4718
COMMUNICATIONS TO THE EDITOR
, Jeaurrs,
P' Figure 2. A is the experimental spectrum; B is the simulated spectrum.
Figure 1.
computer program written for this purpose, it is clear that the interpretation is incorrect. I n order to obtain an improved interpretation, several deuterated compounds based on the 2,4,5triphenylimidazolyl radical were prepared and their esr spectra recorded. The spectrum of 2,4,5-trideuteriophenylimidazolyl contained five partially resolved lines, corresponding to a nitrogen splitting constant of about 1.63 G. To indicate the relative magnitudes of the different proton splittings in the phenyl rings, a blcLachlan2 calculation was performed using the following parameters: hi+ = 0.5 ~ C C N = 1.0 X 1.2. Molecular models showed that the 4- and 5-phenyl rings were twisted out of coplanarity with the imidazolyl ring. The resonance integral between the rings was set to 0.76p corresponding to an angle of twist of about 41". The spin densities calculated by this method are given in Table I, together with the
(2) A. D. McLachlan, Mol. Phys., 3, 233 (1960).
DEPARTMENT OF CHEMISTRY OF NOTTINGHAM UNIVERSITY ENGLAND NOTTINGHAM, RECEIVEDJULY 8, 1968
M. A. J. WILKS M. R. WILLIS
Effect of Dimer Formation on the
Table I
Spin density Splitting, G
and the fully deuterated material. The deuterium splittings were obtained by reducing the proton splittings by a factor of 6.514 and setting the nuclear spin to 1. The experimental and theoretical plots were in good agreement. The splittings suggest that the paramters used in the preliminary McLachlan calculation are capable of improvement; it is hoped to find parameters which will give a good fit with the spectrum of this compound and with the spectra of substituted compounds.
ortho
meta
0.0427 1.01
- 0,0167 0.38
para
ortho'
meta'
para'
0,045 0,0679 -0,0265 0.072 1.07 1.61 0.63 1.71
Triplet States of Organic Dyes' Sir: Under appropriate conditions most organic dyes will dimerize or form higher aggregates in solution. Although there have been many experimenta12-12 and
calculated splittings assuming a Q of 23.7 G. With the (1) This work was supported by a grant from the U. S. Public Health Service (No. GM 10449) and by an Aerospace Corporation nitrogen splitting constant at 1.44 G, spectra were Advanced Study Grant (R. W. C.). computed for varying Q values. It was then found that (2) (a) Th. Forster and E. Konig, Z. Elektrochem., 61, 344 (1957); the best fit with number, intensity, and separation of (b) L. V. Levshin and V. K. Gorshkov, O p t . Spektrosk., 10, 401 (1961). lines was with a slightly higher proportion of the spin (3) L. A. Ignat'eva, L. V. Levshin, T. D. Osipova, and Y. M. density on the para' proton; the splittings (in gauss) Polukhin, ibid., 13, 219 (1962). are: ortho, 1.45; meta, 0.55; para, 1.52; ortho', 2.33; (4) K. L. Arvan and N. E. Zaitseva, ibid., 11, 38 (1961). meta', 0.92; and para', 2.98. The line shape used was (5) G. P. Gurinovich and T. I. Stelkova, BioJizika, 8 , 229 (1963). 100% gaussian and the line width at half-power for any (6) L. V. Levshin and I. 8. Lonskaya, Opt. Spectrosk., 11, 148 (1961). line was set at 0.15 G. The observed and theoretical (7) L. V. Levshin and E. G. Baranova, ibid., 6 , 31 (1959). spectra are shown in Figure 2. As a further test of the (8) L. V. Levshin and V. G. Bocharov, ibid., 10, 330 (1961). correctness of this interpretation, esr spectra were (9) V. G. Bocharov and L. V. Levshin, Izv. A k a d N a u k SSSR, Ser. calculated for the 2-deuteriophenyl-4,5-diphenylim- F i z . ( B u l l . A c a d . Sci., Phys. Ser.), 27, 591 (1963). idazolyl, the 2-phenyl-4,5-dideuteriophenylimidazolyl, (10) L. V. Levshin, ibid., 29, 1299 (1965). The J o u r n a l of Physical Chemistry
COMMUNICATIONS TO THE EDITOR 77" K MONOMER IO"M
4719 77°K
SPECTRA
I
RHODAMINE B
DIMER
SPECTRA B
10-5y RHODAMINE
03
I
I
10-6M ACRIDINE ORANGE
a.
