1902
The Journal of Physical Chemistry, Vol. 83,
No. 14,
1979
S.-M. Y. Huang and H. D. Gafney
Quenching of Various Triplet Donors by the Geometric ,@Diketonate Isomers of Cobalt (111) and Chromium(II1). Effects of Molecular Structure on the Quenching Efficiency Sze-Ming Y. Huang and Harry D. Gafney* Department of Chemistw, City University of New York, Queens Co//ege,Flushing, New York 11367 (Received November 11, 1978; Revised Manuscript Received March 5, 1979)
The quenching of five low-energy triplet donors, ET 5 2.2 pm-l, by eight geometric P-diketonate isomers of Co(II1) and Cr(II1) has been studied by flash photolysis techniques. The isomers, which have identical electronic spectra, but different molecular structures, have been used to probe the effect of molecular structure during the quenching encounter. The quenching rate constants, k,, were found to be dependent on the energy of the donor and the accessible energy level of the complex, and quenching is thought to occur by an energy transfer mechanism. To satisfy the exothermicity of the process, energy transfer from anthracene and its substituted analogues is limited to population of the metal-centered ligand-field states of the isomers. Previous experiments have shown that the quenching efficiency of the P-diketonate complexes is dependent on the structure of the complex, and energy transfer to the ligand-field states of the complexes magnifies the structural difference. With these geometric isomers, however, no difference in the quenching rates was found. Since these geometric isomers are essentially identical in size, these results suggest that a distinction between structural effects exists, Those structural variations which change the distance between the donor and acceptor will introduce a structural dependence to the quenching rate, while those structural variations which do not change the distance will not introduce a structural dependence to the quenching rate.
Introduction Interest in the photophysical and photochemical processes of a wide variety of compounds has led to progress in understanding the chemistry of the excited state. Of interest are not only those states which are observed in the absorption spectrum, but those states where direct population is forbidden by various selection rules. To probe these spectroscopically forbidden excited states, sensitization and quenching techniques have proven valuable with organic molecules,l and have recently been applied to transition metal complexes.2 In fluid solution, electronic energy transfer between organic molecules is nearly diffusion controlled when spin and energy requirements are satisfied., With transition metal complexes, however, the quenching rates may be three orders of magnitude less than the diffusion-controlled rate.2 Although the quantum mechanical constraints on the processes are independent of whether the system is organic or inorganic, the relatively slow rates often found with transition metal complexes indicate that there are characteristics of these complexes which affect the quenching encounter and ratea2t4 With organic molecules, previous studies have shown that quenching is sensitive to the steric environment of the donor and quencher chromophore^.^ A priori, a similar structural dependence would be expected with transition metal complexes, yet relatively few systematic studies have explored the role of structure of a transition metal complex during a quenching encounter. Hammond and Foss have shown that the acetylacetonate complexes, M ( a ~ a c )were ~ , consistently more effective than the dipivaloylmethanate complexes, M(dpm),, in the quenching of the triplet state of benzophenone.6 Most dramatic are the results with the Co(II1) analogues; Co(acac), quenches triplet benzophenone whereas Co(dpm), does not. These results indicate the importance of steric effects and suggest that the tert-butyl 0022-3654/79/2083-1902$0 1.OO/O
groups of the dipivaloylmethanate ligand are steric hindrances by shielding the unsaturated part of the ligand. Wilkinson and Farmilo studied the quenching of triplet states of 16 organic compounds by tris(@-diketonate) complexes of Fe(III), Ru(III), and Al(III).7 The quenching rate was found to show a marked dependence on the energy of the donor and the energy levels of the quencher. This dependence not only establishes energy transfer as the quenching mechanism, but also yields some insight into the relation between structure and the acceptor energy level. The quenching rate constant of F e ( a ~ a cis) ~3.2 times larger than that of Fe(dpm), when the energy of the donor is sufficient to populate the charge-transfer state of both complexes, and 4.5-6 times larger when the ligand-field states, 4Tzand/or 4T1,are available for quenching. Since ligand-field states are essentially metal-centered and embedded within a ligand sheath, the intimacy of the encounter required for energy transfer apparently magnifies steric differences. Clearly, structural variations which affect the distance of closest approach are important during the quenching encounter. Beyond these results, however, a further refinement of the role of molecular structure is unavailable because of the lack of definitive experimental data. Unlike organic compounds, in which structural modification by alkylation of the chromophore causes little or no change in the absorption spectrum, changes in the coordination sphere of a transition metal complex causes not only the desired structural changes, but concurrent changes in the electronic structure of the complex as well. Often, the inability to completely separate structural modifications from the concurrent electronic changes gives rise to an ambiguity which clouds the interpretation of the results.a In probing the role of molecular structure then, a series of metal complexes must be used where structural modification can be made without a concurrent modification of their electronic structure. This requirement is met by the cis and trans geometric isomers of the unsymmetrically 0 1979 American Chemical Society
Effects of Molecular Structure on the Quenching Efficiency
substituted P-diketonate complexes. Although different in their outer structure and dipole moment, the isomers have identical electronic spectra and are essentially identical in size.g We report here the results of an investigation of the quenching of five low-energy triplet donors, ET I2.2 pm-l, by eight geometric P-diketonate isomers of Cr(II1) and Co(II1).
