J. Phys. Chem. 1984, 88, 1379-1385
-
easily observed and much stronger than the S1 So fluorescence. In fact, for these families of compounds the greater challenge lies in finding the SI So or S2 SI emissions which connect neighbor states, as has been done partially for 1 and 2 in this work and for azulene in earlier ~ o r k . ~ , ~ What decay channel dominates the nonradiative decay of these molecules? It was argued recently' that decay in azulene is controlled primarily by internal conversion but that azulene does not fit entirely within the conventional Englman-Jortner* description of internal conversions within the weak coupling limit. Rather, the S2 SI internal conversion involves C-C skeletal modes as acceptors instead of the usual C-H stretching modes; and the S1 So internal conversion is probably better described in terms of intermediate coupling. Noting that cyc1[3.3.3]azine does not have the strained rings which were thought' to be responsible for the importance of the skeletal vibrations, we might expect cyc1[3.3.3]azine to behave more like the polyacene series than like the azulenes. Using the more recent of Siebrand'sg formulations and adopting the subsequent refinement of the parametrization for polyacenes by Gillespie and Lim,lo we calculate a rate constant of about 2.5 X lo9 s-I for S2 S1 internal conversion in cyc1[3.3.3]azine. For this calculation, we ignore the central nitrogen altogether. This calculated lifetime is a factor of 5 too long to explain the mea-
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(5) Gillispie, G . D.; Lim, E. C. J . Chem. Phys. 1976, 65, 4314. (6) (a) Rentzepis, P. M.; Jortner, J.; Jones, R. P. Chem. Phys. Lett. 1970, 4, 599. (b) Huppert, D.; Jortner, J.; Rentzepis, P. M. Ibid. 1972, 13, 225. (7) Griesser, H. J.; Wild, U. P. Chem. Phys. 1980, 52, 117. (8) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. (9) Siebrand, W. J . Chem. Phys. 1967, 47, 2411. (10) Gillispie, G . D.; Lim, E. C. Chem. Phys. Lett. 1979, 63, 293.
1379
surement exactly; but that should be considered as good agreement as could be expected. Parameters adjusted to fit the polyacene family are not expected to be transferable to other compounds without further adjustment. Consequently, there is no reason at this time to look beyond ordinary internal conversion to account for the decay. The shorter lifetimes of three derivatives of l2can be understood quite well in terms of internal conversion, given the uncertainty in how to account for carbonyl or cyano substituents. The smaller S2-Sl energy gaps in the derivatives account reasonably well for the shorter lifetimes when they are interpreted with the same theoryg and parametrization1° used for the parent compound. It is also known,2 however, that triplet states are formed with some efficiency from higher singlet states of the cyclazines. There is as yet no compelling reason to deny intersystem crossing a role in determining the lifetime of the S2state. In conclusion, there can be no doubt about the assignment of the lower excited states in cyc1[3.3.3]azineand its derivatives. We have observed the predicted S2 SI fluorescence; and we have verified the predicted lifetime of S2. The cyc1[3.3.3]azine system can take its place with the azulenes among organic ring systems as the only two molecular families showing true anti-Kasha photophysics.
-
Acknowledgment. This research was supported in part by the National Science Foundation and by the Swiss National Science foundation, project 2.470-82. We thank Mr. P. Bad0 and Mr. B. Campbell for their help. W. L. was supported by a fellowship from the Swiss National Science Foundation; he appreciates the hospitality of Prof. D. Kearns during an extended stay at the University of California at San Diego. Registry No. 1, 519-61-9; 2, 88766-93-2.
Photoionization Pathways of Cu' in CdCI, and CdBr, Stephen A. Payne and Donald S. McClore* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: June 27, 1983)
We have studied the spectroscopy of Cu' in CdCI2 and CdBrz hosts. At the lowest temperatures, T = 4 K, excitation from the 'A,,(dlo) ground state to the singlet state 'E,(dgs) results in rapid relaxation to the lowest d9s level 3E,(d9s), followed by W phosphorescence. This behavior is also observed in almost all copper-doped alkali halide host systems. But at intermediate temperatures, 4 5 T 5 77 K, the cadmium halide hosts interact differently with the Cu' impurity. This is because the d9s states of Cu' lie within the conduction band of CdC12 and CdBr2, but below that of the alkali halides. The s electron is delocalized away from the copper center and this causes the nearest neighbors to relax toward the Cu' impurity forming an excimer-like species. We propose that luminescence results when the s electron recombines with these excimers. At higher temperatures, T > 7 7 K, the state binding the s electron is thermally unstable and the electron is permanently ionized.
