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J. Phys. Chem. 1996, 100, 4480-4484
Benzophenone-Initiated Photoisomerization of the Norbornadiene Group in a Benzophenone-Steroid-Norbornadiene System via Long-Distance Intramolecular Triplet Energy Transfer Chen-Ho Tung,* Li-Ping Zhang, and Yi Li Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, China
Hong Cao and Yoshifumi Tanimoto Department of Chemistry, Faculty of Science, Hiroshima UniVersity, Kagamiyama, Higashi-Hiroshima, Japan 739 ReceiVed: September 25, 1995; In Final Form: December 15, 1995X
Bichromophoric compound 3β-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5en-17β-yl benzophenone-4-carboxylate (NBD-S-BP) was synthesized, and its photochemistry was examined. The phosphorescence of the benzophenone (BP, donor) chromophore is efficiently quenched by the remote norbornadiene (NBD, acceptor) group. Time-resolved spectroscopic measurements indicate that the lifetime of the triplet state of the BP chromophore is shortened by the NBD group. Selective excitation of the BP chromophore results in isomerization of the NBD group to quadricyclane. All these observations suggest that a long-distance intramolecular triplet energy transfer occurs in the NBD-S-BP molecule. The efficiency and rate constant for this triplet energy transfer were determined to be ca. 22% and 1.5 × 105 s-1, respectively, by steady-state photolysis and laser flash photolysis. This long-distance intramolecular triplet energy transfer is proposed to proceed via a through-bond exchange mechanism.
Introduction Triplet-triplet energy transfer has been extensively examined.1 The mechanism for triplet energy transfer is usually described by Dexter electron exchange interaction.2 The energy transfer rate in this case is proportional to the spectral overlap between donor emission and acceptor absorption and decreases exponentially with increasing donor-acceptor distance. Thus, one might expect that the rate constants of triplet energy transfer will become negligibly small as the donor-acceptor distance increases beyond 5-10 Å.1 However, long-distance triplet energy transfer with great efficiency in bichromophoric molecules in which the donor and acceptor are held at an effectively fixed separation by rigid molecular bridging frameworks has been reported.3-9 For the molecule 1-benzoyl-4-(2-naphthyl)bicyclo[2.2.2]octane in which the benzoyl (donor) and naphthyl (acceptor) spacing is fixed at ca. 7 Å, Zimmerman and McKlevery found that the triplet energy transfer occurred with 100% efficiency.3 Using a rigid steroid spacer, which places the benzophenone (donor) and naphthalene (acceptor) chromophores at a distance of ca. 14 Å, Keller and Dolby found that the triplet energy transfer efficiency is about 35%.4 For the molecule spiro[9,10-dihydro-9-oxoanthracene-10,2′-5′,6′benzindan] in which the anthrone (donor) and naphthalene (acceptor) moieties are rigidly linked and separated by ca. 5.1 Å, Weers and Rentzepis and their co-workers6 determined the rate constant for triplet energy transfer to be about 3 × 1010 s-1. Recently, Schuster and Zhu studied the remote activation of an aroyl azide by long-distance intramolecular electron transfer in an arylamine-steroid-azide system in which the two chromophores are separated by 17.8 Å and proposed that the triplet state of the donor chromophore participates in the remote activation by long-distance energy transfer.7 Morrison and Wu studied the photochemistry of 3R-((dimethylphenylsiX
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4480$12.00/0
