J . Phys. Chem. 1990, 94, 7518-7523
7518
the behavior of the polymer PMMA. Conclusion Under the legitimate assumption that PMMA and 1 behave similarly also with respect to phenomena, which cannot be measured directly, a number of conclusions for the photochemistry and ablation of PMMA can be drawn from this study: ( I ) PMMA type compounds undergo photolysis with a high quantum yield for wavelengths of 248 nm or shorter. (2) The quantum yield Q, for the photolysis at 248 nm is Q, = 0.5 f 0.1. ( 3 ) The photolysis is a one-photon process. (4) The main photoreaction is the side-chain scission of the carboxyl group either as methyl formate or in form of smaller fragments like carbon monoxide and methanol or carbon dioxide and methane. The remaining chain radical preferably stabilizes by elimination to form un-
saturated species. (5) The main-chain scission is observed but is only a minor photolytic process. ( 6 ) Taking the high quantum yield for the photolysis of the ester carboxyl group to several gaseous or volatile fragments into account, it can be concluded that the photochemical contribution to the ablation of PMMA at wavelengths X 5 248 nm is far from negligible.
Acknowledgment. We thank K. Muller and W. Sauermann for their technical assistance and S. Szatmiri and F. P.Schafer for the opportunity to use their source of femtosecond excimer laser radiation at 248 nm. Financial support by Bundesministerium fur Forschung und Technologie (BMFT No. 13N5398/7) is gratefully acknowledged. Registry No. 1, 34372-00-4; PMMA, 901 1-14-7.
Photoexcitation and Photoluminescence Study of Coordination Complexes of Lead Diiodide with Pyridine L. C. Yu-Hallada and A. H. Francis* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109 (Received: February 5, 1990)
Pb12exhibits a lamellar CdI, crystal structure, similar to that of transition-metal dichalcogenides (MX,) and transition-metal phosphorus trichalcogenides(MPX3). When PbIz is exposed to any of a variety of organic Lewis bases, it undergoes a solid-state reaction that produces a material with a substantially altered electronic spectrum. Because the reaction is similar to the intercalation reactions of the lamellar chalcogenide lattices with Lewis bases, the reaction of Pb12 has also been described as an intercalation reaction. Examination of the liquid helium temperature photoexcitation and photoluminescence spectra of the reaction product with pyridine suggests that there are important differences between the reaction products of the lamellar chalcogenide lattices and Pb12 with organic Lewis bases. The latter may be described better as coordination compounds resulting from a heterogeneous reaction between solid Pb12 and the vapor or liquid phase of the organic ligand.
1. Introduction Lead diiodide is a layered compound whose crystals form hexagonal lattices with the Cd12 structure. Each layer consists of nearly octahedrally coordinated divalent lead ions, sandwiched between sheets of iodide ions. The I-Pb-I unit is held together by strong (intralamellar) metal-halogen bonds. In contrast, the bonds between the neighboring molecular layers are weak van der Waals bonds. The structure is similar to that of the transitionmetal dichalcogenide (MX,) and phosphorus trichalcogenide (MPX3) lattices. It is well-known that these transition-metal lattices undergo reversible, topotactic solid-state reactions with a variety of organic Lewis bases.’ These reactions involve the intercalation of the interlamellar interstices or the van der Waals gap (VWG). In the process of intercalation, the host lattice may expand considerably along the stacking axis but the intralamellar bonding is not significantly altered. The reactions are frequently reversible under relatively mild conditions. Reactions of Pb12 with organic Lewis bases have been studied for many years. The reactions are generally carried out under conditions similar to those used for the intercalation of MX2 or MPX, lattices and result in the uptake of considerable amounts of the Lewis base by the solid Pbl, reactant. The products have generally been described as intercalation compounds, and there is extensive literature on the subject of their preparation and properties. While there are structural similarities between the PbI, lattice and the MX2 and MPX, lattices, the physical effects of reaction with Lewis bases are dramatically different. For example, large ( 1 ) See for example: Whittingham, M. S. In Intercalation Chemisrry; Whittingham, M. S.. Jacobson, A. J., Eds.: Academic Press: New York,1982.
