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three CC bonds), a non-nearest-neighbor interaction easily seen to be considerably more effective in the cis conformation than in the gauche or trans conformation. This type of through-space interaction has previously been labeled laticyclic hyperconjugation by Paddon-Row and Jordan in the context of diene D/A systems.! la.28e The relative strength of ?r coupling afforded by the bicyclooctyl (3) and other alkyl spacer groups has been the subject of conFrom Table VI we flicting reports in the see that the coupling in 3 is weaker (Le., the magnitude of TKT is smaller) than in the cyclohexyl system in its lowest-energy conformation (chair, with xcos2 Bi = 1.5) and similarly in the single alkyl chain (5), for both anion and cation cases. However, the coupling via the cyclohexyl and alkyl spacers is very sensitive to conformation, and the entire range of -TKT values for different conformations is seen to bracket the bicyclooctyl values.59 Finally, we consider the bearing of stereochemistry on the relative coupling strength in radical anion and cation systems. For the lowest energy single-chain spacer (trans, with cos2 B = l), somewhat greater magnitude of coupling is found for the cation, whereas the order switches for the case of spacer bridges with gauche and cis conformations, both cyclic (cyclohexane and bicyclooctane) and acyclic (butane), provided that the D/A groups maintain the lowest energy conformation (Le., Ccos2 Bi = 1.O or 1.5). In kinetic studies using spacers based on the chair cyclohexyl no difference in falloff of coupling with D/A separation could be detected in comparisons of anion vs the corresponding cation systems. However, in theoretical studies based on dienes coupled by norbomyl spacers, falloff for radical anions was found to be faster than for corresponding radical cations.28e,f
and through-bond (Tz:), using perturbation theory as formulated by RatnerZotogether with a localized orbital basis represented by natural bond orbitals (NBO’s) as defined by Weinhold et a1.40 The overall coupling has been shown to arise from interference among a large number of competing pathways, none of which is strongly dominant. Nearest-neighbor pathways of the McConne11I6 type are significant for transfer in some radical cation systems, but are frequently of very minor significance in comparison with lower-order superexchange pathways, often with contributions differing in sign from the overall TKT values. These latter conclusions are generally consistent with those based on studies involving different saturated spacer groups by Naleway et alaz9(for radical anions) and Jordan and Paddon-Row28e(for radical anions and cations). We find that transfer in radical anions and cations is generally dominated respectively by electron and hole pathways, but both mechanisms are found to be significant in both types of transfer. A number of transferability relationships have been identified for generic pathway types, and the important influence of stereochemistry on coupling has been illustrated, with regard to both orientation of donor/acceptor groups relative to the spacer and internal conformation of the spacer, showing the competition between coupling via the carbon framework and via C H bonds (i .e., hyperconjugation).
Acknowledgment. This research was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the US. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. We thank Dr. John Miller for communicating some of his recent results to us during the course of the present research, and for supplying us with a preprint of ref 29. We also benefited from several discussions with Professor K. D. Jordan, who supplied preprints of refs 28d and 28e prior to publication.
V. Summary Effective transfer integrals ( T ) have been evaluated for u- and *-type electron and hole transfer in radical ion systems comprising methylene donor/acceptor groups linked by various saturated organic spacer groups. The T values have been calculated on the basis of ab initio self-consistent-fieldwave functions for the radical ion states, obtained either directly for the system of interest ( T E ~ ) or from the associated neutral state via Koopmans’ theorem ( T K ~ ) , and employing either minimal (STO-3G) or split-valence (3-2 1G) basis sets. The TKTvalues have been decomposed into additive contributions from individual pathways, both through-space (T:?)
Appendix A Sensitivity of Trtb to Rydberg Contributions (3-2lC Basis). Table VI1 exhibits the sensitivity of Tpathto Rydberg contrib u t i o n ~ ~by’ ~showing the results (through fourth and fifth order) in which all pathways involving Rydberg orbitals are excluded or in which the influence of the Rydberg orbitals on the valence NBO Fock matrix is included via the partitioning methods3]
(58) Paddon-Row, M. N. New. J . Chem. 1991, I S , 107-116. (59) It should also be noted (see Table I), that the coupling in 3 displays a significant sensitivity to basis set.
Appendix B Table VI11 presents low-order contributions to Tpth for the 3-21G basis.