loxy)-5R-androstane-11,17-dione and 3R-((dimethylphenylsiloxy)-5R-androstan-17-one in which the (dimethylphenyl)siloxy (donor) and 17-keto (acceptor) groups are separated by 11.6 Å and estimated the efficiency of the intramolecular triplet energy transfer having a minimum value of ca. 30%.8 The high efficiency of the long-distance intramolecular triplet energy transfer occurring in rigid bichromophoric molecules was rationalized by a through-bond exchange mechanism.3-11 Closs and his co-workers studied the triplet energy transfer between 4-benzophenonyl (donor) and 2-naphthyl (acceptor) connected with cyclohexane and decline rings and found that the rate constant of triplet energy transfer via a through-bond mechanism decreases exponentially with increasing the number of the bridging C-C σ bonds.9 Most studies of intramolecular triplet energy transfer in the literature concern photophysical processes. There are only a few examples of application of long-distance intramolecular triplet energy transfer to activate a remote donor group for chemical reaction.7,8 Furthermore, with few exceptions,6,9 the absolute rates of intramolecular triplet energy transfer have never been measured directly in liquid solution. In the present work we study the photochemistry of the bichromophoric compound, 3β-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17β-yl benzophenone-4-carboxylate (NBDS-BP) as shown in Chart 1. In this molecule, we could selectively excited the benzophenone chromophore (BP). After intersystem crossing with 100% efficiency, the triplet energy of BP is transferred to the norbornadiene group (NBD) via a through-bond mechanism, resulting in the isomerization of the latter to the quadricyclane (QC) group. The efficiency and absolute rate constant for the long-distance triplet energy transfer were examined by steady-state photolysis and time-resolved spectroscopy. The findings reveal that one can use an “antenna” chromophore to “harvest” photon energy which is then utilized to activate a reactive functional group separated in space from the antenna chromophore. © 1996 American Chemical Society
Benzophenone-Initiated Photoisomerization
J. Phys. Chem., Vol. 100, No. 11, 1996 4481
CHART 1
Figure 1. Absorption spectra of NBD-S-BP (s), A-S-BP (- - -), and MNBD (-‚-) in acetonitrile.
Results and Discussion Synthesis of the Norbornadiene-Steroid-Benzophenone System (NBD-S-BP). The synthesis of NBD-S-BP involved three steps. Reaction of 3β-hydroxy-androst-5-en-17-one with 2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-carbonyl chloride gives the NBD derivative androst-5-en-17-one (NBDS-One). After separation NBD-S-One was reduced by a literature procedure12 to yield a mixture of the distereomers 3βNBD-androst-5-en-17β-ol (NBD-S-17β-Ol) and 3β-NBD-androst-5-en-17R-ol (NBD-S-17R-Ol), with the former as the predominant product. Assignments of NBD-S-17β-Ol as the 17-C-βOH alcohol and NBD-S-17R-Ol as the 17-C-ROH alcohol were supported by 1H NMR resonance at 3.32 and 3.68 ppm, characteristic of 17-C-RH and 17-C-βH, respectively.13 Fortunately, isomeric alcohols NBD-S-17β-Ol and NBD-S-17ROl may be separated by chromatography. Since the amount of the purified NBD-S-17R-Ol was insufficient for a photochemical study, we only used NBD-S-17β-Ol as a starting material to prepare NBD-S-BP by its reaction with benzophenone-4carbonyl chloride. Absorption and Emission Spectra. To search for evidence of ground-state interactions between the donor BP and the acceptor NBD groups in NBD-S-BP, the absorption spectra of this compound and the models for the donor, 3β-acetoxyandrost5-en-17β-BP (A-S-BP), and for the acceptor, dimethyl bicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate (MNBD), in acetonitrile were examined and are shown in Figure 1. The spectrum of NBD-S-BP is essentially identical with the sum of the spectra of MNBD and A-S-BP, indicating the absence of measurable interaction between the NBD and BP chromophores of NBDS-BP in the ground state. Significantly, the absorption of the BP group extends to longer wavelength than does that of the NBD group, suggesting that singlet-singlet energy transfer from the excited BP chromophore to the NBD group is endothermic and, consequently, unlikely. Furthermore, this fact permits the selective excitation of the BP moiety in the bichromophoric compound NBD-S-BP. The emission spectra of NBD-S-BP and A-S-BP in glassy methyltetrahydrofuran at 77 K were studied and are shown in Figure 2. No fluorescence from these two compounds was
Figure 2. Phosphorescence spectra of NBD-S-BP (s) and A-S-BP (- - -) in 2-methyltetrahydrofuran at 77 K. λex ) 355 nm; [NBD-S-BP] ) [A-S-BP] ) 10-5 M.
