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our observation of only three microwave transition regions, which, furthermore, were internally consistent (Le., 2E + ( D - E ) = D E). This outcome motivated us to reinvestigate the parent compound 2,2’-biquinoline in n-decane. Table I includes these results as well. Monitoring the 489-nm phosphorescence emission, we observed only one set of triplet sublevel transitions when microwave low-pass filters were judiciously employed. They comprised an internally consistent set of values at 1921, 2223, and 4144 MHz. This connectivity was further confirmed by double-resonance experiments: for all three transitions, saturating any one of the sublevel transitions affected the relative magnitude of the ODMR intensity of each of the two remaining peaks. The earlier work on 2,2’-biquinoline can be reconciled vis-84s our current results by noting that harmonics of the intended frequency are also transmitted by a microwave sweep generator. To illustrate, we can focus on the most intense satellite of the 489-nm site.8 The observed signal at 641 . I MHz actually produced an ODMR transition at a tripled frequency of 1921 MHz; the observed signal at 960.5 MHz was the result of a true transition at the doubled frequency of 1921 MHz; the observed signal at 2072.2 MHz was the result of its doubled harmonic stimulating the actual transition at 4144 MHz; the reported signal at 2223.9 MHz was a genuine transition that we also observed. Our use of, for example, a 1000-MHz low-pass filter prevents the observation of transitions at 641.1 and 960.5 MHz since the molecular effect occurring at 1921 MHz never reaches the sample. This procedure can be followed for each of the originally reported solvents/si tes. In Table I, we have also summarized the zero-field splitting parameters in terms of D and E values. Parameter D is inversely proportional to 1 / r 3 , where r is the distance separating the two highest energy, unpaired electrons, averaged over the molecular orbital wave function. Parent compound quinoline has been included for reference.’* Our results for biquinoline and for its 3,3’-derivatives provide convincing evidence from two directions that the photoexcited
+
( 1 8 ) Vincent, J .
S.;Maki, A. H. J . Chem. Phys. 1963, 39, 3088
triplet, state in these double molecules is confined to one quinoline moiety only. First of all, the D value of biquinoline is actually slightly larger than that of quinoline. We are compelled to conclude that the triplet spin density is not delocalized over the entire double molecule 2,2’-biquinoline, since a smaller D value is expected if the excitation is dispersed over a molecular framework that is smaller in size by a factor of 2. Secondly, examining the D values for the series of four 3,3’-annelated derivatives reveals them to be comparable. A delocalized triplet state would have been implausible for the nonplanar derivatives compared to the planar n = 1, yet no distinctly different D values were measured. The fact that biquinoline exhibits a D value that is somewhat larger than those of the derivatives is simply a consequence of the fact that biquinoline prefers the anti (trans) conformation to that of syn (cis).I9 Electron correlation would obviously change somewhat when the influential quinoline group rotates by 180’. The power of a nearby quinoline moiety to influence electron correlation is revealed even more strongly in the E values. Zero-field splitting parameter E is the consequence of the difference in the electrons’ average separation along the two in-plane axes and is thus a symmetry indicator. The wide variety of directional influences present in the five biquinoline systems of Figure 1, as compared to that of the reference compound quinoline, reconciles the variability among the data.
Conclusion We have presented two important results via ODMR studies on biquinoline and some annelated derivatives. First of all, 2,2’-biquinoline possesses only one triplet state whose zero-field splitting constitutes an internally consistent pattern. Secondly, by investigating a series of 3,3’-annelated biquinoline derivatives, we have conclusively shown that the biquinoline triplet state is localized on one of the quinoline moieties. Acknowledgment. Financial support for this work was provided by NSF-EPSCoR Grant 8610679. (19) Folting, K.; Merritt, L.
L.Acta Crysfallogr.1977, B33, 3540.