0022-3654/90/2094-75 18$02.50/0
crystals of the chalcogenide lattices may be intercalated and remain intact. When Pb12crystals are reacted with Lewis bases, they rapidly disintegrate and form a finely dispersed powder with grain dimension < 1 pm. Whereas the transition-metal chalcogenide lattices undergo intercalation with significant changes in only the interlamellar spacing, Pb12 adducts with Lewis bases generally exhibit large changes in the intralamellar dimensions as well. Intercalation of the layered chalcogenide lattices with neutral Lewis bases produces relatively slight changes in the electronic spectrum of the host, usually amounting to less than a 0.1-eV shift in the band edge to lower energy.2 In sharp contrast, when solid Pb12 is exposed to pyridine, the absorption edge shifts to higher energy by approximately 0.8 eV. These observations and others suggest that there are substantial differences between the intercalation chemistry of the layered transition-metal chalcogenides and Pb12. In the present work, we examine the low-temperature photoluminescence (PL) and photoexcitation (PE) spectra of compounds derived from treating Pb12 with pyridine. We compare the PL and PE spectra with previously published absorption spectra of PbIz thin evaporated films and colloidal suspensions in order to better characterize the products of the solid-state reaction. The PE spectra are particularly useful in identifying the spectroscopic effects of impurities and observing spectral features near the fundamental absorption edge. 11. Experimental Section
Sample Preparation. Reagent-grade PbIz powder was obtained from the Aldrich Chemical Company and used without further ( 2 ) Cleary, D. A.: Francis, A. H.; Lifshitz, E. J . Luminesc. 1986, 35, 163.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7519
Coordination Complexes of Lead Diiodide
r
1
_j
1:; 2
.
~,
.
-sa. 50.0
150.
m.
950.
4m.
mwERATvIE IN mc1US MEITINSI
I 5Ooa
Figure 1. TGA (curve A) and DTA (curve B) of PbIZ(py)z.
purification. Powdered Pb12was reacted with liquid pyridine (py) at room temperature for periods ranging from 2 h to several days. Reactions were also conducted by exposing Pb12 to pyridine vapor at room temperature. The only difference in the two procedures appeared to be in the rate of reaction. Thermogravimetric analysis/differential thermal analysis (TGA/DTA) of the samples (see Figure 1) always exhibited a single endothermic transition at 101 OC associated with the loss of pyridine. The composition of the sample varied with the method of preparation but was consistent with the formulation Pb12(py), with 0.5 < x < 2.0 as determined by TGA. The presence of a single endothermic decomposition peak in the DTA curve indicates that the material with x < 2.0 is probably a heterogeneous mixture of Pb12 and PbI2(PY)2. Samples were also prepared by using the procedures described by Miyamae et al.3 Pb12 powder (3-4 g) was stirred with 100 mL of a solution of pyridine (10-30 mL) in benzene for 2-4 days to yield a pale-yellow product (Pb12(py)2)with a characteristic thermal decomposition temperature of 101 OC and a weight loss consistent with the chemical formula for the 1:2 adduct. The preparation of a 1:l adduct with a thermal decomposition temperature > 200 O C has also been described. No evidence of this adduct was found in the TGA/DTA analyses. Instrumental Procedure. All samples were characterized by thermogravimetric analysis and differential thermal analysis. These measurements were carried out on a Seiko I thermal analysis system. PE and PL spectra were recorded after cooling the samples to approximately 5 K. The powdered materials were contained in fused silica tubes and cooled in a Janis IODT cryostat by helium gas flowing from a liquid helium reservoir at 4.2 K. The sample temperature was monitored by a resistance thermometer attached to the sample holder assembly. Luminescence was excited by the output from a 750-W tungsten-iodine lamp filtered through a cupric sulfate solution and a Corning CS-7-54 filter. The luminescence was passed through a Corning 3-72 filter, dispersed by a 1-m scanning monochromator, and detected by a refrigerated Hamamatsu R375 photomultiplier. To obtain excitation spectra, a half-meter scanning monochromator was placed in the excitation beam in place of the filter combination. 111. Results Pb12 Photoluminescence and Photoexcitation Spectra. The most stable crystalline modification of Pb12 is the 2H polytype. The low-temperature luminescence of this polytype is shown in Figure 2. The dashed curve illustrates the appearance of the emission obtained from a single crystal at 5 K.4 The highest energy (4963.6-A) band in the 2H plytype spectrum corresponds to the free exciton luminescence (FE). Within experimental error, this band is coincident with the absorption band edge (see Figure 2). The series of bands marked K, X, and F in the 2H polytype spectrum arise from colloidal color centers formed by the clustering (3) Miyamae, H. J . Cryst. Soc. Jpn. 1986, 28, 26. Miyamae, H.; Toriyama, H.; Abe, T.; Hihara, G.; Nagata, M. Acta Crystallogr. 1984.40, 1559. (4) Blonskii, 1. V.;Gorban', I. S.;Gubanov, V. A,; Lyuter, Ya. A,; Poperenko, L. V.; Strashnikova, M . I . Sou. Phys. Solid State 1974, 15, 2439.
