Dec., 1963
1,UMISESCEKCE SPECTR.4 AND
DECAY
Dy-DyCla system without the maximum (similar to Nd-NdCl3) are two more examples of this positive deviation indicating electron exchange. However, a few cases of positive deviation in systems with merely ionic conductance are known (MgCl2 with CaClz or BaClzBand Cd.Cl2 with PbClZ7). Thus it seemed necessary to support the electron exchange explanation for NdX3-MdX2 by evidence showing that systems KdXa-MX2 containing the dipositive ion M2+ similar in size to Nd2+do not exhibit a positive deviation from additivity. Figurle 2 demonstrates that this is indeed the case for the systems NdC13-SrC12 and Nd13-Sr12, and thus may be considered a t least indirect support for the assumption of a small electron exchange contribution (up to approximately 40% of the total conductance in Nd13-NdI:J to the otherwise ionic conducta,nce in the KdX:a-KdXz systems. An interesting observation was made in connection with the reaction between these “electron-exchange” melts and single crystal synthetic sapphire. It had been noted earlier” that the rare earth metal-halide solutions attacked isapphire so rapidly that a conductivity measurement with the capillary cell was impossible. H:owever, it was found that a solution of 33.3 mole % S d in NdBrs (pure NdBrz) did not attack the sapphire cell, as demonstrated by the conductivity remaining constant a t the same value as that obtained with the molybdenum parallel electrodes and by the preservation of the weight of the sapphire cell. Further experiments showed that sapphire was rapidly attacked (6) R. W. Huber, E. V., Potter, and H. W. St. Clair, U. 3. Bureau of Mines, Report of Investigations KO.4858, 1952. (7) H. Bloom and E. Heymann, Proc. Roy. Xoc. (London), AlSS, 392 (1947). (8) H. R. Bronstein, A. 8.Dworkin, and >I. A. Bredig, J. Phys. Chem., 6 4 , 1344 (1960).
TIMESOF 2.0
CHELATED
RAREEARTHIONS
r----I
20
Fig. 2.-Specific
40
I
2717 I
60 80 20 40 MOLE PER CENT DIHALIDE.
I
I
60
80
conductivity in NdXs-SdXg and SdX3-SrXg melts.
by solutions containing 20% Nd in NdBr3, 10 and 30% S d in Nd13, and 30% Dy in DyC13. A solution containing 2% Nd in NdI3 did not attack the cell and in pure Nd12, the sapphire cell showed only minimal attack. An attempt was made to measure the conductivity of the Yb-YbC13 system. However, it was found that the pure molten YbCL attacked the molybdenum crucible. This was probably due to the greater stability of YbClz (similar to EuC12). No further measurements on this system were attempted a t this time.
SPECTRA AND DECAY TIMES OF THE LUMINESCEXCES OBSERVED FROM CHELATED RARE EARTH IOnTSla BY J. J. FREE MAN^^ AND G. A. CROSBY Department of Chemistry, University of X e w Mexico, Albuquerque, N e w Mexico Received J u n e 24, 1963 Emission spectra and decay times of the benxoylacetonate, acelylacetonate, and dibensoylmethide chelates of trivalent samarium, europium, terbium, and dysprosium are reported. The groups of spectral lines observed from these compounds in rigid organic glasses and excited by ultraviolet light are given probable J-J assignments. The luminescence decay curves are analyzed in terms of a two-step decay mechanism and a lower limit of 5 :X: lo6 sec.-1 is established for the intramolecular energy transfer rate constant. The nature of the donor level in the energy transfer process is discussed. Further evidence for quenching of the 4f eledronic excited stahes of chelated rare earth ions by a vibronic mechanism is presented.
