4088
J . Phys. Chem. 1988, 92, 4088-4094
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.
0022-3654/88/2092-4088.$01.50/0
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
Excited-State Chemistry of Diplatinum(I1) Complexes
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4089
TABLE I: Absorption Maxima for the Electronic Transitions in Pt2(pop)qCand Pt,(pcp),&, along with Their Oxidation and Reduction Products
species
MO configuration
Amx 368,435 382, 470 330, 460 330,b 480 310 320 248 C
'An alternate assignment is u(p,) yet measured.
-
420 430
------
u*(d,z) u*(d,z) u(d,z)
u(d,z) u(d,2)
u(dzz) u(d:) u(dzz)
assignment
ref
u(p,) (singlet, triplet) u(p,) (singlet, triplet) u*(d,2), u*(d,2) u(p,) (sing1et)O u*(d,z), u*(d,z) u(p,) (singlet)' u*(dzz) u*(d,z) u*(d,z)
--
2 3 2, 5, 9 this work 9 this work 19
u*(d,z)
u*(d,2)
*(PJ
9
u*(d,2)
*(Pd
this work
u*(p,). bMeasured in 50% aqueous H2S04;this 480-nm band is observed at 460 nm in this solvent. 'Not
respective incident and observing wavelengths were 370 and 514 nm. Rate constants k , (M-I s-I) were obtained from plots of Zo/Z against quencher concentration [Q] using the Stern-Volmer equation: z o / l = 1 + kq~o[Ql The respective T,, values used were 55 ns for Pt2(pcp)4e, and either 1.25 or 0.32 ~.lsfor Pt2(pop),", depending whether the data were ~~~ collected in aqueous solution or in an organic s o l ~ e n t .Removal of oxygen from the aqueous solutions of Pt2(pcp),,- causes no significant change in the triplet lifetime. The plots were linear in all cases, and the slope of the line was obtained from a least-squares fit of the experimental data. Transient difference spectra were obtained with solutions of K,[Pt,(pcp),] in water passed through a Millipore filtration system. All solutions were nitrogen purged for some 5-10 min prior to data collection. The solution samples were changed frequently because of photochemical damage. The transient species were generated either by pulse radiolysis or by laser flash photolysis. Third-harmonic 355-nm ca. 10-ns pulses from a Quantel YG 581 Nd:YAG laser were used to excite samples contained in 0.5-cm path length cuvettes, and transient absorptions were monitored with a conventional xenon lamp, monochromator, photomultiplier arrangement. Digitized signals were passed to a PDP 11/70 computer for analysis. The analysis system has been described more fully elsewhere.6 For the picosecond absorption experiments the excitation was the 30-ps, 355-nm third-harmonic, pulse from a Quantel YG 402 mode-locked Nd:YAG laser. The experimental setup has been for the described previously.' For the determination of, ,X, fluorescence band of Pt2(pcp),' the spectrum was measured with the continuum probe pulse blocked. Spectral resolution was 0.6 nm/diode. The experimental setup for pulse radiolysis used electron pulses of 100-ns duration which were delivered to samples contained in a quartz cell having a 2.4-cm optical path length. Absorptions of transient species produced by the pulse were measured by using a conventional xenon lamp, monochromator, and photomultiplier tube assembly, and the signals were digitized by a Biomation 8100 transient recorder. The absorption of radiation by water produces the primary radicals e, -, OH', and H', with G values of 2.7, 2.7, and 0.55, respectively! Addition of tert-butyl alcohol to the nitrogen-saturated aqueous solution removes both the OH' and H' radicals in reactions that produce the relatively stable tert-butyl alcohol radical. Thus reduction of the added solute by hydrated electrons can be observed without complications from other absorbing species formed from hydroxyl or hydrogen radical attack. For pulse radiolysis conditions where the desired reagent is the hydroxyl radical, the reactions were carried out in aqueous so(4) Alexander, K. A,; Bryan, S . A,; Dickson, M. K.; Hedden, D.; Roundhill, D. M. Inorg. Synrh. 1986, 24, 211-213. ( 5 ) Peterson, J. R.; Kalyanasundaram, K. J . Phys. Chem. 1985, 89, 2486-2492. (6) Foyt, D. C. Comput. Chem. 1981, 5, 49-54. ( 7 ) Atherton, S. J.; Hubig, S. M.; Callan, T. J.; Duncanson, J. A,; Snowden, P. T.; Rodgers, M. A. J. J . Phys. Chem. 1987, 91, 3137-3140. (8) Swallow, A. J. Radiation Chemistry; Longman Group: London, 1973; p 92.
