Francesco Castelli and beslie S.Forster
2122
eferences and Notes
(9) J. L. Bates. Nucl. Sci. Eng., 21, 21 (1965).
(IO) R. M. Berman. Westinghouse Electric Corp., Bettis Atomic Power Labo-
(1) Work performed under the auspices of the U. S. Atomic Energy Commission. (2) (a) L. H. Little, "Infrared Spectra of Adsorbed Species," Academic Press, New York, N. Y., 1966; (b) M. L. Hair, "Infrared Spectroscopy in Surface Chemistry," Marcel Decker, New York, N. Y., 1967. (3) Ya. M. Grigorev, Rum. J. Phys. Chem., 46, 2, 186 (1972). (4) International Atomic Energy Agency. Vienna, Technical Report Series No. 39.. 1965. (5) R. R. Heikes and W.D. Johnston, J. Chem. Phys., 26, 582 (1957). (6) R. A. Wolfe, Westinghouse Electric Corp., Bettis Atomic Power Laboratory, Pittsburgh, Pa., Report No. WAPD-270, 1963. (7) S. Aronson, J E. Rulli, and B. E. Schaner, J. Chem. Phys.. 35, 1382 (1961). (8) S. Amelinckx, Centre d' Etude de InEnergie Nucleaire, MOL, Belgium, Report No. EUR- 1414e, 1965.
uanttum Yields of 2E,
-
ratory, Pittsburgh, Pa., Report No. WAPD-316, 1967. (11) M. Tsuboi, M. Terada, and T. Shimanouchi, J. Chem. Phys., 36, 1301 (1962). (12) S. lida, Jap. J. Appl. Phys., 4, 823 (1965). (13) Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U. S. Atomic Energy Commission to the exclusion of others that may be suitable. (14) L. Lynds, W. A. Young, J. S. Mohl, and G. G. Libowitz, Advan. Chem. Ser., No. 39 (1963);, (15) F. F. Vol'keshtein, The Electronic Theory of Catalysis on Semiconductors," Pergamon Press, New York, N. Y., 1963. (16) D. 0. Hayward and B. M. W. Trapnell, "Chemisorption," 2nd ed, Butterworths, Washington, D.C., 1964.
4A2gLuminescence in K3(Co,Cr)(CN)6 Powders'
rancesco Castelli and Leslie S. Forster* Beparfmentof Chemktty, Universityof Arizona, Tucson, Arizona 85721 (Received February 21, 1974)
-
Thje absolute quantum yields of Cr(CN)63- 2E, 4A2g luminescence have been measured for 3( CO,Cr)(CN)6 powders of varying concentration. Evidence for energy transfer from Co(CN)s3- to @r(CN)63- has been obtained. Surface defects quench the powder luminescence. After correction for the nntermolecular energy transfer and surface quenching, the intramolecular luminescence yield is close to unity a t low Cr3+ concentrations, indicating an intersystem crossing (4Tzg 2Eg)yield of 0.8 ICp2, 5 1.