Conformational and Substituent Effects on Spin Distributions in Porphyrin Cation Radicals M. W. Renner,lP R.-J. Cheng,lb*cC. K. Chang,lb and J . Fajer*,la Department of Applied Science, Brookhaven National Laboratory, Upton, New York I 1 973, and Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received: September 6 , 1990)
Optical, redox, EPR, and ENDOR results are presented for zinc meso-tetraphenyltetrabenzoporphyrin(ZnTPTBP). The molecule is a hybrid of the common synthetic porphyrins zinc tetraphenylporphyrin (ZnTPP) and zinc tetrabenzoporphyrin (ZnTBP). The consequences of the additional peripheral substituents in the hybrid are evident in a previously obtained X-ray structure of ZnTFTBP that shows the macrocycle to be severely puckered to minimize steric interactions between the substituents. The cumulative effects of conformational changes and substituent addition are reflected in the red-shifted optical spectrum and ease of oxidation of the neutral compound, and in a shift of unpaired spin density from a,, (ZnTBP) to aZu(ZnTPP) character in the radical. Significant spin density is found to delocalize onto the phenyl substituents of ZnTPTBP+ and illustrates the shortening of the effective distance between donor and acceptor that may occur in biomimetic complexes used to investigate the distance and energy dependence of electron transfer. Clearly, the introduction of covalent aromatic (or aliphatic) linkages in donor-acceptor models may induce conformational changes in the macrocycles themselves, with concomitant redistributions of unpaired spin densities in the radicals that result from electron transfer.
The ubiquitous roles of porphyrin derivatives in photosynthetic and enzymatic p r o c e s s e ~have ~ ~ ~prompted elegant syntheses of 0022-3654/90/2094-8508$02.50/0
donor-acceptor complexes covalently linked by aromatic or aliphatic bridges4 to assess the dependence of electron-transfer rates 0 1990 American Chemical Society
Let ters
The Journal of Physical Chemistry, Vol. 94, NO. 23, 1990 8509 30
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0.
300’400 500‘6~0’700’800’900’1000 WAVELENGTH ( n m )
Figure I . Optical spectra of ZnTPTBP (-) and of its cation radical (- --) obtained by one-electron electrochemical oxidation in CH2C12with 0.1 M Bu4NC104 a s electrolyte. The spectral changes due to the oxidation develop with sharp isosbestic points. Only one intermediate stage in the oxidation is shown (---). T = 298 K.
on distance and energyS5 EPR and ENDOR studies of porphyrin anion and cation radical^,^,' generated by electron transfer, have demonstrated that the unpaired spin densities extend from the A systems of the radicals onto the substituents, thereby shortening the effective distances between donor and acceptor. For example, in both anion and cation radicals of porphinoid macrocycles with aliphatic substituent^,^-^ the unpaired spin densities extend over several carbons of the aliphatic chains and drop by approximately an order of magnitude for every carbon away from the A system. Similarly, in porphyrin cation radicals with aromatic substituents, unpaired spin density is still detected on the para positions of phenyl rings at the meso positions.6q’0 Recent crystallographic results have established that the introduction of multiple peripheral substituents in porphyrins can lead to steric crowding with the consequence that the conformations of the macrocycles change significantly; Le., the porphyrin skeletons pucker to minimize unfavorable contacts between subs t i t u e n t ~ . I l - ~The ~ electronic effects of these structural changes are readily probed by EPR and ENDOR techniques, and examples are found in chlorin, isobacteriochlorin, and bacteriochlorin cations in which abstraction of electrons occurs from the al, highest ( I ) (a) Brookhaven National Laboratory. (b) Michigan State University. (c) On leave from Department of Chemistry, National Chung-Hsing University, Taichung, Taiwan. (2) The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978, 1979; V O ~I-VI. . ( 3 ) Boxer, S . G. Annu. Rev. Biophys. Biophys. Chem. 1990, 19, 261. (4) Wasielewski, M. R. Photoinduced Electron Transfer; Fox, M. A,, Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, Chapter 1.4; Photochem. Photobiol. 1988, 47.923. Gust, D.; Moore, T. A. Science 1989, 244, 31.