Experimental Section Materials. The preparation of the complexes tris(1phenyl- 1,3-butanedionate)cobalt(III), C ~ ( b z a c ) tris( ~ ; 1phenyl-1,3-butanedionate)chromium(III), C r ( b ~ a c ) ~ ; tris(l,l,l-trifluoro-2,4-pentanedionate)cobalt(III), C~(tfac)~; and tris(l,l,l-trifluoro-2,4-pentanedionate)chromium(III), C r ( t f a ~ )and ~ , the separation of the isomers have been previously described.1° The electronic, infrared, and NMR spectra and melting points of the various isomers were in excellent agreement with published data.g Anthracene, 9-methylanthracene, 9-phenylanthracene, pyrene, and phenanthrene, purchased from Aldrich Chemical Co., were used without further purification since the reported purity is greater than 98%. All solutions were prepared with spectral grade benzene (J. T. Baker). In studying the quenching of the triplet states of these various donors, the general procedure was to prepare benzene stock solutions containing ca. 2 X M of the various donors. The stock solutions were kept in the dark and refrigerated. The absorption spectra and triplet lifetimes were checked periodically, but no change beyond experimental error was detected. Solutions of the isomers of the P-diketonate complexes were always freshly prepared. In a typical experiment, solutions containing 4 X M of the donor and lo4 to M of the quencher were prepared by dilution of the stock solutions and transferred to the degassing bulb of the flash cell. The flash cell was then connected to the vacuum line and the solution degassed by nine freeze-thaw cycles. Flash Photolysis Measurements. The flash photolysis apparatus used in these experiments has been previously described.’l Following the freeze-thaw cycles, the cell was closed off, removed from the vacuum line, and placed in the flash apparatus. To prevent direct photolysis of the quencher in experiments where higher concentrations of the quenchers were used, a Plexiglas filter which transmitted h 2 350 nm was placed between the flash cell and the analyzing lamp. Various parameters, such as photomultiplier voltage, slit width, and alignment of the analyzing beam, were optimized to obtain a full scale deflection on the oscilloscope graticle. The flash was then triggered and the oscilloscope trace was photographed with a Polaroid oscilloscope camera. The kinetic data, the absorbance of the triplet state as a function of time, were taken directly from the Polaroid photograph. During the course of this work, a number of steady-state photolysis experiments were carried out with the previously described equipment.ll Physical Measurements. Infrared spectra were recorded on a Perkin-Elmer 237B grating spectrophotometer, calibrated against polystryene. Ultraviolet and visible spectra were recorded on a Cary 14 spectrophotometer or a Techtron 635 spectrophotometer. The NMR spectra of these diamagnetic Co(II1) complexes were recorded on a Varian A-60.4 spectrometer. Emission spectra were recorded on a Perkin-Elmer Hitachi MPF-2A fluorescence spectrophotometer. Results Since the majority of organic triplets do not phosphoresce in fluid solution, luminescence techniques cannot be
The Journal of Physical Chemistry, Vol. 83, 0 l
ysec 400
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GOO l
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No. 14, 1979 1903 800
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Flgure 1. Oscilloscope traces showing the decay of the triplet state of anthracene at 420 nm; (a) 4.00 X M anthracene, (b) 4.00 X M anthracene and 9.36 X lo-’ M transCo(bzac),.
