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PERS. STESSRTA Y D ,JEKOM.IE:I,. R O ~ E Y R E R G
clarifying observations made by classical methods. Beeckl* found the monolayer capacity for carbon monoxide to be twice that for hydrogen, the VC0/VH2 ratio being 2.0. More recent datalg on evaporated films give a value of about 1.5. This non-integer value finds ready explanation in terms of the infrared data, and it would be of importance if the relative numbers of the two species could be found spectroscopically. To do this, the relative extinction coefficients of the species must be known. The best method of estimating these coefficients is to compare the relative strengths of the bands due to bridged and linearly held CO in various metal carbonyls. Infrared data are available for iron ennacarbonyl,20 iron tetracarbonyl?’ and dicobalt octacarbonyl.22 The spectra in the above papers are not suitable for quantitative calculations of relative optical density. Iron enneacarbonyl was examined in the solid state and a strong Christiansen filter effect23distorted the intensities considerably. In the iron tetracarbonyl spectra, the bridged band overlapped the linear band. For cobalt carbonyl well-resolved spectra are given, but the linear band is split into three components, and comparison with the narrow bridged band could not easily be made from peak optical densitie)q. Until some method is devised for determining relative extinction coefficients, infrared data cannot give the relative numbers of these two species. Hence, the “normality” of a T7co/VH2ratio of 1.3 as inferred by Schuit and van Reijen5 from infrared data on nickel, must be regarded as provisional. (18) 0. Beeck, Adzances zn Catalyezs, 3, 131 (1950). (19) P 31. Gundrv and F C Tompkins, Trans Faraday Soc , 63, 218 (1957). (20) R K Sheline and K S. Pitzer, J . 4m. Chem Soc., 73, 1107 (1950). (21) R K. Sheline, ahzd., 73, 1615 (1951). (22) J W Cable, R S. Nyholm and R K. Sheline, z b z d , 76, 3373 (1954). (23) W. C. Price and K S. Tetlow, J Chem. Phys , 16, 1157 (1948).
Val. 6.5
General Remarks.-The effect of the support 111 changing the ratio of linear to bridged species of CO is probably a function of the electrical properties of the junction formed a t the points where the metal is in intimate contact with the substrate. The properties of these metal-support junctions will vary considerably from oxide to oxide. They are expected to be related to both the semi-conductivity of the particular oxide used and to the nature of the interstitial compounds formed a t the junction. Little reliable information seems available on the semi-conductive properties of finely divided oxides. The oxides used here, although fairly pure by normal standards, are impure by the standards needed to obtain interpretable conductivity data of the type obtained, for example, on germanium. Gntil the electrical characteristics of high area oxides are much more clearly understood, it seems that little is to be gained by theoretical speculations about their electrical characteristics, and of those of the metal-oxide junctions. Evidence is accumulating, especially for alumina and titania, that wide variations in surface properties of these oxides can occur when their method of preparation is changed. Hence, it is unlikely that the results reported here can be extrapolated to other silicas, aluminas and titanias, and such comparisons should not be made. Acknowledgments.-We wish to thank the International Kickel Company for generous financial help for equipment and grants which supported this work. Thanks are also due to Professor Jack H. Schulman for his interest and encouragement during this work, and to Professor R. S.Halford, of the Chemistry Department, for helpful discussions on spectroscopic aspects of this work. We also wish to thank Dr. K.Sheppard (of the University Chemical Laboratories, Cambridge) for helpful discussions on the original idea of using a cell with magnesium oxide windows for adsorption studies.
FLVORESCENCE AND ABSORPTION STUDIES OF REVERSIBLE AGGREGATION I N CHLOROPHYLL‘ BY PERS.STEKSBP AXD JEROME L. ROSENBERG Contributzon ]Yo. 1082 f r o m the Department oj Chemistry, Cniversity of Pittsburgh, Pittsburgh 1J, Pennsylvania RecezLed d u g u s t 4, 1960
Changes in the fluorescence spectrum of chlorophyll a and b have been used to observe reversible changes that these substances undergo a t high concentration and low temperature. In both cases the change is reflected in the appearance of an intense fluorescence band above 700 mp. Absorption spectroscopy confirms the formation of reversibly aggregated species a t temperatures below -100”.