abs
nm Figure 1. Spectrum measured at 7.0 ws after the electron pulse with a nitrous oxide saturated solution;, , ,A = 320 nm for Ptz(pcp),'-.
abs
0
1n m Figure 2. Spectrum measured at 0.5 w s after the electron pulse with tert-butyl alcohol added to a N2-saturatedsolution;, , A, = 430 nm for Pt2(PCP)4+.
lutions containing no tert-butyl alcohol, but the solutions were saturated with nitrous oxide to convert the solvated electrons delivered to the solution into hydroxyl radicals (eq 1). Gas purges eaq-
+ N20
HZO
N2
+ OH' + OH-
(1)
were carried out continuously for 30 minutes prior to the use of the solution in the experiments. Transient absorption spectra were generated from the growth or decay curves recorded as absorbance against time data at a series of successive wavelengths. Data processing was accomplished with the on-line PDP 11/70 computer.6 Results and Discussion Oxidation and Reduction. The complexes Pt2(pop),,- and Pt2(pcp)," are characterized spectroscopically by singlet and triplet absorption bands due to u*(dZ2) n(p,) transitions. For Pt2(pop),,- these bands are observed at 367 and 452 nm, and for P t , ( p ~ p ) ~they & are found at 382 and 470 nm.2 These absorption bands have the respective extinction coefficients (emax) of 4 X lo4 and lo2 M-' cm-I. The ppyrophosphito complex Pt,(p~p),~has been previously found to react with hydroxyl radicals to give
-
4090
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988
abs
Roundhill et al.
7
abs
I xldl 5 -
A
4 A
3. A
580
360
820
2 .
\nm Figure 3. Spectrum measured at 0.05 ps after the laser pulse for an aqueous solution of Pt2(pcp)4e.The broad peak centered around 700 nm is due to esq-.
Pt,(p~p),~-and with hydrated electrons to form P t , ( p ~ p ) ~(eq ~2) .9 These complexes were identified using transient absorption
z
P
t
* ' t A
*
o
+
t
t
OH'
Pt2(PoP)45A,
Pt2(PoP)44-
Ptz(PoP)43-
= 420 nm
,A,
(2)
= 310 nm
spectroscopy, under pulse radiolysis conditions, by the presence of absorption bands at 3 10 and 420 nm for the one-electron-oxidized and -reduced complexes, respectively. Using the same experimental conditions we now find that the p-methylenebis(phosphonit0) complex Pt2(p~p)44will also undergo oneelectron oxidation and reduction to form P t , ( p ~ p ) ~(A, ~ - = 320 nm) and P t ? ( p ~ p ) (A~,, ~ - = 430 nm) (eq 3) (Figures 1 and 2; Pt2(PCP)45A,,
ePtz(PcP)44-
= 430 nm
OH'
Pt2(PcP)43Amax
(3)
= 320 nm
Table I). As confirmation of these assignments we have prepared these mixed-valence complexes by an alternative route. Such a route is available because of the tendency of both Pt,(pop):and Pt2(pcp),"- to undergo photoioni~ation.~~'~ Under laser photolysis conditions we observe the rapid formation of the one-electronoxidized complex, followed by the slower growth of the absorption band at 430 nm due to the one-electron-reduced product. This sequence of steps involves the initial formation of Pt2(pcp)?- by the photoionization step, followed by capture of the solvated electron formed in this process by ground-state Pt2(pcp)," to give Pt2(p~p)45(eq 4 and 5). Although we expect that Pt2(p~p)43~t,(pcp),4-
-
~t~(pcp),4-*2 ~t,(pcp),3-
+
Pt,(p~p),~- e-
-
Pt2(pcp)45-
+ e-
(4) (5)
and Pt2(p~p)45will undergo coproportionation at a rate approaching diffusion control, the two complexes can coexist in this solution because their low concentrations cause the rate of recombination back to Pt2(p~p)44to be sufficiently slow for their observation. Photoionization of Pt2(pcp):-. Although the photoionization of P t , ( p ~ p ) ~in~ aqueous solution has been shown to be a twophoton process,I0the sequence of reactions in (4) is the first report of photoionization by Pt,(p~p),~-in aqueous solution (Figure 3). Laser photolysis of separate aqueous solutions of Pt,(pop):and Pt2(p~p)44which have their concentrations adjusted such that the solutions have equal absorbances at 355 nm leads to the discovery that absorption of the 355-nm incident laser radiation causes a more efficient photoionization with Pt,(pc~)~"than with Pt2(pop)44-. This difference, which is evidenced by more intense absorptions due to eaq-,can be adequately explained on the basis that the lower electronegativity of the methylenic bridge in Pt2( ~ c p ) ~will " lead to a higher electron density at platinum(I1) than (9) Roundhill, D. M.; Atherton, S. J. J . Am. Chem. SOC. 1986, 108, 6829-6831. Che, C.-M.; Atherton, S . J.; Butler, L. G.; Gray, H. B. J . A m . Chem. SOC.1984, 106, 5143-5145. (IO) Cho, K . C.; Che, C.-M. Chem. Phys. Lert. 1986. 124, 313-316.
