Photoexcited Triplet State and Chemical Dynamics in Frozen

324

J. Phys. Chem. 1980, 84,324-336

Photoexcited Triplet State and Chemical Dynamics in Frozen Solutions of Metalloporphyrins. An EPR Optical Study? A. Scherr'

and Halm Levanon"

Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel (Received June 29, 1979)

The EPR line shapes of randomly oriented triplets of MgTPP and ZnTPP which are dissolved in ethanol, toluene, and toluene-pyridine are strongly temperature dependent over a relatively narrow range of temperatures,namely, 10@-180K. These line shape variations cannot be accounted for in terms of a dynamic process within a single species. It is suggested that around the freezing point of the solvent the following equilibrium processes occur for ZnTPP and MgTPP, respectively: PL*z,(D&) + D*zn1(D4)and PL*Mg(D&) P D*M,?(D~) P D*M,~(D~). PL*%(Da)represents the photoexcited triplet of a ligated species of Da symmetry, D*k1(D4)ISa dimer consisting of weakly interacting monomeric subunits of the metalloporphyrin which is monoligated to a solvent molecule. The symmetry of the monomeric subunits is Da and their mutual orientation results in, for the dimer, an overall symmetry of D4.In MgTPP solutions,the f i t dimer D*&1(D4)is similar in structure and symmetry to D*Znl(D4). However, because MgTPP can form biligated compounds the second dimer, D*m, consists of weakly interacting biligated molecules. The structure of the various species are discussed in terms of removing the in-plane degeneracy via the ligation and dimerization processes of the solvent molecules to the central metal in the porphyrin ring. The magnetic parameters and activation energies for the various species involved in the monomer-dimer equilibria are calculated.

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I. Introduction It has been known for some time that randomly oriented photoexcited triplets of magnesium and zinc porphyrins reveal anomalous EPR line shapes which neither reflect an axial nor a nonaxial ~ymmetry.~9~9~ This is demonstrated in Figure 1 where the triplet EPR spectrum of zinc tetraphenylporphyrin, ZnTPP, at 113 K is compared to the calculated spectrum of a randomly oriented axially symmetrical triplet in a glass matrix. Since, to a first approximation, the point group symmetry of ZnTPP in its ground state is D4h5v6the disagreement between the experimental and calculated spectra is apparent. On the other hand, experiments performed at liquid helium temperatures on ZnTPP by the method known as optical detection of magnetic resonance, ODMR, demonstrated unambiguously the existence of a rhombic contribution to the dipolar spin Hamiltonian giving rise to a nonvanishing IEI value.5 In fact, all zero field experiments which have been performed, normally at T I4.2 K, on many metalloporphyrins revealed a significant amount of a rhombic cwtribution to the spin Hamiltonian, which is reflected by a relatively high IEI v a l ~ e . ~ p ~ - ~ The discrepancy between the molecular symmetry and the experimental results, obtained at low temperatures, has been rationalized first by Lhoste et ala2in terms of a superposition of many molecular sites having different IEI values.1° In a later publication, Chan et al.7 presented a qualitative interpretation of the low temperature observations, in terms of a combined Jahn-Teller instability, E j T , with a crystal field effect, 6. Typical values for EJT and 6 in magnesium and zinc porphyrins lie on the order of 50-100 cm-l, respectively.12 In this mechanism, the crystal field will lift E j T by the amount of 6, resulting in the low lying energy conformation to be populated as long as kT 6, both energy levels will become Dedicated to Sam Weissman. 0022-3654/80/2084-0324$01 .OO/O

equally populated, resulting in a vanishing value for IEI. For the 6 values reported in the literature, the spectra around 77 K should already have a vanishing IEI value. This mechanism applied to metalloporphyrins was treated quantitatively by Hoffman annd Ratner.13 From the above discussion, a temperature-dependent study is required in order to check the variation of the IEI value with temperature. Above liquid helium temperatures, ODMR is not applicable,14thus leaving direct EPR as a suitable probe for monitoring line shape variations. In the present study we analyze the randomly oriented triplet EPR spectra of MgTPP and ZnTPP at different temperatures and solvents. We show that the line shape variations cannot be accounted for in terms of one species which undergoes a dynamic JT process. Moreover, we calculate an activation energy in the order of 1000 cm-I for a dynamic process that is responsible for the transition from one type of spectrum, observed at low temperature, T 100 K, to another type of spectrum observed at 180 K. It is shown that the triplet spectral line shape is most sensitive over a narrow range of temperatures around the freezing point of the solvent where the soft properties of the solid matrix are still maintained. These observations are best interpreted in terms of an equilibrium process between two or three photoexcited triplet forms: PL*(Da) P D*1(D4) D*z(D4). The PL*(D4,J species are species characterized by their axial symmetry, D&, and the D*i(D4) species are dimers which are composed of weakly interacting monomeric subunits. These subunits are proposed to be the mono- and biligated species with the solvent molecules. The overall symmetry of the dimer is D4and that of the interacting monomers is DW Optical absorption spectra of the above compounds were found to be most sensitive in the same range of temperatures, in parallel to the EPR experiments. We show that, by lowering the temperature, the absorption spectra reflect changes in the molecular symmetry. These changes in the optical absorption, most pronounced in MgTPP dissolved in ethanol, are interpreted in terms of lifting, to some extent, the in-plane degeneracy.

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0 1980 American Chemical Society

N

The Journal of Physical Chemistty, Vol. 84, No. 3, 1980 925

ODMR Study of Metalloporphyrins

dependent optical measurements. Before each experiment, the optical spectrum was compared to those in the literat~re.'~~~~

h

111. EPR Line Shape Analysis We start by writing the equation of motion of the density matrix for the triplet state (S = l)3'v32 p = rp (1)

9.2

+

where H*p = H p - pH and I'p is the contribution to p by a dynamical process. The Hamiltonian, H, is decomposed into three parts H = HO + HD(Q)+ Hrf(t) (2) where Hois the Zeeman energy, uoS,;H&) is the interaction with the rf field, wlSx cos ut; HD(Q) is the dipolar interaction, which depends on the orientation, Q, of the triplet in the laboratory frame and is commonly expressed as31 HD(Q)= (-l)"Q'"(n)s-Q(') (3)

cQ

200 Gauss -_I__,

P

Figure 1. Comprrrison between an experimental photoexclted triplet EPR spectrum of ,ZnTPPat 113 K (upper trace) and that of a calculated spectrum of axial ijymmeo. In the calculation, the line width parameters AH, (i = X , Y , Z)were chosen as: AHx = AHy = 39.0 G, AHz = 13.0 0, and the zfs parameters are ID1 = 0.0294 cm-', IEI = 0; X , Y . 2 are the canonical orientations.