One of the accepted mechanisms for t,he concentration quenching of dye fluorescence in solution is the formation of non-fluorescent dimers.? The (1) Presented at the Symposium on Molecular Fluorescence, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March, 1960. These studies were aided by a contract between the Office of Naval Research. Department of tile Navy, and the Univera i t y of Pittsburgh, NR304-416. (2) T. Forster, “Fluoreszenz Organischer Verbindungen,” Vandenhoeck und Ruprecht, Gottingen, 1951, p. 252 ff.
stability of such dimers has been confirmed in a number of cases by observations of concentrationdependent changes in the absorption spectrum and in the action spectrum for fluorescen~e.~-~ A more (3) E. Rabinowitch and L. F. Cpstein, J . Am Chem. Soc., 63, 69 (1941). (4) J. Lavorel, J . Phys Chem., 61, 1600 (1957). ( 5 ) T . Forster and E. Konig, Z. Elelztrochenz., 61, 344 (1957). ( 6 ) G. Weber and F. Fli. J. Teale, T~ans.Faraday Soc., 64, 640 (1938).
June, 1!Nil
. b 3 h O R P T I O S STUDIES OF
REVERSIBLE L4GGREG.\TIOY
IN C H L O R O P H Y L L
recent theoretical treat'ment indicates that dimers need not always be non-fluorescent.' In fact, Brody found experimentally t'hat chlorophyll in alcoholic solutsions shows an increased fluorescence under conditions where dimerization might be expected to occur, and that the fluorescence spectrum of the dimer is decidedly different from that of the monomer.8 The following investigation was undertaken to confirm this finding of Brody, to improve the experimental discriminat'ion of the fluorescence spectra in the near infrared, to look for long-lived emission, and to study changes in the absorption spectrum .that, might be correlated with the fluorescence. Experimental Chlorophylls a and b were extracted from spinach, separated, purified, and analyzed by a modification of the procedure of Zscheile and Comar,g and stored in cold ether solution. A4110merized chlorophyll was prepared by air oxidation of an alcoholic solution of chlorophyll a t room temperature. Ethanolic solutions for absorption or emission spectroscopy were thoroughly degassed by the Thunberg technique. At, low t,emperat,ures the solutions were clear glasses. Coucentrations were determined spectrophotometrically by comparing absorbances a t t,he red peaks in ether solution xvith the extinction values of Zscheile and Comar . g Fluorescence \vas obverved from t,he illuminated surface of the cell a t a direction perpendicular to the exciting light. The observed fluorescence spectra were badly distorted by almost complete reabsorption of the principal fluorescence peak. Since the main interest in this work lay in the fluorescence a t longer wave lengths t,han the principal peak, this was not serious. The fluorescence data are reported as observed meter readings corrected for detector sensitivity but not for reabsorption. Absorption spectra vere measured in a Cary-14 recording spectrophotometer. Optical paths :is low as 4 X 1 0 - 3 cm. \yere obtained by inEerting flat strips of glass as spacers into a rectangular crosssection cell. The absorption cell was mounted in an aluminum block which, was immersed in a Dewar containing an amylene-bath cooled by circulating liquid nitrogen through copper coils. Smaller paths were achieved for room temperature experiments without deoxygenation by the optical flat sandwich technique. For measurements of fluorescence spectra the emission was passed through a Bansch and Lomb 250 mm. grating monochromator blazed for a maximum firat-order intensity of 750 nip. Corning filters were used, when necessary, to esclude second-order hnsmission. The monochromator slits were set for a band width of 20 mp. The detector was a DuMont 6911 red-sensitive photomultiplier. The monochromator-detector combination varied in sensitivity by less than 1047 over the region of maximum response, 680 to 790 mp. The sensitivity was SOY0 of maximum at 600 and 880 mp, 37C; at (300, and 2yGat, 1000.