used to probe the quenching encounter. However, the intense triplet-triplet absorption spectra of a large number of organic molecules are known, and the quenching reaction can be followed by monitoring the decay of the triplet formed in a flash photolysis experiment.lJ2 Typical oscilloscope traces where the decay of the triplet state of anthracene was monitored at 420 nm in the presence and absence of the quencher are shown in Figure 1. The triplet-triplet absorption spectrum of 9-methylanthracene and 9-phenylanthracene are not currently available in the literature. Spectra of these triplet states were obtained by monitoring their decay at a number of wavelengths from 350 to -500 nm. The absorbance a t each wavelength, measured at a fixed time, 150 ps, on the decay trace, showed that the spectra were qualitatively similar to that of anthracene. Consequently, the triplet states of these donors were also monitored at 420 nm. The triplet states of pyrene and phenanthrene were monitored at 408 and 482 nm, respective1y.l In the absence of a quencher, previous experiments have shown that the decay of the triplet state, T, is described by the expression4*J3J4 - d[T]/dt = k,’[T] k2’[TI2 (1)
+
+
+
where kl‘ = kl hl* k2[Do] and kp’ = k3 - k2. The constant hl describes radiative and nonradiative decay of T while kl*,dependent on temperature and viscosity, describes quenching due to impurities such as oxygen.15 The term h2[Do]arises from the self-quenching reaction, but is small in comparison to the triplet-triplet annihilation reaction described by k316 Substituting [To]= Ao/d and [T,] = A t / d and integrating eq 1 yields Ao(h1’ + (ki/eOAJ = hl’t (2) A,(ki’ + (ki/EOAO) where A. and A, represent the absorbance at time to and t, E is the extinction coefficient of the triplet-triplet absorption, and 1 is the pathlength of the flash cell, 17 cm. Using searching techniques involving the simplex,34 a computer program then obtains kl’ and k i l t 1 by minimizing the quantity
according to the criterion of least squares. In the presence of a quencher, the second-order term is negligible and eq 1 reduces to - d[T] /dt = kl”[T] (4)
1904
The Journal of Physical Chemisfty, Vol. 83, No. 14, 1979
S.-M. Y. Huang and H. 0.Gafney
TABLE I: Bimolecular Quenching Rate Constantsa for the Quenching of the Triplet State of Organic Donors 9-
donors ET
9-
phenylanthracene methylanthracene 1.47 pm-l 1.47 pm-’
anthracene 1.49 pm-’
pyrene 1.68 pm-’
phenanthrene 2.16 pm-’
2.88 f 0.29 5.26 t 0.53 trans-Co(tfac), 0.47 f 0.05 1.02 t 0.09 0.90 t 0.09 1.03 c 0.09 0.95 t 0.09 3.00 t 0.30 4.35 t 0.44 cis-Co(tfac), 0.49 i 0.05 2.46 f 0.25 4.57 i 0.46 1.20 f 0 . 1 1 0.95 k 0.09 trans-Co (bzac), 0.21 f 0.02 4.02 i 0.40 0.89 f 0.09 2.36 t 0.24 cis-Co(bzac), 0.25 t 0.03 1.03 c 0.09 4.75 k 0.47 1.90 f 0.19 2.79 t 0.28 trans-Cr(tfac), 0.70 f 0.07 1.39 i 0.13 4.82 f 0.48 1.93 t 0.19 2.94 t 0.29 cis-Cr(tfac), 0.82 f 0.08 1.21 t 0.11 3.58 t 0.71 5.66 t 0.60 1.33 i 0.12 1.82 i 0.18 trans-Cr(brac), 0.67 f 0.07 2.75 t 0.55 5.08 ~t 0.51 1.77 i 0.17 cis-Cr(bzac), 0.85 f 0.08 1.22 f 0.11 a Bimolecular rate constant X lo-@ M-’ s-l Values obtained from sensitization experiments, C. A. Parker, “Photoluminescence of Solutions”, Elsevier, New York, 1968, p 315. Reference 1.
ysec
Flgure 2. First-order plots for the decay of various triplets in the presence of the quenchers: 9-methylanthracene and 5.08 X lomeM trans-Cr(tfac),, A; pyrene and 1.16 X M cis-Cr(tfac),, W; anthracene and 5.08 X 10“ M fmns-Cr(tfac),; 0 ;and 9-methybnthracene and 5.13 X lo-’ M frans-Co(tfac),, W.
and k? was obtained directly from plots of log (Ao/A,)vs. time. Representative plots for a number of the donors studied are shown in Figure 2. The bimolecular quenching rate constants, k,, were then obtained from the difference in the first-order rate constants, i.e.