-
Introduction Considerable interest has been evidenced in the photophysical proplerties of Cr(CN)G3-. Studies in fluid systems have been focussed on the identification of the photoreactive stateza and on the evaluation of the efficiency (CpzE) for the nonradiative 4T2* 2Eg(Figure 1)intersystem crossing transition.2b The importance of this complex lies in the failure of C ~ ( C N ) Gto~ -fit into the photochemical pattern apparently followed by other Cr3+ complexes, uiz., the photoaquation yield increases with Dq. An understanding of the anomalous case may provide a basis for rationalizing the more general behavior. In addition to the impetus generated by the concern with the photochemical behavior of C ~ ( C N ) G ~studies -, of this species in the solid state have been prompted by an interest in intermolecular energy transfer. For this purpose stoic h i ~ m e t r i c , ~e.g., Cr(~rea)6~+-Cr(CN)6~-, and nonstoK3(Co,Cr)(CN)6, mixed crystals have i c h i ~ m e t r i c ,e.g., ~ been employed. Our attention was drawn to Ks(Co,Cr)(CN)e systems in connection with an evaluation of absolute quantum yields (@p)of powders containing Cr3+ complexes.6 In contrast to the independence of Cpp with excitation wavelength, as in the case of NaMg(Al,Cr)(C20& 9HzO powders, an apparent wavelength variation was observed for K3(Co,Cr)(CN)G powders. Since the absorption of the K3Co(CN)6 host is important in this instance, a somewhat more involved procedure is required for the determination of Cpp. We have now evaluated the absolute Cpp of Ks(Co,Cr)(CN)6 powders as a
-
The Journal of Physical Chemistry, Vol. 78, No. 21, 1974
function of Cr3+ concentration with the aim of answering the following questions. (1) Is excitation energy transfer from C0(CN)c3- to C ~ ( C N ) Gimportant? ~(2) What is the intersystem crossing efficiency (4Tzgth2Eg)in C ~ ( C N ) G ~ - ? (3) Is Cpp concentration dependent? In addition to these specific points, a more general purpose of this work is to validate the powder technique for absolute Cpp measurements in systems where the host absorption is large. Experimental Section Apparatus. The apparatus used for the measurement of quantum yields consisted of a PEK 100-W Hg lamp, and a Bausch & Lomb (6.6-nm band width) monochromator for excitation. Light intensities reflected and emitted from the sample and from the MgO reference were measured by a photodiode. For more details on the apparatus and a discussion of the method used see ref 6. For the excitation and reflectance spectra, the radiation from a 75-W Xe lamp, passed through a Bausch & Lomb monochromator with 1.65-nm band width, was used as the source. This source was calibrated with a Rhodamine €3 solution quantum ~ o u n t e rThe . ~ same exciting light band width was used for both excitation and reflectance of spectra. The detection system consisted of a 0.25-m Jarrell-Ash monochromator (3.3 nm/mm dispersion) and a Centronic photomultiplier (Q4283SA, red extended S-20 response). The photomultiplier output was dc amplified and recorded. In the reflectance spectral measurements no slits (10-mm aperture) were used on the Jarrell-Ash monochromator, ie., the band
Luminescence Measurernents for K3(Co,Cr)(CN)6Powders
21 23
q. ............. ...........................
co
(CN):-
Cr (CN);.
Figure 1. Energy levels of C O ( C N ) ~ ~ and - Cr(CN)63-. O
width of the detection system was larger than that of the excitation source. The excitation spectra were monitored at 826 nm. The linearity of the detection system was confirmed with calibrated filters. Pulsed measurements were performed with a N2 pulsed laser (AVCO C-950) for 337 nm and N2 laser pumped PBD and BBOT dye lasers for 366 and 436 nm, respectively, for excitation. The pulse widths were 10 nsec or less. The dye lasers were operated in the transverse configuration with line width8 of -3 nm. A sample holder, with 2-mm deep parallel recesses on both sides, similar to that used for the steady-state measurements was used. This holder was mounted rigidly, and by 180" rotation it was possible to quickly interchange two different samples in precisely the same position. In measuring c (see text) a pure K3C~(CN)6 sample in one side of the holder was used as a reference in order to check the constancy of the excitation intensity. The emission from the sample was focussed through 5 cm of a saturated IA&rz07 solution onto the slit of the JarrellAsh monochromator. The slits used were typically 5 mm although 2-inm slits were used for the rise time measurement. The light was detected by a C-31034 RCA photomultiplier with a 1kQ load resistance. The output of the photomultiplier was fed to the probe of a Tektronix 7904 oscilloscope with a 7All amplifier. The response time of the detection system was lesrr than 0.1 psec. For time-resolved spectra the output of the photomultiplier was fed into the 5 0 4 input of a PAR Model 60 boxcar integrator. The angle between the excitation and emission was 45" and the excitation was normal to the powder surface. Except as indicated, all the measurements were performed at iroom temperature, Powder Sample prtparation R,ecrystallized K&r(CN)6 and K ~ C O ( C Nwere ) ~ dissolved in water in the desired mole ratios and reprecipitated in tetrahydrofuran as described in ref 5. The powders obtained were passed through screens and -45 pm powders were used for the measurements. Single crystals were grown by evaporation from aqueous solutions. The Crs+ concentration was determined by dissolving the crystals and measuring the absorbance of the resulting solutions. &Cr(CN)6 was electronic grade from City Chemical Corp. K3C:o(CM)6 was from Alfa Inorganics. PBD and BBOT dyes were from Calbiochem. Results Energy Transfer, Co ( C L V ) ~ ~ -Cr (CN)63-. A comparison of the emission spectra of &Co(CN)e and K3(Co,@r)(CN)6 indicates that the emission a t 700 nm arises, in both systems, from the 3T1, lA1, transition of the Co(CN)s3- moiety. On the contrary, the 826-nm emission of 5% K3(Co9Gr)(CN)6excited at 337 nm contains con*%
-
/
- 02 -
-
04 -
0.6
-
08
10
(p)
Flgure 2. Rise time of 2Eq 4A2, Cr3+ emission in 5% M3(Co,Cr)(CN)6(337-nm excitation).
