Nonphotochemical Hole Burnw and Isoabsorptive Behavior of

466

J . Phys. Chem. 1985,89, 466-470

Nonphotochemical Hole Burnw and Isoabsorptive Behavior of Resorufin in Poly(methy1 methacrylate) Matrices A. F. Childs and A. H. Francis* Department of Chemistry, University of Michigan. Ann Arbor, Michigan 48109 (Received: April 23, 1984) Nonphotochemical hole-burning and thermal-annealing experiments have been conducted on the ionic dye resorufin in a matrix of poly(methy1 methacrylate) (PMMA). Hole-refilling measurements confirm the nonphotochemical nature of the hole-burning process in this material. The temperature dependence of the line shape of the nonphotochemical spectral hole was determined over the temperature range 2-77 K. The experimentaldata suggest that, under the conditions of the experiment, the line shape of the hole is determined predominantlyby spectral diffusion. Isoabsorptive points were observed in the absorption spectrum of the sample during both hole burning and thermal annealing of spectral holes. A model is proposed for the dynamical behavior of resorufin in the vitreous PMMA polymer matrix. The model exhibits isoabsorptive behavior for certain special relationships between the dynamical parameters.

Introduction The number of systems for which nonphotochemical hole burning (NPHB) has been demonstrated is rapidly increasing, and certain common features of the phenomenon are becoming apparent.' Results from the many NPHB experiments conducted to date and discussion of the microscopic theory of NPHB are contained in several recent reviews of the field.'-3 The predominant interest in NPHB has been for line width studies of anomalously fast optical dephasing or the T , and T2 lifetimes of the photoexcited guest absorbers at low temperatures. As the temperature of the matrix increases, line widths may be observed to increase reversibly, reflecting an increase in the rate of optical dephasing p r o c e s s e ~ .In ~ general, experimental observations of the temperature dependence of the line width of holes burned in the absorption spectrum of guest molecules in vitreous matrices have been found to increase as T with 1 < n < T4-* Because NPHB is found in plastic matrices, special difficulties are encountered in measuring the temperature dependence of nonphotochemical holes, and the details of the experimental method used are important to the interpretation of the results ~ b t a i n e d . ~ The - ~ plastic nature of the host matrix gives rise to processes which lead to irreversible line shape changes not related to optical dephasing. These latter processes have been termed "spectral diffusion" and are associated with the ability of the lattice to anneaL9 The present work is concerned in its entirety with the role of spectral diffusion in the temperature dependence of nonphotochemical holes burned in resorufin-doped poly(methy1 methacrylate) (PMMA). Several characteristic features of the nonphotochemical hole-burning line shapes in this sample material suggest a detailed dynamical model for the NPHB process. Experimental Section Thin films of PMMA containing the ionic dye resorufin were prepared by using the procedures described by Marchetti and co-workers.loJ' Resorufin was dissolved to saturation in 40 mL of a basic acetone-methanol (1:l) mixture containing a few drops of hexafluoro-2-propanol. Medium molecular weight PMMA (2.0 (1) Small, G. J. In "Spectroscopy and Excitation Dynamics of Condensed Molecular Systems"; Agranovich, V. M., Hochstrasser, R. M., Eds.; NorthHolland: Amsterdam, 1983; p 515. (2) Haarer, D. Angew. Macromol. Chem. 1982, 109, 267-284. (3) Rebane, L. A,; Gorokhovskii, A. A,; Kikas, J. V. J. Appl. Phys. 1982, 298, 235-250. (4) Hayes, J. M.; Stout, R. P.; Small, G. J. J . Chem. Phys. 1981, 74,4266. (5) Thijssen, H. P. H.; Volker, S.; Schmidt, M.; Port, H. Chem. Phys. Lett. 1983, 94, 537. (6) Cuellar, E.; Castro, G. Chem. Phys. 1981, 54, 217. (7) Friedrich, J.: Wolfrum, H.; Haarer, D. J. Chem. Phys. 1972, 77, 2309. (8) Thijssen, H. P. H.; Dicker, A. I. M.; Vblker, S.Chem. Phys. Lett. 1982, 92, 7. (9) Friedrich, J.; Haarer, D.; Silbey, R. Chem. Phys. Letf. 1983, 95, 119. (10) Marchetti, A.; Scozzafave, M.; Young, R. Chem. Phys. Lett. 1977, 51, 424. ( 1 1) McColgin, W.; Marchetti, A.; Eberly, J. J . A m . Chem. SOC.1978, 100, 5622.