Introduction Electron transfer processes in chemistry seem very simple yet the intimate details of such processes are difficult to discern in most experimental situations and not much is known about them. In the experiments to be discussed in this paper, luminescences accompanying electron capture in a crystalline system are shown to reveal some of the details of an electron transfer step. Work related to ours is usually found in the literature of solid-state physics, but we feel that some basic chemical principles are being illustrated by these studies. The Cu' ion, when doped into CdC12 and CdBr2 host lattices, is ionized to Cu2+with UV light above liquid nitrogen temperature (LNT).I We have systematically examined this process and have encountered facts which indicate that diatomic "exciplexes" are formed upon excitation into the dl0 d9s absorption band of Cu'.
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(1) H. Matsumoto, H. Nakagawa, and H. Kuwabara, J. Phys. SOC.Jpn., 44, 957 (1978).
These species may either decay radiatively to the ground state whereupon the molecule dissociates into ions at normal lattice sites or they may ionize, depending on the temperature. Essentially, the electronically excited Cu' center undergoes a highly asymmetric relaxation such that the resultant local distortion is best described as a diatomic quasi-molecule embedded in a host lattice. We believe that the occurrences of these diatomic exciplexes are examples of a general phenomenon in insulating crystals.
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Background Evidence favoring the d'O d9s assignment of the near-UV bands of Cu+:NaCl has existed for many years.2 The Cu' center in alkali halide hosts has continued to be extensively studied by many worker^^-^ since it represents a simple prototypical impurity (2) D. S. McClure, "ElectronicSpectra of Molecules and Ions in Crystals", Academic Press, New York, 1959. (3) M. Bertolaccini, P. Galiardelli, G. Padovini, and G. Spinolo, J. Lumin., 44, 281 (1976).
0022-3654/84/2088-1379$01.50/00 1984 American Chemical Society
1380 The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
crystal system. This is especially true in the case of the new solid solution, NaF:Cu+.6 The 3d94s excited-state results from the promotion of a d electron from the closed-shell ground-state configuration, 3d1°. The d orbitals of Cu+ retain an electronic distribution similar to that of the free ion. The s orbital also remains localized because of the mildly antibonding interaction of the halogen anions (resulting in even greater spatial confinement) and because of the minimal electronic interaction afforded by the highly ionic alkali metal cation^.^,^ In the cases of L ~ C ~ : C UN + ,a~c ,l ~: C ~ +and , ~ NaF:Cu+,6 the physical behavior is quite simple. Transitions from the ]Al (dl0) ground state to the crystal field split ‘E,(d9s) and 1T2g(cfgs)singlet states are observable spectroscopically, while excitation into either of these states results in radiationless decay to the lowest level of the excited state manifold 3Eg(d9s),followed by the spin-forbidden lAl, emission in the near-UV. In the cadmium halides, CdX,, where X = C1 or Br, the Cu+ center substitutes for the Cd2+ionlo and thus experiences a nearly octahedral environment of halogen anions. To a first approximation the Cu+-X- interaction is the same as in the alkali halides. However, Cu+ ionization is observed to begin at about 3.6 eV in CdC12:Cu+,but not below 6 eV in NaC1:Cu+.ll Part of the reason for this difference is given by comparing the bandgaps of these crystals, 5.8 eV in CdC12,9.0 eV in NaC1.12 The higher electron affinity of Cd2+as compared to Na+ means that the conduction band is wider and lower in CdCl, than in NaCl and photoconductivity is possible at lower energies. A similar case is given in the recent work of Langer et al.13 where Eu2+in CdF, (band-gap 6 eV) and in CaF2 (band-gap 10 eV) are compared. Good evidence is presented which shows that the excited states of EuZ+ lie in the conduction band of CdF2, but not CaF,. Similarly, the excited states of Cu+ are believed to be in the conduction band of CdCl, or CdBr,, leading to the observed photoconductivity. What is striking about the CdX2:Cu+systems is the occurrence of several luminescences below about 80 K and their quenching accompanied by photoionization above. We have already briefly described these luminescences and given an interpretation of them.14 Here we will give a full account of the photophysics of these systems.