Introduction Recent studies of the emission spectra of rare earth chelates have delineated the paths of energy migration within these complex m0lecules.~~3It has been established that the triplet state of a chelate molecule plays a key role in the process by which excitation energy is (1) (a) Presented a t tho 18th Southwest Regional American Chemical Society Meeting, Dallas, Texas, December, 1962; (b) Brookhaven National Laboratories, Upton, L. I., N. Y . (2) (a) R. E. Whan and G. A. Crosby, J . Mol. Spectry., 8 , 315 (1962); (b) G. A. Croaby, R. E. Whan, and J. J. Freeman, J . Phus. Chem., 66, 2493 (1962). (3) G. A. Crosby, R. E. Whan, and,R. RI. Mire, J . Chem. Phys., 34, 743 (19611.
transferred from the excited states of the complex to the 4f electronic states of the chelated rare earth ions. In previous work we have been concerned with the path of energy transfer and have used ion luminescence only as an indicator for the transfer process. In the present work we give a more detailed analysis of the luminescence observed from rare earth ions in a chelate environment. Luminescence spectra and decay times of chelated rare earth ions are reported. This information is used to establish limits on the rate of intramolecular energy transfer in these compounds and to provide further details concerning the electronic levels from which energy transfer occur8.
,J. J. FREEMAN AND G. A. CROSBY
2718 Experimental
Preparation of Rare Earth Chelates.-The following chelates were prepared for luminescence studies: samarium trisbenzoylacetonate dihydrate, SmBa.2Hz0; europium trisbenzoylacetonate dihydrate, EuB3.2H-20; terbium trisbenzoylacetonate dihydrate, TbB3.2Hz0; dysprosium trisbenzoylacetonate dihydrate, DyBs. 2Hz0; samarium trisdibenzoylmethide, SmD3; europium trisdibenzoylmethide, EuD3; terbium trisdibenzoylmethide, TbD3; samarium trisacetylacetonate monohydrate, SmAa mH-20; europium trisacetylacetonate monohydrate, EuA3. HzO; terbium trisacetylacetonate monohydrate, TbAs-HsO; dysprosium trisacetylacetonate monohydrate, DyAs HzO. Details of the preparation of the rare earth trisbenzoylacetonate dihydrates (MBI’2Hz0) and the rare earth trisdibenzoylmethides (MDI) are reported elsewhere.” Rare earth trisacetylacetonate monhydrates (MAS.HzO) were prepared in the following m iiiner Approximately 2 g. of the rare earth oxide (99.97,) was converted to the chloride by treatment with concentrated hydrochloric acid. The excess hydrochloric acid was removed by evaporation to dryness. The chloride was then dissolved in 200 ml. of distilled water and 8 ml. of redistilled acetylacetone was added to this solution. Concentrated ammonium hydroxide was added to the mixture drop by drop with vigorous stirring until precipitation of the white microcrystalline chelate was complete. The precipitated chelate was collected by suction on a medium sintered glass filter funnel, washed with distilled water, air dried at room temperature, and recrystallized from boiling acetone. The recrystallized material was then vacuum dried a t room temperature for 16 hr. Quantitative analyses indicated that the final products exist as the monohydrates. Attempts to remove this extra mole of water by vacuum drying at elevated temperatures were not successful. The rare earth chelates were analyzed for per cent rare earth oxide gravimetrically. After slow ignition of 100 mg. of chelate in an open crucible to a black carbonaceous solid, vigorous heating for another 30 to 45 min. reduced the ash to pure rare earth oxide. Results were within 0.3% of expected theoretical values for chelates of europium, samarium, and dysprosium. Accurate analyses of the terbium chelates could not be carried out by this method because these chelates were reduced upon ignition to mixed oxides of terbium of unknown composition. No other analyses of these compounds were carried out. Spectroscopic Measurements .