chlorophyll a; solution, 8 X 10-3 25"; - (no points), absorbance at -196"; --0--,O-, fluoresfluorescence a t - 196 . cence a t 25'; -O-O--, Fig. 1.-Concentrated
?/I in ethanol;
- - - (no points), absorbance at
peak location and peak height ratio are presumably due to differences in reabsorption. The anomalous increase in the secondary peak in the concentrated solutions at -196' and the occurrence of this peak a t a shorter wave length than the secondary peak for the dilute solution both indicate a new molecular species in cold concentrated solution. The data suggest that a small amount of dimer might exist a t room temperature in the concentrated solution. These results confirm those of Brody in general, except that our new peak is located at a somewhat longer wave length than his. We believe that the discrepancy is due t o Brody's use of a detector with a strongly wave length dependent response. The emission was followed to 1000 mp but no additional peak was found in either dilute or concentrated solutions. Results Also, no delayed emission lasting more than a Chlorophyll a.-The fluorescence spectrum of a milli-second mas detected at any wave length below 5 x X ethanolic solution had its principal 1000 mp, within the limited sensitivity of the detecpeak at 676 mp and a secondary peak at 728 mp a t tor. The absorption spectrum of a concentrated room temperature, the ratio of the peak heights - 196' the peaks shifted to 682 and solution was studied as a continuous function of heing 3.7. 740 m p , the peak ratio t o 3.1, and both peaks were temperature between +25 and - 196'. During sharper.. These shifts are normal for simple tem- the cooling of an 8.2 X loe3 .ill ethanolic solution, new shoulders developed a t about - l O O o on both perature effects. For an 8.2 X X solution the peaks were at, the short and long wave length sides of the peak. ti80 and 734 mH at room temperature and at 695 -4t - 196' these shoulders had developed into defiand 734 mp at -196' (Fig. 1). The peak height, nite peaks at 654 and 705 mp, the main peak being The at 676 mp (Fig. 1). A similar experiment with a ratio decreased from 1.4 to 0.4 at -196'. M solution showed a similar shift of differences between dilute and concentrat,ed solu- 3.5 x tions at room temperature with respect t,o both the main peak and a similar new peak at 654 mp but no new features on the long wave length side of 171 E. G. hIcRae a n d AI. Kasha, J . Ciiem. Phys., 88, 721 (19.58). the main peak. The 705 mp peak in the concen(8) S. S.Brody, Science, 128, 838 (1958). trated solution is thus definitely associated u-ith a (9) F. P. Zsclirile and C . L. Comar, Botnn. G a z . , 102, 463 (1941).
PERS.STENSBY AND JEROME L.ROSENBERG
Vol. 65
as to those above 700 because of reabsorption. The ratios of the two peak intensities were 3.0 a t room temperature and 3.3 a t -196'. I n concentrated solutions, however, a new intense peak occurred a t 718 mp at -196' (Fig. 2), masking the 725 secondary peak observed a t room temperature. M the original At a concentration of 7 X main peak, 675 mp a t room temperature, appeared a t -196' only as a shoulder a t 680 mp. At intermediate concentrations the two peaks were resolved, the new one moving 15 mp toward the infrared. The 718 mp probably corresponds to the same species as Brody's 695 mp shoulder; in fact, our sample showed an apparent shift in the new peak to 695 mp, uncorrected for detector response, if a 1P21 blue-sensitive photomultiplier tube was used. With neither detector, however, could we confirm a new shoulder or peak shifted to the shorter wave length side, as Brody had reported. No additional fluorescence peaks were found in the range extending to 1000 mp. As with chlorophyll a, none of the observed emission had a half-life greater than a millisecond. I n absorption new inflections were observed on both sides of the principal band in concentrated solutions a t -196' (Fig. 2). Under these conditions the main peak was a t 661 mp and the new M shoulders were a t 642 and 695 mp for 5 X Fig. 2.--Concentrated chlorophyll b; solutions: for solutions. absorbance, 5 X 10-8 M in ethanol; for fluorescence, Allomerized Chlorophyll a.