5lW
Yo0
5300
WNELEN6M I N AWSTWUS
Yo0
E' 0
T = 5 K with broad-band UV illumination. (B) PL spectrum of 2H Pb12 polytype Figure 2. (A) PL spectrum of powdered PbIz obtained at
recorded at T = 4.2 K (ref 4).
inso
5550
8250
WAVELEHGW I N ANGSTROMS
6950
7650 E O
Figure 3. (A) Pb12 PL spectrum at T = 5 K. (B) PL of Pb12at T = 5 K after reaction with pyridine. (C) PL spectrum after thermal decomposition of pyridine reaction product.
of intrinsic defects5 By analogy to similar defects in alkali halide lattices, the K, X, and F spectra consist of a characteristic series of bands corresponding to the specific chemical and physical properties of the different colloidal centers. The solid curve (Figure 2) illustrates the PL spectrum of the powdered reagent-grade PbIz used as starting material for the preparation of pyridine adducts. The first, weak band observed at 4975 A in the luminescence of the powder sample corresponds to emission from excitons bound at neutral donors (BE). The intensity of the weak band at about 5015 A in the powder depends upon sample history, suggesting that the line originates from transitions into impurity statesa6 As the temperature increases, the weak BE luminescence intensity decreases rapidly and the longer wavelength X center luminescence that is characteristic of the powdered PbIz material increases in intensity. The luminescence spectra illustrated in Figure 3 were obtained after reaction of the powdered PbIz with pyridine. For comparison, the inset (curve A) illustrates the X band emission of the Pb12 reactant. Curve B, recorded subsequent to treatment with pyridine, illustrates the appearance of a new, broad luminescence band (M) with maximum at approximately 6050 A. The M band luminescence is characteristic of the Pb12(py)2 complex. The broadening of the X center emission band is due to the formation of colloidal centers with different local structures. Even after many hours of exposure to liquid pyridine, some X center luminescence is observed, indicating the presence of residual Pb12. Since some pyridine ligand is rapidly lost from the adduct upon removal from the solvent, small amounts of PbIz are always present in the adduct and give rise to the persistent X center luminescence. After heating (5) Zaldo, C.; Agullo-Lopez, F. J . Phys. Chem. Solids 1983, 44, 1099. (6) Levy, F.; Depeursinge, C.; Thanh, Le Chi; Mercier, A,; Mooser, E.; Voitchovsky, J. P. Unpublished results.
7520 The Journal of Physical Chemistry, Vol. 94, No. 19, 1990
Yu-Hallada and Francis
E O
zmo
3400
Moa
4600
UOO
WAYELWTH IN E O
Figure 4. (A) PE spectrum of PbIz powder obtained at T = 5 K, monitoring 5000-A luminescence. (B) Absorption spectrum of a thin evaporated PbIz film at room temperature (ref 7).
0 @
Lila
loatno
0
Nllropln
0
Cnrbon
Figure 6. Structure of the PbIz(py), complex (ref 3).
WAVELENbTH I N ANbSTRCUS
E O
Figure 5. (A) PE spectrum of Pb12:pyridineobtained by monitoring the 6050-A PL band (M band). (B) PE spectrum after partial thermal decomposition. (C)PE spectrum after complete thermal decomposition. the pyridine adduct to remove ligand and regenerate the PbI, starting material, the spectrum labeled C was obtained. Although the TGA indicates essentially complete removal of the ligand, M band luminescence (characteristic of the complex) is still observed. The relative intensity of the X and the M band luminescence is not directly proportional to the concentration of PbI, and pyridine adduct in the sample. The photoexcitation spectra of the adduct and the Pb12starting material were recorded by selectively monitoring the M band and the X center luminscence, respectively. The PE spectrum obtained by monitoring the X center luminescence is illustrated in Figure 4, curve A where it is compared with the absorption spectrum of a thin evaporated film of PbIJ shown in curve B. The position of the absorption edge agrees well with the position of the band edge in the PE spectrum. The agreement supports the correlation of the X center luminescence with the presence of residual Pbl, in the reaction product. The differences between the high-energy portions of the two curves are due to confinement of the excitation to the surface of the crystal for excitation energies above the band gap. PE spectra obtained by monitoring M band luminescence are illustrated in Figure 5. Spectrum A is obtained after essentially complete reaction with pyridine (as determined by TGA) to form the Pb12(py)2 complex. It consists of a single broad excitation peak at approximately 3700 A. Since the peak is uniquely associated with the M band adduct luminescence spectrum, we associate the A excitation spectrum with the adduct. Spectrum B is generated by partial thermal decomposition of the adduct. The appearance of the PbI, band edge excitation spectrum in~~
(7) AI-Jishi, R.; Coleman, C. C.; Treece, R.; Goldwhite, H. Phys. Reu. B 1989, 39, 4862.