Photoirradiation Effect on Colloidal Cadmium Sulfide Formation Processes Toyoharu Hayashi,* Hiroshi Yao, Shigeru Takahara, and Koichi Mizuma Central Research Institute, Mitsui Toatsu Chemicals. Inc., Yokohama 247, Japan (Received: September 5, 1991; In Final Form: January 9, 1992) Cadmium sulfide ultrafine particles of about 3 nm in diameter were prepared by introducing a H,S/He gas mixture into an acetonitrile solution of cadmium perchlorate, styrene monomer, and o-dicyanobenzene while irradiating the solution with light of wavelengths longer than 440 nm. The UV-vis spectral absorption onset wavelengths of the CdS colloidal solutions were blue-shifted in comparison with those of solutions not irradiated. The UV-vis spectra also showed the appearance of a shoulder due to the exciton absorption of quantum-confined CdS particles. These results demonstrate a new method for preparing size-regulated semiconductor ultrafine particles in the size range where quantum confinement of carriers occurs.
Introduction In the past decade, semiconductor ultrafine particles, wellknown as Q-particles,] have been studied extensively. These studies have covered topics such as photogenerated electron-hole carrier ( I ) See review articles such as: (a) Henglein, A. Chem. Rev. 1989, 89, 1861. (b) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183.
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confinement effects on the electronic states of particles: hot camer effects in photo catalyst^,^ and so on. Semiconductor ultrafine particles have also been studied as third-order nonlinear optical materials with large nonlinearity and high operation speed. Here (2) Brus, L. E. J . Chem. Phys. 1983, 79, 5566; 1984, 80, 4403. (3) Williams, F.; Nozik, A . J. Nature 1984, 312, 21.
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Colloidal Cadmium Sulfide Formation Processes the quantum confinement of charge carriers plays an important role. This role has been shown by measurements of third-order nonlinear optical susceptibilities for CdSSe or CdS ultrafine particles dispersed in glass or organic polymer matrices4 as well as by theoretical studies.s One important requirement for semiconductor ultrafine particles with high-performance nonlinear optical properties is to prepare particles with a narrow particle size distributior6 Various methods have been developed for preparing semiconductor colloids with a controlled size in the range giving rise to quantum confinement effects. Such methods include the use of hexametaphosphate as a stabilizer,' confinement in reverse micelles,8 using porous materials such as zeolite^,^ peptide coating by biosynthetic methods,1° stabilization with phenyl-capped surfaces," and so on. Techiques for separating the materials by size have also been developed.I2 Our study has looked at the effect of photoirradiation on CdS particle formation in colloidal solutions. We found an interesting phenomenon in which light regulates the formation of CdS colloidal particles. To our knowledge, there are no previous descriptions of such a photoirradiation effect on the particle formation processes of semiconductor colloidal solutions. In this paper we would like to report on our preliminary experimental results and describe a model that can explain them.
Experimental Section The reagents Cd(C104),.6H20 (99%, Soekawa Chemicals), Cd(N03)2.4H20(99.99% Kojundo Chemicals), and acetonitrile (special grade, Kokusan Chemicals) were used as supplied. Styrene monomer (Junsei Chemicals) was washed with 5% aqueous NaOH solutions, followed by evaporation under reduced pressure. The o-dicyanobenzene was recrystallized from ethanol solution three times and then filtered through activated charcoal. Colloidal solutions of CdS particles were prepared as follows. A gas mixture of H2Sand He (0.02 vol % H2S) with a flow rate of 54 mL/min was bubbled into a solution of Cd(C1O&.6H20 (2 X lo-' M), styrene monomer (4 X M), and o-dicyanobenzene (4 X 10-3M) in acetonitrile (5 mL). The solution was contained in a quartz cuvette (10" path length) with gas inlet and outlet. The solution was stirred with a magnetic stirrer during the reaction. Before starting the formation of CdS colloidal particles by introducing H2S/He gas, the solution was purged of oxygen by flushing N 2 gas. A high-pressure mercury lamp (500 W, USHIO, USH 500D) was used as the light source for photoirradiation. The light beam was passed through a quartz cell (3-cm path length) filled with water to cut infrared radiation. A shorthwavelength sharp-cut filter (Y-44, Toshiba Glass) was used to limit the light to a sharp maximum around 440 nm. The light intensity was measured with a thermopile-type power meter (30A-P, OPHIR Ltd.). The light intensity was changed in the range from 0.21 to 0.032 W/cm2 by focusing the light with an optical lens. Care was taken to irradiate the entire volume of the solution. UV-vis absorption spectra were measured with a photodiode array-type spectrophotometer (Otsuka electronic, IMUC-7000) using a xenon lamp as the light source.