observed, while a phosphorescence characteristic of the benzophenone chromophore with maxima at 435, 467, and 504 nm and a shoulder at 545 nm was detected for both NBD-S-BP and A-S-BP. The general features of these two phosphorescence spectra are essentially identical. However, the phosphorescence efficiency of the BP group in NBD-S-BP is ca. 20% less than in the model compound A-S-BP. This finding indicates that quenching of the BP phosphorescence by the NBD group in NBD-S-BP operates. Measurements at different concentrations reveal that the quenching is intramolecular. In order to clarify the reason for the long-distance intramolecular quenching of BP phosphorescence by the NBD group in NBD-S-BP, we measured the redox potentials of the model compounds MNBD and A-S-BP. The oxidation potential of MNBD, E(NBD/NBD•+), and the reduction potential of AS-BP, E(BP/BP•-), were determined in acetonitrile to be +1.45 and -1.73 V with respect to SCE, respectively. The free energy change involved in an electron transfer process can be calculated by the Rehm-Weller equation:14
4482 J. Phys. Chem., Vol. 100, No. 11, 1996
Tung et al.
∆G (kcal/mol) ) 23.06[E(D/D•+) - E(A/A•-) - e2/r] - E00 (kcal/mol) (1) E00 is the excited state energy and in this study represents the triplet state energy of the benzophenone-4-carboxylate group (69 kcal/mol). e2/r represents the Columbic energy associated with bringing separated radical ions at a distance r in a solvent of dielectric constant (r ) 10.8 Å in NBD-S-BP; see below). Calculation according to eq 1 shows that ∆G ) 4.15 kcal/mol, suggesting that electron transfer from NBD to the triplet state BP group would be very inefficient if any did occur. Furthermore, in the flash photolysis study, we could not detect any transient absorption attributable to a BP anion radical (see below). On the other hand, the triplet energy of NBD (53 kcal/ mol15) is much lower than that of the BP group (69 kcal/mol16). Thus, triplet-triplet energy transfer from the triplet excited BP chromophore to the NBD group is thermodynamically possible. Therefore, we infer that the quenching of the BP phosphorescence in NBD-S-BP is due to the intramolecular triplet-triplet energy transfer to the NBD group. Flash Photolysis. The evidence for remote intramolecular triplet-triplet energy transfer in NBD-S-BP based on phosphorescence efficiency is further strengthened by flash photolysis study. Pulsed-laser photolysis of NBD-S-BP in degassed acetonitrile by using 355 nm excitation light gives rise to a strong transient absorption spectrum with maximum at 560 nm as shown in Figure 3. This absorption is assigned to the lowest triplet state of the BP chromophore on the basis of the following observations. First, this absorption is essentially identical with that of the alkyl benzophenone-4-carboxylate triplet state independently generated.17 Secondly, in the presence of 1-naphthol, the 560 nm absorption is progressively replaced by that of the 1-naphthol triplet state with a maximum at 430 nm.17,18 Third, the 560 nm species is rapidly quenchable by O2. Analysis of the transient spectrum for NBD-S-BP at 560 nm as a function of time yields a lifetime of the triplet state (τ1) of ca. 1.64 µs. The transient absorption of the model compound A-S-BP is identical with that of NBD-S-BP, and the lifetime of its triplet state (τ2) is ca. 2.17 µs. The shorter lifetime of the triplet state for NBD-S-BP in comparison with that of A-S-BP is consistent with the result of the phosphorescence experiments and indicates that a long-distance intramolecular triplet-triplet energy transfer from BP to NBD chromophores in NBD-S-BP indeed occurs. The rate constant (kET) and efficiency (φET) for this energy transfer can be calculated from τ1 and τ2 according to eqs 2 and 3, respectively. kET and φET were obtained to be 1.5 × 105 s-1 and 0.24, respectively.
