ABSORPTIONSPECTRA O F TETRAHEDRAL MANGANESE COMPLEXES
Jan. 20, 1962 [CONTRIBUTION FROM
THE
107
DEPARTMEST OF CHEMISTRY, MASSACHUSETTS ISSTITUTE OF TECHSOLOGY, CAMBRIDGE 39, MASSACHUSETTS]
Absorption Spectra and Electronic Structures of Some Tetrahedral Manganese (11) Complexes BY F. ALBERT COTTON,^^ DAVIDM. L. GOOD GAME^^
AND
MARGARET GOOD GAME^^
RECEIVED JULY 17, 1961 The visible and near ultraviolet absorption spectra of a number of salts of the tetrahalomanganese(I1) ions, [MnXJaX = C1, Br, I, have been measured in the solid state and in various solvents a t -25". I t also has been observed that several of the compounds exhibit green fluorescence and that two of them are triboluminescent. The sensitivity of the complex anions to solvolysis or other effects in some solvents has been observed and the salutary effect of adding an excess of the RX compound corresponding to R*[MnXd] has been demonstrated. The spectral data are qualitatively in agreement with predictions of ligand field theory. While quantitative treatment could not be carried out for [Mn11]*-, the data for the chloro and bromo complexes have been treated t o obtain values of A, B and C. chloride (4.88 g . , 0.044 mole) also in absolute ethanol (7 Introduction ml.). Pale green needles separated as the solution cooled. The electronic structures of manganese(I1) (3d5) These were filtered off, washed with absolute ethanol and high spin complexes have occasioned much theo- dried in wucuo over sulfuric acid; yield say0, m.p. 152' retical interest,2-8 and there has been considerable (lit.lo 152-155 ') , Anal. Calcd. for CIOH12ChMnNZ:C, 33.64; H, 3.39; experimental study of several of the octahedral one^.^-^ However, despite the presumed existence N, 7.85. Found: C, 33.73; H , 3.36; N, 7.82. of some tetrahedral manganese(I1) c ~ m p l e x e s , j ~ ~-This -Methyltriphenylphosphonium ~ ~ pale green compound wasTetrachloromanganate(I1). prepared in the same way and, more recently, the certainty of the existence of as the pyridinium complex; yield 62YG,,m.p. 216". others,12no systematic study of their spectra has Anal. Calcd. for C18H36C14MnP2:C, 60.74; H, 4.83; been reported, although fragmentary data may be C1, 18.87; P, 8.24. Found: C, 60.56; H , 4.75; C1, 18.63; P, 8.06. f o ~ n d We . ~ have ~ ~ prepared ~ ~ ~ salts ~ ~ of~ the ~ Tetramethylammonium Tetrachloromanganate(I1).-This tetrahalo anions, [MnC14I2-, [MnBr4]*-and [Mnwas prepared by a method analogous to that used for the and recorded their reflectance spectra and pyridinium compound. The complex precipitated as very transmission spectra both as mulls in nujol or hexa- small crystals immediately on mixing the hot ethanolic soluchlorobutadiene and in solution. As will be shown tions of the reactants. It had a very faint yellow color; later, quantitative theoretical analysis of the experi- yield 71%. Anal. Calcd. for C S H ~ ~ C ~ ~C,M27.84; ~ N ~H : , 7.01; N, mental data in terms of simple ligand field theory 8.12. Found: C, 27.38; H , 6.65; S , 7.68. Thecompound ( L e . , using only the Racah parameters B and C, did not melt on heating to 400". and A, the modulus of ligand field strength, as adIt was very slightly soluble in the lower alcohols on heatjustable parameters) is not wholly successful, but ing but insoluble in other organic solvents. nevertheless certain valuable conclusions can be obWhen the preparation was carried out using hydrated tained from the approximate analysis which has manganous chloride the product was pale pink. Moreover, the