dicates that Pb12 is regenerated by thermal decomposition and that energy transfer can occur between the adduct and PbIz with which it is in contact. After heat treatment to remove all labile pyridine ligand, spectrum C was obtained. This spectrum is essentially identical with that of the starting material but is obtained by monitoring residual adduct luminescence. The adduct excitation spectrum at 3700 k is, within experimental error, completely absent from spectrum C, indicating that the residual M band luminescence (Figure 3) is due to trace amounts of the adduct remaining after thermal deintercalation. The high efficiency of energy transfer between PbI, and P b 1 , ( ~ y )suggests ~ that the samples are not heterogeneous mixtures of these components in mechanical contact but rather Pb12 particles whose edges are derivatized by pyridine monodentate ligands. In summary, reaction of PbI, with pyridine yields a 1:2 adduct that contains small amounts of residual Pb12, possibly generated through loss of ligand upon removal from the solvent. The adduct is characterized by a luminescence band peaking at 6050 A and an excitation peak at about 3700 k. The adduct undergoes an endothermic thermal decomposition at 101 O C to regenerate the Pb12 reactant, with residual levels of the adduct. Even after heating to nearly the decomposition temperature of PbI, (402 "C), traces of the complex remain.
IV. Discussion The spectroscopic changes observed when crystalline Pb12 is treated with pyridine are consistent with the formation of a lead-pyridine coordination complex. Pyridine coordination compounds of Pb2+have been known for many years. Heise8 examined the equilibrium between Pb12 and pyridine and reported evidence for the formation of coordination compounds of the form Pb12(py),, with x = 2, 3. These materials were described as white, finely divided solids. Many other adducts of lead(I1) halides with 0-,S-,and N-donor ligands have been e ~ a m i n e d . ~ Recently, pyridine and substituted pyridine complexes of Pb12 have been crystallized1° and full X-ray crystal structures obtained.3,1' The X-ray structure of P b 1 * ( ~ y )indicates ~ that monodentate pyridine ligands replace some bridging iodine, destroying the two-dimensional connectivity of the original lamellar (8) Heise, G. W. J . Phys. Chem. 1912, 16, 373. (9) Wharf, I.; Gramstad, T.; Makhija, R.; Onyszchuk, M. Can. J . Chem. 1976, 54, 3430 and references contained therein. (10) Hihara, G.; Miyamae, H.; Nagata, M. Chem. Soc. Jpn. Chem. Lett. 1986. 2009. (11) Engelhardt, L. M.; Patrick, J. M.; Whitaker, C. R.; White, A. H. Unpublished results.
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7521
Coordination Complexes of Lead Diiodide structure. It is replaced by a one-dimensional structure consisting of infinite helical chains. The lead atom is six-coordinated by two ligand nitrogen atoms and four iodides, bridged pairwise to neighboring lead atoms so that a one-dimensional . .I )Pb(py)z(/)Pb(py)2(! ) P b ( ~ y )... ~ (polymeric chain results. Pyridine is alternately coordinated cis and trans along the chain in the manner illustrated in Figure 6. The large shift of the absorption spectrum upon complex formation arises from a combination of effects. Band calculationsI2 for Pb12 convincingly demonstrate that the band edge transition of the 2H polytype is localized on the metal. Although the 6s lead orbital is mixed to a small degree with iodine 5p orbitals, the transition may be described as essentially a 6s 6p transition of the Pb(1I) cation, in which the 6p orbitals are split by both the crystal trigonal field and spin-orbit coupling. The conduction and valence bands undergo little dispersion along the lattice vector parallel to the stacking axis, so that the transition probability is concentrated in three sharp transitions at approximately 2.5, 3.3, and 3.9 eV. These features are observed as shoulders in the spectrum of the evaporated PbI, film shown in Figure 4B. The spin-orbit contribution to the total p-state splitting of 1.4 eV is estimated to be 0.9 eV, somewhat less than the splitting in atomic lead (1.3 eV). Part of the high-energy shift of the Pb12 spectrum upon reaction with pyridine may arise from the quenching of spin-orbit coupling and the decrease in the trigonal field in the complex. These spectroscopic changes may be qualitatively understood in terms of a simple crystal field model. The lead coordination geometry in Pb12 is that of a trigonal antiprism (D3d).After reaction