Equilibrium Constant K ( K = [A]/[B]) in CH,Cl, as a Function of TemDerature and Concentration RB.HC1 temp, "C
RB.lactone
1.04 X
1.04 X
1.13 X
1.13 X
10-5M -
10-4M
10-5 M
10-4~
0.56 0.83 0.99 1.68 5 22
0.011 0.021 0.067 0.32 0.86 1.73 2.02
0.011 0.023
I
t 20
-20 -40 -60 -80 -100
-110
0.31 0.40 0.50 0.75 1..48 2.83 3.35
ma
1.03 2.57 3.49
a Complete conversion t o A is assumed under these conditions.
"C) (Figure la,b) results in a ca. 60-fold increase in the optical density at the first absorption maximum of zwitterion A at 550 nm (ODs5& This increase corresponds to a 184-fold increase in the equilibrium constant K ( K = [A]/[B]) from I( = 0.011 at +20 "C to K = 2.02 at, -110 "C. In the case of RB-HC1 M) the same temperature change results in a 3.15-fold increase in ODbb0,corresponding to an 11-fold increase in K from K = 0.31 at +20 "C to K = 3.35 at -110 "C. The experimental results are summarized in Table 11. The higher values of the equilibrium constants obtained in RB-HC1 solutions (compared to isolutions of the lactone, under the same conditions of temperature and concentration) are most probably due ito the inherent higher ionic character of RBeHCl. Similar, though smaller effects of cooling are observed in all the other solvents of Table I. Thus, cooling a M) results in methanol solution of the lactone (1.1X a further increase in the proportion of A. The equilibrium constant in thiis sytem increases from K = 3 at -20 "C to K = 11 at -90 In 2-methyltetrahydrofuran, at room temperature, the A + B equilibrium leans heavily toward B. Cooling, hfowever, acts in the same direction as in CHzClzand in CH,OH, K increasing from 3 X lo-, at +20 "C to 0.14 at -150 "C (Table 111, Figure ld,e). Figure If illustrates an additional effect taking place in M solution of the lactone in the nonpolar and a2X aprotic solvent mixture methylcyclohexane-isohexane (1:l). Cooling from -80 to -100 "C produces a steep absorption increase which is further enhanced by cooling to -180 "C. The four component band system obtained is typical of the aggregation dimer of RB.9J0 Thus i n this system, in addition to the A e B process one needs also to consider the 2RB + (RB), equilibrium which at low temperatures favors the formation of the dimer (RE"),. It was also noted that hardly any hyperchromic effect oc-
352
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979
II
1.01
I. Rosenthal, P. Peretz, and K. A. Muszkat
-1
1 550
600
650 h m
X nm
X,nm
>.
65 0 2 n
0
201 F
a
0
X
X ,nm
,nm
400
450
500
550
600
A nrr
Figure 1. Visible region absorption of RB solutions. (a) RB lactone (1.13 X M) in methylene chloride in a 10-mm cell, at following temperatures: 1, +20 OC; 2, -20 OC; 3, -40 OC; 4, -60 OC; 5, -80 O C ; 6, -110 OC. (b) RB-HCI (1.04 X M) in methylene chloride in a 1-mm cell. Temperatures as in (a). (c) RBsHCI in methylene chloride at $20 OC: 1, 5.2 X M in 0.2-mm cell; 2, 1.04 X M in 1-mm cell; 3, 1.04 X M in 10-mm cell; 4, 1.04 X lo-' M in 100-mm cell. (d) RBqHCI, 3.2 X M, in 2-methyltetrahydrofuranin a 10-mm cell at the following temperatures: 1, +20 OC; 2, -20 OC; 3, -40 OC; 4, -60 OC; 5, -80 OC; 6, -100 OC. (e) RB-HCI, 1.3 X M, in 2-methyltetrahydrofuran, in 1-mm cell. The temperatures are 1, +20 OC; 2, -20 OC; 3, -40 OC; 4, -80 OC; 5, -100 O C . (f) RB.lactone, 2 X M, in methylcyclohexane-isohexane 1:l mixture, 10-mm light path. The temperatures are 1, +20 OC; 2, -20 OC; 3, -40 OC; 4, -80 OC; 5, -100 OC; 6, -120 OC; 7, -180 OC.