( 5 ) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. Closs, G. L.; Miller, J. R. Science 1988, 240, 440. (6) Fajer, J.; Davis, M. S. The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979, Vol. IV, Chapter 4. (7) Mobius, K.; Lubitz, W.; Plato, M. In Advanced EPR, Applicationr in Biology and Biochemistry; Hoff, A. J., Ed.; Elsevier: Amsterdam, 1989; Chapter 13. (8) Hanson, L. K.; Chang, C. K.; Davis, M. S.; Fajer, J. J. Am. Chem. Soc. 1981, 103, 663. (9) Renner, M. W.; Forman, A.; Wu, W.; Chang, C. K.; Fajer, J. J. Am. Chem. Soc. 1989, 1 1 1 , 8618. (IO) Huber, M.; Kurreck, H.; von Maltzan, B.; Plato, M.; Mobius, K. J . Chem. Soc.,Faraday Trans. 1990,86, 1087. Huber, M.; Galili, T.; Mobius, K.; Levanon, H. I s r . J . Chem. 1989, 29, 65. ( I I ) Barkigia, K. M.; Chantranupng, L.; Smith, K. M.; Fajer, J. J . A m . Chem. Soc. 1988, 110, 7566. Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner. M. W.; Smith, K. M. J . A m . Chem. Soc.. in press. (12) Medforth, C. J.; Berber, M. D.; Smith, K. M.; Shelnutt, J. A. Tetrahedron Lett. 1990. 26. 3719. (13) Vogel, E.; Kiicher, M.; Lex, J.; Ermer, 0.Isr. J . Chem. 1989, 29, 257. (14) Forman, A.; Renner, M. W.; Fujita, E.; Barkigia, K. M.; Evans, M. C. W.; Smith, K. M.; Fajer, J. Isr. J. Chem. 1989, 29. 57. (15) Cheng, R.-J.; Chen, Y.-R.; Chuang, C.-E. J . Am. Chem. Soc., submitted for publication.
occupied orbital leading to U2Alunradi~als.~,’J~ (For simplicity, the nomenclature associated with D4h symmetry is retained.) In porphyrins, the HOMOs are nearly degenerate (a,, or a2u)resulting in 2Aluor 2A2uradicals, depending on the substituents.6 These radicals exhibit significantly different spin distributions that may affect their reactivities,6*8-’6and are exemplified by Zn tetraphenylporphyrin (ZnTPP, 2A2u)and Zn tetrabenzoporphyrin (ZnTBP, We present here optical, EPR, and ENDOR results for the hybrid molecule Zn tetraphenyltetrabenzoporphyrin (ZnTPTBP see Figure 1) and its cation radical that further illustrate the delocalization of unpaired spin onto the substituents and, in addition, reflect the significant spin redistribution that can be induced in porphyrin radicals by substituents and their concomitant conformational effects. Clearly, the introduction of multiple substituents in biomimetic models is not innocent and, indeed, provides an attractive mechanism for modulating the physical and chemical properties of synthetic porphyrins.