-For emission spectra measurements, the compounds were dissolved to concentrations of ~ 1 0 - 6 M in EMPA solutions (4 parts diethyl ether, 4 parts 3-methylpentane, and 2 parts absolute alcohol by volume). The ether (Mallinckrodt, anhydrous, A. R. grade) and the 3-methylpentane (Phillips pure grade) were distilled from sodium metal ribbon, and the absolute alcohol (U.S.I., absolute, reagent grade) was distilled from magnesium ethoxide. The solutions were introduced into 20-mm. 0.d. quartz tubes and frozen to rigid glasses by immersion into liquid nitrogen. The dibenzoylmethide and benzoylacetonate chelates were irradiated with filtered ultraviolet light from a G.E. AH6 mercury arc lamp. For these complexes filter combinations, selected to pass light corresponding to the first strong absorption peaks, are given elsewhere.3 The acetylacetonate samples were irradiated with filtered ultraviolet light from an Osram 500-watt high pressure mercury arc lamp. h light filter combination consisting of two Corning No, 7-54 Red Purple Corex 9 filters and an aqueous solution of nickel sulfate (100 g. of NiS04.6Hz0/1., 5-em. path length) was used to pass light selectively corresponding to the first strong absorption band of the acetylacetonate chelates which has its maximum at BOO A. The total emission spectra were obtained by observing the sample emission a t right angles to the path of the exciting light. The spectra of the long-lived component of the luminescence of the europium chelates were obtained by use of a modified Becquerel phosphoroscope with a calculated resolution time of sec. All spectra were photographed using a Steinheil GH prism spectrograph employing Kodak 103a-F plates. The spectrographowas adjusted to medium dispersion (fs, 17 A./mm. at 5000 A.), and an argon discharge tube was used as a source for calibration. Densitometer traces were obtained with a JarrellAsh automatic recording microphotometer. Decay Measurements.-Direct measurements of the decay of the rare earth ion luminescences were obtained by flashing the samples with short-lived, high intensity ultraviolet light while monitoring the rare earth ion luminescence intensity
-
.
1701.
67
with a photomultiplier. An Edgerton, Gernieshausen, and Grier FX-12 xenon lamp, operating a t 1joule per discharge (0.08 pf. at 5000 v.) was used for the flash source. Spontaneous discharge of the flash lamp was prevented by the use of a triggered spark gap which consisted essentially of two steel poles separated to such a distance that spontaneous discharge across the poles occurred a t voltages slightly higher than the operating voltage of the lamp. Discharge of the storage capacitors across the steel poles of the spark gap and through the flash lamp wag accomplished by momentarily ionizing the air between the steel poles with an electric spark from a thyratron trigger. Using this arrangement, a flash with a mean life of 2 psec. could be obtained. Light filter combinations used to filter the flash lamp exciting light were the same as those used for photographing the emission spectra. The samples, placed in a quartz dewar filled with liquid nitrogen, were also prepared in the same manner as described above. Light filters between the samples and the photomultiplier were selected to pass sample emissions arising onIy from the radiative decays of the excited rare earth ions. For observing narrow regions of the emission spectra, these filter combinations consisted of a solution of dibenzoylmethane in absolute alcohol (-10-* iM, 1-cm. path length), which absorbed the ultraviolet light from the flash lamp, coupled with selected Balzar narrow band interference filters or a 250-mm. Bausch & Lomb monochromator. The spectral regions monitored are indicated in Fig. 1 and 2. An iris diaphragm was used to adjust the intensity of light falling on the RCA-2020 photomultiplier (for Sm3+, a RCA7102 was used) which monitored the ion luminescence. The photomultipliers were operated at 1080 v. with an anode to ground load resistance of 10,000 ohms resulting in a response time of less than 1psec. The photomultiplier signals were displayed on a Tektronix Model 535 oscilloscopeon which the horizontal sweep trigger was adjusted so that a single horizontal sweep was initiated when the lamp flashed. Time mark signals from a Tektronix Model 180 time mark generator were used as a standard for the horizontal time scale of the oscilloscope. A Polaroid Land camera employing 3000 speed film was used to photograph the oscilloscope traces. The relative intensity of the luminescence from a rare earth ion was taken as the vertical distance on the photograph between the base line and the photomultiplier response to the emitted light.