analysis was not satisfactory. (Found: C, 26.52; H, been made. 6.59; N, 7.39.) The compound could not be recrystalThe results of this work should be of interest in lized. The impurity was not [MnC13(H20)]-, reported by connection with the role of tetrahedrally coordi- TaylorIo for the complex [CaHjKH][MnC13(H20)],since the nated manganese(I1) in a number of technologically infrared spectrum of the pink compound contained no bands attributable to 0-H stretching. important phosphors such as [Zn(Mn) ]2Si04.15 Tetraethylammonium Tetrabromomanganate(II).-This was prepared by the method of JZrgensen' in 42% yield Experimental Preparation of Compounds. Pyridinium Tetrachloromanganate(II).-This complex was prepared by one of the methods described by Taylor.lo A solution of anhydrous manganous chloride (2.52 g., 0.02 mole) in hot absolute ethanol (16 ml.) was added to a solution of pyridine hydro(1) (a) Alfred P. Sloan Foundation Fellow. (b) Departmenr of Chemistry, Imperial College of Science and Technology, London. (c) I\-orthampton College of Advanced Technology, London (2) Y .Tanabe and S. Sugano, J . Phys. Soc. Japan, 9, 753 (1984). (3) L . E. Orgel, J . Chem. Phys., 23, 1004 (1955). (4) C. K . Jprgensen, Acta Chem. S c a d . , 3, 1502 (1054). (5) C. K. Jprgensen, i ! ~ i d . 11, , 53 (1957). (6) L. J. Heidt, G. F. Koster and A. M. Johnson, J . A m C h o n . Soc., 80, 6471 (1958). (7) J. W. Stout, J . Chem. Phys., 31, 709 (1959). (8) R. Pappalardo, ibid., 33, 613 (1960). (9) H. P. de la Garanderie and D. Curie, Compl. Y c n d . . 248, 3151 (1959) (10) F. S. Taylor, J . Chcm. Soc., 20, 699 (1934). (11) L . Naldini and A. Sacco, Gaze. chim. ifal., 89, 2258 (1959). (12) N. S. Gill and R . S. Nyholm, J. Chcm. Soc., 3997 (1959). (13) D . M. L. Goodgame a n d F. A. Cotton, ibid., 3735 (19F1). (14) S. Buffagni and T. M. D u m , Nature, 188, 937 (1960). (15) See H. W. Leverenz, "An Introduction t o Luminescence of Solids," John Wiley and Sons, Inc., New York, PI'. Y., 1950, and F. A. Kraper, "Inorganic Crystal Phosphors" in Ergebnisse der Exakten Naturwissenschaften, Bd. 29, Springer-Verlag, Berlin, 1956, p. 61.
with m.p. 300'. Anal. Calcd. for CleH4aBraMnKz: C, 30.26; H, 6.35; N, 4.41. Found: C, 30.80; H , 6.52; iT,4.47. Tetra-n-butylammonium Tetra-iodomanganate( 11).-A solution of manganous iodide (1.55 g., 0.005 mole) in ethyl acetate (65 ml.) was treated with finely-powdered tetran-butylammonium iodide (3.69 g., 0.01 mole) and the mixture was heated. forming a pale yellow solution and a yellow oil. Addition of absolute ethanol (6 ml.) to the hot mixture produced a homogeneous yellow solution, which was filtered. On cooling, pale green needles separated, which were filtered off, washed with ethyl acetate and dried in vacuo; yield SO%, m.p. 148". Anal. Calcd. for C82Hd4MnN2: C, 36.69; H, 6.93; N, 2.67. Found: C,36.56; H,6.76; N,2.84. Physical Measurements.-Electrolytic conductances were measured with a Serfass bridge, using a conventional cell calibrated with aqueous potassium chloride. The values obtained in n-1 mole-' for approximately lO-3M solutions in nitromethane were the following: (PyH)z[MnC14], 114; (EtlN)2[MnBr4l, 197; (.BGN)z[MnI4], 171. Reflectance spectra were measured using a Beckman DU spectrophotometer with the standard Beckman reflectance accessory and magnesium carbonate as the reference solid. A Gary Model 14 recording spectrophotometer was used to obtain solution spectra and the spectra of crystals mulled in hexachlorobutadierie and Nujol.