(11) Lachish, U.; Shafferman, A.; Stein, G. J . Chem. Phys. 1976, 64, 4205-421 1, (12) The singlet emission band for Pt*(p~p),~has been detected on a picosecond spectrometer at A, = 420 nm. The singlet lifetime of this band has been measured from the rate of growth of the 480-nm triplet absorption band following excitation into the singlet state. The aqueous solution contains 30% isopropyl alcohol to reduce the 510-nm triplet emission band. Data analysis gives a singlet excited-statelifetime of 33 ps, which compares with a value of 44 ps for Pt2(pop):- measured by the same method under identical experimental conditions. These values are a direct estimation of the intersystem crossing (singlet-to-triplet) rates. By comparison, other workers (Stiegman, A. E.; Rice, S . F.; Gray, H. B.; Miskowski, V. M. Inorg.Chem. 1987, 26, 11 12-1 116) find a value of -8 ps for the fluorescence lifetime of Pt,(PoP),4-.
Excited-State Chemistry of Diplatinum(I1) Complexes quinone). Clearly we have suppressed the photoionization pathway by effectively trapping the transient Pt2(pop),,-* with hydroquinone before it can capture a second photon.
Th e Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4091 quenchers, but rather we are observing electron-transfer quenching (eq 7). For the electron-transfer oxidants, methylviologen, and Ptz(p~p),~-*+ PhNO2 Pt2(pop)d3- + PhNOT (7) +
Pt2(pop)44-
+
hu
-
Pt2(pop),4-*
€ HQ
+ e(6) P t 2 ( P O P i ~ -+ HQ+ Pt2(pop):-
We can offer no compelling single reason why Pt2(p~p)4+should show such a greater degree of photoionization than does Pt2The electronegativity difference CH2 < 0 has some merit, but other explanations cannot be eliminated. A second explanation is the possibility (shown later in this paper to be true) that the triplet energy of P t , ( p ~ p ) , ~ is * greater than that of Pt2(pop)?-*. An alternative, and better, reason results from our observation of the bathochromic shifts in the electronic absorption bands on As a consequence, we expect going from Pt2(p0p)4" to Pt2(pc~)4'. that the high-energy absorption band in the state of Pt2(pep),"* will have a higher extinction coefficient at 355 nm (the wavelength of the incident laser light) than will the comparable transition in Pt2(pop)44-. Increased absorption of the second 355-nm photon will result in a more efficient photoionization. Triplet Energies of Pt2(pop)44- and Pt2(pcp)44-. The comparative spectroscopic data for Pt2(p0~)4+and Pt2(p~p)4"-in Table I indicate that the triplet energies of the states are closely similar. Nevertheless, in order to understand the difference in rate constants between various quenchers and the complexes Pt2(pp)44* and Pt2(pcp)?-*, it is useful to make a comparative evaluation of the triplet energies (E,) of these two excited states. In the absence of any experimental spectroscopic data for Pt2(pep)," which allows for the accurate evaluation of the zero-point energy in the spectrum, we have estimated the triplet energies of both P t 2 ( p 0 ~ ) 4 ~and * Pt2(p~p)4"-*from the crossing points of both the triplet absorption (a*(d,z) u(p,)) and emission Using this method we obtain the values of E,(Pt2(pop),'*) = 58.1 kcal mol-] and E,(Pt2(pcp)4e*) = 59.7 kcal mol-I. This finding suggests that Pt2(pcp),+* should be thermodynamically slightly more reactive than Pt2(pop)44-*. Reaction Rates of the Triplet States of Pt2(p~p)44-*and Pt2(pcp)4e* with Quenchers. The rate data, given as the logarithm of the quenching rate constant (log k,), are collected in Table 11. These data have been obtained by application of the Stem-Voimer equation to the emission intensity found for the states of the two complexes in the presence of varying molar concentration of the triplet-state quencher, [Q]. The quenching reagents have been chosen to fall primarily into four compound types; these are (i) alkyl and aryl halides, (ii) electron-transfer reagents, (iii) hydrogen atom donors, and (iv) alkenes, alkynes, and condensed aromatics. From the log k, values in Table 11, it is apparent that, for the large majority of quenchers, the complex Pt2(pcp),@* reacts at a faster rate than does Pt2(pop)46*. This generalization probably reflects the higher electronegativity of Pt2(pcp),"* and masks subtler variations which may affect the reactivity patterns. Alkyl and aryl halides react (log k 5-9) with the ,A2,, states of both Pt2(pop),"* and Pt2(pcp)?