The experimental values of k , calculated from the above expression, are listed in Tabfe I. In these experiments, the triplets were generated by exposure to ca. 250-5 unfiltered flash. The substrates pyrene, phenanthrene, and 9-phenylanthracene were found to be photochemically stable; absorption spectra recorded before and after the flash were identical. Flash photolysis of anthracene and 9-methylanthracene, on the other hand, induces a slight net chemical change. Spectra recorded before and after flash photolysis of benzene solutions M in anthracene or 9-methylanthracene showed a uniform decrease of 8-1270 in the absorbance of each reagent. Previous studies have shown that anthracene and 9methylanthracene can undergo either a photoperoxidation or a photodimerization r e a ~ t i 0 n . l ~ Since the flash experiments were carried out under conditions which involved an exhaustive removal of dissolved oxygen and steady-state photolyses were independent of whether the samples were degrassed or not, the photochemical reaction appears to be a dimerization. The photodimerization reaction occurs through the first excited singlet state of each reagent. To determine whether the metal complexes might be involved in these photochemical reactions, a number of fluorescence quenching experiments were
carried out. In these experiments, benzene solutions containing M anthracene or M 9-methylanthracene were excited at 390 nm and the fluorescent emissions were monitored at 420 nm. The Stern-Volmer plots were linear over a concentration range of 10“ to M in the metal complexes. For the quenching of anthracene by truns-Co(tfa&, a quenching constant of 160 f 17 M-l was found, while that for the quenching of 9methylanthracene by t r u n s - C r ( t f a ~was ) ~ 267 f 23 M-l. Less extensive measurements with the other metal complexes yielded similar values. These results established that the concentrations of the metal complexes used in the flash experiments, S10M5 M, were insufficient to directly interact with these reactive singlet states. Flash experiments where the quenching of the anthracene and 9methylanthracene triplets were monitored also indicate that the geometric isomers are unaffected by possible reaction intermediates. With each geometric isomer, the rate constant for the quenching of the triplet state of these photoactive donors was found to be independent of the number of times the sample was exposed to the flash. Consequently, the photochemical reaction is a minor annoyance in the kinetic analysis, but does not affect the measurement of the quenching rate constant. A more serious difficulty in these experiments is the concealment of a possible difference in the quenching rates by either a thermaP or photo~hemicall~ isomerization of these geometric isomers. The experimental data, however, indicate that these reactions, which could occur during the preparation or measurement of the samples, would not account for the equality in the rate constants found for these geometric isomers. To determine the role of thermal isomerization, solutions containing the various donors and quenchers were stored in the dark at room temperature, 23-24 “ C , for 1 h prior to the flash experiment. Yet, the rate constants obtained from these solutions were within experimental error of values obtained from equivalent samples measured immediately after preparation. As mentioned above, the quenching rate constants are insensitive to the number of times the sample is exposed to the flash. Samples exposed to as many as five flashes yielded rate constants which were within experimental error of values obtained after one flash. This insensitivity suggests then that a photochemical isomerization is not concealing a potential difference in quenching efficiency. The lack of discrimination between these geometric isomers cannot be attributed to an experimental artifact, but must reflect instead, the donor-quencher encounter. Discussion The possibility of using the well-defined triplet state of an organic molecule to sensitize a reaction within a transition metal complex has created an interest in the quenching of organic triplets by transition metal com-
Effects of Molecular Structure on the Quenching Efficiency