0022-3654/85/2089-0466$01 SO10

g) was added to 40 mL of acetone and stirred vigorously to ensure complete dissolution. The saturated dye solution (0.8 mL) was then added to the PMMA solution, and the mixture evaporated to a final volume of 10 mL. Thin films were prepared by coating the polymer solution on the walls of its container and allowing the film to dry for 12 h. The film samples were cooled in a Janis 10 DT liquid-helium cryostat. Absorption spectra were obtained by focusing a lowpower probe beam from a tungsten quartz-iodine lamp through the sample film and dispersing the transmitted light with a I-m Jarrel-Ash scanning monochromator. Temperature measurements were made with a 100-ohm Allen-Bradley resistor which had been calibrated against a commercial germanium resistance thermometer. Spectral holes were burned with a Chromatix CMX-4, flash lamp pumped, dye laser with rhodamine 590 dye. An intracavity etalon was used to narrow the output of the laser to 0.15 cm-I. The holes were burned for 15-20 min at an average power of 10-100 mW/cm2. Other experimental details of the hole-burning and thermal-annealing measurements are given in Figures 3 and 4.

Results The first hole-burning experiments on resorufin in PMMA were conducted by Marchetti and co-workers,'O*''who were able to bum narrow, persistent holes in the inhomogeneously broadened 0-0 transition of the dye at approximately 5850 A using a C W dye laser (O.Ol-cm-' line width). Holes were burned at 1.8 K and were accompanied by vibronic and phonon sideband structure. The hole line widths were about 0.2-cm-' full width at half-maximum (fwhm) and increased with longer irradiation times. The narrowest hole line widths observed in this study (see Figure 2) were produced by burning at the lowest temperatures attainable in our Dewar (1.8 K) and were about 6-cm-' fwhm. This is considerably greater than the 0.2-cm-' f w h holes reported in ref 10 but similar in magnitude to hole line widths observed in studies using materials closely related to the chemical structure of resorufinI2 and values reported from other studies using pulsed laser source^.'^ The difference may be partially or entirely due to the use of a pulsed laser rather than a C W laser.6 The hole line widths were independent of power down to the lowest power levels for which holes could be prepared (-1 mW/cm2). The absorption spectrum of resorufin in PMMA shown as the lower trace in Figure 1 exhibits prominent absorption bands centered at 5860,5675, and 5400 A. The upper curve illustrates the background irradiance of the tungsten-iodine lamp used to obtain the spectrum. When the sample was laser irradiated and the absorption spectrum again recorded, an intense spectral "hole" appeared at 5851 A, coincident with the wavelength of laser (12) Carter, T. P.; Fearey, B. L.; Hayes, J. M.; Small, G. J. Chem. Phys. Lett. 1983, 102, 272. (13) Jankowiak, R.; Bassler, H. Chem. Phys. Letf. 1983, 95, 310.

0 1985 American Chemical Society

The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 461

Resorufin in Poly(methy1 methacrylate) Matrices

HOLE ANNEALING

kYELV"

5850

5800

,

I

I

3500

I

I

I

5000

WAVELENGTH

6500

(A)

Figure 1. Absorption spectra of resorufin in PMMA: (top) spectrum of the background source,(bottom) absorption spectrum a t 4 K, and (center) absorption spectrum after N P H B a t 5950 A.

* ORIGIN HOLE

-zI k-

,PRE-BURN

I

5850

5800

I 5900

WAVELENGTH I R 1

Figure 2. Hole spectral profile and base-line absorption for resorufin in PMMA. The hole was burned and spectra were recorded a t 2 K.