These results are of interest in view of reports which indicate that aggregat,ionand dimerization processes occur with organic ~ n o l e c u l e s , ~porphyrins,17-" ~J~ and chlorophyll~, 23-26 11. Experimmtal Section Solutions, for EPR and optical measurements, were prepared in evacuated quartz tubes, 3-mm i.d., for the former experiments and in optical cells of 1- (or 0.1-) cm optical path, for the latter measurements. In some experiments, to avoid any possible contamination of water (hydrated species), the porphyrin was sublimed into the EPR or optical ~ e l l . 2These ~ precautions in sample preparations were stopped after we had certified that no change in the spectrum was noticed because of the sublimation process. The solvents used were toluene, n-octane (dried over Na-K alloy), toluene-pyridine mixture, and ethanol. All EPR experiments were performed by using light modulation excitation combined with a Varian E-12 spectrometer operated at 100-kHz field modulation. Because of the formation of spurious EPR lines under light irradiation, the CW spectra were taken via a second phase sensitive detection coupled to a-low frequency light modulation of -1 Hz. In all measurements, only the first harmonic representation was recorded.28 Except where stated otherwisle, the sample temperatures were changed, on lowering the temperature in a slow rate of -Q.5 OC min-' in the coinventional way, by passing cold nitrogen through the EF'R cavity. Optical absorption measurements were carried out with a Cary 14 spectrophotometer. A home-built dewar, in which the temperature could be varied between room temperature and 100 K, was used for the temperature-

-

where

DQ"'(Q) = (2/3)'/'D~0,~'~'(Q) + E[aZ,Q(''(Q) + a-2,Q(')(Q)I (4)

sp

(3/2)(Sz2 - (1/3)S2)

S+1(" = =F(SZS* + S*SZ)

s + p = (1/2)S*S* Do,Q(2) and D + 2 , ~ (are 2 ) the Wigner matrices: D and E are the zero field splitting (zfs) parameters which are related to the principal values of the traceless tensor D D = (1/2)(Dxx - DYY)- Dzz

E = -(1/2)(Dxx

- DYY)

(6)

Regarding the dynamic process which contributes to the density matrix equation of motion we shall, in this study, be concerned only with two types of dynamic procesries which under some circumstances are considered to be equivalent. The first is where an intermolecular triplet migration takes place between neighboring molecules (two sites case).33-38The second dynamic process can be described in terms of intramolecular discrete solid like jumps.32 These two processes become equivalent, with respect to the EPR line shape variations, in the following two cases: (a) in the intermolecular energy transfer case, the neighboring molecules are of D2h symmetry, the outof-plane principal axes coincide, and the in-plane principal axes are mutually rotated by 7r/2; (b) in the intramolecular solidlike jumps case, the molecule is of D%symmetry, and one should consider discrete 7r/2 jumps. The EPR line shape analysis in the present study was carried out by applying, to a first order, the density matrix method.sia*a The dynamic process, which describes either of the above two cases, was chosen as the intermolecular energy transfer, where the X and Y principal axes are interchanged between the interacting subunits.34 If we denote A and I3 as two different magnetic sites, intramolecular or intermolecular, the contribution to the equation of motion via the jump (or transfer) process will be given by P A = ~ B A P B- ~ A B P A ('7) PB = ~ A B P A- ~ B A P B we assume that kAB = kBA = k where I l k is the mean

328 The Journal of Physical Chemlstry, Vol. 84, No. 3, 1980

Scherz and Levanon

/\I I

Z n T PP- E THAN OL

Z n T P P - TOLUENE

!

I

Flgure 2. Photoexcited triplet EPR spectra of ZnTPP in toluene and ethanol at different temperatures. All spectra were taken at a light modulation frequency of 1.5 Hz,a microwave power of 20 mW, and a fixed field modulation a m p l i . The receiver gain and temperature are the only experimental parameters during the experiment which were changed. The solid lines are the best fit curves obtained by a superposition of two spectra attributed to two species, PL+n(D,h) and,D*mr(D,). Simulation of the experimental curves were carried out as described in the text. The best fit weighing factors can be derived from Figure 6 and the magnetic parameters are given In Tables I and 11.

lifetime, between the jumps (or transfer), on each site. For isotropic distribution of triplets, the randomly oriented triplet line shape is calculated via the equation

I@) a I m x d Q Tr [S+p+lAB(fL,B)]

(8)

where p+lABrepresents the density matrix in the coupled systems; p+lAB = p+lA + p+lB describes the EPR transition within the AM,= +1 region. The expression for p+lABis given by36

2H - ( H A+ HB) + 2i H(HA + HB)- HAHB- H2 +

where

H = B - -W Y D 3 HA = -(I. - 3nA2) - -E(ZA' - mA2) 2 2 D 3 HB = -(I - 3nA2) + ZE(lA2- ??%A2) 2

(10)

I, m,and n, are the Euler angles between the principal axes and the external magnetic field B , 1/T2 is the effective line width, and k is the exchange rate. A symmetric expression

is obtained for the h M s = -1 transition.

IV. Results and Discussion The triplet spectra of ZnTPP dissolved in toluene as a function of temperature (113-179 K)are shown in Figure 2. It is evident that, in this range of temperatures, changes in the EPR line shape occur. The changes with which we shall be concerned in the present study are those related to the canonical orientations in the Ams = fl region, X, Y , 2. The line intensities in the g N 2.0 region are due to three different sources: (a) 101N 31EI as found in many metallop~rphyrins;~~~J+ (b) absorptions by solvent radicals, (c) double quantum transitions.4l In all cases studied here, the latter two sources to the line intensities will not be further discussed. In the high temperature limit, where an EPR spectrum can still be recorded, an apparent axial spectrum is noticed which changes gradually upon lowering the temperature. At temperatures below -140 K the changes are not as noticeable as those obtained in the high temperature region and, in fact, below 125 K no further changes in the line shape were observed. When the solvent was changed into ethanol, the same general behavior of the spectral line shape vs. temperature was noticed (Figure 2). To check as to what extent these line shape variations are general, we have carried out the same type of experiments on a somewhat different metalloporphyrin, MgTPP, dissolved in toluene and ethanol. The triplet EPR spectra are shown in Figure 3. A common feature of all experimental results is the conspicuous line shape variation in a relatively narrow range of temperatures. As we outlined in the Introduction the apparent axial EPR spectrum in many metallo-

The Journal of Physical Chemistry, Vol. 84, No. 51, 1980 321

ODMR Study of Metalloporphyrins

1

I

I

Flgure 3. Photoexcited triplet EPR spectra of MgTPP In toluene and ethanol at differenttemperatures. All experimental conditlons are as described in caption to Figure 2. The best flt weighing factors for triplet spectra of all MgTPP solutions are glven in Table 111, and the magnetic parameters are given in Tables I and 11.

we can predict the spectrum that should be obtained at porphyrin comlpounds, obtained at high temperatures, was accounted for in terms of dynamic JT e f f e ~ t s . ~This ~ ~ ~ ~ J100 ~ K. Thus, employing the */2 jump mechanism, the high temperature experimental spectrum (upper trace of implies that in the high temperature limit, KT >> 6, the Figure 2) can best be simulated by using zfs parameters two energy levels resulting from the vibronic coupling are ID1 N 319 (101= 0.0269 cm-') and a rate jump of lz = 2.8 equally populated. The communication between the two x lo9 s-l. By applying different values of activation enconformations, causes an interchange between X and Y ergies,43AE,i.e., 10,100, and lo00 cm-', the corresponding principal axes in the two distorted equilibrium positions, calculated rate constants are 2.6 X lo9, 1.5 X lo9, and 4.7 thus giving rise to a vanishing zfs IEI value. On the other x los s-', respectively. In Figure 4 we present the calcuhand, at very low temperatures (T5 4.2 K), it has been lated spectra, using the above three rates. It is evident that shown by Langhoff et aL6that ZnTPP exhibits nonvanneither of those activation energies, describing a dynamic ishing values for both /Dland IEI. Therefore, if a dynamic process within a single species, can account for the exJT is the only mechanism for the line shape variation with perimental observations as shown in Figures 2 and 3, temperature, m e should expect a gradual increase in the There is an unambiguous discontinuity around 125 K IEI value on decreasing the temperature, and the low where the spectral line shape stops to be temperature temperature spectrum, in such a case, should reflect a dependent. nonaxial symmetry.42 It is evident from the spectral beThe same considerations hold for the intramolecularcase havior, as shown in Figures 2 and 3, that this is not the where interaction between species of the same symmetry case for ZnTPP nor for MgTPP in the glass matrices where exists. Again, as in the intramolecular case the EPR these compounds have been studied. spectra, observed at high temperatures, are possible for A somewhat (differentdynamic mechanism, which is also interpreting in terms of an exciton interaction with a high an intramolecular one, is the so-called discrete .n/2 jump rate for energy transfer.'0B6d7However, as predicted by model.s2 If one starts with a Da symmetry, the line shape the calculated curves in Figure 4, in the solid like jump variation with temperature will depend on the rate conmechanism, one should expect at low temperature an EF'R stant for the jump process. Thus, at high temperatures, spectrum of a Da symmetry because of zero energy one should expect a high rate for intraconversion giving transfer rate, i.e., a localized triplet. To summarize, the rise to a line shape approaching an axial symmetry, as cold spectra obtained at 125 K are difficult to interpret indeed was observed in both compounds. Such a mechain terms of either of the above, intramolecular or internism would predict, however, a temperature at which the molecular, dynamic processes. When poly(methy1 methjumps have been slowed down, on the EPR time scale, to acrylate) (PMMC), on the other hand, was used in the such a rate where a static Da spectrum is expected to be observed. It is easy to show that, by applying an Arrhenius present study as a solvent, the EPR spectra remain 8ssentially the same along the entire range of temperatures, behavior to the rate constant, k = A exp(-AE/kT), and exhibiting the low temperature spectra of the type shown taking into account the experimental fact that at 180 K in Figures 2 and 3. an axial spectrum is observed (upper trace of Figure 2),