E
, (D
H
03
02
I\-, 0 2
0. I 0
04
0.6
0.8
IO
(1"s)
Figure 3. Evaluation of the rise time from data in Figure 2.
-
tributions from both the 3T1, IAl, Co(CN)$- and the ZEg 4AzgCr(CN)s3- transit~ons.~ Since the 3T1, state in C O ( C N ) ~has ~ - a much shorter lifetime (-5 psec) than the state (-60 msec), energy transfer from Cr(CN)63C O ( C N ) ~to ~ -c r ( c N ) ~ will ~ - be evidenced by a rise time in 4Azgemission intensity that corresponds to the the 2E, 3T1, lifetime. Due to the overlapping 3T1, lA1, and 4Azg emissions, the direct observation of the rise time is not possible. However, the contribution of the C O ( C N ) ~emission ~can be subtracted from the total emission at 826 nm and 4Azgemission determined. the time course of the 2E, The correction for the 3T1g lAl, emission was made in the following manner. For K&o(CN)e, the ratio of the 3T1, IAl, intensities ( h ) a t 826 and 700 nm is dependent of time after pulsed excitation. By monitoring the decay of 5% K3(Co7Cr)(CN)6at both 700 and 826 nm, the Cr3+ emission can be calculated from the expression, 1 8 z 6 c ' j t ) = Z ~ 2 6 C " + C r ( t ) - kZ700(t), where Za26co+Cr((t) and 1 7 0 0 ( t ) are the measured decays at the indicated wavelengths. Cr3+ emission is negligible a t 700 nm. The computed I a Z 6 C r ( t ) clearly exhibits the delay in reaching the maximum value that is characteristic of energy transfer (Figure 2). At 337-nm excitation, Cr3+ is excited diirectly by absorption into 4T2g( I d ) and indirectly by energy transfer from Co3+ (Ic). On the time scale indicated in Figures 2 and 3, the 2E, decay is negligible and the signal intensity (Imax) remains constant after the Co3+ emission has decayed. If y is the fraction of the Cr3+ emission induced by energy -+
-
-
-
--
--j
The Journalof Physical Chemistry Vol. 78, No. 21. 1974
2124
Francesco Castelli and Leslie S. Forster
-ABSORPTION transfer, then It -. ,y(l - e-t'T)lmax and I d = (1 - y)lmax, .......EXCITATION where the rige time, 7, is the "effective" lifetime of the 100, 1 donor. Since 3826cr = I t I d = (1 - ye+7)Zm, a plot of log (1 - IS26Cr/lmax:) us. t, should yield 7.A good exponential fit is obtained with T = 0.5 psec. Extrapolation to t = 0 leads to y = 0.70 (Figure 3). The 0.5-psec risetime is much smaller than the "normal" 5.0-psec 3T1g lifetime of K&o(CN)g. In K3(Co,Cr)(CN)6, the "1, decay is nonexponential. As the Cr3+ concentration increases, the 5-psec component decreases; it is quite small a t the 10% level and undetectable in the 15% samples. Up to 5%, the :jTIgdecay can be well described by a double --.__ ! --.._ exponential with 6.0and 0.5 ysec components, but at higher 250 300 350 400 450 250 300 350 400 450 Cr3+ levels a 3.2-psec component grows in importance and A(nm) replaces the 5.0-psec decay. Nonetheless, the 0.5-psec comFigure 4. Per cent absorption and relative excitation spectra of K3ponent persists up to 15%Cr3+. The 0.5-psec 3T1, decay is (Co,Cr)(CN)6.The excitation spectra were determined by monitoring thus associated with the 0.5-psec 2E, 4A2grise time. the emission at 826 nm. In view of t,he possible involvement of surface defects (uide infra 1, it i s noteworthy that nonexponential 3T1, Absolute Quantum Yields of 2Eg 4A, Luminescence. dewy is also observed in single crystals of 5% We have demonstrated the applicability of the powder E;3(Clo,Glr)(CN1~ a t room and low temperatures. method for the determination of the luniinescence yields of Time-resolved spectra were also recorded using 337- and ruby and NaMg(AI,Cr)(C204)3 9H20.6 The basic equation 366-nm excitation for pure