-The fluorescence 7 X 10-3 11.1 in ethanol; --- (no points), absorbance at spectrum of concentrated solutions in ethanol was 25'; - (no points), absorbance at -196O, --0--0-,found not to be temperature dependent, although fluorescence at 25"; -@-O--, fluorescence at - 196'. the fluorescence yield increased about threefold on cooling from 25 to -196". new component that is not formed in dilute soluDiscussion tions. In a separate experiment with a concentrated solution the main blue absorption peak Our fluorescence data support Brody's conclushifted from 431 mp a t 25' to 441 mp at -196'. sion that a reversible aggregation of chlorophylls The minor bands in the 500-600 mp region lost a and b occurs in concentrated alcoholic solution. some of their sharpness at - 196'. Our absorption data support this interpretation Because of a recent report of BrodylO that evi- and also indicate that the aggregate forms in apdence for dimerization in concentrated solution preciable amounts only below -100'. The abcan be observed in the absorption spectrum even sence of considerable dimerization a t room temwithout cooling, we made repeated efforts to ob- perature confirms the findings of Watson and serve the effect a t room temperature. Even with Livingston, based on absorption spectra and on the a differential procedure the absorption spectrum of concentration and temperature dependence of selfsolutions up to 8 x 10-3 M was found to be iden- quenching of fluorescence.ll The experiments of tical a t 25' with that of a dilute solution. Solu- Weber and Teale6 and of Forster and Livingston12 tions containing more chlorophyll than this showed on the wave length dependence of fluorescence a broadening of the absorption spectrum on both yield in concentrated solutions, might be intersides of the red band, but all such preparations preted by assuming a few per cent. of a noncontained undissolved pigment. The broadened fluorescent dimer a t room temperature, which spectrum was also observed with purposely dried would escape detection in absorption spectrospreparations, made by allowing solutions to copy.13 Even this view entails some difficulties. evaporate to air-dryness in the cell. According to McRae and Kasha,' the fluorescence Chlorophyll 6.-Similar phenomena were found of a dimer should depend in the fiwt instance on with ethanolic solutions of chlorophyll b. In the relative orientation of the two halves of the dilute solutions the fluorescence peaks were at molecule. An arrangement compatible with the 667 and 715 mp at 25' and a t 665 and 727 mp a t observed strong fluorescence a t low temperature, -196'. The reason for the unexpected slight such as a non-parallel arrangement of the conjudecrease in the wave length of the first peak was gated planes of the two monomeric moieties or a not clear, but we did not attach as much signifi- parallel arrangement with some parallel displacecance to any of the positions of the peaks below 700 ment, would have the same high transmission (10) S. S. Brody, Paper presented at the Symposium on Molecular Fluorescence, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. March, 1960. 5. S. Brody and M. Bmdy, Nature, 189, 647 (1960).
(11) W. F. Watson and R. Livingston. J . Chem. Phys., 18, 802 (1950). (12) L. S. Forster and R. Livinpeton, tbid., SO, 1315 (1952). (13) E. Rabinowitoh, Plant Physiol., 35, 477 (1960).
THELANTHANUM-BORON SYSTEM
June, 1961
probability at room temperature. Then the invocation of dimers to account for room temperature fluorescence quenching would either require differently structured dimers a t high and low temperatures or a single form of dimer to which the ad hoc hypothesis of a strongly temperaturedependent collisional or internal conversional quenching is applied. The indications of absorption spectrum broadening a t room temperature,’O we believe, are due to scattering by suspended particles in saturated solutions. For these reasons we feel that the dimer stable a t low temperature is probably not the prevalent form of chlorophyll important for in vivo photosynthesis. We do not mean to exclude the role of reversible changes in the small fraction of chlorophyll molecules participating chemically in the photosynthetic process. The low stability of the chlorophyll dimer is not surprising. Practically all the authenticated cases of reversible room temperature-stable dye dimers have occurred with ionic dyes, such as thionin, acridine and crystal violet cations and fluorescein and eosin anions. Levinson, et al.,
909
attribute the stability of such dimers to the coulombic forces of ion pairing as influenced by charge delocalization. l3 Non-ionic dimers would then be restricted to those formed only in the excited states by electronic excitation delocalization, such as pyrene,14 or to those whose ground-state stabilization is of the much weaker van der Waals type. This latter category, including chlorophyll, would be expected only a t low temperatures. The relative inability of allomerized chlorophyll to dimerize a t low temperature may be due to steric barriers of the bulky alkoxy substituent a t carbon-10. The changes reported here differ by their concentration dependence from the temperatureand solvent-dependent changes reported by Freed and eo-workers and ascribed to reversible solvation. 15,16 Freed’s measurements were all made with dilute chlorophyll solutions. (13) G. 9. Levinson, W. T. Simpson and W. Curtis, J . Am. Chem. Soc.. 79,4314 (1957). (14) T. Forster, 2. Elektrochem., 69, 976 (1955). (15) S. Freed and K. M. Sancier, J . Am. Chem. Soc., 76, 198 (1064). (16) S. Freed, Science, 126, 1248 (1957).