with pyridine to form the Pb12(py), complex, the D3d coordination geometry is reduced to orthorhombic. The crystal potential about lead may be expanded in terms of spherical harmonics ( y) of order I. Since the p orbitals involve spherical harmonics of order 1 = 1, they are not split by terms in the crystal field of order greater than I = 2. Since we will only consider matrix elements between 6p orbitals, only I = even terms in the expansion need be retained by parity. Moreover, the leading term, is spherically symmetric and cannot affect the crystal splitting of the 6p orbitals but does alter the 6s-6p separation. Rotational invariance of the crystal field requires that the terms with Iml = 1 also vanish. Therefore, the effective crystal potential may be represented as
+4
0
-
G,
u=(Yfi+p(g+y;2}
(1)
The parameters (Y and 6 involve the radial part of the spherical harmonic functions. For D 3 d symmetry, only the first term is retained and leads to a splitting of the pxy and pz orbitals. In orthorhombic or lower symmetry, both terms of eq 1 are retained and the second term removes the remaining px, py degeneracy. Representing the 6p orbitals by the functions (MJ4,I with M I = fl and M, = the non-zero matrix elements of the crystal field are ( 1 ,Mslfi1 1,Ms) = -AI / 2
(lNJGl-l,M,) =
A2
(-1N,l~211N,) = A2
( O h f s l f l l O & f s ) = AI
(2)
where AI is the trigonal field splitting and the magnitude of A2 is proportional to the orthorhombic field distortion. The nonvanishing matrix elements of spin-orbit coupling are (-I,M,~LS~l,-M,) = 21/2X ( 1 ,-M,JLSI-I J4,) = 2II2X
(l,M,lLSI1,M,) = M,M/
(3)
where X is the lead 6p orbital spin-orbit coupling parameter. The (12) Schluter, 1. Ch.; Schluter,
M.Phys. Rev. B 1973, 9,
1652.
-4
0
*4
A2 I
Figure 7. Contour plot of the energy of the lowest Pb2+ 6p state as a function of parameters describing the trigonal field strength (A,), orthorhombic distortion (A2),and spin-orbit coupling (A).
energy eigenvalues of the 6p orbitals in the presence of the crystal field and spin-orbit coupling can be found by diagonalization of the following 3 X 3 matrix. Each eigenvalue is doubly degenerate. lI.112)
-Ail2 A2 0
+X
l-l,llz)
lo?-~lz)
A2
0
-All2 - X 2'12X
2'I2X All2
(4)
Eigenvalues of eq 4 were calculated for a range of values of Al/X and A2/X corresponding to reasonable values of the molecular and crystal field parameters involved. For example, Schluter and Schluter12 estimated values of A, = 0.8 eV and X = 0.3 eV for Pb2+ in Pb12 from analysis of the intensities, polarizations, and energies of characteristic bands in the spectrum of PbI,. Figure 7 shows a contour plot of the eigenvalue of the lowest p state as a function of the parameters AI, A2, and A. Each energy contour corresponds to a band edge shift of 0.4X. Reduction of the coordination geometry about lead from D3dto orthorhombic results in a quenching of the orbital motion of the p electrons and a reduction in the magnitude of the spin-orbit coupling parameter. The general effect is to reduce the p-orbital splitting and shift the cation absorption band to higher energy. In fact, the experimentally observed narrowing of the spectrum of the complex is probably due to the decrease in the porbital splitting. However, the maximum high-energy shift induced by changes in AI, A2, and A is about 0.77h or approximately 0.23 eV if a value of X = 0.3 eV is assumed. The observed shift is about 0.8 eV and must contain a substantial contribution from a decrease in the energy of the lead 6s orbital. This result may be anticipated since the highest energy valence orbital, although predominately lead 6s, has some iodine 5p character. Changes in the coordination environment upon formation of the Pb12(py), complex will alter the 5p admixture. Such a result is not expected with the formation of a Pb12 intercalation compound in which the Pb12 coordination remains substantially unaltered. Substitution by different ligands should produce a range of higher energy shifts approximately in the order of the spectrochemical series established for d-orbital splitting. This is qualitatively observed for a variety of other Lewis base ligands including hydrazine, piperidine, quinoline, and aniline. Related Spectroscopic Studies of Pb12. The electronic spectra of Pb12 have been studied for many years, and a considerable number of works have been concerned with physical and chemical effects on the spectrum. These studies have used material in different physical forms (single crystals, powders, evaporated films, and colloidal suspensions), subjected to different chemical treatments to produce remarkable effects on band intensity and band position. The interpretation of these effects has varied widely and has invoked chemical intercalation of the layered structure, impurity absorption and emission, superlattice formation, and quantum-size effects. It is notable, however, that when spectra