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328 The Journal of Physical Chemistry, Vol. 84, No. 3, 1980

Scherz and Levanon

I

where P is the unligated metalloporphyrin, PL is the monoligated compound (ZnTPP is known to complex with one ligand3'?, []AG represents aggregates which are formed on lowering the temperature, and []M,AG represents a photoexcited triplet of a monomeric species within the aggregate. These latter species will be defined as M*znl(D2h). Under some appropriate conditions, a monomeric species within the aggregate can interact with a neighboring molecule to form a dimer [ID in which energy transfer, in terms of hopping exciton, occurs. It is evident that, at temperatures where diffusional motion of M*Znl(D2h) to form properly oriented dimers is allowed, equilibrium 15 will shift to the right. In this model we neglect the interaction between species of different symmetry. The EPR changes are interpreted, therefore, in terms of the following dynamic process: At high temperatures, the main species which contributes to the spectrum is of D4h symmetry. On lowering the temperature, equilibria 11-13 shift to the right, and the resulting monoligated species in the aggregate is of lower symmetry (D%)than the parent compound because of environmental perturb a t i o n ~ . ~ ~Both ~ ~ " PL and [PL*.*PL]A(;are being photoexcited as monomers, and, if the coupling between the monomeric species with the aggregate is efficient, then the net equilibrium, which is monitored by the triplet line shape variation with temperature, is summarized by the reaction PL*Zn@4h) D*Znl(D4) (16) PL*zn(D4h) represents the monoligated species. The right-hand side of eq 16, D*&l(D4),represents the weakly interacting dimers of the monomeric monoligated species Figure 4. Theoretical EPR s m a , predictedat 100 K, for three different activation energies A€. The rate constants for the exchange process, M*znl(&h) within the aggregate (reaction 15). k , between the two subunits were determined via the relatlon In k a This model implies that at low temperature the cold AEIRT, together with the experimental observation that the high spectrum is due to D*Zn1(D4)in which the rate constant temperature spectrum (upper trace) Is an axial spectrum. The latter for triplet migration is low, but not vanishing, and is spectrum was simulated by using a rate constant k = 2.8 X lo9 s-'. temperature independent, at least down to 4.2 K. By The llne wldth parameters in all the calculated spectra are AH, = AHy applying these ideas, we adopt the multichannel transfer = 19.5G,AH,= 13.0 G;101 = 0.0269 em-', 14 = 0.0090 cm-'. The experlmental curve is that of ZnTPP in toluene at 179 K (upper trace mechanism, proposed by Lemaister et al.13' to explain the of Figure 2). cold spectrum which is obtained at temperatures below 125 K. In their model, the exciton transfer takes place The impossibility to interpret the above observations between sites which are chemically equivalent but magin terms of a dynamic process within a single species innetically inequivalent, i.e., the principal axes of one motrigued us to invoke the participation of more than one nomer are tilted with respect to the neighboring one. For species which undergo an equilibrium process. The ocmost practical purposes, it is sufficient to assume two currence of dynamic processes within aggregates at low channels for exciton transfer with rate constants ko and temperatures, at which the soft glass properties are still kl. The former is the rate constant for transfer from the maintained, is known and have been investigated quite vibrationless level, and the latter is the rate constant for intensively in aromatic r n o l e ~ u l e s . ~ Regarding ~ ~ ~ ~ ~ ~the " ~ ~ transfer from the first excited vibrational level. Because PMMC matrix, it was reported3 that when zinc etioof tunneling, one expects that kl > kw The populations porphyrin was dissolved in it, and the sample was heated of the two vibrational levels are governed via thermal to 330 K, the triplet EPR spectrum was very close to an equilibrium. The ratio between the number of photoexaxial spectrum, This observation probably indicates that cited triplets transferring their energy with a rate ko, ( N the matrix has sufficiently been softened to permit some (ko)),and those transferring their energy with a rate kl, diffusional motion.47 ( N ( k l ) ) ,obeys the Boltzmann relation: N(k1)/N(ko)= In view of the above discussion, and taking also into exp(-AE,/kT) where AE, is the vibronic energy difference account that the solvent molecules are capable to interact, between the two channels. In the present treatment we or, in some cases to ligate with the central metal on the assume that the energy difference between the vibrationp0rphyrin,8,~1~~~ we consider the following reactions, which less level, uo, and the first vibration, ul, is of the order of are temperature dependent in solutions containing ZnTPP a few hundreds of reciprocal wave numbers. In such approximation, mainly uo is populated and thus the energy P+LF?PL (11) transfer occurs with a single rate, KO, which is hardly temperature dependent.51 PL -!E+ PL*(D4h) (12) The species as appear in eq 12 and 14 have, therefore, different EPR spectra, which will respond differently to PL + PL e [PL".PL]AG (13) the incident microwave power. The axial spectrum, which hv was observed at high temperatures, retained its shape up [PL"'PL] AG [pL*"*PL]MAG (14) to 100 mW, whereas the ratio between the peak intensities [pL*"'PL]M,AG e [PL**"PL]D [pL"*PL*]D (15) of the spectra, observed at lower temperatures, were found

*

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The Journal of Physical Chemistry, Vol. 84, No. 3, 1980 320

ODMR Study of Metalloporphyrins

TABLE I: Zfs Parameters, Rates of Exciton Hopping, and Enthalpy of Dimerization for Various Species of Metalloporphyrins in Frozen Solutions compd ZnTPP

ZnTPC MgTPP

n-octane toluene toluene 1 : 2 pyridineethanol toluiene tolulene 1:1 toluenepyridine ethanol

0.0269 0.0283 0.0288

0 0

0.0278

0

0.0281

0

0

0.0301 (0.0322)" 0.0294 0.0272 0.0302

0.0079 (0.0096y 0.0098 0.0090

0.0315f 0.0294 0.0276

0.006gf 0.0076 0.0031

0.0276

0.0084

0.0288

0.0028

0.0295

0.0075

0.0100

1800 1649 1043

8.1 x 107 2.3 x 107 10.3 X l o 7

1400 2426

7.8 x 107 11.2 X lo'

769 (834)g

1.4

8.4 X I O 7

x 107

5.0

x

1.07

Values referred to the monomeric subunits, M*m,(D2h)i = 1 or 2, a Values referred to the axial species PL*(D,h). Exciton hopping rate constant in s-', e Zfs parameters reported by Langhoff et al.5 within the aggregate. In cm-'. g Computed from the changes f Zfs parameters attributed to monomeric species of nonaxial symmetry, see also Figure 13. in the optical absorption in the Soret-band, see also Figures 7 and 10. TABLE 11: Bost Fit Parameters, Line Widthsa (aHi,i = X, Y , Z), and g Values for Simulation of Triplet EPR Spectra

-

______

solvent

___-.

AH,

pL*(D,h) a H y AH,

g

D*m,(D4) aHx A H y AH,

g

M*m,(D,h 1 AHx aHyAHZ

D*m2(D4 1 AH, A H y A H z

g I

ZnTPP toluene 1 : 2 toluenepyridine ethanol

19.5 24

19.5 24

13 12

2.0023 2.0023

24 32

24 32

12 12

2.0084 2.0023

26

26

13

2.0023

24

24

12

2.0084

MgTPP toluene 19.5 19.5 13 1:1 toluenepyridine ethanol 19.5 19.5 13 a Line width values in gauss.