~

I

HOLE BURNING

F t

Ln

z

+ w z

e

5850

Figure 4. A family of absorption spectra showing the region of the spectral hole taken a t different temperatures during the thermal-annealing process.

fin/PMMA.14 Substantial refilling (50%) of an initially burned hole could be achieved by burning a second hole up to 100 cm-' removed from the first to either higher or lower frequencies. Similar results have been reported for NPHB in other materials.14 After thermal cycling of the irradiated sample to temperatures above 100 K, the original base-line spectrum was obtained to within experimental error (1%). Moreover, the hole burningthermal annealing cycle can apparently be repeated indefinitely without a measurable loss of sample absorbance. These observations are consistent with a NPHB process although they do not rigorously exclude a reversible photochemical process. A series of spectra taken at constant temperature (5 K) during sample irradiation reveal an interesting feature of NPHB in the resorufin/PMMA sample. The spectra shown in Figure 3 were obtained by repeated scans of the inhomogeneously broadened electronic origin at different times during the hole-burning process. The burn wavelength corresponds to the X = 5847 A maximum in the family of curves illustrated. An isoabsorptive point is observed at XI = 5827 8, (vI = 17 161 cm-'). A second isoabsorptive point, not shown in the figure, is found to longer wavelengths of 5900 8, (see Figure 2). After a change of approximately 50% in the peak absorbance at the burn wavelength, irradiation was terminated and an additional family of spectra was recorded as the sample temperature was raised from the temperature at which the hole was prepared (Figure 4). Thermal-annealing results in refilling of the spectral hole, but in such a manner that the isoabsorptive points are preserved. Note that these points necessarily lie along the original absorption curve of the inhomogeneously broadened resorufin absorption spectrum. The primary focus of this work is on the requirements for formation of isoabsorptive points in the spectra of nonphotochemical spectral holes in vitreous matrices.

Discussion The inhomogeneously broadened spectral line shape I( v) may be written as a superposition of homogeneous line shapes

ORIGIN BAND

5800

5900

WAVELENGTH [ A i

I

5900

WRVELENGTH 1 R l

Figure 3. A family of absorption spectra showing the region of the spectral hole taken a t different times during the hole-burning process.

irradiation. In addition to the 5851-%1hole, vibronic structure can be seen at 5664 A and from 5300 to 5500 A. The line width of a spectral hole burned at 2 K was found to increase completely irreversibly with sample temperature up to 20 K. Therefore, the line width of the spectral hole in resorufin/PMMA is determined predominantly by spectral diffusion processes and not by optical depha~ing.~ Hole-refilling experiments were performed to confirm the nonphotochemical nature of the hole-burning process in resoru-

I(v) =

CNzgt(4 I

(1)

where Niis the number of sites of the ith type with homogeneous line shape gi(v) and the summation extends over all m sites. Equation 1 may be rewritten a s

I(v) = NT.g(v) (2) where NT is the transpose of an m-dimensional column vector N whose components are the number densities of each site and g is an m-dimensional column vector whose components are the (14) Carter, T. P.; Fearey, B. L.; Hayes, J. M.; Small, G. J. Chem. Phys. Left. 1983, 102, 212-216.

468

The Journal of Physical Chemistry, Vol. 89, No. 3, 1985

homogeneous line shape functions of each site. During nonphotochemical hole burning and thermal annealing, the inhomogeneous line shape function will evolve in time due to the time variation of the site population vector N(t). We adopt a model in which the site interconversion is described by a system of coupled, first-order, homogeneous differential equations. Thus, the population of the ith site is given by the master equation dNi/dt = -CkijNi(t) I

+ CkjiNj(t) J

(3)

Childs and Francis

D = T‘KT

(11)

The solution of eq 4 corresponding to the j t h eigenvalue dj of K is Ni”(t) = Tijnj(0) exp(djt)

(12)

where Tuis the ith component of thejth eigenvector. The general solutions of eq 4 are linear combinations of the form Ni(t) = CTi,nj(0)exp(djt)

(13)

The site interconversion rate constants are defined such that kij is the rate constant for conversion of site i to site j. For the entire system of sites we may write the master equation in matrix form as

The ni(0) depend upon initial conditions and may be determined from the relation

N=KN

n(0) = T ’ N ( 0 )

(4)

where N is an m-dimensional column vector whose elements are the time derivatives of the site populations, Ni, and K is an m X m rate constant matrix whose elements are Kii = -Ckij j

and Kij = kji

(5)