7522 The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 obtained subsequent to different chemical and physical treatments are carefully compared, there are many points of similarity. Accordingly, one may anticipate that these effects may have a common physical origin. The results of some previous investigations are discussed below in relation to the present study. Rybalka and Miloslav~kii'~ reported that evaporated thin films of PbIz reacted easily with saturated pyridine vapor at room temperature, increasing approximately 2-fold in thickness and changing from yellow to colorless. The band edge absorption was determined to shift from 2.5 1 eV for Pb12 to approximately 3.3 1 eV. The spectrum was interpreted in terms of a two-dimensional Frenkel exciton model in which the intralamellar structure of the PbIz lattice was preserved and the material was described as an intercalation compound. Within experimental error, the shift observed by Rybalka and Miloslavskii is identical with that observed in the present study. However, we believe that the behavior observed is not the result of intercalation but rather due to the formation of a PbIz(py)2 complex. Yoffe and co-~orkers'~ carried out investigations similar to those of Rybalka and Miloslavskii using thin evaporated films of Pb12 exposed to vapors of hydrazine and found the band edge absorption shifted to higher energy by 1 eV. Drude absorption by free charge carriers was not observed in the intercalated films. The absence of free carriers in the intercalate suggested that the driving force for the intercalation of the metal halide may be different from that proposed for the transition-metal chalcogenides where intercalation is usually accompanied by charge transfer to the host lattice. In view of the present results, it appears likely that the reaction involves formation of a lead-hydrazine complex rather than intercalation of the lamellar Pb12 lattice. Very recently, the results of new studies of evaporated Pb12 films, intercalated with hydrazine and methylhydrazine, were reported by AI-Jishi et aL7 The changes observed in the band edge absorption were qualitatively similar to those reported previously. These workers point out that the band edge shift could not be attributed simply to an increase of the c-axis spacing, since the shift is smaller for the larger methylhydrazine intercalate. A simple scheme was devised in which the crystal was modeled as a one-dimensional construction of independent lead and iodide slabs. This model produced reasonable agreement between the predicted shift and that observed experimentally. Of course, the results are equally consistent with the formation of a one-dimensional polymeric complex as proposed in the present study. The most extensive studies of the intercalation of PbIz with Lewis bases have been reported by Koshkin and co-workers. Koshkin et al.Is reported the preparation of intercalation compounds with pyridine, aniline, piperidine, quinoline, and other amines. In all compounds, the band edge absorption was observed to shift to higher energy with respect to the host lattice edge. These and subsequent studiesI6 of intercalated Pb12 lattices were interpreted as providing evidence of superlattice formation through ordering of the intercalate molecules along lattice planes. All of the organic solvents used by Koshkin and co-workers are Lewis bases, and the results are consistent with the formation of Lewis base complexes. We are presently studying energy shifts with respect to Lewis basicity of these and similar compounds. The optical absorption spectra of Pb12 in the form of small colloidal particles suspended in a variety of solvents (methanol, acetonitrile, water) have recently received considerable study." The observed spectra depend critically upon cluster size in the range of lateral dimension from 12 to 90 A. This effect was
Yu-Hallada and Francis E O
7
I \
4
i\ I I
(A)
1
2 2800
YO0
4200
W A V E L M B T H IN AN6STRCUS
5000
Moo E O
Figure 8. Comparison of the absorption spectrum of colloidal Pblz (A) and the PE spectrum of PbIz(py)2 (B).
