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-
Jan. 20, 1962
INTRAMOLECULAR
ENERGY TK4NSFER
sized upon our request in Professor Ginsburg’s laboratory.* The compounds were homologous 9-anthryl-l’-napthyl-alkanesin which the anthracene and naphthalene units were joined by saturated chains of one, two and three carbon atoms. The absorption spectra of these substances are given in reference 2 and clearly represent superpositions of naphthalene and anthracene spectra. As a matter of fact they were found to agree within very close limits with the spectrum of a solution containing a mixture of 1-methyl-naphthalene and 9-methylanthracene. Such a solution could therefore be used for calibration in the course of this work. Intramolecular energy transfer resulting in sensitized fluorescence has been previously investigated in rare earth chelates8 and in Complex organic m o l e c ~ l e s . ~The mechanism of radiationless energy transfers between two molecular systems, the sensitiser and the acceptor, has been treated in detail by Foerster5 who also recently reviewed the experimental results in this field.6 The Foerster mechanism undoubtedly also applies to molecules with two independent absorbing systems6 such as those investigated in the present work. It was possible here to demonstrate, both by qualitative experiments, as well as by quantitative measurements, the occurrence of intramolecular energy transfer from the naphthalene to the anthracene unit of the compound molecules (9anthryl-1’-naphthyl-alkanes) . The relative intramolecular energy transfer quantum efficiencies of the three compounds investigated were measured and also the measurement of the absolute magnitudes of these efficiencies was attempted with reasonable results. I n the course of these measurements, the relative fluorescence quantum yields of the compounds were determined. Materials The compounds l-CloH?.(CH2),.C14He-9 (9-anthryl-l’naphthyl-alkanes) with n being 1, 2 and 3 (referred to as I , I1 and I11 respectively from here on) were obtained through the kindness of Professor Ginsburg of this department.2 They had been chromatographed and recrystallized and were used without further purification. Their absorption spectra 1-Methylin solution were reproduced in reference 2. naphthalene was an Eastman-Kodak sample which was, however, colored before purification. It was vacuum distilled and chromatographed and was then a colorless liquid whose solution spectrum agreed well with that found in the literature.’ 9-Methyl-anthracene was prepared from 9anthrone and methyl bromide by a Grignard reaction. The compound was purified by chromatography and by recrystallization. Its solution spectrum agreed with that found in the literature.’ Isopentane and methylcyclohexane were (2) P. Rona and U.Feldman, J . Chem. S O L . ,1737 (1958). (3) S. I. Weissman, J . Chem. P h y s . , 10, 214 (1942). A. N. Sevchenko and A. K . Trofimov, Z h u r . Ekspfl. i Teoref. F i e . , 21, 220 (1951). A. N. Sevchenko and A. G. Morachevsky, Ievest. A k a d . Nauk S.S.S.R. (ser. fie.), 16, 628 (19.51). G. A. Crosby and hl. Kasha. Speclrochim. Acta, 10, 377 (1958). (4) T. T. Bannister, Arch. Biochem. Biophys., 49, 222 (1954). G. Weber and F . W. J. Teale, Trans. Faraday SOC.,64, 640 (1958). G. Weber, Nature, 180, 1409 (1957). (5) T. Foerster, “Fluoreszenz organischer Verbindungen.” Vandenhoek and Ruprecht, Goettingen 1951, Chapter IV, and references therein. (6) T. Foerster. Discussions Faraday Soc , 2 7 , 7 (1959).
IN A NAPHTHALENE-ANTHR4CENE
SYSTEX
173
used as solvents and were purified by passage through o a siIica gel column. They were then transparent to 2200 A .