K&o(CN)6 and 5% employed was K3(C!o,Cr)(CN)Gpowders. In general, these spectra are the quanta emitted sum of .the emission. spectra of Co(CN)s3- and c r ( c N ) ~ ~ - , CPP (1) the relative contributions changing with time. With excitaquanta absorbed tion a t 337 nm the short-time (100 nsec after the excitation where D and C are the correction factors for reabsorption pulse) emission spectra for the two powders are very simiof the emission and absorption of excitation by the host, relar sild the Cr(CN)s3" emission is barely visible. The absospectively. @p refers to excitation into *T* followed by of this, latter signal increases with time in the lute intensity 4A2emismicrosecond range due to energy transfer from C O ( C N ) ~ ~ - . emission from 2E.The overlap between the sion and absorption for the above Cr3+ species is significant On the other band, a t 366-nm excitation the Cr(CN)G3and D was as large as 1.45. On the contrary, 6, as estimatemission inteiisity does not change appreciably in the same ed by the Kubelka-Munk functions,s was within 3% of time range irdicating that a t this excitation wavelength unity because of the small host absorption. In the most of the Cr(CN),:3---emission comes from direct absorpK d c o , C r ) ( c N ) ~powders, the converse situation obtains. tion and that the 4T2g 2Egintersystem-crossing process The overlap between the Cr(CN),j3- absorption and emisis much faster than 1100 nsec. sion spectra is small and the reabsorption correction, D, Additional support for the energy transfer hypothesis never deviates from unity by more than 10% in can also be obtained from steady-state measurements. The K3(Co,Cr)(CN)cpowders. corrected excitation spectrum (826-nm emission) and abThe Kubelka-Munk theory is now unsatisfactory for the sorption spectrum (from reflectance data) obtained from evaluation of C due to the large host absorption.* Furtherthe same powder samples are presented in Figure 4. more, the absorption varies from one powder preparation The ratio of the absorption at 370 nm, where Cr3+ abto another of the same concentration, even in pure sorption dominates, to that a t 310 nm, where Co3+ is the K3Co(CN)6. In spite of this variation, q5p is quite reproduciprincipal absorber, :is not very different for the 5 and 15% 4A2g ble (f5%),but application of the Kubelka-Munk theory repowders. However, the relative intensity of 2E, quires that the host absorption be constant. Not only must emission excited a t 310 nm compared to 370 nm increases a different procedure be devised for the evaluation of C, with the Cr3" concentration. The same pattern is also obbut a correction must be made for the @r3+ emission protained with the 1. and 10% powders. Since the increased duced by energy transfer from Co3+ if the intramolecular Cr3+.emission i s not due to increased Cr3+ absorption , it quantum efficiency, (Pp, is to be computed. Equation 1, as ;an only originate in an energy transfer from Co(CN)s3- to modified to correct for energy transfer to 2E, is c r ( c N ) ~ ~ T'be - . efficiency of the energy transfer increases with Cr3+ concentration, since the likelihood that an acceptor will be close to an excited donor increases with acceptor concentration. where CY is the fraction of light absorbed by Co3+ that is On energetic grounds, lTIg can transfer to either 4T2gor transferred to the 2E state of Cr3+ and is independent of 2E,, but only "E,ca:n be excited by 3T1,. The quenching of excitation wavelength. lAl, emission, coupled with the conthe CO(CN)G"'-"1, Estimation of CY. This quantity is evaluated by plotting