6684
J . Phys. Chem. 1990, 94, 6684-6691
Effects of Steric and Electronic Interactions on Sandwich Complexes
?T?T
Overlap in Lanthanide Porphyrin
Jing-Huei Perng, John K. Duchowski, and David F. Bocian* Department of Chemistry, Carnegie Mellon University. Pittsburgh, Pennsylvania 1521 3 (Received: March 2, 1990)
Electrochemical, infrared, resonance Raman, and optical absorption data are reported for the lanthanide porphyrin sandwich complexes, Ce'V(TPP)2, CeIV(TPP),+, Eu"'(TPP),, Ce'V(TPnP)2, Ce'V(TPnP)2+,and Eu"'(TPnP), (TPP = meso-tetraphenylporphyrin; TPnP = meso-tetrapentylporphyrin). The two Eu"' complexes contain a single hole in the porphyrin x system and are electronically similar to the CeIV sandwich porphyrin cation radicals. Variable-temperature UV-vis and near-infrared (near-IR) spectra are obtained for all four single-hole sandwiches. At high resolution and/or at low temperatures, well-resolved fine structure is observed on the intradimer charge-transfer bands (ca. 1300 and 1100 nm for the TPP and TPnP complexes, respectively). These absorptions are dominated by a single Franck-Condon-active vibration. This vibration is assigned as a mode, QAB, which contains a significant amount of multicenter character and modulates inter-ring separation. Vibronic analysis of the near-1R band contours reveals that multiple system origins (2-3) are present. The ground- and excited-state frequencies of the QAe mode increase and the dimensionless origin shifts along this coordinate decrease as the frequency of the system origin increases. The bluer system origins and higher vibrational frequencies are indicative of larger KT overlap and a stronger metal-porphyrin bond, respectively. I n the case of the Ln(TPnP), complexes, both the relative contributions of the individual progressions to the total intensity of the near-IR band and the frequencies of the RR bands are concentration dependent. In contrast, no concentration dependence is observed for the near-IR and RR bands of the Ln(TPP)2 complexes. Collectively, the spectral data indicate that the extent of AT overlap is influenced by both steric and electronic factors. The steric contributions are determined by the relative orientations of the phenyl or pentyl substituents. Multiple conformations of these groups are accessible both at room and at low temperatures. For certain conformers, steric effects can override electronic factors. As a consequence, certain conformers of the LII(TPP)~complexes exhibit equal or greater AT overlap than certain conformers of the Ln(TPnP), complexes.
Introduction Stacked and bridged porphyrinic systems exhibit unusual optical, redox, electron-transfer, and conductivity properties due to the electronic interactions that occur between the closely spaced x In order to gain a more thorough understanding
( I ) Hoffman, B. M.; Ibers. J. A. Acc. Chem. Res. 1983, 16, 15-21 and references therein. (2) (a) Martinsen, J.; Stanton, J. L.; Greene, R. L.; Tanaka, J.; Hoffman, B. M.; Ibers, J . A. J . A m . Chem. SOC.1985, 107, 6915-6920. (b) Ogawa, M. Y.; Martinsen, J.; Palmer, S.M.; Stanton, J. L.; Tanaka, J.; Greene, R. L.; Hoffman, B. M.: Ibers, J. A. Ibid. 1987, 109, 1115-1121. (3) Turek, P.; Petit, P.; Andre, J.-J.; Simon, J.; Even, R.; Boudjema. B.; Guillaud, G . ; Maitrot, M . J . A m . Chem. SOC.1987, 109, 5119-5122. (4) Nohr. R. S.;Kuznesof, P. M.; Wynne, K. J.; Kenney, M. E.; Siebenman, P. G . J . A m . Chem. Soc. 1981, 103,4371-4377. ( 5 ) Diel, B. N.; Inabe, T.; Lyding, J. W.; Schoch, K. F., Jr.; Kannewurf, C. R.; Marks, T. J. J . Am. Chem. SOC.1983, 105, 1551-1567. (6) Pietro, W. J.; Marks, T. J.; Ratner, M. A . J . A m . Chem. SOC.1985, 107. 5387-5391 (7) Diel, B. K.;Inabe, T.; Taggi, N . K.; Lyding, J. W.; Schneider, 0.;