Experimental Stock solutions of concentrations close to 4 X 10-4iMin a methylcyclohexane-isopentane 4 : 1 by volume solvent mixture were prepared of methyl-anthracene, and compounds I, I1 and 111. A near-equimolar mixture of O-methyl-anthracene and I-methyl-naphthalene in the same solvent was prepared and the concentrations of the solutes so adjusted as to match the optical densities of the solutions of compounds I , I1 and I11 as closely as possible over the entire spectral range of interest here. The exact concentrations of the stock solutions are listed in Table I. The absorption
TABLE I
STOCK SOLUTION CONCENTRATIONS Solution
(1) (2) (3) (4)
Compound I Compound I1 Compound I11 Mixture: 9-Methyl-anthracene 1-Methyl-naphthalene (5) 9-Methyl-anthracene
Concentration (AI)
3.4 3.3 3.4
x x x
10-4 10-4 10-4
4.2 4.8 4.2
x x x
10-4 10-4 10-4
spectra of the solutions of the three compounds and of the mixture containing methyl-anthracene and methyl-naphthalene all were in very close agreement with each other, the spectra of the solutions of I, I1 and I11 being almost identical while that of the mixture agreed with the former within 10% of optical density a t all wave lengths of interest, and all absorption peaks were within 10 A . of those of the three compounds. Experiments were performed with the stock solutions and with solutions obtained by dilution by a factor of 11. All solutions were placed in quartz or Vycor test tubes which were connected to a glass vacuum system by means of graded seals, degassed and sealed off under vacuum. These samples were then used for room temperature experiments as liquid solutions or they were inserted in a quartz Dewar flask containing liquid nitrogen or liquid air. A t the temperature of liquid nitrogen or liquid air the solutions froze to form transparent glasses. The fluorescence of the samples was excited by irradiation with a General Electric high pressure mercury arc (A-H6) filtered by filter combinations “B” and “D” described by Kasha.’ The first of these isolates light suitable for exciting the anthracene molecule in its first absorption band while the second filter isolates light which excited the naphthalene molecule in the region of its strong absorption in the neighborhood of 2800 A. Filter “D” was used as described by Kasha in part of the experiments and the other part of the experiments were performed using a modified filter “D” where the carbon tetrachloride component was replaced by a solution of benzene in iso-octane (10 g. in 150 ml., 1 cm. cell). The benzene solution absorbs strongly a t wave lengths* shorter than 2700 A. whereas the carbon tetrachloride filter component transmits to about 2600 A. The fluorescence spectra were photographed on a medium Hilger spectrograph. The fluorescence intensity was measured by means of a 931A RCA photomultiplier, the current being measured by a galvanometer. The anode currents measured were kept below 50 ~ramp. Corning glass filter number 7380 was found to be most suitable for filtering out the stray exciting light while transmitting all of the anthracene fluorescence. This filter was therefore placed in front of the photomultiplier tube. It also absorbs all naphthalene fluorescence. In a typical series of measurements the following solutions were irradiated and the photocurrent recorded in rapid succession: (1) compound I, (2) compound 11, (3) compound (7) AI. Kasha J . Optical SOC.A m . , 38, 929 (1948) (8) A M Bass, ibid., 38, 977 (1948).
0 . SCHNEPP AND M. LEVY
174
Vol. 84
TABLE I1 VALUESOF B Temperature
Concn., M
Room 3X10-4 Liquid air or liquid nitrogen 3x10-4 Room 2.6X10-& Liquid air 2.6X10-6
Bi5
Bas
Bib
Bza
813
54s
1.1 1 0 . 1 2 . 1 1 0 . 2 1 . 6 i O . 1 5 0 . 6 9 i 0 . 0 7 1 . 3 f 0 . 1 3
1.03=0.1
1.0 i 0 . 1 1 . 0 i 0 . 1 1.0 + o . i 1.1 i O . 1 1 . 9 i K l 1 . 4 z t O . l 0.95+0.1 0.951.0.1 0 . 9 0 f 0 . 1
i.o=+o.i 1.010.1 1.02~0.1
1 . 0 h 0 . 1 1.0 i 0 . 1 0.8 3 ~ 0 . 1 1.3 & O , l 1.05h0.1 1.05iO.1
111, (4) mixture of 9-methyl-anthracene and l-methylnaphthalene, ( 5 ) 9-methyl-anthracene, (6) solvent.