Experimental Section Optical spectra were recorded on a Cary 2300 spectrophotometer. EPR and ENDOR spectra were obtained on a Bruker-IBM ERZOOD spectrometer equipped with an ER 251 double resonance system, a field frequency lock, and an Aspect 2000 data acquisition system. Deuterium N M R spectra were recorded on a Bruker AM 300 spectrometer equipped with a 5-mm broad band probe operating at 46 MHz. Cation radicals for EPR experiments were generated by electron transfer to ZnTPP‘+C104-6 in toluene-d8 (99.9% D, Aldrich), distilled, and dried over sodium. 2H N M R results were obtained in CH2CI2,using AgC104 as oxidizing agent. For optical spectroscopy, the radical was generated by controlled potential electrolysis in CH2C12 at a Pt electrode, with 0.1 M Bu4NC104as electrolyte. The electrochemical cell and the purifications of the solvent and electrolyte have been described previously.” Zn meso-tetraphenyltetrabenzoporphyrin(ZnTPTBP) was synthesized from potassium phthalimide and phenylacetic acid according to Luk’yanets’ method.’* This procedure also yields a substantial amount of the triphenyl derivative that must be separated from ZnTPTBP by ~hromatography.’~ ZnTPTBP-d16 (fused benzene rings deuterated) was prepared and purified in the same manner by using deuterated phthalimide obtained by fusion of urea and phthalic acid-d4 (98%D, Cambridge Isotope Lab.) ZnTPTBP-ISN4was made similarly from urea-I5N (99% lSN, Isotec, Inc.) and phthalic anhydride. Results and Discussion The optical spectrum of ZnTPTBP is shown in Figure 1, (maxima at 650,603, and 454 nm). For comparison, the major absorption bands of ZnTPP are found at 588,548, and 419 nm,” and those of ZnTBP at 627, 580, and 430 nm in CH2CI2.l9 The red shifts are consonant with the significant distortion of the macrocycle revealed by X-ray crystallography for ZnTPTBP in which the ,&carbons of the pyrrole rings deviate by as much as 1 8, from the mean plane of the macrocycle, which is saddleshaped.I5 Similar optical shifts have been reported for other crowded TPP derivatives substituted at the same /3 positions with aliphatic substituents.”*I2 These shifts are attributable to the destabilization of the porphyrin ?r systems and of the HOMOs, with the consequence that the HOMO-LUMO gaps decrease, resulting in red shifts of the first absorption bands.” The cumulative effects of the ring puckering and p substituents on the (16) Fuiita, 1.; Hanson. L. K.; Walker, F. A,; Faier. J. J . Am. Chem. Soc. 1983, 105,-3296. (17) Faier, J.: Fuiita. I.: Davis. M. S.; Forman. A,; Hanson. L. K.; Smith. K. M. Ad;. Chem. Ser. 1982, 201, 489. (18) Kopranenkov, V. N.; Dashkevich, S. N.; Luk’yanets, E. A. Zh. Obshch. Khim. 1981, 51, 2513. (19) Similar values were reported in 1O:l methanol/dimethyl sulfoxide:
Vogler, A.; Rethwish, B.; Kunkely, H.; Hiittermann, J.; Besenhard, J. 0. Angew. Chem., Int. Ed. Engl. 1978. 17, 951.
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8510 The Journal of Physical Chemistry, Vol. 94, No. 23, 1990
SI MUL A T I C N 8 d = 0.5SG 12 F = 0 . 3 7 s l15ih = 1.08G
4
5 G
Figure 2. (a) Second derivative EPR spectrum of ZnTPTBP'+CIO,- in toluene-d,. T = 298 K . (b) Simulation that assumes four nitrogens, aN = 0.78 G , eight o-phenyl protons, aH = 0.54 G , and 12 protons (eight meta and four para phenyl), aH = 0.37 G. Smaller coupling constants on the fused benzene rings are subsumed in the line width of 0.2 G .