-*. This observation is in agreement with the previous qualitative result that Pt,(~op),~reacts with aryl halides (ArX) under photochemical conditions to give Pt2(pop),ArX4- and that the initial step involves halogen atom abstraction.2 In most cases the values of k, with alkyl and aryl halides are some 1-2 orders of magnitude larger for Pt2(pcp),+* than for Pt2(pop)4e*. This rate difference may be due to the relative triplet energies, but in the language of a synthetic chemist we can explain this rate difference as being due to the higher electron density in the pcp complex, which makes it the better reductant toward alkyl and aryl halides. The relative reactivity order PhI > PhBr > PhCl follows the sequence of carbon-halogen bond strengths in the compounds, no quenching being observed with chlorobenzene. Indeed the only chloroaromatics that we have found to react with Pt2(pop),,-* and Pt2(pcp);-* are those that also contain a nitro group bonded to the aromatic moiety. Since these compounds p-CIC6H4N02and o-C1C6H4NO2show lower reactivity than does nitrobenzene itself, it is unlikely that chlorine atom abstraction is occurring with these
-
-
quinones, the quenching rate constants are faster for Pt,(pcp);-* than for Pt2(pop),"*, the difference in log k, being only 1 or 2 units. This difference in reactivity of the two complexes again correlates with the expected higher electron density at platinum(I1) in P t * ( p ~ p ) , ~ - . ~ With triplet energies in the 58-60 kcal mol-l range, the triplet states of these diplatinum(I1) complexes will cleave a wide range of H-M bonds (M = CR,, SiR,, SnR,, PR,, etc). Atom-transfer bond-breaking reactions are expected to be more favorable with bimetallic complexes than with complexes having a single metal center. This results from the formation of a "partial" intermetallic bond in the mixed-valence product "Pt2H", which gives the reaction an additional thermodynamic advantage over monometallic complexes (eq 8).13 As for alkyl and aryl halides, we find that, Pt Pt4-* + H M Pt- -Pt-H4M' (8)
-
+
in most cases, the quenching rate constants for hydrogen atom abstraction are greater for Pt2(pcp),"* than for Pt2(pop),"*. Our data show no compelling evidence for steric selectivity between the various hydrogen atom donors; for example, we find that Ph3CH reacts with Pt2(pop)44-* at a rate which is the same as that found for Ph2CHOH. The relative order of reactivity to Pt2(pop),+* is H,PO, C PH,CH < n-Bu3SnH < Et,SiH, although the difference in rates is small. For Pt2(p~p)44-* the order is now Et,SiH < Ph,CH = n-Bu3SnH < H,PO,, again with the rate constants covering a very small range. For this series of four compounds we estimate that the respective C-H, Si-H, Sn-H, and P-H bond enthalpies are 80, 90, 70, and 77 kcal mol-'. The reactivities of Pt2(pop)44-*and Pt2(pcp):-* with these hydrogen atom donors do not show an increased rate with the lowering of H-M bond enthalpies. However, because of the narrow rate range found with these H-atom donors, and because of the fact that the rate constants have been measured in different solvents in some cases, we believe that we cannot draw any firm conclusions regarding these relative rates. Quenching by Alkenes and Alkynes. The majority of the quenchers used in this study are alkenes and alkynes. These compounds were chosen because of their known propensity to react with the triplet excited states of ketones such as benzophenoneI4 and also because of the extensive literature on the coordination chemistry between alkenes, alkynes, and platinum c o m p l e x e ~ . ~ ~ The