Discussion Emission Spectra of Chelated Rare Earth Ions,When P-diketone chelates of Sm3+, Eu3+, Tb3+, and Dya+ were excited by near-ultraviolet light, the luminescences observed consisted almost entirely of line emission characteristic of the rare earth ions. These emissions were sufficiently intense so that the spectra could be recorded with ease and lifetime measurements could be made on most of these compounds by utilizing the apparatus described above. Densitometer traces of the observed spectra are presented in Fig. 1 and 2. From their line-like structure it is evident that these spectra arise from radiative combinations between 4f electronic states of the rare earth ions. In Fig. 3 we have plotted an energy level diagram for the systems. Both the lowest triplet levels of the chelates and the manifolds of low energy ion states for the trivalent ions are included. The former are derived from phosphorescence measurements on analogous La3+, Gd3+, and Lu3+c~rnpounds,~ and the latter are adapted from the data for anhydrous rare earth chlorides reported by Dieke.4 Only those ion states which give rise to luminescence in chelates are designated here as resonance levels. As discussed elsewhere,2,3 ion luminescence arises from an indirect excitation of the ion resonance level via a radiationless transfer of energy from the triplet system of the chelate to the ion. Since (4) G. H. Dieke, “Speotroscopic Observations of Maser Materials. Advances in Quantum Electronics,” edited by J. R. Singer, Columbia University Press, New York, h‘. Y., 1961.
LUMINESCENCE SPECTRA AND DECAY TINESO F CHELATED RARE EARTH IONS
Dec., 1963
2719 OA
(3
f
P w Y
0
U
-I
m
+(D'
'Fa
W
k
U
-1
a w
Lk
a J w
K
Fig. 1.-Emission spectra of p-diketone complexes of trivalent dysprosium, samarium, and terbium: ///////////, transmission region of possible J-J assignthe filter system used for decay measurements; n, line group arising from indicated J-J transition; r"", ment. x, followed by a given number, is the factor by which the exposure time had to be multiplied in order to record the peaks shown in that spectral region. The time required to photograph the most intense group of lines for a given compound has been chosen as unity. Excitation source, intensity, geometry, etc., were all held constant for a series of exposures (displaced vertically) recorded on the same photographic plate for a particular compound with the exposures requiring progressively longer times. The final trace for a complex is a composite made up of the densitometer traces obtained from the series of exposures taken in the described way. This allows one to obtain an estimate of the relative line intensities for a given compound, although only a semiquantitative one. Yo intensity comparisons cain be made between different chelates from these traces.
emissions from Tb3+, Sm3+,and Dy3+arise from radiative transitions originating a t a unique resonance level for each ion, it is possible to assign most groups of line unambiguously to particular J-J transitions on the basis of energy only. The assignments are indicated on Fig. 1. Such J-J transitions would give rise to a single frequency in a free ion, but the electronic degeneracies of the combining states are partially removed by the electric fields of the surrounding ligands. Additional fine structure of the line spectra arises from the vibronic coupling of the ion with the ligand environment. Such ease in making J-J arjsignmerits is not possible for com-
plexes of the Eu3+ ion, for spectral lines from this ion originate from two excited states ( ~ D I5Do) , both of which are populated by energy transfer. Unique assignments of the strong lines can be made, however, by the simple technique of recording the spectra through a Becquerel phosphoroscope, which effectively eliminates the lines originating from the very short-lived 5D1 resonance level. Lines which appear in both the total emission and phosphorescence spectra can be assigned definitely to 5Do -.t 'F transitions (see Fig. 2). From the energy diagram one expects four additional groups of lines arising from the 5D1level, and the curious in-
J. J. FREEMAN AND G. A. CROSBY
2720
10 A
6000
Vol. 67
A
7000
A.
total emission
3
7--
totol emission
3
Eu
0-
p=c
,
5D0+
7Fq
_ - - - - - _I
CH
c3
z Z
w
emission observed through phosphoroscope
Y
u a
0
-I
m
w
Eu
- - --- - -
p=c. , CH
3t
0-C
Q
c a
J
0
I
I
w
I
I
I
total emission
> -
5D0+
c
'DI
'Do
Fo
- - - - - -' D-o + 7 F 4
+ 'F2
-i 7Fy
U
J W
a
H2?
emission observed through phosphoroscope
5D0+7F0
'Do+
7F2 t
r
Eu3t
P31
1 q. p=c b-
500+ 7F4 -----I 5D047F3
. - l
H2°
--- -- -.