plexes.2 In these experiments, however, definitive experimental evidence which distinguishes between the possible quenching mechanisms is rare. With the majority of metal complexes which have been used as quenchers, the dearth of conclusive experimental data arises from the absence of a sensitized emission or transient absorbance. Thus, it might be assumed that the evidence for a specific quenching mechanism would hardly be convincing. A considerable amount of data has been amassed over the past 20 years, however, which indicates that the principle mechanism for quenching of the donors used in this study by the P-diketone complexes involves an energy transfer process. Hammond and co-workers have shown that the quenching efficiency of the 0-diketone complexes of a number of metal ions does not correlate with the paramagnetism or spin-orbit coupling parameter of the complex.20 Thus, quenching via a magnetic or spin-orbit coupling process does not appear to be the principle mechanism. The absence of a net chemical change arising from the triplet state of the donor or in the quencher also relegates a chemical mechanism to a minor role. On the other hand, the sensitization of the 2E 4A2 phosphorescence in C r ( a ~ a cby ) ~ benzil and anthracene at low temperatures establishes an energy transfer mechanism.21 In a recent extensive study. Wilkinson and Farmilo have shown that the quenching efficiency of Fe(aca&, Fe( d ~ m )R~~, ( a c a c )and ~ , A l ( a c a ~depends )~ on the energies of the excited states of these complexe~.~ This characteristic dependence on the energy of the triplet state being quenched and on the spectroscopically determined energy levels of the complexes establishes an energy transfer quenching m e ~ h a n i s m . ~ The quenching behavior of these Cr(II1) and Co(II1) isomers parallels that found with other P-diketone comp l e ~ e s .The ~ quenching efficiency of these diamagnetic Co(II1) isomers is essentially the same as the paramagnetic Cr(II1) isomers. Thus, quenching via a magnetic mechanism can be ruled outsmWith the exception of anthracene and 9-methylanthracene which underwent a photochemical reaction originating from the first excited singlet state, no net chemical change was detected in these experiments and quenching via a chemical mechanism is ruled out. Although more limited, the data summarized in Table I parallel the results of Wilkinson and Farmilo; the quenching rate depends on the triplet state energy of the donor and the energy levels of the q ~ e n c h e r .It~ seems reasonable then to limit the subsequent discussion to a quenching mechanism which involves energy transfer. For energy transfer to occur, the energy of the donor must be sufficient to populate a given state of the acceptor; otherwise the process is endothermic and improbable.ls2 Since the spin-allowed P* and charge-transfer states of these Co(II1) and Cr(II1) isomers have energies 12.7 pm-l;z quenching of these triplet donors, ET 5 2.2 pm-l, is limited to energy transfer to the lower energy ligand triplet statesz3 and the ligand-field states of the complexes. In these Co(II1) and Cr(II1) isomers, however, an exact determination of the energy of the ligand triplet state, 3L, is not possible. The CofIII) complexes do not emit and the low temperature emission of the Cr(II1) complexes is a metal-centered 2E 4A2transition. A reasonable estimate of the triplet state of tfac- is available from the emission spectrum of La(tfac),. Brinen and co-workers report that the broad structured emission of tfac- spans the energies of 2.041 to 2.353 pm-1.24 Although complexation to Co(II1) or Cr(II1) would alter these energies, the change would be slight, and 2.3-2.4 pm-l is taken as
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The Journal of Physical Chemistry, Vol. 83, No.
14, 1979
1905
TABLE 11: Thermally Equilibrated Energiesa of the Ligand-Field States of the Cr(II1) and Co(II1) Complexes Co(tfac), Co(bzac),
'TI 1.38 1.38
3T1 0.9-1.0 0.9-1.0
4T,
*Eb
3T* 1.2--1:3 1.2-1.3
Cr(tfac), 1.43 1.23 1.24 Cr (bzac), 1.42 a The energies are reported in pm" and are the same for both the cis and trans isomers of the complexes. P. Fleischauer and P. Fleischauer, Chem. Rev., 199 (1970).