16s
F. A. COTTON, D. M. L. GOODGAME A N D MARGARET GOODGAME
4r2(aFJ4
~
i 4, F 3
~
4 ~ z ~ -4 ~
-
35,000 -
-
t
1
1
7-
-
5 30,000C
-
w
___---
-----'T,
_ _ _ _ _ _ - - - - e - -
_ _ _ _ _ _ _- - - -
'0 r--
___.----
-'-.--..--.-4T2(4Dl
4P
'E
4G
4E.4A,[4G)-
---.__ ---__
----___- - - _ _ _
20,000,
('D31 -
25,000-
"
"
I000
"
"
"
2000
I
"
'
3000
-
4T2(4G).
4000
VOl. s4
ganate(I1) displayed triboluminescence, emitting yellow-green light when rubbed with a glass rod. The tetraiodomanganate(I1) and the other tetra1 chloromanganate(I1) complexes did not show this effect. Similar effects have been reported by the present authors13for the tetrahedral manganese(I1) complexes of the types [ M ~ ( ( C O H ~ ) ~ P Oand )~X~] [Mn((Cd%)&sO)&2]. These luminescence effects also seem to be diagnostic of tetrahedrally coordinated Mn(II), although their absence cannot be considered as evidence against tetrahedral coordination of Mn(I1). The molar conductances in nitromethane are within the normal ranges for 1-2 electrolytes in this solvent and thus support the proposed structural formulations. All of the spectral data are collected in Table I.
ABSORPTION SPECTRA
Jan. 20, 1962
OF
TETRAHEDRAL I\f A N C A N E S E
, 0
m
*. -r m
V v1
a
a A L
0
.-
c Y
i r
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e,
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0 0 0.
m N
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-
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COMPLEXES
169
170
F. A. COTTON, D. M. L. GOODGAME AND MARGARET GOODGAME
The energy level diagram indicates that a closely spaced group of three absorption bands, caused by transitions to the states originating from the 4G term of the free ion, should be found in the range 20,000 to 25,000 cm.-'. This group of bands is clearly seen in Fig. 2 , where we also have shown how the observed envelope can be resolved into three approximately Gaussian bands. In making such a Gaussian analysis of the observed absorption, it turns out quite naturally that the uppermost band in this group appears to be definitely narrower than the other two. As Orgella has shown, this is to be expected since the upper, 4E,4A1(4G), and lower, 6Al, states concerned in this uppermost band have the same slopes on the energy level diagram, whereas the other two bands involve excited states with slopes different from that of the ground state. For octahedral complexes, such as the [Mn(&0)6]'+ ion, a significant difference in the widths of the other two bands is also observed, the lower one being the broader as the energy level diagram requires. As Fig. 2 shows, the [MnBr4I2-spectrum can be resolved so as to make the lowest band somewhat broader than the middle one, but because of the inherent uncertainty in the resolution procedure, i t cannot be said that such a result is rigorously required. At the low A values prevailing in the tetrahedral complexes, the difference in the slopes of the 4T1(G)and 4Tz(G)states is less than it is a t the higher A values found in octahedral complexes, so that we do not in fact expect the difference in band widths to be as great. The energy level diagram shows that the next absorption bands should be another closely spaced group of three in the range 25,000 to 28,000 cm.-', and this group is clearly evident in Fig. 2. Again, the figure indicates a Gaussian analysis of the observed envelope, but, in this case, the overlapping of the bands is so severe that no reliable conclusions as to relative band widths are possible. Thus, we are denied any simple means of assigning the 6A1 + 4E(D) transition, which should have the narrowest band. This causes some difficulty when a quantitative interpretation is attempted, as will be seen later. Finally, the spectrum should contain a third set of three bands in the region 36,000 to 38,000 cm.