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J. Phys. Chem. 1996, 100, 362-367
Photoinduced Electron Transfer Processes of CdPS3 Intercalated with Ruthenium Tris(bipyridyl) and Methylviologen Cations R. Jakubiak and A. H. Francis* Department of Chemistry, The UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: July 27, 1995; In Final Form: September 29, 1995X
Cation exchange was employed to intercalate the lattice of CdPS3 with both ruthenium tris(bipyridyl) cation, a good photoreducing agent, and methylviologen cation, an electron acceptor. The host lattice is diamagnetic and transparent in the visible and near-ultraviolet region of the spectrum, allowing excitation and spectroscopic observation of photoinduced electron transfer processes involving the host lattice and the intercalated species. The photoinduced charge transfer between these species was examined using photoluminescence and electron paramagnetic resonance spectroscopy. Cation vacancy centers in the CdPS3 host lattice are shown to be chemically and photochemically active.
Introduction The photoinduced charge transfer reaction between ruthenium tris(bipyridyl) cation (RUBIPY2+) and dimethylviologen cation (MV2+) has been studied in a variety of media, including ionexchange resins,1 silicates,2 micelles,3 cellulose,4 porous Vycor glass,5 oxides,6 and other materials.7 The interest of researchers has often been stimulated by potential energy storage applications8 and efforts have therefore focused on the role of the media in stabilizing the products of the photoinduced reaction against back electron transfer. In the present work we have examined this much studied process in the lattice of the lamellar chalcogenophosphate, broad-band semiconductor CdPS3. The transition metal chalcogenophosphates form a family of compounds with the general empirical formula MPX3 (M ) transition metal and X ) S, Se) that crystallize with a layered structure.9 The layers are built of octahedrally coordinated transition metal cations sandwiched between planes of close packed chalcogen atoms. Because the repeating layers are held together only by weak van der Waals (vdW) interactions, it is possible to introduce molecular cations into the two-dimensional interstices between the chalcogen planes to form intercalation compounds.10 When cations are inserted into the interlamellar spaces, dipositive transition metal cations are displaced from the layers11 leaving metal cation vacancies that compensate the charge of the intercalated cations. In effect, the lattice vacancy defects serve as counterions for the intercalated molecular cations. The solid state chemistry of the MPX3 materials has proven useful for the spectroscopic study of a variety of molecular cations.12 Experience indicates that the intercalated species are only weakly perturbed by the homogeneous environment of the host lattice, making it possible to obtain well-resolved electronic, vibrational, and magnetic resonance spectra. The two-dimensional interstitial space may, in some instances, orient the intercalated species, permitting studies of the polarization of vibrational and electronic spectra or the angular field dependence of magnetic resonance spectra. In the present work we have employed the host lattice of CdPS3 to study photoinduced charge transfer between cointercalated RUBIPY2+ and MV2+. CdPS3 is particularly useful for spectroscopic studies because it is diamagnetic and transparent in the near-UV, visible, and near-IR regions of the spectrum. X
Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0362$12.00/0
The possibility of conducting reactions within the interlamellar spaces is intriguing because the reduced dimensionality of the space may modify the reactivity by restricting the diffusion and the relative orientation of the reacting species. It was hoped that the intercalated cations, separated spatially and constrained to the two-dimensional interstices, might be coupled through the delocalized band states of the host lattice. We have been particularly interested in the role of the chalcogenophosphate lattice in stabilizing the charge-separated products against back electron transfer. Finally, since the states of the CdPS3 conduction band are accessible to wavelengths 125 K with light between 350 and 400 nm, samples of III rapidly turn from their initial red-orange color to a uniform deep blue, the color of the dimethylviologen radical cation MV•+. In addition, the intensity of the characteristic luminescence of RUBIPY2+ rapidly decreases as illustrated in Figure 3. Since the MV•+ absorption spectrum overlaps the RUBIPY2+ photoluminescence spectrum, the decrease in photoluminescence intensity is due to both the oxidation of RUBIPY2+ and absorbance by MV•+. Because only the cointercalated crystals (III) exhibit this photochemistry, it is likely that the process involves the photoinduced transfer of an electron from RUBIPY2+ donors to MV2+ acceptors according to
RUBIPY2+ + hν T *RUBIPY2+
(2)
*RUBIPY2+ + MV2+ T RUBIPY3+ + MV•+
(3)
The products are metastable and persist for several hours at room temperature, and longer at low temperature. Poizat et al.14 have reported that the blue MV•+ radical cation may also be produced in MV2+-intercalated CdPS3 by treatment with n-butyl lithium in tetrahydrofuran solution and has a half-life of approximately 3 weeks at room temperature.14 The photoproduced radical was stable in our samples of III for approximately 1 day at room temperature. It is clear that the kinetics of recombination are highly sensitive to the structure and contents of the intercalated interlamellar spaces. Cooling III to 77 K completely inhibits the production of MV•+. Instead, constant intensity irradiation of III at 85 K produces a time dependent increase of the RUBIPY2+ photoluminescence as illustrated in Figure 3. The nearly 4-fold increase results from the photodepletion of quenching centers. *RUBIPY2+ luminescence is quenched by lattice acceptors centers (A) through both electron and energy exchange pro-
Photoinduced Electron Transfer Processes of CdPS3
J. Phys. Chem., Vol. 100, No. 1, 1996 365
Figure 4. Superposition EPR showing singlet and 13-line spectrum at room temperature.
