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and E parameters smaller than earlier suggested by high-temperature measurements. The reactions of addition to acetylene show significant Arrhenius plot curvatures and activation energies lower than activation energies exhibited by alkyl radicals in similar reactions. These results are in accord with current representations3 of the transition states of this type of reaction. The preferred rate parameters for the modified Arrhenius expressions are given in Table VI.
Acknowledgment. This work has been partially supported by the McDonnell Douglas Independent Research and Development program. S.W.B. is indebted to the US.Army Research Office and to the NSF Grants DAAG29-85-K-0019 and CHE84-03761, which supported part of this work. Registry No. H2, 1333-74-0; C4H,, 86181-68-2; C,H,, 2669-89-8; C2H2, 74-86-2.
Photophysics and Photochemistry of Diphenyl Sulfone. 1. The Triplet Mechanism of Photodlssociation Glauco Ponterini and Fabio Momicchioli* Dipartimento di Chimica. Universitri di Modena, via Campi 183, 41 100 Modena, Italy (Received: June 15, 1987; In Final Form: January 26, 1988)
A laser flash photolysis study of diphenyl sulfone (DPS) was carried out. The measured time-resolved absorption spectra suggest that laser excitation results in the formation of two transients, a fast and a slower one, which were identified as the lowest triplet state TI of DPS and the benzenesulfonyl radical, respectively. Measurement of the decay rate of the T, absorption at variable temperature showed that this state decays via a spin-allowed process, with an activation energy of about 8.6 kcal mol-'. These data were interpreted in terms of a model potential-energy diagram characterized by an avoided curve crossing between the lowest 37rr*state and a 3uu* repulsive state along the stretching coordinate of a C-S bond. The possibility of extending this model to the photodissociation of substituted diphenyl sulfones was checked by carrying out a parallel investigation on p,p cdiaminodiphenyl sulfone (ADPs).
Introduction The photochemistry of diphenyl sulfone (DPS) and closely related compounds has previously been investigated by using steady-state photolysis'-4 or conventional flash photolysiss techniques. From the results of steady-state photolysis experiments, the cleavage of a C-S bond, leading to the phenyl-benzenesulfonyl radical pair, was identified as the primary step in the photodecomposition of DPS. Thermal reactions following C-S homolysis were found to depend on the solvent. As an example, in benzene the phenyl radical was shown to give biphenyl by intermolecular reaction with the ~ o l v e n t ,while ' ~ ~ ~in~ acetonitrile the photolysis of para,para-disubstituted DPS's yielded para,para-disubstituted biphenyls by recombination of the para-substituted phenyl radicals: Also, the formation of benzenesulfinic acid in the photolysis of DPS in benzene has been interpreted as the result of the benzenesulfonyl radical ev~lution.~ Conventional flash photolysis of various aromatic sulfones, DPS included, in CC14sshowed a common transient absorption with a maximum near 330 nm, which was attributed to the arenesulfonyl radicals. The observed second-order decay kinetics of the arenesulfonyl radicalsSwas found to be consistent with the disproportionation mechanism proposed for the decay of tosyl radicals in CC14.2 To sum up, analysis of the photoreaction products and detection of long-lived transients have provided indirect, yet clear, evidence that the photodecomposition of DPS and related compounds proceeds via homolytic cleavage of one of the two formally single bonds connecting the sulfur atom to an aromatic ring. On the (1) Kharasch, N.; Khodair, A. I . A. Chem. Commun. 1967, 98. ( 2 ) da Silva CorrBa, C. M. M.; Waters, W. A. J . Chem. Soc. C 1968, 1874. (3) Nakai, M.; Furukawa, N.;Oae, S.; Nakabayashi, T. Bull. Chem. SOC. Jpn. 1972, 45, 11 17. (4) Abdul-Rasoul, F.; Catherall, C. L. R.; Hargreaves, J. S.; Mellor, J. M.; Phillips, D. Eur. Polym. J . 1977, 13, 1019. (5) Thoi, H. H.; Ito, 0.;Iino, M.; Matsuda, M. J . Phys. Chem. 1978, 82, 314.