Hanack, M.: Kannewurf, C. R.; Marks, T. J.; Schwartz, L. H. J . Am. Chem. SOC.1984, 106, 3207-3214. (8) Collman. J . P.; McDevitt. J. T.; Leidner, C. R.; Yee, G . T.; Torrance, J . B.; Little. W. A. J . A m . Chem. SOC.1987, 109, 4606-4614. (9) Hale. P. D.; Pietro, W. J.; Ratner, M. A,; Ellis, D. E.; Marks, T. J. J . Am. Chem. SOC.1987, 109, 5943-5947. (10) Gouterman, M.; Holten, D.; Lieberman, E. Chem. Phys. 1977, 25, 139-153 ( I I ) Hunter, C. A.; Sanders, J. K. M.; Stone, A. J . Chem. Phys. 1989. 133, 39 5-404. (12) Schick, G . A.; Schreiman, I . C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J . A m . Chem. SOC.1989, 1 1 1 , 1344-1350. (13) Osuka, A.; Maruyama, K. J . Am. Chem. SOC.1988,110,4454-4456. (14) (a) Yan, X.;Holten, D. J . Phys. Chem. 1988,92, 409-414. (b) Bilsel, 0.; Rodriguez. J.; Holten, D. J . Phys. Chem., in press. ( I S ) (a) Donohoe, R. J.; Duchowski, J. K.; Bocian, D. F. J . Am. Chem. SOC.1988, 110.61 19-6124. (b) Duchowski, J. K.; Bocian, D. F. J . Am. Chem. SOC.1990. 112, 3312-3318.
0022-3654/90/2094-6684$02.50/0
of the nature of such AT interactions, we have been investigating complexes of the form Ce1v(porphyrin)2,Ce'v(porphyrin)2+, and L n " ' ( p ~ r p h y r i n ) ~The . ~ ~ trivalent lanthanide sandwiches contain a single hole in the porphyrin x system and are electronically similar to the Ce1v(porphyrin)2+ complexes.i6b,eyf [Collectively, we refer to these as single-hole complexes.] The AT overlap is particularly large in all of the lanthanide porphyrin sandwich complexes because the spacing between planes defined by the nitrogen atoms of each ring is considerably less than the van der Waals distance, while the separation between the average planes of the macrocycles is comparable to this distance.16b*eOur previous studies of the sandwich dimers focused primarily on complexes with octaethylporphyrin (OEP).IS These studies provide further evidence in support of the proposal of Buchler and co-workersi6 that the extent of x x interaction can be modulated by varying the inter-ring separation via altering the radius of the lanthanide ion. However, the x x overlap is sufficiently strong in Ce(OEP),+ and all the Ln"'(OEP), complexes that the hole in the x system is completely delocalized over both rings on the vibrational and electronic time scales. Our studies also suggested that the extent of x~ interaction can be modulated by steric interactions which are dependent on the conformation of the ethyl substituent^.'^^ In order to investigate further the effects of electronic and steric interactions on the X R overlap in the lanthanide porphyrin sandwich complexes, we have examined in detail the UV-vis, near-infrared (near-IR), infrared, and resonance Raman (RR) spectra of a series of meso-substituted systems including Ce"(TPP)?, Ce'V(TPP)2+,EuI1'(TPP),, Ce1V(TPnP)2,Ce'V(TPnP)2+, (16) (a) Buchler, J. W.; Kapellmann, H.-G.; Knoff, hi.; Lay, K.-L.; Pfeifer. S. Z . Naturforsch. B. Anorg. Chem., Org. Chem. 1983, 388, 1339-1345. (b) Buchler, J. W.; Knoff, M . In Optical Properties and Structure of Tetrapyrroles; Blauer, G., Sund, H., Eds.; de Gruyter: West Berlin, 1985; pp 91-105. (c) Buchler, J. W.; Elsasser, K.; Kihn-Botulinski, M.; Scharbert, B. Angew. Chem., Int. Ed. Engl. 1986, 25, 286-287. (d) Buchler, J. W.; DeCian, A,; Fischer, J.; Kihn-Botulinski, M.; Paulus, H.; Weiss, R. J . A m . Chem. SOC.1986, 108, 3652-3659. (e) Buchler, J. W.; De Cian, A,: Fischer, J.; Kihn-Botulinski, M.; Weiss, R. Inorg. Chem. 1988, 27, 339-345. ( 0 Buchler, J. W.; Scharbert, B. J . A m . Chem. SOC.1988, 110, 4272-4276. (9) Buchler, J . W.; Loffler, J. 2.Naturforsch., in press.
