J. Phys. Chem. 1994,98, 8279-8288
8279
Hole Localization or Delocalization? An Optical, Raman, and Redox Study of Lanthanide Porphyrin-Phthalocyanine Sandwich-Type Heterocomplexes Thu-Hoa Tran-Thi'lt and Tony A. Mattiolit Laboratoire de Photophysique et de Photochimie. CEA- CNRS URA 331, SCM/DRECAM/DSM, and Section de Biophysique des ProtCines et Membranes, CEA-CNRS URA 1290, DBCM/DSV, Centre d'Etudes de Saclay, 91 I91 Gif-sur- Yvette Cedex, France
Driss Chabach, Andre De Cian, and Raymond Weiss Laboratoire de Cristallochimie et de Chimie Structurale, CNRS UA 424, Institut Le Bel, Uniuersitb Louis- Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, France Received: August 9, 1993; In Final Form: May 2, 1994'
A systematic electrochemical, redox, optical absorption, and resonance Raman spectroscopic study of 22 gadolinium and cerium double- and triple-decker tetraphenylporphyrin-phthalocyanine (TPP-Pc) complexes is reported. The work is aimed a t elucidating the extent of the localization or delocalization among the chromophore ligands in the mixed complexes, where there exists a charge imbalance between the metal centers and the dianion Pc/TPP ligands, as well as in different oxidation states for these types of complexes. For mixed double- and triple-decker complexes, IPc(M)TPPI and ITPP(M)Pc(M)TPPI (where M = GdIII, CeIV), in which the metal cannot be oxidized, it appears that the ligand preferentially oxidized is the Pc moiety. This preferentially localized hole in the phthalocyanine results in new near-infrared transitions which show maxima around 12001300 nm for the double-decker complexes and shifts to 1800-2300nm for the triple-decker derivatives. In this case, the removal of an electron from a complex redox orbital which is rich in Pc character is easily identified by the accompanying changes in the optical absorption characteristic of a Pc'- species and is also seen in FT Raman spectra (excited at 1064 nm) by a significant (>15 cm-l) upshift of a redox-sensitive Raman band a t ca. 1500 cm-l. The case of the cerium(II1) derivatives, where the metal can also be oxidized, is more complex. It appears that the metal is preferentially oxidized in the symmetric triple decker with the two outer PCS. In contrast, the interpretation of the data obtained for the second triple-decker compound with the inner Pc is more complex. This point is thoroughly discussed.
1. Introduction Electronic interactions between porphyrin-like ?r systems held in close proximity play a crucial role in various systems such as photosynthetic bacteria or molecular metalloorganic conductors. In this regard, lanthanide porphyrin1or phthalocyanine*sandwich complexes as well as "electrostatic" complexes3 are attractive models, and an understanding of their optical properties may provide insight into how the electronic properties of dimeric or multichromophoricspecies influence the initial stagesof the charge separation process. Among the various rare-earth and transition-metal complexes, thecerium derivatives are the most attractive compounds. Owing to the ease of oxidation or reduction of the cerium itself, the bi~(porphyrins),~ bi~(phthalocyanines),~ and mixed-ligand complexes,6s7 which are electrochromic, can display extra colors in comparison with the other lanthanide(II1) derivatives.'v2 The existence of such redox sites could permit a better understanding of the interaction of metal orbitals with those of the two strongly coupled chromophores. As a matter of fact, the metal-to-ring or ring-to-metal charge transfer (MRCT or RMCT) could provide effective routes for deactivation of the normally emissive ring (r,?r*) states of the bi- or trichromophoric complexes. Moreover, disymmetriccomplexes, such as porphyrin-phthalocyanine dimers and trimers whose individualchromophores display very different optical and redox properties, can provide further evidence for the problem of hole (or charge) localization or delocalization in multichromophoric systems. Vibrational spectroscopy,and in particular resonance Raman spectroscopy, is a valuable tool in analyzing the extent of (de)t Laboratoire de Photophysique et de Photochimie. t 0
Section de Biophysique des ProtBnes et Membranes. Abstract published in Advance ACS Abstracts, July 15, 1994.