2.0023 2.0023

24 26

24 26

12 13

2.0084 2.0023

13

:13

13

2.0023

26

26

13

2.0023

36

36

12

2.0023

13

13

13

2.0140

22

22

13

2.0023

to be sensitive to the microwave power. The spectrum which is assigned to PL*Zn(D4h) is a pure axial spectrum which is easily calculated. The spectrum of D*znl(D4)is obtained by employing the general equation for the line shapes (eq 8). In this dimer we assume that the principal axes of X , Y in the molecular plane of one subunit are tilted by n / 2 relative to the other monomer's axes, while the 2 principal axes of both monomers coincide. As discussed above, triplet exciton hopping between the two subunits is thuls equivalent to discrete n / 2 solid like jumps. Therefore, the spectral line shape associated with the 2 principal axis is unaffected by the dynamic process. The interchange of X and Y principal axes between the subunits will affect, therefore, the spectral line shape. Thus, when ko = 0 irnplying localized triplets, a pure nonaxial spectrum is exlpected to be observed. On the other hand, when ko >> IX - Yl a pure axial spectrum will result because of complete delocalization of the triplet exciton. The solid lines superimposed on each spectrum in Figures 2 are the best fit spectra obtained by a superposition of two line shapes with different concentrations for the axial species and the weakly interacting dimers. The best fit analysis was carried out in three consecutive steps: (a) the zfs parameter ID1 and the line widths values AHi(PL*), i = X,Y , 2, wore first determined for the axial species, PL*, from the high temperature spectrum; (b) the zfs parameters 101and IEI, the rate constant KO together with the line widths values AHi (D*) i = X,Y, 2,for the dimer D*, were deterrnined from the low temperature spectrum; (c) the best fit values obtained in steps a and b were chosen as fixed parameters and the line shape variation with temperature were simulated by changing the ratio, D*al(D4)/PL*:&14,J which was the only free parameter in the final calculation (cf. Figure 7). The values of the

zfs parameters and ko are given in Table I and the values for the line width parameters, AHi,are given in Table 11. When different g factors were necessary to be assigned1 to the various species, one component was arbitrarily chosen to have g = 2.0023. The relative shifts are given in Table I1 (this analysis does not take into account g anisotropies). In the analysis of the results for MgTPP, one should consider the occurrence of biligation to the central metal: PL L -8 PL2 (17)

+

together with the fact that unligated MgTPP is not known to exist. In terms of the model presented above, we add the analogous reactions describing the aggregation and triplet migration within the aggregate PL2 + PL2 -8 [PL2'*'PL2]AG

-

(18)

hw

[PIA2"'PL2]AG [PL*2"'PL2]M,AG (19) [PL*2"*PL2]M,AG F! [PL*2"*PL2]D F! [PL2***PL*2] D (20) For MgTPP in toluene, the triplet line shape variation with temperature was accounted for in terms of two species only, namely, PL*,(D,) and D*MBl(D4) (for the chemical identification of these species see section V). The solid lines, superimposed on the experimental curves shown in Figure 3, are the best fit calculated curves obtained as described above in the analysis of ZnTPP. The analysis of the results for MgTPP in ethanol (Figure 3) was unable to simulate the experimental curves without taking into account the participation of a third species in the dynamic process. In view of the spectrophotometric results which will be described in the next section, we propose the following interpretation of the triplet EPR line shape variations vs. temperature. Between 167 and 125

330

The Journal of Physical Chemistty, Vol. 84, No. 3, 1980

Scherz and Levanon

TABLE 111: Best Fit Weighing Factors for Various Components of Which the Triplet EPR Spectra Were Simulated

~~

167 158 146 138 125 115 178 174 168 161 157 149

200

172 166 162 150 140 126

Gauss

MgTPP in Ethanol 0.15 0.6 0 0.12 0.07 0.05 0.03 0.02

0.6 0.5 0.42 0.27 0.23

0 0 0 0 0.2

MgTPP in Toluene-Pyridine 0 0.30 0 0 0 0 0 0

0.20 0.12 0.10 0.10 0.10

0 0 0.20 0.30 0.40

MgTPP in Toluene 0.33 0

0.67 0.55 0.50 0.30 0.09 0.02

0.45 0.50 0.70 0.91 0.98

0 0 0 0 0

0.25 0.28 0.43 0.53 0.70 0.55 0.70 0.80 0.88

0.70 0.60 0.50

0 0 0 0 0 0

Flgure 5. Triplet EPR spectra of the monomerlc specles, PL*,,&D,,,), M'hAp,(DPh), and M*,,,&12,,), which participate In the equilibria processes of MgTPP dissohred in ethanol. The magnetic parameters (131,(E(,and g are given in Tables I and 11, except for the line widths AH,(i = X , Y , 2)which were chosen identical (13.0 G) for the three canonical orientations for all three species.

K, the overall spectrum is a superposition of three spectra according to the equilibrium PL*M,(D~~) D*M,I@~) D*M,Z@~)

(21)

where D*?(D4), i = 1,2, are the corresponding dimers as presented y eq 15 and 20, respectively. Below -125 K, the dimerization process, which is orientation dependent16*6*46 (eq 20), is disturbed. Therefore, the contribution of the left-hand side of eq 20, the monomeric species within the aggregate [PL*&%&AG = M*w(D%), to the overall spectrum becomes appreciable. It is expected, therefore, that on further lowering the temperature the EPR spectrum will remain unaltered as indeed was verified in our experiments. I 1 I I I I 1 The ratio [PL2"'PLz]AG/([PL] + [PL"*PL]AG]was de6.0 7.0 8.0 9.0 10.0 termined from the spectrophotometric study (section V). IITX io35( We expect, therefore, to obtain a similar ratio from the Flgure 6. Semilog plot of [D',,(D,)]/[PL",,(D,,)1 (eq 18) vs. 1 / T EPR calculations, namely [D*M,Z(D~)]/([D*M~~(D~) + in toluene (A),toluene-pyrkline (1:2), (O), and ethanol (MI. The ratlo PL*M,(D4h)]). By employing the concentrations of the was determined from the best fit analysls of the experimental curve three species as a free parameter, we have synthesized the as described in the text. triplet EPR spectra between 167 and 125 K (Figure 3). In Figure 5 we show the calculated triplet EPR spectra of The lower trace spectrum, however, (115 K) could be the monomeric subunits which compose the various dimers synthesized only by a superposition of four lines attributed, in MgTPP-ethanol solutions. These monomers are axial respectively, to PL*w(D4h), D*,1(D4), D*~p@4),and (top) and nonaxial (middle and bottom). M*%z(P2h). The addition of the latter species is interTo check upon the basic assumption that equilibria 16 preted in terms of the experimental procedures. On slow or 21 are responsible for the line shape variation with cooling the relative concentration of the biligated species temperature, we have carried out experiments in which increases, and at low temperature (115 K), because of steric ZnTPP or MgTPP were dissolved in solutions containing hindrance, the biligated monomers in the aggregate, pyridine, which is known to be an efficient complexing M*mz(Dzh),will increase in concentration relative to the agent.30-48In Figure 8 we show the EPR spectra of ZnTPP biligated dimers D*w(D4). It should be noted, therefore, that the relative populations [PL*&(D?h)]:[DM,I(D~)]: and MgTPP dissolved in a mixture of toluene-pyridine + [M*~,2(&h]) are close with those from as a function of temperature. The ratio between the {[D*M,Z(~~)] various components of the overall spectrum vs. temperawhich the 125-K spectrum was synthesized (see Table 11). I