TABLE 111: Equilibrium Constant K ( K = [ A]/[B]) in Methanol and Methyltetrahydrofuran as a Function of Solvent and Temperature temp, "C
methanola
+ 20
3.07 4.30 5.73 7.75 9.93 11.5
-20 -40 -60 -80 -90 -100 -120
methyltetrahydrofuranb 3 x lo-* 4.9 x lo-' 6.2 X 7.8 X lo-' 9.2 X 10.' 11.1 x lo-' 12.5 X 10" 14.3 X lo-'
-150
RB.lactone ( 1 . 1 3 X l O - ' M ) .
b
RB.HC1 (3.3 X
MI.
curred in solutions of the lactone in toluene cooled to -90 "C, Le., slightly above the melting point of this solvent. In this case the required low temperature region for an appreciable shift of B A is not attained above the melting point. However the same solution frozen at liquid nitrogen temperature became strongly red colored. The enthalpy changes for the process B e A for solutions of RB-lactone and RB-HC1 are listed in Table IV. The negative values of the enthalpy changes indicate that
TABLE IV: Enthalpy Changes for the Process B --f A (kcal/mol) solvent methylene chloride methylene chloride 2-methyltetrahydrofuran
RB.HC1 RB.lactone -2.0 -4.1 -0.65
-3.1 -2.5
concn, M 10-5
10-4 3 x 10-5
in these systems the zwitterionic form is the more stable. (c) Effect of Solute Concentration on the A e B Equilibrium. Solutions of RB-HC1 show a clear cut concentration dependent hyperchromic effect. Figure IC shows the dependence of OD550 on concentration of solutions of RB.HC1 in CH,C12. The product optical pathlength X concentration (IC) was kept constant in all four spectra. Thus going from a lo4 M solution (1 = 100 mm) to a 5 X lo4 M solution ( 1 = 0.2mm), OD550 increases from 0.2 to 1.1. This corresponds to a 16-fold increase in K , from 0.14 to 2.2. This effect is also evident at other temperatures (Table 11). The hyperchromic effect of solute concentration can be explained by the intramolecular stabilization of the zwitterion A, in a manner similar to the stabilization gained by hydrogen bonding in protic solvents.
The Journal of Physical Chemistry, Vol. 83, No. 3, 7979 353
Silanization Effects on TiO, Electrodes
The present discussion of the factors which affect the A ~ t B. equilibrium provides a straightforward explanation of several little-understood occasional observations on RB solutions reported in literature. Thus the absorption of RB in ethanolg or water1',12 decreases on heating. Conversely a large hyperchromic shift is shown at low temperatures in EPA or alcohol." In the former solvent the absorption is strongly dependent on con~entration.~ Finally the output power from a flashlamp-pumped RB dye laser is increased as the temperature of the dye solution is reduced, It was also noted that the lasing ability of rhodamine 6 G (whose carboxyl group is blocked by esterification and thus unable to lactonize) was unaffected by co01ing.l~Since only the zwitterion can serve as a lasing medium, this result is at least qualitatively not unexpected in light of our results. Preliminary results obtained with other free carboxyl-rhodamine dyes, e.g., rhodamine 110 and rhodamine 19, indicate a behavior similar to that of rhodamine B. Indeed
we suggest that lactonezwitterion equilibria play a general role in both phlotochemistry and photophysics of other xanthene dyes substituted a t the 9 position with a phenyl-2-carboxylic acid group. References and Notes (1) K. H. Drexhage in "Dye Lasers", E. P. Schaffer, Ed., Springer, Berlin, 1973,Chapter 4. (2) E. Noelting and K. Dziewonski, Chem. Ber., 38, 3516 (1905). (3) D. Deutsch, 2 . fbys. Chem., 138,353 (1928). (4) R. W. Ramette and E. B. Sandell, J. Am. Cbem. SOC.,78, 4872 (1956). (5) H. P. Lundgreri and C. H. Binkley, J. folym. Sci., 14, 139 (1954). (6) U. K. A. Klein and F. W. Hafner, Cbem. fbys. Left., 43, 141 (1976). (7) I. Rosenthal, Opt. Commun., 24, 164 (1978). (8) See, e.g., E. Fischer, Mol. Pbotocbem., 2 , 99 (1970). (9) J. E. Selwvn and J. I. Steinfeld. J . fhvs. Cbem., 76, 762 (1972).