kET ) 1/τ1 - 1/τ2
(2)
φET ) 1 - τ1/τ2
(3)
Steady-State Photolysis. The photosensitized valence isomerization of NBD to QC has been the subject of intense experimental and theoretical investigation19 in view of its significance in solar energy storage 20 and mechanistic interests.21 Benzophenone and some other aromatic ketones are known to sensitize NBD f QC isomerization through triplettriplet energy transfer.21b,22 Thus, study of the intramolecular photosensitized isomerization of the NBD group in NBD-S-BP may provide evidence of remote triplet-triplet energy transfer. Irradiation with λ > 350 nm of a 2.5 × 10-5 M solution of NBD-S-BP in acetonitrile at room temperature leads to valence isomerization of the NBD group to QC (QC-S-BP) as shown in Chart 1. Under this condition only the BP chromophore
Figure 3. Transient absorption spectra of BP triplet state formed upon laser photolysis of NBD-S-BP (O) and A-S-BP (4) in acetonitrile 0.5 µs after the laser pulse.
SCHEME 1
absorbs the light. Thus, the isomerization of NBD to QC must be attributed to energy transfer. The yield of the isomerization product is 100% on the basis of the consumption of the starting material.23 The assignment of the product as the quadricyclane derivative relies mainly on its 1H NMR spectrum, which is in close agreement with that reported in the literature.15 Measurements of product formation at different concentrations demonstrate that the isomerization of the NBD group in NBD-S-BP is induced by intramolecular photosensitization. On the basis of the experimental results mentioned above, the primary photophysical and photochemical processes in NBD-S-BP can be expressed by Scheme 1. The quantum yield of this intramolecular photosensitization isomerization, φiso(NBDS-BP), can be calculated according to
φiso(NBD-S-BP) ) φisc‚φet‚φiso(NBD)
(4)
where φisc represents the quantum yield of the intersystem crossing from the singlet to the triplet excited state of the BP group and is assumed to be unity.1 φiso(NBD) represents the quantum yield of the isomerization reaction of the NBD triplet state; φiso(NBD-S-BP) was determined to be 0.038. To obtain φiso(NBD), we irradiated with λ > 350 nm a solution of benzophenone (10-2 M) in acetonitrile in the presence of MNBD (1 × 10-2 M). Under this condition we could exclusively excite benzophenone. The intersystem crossing efficiency for benzophenone is unity as mentioned above. Since the quencher (MNBD) concentration is high, we assume that all of the benzophenone triplet energy was transferred to MNBD. Thus, measurement of the yield of the isomerization product of MNBD allows determination of φiso(NBD). It is found that φiso(NBD) ) 0.185. This in turn gives φET as 0.21 according to eq 4. This value is very close to those obtained by phosphorescence (0.20) and flash photolysis experiments (0.24) mentioned above. Mechanism of BP to NBD Group Intramolecular TripletTriplet Energy Transfer. The flash photolysis and photosensitization reaction experiments reveal that excitation of the BP chromophore in NBD-S-BP results in a remote intramolecular triplet-triplet energy transfer to the NBD group and subsequently leads to the isomerization of the latter group to QC (Scheme 1). The efficiency of such triplet-triplet energy transfer is ca. 22%, and the rate constant is ca. 1.5 × 105 s-1.