0 1990 American Chemical Society
Lanthanide Porphyrin Sandwich Complexes
The Journal of Physical Chemistry, Vol. 94, No. 17, I990 6685 near-IR spectra, the spectral resolution was approximately 50 cm-1. Resonance Raman (RR) spectra were acquired by using instrumentation described e1~ewhere.l~The samples were dissolved in dichloromethane or suspended in compressed pellets with a supporting medium of Na2S04(1 :IO ratio). For all the complexes, the incident laser power was approximately 35 mW and the spectral slit width was approximately 3 cm-'. Infrared spectra were recorded of samples in compressed pellets with a supporting medium of KBr. The spectra were obtained on a Nicolet FT SDXB spectrometer at a spectral slit width of approximately 4 cm-I. B. Near-IR Band Simulations. The vibronic analysis of the near-IR band contours was performed as previously described.Isb The intensity of an individual member of the vibronic progression is given by I,,,, = M~~ZI(FC),#(V~~ + mv, nu,) e x p ( - n h u g / W / X exp(-nhv,/kr)
(1)
n
I
300
500
700 Wavelength (nm)
Figure I . Room temperature absorption spectra of (a) Ce'V(TPP)2, [ A (nm) (log emax): 394 (5.38). 480 (4.83), 539 (4.66), 625 (4.00)j; (b) Ce'V(TPP)2+, [ A n m (log cmaX): 381 (5.04), 520 (4.30). 1310 (3.93)]; (c) Eu"'(TPP), [ A nm (log cmaX): 402 (5.10). 510 (3.87), 1330 (3.75)] in dichloromet hane.
and E U " ' ( T P ~ P )(TPP ~ = meso-tetraphenylporphyrin, TPnP = meso-tetrapentylporphyrin). The electron-donating capabilities of the pentyl groups of the TPnP macrocycles should enhance the extent of x x overlap in these sandwiches relative to that which occurs in the TPP complexes. Steric interactions between the bulky phenyl substituents on opposite TPP macrocycles should further attenuate the x x overlap in these sandwiches. [The decreased a x interactions are not sufficient, however, to result in a localized hole for the Ln(TPP)2 c ~ m p l e x e s . ' ~These ~] studies, in conjunction with our previous work on Ln(OEP)2 sandwiches, provide further insight into the nature of the x x interactions in this class of complexes.
Methods A . Experimental Procedures. The Ln(TPP)2 and Ln(TPnP), complexes (Ln = Ce'" and Eu"') were prepared by refluxing H2TPP (Midcentury) or H2TPnP and the appropriate Ln"'(acetylacetonate),.xH20 (Alfa, 99.9+% rare earth oxide) in 1,2,4-trichlorobenzene (Aldrich, 99+%) under an inert atmosphere.'6b~d~e~g H2TPnP was prepared by the method of Lindsey et al." and was the generousa gift of Drs. R. W . Wagner and J. S. Lindsey. Oxidation of the Ce" complexes was carried out both chemically and electrochemically. Chemical oxidation was effected with phenoxathiinylium hexachloroantimonate in 1,2-dichloroethane (Aldrich, 99+%) under an inert atmosphere.'6cJs Electrochemical oxidations were performed in dichloromethane solutions by using procedures previously described.Isa Purification of all the complexes was performed according to the procedures of Buchler et Absorption spectra were collected on a Perkin-Elmer 330 grating spectrophotometer. Routine room temperature spectra were obtained by using dichloromethane (Fisher Optima Grade) as the solvent. Variable-temperature absorption spectra were obtained with the aid of an APD Cryogenics DE-202 Displex closed-cycle refrigeration system. All samples used in the variable-temperature studies were prepared in a 1:l mixture of dichloromethane-d2 and toluene-ds (all solvents Aldrich, 99+ atom 3'% D) with the exception of Ce'V(TPP)2+which was prepared in dichloromethane-d2 due to poor solubility in the mixture. For all (17) Lindsey, J . S.; Schreiman, I. C.; Hsu,H.C.; Kearney, P. C.; Marguerettaz, A . M. J . Org. Chem. 1987, 52, 827-836. (18) Gans, P.; Marchon, J.-C.;Reed, C. A.; Regnard, J.-R. Now. J . Chim.