Figure 4. (a) Second-derivative EPR spectrum of the radical of ZnTPTBP-I5N4in toluene-d, at 298 K. (b) Simulation that assumes four nitrogens, aisN = 1.08 G, eight protons, = 0.54 G, and 12 protons, aH = 0.37 G ; line width = 0.18 G. TABLE I: EPR Data for Zinc Porphyrin Cation Radicals: Hyperfine Coupling Constants (in gauss) atoms ZnTBP ZnTPTBPb ZnTPOEP Ns 95% of the parent compound. As shown in Figure 1, the spectrum of the radical exhibits a series of weak, near-infrared transitions that resemble those observedi6for ZnTPP'+ and that are clearly distinct from the spectrum reported for ZnTBP'+, which does not extend beyond 700 Although optical spectra of radicals with closed-shell metals seem to reflect the orbital occupancy of the HOMOs, i.e. 2A,, vs 2A2u,definite assignments are based on EPR studies that correlate unpaired spin densities at specific sites of the porphyrins with theoretical calculations that predict high spin densities at the nitrogens and meso positions for 2A2uversus high spin densities at the a carbons adjacent to the nitrogens and some spin density on the @ carbons for 2Alu.6Thus, ZnTPP" and ZnTBP'+ have been assigned to 2A2uand 2A,, radicals, respectively.* The EPR spectrum of ZnTPTBP'+C104-in toluene-d8 is shown in Figure 2. The spectrum can be assigned on the basis of 'H ENDOR, 'H NMR, and isotopic substitutions with 2H and ISN (vide infra) to four nitrogens with hyperfine coupling constants, al4~ = 0.78 G and two sets of 8 and 1 2 protons, with aH= 0.54 and 0.37 G, assigned to the 8 ortho protons and 12 meta and para protons on the phenyl rings, Le., a 2A2uprofile. A simulation based on these assignments yields reasonable agreement with the ex-
perimental spectrum (Figure 2). The assignments are based on the following data: 1. IH ENDOR spectra of ZnTFTBP'+ at 230 K reveal two proton resonances with aH = 0.37 and 0.56 G. The same spectrum is obtained if the fused benzene rings are deuterated ( ZnTPTBP-d16,Figure 3), indicating that the protons observed are located on the meso phenyl rings. The relative intensities of the peaks are also consistent with their assignments to 8 (ortho) and 12 (meta and para) phenyl protons. 2. 2H N M R results for ZnTPTBP-d16*+show only small chemical shifts when compared to the neutral compound, again indicating that the spin densities on the fused rings are negligible. (Data are not shown.) The coupling constants are estimated to be C0.02 G to be compared with ENDOR values of 0.55 and 0.32 G found for ZnTBP'+ at the same positions.* 3. The I4N coupling constants deduced by simulation of the ZnTPTBP'+ spectrum of Figure 2 were confirmed by isotopic ( I I S ~ / ( I M ~ = 1.40). The EPR substitution with ISN ( I = spectrum of ZnTPTBP-'SN,*+ is shown in Figure 4 along with a simulation that assumes 8 and 12 protons with aH = 0.54and 0.37 G, as above, and four nitrogens: aisN = 1.08 G. The latter value agrees within 1% with the value obtained from the simulation of the I4N data. It is instructive to compare the present EPR results with other Zn cation radical data to assess the effects of substitutions and ring puckering. As shown in Table I, ZnTBP'+ displays a spin profile typical of 2Ai, radicals. Introduction of the phenyl rings at the meso positions redistributes the spin density to that of 2A2u radicals. In addition, the conformational puckeringI5 that results from the crowding of the substituents also tends to shift spin density onto the phenyl rings. This trend probably arises from the fact that the phenyl rings roll more into the porphyrin plane to minimize steric interactions with the @ pyrrole substituents."*15 This steric effect may also contribute to the decrease in spin densities at the nitrogens observed in the series of ZnTPP, ZnTPOEP, and ZnTPTBP, in which only the ZnTPP macrocycle is essentially planar, and where the nitrogens lie progressively more
J. Phys. Chem. 1990, 94, 851 1-8522 out of the skeletal Note that these unpaired spin redistributions are not simply of theoretical interest. Increased spin densities at the nitrogens are transmitted to the metal and its axial ligand(s), favoring possible interactions with molecules located axially above the plane of the porphyrin,16 whereas spin density shifts onto the phenyl rings will enhance interactions with acceptors covalently linked via those substituents.'O The data presented here and the cumulative body of experimentalll-15 and theoreticall'*20results recently obtained for the effects of substituents and conformations on the properties of porphyrin derivatives lead to the following conclusions: I . The above results provide further evidence of spin delocalization onto peripheral substituents in porphyrin radicals. The effective distances between donor-acceptor pairs are therefore shorter than assumed from macrocycle edge-to-edge distances. (Obviously, the same arguments apply to electron transfer in
protein^.^^^*^^^') (20) Gudowska-Nowak, E.; Newton, M. D.; Fajer, J. J . Phys. Chem. 1990, 94, 5795.