ACRIDINE ORANGE
IO-5yEOSIN
1 0 - 5 ~EOSIN Y
WAVELENGTH
I O - 5 ~
(%)
Y
WAVELENGTH
(8)
b.
Figure 1. a, Spectral properties of dye monomers a t 77°K; absorption (A), phosphorescence (P), and phosphorescence excitation (PE) spectra; solvents, 1 :1 ethanol:water containing 10 M LiBr. The phosphorescence emission spectra were measured using a n RCA 7265 photomultiplier. b, Spectral properties of dye dimers a t 77°K; absorption (A), phosphorescence (P), and phosphorescence excitation ( P E ) spectra; solvents, aqueous solution with 10 M LiBr (Rhodamine B, acridine Orange), or 10 M LiCl (Eosin Y). For comparison with the direct absorption spectra, the PE spectra have been corrected for the wavelength dependence of the intensity of the exciting light. The phosphorescence emission spectra have not been corrected for the spectral response of the photomultiplier.
t h e o r e t i ~ a l ~ ~ -studies ~' of dimerization effects on We obtained monomer and dimer phosphorescence excited singlet state properties, little is known about the emission spectra for these dyes (see Figure 1) by excieffects of dimerization on the triplet state properties of tation of solutions known to contain primarily monomers dyes. There are several reports of dye phosphoresor dimers. While we chose experimental conditions cence which are attributed to dimer e m i s s i ~ n but , ~ ~ ~ favorable ~~ to the observation of either monomer or the evidence is more suggestive than conclusive. In dimer phosphorescence emission, we felt, for several this communication we present the first unambiguous reasons, that it was necessary to prove that the emission experimental evidence for both monomer and dimer spectra were authentic. First of all these dyes are phosphorescence from three organic dyes, describe the (11) L. V Levshin and D. M. Akbarova, Zh. Prikl. Spektros., 2, conditions used to obtain these emission spectra, and 43 (1965). use these data to obtain information regarding inter(12) L. V. Levshin and D. M. Akbarova, ibid., 3, 326 (1965). molecular interactions in the triplet state. (13) E. G . McRae and M. Kasha, J . Chem. Phys., 28, 721 (1958). The 77°K monomer absorption spectra of the three (14) E. G McRae and M. Kasha, Bulletin 10, Institute of Molecular Biophysics, U. 8. Atomic Energy Commission, July 1963. dyes presented in Figure l a were obtained using dilute (15) M. Kasha, H. R. Rawls, and M. A. El Bayoumi, Bulletin 30, (10-0 to 10-6M) solutions in aqueous 10 M LiCl or Institute of Molecular Biophysics, U. S. Atomic Energy Commission, Sept 1966. LiBr to which ethanol was added in order to suppress (16) J. N. Murre11 and J. Tanaka, Mol. Phys., 7, 363 (1964). dimer formation. The 77°K dimer absorption spectra (17) E. G. McRae, Aust. J . Chem., 14, 329 (1961). shown in Figure l b were obtained using aqueous salt (18) Y . V. Morozov, Biofizika, 8, 388 (1963). solutions without ethanol. Comparison of our low(19) G. S. Levinson, W. T. Simpson, and Curtis, J . Arner. Chern. temperature spectra with previously published and Soc., 79, 4314 (1957). confirms our assignment assigned 300°K spectraZ&,20,21 (20) V. Zanker, 2. Phys. Chem. (Leipzig), 199, 225 (1952). of the low-temperature monomer and dimer spectra. (21) V. Zanker, ibid., 200, 250 (1952). Volume 78, Number 13
December 1968
4720 difficult to purify. Secondly, identification of the emitting species is complicated by the fact that the phosphorescence from one species (monomer or dimer) is generally much brighter than the other. Finally, apparent changes in phosphorescence spectra accompanying changes in the dye concentration or solvent might be due to experimental artifacts (re-absorption effect?). To eliminate these possible complications and to establish the identity of the emitting species, we simply measured the phosphorescence excitation (PE) spectra of optically dilute ~ a m p l e s . ~In~ ,the ~ ~ PE measurements the intensity of phosphorescence is monitored at the peak of the phosphorescence while the wavelength of the exciting light is continuously varied throughout the region in which the sample absorbs. Since the PE spectrum obtained under these conditions is essentially identical with the true absorption spectrum of the phosphorescing species, comparison of the PE spectrum with the known monomer or dimer absorption spectrum serves to identify the phosphorescing species. This comparison is shown in Figure l a and l b and convincingly establishes that the spectra reported here are authentic monomer or dimer phosphorescence spectra. 