Benzophenone-Initiated Photoisomerization It has been well established that triplet-triplet energy transfer proceeds via the Dexter exchange mechanism, and its rate decreases exponentially with increasing of the distance between the donor and acceptor.12 This energy transfer is normally expected to become very inefficient as the donor-acceptor distance increases beyond 5-10 Å. In this study the BP and NBD chromophores in NBD-S-BP are separated by 10 C-C σ bonds. We used the Alchemy II program to calculate the energies of the two general conformations, extended and bent, of NBD-S-BP and found that the extended conformation has the lowest energy. In this conformation the edge-to-edge distance between the BP and NBD groups defined as the distance between the 3β-O and 17β-O atoms is 10.8 Å, and the center-to-center distance (between the C-2 in norbornadiene and the carbonyl group in benzophenone) is 19.1 Å. At such separation between the chromophores triplet energy transfer via a through-space exchange process would be very inefficient. Thus, we believe that a through-bond exchange mechanism operates in the intramolecular triplet-triplet energy transfer in NBD-S-BP. The long-distance through-bond exchange triplettriplet3-9 and singlet-singlet10,24-26 energy transfer is now well precedented. Conclusions Flash photolysis and photochemical reaction studies demonstrate that triplet energy of the BP group in NBD-S-BP is transferred to the NBD group with 22% efficiency, leading to the valence isomerization of the NBD group to QC. This remote activation of the NBD group occurs by intramolecular triplettriplet energy transfer via a through-bond exchange process. The findings reveal that one can use an antenna chromophore to harvest photon energy which is then utilized to initiate chemical reaction of a functional group separated in space from the antenna chromophore. Experimental Section Material. Unless otherwise noted, materials were purchased from Beijing Chemical Work and were used without further purification. Spectral-grade cyclohexane, 2-methyltetrahydrofuran, and acetonitrile were used for absorption and emission spectra, flash photolysis, and steady-state photoirradiation measurements. NBD-S-One, NBD-S-17R-Ol, NBD-S-17β-Ol, NBD-S-BP, and A-S-BP were synthesized and identified by elemental analysis, IR, MS, and 1H NMR spectroscopies (Supporting Information). Instrumentation. 1H NMR spectra were recorded at 300 MHz with a Brock spectrometer. MS spectra were run on a VG ZAB spectrometer. UV spectra were measured with a Hitachi UV-340 spectrometer. IR spectra were run on a PerkinElmer 983 spectrometer. Steady-state phosphorescence spectra were recorded on either a Hitachi EM850 or a Hitachi MPF-4 spectrofluorimeter. HPLC was performed on a Varian VISTA 5500 liquid chromatograph with a Lichrosorb RP 18 column. Phosphorescence Measurements. Phosphorescence studies were performed in 2-methyltetrahyarofuran at 77 K, and the sample solutions were degassed by at least three freeze-pumpthaw cycles at a pressure of 5 × 10-5 Torr. The excitation wavelength was 355 nm. For comparison of the emission efficiency of NBD-S-BP with A-S-BP, the spectra were run using solutions with identical optical density at the excitation wavelength. The relative emission efficiencies were measured from the peak areas of the emission spectra. Redox Potentials of BP and NBD. The redox potentials of BP and NBD were determined by cyclic voltammetry in