the thermally equilibrated energy of the 3L state of these tfac- isomers. Similarly, the thermally equilibrated energy of the 3L state of these bzac- complexes is taken from the phosphorescence spectrum of benzoylacetone to be 2.1-2.2 pm-1.25
The remaining lower energy excited states to be considered are the spin-allowed and spin-forbidden ligandfield states. Molecular orbital calculations indicate that the 4T1state of Cr(acac)3or the lT2state of C o ( a ~ a clie )~ a t 2.3 and 2.5 pm-l, respectively.22b Although the ligand-field strengths of bzac- and tfac- are slightly less than that of acac-, these states may be involved in the quenching of phenanthrene, but are too energetic to be involved in the quenching of the other donors. The thermally equilibrated energies of the other ligand-field states, summarized in Table 11, were estimated by applying the Fleischauer-Adamson criteria to the absorption and/or emission spectra of the complexes.26 The quenching rate constants, k,, listed in Table I parallel previous results in that the quenching efficiency depends on the triplet state energy of the donor and the energy levels of the q ~ e n c h e r s . ~The energy of the phenanthrene triplet, ET = 2.16 pm-', is sufficient to populate the 3L state of the bzac- ligand. In view of Wilkinson and Farmilo's results, which indicate that an increase in k , occurs when the energy of the donor is sufficient to populate ligand-localized or charge-transfer state^,^ the large value of k, found with phenanthrene might then be attributed to energy transfer to the ligand triplet state. The difficulty in this interpretation, however, arises from the trifluoroacetylacetonate chelates. The values of k , found with the M ( t f a ~complexes )~ are within experimental error of those found with the M ( b ~ a c ) ~ complexes, yet the estimated energy of the tfac- triplet is approximately 0.1-0.2 pm-' higher in energy than the phenanthrene triplet. To quantify the uncertainity in the above estimate of the 3L state energy is difficult, but the similarity of the kq's suggests that quenching by the M ( t f a ~complexes )~ may also involve population of the ligand-localized triplets. The rate constants describing the quenching of pyrene are smaller than those found with phenanthrene, but larger than those found with anthracene. Since energy transfer from anthracene is limited to population of the ligand field states of the isomers, the larger k,'s suggests that states other than the ligand-field states are involved in the quenching of pyrene. The energies of the 3L states or the spin-allowed charge transfer states, E > 2.7 pm-1,22bindicate that these states would not be involved in the quenching of pyrene. Associated with the spin-allowed charge transfer states are the corresponding spin-forbidden states, but assessment of the energies of these states flounders in the absence of exact values for the exchange integrals and the thermally equilibrated energy of the corresponding spin-allowed state. Crude calculations suggest that the spin-forbidden
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The Journal of Physical Chemistry, Vol. 83,No. 14, 1979
charge transfer states could lie as low as 1.6 prn-l, but we doubt that this is a wholly adequate explanation of the increase in the quenching rate. Independent of the question of the acceptor level, however, the rate constants listed in Table I show that no discrimination of the isomers occurred during the quenching encounter. With anthracene and its substituted analogues energy transfer must occur to the ligand-field states of the isomers to satisfy the necessary exothermicity of the process.’ Additional distinctions between the various ligand-field states of these Co(II1) complexes might be made on the basis of spin conservation. In our opinion, however, such an assignment is dubious since it is based on the assignment of formal spin states which itself is tenuous if the spin-orbit coupling is large.27 Suffice it to say that energetics demand the involvement of the ligand field states where previous experiments have indicated that energy transfer requires a more intimate encounter and apparently magnifies structural difference^.^ In energy transfer to the 4T1and 4T2states of Fe(dpm), and Fe( a ~ a c )the ~ , rate constant for the quenching of anthracene by F e ( a ~ a c is ) ~7.6 f 0.5 X loa M-l s-l while that for quenching by Fe(dpm), is 1.5 f 0.2 X lo8 M-l s-l.’ This difference in the rate constants shows how critical a close approach is for energy transfer to occur to a ligand-field state. Quite different results were found in these experiments. The rate constants for quenching by the various pairs of cis and trans isomers (Table I) were identical within experimental error. Thus, during the energy transfer process to the ligand-field states of these geometric isomers, no discrimination occurred. The lack of any difference in the quenching efficiency of these isomers cannot be attributed to the leveling effect of diffusion. The rate constants listed in Table I are less than the theoretical diffusion limit which in benzene at 22 “C is 1 X 1O1O M-l s-l.