Figure 5. EPR spectra: (a) 13-line hyperfine multiplet spectrum of selected crystals of III (unilluminated); (b) simulated spectrum with aN ) 7.054 G and g ) 2.006.
cesses.
*RUBIPY2+ + A f RUBIPY3+ + A-
(4)
*RUBIPY2+ + A f RUBIPY2+ + *A
(5)
The concentration of acceptors is depleted at low temperature by trapping photoelectrons to form long-lived A- centers that do not efficiently quench RUBIPY2+ luminescence. Impurities such as Fe3+ and Cr3+ are known to be extremely efficient at quenching RUBIPY2+ luminescence and were found by neutron activation analysis to be present at the ppm level in the CdPS3 host lattice. Much higher concentrations of intrinsic Cd2+, [VCd]0, and [VCd]- acceptors (V represents cation vacancies) are also present. An interesting change occurs in the MV2+ intercalation compound (II) over a period of several months. Stored at room temperature, the initially photoinactive crystals acquire a faint blue hue and become photoactive. Illumination with blue light causes the rapid development of a deep blue color. It is likely that the process involves photoreduction of MV2+ by cation vacancies:
[VCd]2- + MV2+ + hν f [VCd]- + MV•+
(6)
It is not known why this material is initially inactive and requires incubation for several months. It is possible that in the freshly prepared material the vacancies are not adjacent to the interlamellar cations and that diffusion of the vacancies within the lamella establishes a more favorable relation for charge transfer. EPR Spectra. EPR spectra of the photochemically active cointercalation compound (III) were recorded at room temperature prior to irradiation. The spectrum shown in Figure 4 is typical of III and appears to consist of a superposition of a 13-line hyperfine multiplet and a singlet, both having g ≈ 2. The relative contribution of each spectrum to the total signal varied between samples, with some samples exhibiting only the hyperfine multiplet spectrum shown in Figure 5(a). The
Figure 6. Effect of illumination at room temperature upon the EPR spectrum of III: (a) before illumination; (b) after 30 min of illumination.
spectrum is nearly isotropic, with only the relative intensity of the component spectra changing as a function of field angle. Comparison of the singlet spectrum with the EPR spectrum of MV2+-intercalated CdPS3 (II) suggests that the resonance is due to MV•+. Crystals of II exhibit a weak, broad (g ) 2.006, ∆Hpp ) 14 G) singlet believed to be due to the reduction of the intercalate by lattice vacancies. The g-value and line width are similar to that reported previously for the MV•+ cation radical.28 Recently, Xiang and Kevan reported a broad singlet (g ) 2.004, ∆Hpp ) 15 G) in the EPR spectrum of methylviologen photoreduced in a silica gel host.29
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The analysis of the 13-line hyperfine spectrum is more problematic. We were able to generate a reasonable simulation30 of this spectrum by assuming g ) 2.002 and six equivalent nitrogen nuclei having a nuclear hyperfine coupling constant of aN ) 7.05 G (Figure 5b). The spectrum is qualitatively similar to that obtained by Saji and Aoyagui31 for the chromium tris(bipyridyl) compound, Cr(bipy)3+ in solution at room temperature. The comparison suggests that the 13-line hyperfine spectrum, in both instances, arises from coupling to six equivalent nitrogen nuclei of the three equivalent bipyridyl ligands. The comparison to Cr(bipy)3+ notwithstanding, there are several difficulties in assigning the spectrum shown in Figure 5a to either an oxidation or a reduction product of the RUBIPY2+ intercalate. In particular, the spectrum is markedly different from those reported previously for RUBIPY3+ in which the unpaired electron resides in an orbital with considerable metal character, producing an anisotropic g-value that deviates substantially from the spin-only value because of unquenched ruthenium spin-orbit coupling. Because the electron occupies an orbital localized on the metal, the ligand hyperfine coupling is small and the ligand hyperfine structure is not resolved. RUBIPY+ may be formed by electron transfer from cation vacancy centers. Cadmium cation vacancies, [VCd]2-, are created in abundance by the cation exchange intercalation process (eq 1). These vacancies are present in the unintercalated CdPS3 at lower concentrations and may be detected by their characteristic luminescence observed with maximum intensity at 500 nm.32 The vacancies can diffuse within the lamella and are likely to be positionally correlated with the intercalate cations. The [VCd]2- centers are good electron donors and are likely responsible for the reduction of intercalated molecular cations by the following reactions:
RUBIPY2+ + [VCd]2- f •RUBIPY+ + [VCd]-
(7)
MV2+ + [VCd]2- f MV•+ + [VCd]-
(8)
The unpaired electron of the reduced species RUBIPY+ is expected to occupy a ligand π orbital and exhibit a g-value close to the free-electron value that is observed. However, RUBIPY+ EPR spectra do not typically exhibit the resolved hyperfine structure evident in Figure 5a. The absence of hyperfine structure has been shown33 to be associated with relaxationinduced line broadening due to activated hopping of the unpaired spin between equivalent ligand π orbitals. In some instances resolved nitrogen hyperfine spectra have been obtained for the one-electron reduction products of mixed ligand RuL32+ (L ) R-diimine) complexes34 In these examples, the unpaired spin is localized in a “spatially isolated” redox orbital of a single ligand and the hyperfine spectrum is associated with the isolated ligand. The nitrogen hyperfine coupling constants observed in mixed ligand complexes are typically 3-4 Gauss, considerably smaller than exhibited in Figure 5a. In view of the results of previous studies, we do not have a satisfactory interpretation of the 13-line hyperfine spectrum. The RUBIPY2+ luminescence spectrum is only weakly perturbed, suggesting that the host lattice does not severely alter the metalligand interactions of the intercalate. In the absence of strong lattice-ligand interactions it is difficult to understand why the electronic structure of RUBIPY+ should be strongly modified from that observed in many previous EPR studies. We note that the strength of the EPR signal suggests that the concentration of the active species in the unilluminated host is quite low and that the spectrum may arise from a small concentration of highly perturbed reduced species.