comitant rise of the Cr(CW)s3- 2E, 4A2gemission, clearly the decay curves of K&o(CN)6 (Cr = 0) and demonstrates the im.portance of the energy transfer process K3(Co,Cr)(CN)6powders for emission monitored at 700 nm 3T1, -t- 4A2g 'AI, 2E,. In the following discussion ener(Co3+ emission). If the t = 0 intensities are normalized to 3T1, gy transfer from "TI, will be ignored and the lT1, compensate for differences in lT1, lA1, absorption, the intersystem crossing efficiency in C O ( C N ) ~ will ~ - be asareas are proportional to the total number of 3 T ~ , lAl, sumed to be unity. We will show later that the results supquanta emitted (Figure 5 ) and a can be computed from the port this assumption. expression
+
-
-
=(
-
-
-+
-
4.
-
+-
The Journalof Physical Chemistry, Vol. 78. No. 21. 7974
-
-
-
Luminescence Measurements for Ks(C0,Cr) (CN)6 Powders
21 25
TABLE I 337 nm
cr3+ concn, %
1 5 10 15 a
366 nm
9.
6PD
C
@'pa
@P/PQ
0 PD
C
0.06 0.28
0.06 0.24 0.21 0.16
0 0.10 0.13 0.17
1.0 0.68 0.34 0.23
1.0 0.92 0.55 0.42
0.22 0.47 0.26 0.16
0.19 0.55 0.75 0.80
0.51
0.65
apt2 0.92 0.70 0.29 0.1'7
@P/P"
0.92 0.93 0.48 0.37
Computed for axE= I.
Computation of @p. It was convenient to measure +p a t the emission wavelengths of the Hg lamp (366,404, and 436 nm). The reabsorption corrections, D, were computed as described in ref 6 and the relevant quantities are listed in Table I. Each @p represents the average of a t least three determinations with different samples. Except for the 1% powders where C and a are small and errors as large as 15% are possible, the @p values are reliable to 5%. In Table I1 the effect of Cp2E on Cpp computed from eq 1' is illustrated. The value of q5p a t 337 nm was obtained by a comparison of' the 826-nm excitation (E) and absorption (a) spectra of a given sample at 337 and 366 nm (Figure 4). In all cases the 3T1, ]AI, C O ( C N ) ~ emission ~was negligible at 826 nrn and the expression employed was
I O
075
-
o
2
i
4
3
5
6
7
8
9
(p)
Figure 5. 3T1g 'A ig cC)((>N)63- decays from K3(CO,Cr)(CN)6 powders. All curves arbiirarily normalized to 1.0 at t = 0 (337-nm excitation). -+
Q
(Acrzo
(21
Acr)/ACr=O
-
where A = j 0 - 1 dt. hi using eq 2 to calculate a , we assume that 3T1, lA1, quenching is due solely to energy transfer * has unit efficiency. These assumptions are and l T l y ~3T1, necessary because only the 3T1, state quenching is monitored by the measurements. The radiative rate constant for this transition is unaffected by changes in the Cr3+ concentration as is shown by the near constancy of 1 7 0 0 ( 0 ) (cf. next section). The a values are included in Table I. Determination of 6. The t = 0 intensities of the 700-nm emission are proportional to the light absorbed by c o ( c N ) ~ ~ - The . fracl,ion of the absorption due to Cr(CN)$- is then
c:1-
a,,,o
~ ~ 7 o o ~ ~ ~ l c r
(3)
[I7,o(O)l,r=,
where ac,=o arid acr are the fractions of incident light absorbed by the &Co(CN)6 and K3(Co,Cr)(CN)6 powders lAl, t (Figure 4) and the 1 ' s are the corresponding 3T1, = 0 Co(CN)& emissions a t 700 nm. The variation of 1700(0)at 337 nm is less than 15%in the 0-15% Cr3+ range. The values of C computed from eq 3 for both 337- and 366-nm excitation are listed in Table I for K3(Co,Cr)(CN)6 samples of varying C'r3-t concentration. Implicit in eq 3 is the condition that the host absorption is due mainly to a situation likely to obtain at 337 and C O ( C N ) ~ centers, ~366 nm where thc Co(CN)e3- molecular absorption is high compared to the nonspecific absorption of the powder. The reabsorption at 700 nm 11snegligible in all samples.