ODMR Study of Metalloporphyrins

tures is given in Figure 6 and in Table 111. The main features of the spectral changes are the same as those in toluene without pyridine. This is not surprising if the ligation processes (reactions 11and 16) are the main source for these changes. Because the reactions are shifted to the right on addition of pyridine, the relative concentration of PL (or PL2 for MgTPP) will increase. In terms of the EPR results for ZnTPP, the contribution of PL*Zn(D4h) to the overall spectrum, particularly at high temperatures, will be low as compared to solutions free of pyridine (Figure 6). For MgTPP solutions in the presence of pyridine, the biligation process is achieved at relatively higher temperatures. It is reasonable to assume that the axial species f'L*Mg(D4h) does not contribute to the EPR spectral line shape, and, as in ethanol solutions, M*w(Da) species will also contribute to the spectra. The calculated curves in Figure 8 are best fit lines in which the ratio [D*~,l(D4)]:[?fl*~ ~ ( D ~ ~ ) I : [ D * M Was ~ ~ (chosen D ~ ) I BS the free parameter. $he magnetic parameters and the ratio between the v,arious species at different temperatures are given in Tables 1-111. Comparison between the experimental spectra shown in Figures 2 amd 3 to those in Figure 8 shows that the apparent spectral width in solutions free of pyridine is larger than those obtained in solutions containing pyridine. I t is also noticeable that the spectral width in the latter solutions does not alter with temperature. This is contrary to the behavior of ZnTPP and MgTPP in toluene and ethanol where1 a decrease in the spectral width on increasing the temperature is noticed. This implies that the zfs parameter 101of the porphyrin-pyridine complex is smaller than that of the porphyrin-toluene or porphyrinethanol complexes (see Table I). Moreover, the constancy of 101with temperatures, in the pyridine solutions, indicates that equilibrium 17 has been shifted mostly to the right. From the ratio between the species' concentration we have determined the enthalpy of activation, AH, for reaction 16 in which two species are involved (Figure 6). In MgTPP solutions, where more than two species participate in the equilibria, we plot the ratio A/B vs. 1/T, where A/B denotes the ratio between the different species (Figure 7). In the range of temperatures, where still diffusional motion is allowed, the contribution to the overall spectrum is mainly due to two species (cf. Table 111). Notice that the transition in to luene-pyridine solutions occurs at higher temperature than that in pure toluene. This is in line with the higher melting point of former solutions. The values for AH are in the order of 1000 cm-l which are reasonable for processes of this type. For example, Langelaar et al.46@ calculated equilibrium constants for the reactions of the type A* + A F! Az* (22) where A and AdCare the ground and excited triplet states of aromatic molecules. The enthalpy of activation for these types of reactions were found to be on the order of 400-1200 cm-l. This type of comparison is important ~ ~ i ~the ~ nominal because reaction 22 was s t ~ d i e d below freezing point of the solvent used (ethanol). If reactions 15 and 20 are also affected by the properties of the solvent around its freezing point, one should expect that the EPR spectrum will depend on how equilibrium concentrations have been achieved. The spectral changes shown in Figurcss 2, 3, and 8 are due to a slow rate of cooling. On the other hand, if first the solution's temperature is suddlenly lowered such that diffusional motion is frozen and the EPR spectra are started to be taken on increasing the tlemperature, from about 100 K, the line

The Journal of Physical Chemistty, Vol. 84, No. 3, 1980 331

A

t

60

68 76 V T X lo3 "K

Figure 7. Semilog plot of A/B vs. l/Tin toluene (A), toluene-pyrkjine (1:l) (+), and ethanol (a). For toluene solutions A = [Dour(D4)]and B = [PL*m((D4h)]; for toluene-pyridine and ethanol solutions A = [D"&D3I + [M*klp2(Ddl and B = [De&4)1 + [K't@,h)l. best fit weighing factors for ethanol, toluene, and toluene-pyridine solutions are given in Table 111. For comparison we show the plot where A = [&**.&]A@ B = ([PL] -I- [Pb*k]AQ)for MgTPP in ethanol (a), as taken from the ratio between the absorptions at 420 and 432 nm (cf. Figure 10).

'p

shape variations will obey a different trend compared to those shown in Figures 2,3, and 8. We demonstrate this effect on Mg'I'PP dissolved in ethanol (Figure 9). When this solution was dropped into a temperature of 103 IC, a somewhat different (less resolved) spectrum from the corresponding one shown in Figure 3 was obtained. On increasing the temperature to -145 K, except for line intensity, the line shape was hardly affected. A further increase in temperature resulted in the same spectrum as that obtained by the slow cooling process (cf. Figure 3). This set of experiments, which was found to be completdy reversible, indicates that above -145 K the equilibrium process (eq 21) starts to be operative again. It is noteworthy that this temperature is very close to that chosen by Langelaar et al.4sp4 as the critical temperature in ethanol above which excimers formation is possible. Unfortunately, the low solubility of the metalloporphyrins together with the limited sensitivity of the EPR detection prevented us from carrying out these experiments, as one should anticipate, with varying porphyrins concentrations.

V. Optical Measurements The natural corroboration measurements to the EPR results would be those obtained via optical spectroscopy experiments. At room temperature, in all solvents used, no indication of ground state dimers was noticed over a wide range of concentrations: 10-3-10-6 M. Additionally, only with ethanol the temperature could be lowered below its freezing point without cracking the glass and ruining the transparent properties. We, therefore, performed optical experiments with the metalloporphyrins dissolved in ethanol in order to examine equilibria 11 and 17 and their temperature dependence. In Figure 10 we show the absorption spectra of MgTPP dissolved in ethanol as a function of temperature. Inspection, for example, of the Soret region shows that, between room temperature and -213 K, the spectrum is

The Journal of Physical Chemlstry, Vol. 84, No. 3, 1980

332

A

~

~

Scherz and Levanon

~~

ZnTPP/ TOL- P Y R

Figure 8. Photoexcitedtriplet EPR spectra of ZnTPP in toluene-pyridine (1:2) and MgTPP in toiuene-pyridine (1: 1) at different temperatures. All experimental condltions are as described in caption to Figures 2 and 3.

A

M g T P P - ETHANOL

tant decrease of the 420-nm band. We interpret these changes in terms of eq 11 and 17 which describe the ligation process by applying the results of Miller and Dorough30who claimed that the optical absorption spectra of MgTPP monopyridinate (PL) is almost the same as the absorption spectrum of its parent comWe assume, therefore, that MgTPP in ethanol down to about 203 K is mostly in the monoligated form and the 424-nm band we attribute to PL. On further lowering the temperature, the subsequent reaction PL L e PL2 is taking place and the new absorption at 432 nm we attribute to PL2 in its aggregated form, [PL2-.PL2lAG. At temperatures below 130 K the equilibrium is strongly shifted toward PL2. From the temperature dependence of the absorption spectra, at two wavelengths, we determined the enthalpy of activtion to be -850 cm-l (Figure 7), a value which is close to that obtained from our EPR analysis, 769 cm-’. In Figure 10 we also show the absorption spectra of MgTPP in the &-band region of the spectrum, 650 nm > X > 500 nm. The temperature effect is basically the same as in the Soret band. The changes in the spectra, which are completely reversible, indicate that the equilibrium process may be manifested on each of the vibrational bands Qo2, QO1, and QW27 Thus, on cooling the solution, the room temperature spectrum gradually changes from exhibiting main absorptions at 523, 560 and 600 nm to 535, 572, and 615 nm at 133 K, respectively. By the same argument, given above for the Soret region measurements, we attribute these changes, in the absorption, to the dynamical process given by eq 11 and 17. Inspection of the spectra at low temperatures clearly indicates that the Qol and Qo2 bands are hardly changed in their intensity on lowering the temperature whereas the Qoowhich is red shifted by 15 nm is strongly increased. These new features in the resulting spectrum are typical of “chlorin-like” species.53 In other words, it may indicate that the initial degeneracy in MgTPP dissolved in ethanol