1981, 5, 203-204.
where M is the electronic transition moment and FC are the Franck-fondon overlaps given by the Manneback relationsZo (010) =
exp[-A2/2(1
+ R)]
(2)
+ Iln) = -b[m/(m + I)]1/2(m- Iln) + a[n/(m + I)]'/2(mln - 1 ) - c(m + I)-l/2(mln)(3) (mln + 1 ) = b [ n / ( n + I)]'/2(mln - 1 ) + u[m/(n + I ) ] ' / 2 ( m - Iln) + d(n + I)-'/2(mln) (4) (m
where R =
ug/u,
+ 1) b = 2R'/2/(R + 1) c = A(2R)'iz/(R + 1) d = 2'I2A/(R + 1) u = (R - I ) / ( R
(5) (6) (7)
(8) (9)
Here n and m are the vibrational quanta in the ground and excited electronic states, respectively; ug and Y, are the vibrational frequencies in these two states; voo is the frequency of the electronic transition; A is the origin shift expressed in the ground-state dimensionless coordinate.
Results A . Electrochemistry. The first and second ring oxidations of Ce(TPnP), occur at f0.40 and +0.87 V [versus Ag/AgCl(saturated KCI)], respectively. The first reduction (metal centered) occurs at -0.46 V. All of these potentials are cathodic of the analogous waves observed for Ce(TPP), (oxidations at +0.79 and + I . 19 V; reduction at -0.16 V). In contrast, the redox potentials for Ce(TPnP)2 are all anodic of the analogous waves observed for Ce(OEP)2 (oxidations at 0.26 and +OS8 V; reduction at -0.51 V).Isa The first oxidation potentials observed for all the CeIV sandwich complexes are anodic of those of the analogous Zn" porphyrin monomers.21~22This latter result reflects the effects of x x interactions between the rings of the sandwiches.I6' The general trend observed in the first oxidation potentials of the CeIV sandwiches (TPP >> TPnP > OEP) is not maintained in the monomeric Zn" complexes for which TPP > OEP > TPnP. This is due to the fact that the CeIV ion interacts with the redox orbital of the 2A2ucations (TPP and TPnP) but not with the redox orbital of the 2Al, cations (OEP).ISa This interaction results in an anodic shift in the potentials of the former ions which offsets the potential (19) Donohoe. R. J.; Atamian, M.; Bocian, D. F. J . Am. Chem. SOC.1987, 109, 5593-5599. (20) Manneback, C. Physica 1951, 17, 1001-1010. (21) Felton, R. H.In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. V, pp 53-125. (22) Atamian, M.; Wagner, R. W.; Lindsey, J. S.; Bocian, D . F. Inorg. Chem. 1988, 27, 1510-1512.