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Experimental Section A variety of spectroscopic techniques was employed in this study. All absorption measurements were made with a Cary 14R spectrometer. So that the formation of photoproduct resulting from irradiation could be studied, a small mirror could be inserted into the sample compartment to deflect the light from a 1000-W high-pressure mercury lamp equipped with a 7-54 Corning filter onto the sample, which was located in an Air Products continuous flow helium Cryotip. The emission and excitation spectra were recorded with lock-in techniques at a modulation frequency of 200 Hz. To obtain emission spectra, we passed light from a high-pressure mercury lamp through a monochromator, a chopper, and lastly an interference filter which ensured that no unwanted light reached the sample. The luminescence was collected, passed through a monochromator, and then detected by a Hamamatsu R446 photomultiplier tube (PMT). Corning cutoff filters were used (4) R. Oggioni and P. Scaramelli, Phys. Status Solidi, 9,411 (1965). (5) J. Simonetti and D. S. McClure. Phvs. Reu. B. 16. 3887 (1977). (6j S. A. Payne, A. B. Goldberg, and D. S.’McClure, i.Chew. Phis., 78, 3688 (1983). (7)’J. G.’Harrison and C. C. Lin, Phys. Reu. 8, 23, 3894 (1981). (8) C. Pedrini, Phys. Status Solidi b, 87, 273 (1978). (9) H. Chermette and C. Pedrini, J . Chem. Phys., 75, 1869 (1981). (10) K. Ka’no, S. Naoe, S.Mukai, and Y . Nakai, Solid State Commun., 13, 1325 (1973). (11) B. R. Soller, M. Voda, and D. S. McClure, J . Lumin., 24/25, 201 (1981). (12) W. H. Strehlow and E. L. Cook, J . Phys. Chem. Data, 2, 163 (1973). (13) M. Godlewski, D. Hommel, J. M. Langer, and H. Przybylinska, J . Lumin., 24/25, 217 (1981); D. Hommel, J. M. Langer, and B. KrukowskaFulde, Phys. Status Solidi a, 31, K81 (1975). (14) S . A. Payne and D. S. McClure in “Photochemistry and Photobiology”, Vol. 2, A. Zewail, Ed., Harwood Academic Publishers, Switzerland, 1983.
Payne and McClure
,\ 0. 50
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61
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ABSORPTION 1-
:
c 2 EJ
61
1
61
m
eV iri Id Figure 1. Absorption and emission spectra: absorption, broken curves at temperatures indicated, CdC12:Cu+is 0.34 mm thick, A (absorbance) = 1.5 a t 2 K peak, CdBr2:Cu+ thin sliver used, A = 0.37 at 77 K peak; emission, solid curves, excited at 290 nm. *
ri
6
to keep the exciting light out of the monochromator and to remove higher order effects of the grating. The lock-in output was fed into a strip-chart recorder. For the excitation spectra, the light from a 2200-W Xe lamp was filtered, monochromated, and chopped before impinging on the sample. A small fraction of the light deflected to a 1P28 PMT provided a reference. The emission range desired was selected with several Corning filters and measured by the R446 PMT, whose signal was then phase-sensitively detected. The signal and reference were electronically ratioed, and the result was sent to a strip chart recorder. The wavelength dependence of the emission monochromator and PMT in the luminescence setup and the reference PMT in the excitation experiment were calibrated in separate experiments using a thermopile. The band pass of the scanning monochromator, typically 1 nm, was always much less than the spectral width of the features being studied. The sample was immersed in the cryogenic fluid in a double-walled dewar that could be filled with liquid helium and pumped below the lambda point, or simply filled with liquid nitrogen. The emission intensity and lifetime data were obtained with a 7-11s UV pulse produced by the doubled beam of a N, laserpumped dye laser. The fundamental was removed with a 60’ prism and a Corning 7-54 filter. A small fraction of the UV beam provided a reference. A calibrated chromel-alumel thermocouple measured the temperature of the sample in a double-walled dewar. After a charge of liquid helium boiled off, a series of luminescence lifetimes were measured as the sample temperature slowly rose. The Cu+ emission desired was isolated with several appropriate Corning filters. A Hamamatsu R446 PMT in conjunction with the Biomation 8100 digitized the signal while the Nicolet 1170 added 1024 transients together. With the help of the Molectron 242 interface, the data were stored on magnetic tape by the Hewlett-Packard 9825A desktop computer. A hard copy was provided by a HP 7225A plotter. With the above experimental arrangement the transients obtained could later be fitted to an exponential function of time to give the emission time constant. The data could also be integrated and normalized to laser power to provide the emission intensity vs. temperature plot.