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other hand, no direct observation of this photochemical primary process has been reported so far.6 As a first step in a detailed investigation of the photophysics and photochemistry of DPS, we report here direct evidence for the involvement of the lowest triplet state (T,) in the photodecomposition of this compound. By laser flash photolysis experiments we show that the decay of the T, state of DPS results in the formation of benzenesulfonyl radical via cleavage of a C-S bond. The kinetic parameters of the T, decay in two different solvents are interpreted with reference to a model potential-energy diagram similar to the one first suggested by Portera (but see also ref 9) to describe the photodissociation of toluene yielding a benzyl radical and an H atom. The possibility of extending the proposed model to the triplet dissociation of substituted diphenyl sulfones is finally demonstrated by a parallel investigation on p,p'-diaminodiphenyl sulfone (ADPs) in benzene and glycerol, based on flash photolysis data and an analysis of the phosphorescence decay kinetics.
Results Diphenyl Sulfone. Two transient absorptions were observed in laser flash photolysis experiments on ethanol and cyclohexane solutions of DPS. Their difference spectra, measured in cyclohexane at 100- and 650-11s delay times, are shown in Figure 1. Very similar results were obtained in ethanol. The absorption characterized by a maximum at 440-450 nm grows together with the fluorescence decay (the observed rise time, 20-25 ns, reproduced the instrumental time resolution) and decays exponentially ( 6 ) A pulsed-laser photolysis study of the photodecomposition kinetics of aromatic sulfones has been published.' That study, however, dealt with arylalkyl sulfones, compounds significantly different from those considered in this work. (7) Gould, I. R.; Tung, C.; Turro, N. J.; Givens, R. S.; Matuszewski, B. J . Am. Chem. SOC.1984, 106, 1789. (8) Porter, G. Chem. Soc., Spec. Publ. 1958, 9, 143. (9) Michl, J. Top. Curr. Chem. 1974, 46, 1.
0 1988 American Chemical Society
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b
J(nm) Figure 2. Phosphorescence spectra of DPS (---) and ADPS in 1:l ethanol-methanol at 77 K. The 0-0 transition energy of D P S was assumed to correspond to the shortest wavelength peak of the spectrum. For ADPS, the 0-0 transition was assumed to fall between 390 and 407 nm. (-e-)
described in ref 2 for the tosyl radical in CCl, solution. A similar explanation can be advanced for the second-order component of the C6H5S0, radical decay in ethanol. In fact, benzenesulfonyl radical disproportionation is operative in alcohols too, even if products are different from those obtained in CC14.]* Our observation of a first-order additional component in the decay of the 320-nm absorption in ethanol (T N 13 ps) may reflect the occurrence of processes like loss of SO2 and/or hydrogen abstraction from the solvent. Given the precursor-successor relationship between the DPS lowest triplet and the benzenesulfonyl radical, it was interesting to characterize the kinetics of the triplet decay, in order to elucidate the triplet pathway of the photodissociation of DPS. As previously mentioned, the room-temperature triplet lifetime was found to depend on the solvent purity. The Arrhenius plots of the rate constant for the triplet decay of DPS in glycerol and anhydrous and 95% ethanol all reproduced very closely the behaviors of the viscosities of these solvents with temperature: the plots were linear between 230 and 300 K and their slopes compared very closely with the solvent activation energies for viscous flow as deduced, in the same temperature interval, from the viscosity data of ref 13a. Thus, in those solvents, in spite of our attempts at purification, the triplet decay was always diffusion controlled, as is expected for efficient impurity-induced quenching. The difficulty of removing such a nonreactive relaxation channel was probably due to the high value of the T I energy of DPS. This was estimated to be about 79.2 kcal mol-' from the 0-0 transition energy of the phosphorescence spectrum measured at 77 K in an ethanolmethanol 1:l mixture (Figure 2). As a direct consequence, virtually any impurity present in the solvent may act as a quencher. Things were quite different, though not yet fully satisfactory, in methanol and butyl chloride. The Arrhenius plots of the triplet decay rate constants measured in these solvents between 200 and 31 3 K are reported in Figure 3. In both solvents, the experimental points were best fitted by expressing the rate constants as the sums of two Arrhenius contributions. The first contributions are characterized by preexponential factors of 3 X lo7 and 9.6 X lo6 s-I and by activation energies of 2.1 and 1.46 kcal mol-' in methanol and butyl chloride, respectively. These contributions prevail at low temperatures and clearly reflect the occurrence of ~~~
(10) Chatilialoglu, C.; Griller, D.; Guerra, M. J . Phys. Chem. 1987, 91, 3747. (11) Cercek, B.; Kongshaug, M. J . Phys. Chem. 1970, 74, 4319.