(4) (a) Jain, R. K.; Lind, R. C. J. Opt. Soc. Am. 1983,73,647. (b) Wang, Y.; Mahler, W. Opt. Commun. 1987, 61, 233. (5) Hanamura, E. Solid State Commun. 1987, 629465. (6) Takagahara, T. Phys. Reu. B 1987, 36, 9293. (7) Fojtik, A.; Weller, H.; Koch, U.; Henglein. A. Eer. Bunsen-Ges. Phys. Chem. 1984, 88, 969. (8) Meyer, M.; Wallberg, C.; Kurihara, K.; Fendler, J. H. J . Chem. SOC., Chem. Commun. 1984,90. (9) Wang, Y.; Herron, N. J. J . Phys. Chem. 1987, 91, 257. (10) Dameron, C. T.; Winge, D. R. Inorg. Chem. 1990, 29, 1343. (11) Steigerwald, M. L.; Alivisatos, A . P.; Gibson, J. M.; Harris, T. D.; Kortan, R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J . Am. Chem. SOC.1988, 110, 3046. (12) (a) Fischer, Ch.-H.; Weller, H.; Katsikas, L.; Henglein, A. Lmgmuir 1989, 5, 429. (b) Eychmiiller, A.; Katsikas, L.; Weller, H. Langmuir 1990, 6, 1605.
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2861
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WAVELENGTH (nm) Figure 1. Upper part: UV-vis absorption spectra of CdS colloidal solutions obtained with photoirradiation and without photoirradiation. Experimental conditions are described in the text. Irradiation light intensity: (a) no irradiation; (b) 0.032 W/cm2; (c) 0.21 W/cm2. Lower part: first-derivative absorption spectra of upper part to clearly show the position of the absorption edges.
Results and Discussion When the CdS colloidal solution was prepared under irradiation with light (>440 nm), the UV-vis absorption onset wavelength was blue-shifted from 500 nm for the unirradiated solution to 450-480 nm depending on the irradiating light intensity, as shown in Figure 1. High-resolution analytical transmission electron microscopy (JEOL, JEM2010,200 kV) was used to obtain high-magnification photomicrographs and electron beam diffractograms. The electron diffraction patterns agreed better with a hexagonal crystal structure for CdS (ASTM standard) rather than a cubic structure. Electron photomicrographs revealed coagulated particles of about 3 nm in diameter. This size corresponds to the UV-vis absorption band edge or shoulder wavelength of the CdS colloidal solution. Distinct differences in the morphology and spectra of particles prepared with and without photoirradiation were observed. The irradiated samples contained very few particles larger than 3 nm in diameter, while the unirradiated samples contained extremely large particles up to several tens of namometers in diameter. The observed changes in the absorption spectra are similar to the spectral changes observed for CdS colloidal solutions not containing large particles, as reported in the study on size separation by electrophoresis gel chromatography.I2 Considering that the appearance of an exciton absorption shoulder was more pronounced in our spectra, the size distribution of our colloidal particles is narrower than that from the above size separation method. As is well-known, when semiconductor particles become smaller than a characteristic size related to the exciton Bohr radius, the band gap energy or exciton transition energy becomes larger due to the quantum confinement of electron-hole carriers. The particle size in which the electronic character changes corresponds uniquely to the wavelength of the exciton absorption for a specified semiconductor. Because the solution of styrene, cadmium perchlorate, and o-dicyanobenzene in acetonitrile was transparent above 380 nm, only the CdS particles larger than a certain size can absorb the irradiating light. Therefore, when the CdS particles grow to a size corresponding to the irradiation light wavelength, they absorb the irradiating light. This causes the particles to undergo photocatalytic reactions on or near their surface, thus preventing the particles from further growing. The inhibition of particle growth leads to an apparent shift of the UV-vis absorption onset wavelength of the colloidal solution to shorter wavelengths compared to the case of unirradiated samples. The clear shoulder in the absorption band edge is due to excitons. This appears only
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when the inhomogeneous broadening due to the particle size distribution is reduced. The photocatalytic polymerization of styrene is the most probable candidate for a photocatalytic reaction. The existence of styrene monomer was indispensable to the observation of the photoirradiation effect. In addition, the photoirradiation effect was enhanced by increasing the styrene monomer concentration. Photocatalytic polymerization of styrene would create a polystyrene coating on the surface of the CdS particle. The polymer layer prevents the particles from growing further. In spite of our efforts to obtain evidence for the formation of polystyrene by using such methods as FT-IR analysis, no trace of polystyrene has yet been detected. This is probably due to the low concentration of polystyrene on the surface. This model describing the polymerization of styrene on the surface of the growing particles thus remains hypothetical at present. It was initially thought that o-dicyanobenzene was necessary as an electron acceptor or mediator for generating active radicals for subsequent reactions or by simply making holes more reactive by removing electrons. However, the experimental results on the contribution of each component in the solution revealed that o-dicyanobenzene was not indispensable as long as enough styrene monomer is in the solution. As the irradiating light intensity increased, the wavelength of the absorption onset shifted to shorter wavelength, approaching 440 nm, as shown in Figure 1. With 0.21 W/cm2 intensity light, the onset wavelength almost coincided with the shortest wavelength (440 nm) of the irradiating light. This indicates that photoirradiation is effective for preventing the CdS particles from growing over a size corresponding to the shortest wavelength of the irradiating light. The photoirradiation effect is expected to become more effective when the CdS formation rate is small enough so that the photocatalytic reaction can take place before the growing particles have become larger than the size corresponding to the irradiation light wavelength. In fact, decreasing the H2Sconcentration in the H2Sand He gas mixture from 0.25 to 0.02 vol % resulted in a shift of the absorption onset wavelength to shorter wavelengths.