-
Since 1.0 < D < 1.1at 366 nm, D 1 a t 337 nm where the penetration depth is less. The Cpp values for 337 nm are also collected in Tables I and 11. Surface Luminescence. The 2E, decay from single crystals of Ks(Co,Cr)(CN)6 is accurately exponential up to 25% Cr3+.9 In 1%powders the emission decay is likewise exponential, but as the Cr3+ concentration increases, a fast 2E, 4A2scomponent is observed (Figure 6). The absence of the fast component in single crystals suggests an origin in the powder surface. In the steady-Btate measurements the yield from the surface centers will be less than 1%of the bulk emission, but the absorption by the surface sites will decrease Cpp. If all the properties, except the lifetime, are the same for the surface and bulk species, the t = 0 intensities of the fast and slow components will be measures of the light absorbed by the two types of centers. The correction factors, p, are calculated from
-
-
where IfsZ6(O)and 1,826(0) are the t = 0 intensities for the *&,. The corrected fast and slow components of 2E, quantum yields, @PIP, are included in Tables I and I1 and p decreases with the Cr3+ concentration. Although /3 is almost the same at 337 and 366 nm, it becomes smaller at longer wavelengths and other faster components appear in the decay curves of 25% powders. For the 1%powder and for crystals with Cr"+ concentration as high as 5%,the decay remains exponential (the lifetimes of crystals with higher Cr3+ concentrations were not measured). At excitation wavelengths of 404 and 436 nrn the values of Cpp calculated from eq 1 (C determined by the Kubelka-Munk function) are 0.50 and 0.1 I respectively, for the 5% powder. The decrease, from @p = 0.7 found at 337and 366-nm excitation, is probably due to the decrease of p The Journal o f Physical Chemistry, Vol. 78, No. 21. 1974
21 26
Francesco Castelli and Leslie S.Forster
TABLE II &337
cr3+concn, %
@P337/P
92g 1
1 5 10 15
1.0 0.68 0.34 0.23
0.8
0.5
1
0.8
0.5
1
0.8
0.5
1
0.8
0.5
0.8 0.58 0.28 0.19
0.50 0.40 0.19 0.13
0.92 0.70 0.29 0.17
0.88 0.66 0.28 0.17
0.77 0.59 0.25 0.15
1.0 0.92 0.55 0.42
0.80 0.78 0.45 0.35
0.50 0.54 0.30 0.23
0.92 0.93 0.48 0,37
0.88 0.88 0.46 0.36
0.77 0.78 0.41 0.33
the Cr3+ emission excited by energy transfer should exhibit a 0.5psec rise time. As described above, the fraction of Cr3+ emission due to energy transfer was calculated from Figures 2 and 3 as 0.7 for 5% KdCe,Cr)(CN)6. y can also be calculated from the equation
67
5
+P366/P
~-
__
A
y = a(1 - C ) / [ a ( l -- c) 4- C@2
E
2c--7---
05
10
I5
20
(m SI
-*4A29 Cr(CN)e3- decays from K3(CO,Cr)(CN)6POWders (337-nm excitation).