-' due to transitions to the states arising from the 4F term. These transitions are less easily observed than the six already discussed because they occur in a region where solvent absorption, absorption by some of the organic groups in the cations and charge transfer absorption by the [MXr]'- complexes themselves become prominent and tend to obscure the relatively weak d-d bands. In all three of the [MnX412-species a band we can assign to the 6ill + 4A2(4F) transition has been observed and there is some indication of the -.t 4T2(4F) transition for [MnBr4lZ-. Other small bands or shoulders have been observed from 32,000 to 35,000 cn~.-' which, for reasons which are explained later, we prefer to assign to transitions ha.ving doublet upper states. (18) For discussion and references see L . E. Orgel, "An Introduction t o Trdnsition hIetal Chemistry," l l e t h u e n and Co., Ltd , London, l!iW
Vol. 84
For the [MnCl4lZ-and [Mn14]'- ions, the experimental observations were much the same, differing in no essential way from those described for the [MnBr4I2- ion, and the qualitative features of the assignments we have made are therefore also essentially the same. I t is interesting to note that for [MnC14IZ- we observe a small splitting in the uppermost band of the low energy triplet (at -23,200 cm.-'). Since we assign the corresponding unsplit band in [MnBr4I2- to the transition to the accidentally degenerate 4E(4G)and 4A1(4G)states, the splitting observed in the spectrum of [MnC14I2can be readily explained as due to a slight resolution of this degeneracy, as is observed in various octahedral species, e.g., [Mn(H20),~]~-.The observation of this splitting provides good evidence for the essential correctness of our assignments in the low energy triplet since other assignments (for example, assuming a large 4E(4G)-4A1(4G) splitting in all cases and assuming that the transition to either 4T1(4G)or 4Tz(4G)is weak and unobserved) afford no convenient explanation for this splitting. For all three [MnX4I2- ions the intensities of the bands are markedly greater than for octahedral complexes. They are, in fact, in accord with the apparently general ratio of approximately loz between the intensities of bands in the tetrahedral and octahedral complexes of other metal ions, such as Co(II)l7and Ni(II).I6 It is noteworthy, however, that the band intensities in the [MnI4I2- spectrum are significantly greater, by a factor of 5 to 10,than those in the [MnC14lZ-and [MnBr4I2-spectra. I n attempting to account quantitatively for the spectral data for the [MnX4I2-ions, we have been guided by the published studies of the spectra of octahedral Mn(I1) complexes, particularly by the work of Heidt, Koster and Johnson6 (hereafter HKJ) on the [Mn(H2O)6l2+spectrum. HKJ found that using only the parameters B, C and A they obtained the best fit to the data on [Mn(HzO)6]2+ by assigning these the values 671, 3710 and 8480 cm.-' respectively. Two comments on their work should be made before turning to our analysis of the [MnX412- spectra. (1) Although HKJ's calculated band energies agree with the observed ones within, on the average, 5y0 of the total energies, the errors constitute much larger percentages of the differences in energies of the upper states among themselves. (2) The treatment of the term separations using only the two Racah parameters gives a very poor prediction of the position of the 4P term of the free ion (-8.75y0 too high) and of the (Tl(P) state of the [Mn(I-h0)6]'+ ion (-5.7% too high). The energy level diagram for the [MnBr4I2- ion, which is shown in Fig. 1, was constructed in the following way. The assignment of the three bands in the 25,000 to 28,000 cm.-l region could be made in one of the following three ways, where we list the upper states in order of decreasing energy: (1) ITI(P), 4E(D), 4 T ~ ( D )(2) ; 4E(D), 'Tl(P), 4T2(D); ( 3 ) 4E(D), 4Tz(D),4T1(P)..Assignment 2 can be eliminated immediately since the 4E(D)-4T~(D) separation would require a A value twice as great as the A value suggested by the separations in the states arising from the 4G term. Assignment 3 was then eliminated in favor of (1) because from the
Jan. 20, 1962
ABSORPTION SPECTRA OF TETRAHEDRAL MANGANESE COMPLEXES
171
TABLE I1 COMPARISON OF OBSERVED AND CALCULATED ABSORPTION MAXIMA(CM.-1) (CM.-l) USED' Upper states of absorptions a n d parameters used
Calcd.