+

-

162 K 2 0 0 Gauss c-------*

I

I

Figure 9. Photoexcited triplet EPR spectra of MgTPP in ethanol at different temperatures. The way in which the temperature of each spectrum was taken is described in the text. All experimental conditlons are as described In the caption to Figure 3. No simulation of these spectra were carried out.

hardly affected. On further lowering the temperature, a new absorption band appears at 432 nm with a concomi-

The Journal of Physical Chemistty, Vol. 84, No. 3, 1980 333

ODMR Study of Metalloporphyrins

400

420

1 500

550

600

I

1

I

1

1

X(nm

I.5

9

0

I

.c

0.5

Figure 10. Optical absorption spectra of MgTPP, dissolved In ethanol, at three temperatures: 300 K (solid curve); 154 K (dashed curve); 133 K (dotted curve). The concentrations of MgTPP are 5.4 X lo-' M for the Q-band region end 1.4 X lo-' for the Soret-band region. X(nm)

400

420

I

I

500 I

550

600

I

I

I .z

I .o 0

0

0.5

Figure 11. Optical absorption spectra of MgTPP, dissolved in various solvents, at 300 K ethanol (solid curve); ethylene glycol (dotted curve; methanol (dashed curve): pyridine (dasheddotted curve). The concentratlons of MgTPP are 6.5 X M for the Q-band region and 2.6 X "IO-' M for the Soret-band reglon.

has been slightly removed. This implies that the low temperature alx3orption spectrum reflects a nonaxial symmetry of MgTPP, which is in accord with our interpretation of the EPR results from which we invoke the existence of nonaxial species of Dahsymmetry that undergoes the dynamic process to produce the photodimers. Unfortunatt?ly, we could not perform these types of experiments with MgTPP dissolved in toluene. Nevertheless, we can make use of X-ray and optical data which indicate that toluene can complex with ZnTPP6 and zinc mesoporphyrin M." In analogy, it is reasonable to assume that toluene can complex with MgTPP as well. As to what extent an additional toluene molecule can interact with MgTPP to form PL2 i s still questionable. Neverthless, we

suggest that also in toluene the axial symmetry is removed through the existence of a ligand field effect, giving rise to the same experimental patterns as in ethanol. To check as to what extent the ligation process in MgTPP to form PL2 occurs in other solvents, we have compared the absorption spectra of MgTPP at room temperature in different solvents. The solvent effect on ithe absorption spectra is quite intriguing and is demonstrated in Figure 11. It is clear that changing the solvents affects the spectrum in a similar way as the temperature does, Le., pyridine > methanol 2 ethylene glycol > ethanol. The temperature effects on the absorption spectrum of ZnTPP dissolved in ethanol are not as strong as in the C i m of MgTPP. This may be due to the fact that ZnTPP does

334

,

,

The Journal of Physical Chemistry, Vol. 84, No. 3, 1980 kinrnl

40,O

420

5pO

5,SO

6r)O

,

Scherz and Levanon

Zn TPP

00.5

2.50

2.40

2.00

I90

1.80

1.70

3 K M l ” 164,

Flgure 12. Optical absorption spectra of ZnTPP (upper trace) dissolved in ethanol at two temperatures: 300 K (solid curve); 103 K (dashed M for the Q-band curve). The concentrations of ZnTPP are 4.5 X region and 1.8 X lo-’ M for the Soret-band region. The lower trace spectra are those of ZnTPC at 300 K (solid curve) and 103 K (dashed curve). The concentrations of ZnTPC are 2.2 X M for the Q-band region and 4.1 X IO-’ M for the Soret-band region.

not complex with more than one ligand.30i48*49 By comparing the absorption spectra of ZnTPP in the Q-band region a t room temperature in benzene30 and ethanol (Figure 12), it is clear that the -600-nm absorption band in the latter solvent is stronger. This may indicate that ZnTPP in ethanol exists, in part, as the monoligated species PL. Decreasing the temperature results in a relatively stronger increase in the -600-nm peak compared to the -570-nm peak (Figure 12). Similar to the case of MgTPP, this enhanced absorption in the -600-nm peak, which occurs mainly around the freezing point of the solvent, may indicate the contribution of a “chlorin-like” structure to the overall spectrum. This implies that the degeneracy Q, and Qy is slightly lifted giving rise to nonaxial species. We correlate, therefore, the spectrophotometric and the triplet EPR results by attributing the absorption changes due to “chlorin-like” species as to the transformation from axial species PL(D4h) to nonaxial species in the aggregated form [pL”*PL]AGor [PL2’*’PL21AG both in ZnTPP and MgTPP. Inspection of the absorption changes, in the Soret region shown in Figure 12, reveals that the main absorption, A,, = 421 nm, at 293 K is red shifted on decreasing the tem= 425 nm. Moreover, below perature to 128 K to A,, 150 K a new absorption appears, as a shoulder, at -418 nm indicating a splitting in the spectrum of the same type as observed in MgTPP. The energy difference, however, between the two absorptions is smaller for ZnTPP as compared to MgTPP, 232 cm-l vs. 550 cm-l, respectively. This splitting at low temperatures is similarly interpreted, as in the case of MgTPP, i.e., the formation of “chlorinlike” species.53 Preliminary results on ZnTPC (zinc tetraphenylchlorin), which obviously lacks a fourfold symmetry, exhibit optical line shape variations with temperature which supports our interpretation. In the lower trace of Figure 12 we show the absorption spectra of ZnTPC at N

200

Gauss U

Figure 13. Photoexcited triplet EPR spectra of ZnTPP in n-octane at 113 K (upper trace) and of ZnTPC in toluene at 157 and 141 K. Except for the microwave power which is 100 mW for the upper trace spectrum, the experimental conditions are as described in caption to Figure 2. The apparent change In the spectral shape and width, in ZnTPC, with temperature is probably due to a superposition of two spectra (monomers and dimers). The ZFS parameters, given in Table I, are those computed from the 157-K spectrum.

room temperature and at 120 K. The splitting of the main absorption band in the Soret region, on decreasing the temperature, is apparent and is almost identical with that of ZnTPP except, of course, for the resolution which is more conspicuous in ZnTPC. The difference in the absorption spectra between the two compounds in the Q spectral region is less conspicuous and probably due to the very low extinction coefficient of the Q transition (-620 nm) in the “chlorin-like” species of ZnP!I‘P. These results are in accord with our EPR results of ZnTPP solutions from which we deduced the existence of species exhibiting D2hsymmetry. If the splitting of the absorption band in the Soret region in ZnTPC and ZnTPP is attributed to an exciton coupling mechanismlgthen an appropriate explanation for such an effect is via the formation of dimers. One should expect, therefore, to observe the corresponding changes in the triplet EPR spectra of ZnTPC on lowering the temperature. This is shown in Figure 13 where the triplet EPR spctrum of ZnTPP dissolved in n-octane is compared to the EPR spectra of ZnTPC dissolved in toluene,55at two temperatures, I t is shown unambiguously that the main contribution to the EPR spectrum of ZnTPP is attributed to species of Dahsymmetry with almost the same zfs parameters, ID1 and lEl, with which we simulated, by a best fit analysis, the EPR spectra shown in Figure 2.% Regarding ZnTPC, as expected, the high temperature spectrum is mainly due to monomers of nonaxial symmetry, whereas the low temperature spectrum reflects a mixture