3
I
20
I
19
I
I
1
17
I8
cm-' Fig. 2.-Emission
x
16
15
14
lom3.
spectra of p-diketone complexes of trivalent europium. (For explanation Bee Fig. 1.)
tensity changes in the region of 6700-7000 A. occurring in the emission spectra of EuA3 lead one to suspect that lines arising from the jD1 level are present for at least this one compound. Other expected transitions are evidently so weak, however, that they cannot be discerned by differences between the total emission spectra and the phosphorescence spectra. Rate Constants for Intramolecular Energy Transfer.-The semilogarithmic plots of the luminescence decays exhibited by all the compounds studied possess , typical plot (DyB8.2H20) the same salient features. 4 is given in Fig. 4a. The logarithm of the intensity of the luminescence rises rapidly after the excitation flash, reaches a maximum, and falls off slowly, eventually becoming linear with time. Such a radiation intensity curve is indicative of a
two-step first-order exponential decay mechanism (see Fig. 4b) in which the initial nonradiative step Kith rate constant XI is followed by a radiative step with rate constant Xz. The radiation intensity ( I ) as a function of time is given by the equation
(assuming no population of the radiating state at zero time).5 At the time, t, at which the curve reaches a maximum, the following relationship holds
( 5 ) G. Friedlander and J. W. Kennedy, "Nuclear and Radioohernistry," John Wiley and Sons, Ino., New York, N. Y., 1960, Chapter 5 .
Dee., 1963
LUMINESCEKCE SPECTR-4
DECAY TIMES OF
ASD
CHELATED
RAREE A R T H
2721
IOKS
-0
-
7 F6 5 4
-3 -2
:FO
2
3
4
'b2
-'3/*
-5
I
7
FO L
sm3+
Fig. 3.-Energy
E"3+
-
Tb3+
Dy 3+
level diagram for rare earth chelates: - - -, triplet energy level for the complexes; ---, rare earth ion resonance level in chelates.
As can be deduced from eq. 1, the rate constant A, corresponding to the slower of the two steps can be obtained by measuring the slope of the linear region of the plot of In I against t. A measure of A, and t, may then be used to calculate the rate constant Af for the faster of the two steps. Because of the symmetry of the equations with respect to AI and A 2 one can identify AI with A, and A 2 with Af or v i c ~versa. Additional information is needed to make the correspondence unique. The mean lives, r8 and q (reciprocals of the rate constants), of the long-lived and short-lived states, respectively, as calculated from the decay curves of the rare earth chelates are summarized in Table I. I n this table we have also Iisted the mean lifetimes reported by Dieke and Hall6 for the decay of rare earth ion emissions when the ions are incorporated in inorganic crystals and excited directly by absorption of ultraviolet light. As can be seen from the table, there is a rough correspondence between the radiative lifetimes for the ions in inorganic salts and the rs derived from measurements on rare earth chelates. We therefore identify ra with the radiative step of the two-step decay mechanism (r8 = 1/Az, Fig. 4b). rs therefore represents the mean life of the resonance level of the rare earth ion. The quantity r f , which is on the order of 2 psec. for all the chelates studied, is assigned to the nonradiative step ( q = l / A l ) . rf must be associated with the ratedetermining step in the chain of events for the transfer (6) G . H. Dieke and L. A. Hall, J . Chem. Phys., 27, 465 (1957).