6686
Perng et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 405 x 10
x 3
>
5z w
I-
z_
>
a a a
t m U
a
Wavelength (nm)
Figure 2. Room temperature absorption spectra of (a) Ce'V(TPnP)2, [A nm (log 395 (5.06), 4 8 4 ( 4 . l l ) , 556 (3.70), 668 (3.58)]; (b) Ce'V(TPnP)2f, [ A nm (log emax): 378 (4.75). 537 (3.58), 1087 (3.45)]; ( e ) Eu"'(TPnP)2, [ A nm (log cmax): 405 (4.81), 558 (3.47), 1100 (3.48)J
Figure 4. Variable-temperature near-lR spectra of Eu"'(TPP)~in dichloromethane-d2:toluene-d8(1:l). Cdrpnp,;
in dichloromethane.
29 K Y
100 K
- I -1
! I
306 K '
ENERGY
( c m l x 10.3)
Figure 3. Variable-temperature near-IR spectra of Ce'V(TPP)2+ in dichloromethane-d,.
. 6
--,
7
8
9
1011
ENERGY c s - 8 x I O j l
of Ce(TPnP), such that it is more negative than that of Ce(OEP),. B. Absorption Spectra. The room temperature absorption spectra of Ln(TPP), and L I I ( T P ~ P )complexes ~ are shown in Figures 1 and 2, respectively. The general features of the spectra of the two neutral Eu"' complexes are similar to those of the CeIV porphyrin ?r cation radicals. This is indicative of the hole which resides in the a system of the former species.'&*' The fact that the hole is delocalized in all four single-hole complexes is evidenced by the appearance of a single set of bands in the spectra rather than of two sets of bands due to separate neutral and oxidized chromophores isolated within a single molecule.2f26 The near-IR (23) Edwards, W. D.; Zerner, M. C. Can. J . Chem. 1985,63, 1763-1772. (24) Heath, G. A.; Yellowfees, L. J.; Braterman, P. S. J . Chem. Soc., Chem. Commun. 1981, 287-289. ( 2 5 ) Elliot, C. M.; Hershenhart, E. J . Am. Chem. SOC.1982, 104, 7519-7526. (26) (a) Angel, S. M.; DeArmond, M. K.; Donohoe, R. J.; Wertz, D. W. J . Phys. Chem. 1985,89,282-285. (b) Donohoe, R. J.; Tait, C. D.; DeArmond, M. K.; Wertz, D. W. Specrrochim. A d a 1986,42A, 233-240. (c) Tait, C. D.; M a q u e e n , D. B.; Donohoe, R. J.; DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. J . Phys. Chem. 1986, 90,1766-1771. (d) Donohoe, R. J.; Tait, C. D.; DeArmond, M. K.; Wertz, D. W. Ibid. 1986, 90, 3923-3926. (e) Donohoe, R. J.; Tait. C. D.; DeArmond, M. K.; Wertz. D. W. Ibid.1986, 90. 3927-3930.
Figure 5. Variable-temperature near-IR spectra of Ce'"(TPnP),+ in dichloromethane-d2:toluene-d8 (1 :I).
absorption bands, characteristic of the single-hole complexes, are observed at ca. I100 nm in the Ln(TPnP)* complexes and at 1300 nm in the Ln(TPP)2 systems. Variable-temperature near-IR spectra of Ce'V(TPP)2+, EU"'(TPP)~, Ce'V(TPnP)z+, and Eu111(TPnP)2are shown in Figures 3,4, 5,and 6, respectively. Very low temperature spectra could not be obtained for the single-hole Ln(TPP), complexes due to cracking of the solvent glass. The spectra shown in the figures were recorded in deuteriated solvents which allows resolution of the fine structure present on the band contours. As we have ~ ~ near-IR previously observed for Ln(OEP)2 c o m p l e x e ~ , 'the contours of the TPP and TPnP sandwiches are dominated by progressions in a single, relatively harmonic mode. We have assigned this Franck -Condon active vibration as a mode, QAB, which contains a significant amount of multicenter character and which modulates inter-ring separation. As in the case of the Ln(OEP)2 complexes, the progressions in the QAe modes of the Ln(TPP), and Ln(TPnP)2 complexes appear to be built off more than one system origin. Simulations of the near-IR band contours indicate that in some cases as many as three system origins are
Lanthanide Porphyrin Sandwich Complexes
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6687
h
1/
N" I
6
,
7
P
t@ 8
L\
s
I
L
k 9 1 0 1 1
ENERGY (cm-1x IO 3)
Figure 6. Variable-temperature near-IR spectra of E u " ' ( T P ~ P )in ~ dichloromethane-d2:toluene-d* (1 :l).