ODMR Study of IMetalloporphyrins

of two species;, monomers and dimer^.^'

VI. Concluding Remarks We have demonstrated in the present study that ZnTPP and MgTPP, dissolved in glass matrices, can interact to form loose dimers, which undergo dynamic equilibrium of the type monomer F! dimer. This equilibrium process is unique since it proceeds around the freezing point of the solvents in which the metalloporphyrins are embedded. Also, we show that d.ifferent species which undergo dynamic equilibria together with triplet energy transfer are the causes of anomalous triplet EPR line shape of the metalloporphyrin studied herein. We believe that this reaction is general and applicable to other metalloporphyrins, porphyrins, and probably to photosynthetic pigments. The invokement of the above dynamic process, which is temperature dependent, rules out the dynamic JT process as respoinsible for the triplet line shape variation within H single species. This, however, does not exclude the possibility of a JT mechanism as the cause for removing the in-plane molecular degeneracy in inert solvents as was reported for some metalloporphyrins in oriented. crystals of n-a:tane.795u It may well be that at temperatures below 4.2 K the interaction between the monomers are sufficiently low, ko = 0, resulting mainly in species of ,D2h symmetry. An EPR study under these conditions may settle this question. As already ]mentioned, the formation of noncovalently linked dimers has been studied previously, mainly by optical methods, In some cases, however, the formation of these types of dimers may escape optical absorption det e c t i ~ n In . ~ that ~ respect, we demonstrate that triplet EPR line shape analysis provides an efficient tool to study the monomer-dimer equilibrium. The structure of the dimer we deduced from the analysis of the EPR r e d s should be taken cautiously, and further experimental situdies by different techniques and probably some more refined calculations are necessary to determine uniquely the oxact structure and mechanism for the dimerization processes. Acknowledgment. We are grateful to Professor A. D. Adler for supplying mi with the metalloporphyrins for this study. The stimulating discussions with Professors Z. IAZ and R. Lefebvre are highly appreciated. Ms. V. Grebe1 and Mr. 0. Gonen helped us in running the optical measurements and in the curve-fitting analysis, respectively. We are grateful to Professor S. I. Weissman for sending us Dr. A. L. Shain’s thesis. References and Notes (1) In partial fulfillment of the requlrements for the Ph.D. Degree at the Hebrew University of Jerusalem, Israel. (2) J. M. Lhosto, C. Helene, and M. Ptak In “The Triplet State”, A. B. Zahlan, Ed., Cambridge Unlversity Press, New York, 1967, p 487. (3) Z. P. Gribovsi and L. P. Kayushin, Rws. Chem. Rev., 41, 154 (1972). (4) B. M. Hoffman, J . Am. Chem. Soc., 97, 1688 (1975). (5) S. R. Langhoff, E. R. Davidson, M. Gouterman, W. R. Leenstra, and A. L. Kwiram, J. Chom. Phys., 82, 169 (1975). (6) W. H. Schekit, M. E. Kastner, and K. Hatano, Inorg. Chem., 17, 706 (1978). (7) I. Y. Uwn, Mi. 0. Van btp, T. J. Schaafsma, and J. H. Van der Waals, Mol. Phys., 22, 741, 753 (1971). (8) We are grateifulto Dr. Kleibeuker for sending us his thesis: Agricultural Universlty, VVagenlngen, 1977. (9) G. Jansen and J. H. Van der Waals, Chem. Phys. Len., 43, 413 (1976). (10) In thelr anaiysls (ref 2) they employed the semiquantltative analysis of De Grmt et ai.” who Interpretedthe anomalous triplet spectrum of triiphenaieinylium cation. (11) M. S. De &cot, I. A. M. Hesselmann, and J. H. Van der Waals, Mol. Phys., 10, 241 (1965). (12) G. W. Canters, G. Jansen, M. Noort, and J. H. Van der Waals, J . Phys:. Chert., 80, 2253 (1976).

The Journal of Physical Chemistry, Voi. 84, No. 3, 1980 335 (13) 8. M. Hoffman and M. A. Ratner, Mol. Phys., 35, 901 (1978). (14) I t has been reported recently by Chan et al. (Chem. Phys. Len., 81, 465 (1979)) that under certaln experimental conditlons OOMR can be carrled out at temperatures higher than 4.2 K. (15) H. C. Brenner, J. C. Brock, and C. B. Harris, Chem. Phys., 31, 137 (1978). (16) M. Martinaud and Ph. Kottis, J . Phys. Chem., 82, 1497 (18178). (17) A. MacCragh, C. B. Storm, and W. K. Koski, J . Am. Chem. Soc., 87, 1470 (1965). (18) R. F. Pasternack, P. R. Huber, P. Boyd, G. Engasser, L. Francestmi, E. Gibbs, P Faseila, G. C. Venturo, and L. de C. Hinds, J. Am. Chem. Soc., Q4, 4511 (1972). (19) K. A. Zacharlasse and D. G. Whitten, Chem. Phys. Len., 22, 527 (1973). (20) M. Gouterman, D.Holten, and E. Lleberman, Chem. Phys., 25, 139 (1977). (21) J, A. Anton, P. A. Loach, and Govlndjee, Photochem. Photohioi., 28, 235 (1978). (22) R. J. Abraham, F. Eivazi, H. Pearson, and K. M. Smith, J. Chem. Soc.. Chem. Commun.. 698, 699 (1976). (23) L. L. Shipman, T. M. Cotton, J. R. N&ris, and J. J. Katz, Proc. /Vat/. Acad. Sci. U . S . A . , 73, 1791 (1976). (24) F. K. Fona. V. J. Koester, and L. J. Galloway, 4. Am. Chem. Soc., . -99, 2372-(1977). (25) N. Periasamy, H. Linschl, G. L. Closs, and S. G. Boxer, Proc. IW/. Acad. Sci. U.S.A., 75,2563 (1978); N. Perlasamy and H. Linschitr, J . Am. Chem. Soc., 101, 1056 (1979). (26) P. A. Loach In ”Progessin Bioorganic Chemistry", Vol. 4, E. T. KEilser and F. J.-Kerdy, Ed., Wiley, New York, 1978, p 89. (27) For the description and definition of the optical spectra of teitraphenylpphyrins, see,e.g., L. Edwards, D. H. Dolphln, M. M m n , and A. D. Adler, J . Mol. Spectrosc., 38, 16 (1971); see also ref 52. (28) For the desalptkn of the eqmlmentai tedWrLque, see,e.g., H. Lewinon and S. I. Weissman, Isr. J. Chem., 10, 1 (1972). (29) 0. D. Dorough, J. R. Miller, and F. M. Huennekens, J. Am. Chem. Soc., 73, 4315 (1951). (30) J. R. Miller and G. D. Dorough, J. Am. Chem. Soc.,74, 3977 (19152). (31) P. W. Atkins and 0. T. Evans, Mol. Phys., 27, 1633 (1974). (32) A. Baram, 2. Luz, and S. Alexander, J. chem.phys., 64,4321 (19176). (33) H. Sterniictit and H. M. McConnei, J. Chem. Phys., 35, 1973 (19161). (34) A. Hudson and A. D. McLachlan, J. Chem. Phys., 43, 1518 (1985). (35) Y. Marechal, J . Chem. Phys., 44, 1908 (1966). (36) A. L. Shah, Thesis, Washington University, St. Louis, Mo., 1969. (37) J. P. Lemalstre and Ph. Kottis in “Electron Spin Resonance”, 1.. T. Muus and P. W. Atkins, Ed., Plenum, New York, 1972, p 483 and references therein. (38) R. H. Clarke, R. E. Connors, H. A. Frank, and J. C. Hoch, Chlem. Phys. Len., 45, 523 (1977). (39) J. Kaplan, J . Chem. Phys., 28, 278 (1958). (40) S. Alexander, J. Chem. Phys., 37, 967, 974 (1982). (41) W. A. Yager, E. Wasserman, and R. M. Cramer, J . Chem. Phys., 37, 1148 (1962). (42) Such a behavior has recentfy been verified where a continuous change from an axlal to nonaxial EPR spectrum of (cis,c/s-1,3,5-triaminocyclohexarre)copper*+ has been observed: J. H. Ammeter, H. B. Burgi, E. GAmp, V. Meyer-Sandrin, and W. P. Jansen, Inorg. Chum., 18, 733 (1979). (43) These values, for A€, have been arbitrarily chosen. However, they cover a wide range of activation energies which may be responelble for the dynamical processes described here. (44) J. Fer-, J. Chm. Phys., 44,2877 (1966), and references themin. (45) J. Langelaar, R. P. H. Rettschnlk, A. M. F. Lambroy, and G. J. Hoyijnk, Chem. Phys. Len., 1, 609 (1968). (46) J. Langelaar, 0. Jansen, R. P. H. Rettschnk, and 0. J. Hoytink, Chsm. Phys. Lett., 12, 86 (1971). (47) I t has recently been reported that in PMMC triplet-triplet annihilation proceedsvia a d h b e mechanism at elevated temperaturesaround 300 K. F. E. El-Sayed, J. R. MacCaiium, P. J. Pomery, and T. M. Shepherd, J. Chem. Soc., Faraday Trans. 2 , 1, 79 (1979). (48) C. Klrsky and P. Hambrlght, Inorg. Chem., 9, 958 (1970). (49) 0. C. Vogel and B. A. Beckmann, Inorg. Chem., 15, 483 (1975), and ref 3 therein. (50) M. Werman, 6. S. Yamanashi, and A. L. Kwiram, J. Chem. phys., 58, 4073 (‘1972). (51) One should not completely exclude the posslbllity of a temperature dependency of ko, Le., lncreaslng of the exciton hopping rate wlth temperature. However, dissociation of the dlmerlc form (eq 10 or 21) on lncreaslng the temperature Is probably the &mhantprecess. (52) However, since MgTPP In solution does not exist in L unllgated form, we assume that In our experiments the monoligatedspecks, wtilch already exists at room temperature, is MgTPP-ethanol. I n the experiments of Miller and Dorough3Othe parent compound Is rniost probably MgTPP.H20. The addition of ethanol to soiutlons of zinc and magneslum porphine In n-octane are known to affect the high resolution optical spectra and are dlscussed by G. Jansen and M. Noort, Spectrochim. Acta, Part A , 32, 747 (1976). (53) K. M. Smlth in “Porphyrins and Metalbporphyrlns”, K. M. Smith, Ed,, Elsevier, New York. 1975, p 25. The absorption spectra of MgTPC and ZnTPC at room temperature are glven in ref 30.