-, rare earth
ion level;
(a)
L IO 20 30 I
I
I
40
50
TIME p s . Fig. 4.-(a) Semilogarithmic plot of the luminescence decay of DyBs.2HdI at 77" K.; (b) schematic energy level diagram showing a two-step first-order decay mechanism: .-+, nonradiative step; +, radiative step.
of energy from the flash lamp to the resonance level of the rare earth ion. There are three distinct rates to be considered : (a) the rate of pumping the singlet state by the flash; (b) the rate of energy migration from the singlet system t o the triplet system of the complex; and (c) the rate of energy transfer from the triplet
27 22
J. J. FREEVAN ASD G. A. CROSBY
Vol. 67
system to the rare earth ion resonance level. (For a detailed discussion of energy migration in rare earth chelates see Whan and Crosby,2aespecially section c of Results and Discussion.)
level; and kt is the rate constant for energy transfer. Using T P = 2 X sec., kt 2 5 X lo5 sec.-l, we obtain a negative value for k,, an impossibility. We conclude that energy transfer to a rare earth ion does not originate from the same excited triplet level of the comTABLE I plex as that one from which phosphorescence occurs. MEAN LIVES CALCULATED FROU THE LUMINESCENCE DECAY It is generallv accepted that phosphorescence originates CURVES OF COMPLBXED RAREEARTH IONS from the lowest triplet level in a molecule7 Energy e Mean lifetimes, fisec.a transfer to the rare earth ions must then originate at MBa.2HaO MDs in MArHnO MCls.6some higher level of the triplet system. This higher Rare i n EMPA EMPA @ in EMPA Ht0 solid level may be an excited vibrational level of the Iowest earth Transition @ 77OK. 77'K. @77OK. @77OK. ion observed Tf 7s Tf 7s rf T(radiativa) triplet or possibly a higher electronic level of the triplet Tb*+ 'D4+'Fg 2 630 2 464b 2 903 487' system. For either alternative the transfer process Eu'+ W p +'F'E"z 2 430 1 361 2 564 120" must compete favorably with internal conversion within Sm*+ * F S / , + ~ H ~3/ ~ 14.1 2 20.1 . , . . . . e -10" the manifold of triplet levels of the molecules. The Dya+ ?-+6H~8/2 2 12.3 . . ...d .. . ..' -10' latter process is generally assumed to have a rate conProbable error 1 5 7 , . Semilog plot of decay is curved stant of -10l2 sec.-l, which mould dictate a comparable Dielre initially; T~ calculated from linear region a t long times. value for the rate constant for intramolecular energy and Hall (see ref. 6). No Dy3+ luminescence observed from transfer, Le., ~ 1 0sec.-l. ' ~ this chelate. a Rare earth ion luminescence observed but intensity of emission too weak for lifetime measurements. We wish to emphasize that the arguments of this section are based wholly upon the assumption that the Since it has been s h ~ w that n ~ intersystem ~ ~ ~ crossing molecular phosphorescences observed through the (step b) competes favorably with fluorescence in rare phosphoroscope arise from those same molecular earth chelates, it can be concluded that the rate conspecies in which energy transfer is occurring. There is stant for intersystem crossing is of the same order of still the possibility that the weak molecular emissions magnitude or greater than the rate constant for radiawhich we observe are originating from some dissociative tive decay of the singlet state, which places the interspecies in solution although the structure of these molecsystem crossing rate at approximately 108 sec.-l or ular spectra support our contention that they do indeed greater. Step b is not rate determining. arise from the species in which transfer is occurring. For all compounds the measured rate constant Xf = Decay of 4f Electronic Excited States.--As pointed X I is -5 X lo5 sec.-l, which coincides with the measout above, the magnitudes of the measured radiative ured rate constant for decay of the excitation flash. decay times for a particular rare earth ion in various We thus identify step a as the rate-determining step. chelates are comparable to those measured by Dieke This immediately places a lower bound of -5 X l o b and Hall in hydrated chloride crystals. Two generalsec.-l on the rate constant for step c, the intramolecular izations from the data presented in Table I are obvious. energy transfer process. The decay times for an ion are Ionger in chelates than they are for the hydrated chloride; for a given ligand Recently, stimulated emission has been reported the magnitudes of the measured lifetimes of the ions from the 5Do-t.'F2 transition of Eu3f in EuB3.