Experimental Results When the stock solution 4 containing a near-equimolar mixture of 9-methyl-anthracene and 1-methyl-naphthalene was irradiated through Kasha’s filter “D,” two sets of fluorescence spectra were obtained on the spectrographic plates. The shorter wave length fluorescence consisted of broad bands which lay between 3160 and 3400 A. The longer wave length fluorescence was similarly banded and lay between 3880 and 4740 A. These spectra are easily identified to be the fluorescence spectra of naphthalene and anthracene respectively. When, on the other hand, solutions of the compounds 1, I1 and 111 are irradiated through the same filter, only the anthracene fluorescence could be photographed and all attempts to obtain the naphthalene emission were unsuccessful. The same results were obtained both a t room temperature and for the rigid glass solutions a t liquid nitrogen temperature. Tables I1 and 111 list the values of certain ratios which are calculated from the experimental measurements. The ratio B x , represents the relative quantum yields of the solutions on direct excitation of the anthracene part of the molecule in the case of solutions 1, 2 and 3 of the methylanthracene molecule itself in the case of solutions 4 and 5. ( x ) and (y) are the fluorescence intensities as measured by
I B = ka,Nn
On the othcr hand if the anthracene fluorescence intensity on excitation of the naphthalene group be ID and the number of quanta absorbed by the naphthalene group be N D ,then we define a transfer coefficient k T by the equation In
The product k T . N D gives the number of quanta which come to reside on the anthracene group after transfer from the naphthalene group. If two solutions of very closely similar optical density are irradiated, we can assume that the number of quanta absorbed is the same. As a result, we can calculate ratios of the ka’s and the JET’S for the different compounds and the mixture investigated here from the Auorescence intensity measurements. TABLE I11 VALUESOF Temperature
Concn., M
Liquid air or liquid nitrogen Room Liquid air
Daa
Di3
3 x 10-4 1 . 0 i . I 1.1 i .i . l 5 1 . 2 5 i .1 2 . 6 X 10-5 0 . 8 .l 1.O j; . I 2 . 6 X 10-5 1 , 0 5
*
Do
.o
*
.I
.O & .1 .O =t .1
If the subscripts x and y denote different compounds, we have k ~ x . 1 % ~
IBX
Bxy
through filter ‘ID” (subscript D) and ( x ) and ( y ) may, inpractice, be (l), ( 2 ) , ( 3 ) or (4) or in other words the solutions of compounds I, I1 and I11 or the solutions of the mixture of methyl-anthracene and methyl-naphthalene. In order for the desired ratio to be obtained, the measured ~ be corrected for the fluorescence intensity [ ( x ) - ( 6 ) ] must anthracene fluorescence resulting from direct excitation of the anthracene group by light transmitted by filter “D.” This correction is the fluorescence intensity of solution 5 or ~ for relative quantum yield of the value [(5) - ( 6 ) ]corrected the solution studied ( x ) and ( 5 ) or Rx5. Implicit in this calculation is the assumption that the quantum yields for the anthracene group remain constant for exciting light wave lengths4 between 4000 and 2600 b .
D
3 X 10-4 0 . 7 0 i 0.05 1.2.5 f 0.15 0 . 0 z t 0 . 1
Room
the photocurrent obtained when any of these solutions are irradiated through filter “B” (subscript B ) . (6) represents the photocurrent obtained under the same conditions when a sample tube containing the pure solvent is substituted in the sample holder. D,, expresses the ratio of the anthracene fluorescence intensities of two solutions when the naphthalene group is excited. Here the solutions are irradiated
= kQ.kT.Nn
=
;7
=
m G
=
kox