attributed to quantum confinement of charge carriers. By using the techniques described by Sandroff and co-~orkers,'~ colloidal suspensions of small particles were prepared. Small volumes of the suspensions were evaporated on substrates to permit characterization of the size distributions using TEM. The particle size distributions varied widely. In the smallest distributions, the dominant size was about 50 A. The PE and PL spectra of the supported particles were obtained and compared with the spectra of the starting powder (ca. 1 p ) and crystalline material. No significant differences were observed in the spectra. When the absorption spectra of the colloidal suspensions (in 2-propanol) are compared with the PE spectrum of Pb12(py)2(Figure 8), the band edge absorption occurs at approximately the same energy. The comparison suggests that the colloidal spectra are influenced by solvent complexation. Wang and HerronI8 have recently examined chemical effects on the optical properties of semiconductor particles, including colloidal suspensions of Pb12. These authors observed that several prominent absorption bands in solutions containing dissolved PbI2 arise from I-, 13-, and I2 coexisting in equilibrium. The spectra of these species are very similar to the literature-reported spectra of colloidal PbIz. They note that the optical properties of PbI2 may be modified by mechanisms other than quantum-size effects or intercalation, in particular, by strong solvent interactions between the solid and many commonly used oranic solvents. Micic et al.I9 suggest that colloidal suspensions of Pb12 in alcoholic solution are probably composed of Pb(0H)I and that the spectrum is dominated by the absorption of I2 in solution. We have conducted PL and PE spectroscopic studies of PbI2 samples after various chemical treatments in order to test the hypothesis that features observed in the absorption spectrum arise from species such as I-, 12, or 1). Samples of powdered Pb12 were washed with solutions containing high concentrations of I2 and the anions I- and I< and then were dried in air. No appreciable differences were observed between the PL and PE spectra of treated and untreated samples. Although the impurity species certainly contribute to the absorption spectrum in the region 2000-5000 A, the excitation of the impurity centers evidently does not activate luminescence from the host lattice. Therefore, the impurity bands do not appear in the PE and PL spectra of the host lattice. We conclude that the characteristic 3700-A absorption is not due to contamination by reaction byproducts. V. Conclusion
(13) Rybalka, A. 1.; Miloslavskii, V. K. Opt. Spectrosc. 1976, 41, 147. (14) Ghorayeb, A. M.; Coleman, C. C.; Yoffe, A. D. J . Phys. C: Solid State Phys. 1984, 17.71 5. Friend, R. H.; Yoffe, A. D. Ado. Phys. 1987,36, I. (IS) Koshkin. V. M.; Mil'ner, A. P.; Kukoi, V. V.; Zabrodskii, Yu. R.; Dmitriev, Yu. N.; Brintsev, F. I. Sou. Phys. Solid State 1976, 18, 354. Koshkin, V. M.; Kukol', V. V.; Mil'ner, A. P.; Zabrodskii, R.; Katrunov, K. A. Sou. Phys. Solid State 1977, 19, 939. (16) Katrunov, K. A.; Koshkin, V. M.; Mil'ner, A. P.; Shevchenko, S. 1.; Sou. J . Low Temp. Phys. 1978. 4, 260. (17) Sandroff, C. J.; Kelty, S. P.; Hwang, D. M. J . Chem. Phyys. 1980.85, 5337.
Although there is considerable variation in the intercalation chemistry of lamellar solids, certain general criteria for the formation of true intercalation compounds are accepted:' (i) subsequent to intercalation, the atoms of the host remain held by valence forces to form two-dimensional layers that are substantially rigid; (ii) two-dimensional patterns of atoms in each rigid layer (18) Wang, Y.; Herron, N. J . Phys. Chem. 1987, 91, 5005. (19) Micic, 0. I.; Zongguan, Li; Mills, G.; Sullivan, J. C.; Meisel, D. J . Phys. Chem. 1987, 91. 6221.
J. Phys. Chem. 1990, 94,1523-1529
7523
occurs through a heterogeneous equilibrium between the solvent (pyridine), solid PbIz, and solid Pb12(py)2. The solid Pb12 reactant
show correlations with those in neighboring layers in the perpendicular direction, which may or may not be systematically changed when an intercalate penetrates between them; and (iii) intercalation is largely, but not entirely, reversible. True intercalation compounds of PbIz that satisfy these broad criteria may indeed exist; however, it is not clear from the available evidence that any have yet been prepared. The experimental evidence strongly suggests that the compounds formed by reaction of Pb12 with Lewis bases are not true intercalation compounds but rather are best described as coordination complexes. The host lattice lamellar structure is not preserved in the product, and the extended electronic band structure of the lattice is replaced by the electronic properties of a coordination complex. The reaction
is converted to the solid Pb2(py)z product without appreciable dissolution of either solid phase. The weakly bound Lewis base complexes may be easily thermally decomposed to yield starting products, giving the appearance of a reversible intercalation reaction.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research fund, administered by the American Chemical Society, and to the National Science Foundation for funds used in the execution of this research. Partial support was provided by funding from the NATO Scientific Affairs Division.
Photoinduced Hydrogen Abstraction Reactions of Azanaphthalenes in Single Crystals of Durene Proceeding via Tunneling Nagahiro Hoshi, Seigo Yamauchi? and Noboru Hirota* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, 606 Japan (Received: February 26, 1990)
The rate constants of photoinduced hydrogen abstraction reactions of three azanaphthalenes, namely, quinoline, quinazoline, and isoquinoline,are studied in single crystals of durene and perdeuterated durene from measurements of the phosphorescence decays and intensities at higher temperatures. It is found that the reaction rate constants are quite different among these molecules. From the temperature and deuterium effects on the reaction rate constants, it is suggested that tunneling plays an essential role in these reactions as in the case of quinoxaline in durene. The experimental results are simulated reasonably well by the golden rule treatment of tunneling proposed by Siebrand et al.