~
(12) (a) Badr, M. Z. A.; Aly, M. M.; Fahmy, A. M. J . Org. Chem. 1981, 46, 4784. (b) Bruni, M. C.; Ponterini, G.; Scoponi, M., part 2 of this work, submitted for publication in J . Phys. Chem. (1 3) (a) Handbook of Chemistry and Physics, 56th ed.;Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1975-1976. (b) Handbook of Chemistry and Physics, 44th ed.; Hodgman, C. D., Ed.; CRC Press: Cleveland, OH, 1963.
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21
01
12-
'I
1
1l o
.2
a2
'
35
4.0
4.5
3.5
4.0
4.5
lo3 KIT Figure 5. Arrhenius plots of the activated contributions to the phosphorescence decay rate constants of ADPS in glycerol (left) and 20-80% water-glycerol mixture (right). T~ was 0.8 s in both solvents. decay. This longer lived absorption is strongest in the 310-370-nm region and, by analogy with DPS, we take these observations as indicative of the formation of a p-anilinesulfonyl radical. In fact, the absorption spectrum of the latter is expected to be not much different from that of the benzenesulfonyl radical, owing to the nature of its lowest energy electronic transition, which involves the unpaired electron and is "localized" on the SO, m ~ i e t y Also, .~ steady-state photolysis experiments carried out in deaerated ethanol showed that benzidine is among the products, thus suggesting that, in this molecule too, the homolytic cleavage of a C-S bond, with formation of the anilino-anilinesulfonyl radical pair, is the primary photochemical event.14 While the spectral evolution in the 3 10-370-nm region did not enable us to establish with certainty the existence of a precursorsuccessor relationship between the TI state of ADPs and the anilinesulfonyl radical, an analysis of the decay kinetics of the phosphorescence of ADPS in glycerol gave us valuable information supporting this hypothesis. The Arrhenius plot of the phosphorescence decay constants in the 220-300 K temperature range is shown on the left side of Figure 5. A least-squares fit yields an activation energy of 13.2 kcal mol-' and a preexponential factor of 3 X lOI3s-'. In order to check that these values did not reflect the occurrence of a diffusion-controlled process, we measured the same phosphorescence decay constants in a water-glycerol mixture, 20-80 wt %, whose viscous-flow activation energy is about 5 kcal mol-' lower than that of anhydrous glycer01.I~~ The results are shown on the right side of Figure 5 and yield an activation energy of 13 kcal mol-' and a preexponential factor of 7.8 X lOI3s-'. No relevant lowering of the activation energy is thus observed, a fact which rules out the possibility that the considered process be a diffusion-controlled one. By analogy with DPS, we assume that these values of activation energy and preexponential factor refer to an intramolecular spin-allowed process, though they may also be affected by solvent properties. Discussion In this section we provide an interpretation of the kinetic parameters obtained for the triplet decays of DPS and ADPS, in terms of the model potential-energy diagram shown in Figure 6. This model derives from the one originally constructed by Porter for the photolysis of toluene (C,H5CH3 C,H5CH, + H)* and subsequently improved by Mich19 with the explicit introduction of curve crossing avoidance. Briefly, Figure 6 shows the potential energy of the ground state and two relevant excited triplet states as a function of the assumed dissociation coordinate, i.e., the stretching of one C-S bond. The notation of the excited states refers to a hypothetical MO basis set formed by C T ( U * ) orbitals localized on the bonds and T ( T * ) orbitals of the two phenyl * the lowest excited triplet state chromophores. Thus, 3 ~ rindicates
-
(14) Ponterini, G., unpublished results.