Results In Figure 1 the absorption spectra of the Cu+ impurity in CdCl, and CdBr, are shown on the right hand side. These spectra agree well with those observed by Japanese workers.’JS The major absorption band is at 4.37 eV in CdClz and 4.14 eV in CdBr, at liquid nitrogen temperature. N o other bands associated with Cu+ (15) S . Naoe, K. Kan’no, and Y .Nakai, J. Phys. SOC.Jpn., 42, 1609 (1977).
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1381
Cu+ in CdC12 and CdBrz are observed to lower energy nor does rapid cooling of the crystal from 400 OC change the shape of the bands. The absorption is due to the atomic dIo d9s transition. The lack of temperature dependence of the oscillator strength indicates that the Cu+ center experiences a static perturbation which destroys the center of inversion. Since Cu+ has been shown to be at the CdZ+site,I0 the charge compensator may be responsible for the lowering of the symmetry. Alternatively, the well-known "off-center" effect of Cu+ could be the cause.1q2 The ESR spectrum of CdC12:Cu+crystals UV irradiated to produce Cu2+ has shown that the impurity is in a nearly octahedral environment,I0 that is, no compensator site is nearby. For this reason and others to follow we believe that the charge compensator has a minor effect on the absorption spectrum and, therefore, that the Cu+ equilibrium position is off-center, probably along the c axis in CdCl, and CdBr2, in analogy to many alkali halides. This displacement is probably facilitated because the cadmium halides are less ionic than the alkali halide@ and hence more deformable. The resultant C,, symmetry provides allowedness for the parity s transition. forbidden d The photoionization and photoemission processes would be more sensitive to the presence and nature of the compensators than is the absorption band. We know very little about the compensators but will discuss the relevant data at various points in this section and summarize it at the end. Also shown in Figure 1 are the emission spectra of CdC12:Cu+ and CdBr2:Cu+ at LNT. There are three bands in the chloride and two in the bromide. We shall refer to the highest energy band at 3.33 and 3.08 eV in the chloride and bromide, respectively, as the UV band, the next band at 2.87 and 2.76 eV as the blue band, and the 1.75-eV band as the red band. The relative intensity of the emission bands is somewhat sample dependent. The red to blue ratio in CdC12 is reproducible to &20%, while the blue to UV ratio in the bromide is repeatable to &50%. The nonreproducibility of the emission spectra could lead us to believe that the different peaks are due to differently compensated Cu+ in different crystals. Nevertheless, our results are close to those of Matsumoto et al.,I indicating that the extra luminescences are not due to unknown impurities. In addition we have codoped Cu+ into CdC12 with such ions as Y3+, Na+, and La3+ and have observed only minor changes in the emission and absorption spectra. These emission spectra are very surprising since Cu+ exhibits a single emission band in almost all alkali halide crystal^,^^^^-^^ NaBr being the exception. A single band is expected since the ,Eg state, derived from the ,Datomic term, is the only reasonable state responsible for the emission. Since the UV emission band is at the highest energy, and appears a t the lowest temperatures where other processes are frozen out, we assign it to the ,E, ]A,, transition. The excitation spectra of the five emission bands are in Figure 2. The excitation spectrum of CdC12:Cu+ resulting from monitoring the UV band at 2 K is exactly the same as the spectrum obtained by monitoring the red band at 77 K. (At 2 K the UV band is large and the red band has little intensity.) Thus, the large shift of the UV vs. the red emission bands, 1.58 eV, is accompanied by absolutely no change in the absorption. Hence, these two emissions must originate in processes beginning at the same excited state and therefore also in the same copper species (compensated or not). Note that the blue excitation spectrum is qualitatively different from the other two in the chloride in having a dip at the peak of the absorption spectrum. This indicates that the relaxation from the initially excited state to the state which emits blue light (hereafter, the blue emitting species) either does not involve a thermally equilibrated process or may occur in a differently compensated Cu' ion. But the absence of any shift in the centroid
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-
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(16) C. Sugiura, J . Chem. Phys., 62, 1111 (1975). (17) C. Pedrini, Phys. Stat. Solidi b, 87 (1978). (18) A. B. Goldberg, D. S. McClure, and C. Pedrini, Chem. Phys. Lett., 87, 508 (1982). (19) C. Pedrini and B. Jaquier, J . Phys. C, 13, 4791 (1980). (20) R.L. Bateman and W. J. Van Sciver, Phys. Status Solidi b, 46, 779 (1971).