alkenes chosen have a wide range of substituents, leading to the availability of rate constant data in Table I1 which can be used to probe electronic, steric, and triplet energy effects on the reactivities. The quenching pathways include energy transfer, hydrogen atom abstraction, and diradical formation. The range of rates (k,) range from 0 ( 2-hexene > trans-3-hexene > tetramethylethylene, which is the expected order for alkene coordination to ~ 1 a t i n u m . lBy ~ analogy with benzophenone and other ketones, our data can be explained by a quenching mechanism which involves the formation of a diradical The second step in this inner-sphere mechanism involves the collapse of this intermediate back to the alkene and the ground-state complex Pt2(pop);- or Pt2(pcp)4e (eq 11). The alkene quenching rates Pt;-*
-
+ RCH=CH2
-+
h
2 Pt,(pcp):-*
Ptz(pcp):-.NQS
NQS kkq
1
Ptz(pCp):-*NQS*
= c(Ptz(pcp)4'),
@o/@
(1
for these simple alkenes that have no heteroatom substituents fall in the range of log k, = 5-8. (c) Two other plausible mechanisms are ligand-to-metal-charge transfer (LMCT) and hydrogen atom abstraction. The first pathway represents a feasible option because alkene binding to platinum(I1) will primarily involve donation of electron density from the rr-bonding orbital of the alkene to an empty orbital on platinum. As a test of this hypothesis of LMCT quenching, we have used 4-pentenol as a triplet-state quencher and have analyzed the organic products obtained after long duration (-24 h) photolysis. We observe no formation of 2-methyltetrahydrofuran under these experimental conditions, a product expected to be rapidly formed if any radical cation were generated by electron transfer to the triplet excited state of Pt2(pop)44-* (eq 12). We must conclude therefore that such an electron-transfer pathway does not occur for 4-pentenol.
+
m s , KJr
If e(Ptz(pcp),'.NQS)
Anm Figure 6. Triplet excited-state spectrum of tetraphenylethylene formed by energy transfer from Pt2(pop).,"-*.
RCH-CH2-Pt-Pt4Pt24- RCH=CH2 ( 1 1 )
(18) Roundhill, D. M.; Shen, Z.-P.; Atherton, S. J. Znorg. Chem. 1987,26, 3833-3835. (19) Bryan, S. A. Ph.D. Thesis, Washington State University, 1985. (20)An interesting situation occurs with 1,2-naphthoquinone-4-suIfonic acid (NQS) and Pt2(pcp),'* where the apparent quenching rate constant is 3.6 X 10" M-I s-l. This value exceeds that usually accepted for a diffusion-controlled reaction (- loLos-l). A reaction rate that exceeds the solution diffusion rate is feasible for a situation where the triplet excited state and the quencher NQS are already in close proximity prior to reaction. Such a preequilibrium step (see Balzani, V.;Moggi, L.; Manfrin, M. F.; Bolletta, F.; Laurence, G. S. Coord. Chem. Rev. 1975, 15, 321-433) is Pt2(PCP):-
1
I
-
then
+ kq~o[NQSI)(l+ KWQSI)
+ (k,ro + K)[NQSl + (kqK70)[NQSJZ
The experimental value of 3.6 X 10" M-' s-I is therefore the value for k, + K. If the quenching rate constant k, approaches the diffusion limit, the equilibrium constant K has a value of approximately 1O'O. This mechanism involving a preequilibrium step should show a quenching rate which is increasingly dependent on the quadratic terms of this equation at high concentrations of NQS. Because of the high quenching rate, we cannot observe significant emission intensity when [NQS] > 6 X lo4 M, which is not sufficiently large to cause deviations from linearity in the Stern-Volmer plot. These data for 1,2-naphthoquinone-4-sulfonicacid provide the first evidence for preassociation between ground-state Ptz(pcp)2- and quencher. Evidence for association with Ptz(pop)4k has been observed in vesicles (see Hurst, J. K.;Thompson, D. H. P.; Connolly, J. S . J . Am. Chem. SOC.1987, 109, 507-515). Because of the hydrophilic ligand surface of Ptz(pcp)4', it is likely that 1,2-naphthoquinone-4-sulfonicacid associates with the complex by hydrogen bonding with the sulfonic acid groups. If quenchers hydrogen bond it is possible for energy- or into the first solvation sphere of Pt2(p~p)44-, electron-transfer quenching to occur at a rate that apparently exceeds the diffusion rate, because of the large association constant K.