For nonphotochemical hole burning, we shall require that the dynamical system be “closed” so that the total population of all sites is constant. Additionally, the condition that det K = 0 ensures that a t least one equilibrium solution of eq 4 e x i ~ t s . ’ ~ J ~ We are specifically interested in isoabsorptive points (vI) in the inhomogeneous line shape function during NPHB and thermal annealing. For these points

J

(14)

If the dynamical system obeys detailed balance, then the eigenvalues of K a r e real and n ~ n p o s i t i v e . ’An ~ ~ equilibrium ~~ solution is one whose eigenvalue is zero. The requirement that det K = 0 ensures that at least one zero eigenvalue and therefore one equilibrium solution exists. We designate the eigenvalue of the equilibrium solution dE = 0 and its corresponding eigenvector T(0. For initial conditions corresponding to displacement from equilibrium along the direction of a single (kth) eigenvector

njzk(0)= 0 and nk(0) # 0

(15)

and the solution of eq 4 is simply N(t) = T(&)nk(0) exp(dkt)

(16)

Substitution of this result into eq 8 yields

(T(k))T.g(vI)nk(0)dk exp(dkt) = 0 From eq 2 , we have that Therefore, we identify two sources of variation in the inhomogeneously broadened line shape function. The first term in eq 7 represents line shape changes due to changes in site occupancy. In the vector notation employed here, this corresponds to a change in the orientation of the site population vector with time. This process is termed “spectral diffusion”. The second term in eq 7 represents the effect on the inhomogeneous line shape of changes in the homogeneous line widths of the component site line shape functions which might be caused by variations in the relaxation time of photoexcited sites. This effect has been of major interest in NPHB studies. We are interested here in pure “spectral diffusion” or line shape changes dominated by the first term of eq 7. If g = 0, then at an isoabsorptive point

i(v,) = NT.g(vl) = 0

(8) This condition is trivially satisfied for the equilibrium orientation of the site population vector (N = NE) since NE = 0 at each frequency in the absorption band (0 is a null vector); it is also satisfied trivially at all times for frequencies outside the absorption band where g = 0. We are interested in the existence of isoabsorptive points within the inhomogeneous absorption line shape for arbitrary directions of N. To examine the nontrivial requirements for isoabsorptive behavior, we carry out a linear transformation to a new coordinate system in which eq 4 is in diagonal form n = TIN (9) T i s an m X m matrix whose columns are eigenvectors of K . In the transformed coordinate system, eq 4 becomes

n=Dn (10) D , an m X m diagonal matrix whose elements are the eigenvalues of K , is given by (15) Hearon, J. Z. Bull. Math. Biophys. 1953, 15, 121. (16) Rosen, R. “Dynamical Systems Theory in Biology”;Wiley-hterscience: New York, 1970.

(17)

-

This is satisfied trivially at all times for the equilibrium eigenvector (dk = dE = 0), outside the absorption band (g = 0), and as t m for all k # E (dk < 0). Nontrivial solutions are obtained from the requirement that

(T(k))T*g(vI) =0

(18)

Since CiT/k)= 0 for all k # E, it is easy to show that the mean value of eq 18 is zero. Therefore, there must exist at least one value of vI for which eq 18 is satisfied. Thus, displacements from equilibrium along a single eigenvector of K yield at least one isoabsorptive point. Physically, this occurs because for these displacements all site populations evolve in time in fixed relation to one another. Displacement of the site population along directions other than eigenvectors of K will, in general, not produce isoabsorptive points. The Two-Site System. Isoabsorptive points are frequently observed in the spectra of binary mixtures (isosbestic points) which are functionally equivalent to a two-site model.” For two sites, K is a 2 X 2 matrix with two eigenvalues ( d , and d2) and two eigenvectors (T(’)and T(2)). One eigenvalue and one eigenvector = TE); must correspond to the equilibrium condition (d, = dE, T(’) thus, the only possible displacements from equilibrium correspond to pure eigenvector displacements for which n2(0) # 0. For the two-site model, T(2)may be written without loss of generality as T(2) = (1,-1) (19) The isoabsorptive condition (eq 18) becomes (1,-1)($;) = 0

(20)

or gl(v) = gz(v). Thus, the only requirement for an isoabsorptive point at v l in the spectrum of a two-component (two-site) system is that the individual component line shape functions intersect. Each point of intersection will correspond to an isoabsorptive point in the spectrum of the twc-component system. Note that this result is independent of the values of the elements of K . It is clear that, for three or more components, simultaneous displacement of the site population vector along two or more (17) Hirt, R. C.; King, F. T.; Schmitt, R. G. A n d . Chem. 1954, 26, 1270.