Photophysics and Photochemistry of Diphenyl Sulfone
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4087 are both in the range of values expected for a spin-allowed process, as is the case of the crossing over the energy barrier in the TI state. The increase of the activation energy (from -9 to 13 kcal mol-’) on going from DPS in methanol or butyl chloride to ADPS in glycerol is also in agreement with the proposed model. In fact, as a result of NH2 substitution at the two para positions the 3aa* state (i.e., the 3L, state of the phenyl groups) will be sensibly stabilized, while the 3uu* state is expected to be nearly unaffected in view of its localized nature. In support of that, from the position of the 0-0 band in the phosphorescence spectrum of ADPs in 1:l ethanol-methanol (Figure 2) the energy of the lowest triplet turned out to be 70-73 kcal mol-’, 6-9 kcal mol-’ lower than the triplet energy of DPS in ethanol-methanol. Therefore, a substantial lowering of the %7r* curve relative to the 3uu* one is expected to occur on going from DPS to ADPS. As is evident from Figure 6, this relative downshift of the diabatic %A* curve will cause an increase in the energy barrier in the adiabatic TI surface, provided that the coupling between 3uu* and 3aa*states at the crossing point does not become so large as to offset the effect of the aforementioned phenomenon. Thus, all the observations concerning the triplet decay kinetics of both DPS and ADPS are qualitatively well accounted for within the model potential-energy diagram of Figure 6. Extension of the proposed model to the triplet mechanism of the photodissociation at one C-S bond in other DPS derivatives may easily be envisaged and, in line with the arguments advanced in the case of ADPS, the activation energies should be expected to depend on the substituent effect, Le., on the extent of the stabilization of the lowest 3a7r*state induced by the substituents. Such a prediction is quite interesting since it suggests that the photostability of DPS’s may be controlled by introducing appropriate substituents in the phenyl rings. To summarize, the above discussion shows that the reactive decay kinetics of the TI state of DPS and ADPS can both be rationalized in terms of a thermally induced passage from the lowest 3 a ~state * to the repulsive 3uu* state of a C-S bond, through an avoided crossing region. The starting point of the triplet reaction, Le., the lowest )aa* state, was implicitly assumed to be reached from the directly excited SI (a**)state via SI T, intersystem crossing and subsequent fast T, T, internal conversion. Although the results presented here clearly demonstrate the consistency of this scheme, it would be interesting to investigate the possibility that cleavage of a C-S bond may proceed directly from the S1 state. Indirect signs for a parallel singlet pathway to photodissociation emerged from the observation that some benzenesulfonyl radical absorption was already present during the laser flash. A direct experimental study, involving complete analysis of radiative, nonradiative and reactive decay of the Si state of DPS, has been carried out in our laboratory. The most relevant conclusion of this study is that the main S1 decay channel is an activated S1 T, intersystem crossing ( E , N 2 kcal mol-’). It is also suggested that population of a C-S dissociative triplet state and, eventually, homolytic C-S bond cleavage follow this intersystem crossing, thus explaining the observed singlet contribution to the DPS steady-state photolysis. A detailed presentation of the results of this experimental investigation and their rationalization in terms of a simple extension of the potential-energy diagram of Figure 6 will be the subject of part 2 of this work.
-
Figure 6. Model potential-energy curves of the ground state and two relevant excited triplet states of DPS along the stretching of one C-S bond (see text for a detailed description). Energies are given in kcalories per mole.
of DPS involving the a electrons of the phenyl rings and 3uu* indicates a higher energy triplet involving the bonding and antibonding orbitals of a C-S bond.15 During dissociation, the u and u* orbitals become the nonbonding orbitals of the benzenesulfonyl and phenyl radicals, while the a and a* orbitals of the phenyl groups remain almost unchanged. As a consequence, in a zeroth-order description u(C-S)-*(phenyls) molecular orbital intersections will take place at some point along the reaction coordinate and this will cause in turn a crossing between the sloping down 3uu* repulsive curve and the steeply rising curve of the lowest 3mr* state (dashed lines). At a higher level of description, where u - ~interactions are allowed for, the 3uu* and %T* zeroth-order states mix sensibly at and around the intersection point, and so the curve crossing becomes avoided (solid lines), Connected with the avoided crossing, an energy barrier will arise in the T, state adiabatic curve, the height of which will depend on the relative position of the 3uu* and %a* curves as well as on the extent of the interaction between the diabatic states. The electronic nature of the T, state changes drastically at the barrier top: it is m*on the left, in the equilibrium geometry region, but it becomes uu* on the right, in the stretched C-S bond region. Thus, the passage across the barrier from left to right appears to be characterized by “migration” of the triplet excitation from the phenyl a systems, where it is initially acquired, to the stretched C-S u bond.I8 Once the barrier is overcome, the molecule will undergo fast homolytic dissociation following the repulsive 3uu* potential curve. In conclusion, Figure 6 suggests that (a) the triplet pathway of the DPS photodissociation proceeds via activated fission of a C-S bond on the adiabatic TI surface and (b) the involved energy barrier corresponds to an avoided * a C-S repulsive 3uu* crossing between the lowest phenyl 3 ~ aand diabatic states. The preexponential factors of the measured triplet decay rate constants, about 4 X 10l2s-’ for DPS and 3 X l O I 3 s-’ for ADPS, (15) A CS-INDO CII6 calculation of the electronic spectrum of DPS carried out recently in our laboratory1’ substantiates this scheme. In fact, a
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triplet state clearly related to the 3uu* state envisaged in our model (Figure 6 ) was predicted to lie 15 kcal mol-I above the lowest triplet state having mr* (La) nature. (16) Momicchioli, F.; Baraldi, I.; Bruni, M. C. Chem. Phys. 1983.82, 229. (17) Dainese, D. Graduation Thesis, Department of Chemistry, University of Modena, Italy, 1986. (18) According to the classification proposed by SalemI9 this avoided crossing belongs to type E (migration of the excitation from one region to the other of the molecule). However, from a different point of view (molecular orbital intersection) it might be considered as a type C avoided crossing. (19) Salem, L. Electrons in Chemical Reacrions: First Principles; Wiley-Interscience: New York, 1982; Chapter 5.