J. Phys. Chem., Vol. 100, No. 11, 1996 4483 acetonitrile with respect to an SCE in the presence of 0.1 M tetraethylammonium perchlorate as the supporting electrolyte. Laser Flash Photolysis. The laser flash photolysis system has been described elsewhere.27 The pump light source was the third harmonic (355 nm) of an Nd:YAG laser (SpectraPhysics, GCR-11-1). The probe light source was a xenon arc lamp (Vshio, UXL-500-0). The probe light transmitting through the sample cell was fed to a detection system which consists of a monochromater (Ritsu, MC-10L), photomultiplier (Hamamatsu, R928), digital oscilloscope (Tektronix, 2440), and microcomputer. The decay curve, were analyzed by using nonlinear least-squares fitting. Photoirradiation and Product Analysis. Photoirradiation was carried out in a Pyrex reactor, and the samples were purged with nitrogen. A 450-W Hanovia high-pressure mercury lamp was used as the excitation source. A UVD-36B glass filter was used to cut off the light with λ < 350 nm. After irradiation the solvent was evaporated from the samples under reduced pressure. The product was separated from the starting material by preparative thin-layer chromatography and characterized by 1H NMR and mass spectroscopies. Product yields were determined by analysis of the 1H NMR spectra and by HPLC analysis. Quantum yields for intramolecular photosensitization isomerization of the NBD group in NBD-S-BP (φiso(NBDS-BP)) and the efficiency of isomerization of the NBD triplet state (φiso(NBD)) were determined by using a benzophenone/ benzohydrol system for actinometry (φ ) 0.74 in benzene).28 Acknowledgment. We thank the National Science Foundation of China for financial support. Supporting Information Available: Synthesis procedures and data of melting points, elemental analyses, mass spectroscopies, infrared spectroscopies, and 1H NMR of 3β-((2(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-dien-3-yl)carboxy)androst-5-en-17-one (NBD-S-One), 3β-((2-methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17βol (NBD-S-17β-Ol), 3β-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-dien-3-yl)carboxy)-androst-5-en-17R-ol (NBD-S-17ROl), 3β-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-dien-3yl)carboxy)androst-5-en-17β-yl benzophenone-4-carboxylate (NBD-S-BP), 3β-((5-(methoxycarbonyl)teracyclo[3.2.0.0.2,7.04,6]heptyl)carboxy)androst-5-en-17β-yl benzophenone-4-carboxylate (QC-S-BP), and 3β-acetoxyandrost-5-en-17β-yl benzophenone-4-carboxylate (A-S-BP) (3 pages). Ordering information is given on any current masthead page. References and Notes (1) For a review of energy transfer, see: Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cumming: Menlo Park, CA, 1978; Chapter 9. (2) (a) Dexter, D, L. J. Chem. Phys. 1953, 21, 836. (b) Katz, J. L.; Jortner, J.; Chol, S. I.; Rice, S. A. J. Chem. Phys. 1963, 39, 1897. (3) Zimmerman, H, E.; Mcklevery, R. D. J. Am. Chem. Soc. 1971, 93, 3638. (4) (a) Breen, D. E.; Keller, R. A. J. Am. Chem. Soc. 1968, 90, 1935. (b) Keller, R, A. J. Am. Chem. Soc. 1968, 90, 1940. (c) Keller, R. A.; Dollby, L. J. J. Am. Chem. Soc. 1969, 91, 1293. (5) Tung, Z.-H.; Yang, G.-Q.; Wu, S.-K. Acta Chim. Sin. (Engl. Ed.) 1989, 450. (6) Mak, A. H.; Weers, J. G.; Hilinsky, E. F.; Milton, S. V.; Rentzepis, P. M. J. Chem. Phys. 1984, 80, 2288. (7) Zhu, Y.; Schuster, G. B. J. Am. Chem. Soc. 1993, 115. 2190. (8) Wu, Z.-Z.; Morrison, H. J. Am. Chem. Soc. 1992, 114, 4119. (9) Closs, G. L.; Piotrowiak, P.; Maclnnis, J. M.; Fleming, G. R. J. Am. Chem. Soc. 1988, 110, 2652. (10) Oevering, H.; Verhoeven, J. W.; Paddon-Romw, M. N.; Cotsaris, E. Chem. Phys. Lett. 1988, 143, 488. (b) Kroon, J.; Oliver, A. M.; PaddonRow, M. N.; Verhoeven, J. W. J. Am. Chem. Soc. 1990, 112, 4868.