The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 469

Resorufin in Poly(methy1 methacrylate) Matrices

+

eigenvectors becomes possible (nk(0) # 0 and nj(0) 0). For these displacements, isoabsorptive behavior will not be observed, since the site populations will relax to the equilibrium site distribution (NE)with a compound time constant and the individual site populations will not be linearly related to each other at all times during the return to equilibrium. We conclude that a two-site system may easily exhibit isoabsorptive points while systems with more than two components will not exhibit isoabsorptive points since displacement from equilibrium, in general, will not occur along the eigenvectors of K . The Multisite Lumped System. The resorufin/PMMA sample is a multisite system which exhibits behavior characteristic of a two-site system: isoabsorptive points in the absorption spectrum for arbitrary displacements from equilibrium during hole burning and thermal annealing. It follows that the experimental results may be described by a master equation similar to eq 4 but of lower dimension

N=m

(21)

where N is a two-dimensional column vector obtained from the original m-dimensional concentration vector N by a “proper lumping” of its elements.1s-21 In this procedure, the m-tuple cpmposition vector N is transformed into a two-dimensional vector N by means of a 2 X m matrix M of rank 2. That is

Figure 5. A four-site model illustratingthe lumping procedure. Relative first-order rate constants for site interconversion are given.

although it is not intuitively obvious, we may replace the four-site system with the lumped two-site system shown. The lumping is exact and independent of the values of the undesignated rate constants. For any initial values of the concentrations, sites 1 and 2 will approach their equilibrium values maintaining constant proportion to one another; sites 3 and 4 will behave in a similar fashion. Thus, the absorption spectrum of the four-component mixture will exhibit an isoabsorptive point at all uI for which

N=MN

The matrix M lumps all the sites in the system into two sets depending upon whether the site population decreases (set A ) or increases (set E ) upon displacement of the site population vector from equilibrium. Stationary sites do not contribute to the dynamical behavior of the inhomogeneous line shape and are therefore excluded from consideration. The matrix M has elements mai = 1 if the ith site belongs to the a set, and 0 otherwise. An m-dimensional system described fully by eq 4 is exactly lumpable by a matrix M if and only if there exists a matrix K such that the kinetic behavior of the lumped system can be described by eq 21. Wei and Kuo18 have shown that for a first-order kinetic system to be exactly lumpable it is necessary and sufficient that

MK = KM

(23)

It may be shown2’ that this is equivalent to the requirement that

Thus, in order for the M lumping to be exact, the sum of all interset rate constants connecting any single site of the A set with sites belonging to the E set must a constant ( k A B ) for all members of the A set. A similar relation holds for any member of the E set. For the lumped system, the condition for formation of an isoabsorptive point at uI (eq 18) becomes where g is a two-component vector whose elements, g, and gg, are the line shape functions of the A set and E set, respectively. Isoabsorptive points will be observed for values of ul such that &(VI) = &(VI). Since g = Mg, the requirement for the formation of an isoabsorptive point at vI is that

A simple numerical example is useful for illustrative purposes. Consider the four sites interconnected by first-order rate constants as shown in Figure 5 . The rate constants chosen satisfy eq 24 if we lump site 1 with 2 (set A) and site 3 with 4 (set E). Then, (18) (19) (20) (21)

383.