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J. Phys. Chem. 1980, 84, 336-338

(54) A. H. Corwin, D. G. Whitten, E. W.Baker, and G. Kleinspehn, J. Am. Chem. Soc., 85, 3621 (1963). (55) Because of the low solubility of ZnTPC in n-octane we show its EPR

triplet spectrum in toluene only. (56) Notice that part of the upper trace spectrum of Figure 13 is due to the photodimers, arid the unlabeled peaks between Y and X canonical

orientations are due to the exchange process. (57) Work in progress in this laboratory. (58) J. A. Kooter and J. H. Van der Waals, Mol. Phys., 37,997 (1979). (59) It was Indicated in ref 19 that no signs of dimer formation were noticed when solutions of ZnTPP, dissolved in methyicyclohexane, were coded to 140 K.

Infrared Spectra of NO Adsorbed on a Low-Area Pt Surface in the Presence of High-pressure Gas-Phase NO Douglas S. Dunn, W. G. Golden, Mark W. Severson, and John Overend” Department of Chemistty, University of Minnesota, Minneapolis, Minnesota 55455 (Recelved July 13, 1979)

An adsorbate has been established on low-area Pt surfaces by exposing them to gas-phase NO at pressures of 50 torr. Infrared spectra of the adsorbate have been recorded in the wavenumber range 3600-1000 cm-l and spectral features have been observed in the region expected for the NO stretching mode (ca. 1700 cm-’ and also at ca. 1335 cm-’). When the pressure of the gas-phase NO is reduced in stepwise fashion to lo-* torr, the spectra change significantly. Spectra of an adsorbate formed when a Pt surface is in equilibrium with gas-phase N20 are also shown.

Although several laboratories, including our own, have Recognizing that IRRAS can be applied at relatively previously reported the measurement of the infrared rehigh pressures, we have concentrated on the development flection absorption spectra (IRRAS) of adsorbates on of IRRAS as a technique for the study of molecular species low-area metal surfaces,l-’ these measurements have fopresent on a catalytic surface under engineering conditions. cused almost exclusively on the study of adsorbed CO. It In a series of experiments preliminary to a study of the catalytic oxidation of ammonia on a Pt surface, we have is generally apparent that CO adsorbed on a metal surface has an unusually large oscillator strength. In the spectrum measured the IRRAS spectra of an adsorbate formed from of CO on Pt(ll1) reported by Shigeishi and King1 absorgas-phase NO at pressures of 50-10-5 torr. bances as high as 6% at the CO band maximum were The spectrometer used for these experiments is the same measured at saturated coverage. There may, indeed, be as that described in ref 6 and 7. The sample cell, configured as a stirred-tank reactor, is constructed from 316 other surface species which have oscillator strengths as stainless steel. The windows and the inlet and outlet ports large as that of CO, but we intuitively expect most surface are sealed to the body of the cell with Viton “0” rings. All species to give much weaker absorption bands. Over the inlet and outlet lines are trapped at either liquid N2 or past year we have been able to achieve substantial imdry-ice acetone temperature. The substrate, a piece of Pt provements in both the signal-to-noise ratio and the dyfoil 5 mm X 25 mm, was spot welded to Pt rods which namic range of our IRRAS apparatus with the result that were, in turn, connected by stainless steel sleeves to we are now able to measure absorbances as low as loa with tungsten feedthroughs. The Pt foil could be heated rean integration time of a few seconds per resolution element. sistively. Substrate temperatures were measured with a This increase in instrumental sensitivity has significant Pt-Pt-Rh thermocouple spot welded to the foil. The implications since we are now able to measure bands in cleaning procedures we adopted for these experiments were the IRRAS spectra of adsorbates with absorbances of similar to those described previ~usly.~ The foil was heated between 0.1 and 0.01%. This gives us the ability to observe at 1300 K for about 15 min in lom3torr of O2 to produce IRRAS spectra of a wide variety of adsorbate structures. an oxide surface. This was further treated by heating at Of course, the IRRAS experiment remains restricted by 650 K in torr of H2for 15 min to produce a reduced the perpendicular dipole selection rule and we expect to surface, presumably with a hydrogenic adlayer. The see only those transitions which have a component of the cleaning procedure was repeated prior to each set of exelectric dipole transition moment parallel to the surface periments. Most of the measurements were made on a normal. well-annealed Pt foil which is expected to be mostly The present sensitivity of our IRRAS measurements polycrystalline Pt(ll1). A few experiments were carried approaches that attainable in electron energy-loss specout before the foil had been annealed. troscopy (ELS). The two techniques are, however, comIn each experiment the spectrum of the reduced Pt plementary; because of instrumental limitations our IRsurface was first scanned and the spectrum was stored RAS measurements cannot be made at low wavenumbers digitally. This background was subtracted from subse(