Raman Wft (an-3 Figure 8. High-frequency regions of the B-state excitation R R spectra of the Ln(TPP), complexes in dichloromethane solutions.
TABLE I: Resonance Raman Frequencies (cm-I) of the Ln(TPnP)Z and Ln(TPP), Complexes complex Ce(TPP)2" Ce(TPP)2+ Eu(TPP): Ce(TPnP)2 Ce(TPnP),+
Vin
1546 1548c 1566 (sh) 1549 1558
Y?
Y7.a
YA
1543 1535 (sh) 1 539e 1544 1539d 1541
1355 1351 1358 1354 1347 1352
1344 1331 1344 1345 1334 1342 (sh)
Eu(TPnP),
(1549)c
6
7
8
9
1011
(1535)
(1348) (1331)
"Spectra of all Ce complexes obtained with ,A = 363.8 nm. bSpectra of all Eu complexes obtained with A,, = 406.7 nm. C T h e 1548- and 1539-cm-'bands overlap and are not clearly resolved (see Figure 8). dObserved with A,, = 406.7 nm. CFrequencies in parentheses observed at high concentrations (see Figure 9).
ENERGY ( c m l x 10.3)
cycling.'5b Analogous studies on the Ln(TPP), complexes yield P)~ the relative identical results. For the L ~ I ( T P ~ complexes, populations of the different forms are also independent of the present (vide infra). As was observed for the L ~ I ( O E Pcomplexes, )~ solvent type and of temperature cycling; however, the relative the relative contributions of the individual progressions to the total populations are significantly influenced by concentration. This intensity of the band are dependent on temperature. This is is illustrated for E u " ' ( T P ~ P ) ~in Figure 7 . At various concenparticularly apparent in the near-IR spectra of the L I I ( T P ~ P ) ~ trations less than - 5 X lo4 M, the near-IR contours are escomplexes (Figures 5 and 6). For example, a progression on the sentially identical. As the concentration is increased above this red side of the near-IR contour of the Eu"'(TP~P)~complex gains value, the general appearance of the contour changes. At very intensity as the temperature is lowered from 308 to 100 K and high concentrations the band shape is completely different from then loses intensity as the temperature is lowered further. Comthat observed at lower concentrations. These concentration-deplicated temperature-dependent behavior of this type has also been pendent effects are not manifested a t all in the UV-vis spectra observed for the near-IR bands of the L ~ I ( O E P complexes. )~ This of the L ~ ( T P I I P )complexes. ~ C. RR Spectra. The high-frequency regions of the B-state observation led us to suggest that the multiple system origins may arise as a consequence of different orientations of the substituent excitation RR spectra of the LII(TPP)~and Ln(TPnP), complexes groups with respect to the planes of the porphyrin m a c r ~ y c l e s . ~ ~ ~are shown in Figures 8 and 9, respectively. RR spectra were also The general features of the near-IR band contours of the LII(TPP)~ obtained at a number of other excitation wavelengths throughout and Ln(TPnP), complexes suggest that the phenyl and pentyl the blue and yellow-green regions of the absorption spectrum (not substituents are also capable of assuming more than one stable shown). The frequencies of selected vibrational modes are sumconformation at both room and low temperatures. marized in Table I. In general, the high- and low-frequency R R spectra of the single-hole complexes appear to be comprised of In the case of the L ~ I ( O E Pcomplexes, )~ the relative populations of the various forms at a given temperature are independent of a single set of bands which is indicative of the delocalized hole solvent type, sample concentration, and the effects of temperature in the porphyrin a system.Is The appearance of a single set of
Figure 7. Variable-concentration near-IR spectra of Eu"'(TPnP), in dichloromethane-d2:toluene-d8 (1:l) at 22 K.