In order to test the feasibility of a quenching pathway occurring by hydrogen atom abstraction, we have used transient difference laser spectroscopy to investigate the first products formed from the reaction of Pt,(p~p),~-*with allyl alcohol. This alkene is chosen because the relatively stable allyl radical results in a weaker allylic C-H bond (82 kcal mol-') than is found with other alkenes. The transient difference spectrum of this solution shows the presence of two products, Pt2(pop),H4-, with A, = 340 nm formed by hydrogen atom abstraction, and a second product with A, = 310 nm which we tentatively assign to Pt2(pop),R4- ( R = CH2CHCH20H). This product is the one that we expect to be formed from the diradical quenching reaction (eq 13). Ptz(pop):-*
+ CHz=CHCH20H
P t 2 ( p ~ p ) 4 H 4 - +CHz=CH;HOH p)CHz6HCHzOH4- ( 1 3 )
(d) For the other alkenes in Table I1 it is difficult to ascertain which quenching mechanism is dominant. For 1,3-~yclohexadiene, which has the low triplet energy of 52.4 kcal mol-', energy transfer will likely be a major contributing pathway. For butenediol and ethyl vinyl ether the presence of an oxygen heteroatom will favor both hydrogen atom abstraction and diradical formation, and it is probable that each pathway makes a contribution to the quenching mechanism. (e) For a further group of alkenes such as styrene, 1,l-diphenylethylene, fumaronitrile, and maleic acid, which have no bonds to hydrogen that can be cleaved by Ptz(pop)4e* and which have high triplet energies, fewer pathways are available. The slow quenching rate for maleic acid makes an electron-transfer (MLCT) pathway unlikely, and for these alkenes it appears that diradical formation or a simple collisional deactivation of the triplet
4094
J . Phys. Chem. 1988, 92, 4094-4099
s t a t e is t h e most likely pathway. (f) For t h e alkynes a similar series of pathways a r e feasible. All t h e r a t e constants fall in a n a r r o w range, and oxygen heteroatom substituents do not result in significantly faster quenching rates. Indeed, one of t h e largest bimolecular quenching r a t e constants is found for diphenylacetylene, a quencher that can only realistically react by a diradical intermediate or by collisional deactivation of t h e triplet s t a t e .