% Furthermore, the rate constants for the quenching of anthracene found in these experiments are similar to those found for Fe(acac), and Fe( d ~ mwhere ) ~ the rate constants differed by a factor of 5.7 The rate constants listed in Table I show that the quenching rate does depend on the substituents attached to the parent anthracene chromophore. Since the energies of anthracene and its substituted analogues are essentially identical and more than adequate to populate the lower energy ligand-field state of the isomers, the difference is not due to energetics. Rather, the difference is thought to arise from some steric hindrance during the quenching encounter. The small size of the methyl substituent in 9-methylanthracene would be expected to cause only a slight perturbation on the quenching encounter. Consistant with this expectation, there is very little difference in the rate constants found with anthracene and 9methylanthracene. The smaller rate constants found for the quenching of 9-phenylanthracene as compared to anthracene or 9methylanthracene indicate that the phenyl substituent causes additional steric hindrance to the energy transfer process. The absorption spectrum of 9-phenylanthracene is slightly red shifted as compared to that of anthracene. Although this indicates that some conjugation between the phenyl substituent and the parent chromophore exists, inspection of molecular models indicates that the phenyl group is not planar with the parent chromophore. In the similar molecule 9,10-diphenylanthracene,the angle between the plane of the phenyl group and the plane of anthracene has been reported to be 60°.1,29 Consequently, an extension of the anthracene ?r system through conjugation of the phenyl substituent is unlikely or inefficient
S.-M. Y. Huang and H. D. Gafney
and the effect of this substituent is principally one of steric hindrance. A number of flash experiments were tried with 9,10-diphenylanthracene, but the intersystem crossing yield for the molecule is too small to obtain reliable kinetic data.lJ* We find it surprising that even with anthracene and its substituted analogues where the smaller rate constants imply a necessity of additional collisions for energy transfer to occur that no discrimination of these isomers occurred. Regardless of which excited state of the isomers is the final acceptor level in the energy transfer step, these results are quite different from those found with the acetylacetonate and dipivalocylmethanate c ~ m p l e x e s .In ~ ~these ~ complexes, however, the steric difference is due only to the size of the substituent; the larger substituent, a tert-butyl group, appears to prevent a close approach of the quencher to the donor. These geometric isomers, on the other hand, are essentially identical in size and differ only in the arrangement of the asymmetric ligands about the metal ion. There are two possible explanations to account for the difference found with the acetylacetonate and dipivaloylmethanate complexes and the results found in these experiments. The first explanation arises from the theory of exchange energy transfer. The equation defining the rate constant for energy transfer, ket, has the form30
Assuming the Bohr radii, L, of the initial and final electronic states of the donor and acceptor are the same, 1A,31 and the separation distance, R, is that given by the van der Waals radii of a methyl group, 2.0 A, and a tert-butyl group, 3.15 A,32the ratio ketacac/hetPm = 10. Considering the approximations made, this is in reasonable agreement with the value of 5.1 obtained from the ratio of rate constants for the quenching of triplet anthracene by Fe(acad3 and F e ( d ~ m ) ~These . ~ geometric isomers of Co(II1) and Cr(II1) are essentially identical in size.g Consequently, there would be no difference in the separation distance and according to eq 6 no difference in the quenching rate constants would be expected. This explanation, however, implies that a distinction between structural effect exists. Provided the structural variation per se is not involved in the energy migration pathway, those structural variations which change the distance of separation between the donor and acceptor will introduce to the efficiency of energy transfer a structural dependence. Those structural variations which do not change the distance of separation between the donor and acceptor, Le., differences due to the orientation of the ligands about the metal ion, will not introduce a structural dependence. The alternative explanation arises from the concept of the “conducting ability” of a ligand.2,33Previous studies of the quenching of various organic and inorganic donors have shown that quenching efficiency can be significantly increased by changing one or more of the ligands in the coordination sphere of the q ~ e n c h e r Although . ~ ~ ~ ~ not ~~~~ conclusive, it does suggest, in our opinion, that quenching may be specific to a given metal-ligand pair or a specific part of the quencher. With these geometric isomers, however, the structural differences arise from the complexes as a whole, but are negated if quenching reflects a specific part of the isomer such as a metal-ligand pair. Within this hypothesis then, no difference in the quenching efficiencies of the isomers would be expected. Yet, the difference in the quenching efficiency of Fe(acac), and Fe(dpm)3would still be expected since the substituents on the ligands cause differences in the distance of sepa-
ESR Study of Spin Labels Attached to HPMA
ration between the donor and acceptor. Intuitively, a structural dependence is expected, yet these results suggest that a dependence will be found only when the structural difference between two quenchers is sufficient to change the distance of separation between the donor and the acceptor.