Kuo, J. C. W.; Wei, J. Ind. Eng. Chem. Fundam. 1969, 8, 124. Wei, J.; Kuo, J. C. W . Ind. Eng. Chem. Fundam. 1969, 8, 114. Ozawa, Y.Ind. Eng. Chem. Fundam. 1973, 12, 191. Grimmelmann, E.K.;Lohr, Jr., L.L.Chem. Phys. Lett. 1978, 54,

In a similar manner, the spectrum of the resorufin/PMMA sample, which is fully described by an m-dimensional concentration vector, is divided into the spectrum of the depleted sites (set A ) and the sites whose population is increased (set E ) . Note that the location of an isoabsorptive point is independent of the individual values of the elements of K . This is in agreement with the observation that hole burning and thermal annealing produce identical isoabsorptive points although the dynamical processes are described by different Kmatrices. The location of isoabsorptive points depends only upon the site lumping which is evidently determined during the hole-burning process and is, therefore, the same for the annealing process.

Summary and Conclusion Nonphotochemical hole-burning and thermal-annealing experiments have been performed on the ionic dye resorufin in a PMMA matrix. Two isoabsorptive points were observed in the absorption spectrum of the dye during the hole burning and thermal annealing. This is an unexpected result, since it is generally held that isoabsorptive points are a characteristic of the spectra of a two-component mixture and that hole burning is a characteristic of a multisite sample. Therefore, we have examined conditions necessary for the formation of an isoabsorptive point using a first-order kinetic model of the dynamical system of interconverting sites in a vitreous matrix and concluded the following: (1) An isoabsorptive point is possible only if the dynamical system is closed. Thus, the observation of an isoabsorptive point is consistent with the nonphotochemical nature of the hole-burning process in PMMA/resorufin. (2) An isoabsorptive point is easily formed in a two-component system but is also possible in a multicomponent system for pure eigenvector displacements of the site population vector. This, however, is a situation not likely to be realized experimentally. (3) The PMMA/resorufin sample exhibits characteristics of a two-component system in its dynamical behavior. Thus, the absorption spectrum can be fully described by a two-dimensional master equation. (4) The first-order rate constants which parameterize the dynamical system during the hole burning or thermal annealing must satisfy the following relations for all levels of illumination and sample temperature investigated in order for the reduction in dimensionality to occur:

c kji = k ,

iEA

for all j E B

J. Phys. Chem. 1985,89, 470-474

470

Equations 27 and 28 establish the most general relationship between the phenomenological first-order rate constants of the dynamical system of interconverting sites which will result in a reduction in the dimensionality of the master equation to 2-fold. The question naturally arises as to the nature of the microscopic physical processes associated with hole burning and thermal annealing which will result in eq 27 and 28. Any phenomenological kinetic model admits a number of possible mechanisms. We discuss only one possible mechanism here. The rate constants k , which appear in the master equation are associated with a variety of processes for interconversion between sites, including classical thermally activated processes, quantummechanical tunneling, and photoassisted processes. If we neglect tunneling events and assume that at sufficient low ambient temperatures all thermally assisted processes are quenched in the resorufin/PMMA sample, then none of the sites populated (set

B ) during hole burning reconvert to a site belonging to the depleted set (set A ) . Thus, all kji = 0 and eq 28 is trivially satisfied. To satisfy eq 27, we must make a further assumption about the nature of the hole-burning process. If it is assumed that the high instantaneous energy density associated with the use of a pulse laser for hole burning produces highly energetic sites in which all of the initial intermolecular structural information which characterized the initial site is lost, then it follows that the sum of all rate constants for the decay of these sites into the distribution of final sites (set B ) must be a constant for all members of the set. If the relaxation event is the rate-determining step, then eq 27 is satisfied. Further experiments are currently in progress to test this hypothesis by using low-power C W sources for hole burning in resorufin/PMMA. Registry No. PMMA (homopolymer), 901 1-14-7; resorufin, 635-78-9.