Acknowledgment. We t h a n k R . H. Schmehl and M. A. J. Rodgers for helpful discussions. W e t h a n k t h e Louisiana Board of Regents and t h e donors of t h e Petroleum Research Fund, administered by t h e American Chemical Society, for financial support. The C e n t e r for F a s t Kinetics Research is supported jointly by t h e Biotechnology Branch of Research Resources of NIH (RR 00886) and by t h e University of Texas a t Austin. Registry No. Pt,(pop)?-, 8001 1-25-2; Pt,(pcp),,-, 114763-58-5; Pt,(pcp)>-, 114763-59-6; Pt,(p~p),~-,114763-60-9; MeI, 74-88-4; EtI, 75-03-6; n-PrI, 107-08-4; (n-heptyl)I, 4282-40-0; i-PrI, 75-30-9; (CH2Br),, 106-93-4; PhCI, 108-90-7; PhBr, 108-86-1; PhI, 591-50-4; p BrC,H,OH. 104-92-7: p-BrC,H,OMe, 104-92-7: 1 -BrC,H,Bu-t-4,
3972-65-4; o-BrC6H4Me,95-46-5; p-BrC6H4F,460-00-4; p-BrC6H4CN, 623-00-7; p-BrC6H4NO2, 586-78-7; p-C1C6H4NOz, 100-00-5; oClC,H,NO,, 88-73-3; C,F,Br, 344-04-7; C6H5CH,0H, 100-51-6; PhzCHOH, 91-01-0; Ph3CH, 519-73-3; H3P0,, 6303-21-5; H3P03, 13598-36-2; Et,SiH, 617-86-7; n-Bu3SnH, 688-73-3; MVZ+,4685-14-7; MeNO,, 4685- 14-7; C6HSN0,, 98-95-3; m-C&(N02),,99-65-0; NaCI, 7647-14-5; NaBr, 7647-15-6; NaI, 7681-82-5; isopropyl alcohol, 67-63-0; 1,4-naphthoquinone, 130-15-4; 1,2-naphthoquinone, 2066-93-5; anthraquinone-2,6-disulfonic acid, 84-50-4; ferrocene, 102-54-5; hydroquinone, 123-31-9;ascorbic acid, 50-81-7; 1-hexene, 592-41-6; 2-hexene, 592-43-8; trans-hexene-3, 13269-52-8; I-heptene, 592-76-7; 1-octene, 11 1-66-0; cyclohexene, 110-83-8; cyclooctene, 931-88-4; cyclododecene, 1501-82-2; 1,5-~yclooctadiene,11 1-78-4; 1,5-hexadiene, 592-42-7; trans-stilbene, 103-30-0; cis-stilbene, 645-49-8; fumaronitrile, 764-42- 1; 3,4-dihydroxy-3-cyclobutene-1,2-dione, 2892-51-5; butadienol, 12542-32-4; trans,trans-l,4-diphenyl-l,3-butadiene, 538-8 1-8; 3-buten-2-one, 78-94-4; I ,3-cyclohexadiene, 592-57-4; 2,3-dimethyl-2-butene, 563-79-1; styrene, 100-42-5; 1,l-diphenylethylene, 530-48-3; ethyl vinyl ether, 109-92-2; bicyclo[2.2.11hepta-2,5-diene, 121-46-0; norbornylene, 498-66-8; maleic acid, 110-16-7; allyl alcohol, 107-18-6; 1-hexyne, 693-02-7; butynediol, 11070-67-0; acetylenedicarboxylic acid, 142-45-0; Cz(CMe20H),, 14230-3; diphenylacetylene, 501-65-5; benzene, 71-43-2; toluene, 108-88-3; naphthalene, 91-20-3; anthracene, 120-12-7; phenanthrene, 85-01-8.
High-Temperature Decomposition of Tetramethyldioxetane: Measurements of Gas-Phase Chemiexcitation Yields M. A. Tolbert, D. L. Huestis, and M. J. Rossi* Chemical Physics Laboratory, S R I International, Menlo Park, California 94025 (Received: December 3, 1987)
The yields of electronically excited acetone from high-temperature decomposition of gas-phase tetramethyldioxetane (TMD) a r e reported. High temperatures a r e achieved by using infrared-sensitized excitation of TMD. Emission studies a r e used = 0.017 (h0.008). This value is approximately a factor of 2 higher than to obtain the yield of excited singlet acetone, that reported in solution. A lower limit for the yield of triplet acetone is determined by using transient absorption studies, aT 0.10 (k0.03). This value is comparable to previous results in solution. The lifetime of triplet acetone observed in the present experiment is approximately 5 ps, almost 2 orders of magnitude shorter than the lifetime of thermalized triplet acetone a t low pressures. The short lifetime is attributed to electronic quenching by the infrared-absorbing gas CH3F,with an estimated rate constant of 5 x 1 0 - l ~cm3/(moIecuIe s).