Figure 6. 2E,
at longer wavelengths. In the case of 404- and 436-nm excitation, it is more reasonable to use the Kubelka-Munk function in order to calculate C because of the smaller absorption of C'o(CN 1 ~ for ~ - 25% K3(Co,Cr)(CN)6 powders; for these powders C was -0.93. At 404- and 436-nm excitation, increasing the concentration of Cr3+ from 5% also led to a large decrease in @p. For 1 hese excitations it was not possible to calculate the correction factor p, but it decreases a t the longer wavelengths. The possibility that surface defects affect @p by absorbing part of the incident light is supported by the 77'K measurements. While @p at 366 nm, a wavelength close to the abscrption nraximum, did not change with temperature, lowering the tempeirature to 77°K led to a large decrease in @pa t longer wavelengths. For the 5% K3(Co,Cr)(CN)6powder, the valws o f @p are 0.16 and 0.016 a t 404 and 436 nm, respectively. Moreover, these values change with time when irradialed, becoming as large as 0.7 a t 404 nm and 0.22 a t 436 nm for the 5% powder. In the emission spectrum new bands appeared in the 750-800-nm range upon irradiation their relative intensities depending on the excitation wavelength. These phenomena, which are reversible with temperature, were found for all the powders. However they were absent i n single crystal samples. Discussion
Ertergy Transfer Although Kirk, et al.,5 did not completely exclude Co3+ Cr3+ energy transfer, they did not believe that energy transfer was responsible for a major portion of the Cr(CN)63- 2Eg-,4Azg emission. However, the observation of the 0.5-psec rise time for a significant $Azgemission clearly implicates energy part of the transfer as an impoytant source of Cr3+ luminescence in the mixed crystals when the lT1, lA1, Co3+ band is the principal absorber. As discussed below, we attribute the 0.5psec componmt in the Co3+ 3T1, decay to Co3+ with one nearest neighbor site occupied by Cr3+. Thus, the bulk of M+
-
-
The Journal of Physical Chemistry, Vol. 76. No. 21, 1974
I
(4)
In deriving eq 4 we have again assumed that the lTlg 3T1gintersystem crossing efficiency is equal to unity. At 337-nm excitation y, as obtained from eq 4 and the data in Table I, is 0.72 if @zE = 1 and 0.83 if @zE = 0.5. Analysis of the 3T1g lAlg C O ( C N ) ~decays ~as a function of Cr3+ concentration provides insight into the distance dependence of the energy transfer. The single exponential decay ( 7 = 5.0 psec) observed in pure K3Co(CN)6 becomes increasingly nonexponential as the Cr3+ concentration is increased. The intensity of the 5.0-psec component diminishes and the 3.2-psec component grows. In addition to the slow components, a fast (0.5 psec) decay is observed in the mixed crystals. As the Cr3+ concentration is increased further, additional components are observed, but the 0.5-psec component is still evident up to 15% Cr3+. K3Co(CN)6 is polytypic and crystallizes in either orthorhombic or monoclinic lattices.1° The differences between the Co-Co distances in both lattices are small and for our purposes can be ignored. Each Co has six nearest neighbors with Co-Co distances of approximately 7 A. The second nearest neighbors at 8.5 A are separated by an intervening K+ ion, but there are two Co sites a t 9.2 A without any intervening K+. In the mixed crystals, Cr3& occupies the Go3+ sites substitutionally. If the six 7-A sites are all occupied by Co3+, the 3T1glifetime is 5.0 psec. We ascribe the 0.5-psec lifetime to Co3+ with a single Cr3+ a t 7 A. Multiple occupancy of 7-A sites then reduces 7 to a still smaller value. The fraction of the more distant sites occupied by Cr3+ increases rapidly with Cr3+ concentration and the less efficient transfer to these more distant Cr3+ will merely serve to reduce the 5.0-psec component to 3.2 psec. The likelihood that all six 7-A sites surrounding an excited Co3+ are occupied by Co3+ is ( X C , ) (XC, ~ is the Co/(Co Cr) atom