-- --
--
Experimental Section All the laser flash photolysis measurements were performed with a “single shot” approach on equipment already described.20 DPS was excited at 249 nm with a Kr-HF laser, while 308-nm excitation with a Xe-HCl laser was used with ADPS. Real-time acquisition and processing of the signals with a boxcar averagerZ0 could not be performed because of the rapid sample photodegradation and the unavailability of a suitable flow system for deaerated solutions. Small amounts of solution (- 10 mL) were used, frequently checked and replaced when photodegradation (20) Longoni, A,; Ponterini, G. Appl. Spectrosc. 1986, 40, 599.
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reached 4-5%. In the Arrhenius experiments, temperature control was achieved by means of a Thor C-610 cryostat adapted for liquid samples. The phosphorescence decay rates were measured by exciting the low-temperature samples with the excimer laser and by monitoring the emitted phosphorescence by means of a photomultiplier connected with a storage oscilloscope. Each point of the reported difference spectra represents the average of three to five measurements. The triplet decay rates used for the Arrhenius plots were typically the average of three to four determinations. The phosphorescence spectra of DPS and A D P s in 1:1 ethanol-methanol were measured at liquid nitrogen temperature with a Jobin-Yvon JY3CS spectrofluorimeter and were corrected for the instrumental response curve. DPS was synthesized according to the procedure given in ref 21 and recrystallized twice from ethanol. Glycerol (Carlo Erba, RPE) was extracted three to four times with cyclohexane and methylene chloride and once with acetonitrile; it was subsequently (21) Kniisli, E. Gazz. Chim. Iral. 1949, 621; 1950, 522.
"dried" by keeping it under vacuum for 24 h. All the other solvents were Merck UVASOL. Hydrocarbons were passed through activated silica columns. Alcohols were distilled from CaH,; the central fractions were collected and kept under nitrogen atmosphere. Samples were degassed by three to four freeze-pumpthaw cycles for room-temperature experiments and by thorough nitrogen bubbling for T-dependence kinetic measurements. The two methods were checked to give the same room-temperature DPS triplet lifetimes in the same solvents. Glycerol solutions were degassed by gently heating them under vacuum for a few hours before use. Acknowledgment. Dr. M. Scoponi and Mr. M. Bandiera are thanked for adapting and calibrating the temperature control system. Dr. D. Iarossi is warmly thanked for synthesizing and supplying DPS. Helpful discussions with Dr. M. C. Bruni are gratefully acknowledged. This work was financially supported by the Minister0 della Pubblica Istruzione (Roma). Registry No. DPS, 127-63-9;ADPS, 80-08-0; C,H,SO,, 3401 4-44-3; p-anilinesulfonylradical, 85 121-74-0.