1. 0 0
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Figure 2. Excitation and absorption spectra: for CdCl@+, broken curve, absorption, 2 K; X's, UV band monitored at 2 K or red band at 77 K, peak absorption is 0.20; circles, blue band at 77 K for CdBr2:Cu+, broken curve, absorption, 77 K, X's, UV band at 2 K; circles, blue band monitored at 77 K; A = 0.37 at peak.
of the excitation spectrum relative to the absorption spectrum lends support to the first explanation. Hence the mode of relaxation is dependent on excitation energy. For the bromide, Figure 2, both the blue and UV excitation spectra are similar to the absorption band, even though the two emission bands are separated by 0.32 eV. In order to examine the kinetics of state-to-state energy transfer, we studied the intensity of the emissions as a function of temperature. Our intensity data are largely in agreement with those of Matsumoto et a1.l Since we integrated the emission transient resulting from a short exciting pulse, our intensity data are noisy but are far more selective because the lifetime is a signature of an emission and hence may easily be separated from other interferences. Thus our results cover the complete temperature range and go over more orders of magnitude in intensity. In addition, we have obtained the emission lifetime vs. temperature curves. The emission lifetimes and intensities as functions of temperature are shown in Figures 3 and 4. At the higher temperatures 70-150 K, the intensity and lifetime tend to behave in the same way with temperature. This is expected for a state in which the emission is competing with a parallel nonradiative process. 111 our case, one of the nonradiative processes is ionization as will be shown later. Both intensity and lifetime show an extended plateau region in both compounds. The value of the lifetime in the plateau region must be the natural lifetime of the emitting state, as it is highly unlikely that a steady-state balance between the radiative and a nonradiative process could be maintained over a long temperature range. At the lowest temperatures, 2-30 K, the red and blue lifetimes fall while their intensities rise with temperature, in contrast to their high-temperature behavior. In this temperature range, the feeding rates to the emitting levels are less than their radiative rates. At low temperature the UV lifetime and intensity fall together. In the case of CdBr,:Cu+, the UV emission also appears at higher temperatures, unlike the case of CdC12:Cu+.
..
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itter. 1-0°
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Figure 4. Emission intensity and lifetime vs. temperature for CdBrzCut: upper, emission intensity, X's, UV band, circles, blue band; lower emission lifetime, X's UV band, circles, blue band; solid curves are theoretical fits (eq 1 and 2 in text) while broken curves are smooth lines drawn
through data. In the upper sections of Figure 5 we show the result of UV irradiation of CdCl,:Cu+ and CdBr2:Cu+ crystals at various temperatures. The product bands observed correspond to the charge-transfer bands of CuZt. So we see that the threshold for
ionization in CdClz:Cu+ is between 79 and 106 K and that for CdBr2:Cu+is between 70 and 89 K. These temperatures correspond to those at which luminescent quenching begins as shown in Figures 3 and 4: 81 K for the chloride red band and 71 K for the bromide blue band. We conclude that the red and blue emitting species thermally ionize.. The photoconductivity data in ref 14 also support this conclusion. We can now draw a very simple kinetic scheme to describe the salient characteristics of the data. As shown in Figure 6, the blue or red level can be fed in two ways, (1) by thermal activation from 3E,with rate constant k,, or (2) by a nonthermal process indeWe assume that the natural lifetime pendent of temperature, kt. of the emitting state is independent of temperature, k, = l / ~ ~ , and that the state may thermally activate an electron into the conduction bnad, k,. The ionization therefore competes with the emission. Since the lifetime of the 3E,triplet state is often known to change rapidly at low t e m p e r a t ~ r e ' due ~ ~ ~to' ~the ~ ~existence (21) C . Pedrini, Solid State Commun., 38, 1237 (1981).
Cu+ in CdCl, and CdBr,
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1383
TABLE I: Parameters Describing the Temperature Dependence of the Emission Lifetime and Intensitva
UV; CdCl,
3.33
blue;CdCl, 2.87 2.42 X l O I 3 256