8511
2. Peripheral substitutions are not innocent. Significant conformational changes may occur to minimize steric effects between substituents. 3. Conformational effects due to steric crowding can affect optical, excited state, redox and EPR proper tie^."^'^ Examples were provided here. 4. Conformational changes provide an attractive mechanism for modulating a wide range of physical and chemical properties of porphyrins. These can be induced by synthetic manipulations in vitro".'5 and may be enforced by different protein frameworks imposed by nearby residues, axial ligands, and hydrogen bonding in vivo.2o Acknowledgment. This work was supported by the Division of Chemical Sciences, U S . Department of Energy, under contract DE-AC02-76CH00016 at BNL, by a National Institutes of Health grant, GM 36520, at MSU (C.K.C.), and by the National Science Council of Taiwan (R.-J.C.). (21) Gingrich, D. J.; Nocek, J. M.; Natan, M. J.; Hoffman, B. M. J . Am. Chem. SOC.1987, 109, 7533.
FEATURE ARTICLE Low-Energy Helium Scattering from Ordered Physisorbed Layers of Polar Molecules P. A. Rowntree, G.Scoles,* Department of Chemistry, Princeton University, Princeton, New Jersey 08544- 1009
and J . C. Ruiz-Suirezt Department of Physics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 (Received: March 6, 1990; In Final Form: May 23, 1990)
Atomic beam diffraction is discussed as a useful method for the structural characterization of mono- and multilayers of gases physisorbed on single-crystal surfaces with some emphasis on experimental techniques. The method is applied to the study of physisorbed phases of polar molecules on the (0001) face of single crystal graphite. On bare graphite CH3F forms a commensurate d 3 X d3R30° structure with all the molecular dipoles parallel to each other while CH3CI and CH3Br form uniaxially incommensurate phases with two molecules per unit cell that have the dipoles antiparallel to each other. If Xe-covered graphite is used as a substrate, the unit cell of CH3Fdoubles with the two dipoles becoming also antiparallel, while the study of CH3CI and CH3Br is complicated by layer mixing phenomena. HCI on graphite forms an incommensurate closed packed structure, rotated by 30° with respect to the substrate lattice, with a lattice parameter of 3.80 A. On the same substrate ammonia shows instead ringlike diffraction patterns indicativeof a two-dimensional polycrystalline phase in which the crystalline domains have a large degree of azimuthal disorder. This observation and the value of the basic unit cell derived from it are in excellent agreement with the computer simulation results of Cheng and Steele, which shows that molecule-molecule and molecule-surface interactions are at present known, at least for a few systems, with a satisfactory degree of precision. The ensemble of these results shows the usefulness and versatility of atomic beam diffraction for the study of moleculesurface interactions and of the very large variety of structures and types of behavior of the mono- and multilayer phases of molecular gases adsorbed on single-crystal surfaces. In view of the current strong interest in the chemistry and photochemistryof molecules on surfaces, this structural knowledge should prove to be extremely useful for the interpretation of a very large class of physicochemical phenomena.
Introduction As part of the growing general interest in surface science, the study of physisorbed layers on well-characterized surfaces has expanded rapidly in recent years and has grown to overlap with
equally active areas of research such as the study of wetting phenomena, surface dynamics, interIlKkdar f o r m , and the study Of phase transitions in two dimensions* At the center Of that interest we find the adsorption of the rare gases onto
'Author to whom correspondence should be addressed. 'Present address: Centro de Ciencias de la Atmdsfera, Circuit0 Exterior, Ciudad Universitaria, C.P. 04510 Mexico.
2307.
(1) Ellis, T. H.; Iannotta, S.; Scoles,G.; Valbusa, U.Phys. Rev. 1981, B24, (2) Shaw, C. G.; Fain, Jr., S.C.; Chinn, M. D. Phys. Rev. Leu. 1978,41, 955.
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