4484 J. Phys. Chem., Vol. 100, No. 11, 1996 (11) Cvitas, Y.; Kovae, B.; Pasa-Tolic, Lj.; Ruscic, B.; Klasinic, L.; Knop, J. V.; Bhacca, N. S.; MeGlynn, S. P. Pure Appl. Chem. 1989, 61, 2139. (12) (a) Chinn, L. J. J. Org. Chem. 1965, 30, 4165. (b) Norymberski, J. K.; Woods, G. F. J. Chem. Soc. 1955, 3426. (13) Bridgeman, J. E.; Cherry, P. C.; Clegg, A. S.; Evans, J. M.; Jones, E. R. H.; Kasal, A.; Kumar, V.; Meakins, G. D.; Morisawa, Y.; Richards, E. E.; Woodgate, P. D. J. Chem. Soc. 1970, 250. (14) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259. (15) Wu, Q.-H.; Zhang, B.-W.; Ming, Y.-F.; Cao, Y. J. Photochem. Photobiol., A 1991, 61, 53. (16) The triplet energy of benzophenone-4-carboxylate was estimated from the 0-0 band of the phosphorescence spectrum of NBD-S-BP in acetonitrile. This value is in good agreement with the literature. (17) Cao, H.; Akimoto, Y.; Fujiwara, Y.; Tanimoto, Y.; Zhang, L.-P.; Tung, C.-H. Submitted for publication in Bull. Chem. Soc. Jpn. (18) Shizuka, H.; Hagiwara, H.; Fukushima, M. J. Am. Chem. Soc. 1985, 107, 7816. (19) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401 and references therein. (20) (a)Yoshida, Z. I. J. Photochem. 1985, 29, 27. (b) Harel, Y.; Adamson, A. W.; Kutal, C.; Gutsch, P. A.; Yasufuku, K. J. Phys. Chem. 1987, 91, 901. (c) Basu, A.; Saple, A. R.; Sapre, N. Y. J. Chem. Soc., Dalton Trans. 1987, 1797. (d) Orchard, S. W.; Kutal, C. Inorg. Chim. Acta 1982, 64, 195. (21) (a) Schwarz, W.; Dangel, K. M.; Jones, G., II; Bargan, J. J. Am. Chem. Soc. 1982, 104, 5686. (b) Arai, T.; Oguchi, T.; Wakabayashi, T.;
Tung et al. Tsuchiya, M.; Nishimura, Y.; Oishi, S.; Sakuragi, H.; Tokumarce, K. Bull. Chem. Soc. Jpn. 1987, 60, 2937. (c)Takumemans, A. H. A.; den Ouden, B.; Bs, H. J. T.; Mackor A. Recl. TraV. Chim. Pays-Bas 1985, 104, 109. (22) (a) Murov, S.; Hammond, G. S. J. Phys. Chem. 1968, 72, 3797. (b) Hammond, G. S.; Wyatt, P.; Deboer, C. D.; Turro, N. J. J. Am. Chem. Soc. 1964, 86, 2532. (23) Breslow reported that the triplet state of benzophenone chromophore linked to a cholestanol skeleton can abstract hydrogens from the latter. In this study we could not detect any hydrogen-abstraction product; see: Breslow, R. Acc. Chem. Res. 1980, 13, 170 and references therein. (24) Zimmerman, H. E.; Goldman, T. D.; Hirzel, T. K.; Schmidt, S. P. J. Org. Chem. 1980, 45, 3933. (25) Wu, Z.-Z.; Nash, J.; Morrison, H. J. Am. Chem. Soc. 1992, 114, 6640. (26) For contrasting views on long-range through-band exchange energy transfer, see: (a) Ser, S.; Rubin, M, B. Chem. Phys. Lett. 1988, 150, 177. (b) Overing, H.; Verhoeven, J, W.; Paddon-Row, M. N.; Cotsaris, E.; Hush, N. S. Chem. Phys. Lett. 1988, 150, 179. (27) Tanimoto, Y.; Takashima, M.; Itoh, M. Bull. Chem. Soc. Jpn. 1989, 62, 3923. (28) Hammond, G. S.; Moore, W. M.; Eaker, W. P. J. Am. Chem. Soc. 1961, 83, 2795.
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