2 4 From these data it is evident that with Eosin Y and Rhodamine B (i) the shape of the phosphorescence spectrum is the same for both the monomer and dimer, and (ii) the dimer phosphorescence is red shifted only 100-200 relative to the monomer. We conclude from (i) that the intermolecular separation is the same in both the ground and excited triplet states of these dimers (change much less than -0.1 8 judging from studies on pyrene excimersZ6)and from (ii) that the lowering in energy accompanying dimerization is only 300-500 cm-* greater for the triplet state than for the ground state. Conclusion ii also applies to Acridine Orange, but the observed change in the shape of the phosphorescence emission spectrum accompanying dimerization suggests that there is a significant change in the intermolecular separation upon excitation of the Acridine Orange dimers. When PE measurements are combined with measurements of the relative intensities of monomer and dimer phosphorescence, it may also be possible to evaluate the effect of dimerization on various nonradiative processes in these dyes.Ia
COMMUNICATIOI~S TO THE EDITOR Correlation of AH and A S for the Association of the Rare Earth(II1) Ions with Fluoride’
Xir: In the extensive literature on the stabilities of rare earth complexes in solution, the absence of fundamental relationships representing their behavior is disappointing. Moeller, et U Z . , ~ attributed this situation to the concentration of effort on relatively complicated organic and polydentate ligands before the behavior of simple ligands was well understood. An important contribution has recently been made toward alleviating this deficiency by Walker and Choppin,3 who reported stability and calorimetric data on the first stepwise association reactions of the tripositive rare earths cations with fluoride ion. Walker and Choppin pointed out the irregular variation with the atomic number (2) of the AH and A S associated with complex formation. They accounted for the differences in the variation of AH for fluoride complexes and the variation of AH for acetate complexes in terms of the greater amount of covalent character in bonding of the acetate ion to the rare earths. They attributed the relatively large A S values associated with the formation of the fluoride complexes to a “dehydration” of the fluoride ion on complexing. Linear correlations of enthalpy changes with entropy changes were first pointed out in 1938 by Barclay and Butler4 for a number of Vaporization processes. A similar but limited correlation of enthalpy and entropy changes has been made with early data for some ionpair-formation reactions by Duncan and Kepert,6 and more recently Kazakov, et ~ 1 . ~have 6 reported such a correlation for AH and AX* of activation among various reactions involving actinide ions, but no attempts have been reported to test such a correlation for an extended series of cations with a single, simple ligand for which wide variations in AH and A S exist. We wish to point out the very striking linear correlation which exists between AH and A S for the rare earth-fluoride complexes, LnF2+ (Figure 1). Over the relatively wide ranges of enthalpy and entropy changes involved, the following expression represents the A S and AH values of Walker and Choppin within experimental error
*
(1) Research sponsored by the U. 9. Atomic Energy Commission under contract with the Union Carbide Corp. (2) T. Moeller, D. F. Martin, L. C. Thompson, R. Ferrus, G. R. Feistel, and W. J. Randall, Chem. Rev., 65, 1 (1965). (3) J. B. Walker and G. R. Choppin in “Lanthanidelhctinide Chemistry,” P, R. Fields and T. Moeller, Ed., Advances in Chemistry Series, No. 71, American Chemical Society, Washington, D. C., 1967, pp 127-138. (4) I. M. Barclay and J. A. V. Butler, TTan8. Faraday Soc., 34, 1445 (1938). DEP.4RTMENT O F CHEMISTRY RICHARD W. CHAMBERS ( 5 ) J. F. Duncan and D. L. Kepert in “The Structure of ElectroUNIVERSITY OF CALIFORNIA DAVIDR. KEARNS lytic Solutions,” W. J. Hamer, Ed., John Wiley and Sons, Inc., New York, N. Y., 1959, pp 380-400. RIVERSIDE, CALIFORNIA92502 (6) V. P. Kazakov, B. I. Peshchevitskii, and A. M. Erenburg, RECEIVED AUGUST16, 1968 Radiokhimiya, 6, 291 (1964).
(22) A. Marchetti and D. R. Kearns, J . Amer. Chem. Soc., 89, 768 (1967). (23) D. R. Kearns and W. A. Case, ibid., 88, 5087 (1966). (24) The “extra” long wavelength peaks in the dimer P E spectra of Acridine Orange and Eosin Y (5000 and 6100 b, respectively) were shown to be due to excitation of residual monomers. (25) J. B. Birks, Paper presented at 4th Molecular Crystal Symposium, July 9-12, 1968, Enschede, Netherlands.
The Journal of Physical Chemistry