1. Introduction Studies of chemical reactions in single crystals have several advantages over those in liquid and gas phases, because orientations and distances of the reacting molecules are fixed. Although there have been many studies on photochemical reactions in single crystals,'S2 most of them are the reactions between the same kind of molecules in neat crystals. There are only a few examples of the reactions between different species in mixed crystals. There are also very few cases where kinetic data of the reactions were obtained in single crystals. Stehlik's group is one of the few groups that have studied the reactions in mixed crystals. They found that tunneling is involved in the photoinduced hydrogen abstraction reactions of acridine and phenazine in single crystals of f l u ~ r e n e . ~ . ~ By measuring the reaction rate constants very accurately, they showed that a lattice mode of the fluorene crystal promotes tunneling at low temperatures (33-72 K).5 The photochemical reaction of quinoxaline (Qx) in a single crystal of durene (D), Qx/D, studied in our laboratory is another example of the reaction in the mixed crystal. We reported the following facts in previous (1) The initial process of this reaction is a hydrogen abstraction reaction from the durene methyl group to produce a quinoxalinyl radical ('QxH) and a duryl radical. (2) The precursory excited state of this reaction is the lowest excited triplet state of Qx. (3) The phosphorescence decay rate constant (kT)of Qx increases drastically at higher temperatures (>-IO0 "C) because of the increased reaction rate constant ( ~ T R ) . (4) The Arrhenius plot of kTR(log kTRvs 1 / T ) deviates from a straight line and a large deuterium isotope effect of the host is observed. These results suggest that tunneling plays an important role in the reaction of Qx/D. 'Present address: Chemical Research Institute of Non-aqueous Solutions, Tohoku University, Katahira, Sendai, 980, Japan.
0022-3654/90/2094-7523$02.50/0
In this report we have investigated hydrogen abstraction reactions of a series of azanaphthalenes, quinoline (Q), quinazoline (Qz), and isoquinoline (IQ) in durene single crystals in order to examine the generality of the reaction and to confirm the tunnelling mechanism. Since, in the excited states, these molecules, and more specifically their nitrogen atoms, are known to abstract hydrogens from alcohols,&12it is expected that all of them abstract hydrogen atoms from durene in the initial stages of the reactions in the mixed crystal. Here we first show that kT'S of these azanaphthalenes show drastic increases at higher temperatures. These increases are attributed to the photochemical reactions in the triplet states, and rate constants, kTR's, are determined. The drastic decrease of kTR'scaused by deuteration of durene confirms that the initial stage of the reaction is hydrogen abstraction from durene. The temperature dependence and the deuterium isotope effect on kTR'ssuggest that tunneling is important in Q/D and (1) Gavezzotti, A.; Simonetta, M. Chem. Reu. 1982, 82, 1. (2) Ramamurthy, V.; Venkatesan, K. Chem. Reo. 1987, 87, 433. (3) Colpa, J. P.; Prass, B.; Stehlik, D. Chem. Phys. Lett. 1984, 207, 469. (4) Tietje, M.; Borczyskowski, C. V.; Prass, B.; Stehlik, D. Chem. Phys. Lett. 1986, 127, 475. (5) (a) Prass, B.; Colpa, J. P.;Stehlik, D. J . Chem. Phys. 1988,88, 191. (b) Prass, B.; Colpa, J. P.; Stehlik, D. Chem. Phys. 1989, 136, 187. ( 6 ) Yamauchi, S.; Terazima, M.; Hirota, N. J . Phys. Chem. 1985, 89, 4804. (7) Hoshi, N.; Yamauchi, S.; Hirota, N. J . Phys. Chem. 1988, 92,6615. (8) Allan, G.; Castellano, A.; Catteau, J. P.; Lablache-Combier, A. Tetrahedron 1971, 27, 4704. (9) Bent, D. V.; Hayon, E.; Moorthy, P. N. J . Am. Chem. Soc. 1975.97, 5065. (IO) Basu, S . ; McLauchlan, K. A.; Sealy, G.R. Chem. Phys. Lett. 1982, 88,84. ( 1 1 ) Basu, S . ; McLauchlan, K. A.; Ritchie, A. J. D. Chem. Phys. 1983, 79, 95. (12) Yamauchi, S.; Hirota, N. J . Phys. Chem. 1984, 88, 4631.
0 1990 American Chemical Society