Triplet Excited-State Chemistry of Dlplatinum(I I ) Complexes. Comparative Spectroscopy and Quenching Rate Constants between the Tetrakk(p-pyrophosphito)diplatinate(I I ) and the Tetrakis[p-methylenebis(phosphonfto)]diplatlnate( I I ) Tetraanlons D. Max Roundhill,*JPZhong-Ping Shen,'. Christopher King,lPand Stephen J. Athertonlb Department of Chemistry, Tulane University, New Orleans, Louisiana 701 18, and Center for Fast Kinetics Research, ENS Annex 16N, The University of Texas at Austin, Austin, Texas 78712 (Received: August 17, 1987: I n Final Form: December 11, 1987)
By use of transient difference spectroscopy, the complex Pt2(pcp)44-(pcp = HO(O)PCH,P(O)OH') has been shown to undergo one-electron oxidation and reduction to give Pt2(p~p)43(A,, = 320 nm) and Pt2(p~p)45(A,, = 430 nm), respectively. Comparative quenching rate constants for the reactions of the 3A2ustates of Pt2(pop)4e* (pop = HO(0)POP(O)OH2-) and Pt2(p~p)44-* with alkyl and aryl halides, hydrogen atom donors, electron-transfer reagents, and alkenes and alkynes, show that Ptz(pcp)2-* is the more reactive of the pair of complexes. The triplet energies of Pt2(pop)?-* and Pt2(p~p)44-* are estimated to be 58.1 and 59.7 kcal mol-', respectively. By transient difference spectroscopy, tram-stilbene, diphenylacetylene, and tetraphenylethylene have been shown to react with Pt2(pop)2-* by energy transfer.
Introduction From a series of published earlier studies, it is clear that the triplet excited state of the tetrakis(p-pyrophosphit0)diplatinate tetraanion, Pt2(pop)4e (pop = HO(0)POP(O)OH2-), reacts with added quenchers by mechanisms that involve the platinum complex as an oxidant, reductant, or atom-transfer reagent2 A structurally analogous complex is t h e tetrakis[p-methylenebis(phosphonito)]diplatinate(II) tetraanion, Pt2(p~p)44-(pcp = HO(0)PCH2P(O)OH2-), which differs from Pt2(pop)," in having a methylenic group rather than an oxygen atom in the ligand bridge.3 The electronic spectroscopy, both absorption and emission, of the two complexes are closely similar, the only major (1) (a) Tulane University. (b) Center for Fast Kinetics Research. (2) Che, C.-M.;Butler, L. G.; Gray, H . B. J . Am. Chem. Soc. 1981, 103, 7796-7797. Nocera, D. G.; Maverick, A. W.; Winkler, J. R.; Che, C.-M.; Gray, H. B.ACS Symp. Ser. 1983, 211, 21-33. Heuer, W. B.; Totten, M. D.; Rodman, G. S.;Hebert, E. J.; Tracy, H. J.; Nagle, J. K. J . Am. Chem. SOC.1984,106,1163-1164. Roundhill, D. M. J . Am. Chem. SOC.1985,107, 4354-4356. Roundhill, D. M.; Atherton, S . J. Inorg. Chem. 1986, 25, 4071-4072. Roundhill, D. M.; Atherton, S. J.; Shen, Z.-P. J. Am. Chem. SOC. 1987,109,6076-6079. Vleck, Jr., A.; Gray, H. B. J . Am. Chem. Soc. 1987, 109, 286-287. Roundhill, D. M.; Dickson, M. K.; Atherton, S. J. J . Organomet. Chem. 1987, 335, 413-422. (3) King, C.; Auerbach, R. A.; Fronczek, F. R.; Roundhill, D. M. J Am. Chem. SOC.1986, 108, 5626-5627.
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difference being that the triplet lifetime of Pt,(pcp),"* in aqueous solution at ambient temperature under anaerobic conditions is 55 ns, whereas that of Pt2(pop),4-* is 9.5 p s . No photochemical reactions of Ptz(pcp)2- have yet been reported. In this paper we report for the first time the detection by transient difference spectroscopy of the one-electron-oxidized and -reduced complexes Pt2(pcp)2- and Pt2(pcp):- and compare their spectroscopic properties to the pop analogue complexes. In this work a detailed evaluation of the relative reactivities of Pt2(pop)4"* and Pt2(pcp):-* has also been made by comparing their respective rate constants with a wide range of triplet-state quenchers. The quenchers used are alkyl and aryl halides, hydrogen atom donors, and alkenes and alkynes. The results are used to show that Pt2(pcp)44-*has a higher kinetic reactivity as a reductant than does Pt2(pop),4-*.
Experimental Section The complexes K4[Pt2(p~p)4] and K4[Pt2(p~p)4] were prepared by published procedure^.^^^ Organic solvents and quenchers were of reagent grade, and aqueous solutions were prepared from water which was purified by distillation from a glass container. Aerated solutions were therefore used in the quenching rate measurements. Emission intensities were measured on a SPEX fluorolog fluorometer with data analysis on a DATAMATE processor. The 0 1988 American Chemical Society