We also investigated the possibility that previously-grown particles undergo photobleaching, giving rise to smaller particles. When a previously-prepared CdS colloidal solution was irradiated without the presence of H2S, almost no change in the optical absorbance was observed. Thus, photobleaching does not occur easily. It was found by us that counteranions play an important role in photobleaching. We found that different cadmium compounds gave different results for the photoirradiation effect. When Cd(N03)2.4H20was used instead of Cd(C104)2.6H20,no photoirradiation effect was observed. Furthermore, the colloidal solution of CdS derived from the nitrate suffered from photobleaching. That is, a decrease in the absorbance of the solution over the entire spectral range was observed when irradiated. It is known that different cadmium compounds give different size distributions of colloidal particles. It is also well-known that the perchlorate counteranion primarily used in the present experiments shows little or no complexing tendency toward metal cations in comparison to the nitrate ion.13 This might explain the different behavior observed for the photobleaching experiments of CdS colloidal solutions. The nitrate ion might have a stronger tendency to coordinate metal cations, thus leading to an acceleration of the photobleaching process. Several items still remain that need to be studied. These include exact characterization of the particle size distribution of irradiated and unirradiated samples, as well as confirming the mechanism controlling particle growth. The type of photocatalytic reactions occurring also needs to be studied further to clarify the origin and other details of the absorption onset shift induced by photoirradiation.
Acknowledgment. This work was supported by N E D 0 (New Energy and Industrial Technology Development Organization). Registry No. CdS, 1306-23-6; H,S, 7783-06-4; Cd(C104)2,13760styrene, 100-42-5; o-dicyanobenzene, 91-15-6.
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(13) Moeller, T. Inorganic Chemistry, Modern Asia ed.;John Wiley & Sons: New York, 1970; p 237.
Recombination Luminescence Quenching of Nonstoichiometric CdS Clusters by ZnTPP J. Chrysochoos Department of Chemistry, The University of Toledo, Toledo, Ohio 43606 (Received: September 9, 1991; In Final Form: November 26, 1991)
The recombination luminescence of CdS(e-/h+) clusters is quenched effectively by ZnTPP. The quenching procass is described by static interactions at low [ZnTPP], leading to a quenching constant (KQ),and by Langmuir isotherms, leading to ranging from 5 X lo4 to 2.5 X IO5 M-I, increase slightly adsorption-desorption equilibrium constants K . Values of (KQ), with decreasing cluster size (or increasing Eg value) at constant CdS cluster composition. On the other hand, values of (KQ) decrease slightly with increasing Cd2+nonstoichiometry (Le., Cd2+excess) at constant Eg value. Values of K range from 1.5 X lo5 to 9.5 X lo5 (Le., kads>> kdss), implying a strong binding of ZnTPP on the surface of CdS clusters. Treatment of recombination luminescence quenching by Poisson statistics indicates that an average CdS cluster consists of at least 100-150 CdS units (Le., 25-30-A diameters). Fluorescence quenching of ‘ZnTPP*(S,) by CdS is less efficient, and it is dependent upon the size of the cluster. Values of kQ70(’ZnTPP*)range from 0 to 2 X IO3 M-l for Eg values varying from 3.1 to 2.9 eV.
Introduction During the past decade scientists from different disciplines demonstrated a very keen interest in the physical and chemical properties of semiconductor clusters. Although investigations in semiconductor clusters encompass a wide range of topics, two aspects of particulate semiconductors have attracted the interest of many investigators: optical properties (linear and nonlinear)
and interfacial electron transfer. The former relates to the significance of such species in electronic materials and devices whereas the latter to the role of semiconductor clusters as photocatalysts. Differences in the behavior of semiconductor clusters and that of bulk semiconductors are linked to both the “space confinement” imposed by the former on charge carriers (e-/h+), leading to “size quantization” effects, and the effect of surface
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