Upon irradiation, the hyperfine spectrum is rapidly lost and the singlet spectra increase dramatically in intensity because of the photoproduction of MV•+ (see Figure 6). The spectrum of the irradiated crystal appears to be a superposition of two singlets. It is not clear whether these arise from MV•+ in different crystallographic environments or are the separate resonances of MV•+ and photoinduced [VCd]- centers. It was not possible to detect a resonance with an anisotropic g-value significantly differing from 2 that might be plausibly attributed to the production of RUBIPY3+. After illumination, the photoinduced EPR signals decay over a period of several hours at room temperature. Conclusion RUBIPY2+ and MV2+ may be cointercalated into CdPS3 by a cation exchange mechanism. The photoluminescence of the cointercalation compound indicates that the RUBIPY2+ intercalate is located in a relatively homogeneous but sterically constrained environment that is modified by the presence of the MV2+ cointercalate. The lattice basal plane spacings obtained from XRD measurements suggest that at least some of the intercalate is segregated within the host lattice. Cadmium cation vacancies created during the intercalation process serve as counterions for the molecular cation intercalates and act as electron donor centers. At high concentration, the vacancies reduce a portion of the dipositive cation intercalate, generating small concentrations of paramagnetic MV•+ and a paramagnetic reduction product of RUBIPY2+. The formal oxidation state of the product is unknown, but it exhibits an unusual hyperfine spectrum consistent with an unpaired electron spin delocalized over six equivalent nitrogens. Prior to illumination, the intercalated species are present predominately as the dipositive cations. There is no indication of a significant direct reaction between the cointercalates in the absence of illumination. Upon illumination at room temperature a rapid electron transfer from the RUBIPY2+ donor to the MV2+ acceptor is observed. The products of the photoreaction are depleted by back electron transfer over a period of hours. Intrinsic acceptor centers in CdPS3 partially quench RUBIPY2+ luminescence at room temperature, leading to a decreased high-temperature luminescence efficiency. At 77 K the acceptor centers trap photogenerated electrons and are no longer efficient quenching centers, leading to an increase in RUBIPY2+ luminescence. The photoinduced electron transfer from RUBIPY2+ to MV2+ is completely quenched at 77 K, leading to the belief that the process in CdPS3 involves a thermally activated step. The thermally activated nature of the process suggests that the transfer is indirect, involving states of the host lattice, rather than direct via an overlap of the donor and acceptor valence orbitals. Acknowledgment. The authors wish to express their appreciation to Professor Rene Clement for helpful discussions during the course of this research. The authors acknowledge support from the National Science Foundation (DMR 8818371). References and Notes (1) (a) Thornton, A. T.; Laurence, G. S. J. Chem. Soc., Chem. Commun. 1978, 408. (b) Lee, P. C.; Meisel, D. J. J. Am. Chem. Soc. 1980, 102, 5477. (c) Kurimura, Y.; Nagashima, M.; Takato, K.; Tsuchida, E.; Kaneko, M.; Yamada, A. J. Phys. Chem. 1982, 86, 2432. (d) Miyashita, T.; Arito, Y.; Matsuda, M. Macromolecules 1991, 24, 872. (2) (a) Ogawa, M.; Inagaki, M.; Nobuhiro, K.; Kuroda, K.; Chuzo, K. J. Phys. Chem. 1993, 97, 3819-3823. (b) Ghosh, P. K.; Bard, A. J. J. Phys. Chem. 1984, 88, 5519-5526. (b) Joshi, V.; Ghosh, P. K. J. Am. Chem. Soc. 1989, 111, 5604-5612. (c) Joshi, V.; Ghosh, P. K. J. Chem. Soc., Chem. Commun. 1987, 789-791. (d) Turro, N. J.; Kumar, C. V.; Grauer, Z.; Barton, J. K. Langmuir 1987, 3, 1056-1059.
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