Introduction The high yield of electronically excited products from t h e decomposition of tetramethyldioxetane (TMD) has stimulated extensive studies of this intriguing molecule. In addition t o numerous and in recent work has studies in t h e g a s
addressed t h e decomposition of solid-phase TMD as a possible source of a short-wavelength chemical laser.21 These many studies have shown t h a t decomposition of TMD results in t h e formation of two molecules of acetone, one of which may be electronically excited (reaction 1).
TMD (1) Bogan, D. J.; Durant, Jr., J. L.; Sheinson, R. S.; Williams, F. W. Photochem. Photobiol. 1979, 30, 3.
(2) Bogan, D. J. In Chemical and Biological Generation of Excited States; Adam, W., Cilento, G., Eds.; Academic: New York, 1982; p 37. (3) Cannon, B. D.; Crim, F. F. J . Am. Chem. SOC. 1981, 103, 6722. (4) Haas, Y.; Yahav, G. Chem. Phys. Lett. 1977, 48, 63. (5) Haas, Y.; Yahav, G. J . Am. Chem. SOC. 1978, 100, 4885. (6) Haas, Y.; Ruhman, S.; Greenblatt, G. D.; Anner, 0. J . Am. Chem. SOC. 1985. 107. 5069. (7) Ruhman, S.; Anner, 0.; Haas, Y. J . Phys. Chem. 1984, 88, 6397. (8) Farneth, W.; Flynn, G.; Slater, R.; Turro. N. J. J . Am. Chem. SOC. 1976. -, 98. -. 1877. (9) Brown, J. C.; Menzinger, M. Chem. Phys. Lett. 1978, 54, 235. (10) Bottari, F.J.; Greene, E. F. J . Phys. Chem. 1984, 88, 4238. (1 1) Adam, W. In Chemical and Biological Generation of Excited States; Adam, W., Cilento, G., Eds.; Academic: New York, 1982; p 115. (12) Adam, W.; Zinner, K. In Chemical and Biological Generation of Excited States; Adam, W., Cilento, G., Eds.; Academic: New York, 1982; p 153. (13) Turro, N. J.; Lechtken, P.; Lyons, A,; Hautala, R. R.; Carnahan, E.; Katz, T.J. J . Am. Chem. Sot. 1973, 95, 2035. (14) Turro, N. J.; Lechtken, P.; Schore, N. E.; Schuster, G.; Steinmetzer. H. C.; Yekta, A. Arc. Chem. Res. 1974, 7, 97. ( 1 5 ) Lechtken, P.; Yekta, A.; Turro, N . J . J . Am. Chem. SOC.1973, 95, 3027. ~~
0022-3654 / 8 8 1'2092-4094$01.50/0
---*
A*
+ A0
(1)
The chemiexcitation yields of singlet acetone, 'A*, and triplet acetone, 3A*, have previously been measured in solution a t temperatures in t h e range 325-360 K." The results indicate t h a t 30-50% of the reactions lead to 3A*, 0.04-0.08% of the reactions form 'A*, and t h e remainder form only ground-state acetone, A,. T h e chemiexcitation yields have not been measured for decomposition of either solid-phase TMD or gaseous TMD a t elevated temperatures (>500 K). T h i s is unfortunate because these quantities a r e essential in assessing t h e possibility of a chemical ~~~
~~
~
~~
~~~~
(16) Turro, N. J.; Lechtken, P. J . Am. Chem. SOC.1972, 94, 2886. (17) Steinmetzer, H. C.; Yekta, A.; Turro, N. J. J . Am. Chem. SOC. 1974, 96, 282. (18) Turro, N. J.; Lechtken, P.; Schuster, G.; Orell, J.; Steinmetzer, H. C.; Adam, W. J . Am. Chem. SOC. 1974, 96, 1627. (19) Adam, W.; Duran, N.; Simpson, G. A. J . Am. Chem. SOC.1975, 97, 5464. (20) Smith, K. K.; Koo, J. Y . ;Schuster, G . B.; Kaufmann, K. J . Chem. Phys. Lett. 1977, 48, 267. (21) Tolbert, M. A,; Spencer, M. N.; Huestis, D. L.; Rossi, M. J . J . Photochem. 1988, 42, 73.
a 1988 American
Chemical Societv