Acknowledgment. We thank Professor Jerry Koeppl and Dr. Gary Stein for their assistance in developing the computer programs to analyze the flash data. Financial support of this research from the Research Corporation and the Research Foundation of The City University of New York is also gratefully acknowledged. References and Notes J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, New York, 1970. V. Balzani, L. Moggi, M. F. Manfrin, F. Bolletta, and G. S. Laurence, Coord. Chem. Rev., 15, 321 (1975). (a) A. A. Lamola and N. J. Turro, "Energy Transfer and Organic Chemistry", Wiley-Interscience, New York, 1969; (b) N. J. Turro, J. G. Dalton, and D. S. Weiss, "Organic Photochemistry", Vol. 1, 0. L. Chapman, Ed., Marcel Dekker, New York, pp 1-62. (a) H. Linschitz and L. Pekkarinen, J . Am. Chem. SOC.,82, 2411 (1960); (b) P. Pfiel, ibid., 93, 5359 (1971). (a) C. C. Wasmer et ai., J . Am. Chem. SOC.,97, 4864 (1975); (b) K. Yekta and N. J. Turro, Chem. fhys. Lett., 17, 31 (1972); (c) K. Janda and F. S. Wettack, J . Am. Chem. SOC.,94, 305 (1972); (d) C. Ouannes, R. Bengelmans, and G. Rossi, ibid., 95, 8472 (1973). G. S. Hammond and R. P. Foss, J. fhys. Chem., 66, 2577 (1962). F. Wllkinson and A. Farmilo, J. Chem. SOC.,Faraday Trans. 2 , 604 (1976). F. Bolktta, M. Mastri, L. Moggi, and V. Balzani, J . Am. Chem. Soc., 95, 7864 (1973). R. C. Fay and T. S.Piper, J . Am. Chem. SOC.,84, 2303 (1962); 85, 500 (1963).
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Electron Spin Resonance Study of Anisotropic Rotational Reorientation of Spin Labels Attached to the Side Chains of Soluble Poly(methacry1amide)-Type Copolymers J. Pilaf, J. Labskf, J. Kllal, Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6, Czechoslovakia
and J. H. Freed" Department of Chemistry, Cornell University, Ithaca, New York 14853 (Received December 7, 1978)
Soluble copolymers of N-(2-hydroxypropy1)methacrylamide(HPMA) with N-methacryloylated w-amino acids, which form side chains with 4-aminoperdeuterio-2,2,6,6-tetramethylpiperidinyl-l-oxy (PD-Temp-NH2)attached to their end, were investigated. The EPR spectra of methanolic solution of these copolymers recorded at 313-173 K were analyzed by using a model of anisotropic but axially symmetric rotational reorientation of the spin label. An analysis of these spectra in the motional narrowing region showed that the symmetry axis z' of the rotational-diffusion tensor describing this rotational reorientation was an axis close to that of the N-0. bond, and that, depending on the type of side chain, the rotational reorientation about this axis is four to six times faster than about the remaining two axes. The correlation time, TR, characterizing the rate of rotational reorientation of the spin label decreases monotonically with increasing side chain length. Linear log rR vs. 1 / T dependences were obtained and were characterized by a low activation energy, E = 15 f 2 kJ/mol, that was the same within the limits of experimental error for all the copolymers. Slow motional spectra were given a preliminary interpretation with a model of very anisotropic rotational reorientation. I. Introduction The EPR method is able to provide information on the rotational reorientation of radicals of the nitroxide type, whose characteristic feature consists of anisotropy in their g tensor and hyperfine interaction tensor AN of the nitroxide group nitrogen atom.laSb An investigation of the E P R spectra of stable nitroxide radicals bound at the ends of side chains of polymers allows one to obtain information
on the way in which the rotational reorientation of a nitroxide (spin label), characterized by a temperature-dependent correlation time, is affected by the polymer, side chain, and solvent.2-6 Copolymers of N-(2-hydroxypropy1)methacrylamide (HPMA) with 3 mol % of 4nitrophenol esters of N-methacryloylated o-amino acids were chosen for the investigation. These amino acids form side chains of copolymers a t the end of which 4-amino-
0022-3654/79/2083-1907$01.00/00 1979 American Chemical Society