(fOi
R. W. Chimbers, T. Kajiwara, and D. R. Kearns, J . Pbys. &em:, 78, 380 (1974). (11) W. E. Speas, f b y s . Rev., 31, 569 (1928). (12) L. V. Levshin and V. K. Gorshkov. ODt. Spectrosc., 10,401 (1960). (l3j R. B. Huth, G. I . Farmer, and M. R. Kagan, J. Appl. Phys., 40, 5145
(1969).
Chemically Modified Electrodes, 12. Effects of Silanization on Titanium Dioxide Electrodes Harry 0. Finklea and Royce W. Murray* Kenan Laboratories of Cbemistty, University of Nortb Carolina, Chapel Hill, North Carolina 275 14 (Received August 25, 1978) Publication costs assisted by the Office of Naval Research
The binding of a monolayer or less of organosilane to the surface of a TiOzelectrode has little discernable effect on semiconductor properties such as flat-band potential, apparent doping level, hole oxidation of water, and reduction processes via a surface state. The attached silane is stable toward hole oxidation. The results indicate the presence of unreacted Ti-OH groups on the silanized surface.
properties. Kuwana et a1.26and Hawn and A r m ~ t r o n g ~ ~ Titanium dioxide is a wide band-gap semiconductor which has received much recent attention1-13 due to its observed a change in the slope of a Mott-Schottky (MS) plot upon silanization of SnOz, indicating a lower doping stability as a photoelectrode.' Holes generated in the level. In accord with these results, Srinivasan and Lamb2* valence band by illumination are capable of oxidizing noted a decrease in Sn02thin film conductance when the water2 or solutes within the electrolyte3s4without lattice dissolution. Both processes have connotations in energy surface was silanized. However, a later p u b l i ~ a t i o non ~~ conversion technology. More recently, unusual photothe same system revealed no change in the MS slope. The catalyzed oxidations and reductions have been reported highly doped Sn02 electrodes make separation of solution using T i 0 2 p ~ w d e r s . l ~A- further ~~ advantage with TiOz double layer capacitance variations from electrode space lies in the ease of fabrication of polycrystalline electrodes charge capacitance changes difficult. Sensitization phoby chemical vapor d e p ~ s i t i o n . ~ tocurrents have been observed3b32for silane-bound dyes Chemical modification of semiconductor electrode on S n 0 2and Tic&, and one very interesting report exists surfaces has the same attractions of covalent attachment for a derivatized semiconductor (n-Si a silylferrocene) of selected molecular species to the surfaces of electrodes irradiated with bandgap light.33 with metal-like behavior, e.g., the possibility of tailoring We report herein results of a study of effects 02 silanelectrode properties in a predictive manner. Ti02, like ization on such TiOZ semiconductor properties as phometal oxides such as Pt/Pt0,21 Sn02,22and R u O ~ , ~ ~tocurrent generation under bandgap ilhjmination, flatpossesses surface hydroxyl groups24capable of reacting band potential, interfacial capacitance, and electron with an organosilane reagent: transfer from the conduction band to a solution species via a surface state. TiO,--OH + XSiR, Ti0,i-OSiR, + HX
+
+
.X = C1, OMe, OEt
An electroactive site can then be assembled on the immobilized reactive functionalities of the organosilane such as amine or p ~ r i d y l . ~ ~ Little work has been done so far to characterize the effects of surface silanization on semiconductor electrode 0022-3654/79/2083-0353$01 .OO/O
Experimental Section Titanium dioxide single crystal electrodes were fabricated from a boule (N. L. Industries, Inc.) by slicing wafers approximately 1mm thick perpendicular 1.0 the c axis, and cutting disks 6 min in diameter from the slices. The 6-imm diameter allowed insertion of the disk into the E X A
0 1979 American
Chemical Society