[MnBrdZ-
Obsd.
Calcd.
AKD THE
Ohsd.
ELECTRONIC STRUCTCRE PARAMETERS
Calcd.
[MnI~lz-
Obsd.
.... 21,300 21,350 21,300 21,200 21,500 . . . .b 22,600 22,180 22,700 22,400 (23,000) (23,000) (23,200) (23,200) (22,500) (22,500) 26,000 . . . .b 26,200 25,900 26,400 26,300 (27,100) (26,900) (26,900) (26,750) (26,750) (27,100) 28,100 27,750 27,700 28,800 27,900 . . . .b 36,500 36,150 38,000 -38,000 37,400 -36,000 Not obsd. 36,700 36,150 38,300 hTot obsd. . . . .b Not obsd. 'T@) 37,500 -37,400 39,300 Not obsd. . . . .b B 536 .... 558 .... 667 .... C 3,530 .... 3,524 .... 3,250 .... A 3,100 .... 3,600 .... Not calcd.b .... Energies in parentheses are field-strength-independent ones which were used to assess B and C. As explained in t h e text, the data are insufficient to permit any reliable estimate of A. "I(G) 'TAG) 4E,'A~(G) 'TdD) 4E(D) 4T1(P) 4A~(F) 4T1(F)
b
positions of the "(D) and 4E(G) states in assign- intensities of the bands, which already have been ment 3 one obtains a B value of 671 cm.-l and a C noted, several aspects of the quantitative treatvalue of 3257 cm-I. This seems unlikely when ment of the data merit comment. The manner in which the position of the 4Pstate compared with the B and C values obtained by HKJ for [Mn(Hz0)6]2+,since it means that the entire has been manipulated is empirically and not theodifference between [MnBr4I2- and [ M n ( H ~ 0 ) 6 ] ~ +retically dictated. However, from a theoretical is in C with no difference in B. Assignment 1, viewpoint, i t is not without plausibility. Our treathowever, gives a B value of 536 cm.-l and a C ment merely amounts to introducing a third advalue of 3530 cm.-l, both lower, by about the same justable parameter in addition to B and C, and it cannot be considered surprising, or indeed unlikely, fraction, than those for the [Mn(Hz0)6]2+ ion. Having selected this assignment, the B and C that only two interelectronic repulsion parameters values were used to calculate the positions of the fail to fix precisely the relative energies of all five free ion terms for A equal to zero. The energy of terms. the 4P term of the ion in zero field then was lowered I t is also necessary to point out explicitly that we by 7.5%. An arbitrary value of C, chosen so as to have chosen to assume that those higher energy fit this placement of the 4G, 4Pand 4F terms relative absorptions which fall below the calculated enerto one another (but not their absolute placement) gies of the 'j-41 + 4A2(4F)transitions are either exthen was used, together with the B value derived traneous or are due to transitions having doublet from the 4E(D)-4E(G) separation, in the secular upper states. The alternative would be to assume equations of Tanabe and Sugano2 for the 4T1and that the calculation of the energy of the 4F term 4T2levels. These equations were solved for A using B and C values obtained from the energies of values of 1000,2000,3000and 4000 cm.-'. By this the 4G and 4D terms is very seriously in error. On procedure the relative energies of the states arising the basis of the successful results which have been from the quartet terms of the free ion were obtained obtained for the free ione and for octahedral to a good approximation. Their absolute positions Mn(I1),6--8 this seems the more unlikely continrelative to the ground state then were determined gency. In their treatment of [Mn(H20)6]*+, HKJ using the exact B and C values given by the energies explicitly suggested that a weak band a t 26,500 of the 4A1+ 4E, 4A1(4G)and 4A1+ 4E(4D)transi- cm.