* Pumpare directly related to the magnitudes of the energy ing of the ion was carried out by intramolecular energy gap separating the lowest resonance leirel of each ion transfer. The attainment of population inversion of from the next lower electronic state. This connection Eu3+ in EuB3 means that the transfer rate constant between Iifetimes and energy gaps was noted by Dieke from complex to ion is certainly greater than the rate and Hall for hydrated inorganic salts. These investiconstant for decay of the ion. The observation of stimugators also observed that the intensity of the emitted lated emission is consistent with our experimentally light was strongest for those ions exhibiting the longest determined lower limit of 5 X lo5sec.-l for the transfer decay times. These same connections between emisconstant. sion intensities and magnitudes of decay times hold Nature of the Transferring Levels.-As reported true for the complexes studied here. Although the earlier,2a triplet-singlet emission (molecular phospostulated vibronic coupling mechanism of Dieke and phorescence) has been observed through a Becquerel Hall appears to explain the spectral characteristics for phosphoroscope for a t least three of the chelates studied chelates, the greater intensities and longer lifetimes here (SmD3, SmB3, DyB$. Since the resolution time observed from rare earth chelates compared with the of the phosphoroscope was -lo-* sec., it can be conhydrated ions indicate that the quenching mechanism is cluded that the mean lifetimes of these phosphorescing less efficient in chelates than in hydrated salts, a t least (triplet) levels are no shorter than 20 psec. a t 77 OK. If one assumes that molecular phosphorescence and Recently several papers have appeared which report energy transfer to the ion both originate a t the same lifetime data for chelated rare earth i o n ~ . ~ - l lWith the triplet level, one has exception of the value of 470 psec. reported for EuB3 in EPA a t 90" K. by Bhaumik, et al., the published values 1 TP = cannot be expected to agree with those reported here. kr kt We find that the measured decay times are a sensitive function of the physical state of the compounds, the where r p is the measured lifetime of phosphoresence; IC, is the rate constant for radiative decay of the triplet 78
+
I__
(7) M. Kasha. Dzscussions Faraday Soe., 9, 14 (1960). (8) A. Leinpicki and H. Yamelson, Physics Letters, 4, 133 (1963).
(9) F. F. Rieke and R. .4llison, J . Chern. Phys., 37, 3011 (1962). ( l o ) E. Nardi and 9. Yatsiv, tbid., S I , 2333 (1962). (11) M. L. Bhaumik, H. Lyons, and P. C. Fletcher, zbid., 88, 568 (1963).
Dec., 1963
LUMISESCESCE SPECTRA AXD DECAPTIMESOF CHELATED RAREEARTHloxs
nature of the solvent used, the temperature, and the method of preparation employed. Studies on solid rare earth complexes are in progress.
2’723
Acknowledgment.-The research presented in this communication was supported by Saiidia Corp., a prime subcontractor to the Atomic Energy Commission.
CAR.BON-13 CHEMICAL SHIFTS OF para-DISUBSTITUTED BENZENES BY GEORGE B. SAVITSKY Department of Chemistry, University of California, Dacis, Califoinia Receiced June 27, 1963 The additiivity relations of C13chemical shifts in polysubstituted benzenes previously established by Lauterbur for two electron releasing groups (CH,, OCH,) were also found to hold for three other electron releasing groups (C1, 13r, F) and one electron withdrawing group (COCH3) in a series of para disubstituted benzenes of the type p-C6H4X2. The deviations from these additivity relations in three compounds (anisaldehyde, p-chloroanisole, and p-bromoanisole) which were expected on theoretical grounds were found to be of the order of magnitude not exceeding f 2 p.p.m. and could be readily masked by solvent effects. It is concluded that additivity relations should be helpful in the assignment of C13 n.m.r. spectra of polysubstituted benzenes. However, it seems doubtful that the interpretation of small deviations from additivity relations can be used as a tool for the study of T electron density distribution in the benzene ring before solvent effects are better understood and techniques of higher precision in measuring C13 chemical shifts are developed.