ratio). If this analysis is correct, the fraction of the t = 0 emission associated with the long-lived emission (either 5.0 or 3.2 psec) should also be (XcOl6.The agreement demonstrated in Table HI is better than might be expected. Intersystem Crossing Efficiency in Clr (CN)e3- (@zE). Although the 2E, lifetime of &(Co,Cr)(CN)6 decreases somewhat as the temperature is increased from 77'K to room temperature, it has been found that this increase is caused by a change in the radiative 2Eg 4Azg rate.ll Once the molecule reaches 2E,, it radiates. Consequently @p = (PzZ. The quantum efficiency of the radiationless 4Tzg ZE,
-
+
-
-
Luminescence Measurements for K3(Co,Cr)( C N ) 6 Powders
TABLE I1 @r3+ concn, %
Ratio between the slow component and the total Co emission at t = On
(XJ
1 5 10 15 25
0.94 0.70 0.46 0.37 0.19
0.94 0.74 0.53 0.38 0.18
T h e r a t i o was calculated by extrapolating the exponential t a i l the emission decay is nonexponentia! in the 10% K3(Co,Cr)(CN)6 powder, the value for t h i s sample is the least reliable. a
of
the decay back Do t =: 0. Because t h e t a i l of
process in C ~ ( C N ) Ghas ~ - been variously estimated as 0.15 and 10.5.2bJ2The small @2E in K3(Co,Cr)(CN)6 was based on a coniparison of the single crystal absorption and excitation spectra.ll This measurement was apparently erroneous since we have repeated the excitation spectrum of this system and found that 0.5 5 @zE 5 1.0. An upper limit of @2E (0.5) was obtained from the measurements of the b e n d sensitized luminescence of Cr(CN)e3- in a fluid solution at -1130.12 However, the data upon which this value was based scatter considerably and @2E = 1 is consistent with the measurements. This leaves the comparison of the direct and R u ( B i p y r ) ~sensitized ~~ luminescence at room temperature in dimethylformamide +zE 5 0.5, as the strongest basis for the belief that @zE < 1.2bThe evidence to support 5 1 is as follows. (11) For the 1% K3(Co,Cr)(CN)6 powder, where /3 = 1, @p’sare equal (within 10%) for both 337and 366-nm excitation. If @2E = 0.5, the wavelength independence disappeargi. (2) For the 5% powder, @p is the same a t 337 and 366 nm only if @zE 20.8 and then @p/@N 1. The invariance of @pl@to excitation wavelength and concentration (up to 5% Cr3+)supports the conclusion that 0.8 5 @zE 5 1. (3) In spite of very different values of C, @p/P are the same for the 3 0 and 15%powders a t 337 and 366 nm if @zE 0.8. (4) The agreement between y computed from eq 4 and directly from the rise time obtains only if @zE 2 0.8. Equation 4 also depends upon the assumption that ‘TI, M* “1, is unity. ( 5 ) At 404 nrn @p = 0.5 for the 5% powders. At 366 nm /3 := 0.75. We have not quantitatively determined /3 a t 404 nm, but this quantity definitely decreases at the longer wavelengths. These results again support the belief that 0.8 2; 43zE 5 1. To summarize, two aissumptions, uiz., (i) all Co3+ 3T1, quenching is due to energy transfer, and (ii) lT1, w* 3T1, has unit efficiency, and one parameter (QE) are employed in the calculation of +p by eq 1’which involves the computation of C and a by eq 2 and 3, respectively. When @zE 2 0.8, @PI@is the same with 337- and 366-nm excitation at all Cr3+ concentrations, but better agreement is obtained for =e 1. Concordance between the the 6% powder when @p/P value for the 6% powders at 404-nm excitation, a quantity that does not depend upon assumptions i and ii, and the @p/p values calculated from eq 1’provides further evidence for the validity of the assumptions.
21 27
It is possible that there is a real difference between @ZE in the solid state and in solution.2b Photosolvation has been reported to take place in the 4T2gstate,2a but the quantum yield of this process is only 0.1. Perhaps this value is not the yield of the primary reaction and C I - ( C N ) ~ ~is- reformed after a dissociative process, thus reducing (PzE at room temperature. In any case, the 4T2glifetime is