Laser Photolysis Studies on the Electron-Transfer Reaction from the Photoexcited Triplet State of Chloroindium( I I I ) Tetraphenylporphyrin to Methylviologen in Methanol Solutions Mikio Hoshino,* Hiroshi Seki, Solar Energy Research Group, The Institute of Physical and Chemical Research, Wako, Saitama 351, Japan

and Haruo Shizuka Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received: May 15, 1984; In Final Form: October 3, 1984)

Laser photolysis studies were carried out for chloroindium(111) tetraphenylporphyrin, ClIn"'TPP, in methanol solutions. The triplet states of (1n"')'TPP and methylviologen, MV2+,were found to form a triplet exciplex with an association constant of 6.5 X IO2 M-I. The triplet exciplex partly dissociatesto the cation radical of (In"')'TPP, [(In"')+TPP+.], and methylviologen cation radical, MV'., followed by the back electron transfer from MV+. to [(In"')+TPP+.] to regenerate MV2+and (1n"')'TPP. The triplet exciplex reacts with triethanolamine,TEA, presumably to produce a new triplet exciplex, 3[(In111)+TPP(TEA)(MV2+)], in which a TEA molecule is considered to occupy the axial position. No ionic dissociation from this triplet exciplex was M MV2+gives rise to the formation observed. Photolysis of the methanol solution of CIIn"lTPP containing 0.5 M TEA and of MV+. as a final product. The absorption spectroscopic study revealed that C1In"'TPP in a methanol solution at 0.5 M TEA is transformed to [In111TPP(TEA)2]+[C1-], in which two TEA molecules are located in the axial positions. On the basis of the laser photolysis study the triplet state of [In"'TPP(TEA),]+ is confirmed to undergo efficient electron transfer toward MV2+, resulting in the formation of MV'..

Introduction The excited states of metalloporphyrins in solutions have long been recognized to undergo the electron-transfer reaction in the presence of suitable electron Particular attention has been paid to electron transfer between the photoexcited porphyrins and quinones in order to elucidate the primary photochemical processes in photosynthesis.5* Recently solar energy

utilization has become one of the important subjects of photochemistry. From this viewpoint metalloporphyrins serve as useful photosensitizers because they efficiently absorb visible light from the s ~ n . ~ - I * Metalloporphyrins in the ground or the excited state are known to form molecular complexes with electron donors or acceptor^.'^-'^ A few studies have been carried out on the role of these molecular complexes in photochemistry.'6*17 For example, the triplet states

(1) Quinlan, J. J. Phys. Chem. 1968, 72, 1797-1799. (2) Holten, D.; Windsor, M. W.; Parson, W. W.; Gouterman, M. Photochem. Photobiol. 1978, 28, 951-961. (3) Seely, G. R. Photochem. Photobiol. 1978, 27, 639-654. (4) Shiozawa, M.; Yamamoto, H.; Fujita, Y. Bull. Chem. SOC.Jpn. 1977,

(9) Harriman, A.; Porter, G.; Richoux, M. C. J. Chem. SOC.,Faraday Trans. 2 1981, 77, 833-844. (10) Okura, I.; Thuan, N. K. J. Chem. SOC.,Faraday Trans. 1 1980.76,

50. 2177-2178. -- -

2209-221 1. .. __..

- - .( 5-)-Netzel, T. L.; Bergkamp, M.A.; Chang, C. K. J. Photochem. 1981, 17, 451-460. (6) Lever, A. B. P.; Ramswamy, B. S.; Licoccia, S. J. Photochem. 1982,

-19 - , -171-1112 . - - - -.

(7) Netzel, T. L.; Bergkamp, M. A,; Chang, C. K. J. A m . Chem. SOC. 1982, 104, 1952-1957. ( 8 ) Migita, M.; Okada, T.; Mataga, N.; Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S . Chem. Phys. Lett. 1981, 84, 263-266.

(11) Matsuo, T.; Ito, K.; Takama, K. Chem. Lett. 1980, 1009-1012. (12) Kiwi, J.; Gratzel, M. J. Am. Chem. SOC.1979, 101, 7214-7217. (13) Gouterman, M.; Stevenson, P. E. J. Chem. Phys. 1962, 37, 2266-2269. (14) Roy, J. K.; Carroll, F. A.; Whitten, D. G. J. Am. Chem. SOC.1974, 96, 6349-6355. (15) Barry, C. D.; Hill, H. A. 0.;Mann, B. E.; Sadler, P. J.; Williams, R. P. J. J. A m . Chem. SOC.1973, 95, 4545-4551.

0022-3654/85/2089-0470$0lSO/O0 1985 American Chemical Society