Introduction The analysis of the CI3 n.m.r. spectra of 11 monosubstituted benzenes by Spiesecke and Schneider’ has led to the conclusion that a-electron densil y dominates the shieldings a t the para positions. They also found that the meta carbon shifts mere small and relatively iiisensitive to the iiature of the substituent. They attributed the shifts on the remaining carbons to inductive and magnetic anisotropy effects. Lauterbur’s extensive studies on polysubstituted benzenes2-6 amply demonstrated that the effects of the two electron releasing groups, namely, a weakly electron releasing methyl and a strongly electron releasing methoxy group, were additive in para and meta positions. The “nearest neighbor” effects seemed to interfere in ortho positions. Thus, barring steric and “ortho” effects the resonance contribution of at least two groups sludied so far seem to be practically independent of contributions to shielding of other substitutents which are meia and para to them. The purpose of this work is to extend the investigation of additivity relations to other substituents, including the electron withdrawing groups. It was decided to limit this investigation to para disubstituted compounds, since two groups in para positions are the least likely to interfere with one another by long range anisotropy effects, as well as to substituents whose effects were carefully and unambiguously determined by Spiesecke and Schneiderl with the aid of deuterium substitutions in rnonosubstituted benzenes. If we denote the chemical shifts in monosubstituted benzenes, C6136X, relative to benzene, by X,, X,, X,, and X, 011 the substituted carbon, ortho, meta, and para carbon, respectively, and the chemical shifts on the para disubstituted benzenes, p-CaHiXY, by XIs, Yts,XIo,Y‘, on the carbons substituted by X and Y, and on the carbons ortho to X aiid t o Y, respectively, the additivity relations of chemical shifts imply (1) H. Spiesecke and W. G. Schneider, J . Chem. Phvs., 35, 731 (1961). (2) P. C . Lauterbur, J . Am. Client SOG 85, 1838 (19fil). (3) P C. Lauteibnr, zbzd. 83, 1846 (1961). (4) P.C . Lauterbui, J Chem. Phys. 38, 1406 (1963). ( 5 ) P. C. Lauterbur zbrd , 38, 1415 (1963). (6) P.C. Lauterbur, %bad., 35, 1432 (1963).
XI,
X’,
x,+ Y, = x,+ Y , =
+ Yo Y’, = X, + Y,
TI, In case X
=
=
X,
Y these relations simplify to XI,
=
X‘osrn
x,+ x,
=
Xo
+ Xm
In addition, the spectra of p-CsH4Xz compounds lead to completely unambiguous assignments. Iii this case the ring carbons fall into two groups, namely, two equivalent carbons substituted by X which result in a single resonance peak (except for X = F, in which case it is split into a doublet) and four remaining equivalent carbons resulting in a resonance signal split into a doublet by the neighboring hydrogens with J C H 160 C.P.S. Thus the spectra of p-C6H& conipounds offer a very reliable check on the additivity relations for various groups. Unfortunately, some of the compounds of this type are solids which are sparingly soluble in various organic solvents, thus precluding n.m.r. studies without isotopic enrichment. These experimental limitations restricted this work to the measurement of four such compounds (X = C1, Br, F, and COCH,). Attempts to detect measurable signals in various organic solvents have failed so far for pdiiodobeiizene, p-dinitrobenzene, and p-phenylenediamine. In case of the mixed para disubstituted benzenes pCBH,XY, the interpretation of the spectra is no longer unambiguous, there being now four different signals (two singlets corresponding to XI, and Y’,carbons and two doublets corresponding to XI, and Y’, carbons), The best one can do in this case is to compare the experimentally observed values to the values calculated, assuming additivity relations 1-4, which are the nearest to them in position. Since most of Lauterbur’s work w~bsdone with electron releasing groups (except with nitrotoluenes,6 in which the signal correspondiiig to carbon bonded to the nitro group could not be de-