6688
Perng et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
I
lob0
'
1200
'
1400 ' 1200
cm Figure 10. IR spectra of the Ln(TPnP), complexes in KBr pellets. '
TABLE 11: Near-IR Band Simulation Parameters for Ln(TPnP)* and Ln(TPP), Complexes
Ce(TPnP),+
Eu(TPnP), Conformer I
~oOU "g i'e
1 6 IJ loo0
*w
Figure 9. High-frequency regions of the B-state excitation spectra of (a) Ce'V(TPnP)2,A,, = 363.8 nm; (b) CeLV(TPnP),+, A,, = 363.8 nm; (c) Eu"'(TPnP),, A,, = 406.7 nm, concentration M; (d) Eu"'M; (e) Euil'(TPnP)t, kX (TPnP),, A, = 406.7 nm, concentration = 363.8 n m . concentration M in dichloromethane solutions.
"8
RR bands also indicates that the multiple forms observed in the near-IR spectra do not exhibit substantially different porphyrin skeletal-mode frequencies. The direction of the frequency shifts observed upon oxidation of Ce(TPP), and Ce(TPnP), is similar to that reported for monomeric metallo-TPP c~mplexes.~' In particular, the uZ, q0,and v4 bands of the oxidized complexes are down-shifted relative to those of the neutral species. [It should be noted that polarization measurements indicate that the strong band observed near 1543 cm-l in the neutral CeIV complexes is v,, whereas the strong band observed in this region of the oxidized complexes is predominantly vl0.] These shifts are consistent with an 2A2uground state for the Ce"'(TPP),+ and CeiV(TPnP),+. The general features of the RR spectra of the Eu"' sandwiches are similar to those of the oxidized Ce" complexes which suggests an ,A2,, ground state for these neutral single-hole complexes. The nature of the ground state is confirmed by the electron paramagnetic resonance (EPR) spectra of the Eu"' sandwiches (not shown). Although the existence of multiple forms is not clearly evident in the RR spectra, the observation that the near-IR band contours of the Ln(TPnP)2 complexes are influenced by concentration prompted us to investigate this effect further. The RR spectra observed for Eul[I(TPnP), at low versus high concentrations with XFx = 406.7 nm are compared in Figure 9, c and d, respectively. Figure 9e shows the spectrum obtained at high concentrations with kx= 363.8 nm. At high concentrations, a second set of RR bands is observed for which the frequencies of the u2, u I o , and u4 bands are lower than those observed at low concentrations. This second set of bands probably corresponds to the form which gives rise to the unusual near-18 band contour observed at high concentrations (see Figure 7 , top). D. IR Spectra. The IR spectra of the single-hole Ln(TPP), complexes have been previously reported by Buchler and coworkers.'6f*g These complexes exhibit characteristic "oxidation"
UoO
-
- -
(27) Czernuszewicz, R. S.;Macor, K . A., Li, X-Y.; Kincaid. J . R.; Spiro, Am. Chem. SOC.1989. 111, 3860-3869.
T.G . J .
Eu(TPP),
6970 315 194 3.10 0.059
6857 3 20 210 3.20 0.060
7358 380 240 2.70 0.047
7697 380 260 3.10 0.053
8328 460 3 20 2.30 0.036
8455 430 310 2.80 0.045
0.4/1/0.3/0
0.5/1/0.2/0
Conformer 11
1700
Raman Wfi (mi')
Ce(TPP),+
i'e
1 rir
7610 370 246 3.15 0.056
7606 370 256 3.15 0.056
Conformer 111 1'8 "e
1 hr
8350 460 306 3.06 0.049
8350 450 325 3.08 0.050
Conformer IV VoO "8 "e
A 6r
c,jc,,/ c,, I
9570 550 420 2.00 0.029 010.1j l l O . 5
0/0.3/1/0 (0/0.6/ I / O ) e
-
M. "All frequencies in em-'. b6r in A. CTotalconcentration "Temperatures: 29 K , Ce(TPnP),+; 22 K, Eu(TPnP)*; 150 K, Ce(TPP),+: 90 K. Eu(TPP),.