THE LANTHANUM-BORON SYSTEM’ BY ROBERT W. JOHNSON AND -4. H. DAANE Institute f o r A t o n i c Research and Department of Chemistry, Iowa State University, Ames, Iowa Received August 11 1960 ~
From thermal, metallographic, X-ray and electrical resistance data a phase diagram is proposed for the lanthanum-boron system. Two compounds are forTed, LaB4 and La&. The former has a very narrow range of homogeneity and decomposes peritectically a t 1800 i 15 The crystal system is tetragonal, and the com ound is a metallic type conductor. LaB6exists in the range 85.8 to 887, boron, melts above 2500”) and has a simple cubic Tattice. The color of this compound changes with composition, going from purple to a bright blue with increasing boron content. The addition of boron t o lanthanum has no measurable effect on the melting point or transition points of the metal. The addition of lanthanum to boron appears t o have very little effoect on the melting point of boron. There is metallographic evidence for an allotropic transformation in boron above 2100 Evidence also is given for a new compound CaB4, which appears to be isomorphous with LaB,.
.
.
Introduction The past decade has seen an increasing interest in compounds between transition metals and boron, carbon, nitrogen and silicon. This group of refractory compounds is under study not only because they possess useful properties, but also because knowledge about the nature of their bonding is expected to contribute to the understanding of metallic bonds. Work on metal-boron systems has been hampered until recent years because elemental boron was not available in sufficient purity to permit reliable experimental results to be obtained, and as a result the state of knowledge of borides is less developed than that of carbides, nitrides and silicides. There is additional incentive for study of borides rather than the other refractory compounds because “electron deficient” bonding found in the boron hydrides and their derivatives is present in the boron “frameworks” of some borides. It would be desirable to seek a common basis for iiriderstanding the boron bonding in both sets of compounds. (1) Contribution No. 908. Work was performed in the Anies Laboratory of the U. S. Atomic Energy Cornmission.
The employment of the lanthanides (and scandium and yttrium) in the study of a set of compounds such as borides can be very useful because the size of the metal atom can be varied while other factors are nearly constant, thus helping the investigator to distinguish between size effects and effects due to other factors. The study of the lanthanum-boron system was undertaken for the above reasons, and also as a part of a program of investigation of the effects of interstitial type elements on the properties of the rare-earth metals. Kiessling,2 Kieffer and Benesovsky, and Robins4 have reviewed borides and other refractory compounds, and references 5-17 include most of the recent work on rare-earth borides. (2) (a) R. Kiessling, Acta Chim. Scand., 4, 209 (1960); (b) Powder Metallurgy, No. 3 (1959). (3) Kieffer and Benesovsky, ibid., No. 1/2 (1958). (4) D. A. Robins, ibid., No. 1/2 (1958). ( 5 ) L. Brewer, D. L. Sawyer, D. H. Templeton and C. €1. Dauben, J . Am. Ceram. Soc., 34, 173 (1951). 16) J. M. Lafferty, J . Appl. Phys., 22, 299 (1951). (7) F. Bertrtut and P. Blum, Compl. rend., ‘234, 2621 (1852). (8) A. Zalkin and D. H. Templeton, Acta Cryst.. 6,269 (1963). (9) H. C. Longuet-Hipgins and M. De V. Robert.?, Proc. Roy. SOC. (London), A!d24, 336 (1954).