-' is due to a transition with a doublet upper tions. The experimental results are compared with state, and their spectrum also contains one, and those which can be read from the energy level dia- perhaps two, other weak shoulders which cannot be gram for A equal to 3100 cm.-l in Table 11. assigned in any other way. It may be seen in Fig. 2 For [MnC14I2-, quantitative treatment of the that the band at 36,100 cm.-' in [MnBr4I2- is about data was carried out in the same fashion and the as narrow as those corresponding to the other results are recorded in Table 11. transitions with horizontal upper states. With [MnI4I2-, serious difficulties were enThe values of A obtained for [MnC14I2- and countered and quantitative treatment could not be [MnBr4I2- are in accord with the accepted positions carried out to any significant extent. Poor resolu- of C1- and Br- in the spectrochemical series.lg The tion makes i t impossible to assign enough of the spectral data for these two complexes also show proper low energy bands to permit any meaningful that Br- is more nephelauxetic than is C1,- in assessment of the A value. Values of B and C were agreement with the accepted order.I9 While there obtained from the energies of bands to which 4E, is not a great deal of information available on the 4A1(4G)and 4E(4D)were assigned as upper states. position of the iodide ion in the nephelauxetic From these, the position of the band with the 4A2 series, the results of our spectra studies16.17of the (F) upper state was predicted in fair agreement with COX^]^- and [NiX4I2- ions led to the conclusion an observed absorption band. (19) See T. M. Dunn in "Modern Coordination Chemistry," J. Conclusions.-Aside from the qualitative con- Lewis and R . G. Wilkins, Editors, Interscience Publishers. New York. clusions as to general regions of absorption and the N. Y., 1960, for discussion and references.
172
0. SCHNEPP
.4ND
that it is probably somewhat more nephelauxetic than C1- or Br-. The present data on [MnI4I2are too limited and inaccurate to offer any definite evidence on this point. There is a notable discrepancy between our A value of 41600 crn.-l and Buffagni and Dunn's valueI4 of 2650 cm.-' for the [MnC14]2- ion. So large a discrepancy obviously requires comment. First, it should be emphasized that the difference in A values is due entirely to a difference in interpretation of data; there are no significant differences between Dunn's data and ours. It also should be noted that the exact numerical value of A is of somewhat limited significance, since i t is impossible, within the theoretical framework of the Tanabe and Sugano treatment, to obtain perfect agreement between calculated and observed energies of the lower six transitions for any reasonable A value. This is because the ratio of the separations 4T2(4G)4T1(4G)and 4E,4A1(4G)-4T2(4G) is theoretically of the order of -3 throughout the range of A from 1000 to 4000 cm.-l (see Fig. l), whereas, experimentally, this ratio is -1. Finally, it also must be recognized that the slopes of the energy lines for all of the lower states, on which the evaluation of A depends, are not great, so that the sum of squares of deviations has no pronounced minimum in the range of reasonable A values. The center of the broad shallow minimum is, however, a t -3600 em.-'. The sum of squares of deviations a t A = 2650 cni.-' is about two times greater than at A = 3600 tin.-'. As a form of independent support for a A value of -3600 em.-' in preference to one of -2600 cm.-l, we have made a comparison of A values for [MnC14]*-, [ C O C ~ ~and ] ~ -[NiCl4I2-with those for the corresponding hexa-aquo ions, which Buff agni
[CONTRIBUTION FROM
THE
VOl. 84
M. LEVY
and Dunn regarded as offering support for their results. Their results and ours for the [CoC14I2- and [NiC14I2-systems have previously been shown to be in satisfactory agreement.16v17A v alues for the aquo ions have been taken from the sources noted in Table 111. It can be seen that, with A equal to -3600 for [MnCl4I2-, the At/Ao ratio for Mn(I1) is within 4y0 of the average value of this ratio for Co(I1) and Ni(I1) while with A equal to 2650 cm.-', the ratio is 2.5% lower. TABLE I11 COMPARISON OF A VALUESIN [MCla]*- AND [M(H20)8]z+ SPECIES FOR M = Mn, Co AND Ni Metal
Ao in [M(H20)sl*
Mn
8500"
+
Ab in [MClr 1%-
AdAt
265Od 0 312 3600" 423 co 8300b 3200' ,386 ,427 Ni 8500" 3625s a From ref. 6. bObtained from the results of C. J. Ballhausen and C. K. Jgrgensen, Acta Chem. Scand., 9, 397 (1958), using an energy level diagram computed by A. D. Liehr from the complete matrices of J. C. Eisenstein, J . Chem. Phys., 34, 1628 (1961); -4 D. Liehr, private communication, presented in part a t the Symposium on Molecular Structure and Spectroscopy, Ohio State University, Columbus, Ohio, June 12-16, 1961 (to be submitted to J . Phys. Chem.). CFrom A. D. Liehr and C. J . BallFrom ref. hausen, Ann. Phys. ( N . Y . ) , 6 , 134 (1959). 14. e Present work. Average of values in ref. 14 and 17. 0 Average of values in ref. 14 and 16.
Acknowledgments.-Financial support of this work by the United States Atomic Energy Commission under Contract No. AT(30-1)-1963 is gratefully acknowledged. We thank C. K. Jgrgensen and A. D. Liehr for useful correspondence.
DEPARTMENT O F CHEMISTRY, ISRAEL INSTITUTE
OF
TECHNOLOGY, HAIFA, ISRAEL]
Intramolecular Energy Transfer in a Naphthalene-Anthracene System BY 0. SCHNEPP AND M. LEVY RECEIVED JULY 6, 1961 Intramolecular energy transfer has been demonstrated in three homologous 9-anthryl-1'-naphthyl-alkanes in which the anthracene and naphthalene units were joined by saturated chains of 1, 2 and 3 carbon atoms. The absorption spectra of these compounds bear evidence that the two aromatic x-electron systems do not overlap appreciably. The naphthalene group of the compound molecule was excited by appropriately filtered radiation and the resulting fluorescence was characteristic of the anthracene group. From the comparative fluorescence intensity measurements relative as well as absolute values of the quantum efficiencies of the energy transfer process were calculated. The relative fluorescence quantum yields were also obtained.
Introduction The absorption spectrum of naphthalene' begins a t 3100 A. with a very weak absorption system ( e = 200) which fuses into a much stronger system with maximum a t 2770 A. ( e = 5000) and a very intense absorption band appears a t 2200 A. ( E = lo5). The absorption spectrum of anthracene' begins with a medium absorption a t 3750 A. (e 7 SOOO) and has a very intense peak a t 2530 A. (e = 1.6 X lo5). It is clear from these data that (1) I< A. Friedel and M. Orchin, "Ultraviolet Spectra of Aromatic Compounds," John W l e y and Sons, Inc.. N e w York, N Y., 1951.
the intense peaks of these two compounds do not overlap and it should therefore be possible to excite one of the substances by irradiation in the presence of the other. This is only approximately true, however, since the absorption coefficient of anthracene a t 2800 A. still has a finite value, even though i t is much smaller than that of naphthalene a t this wave length. It was believed that in view of the discussion of the last paragraph a molecule consisting of nonconjugated anthracene and naphthalene units would be very suitable for a study of intramolecular energy transfer. Such molecules were synthe-