localization of the resulting positive charge once a porphyrin or phthalocyanine complex has been 0xidized.1b~c~*.~ Vibrational modes of these molecules are sensitive to the removal of an electron from the r conjugation pathway. By tuning the Raman excitation wavelength near to, or within, a specific electronic absorption band of a chemical species, one may selectively enhance the Raman activity of vibrational modes of that species. This also helps in identifying the nature of a specific absorption band. In addition, the complexes studied here exhibit an absorption band at ca. 1200nm when there exists a charge imbalance between the metal centers and the dianion phthalocyanine/porphyrin ligands. Thus, this absorption band is associated with one or several partially oxidized phthalocyanine/porphyrinligands. With Fourier transform Raman spectroscopy, which uses 1064-nm radiation to excite the Raman spectrum, one should obtain a genuine resonance condition with the species giving rise to this absorption band. Radiation at 1064 nm is ca. 1100 cm-1 higher in energy than 1200 nm and thus falls in the vibronic satellite region of such a transition. In this paper, we report the electronic absorption and Raman spectra of cerium and gadolinium bis- and tris(porphyrinphthalocyanine) complexesin their neutral, oxidized, and reduced forms. This work is aimed at elucidating the extent to which a hole could be delocalized over the chromophores in oxidized double- and triple-decker porphyrin-phthalocyanine complexes. The optical and Raman spectra are compared to those of the free base and gadolinium monomers and discussed. 2. Experimental Section
Syntheses. The neutral I(TPP(CeI")Pcl (TPP = tetraphenylporphyrin, Pc = phthalocyanine dianions),ITPP(Ce1l1)Pc(Ce1I1)TPPI, and IPc(CeIII)TPP(CeIII)Pclwere synthesizedas previously
0022-365419412098-8279$04.50/0 0 1994 American Chemical Society
8280 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 described.637 The synthesis of the gadolinium derivatives IGdIIIPc(acac)l, 1TPP(Gd1I1)Pc(,(Gd1I1(Pc)2l,(Gd111H(Pc)21,and (Pc(GdlI1)Pc(GdlI1)TPPIare described as follows. Synthesisof IPc(CdI11)TPPI. A 1,2,4-trichlorobenzene (1,2,4TCB) solution containing 300 mg (0.57 X M) of dilithium phthalocyanine (Li2Pc) and 545 mg (1.2 X l e 3M) of gadolinium acetylacetonate (Gd(acac)rnH20) was heated to 120 "C during 3 h under a slow stream of argon to form the mono(phtha1ocyanhato)-(acetylacetato)gadolinium(III) derivative. To this solution was added, after cooling under argon, 370 mg (0.6 X 10-3M) of tetraphenylporphyrin (HzTPP), and the wholemixture was refluxed for 8 h. Addition of hexane, after cooling of the solution, yielded a crude solid. Filtration, extraction with chloroform (several times), and concentration under vacuum gave a solution which was chromatographed on a silica gel column. Elution with toluene yielded successively a red-brown tetraphenylporphyrin-phthalocyanine gadolinium(111) double-decker derivative (Pc(Gd)TPPI (major fraction), an olive-green bis(tetraphenylporphyrin-phthalocyanine)-bis(gadolinium( 111)) triple-decker complex ITPP(Gd)Pc(Gd)TPPI (minor fraction), and the blue bis(phthalocyaninato)gadolinium(III) double-decker anionic sandwich IGd(Pc)zt (minor fraction). The major fraction was dried under vacuum, and the red-brown mixed-ligand complex IPc(Gd)TPP( was precipitated by addition of hexane (475 mg, 65%). This compound is stable in air and gave analytical and spectral results in agreement with a 1:1 porphyrin-phthalocyanine gadolinium formulation. Synthesis of (TPP(Gd)Pc(Gd)TPq.A mixture containing 170 mg (0.326 X 10-3 M) of LizPc and 590 mg (1.3 X 10-3 M) of Gd(a~ad)~.nHzO in 70 mL of dry 1,2,4-TCB was stirred and heated to 120 OC under a slow stream of argon for 3 h. After cooling, a solid sample of H2TPP (500 mg, 0.81 X M) was added, and the mixture was refluxed under argon for 18 h. Cooling of the solution and addition of hexane yielded a crude solid. Chromatographyon a silica gel column with toluene as the solvent gives three fractions. The first one, in minor quantity, corresponds to (Pc(Gd)TPPI. Thesecond and the third fractions contain both lTPP(Gd)Pc(Gd)TPPI (major fraction) and ITPP(Gd)Pc(Gd)Pcl. Addition of hexane to a solution of the major fraction gives a brown-green precipitate (520 mg, 78%). On the basis of analytical results, which showed the presence of two gadolinium cations together with a porphyrin to phthalocyanine ratio of 2:1, we formulate this compound as ITPP(Gd)Pc(Gd)TPPI. Synthesis of lTPP(Gd)Pc(Gd)TPP(+(SbC&(-. A solution of ITPP(Gd)Pc(Gd)TPPI (30 mg, 1.4 X l0-5M) in CH2C12 (15 mL) was treated with 1 equiv of phenoxathiine hexachloroantimonate. The oxidation process was accompanied by a color change from brown-green to red-brown. The solvent was removed under vacuum. Recrystallization by slow diffusion of hexane into the CHzCl2 solution yielded 22 mg (63%) of ITPP(Gd)Pc(Gd)TPP(+ISbCl&. The identity of the complexes was confirmed via UV-visible, magnetic resonance spectroscopy, and X-ray structure determin a t i o n ~ .Oxidation ~ and reduction of the compounds werecarried out either chemically in situ using respectively Fe(C104), and hydrazine in the presence of N(Bu)4PF6 or electrochemically with formation of salts with the corresponding counterions IN(Bu)41+ or IPf&. Dichloromethane (DCM), 1,Zdichloroethane (1,2-DCE), dimethylacetamide (DMA), chloronaphthalene, and dimethyl sulfoxide (DMSO) of spectroscopic grade were used for all measurements. The supporting electrode is N(Bu)&104, and Pt disk and standard calomel electrodes were used as references. The ground-state absorption spectra from UV to nearinfrared were recorded on Perkin-Elmer A5 and Beckman UV5240 spectrophotometers. Fourier transform (FT) Raman spectra were recorded, at room temperature, with a Bruker IFS 66 interferometer coupled to a Bruker FRA 106 Raman module equipped with a continuous
Tran-Thi et al.
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h (I" Figure 1. UV and visible electronic absorption spectra of (-) IGd111(Pc2-)(acac)lin C2H4C12, (- - -) (Gdtt1(Pc'-)(acac)l+(PF6~ in C2H4IGdtt1(Pc*-)(acac)l+IPF6~ in C2H&12 after 24 h. (212, and (-e)
diode-pumped Nd:YAG laser operating at 1064 nm.8 Typical laser power was ca. 150 mW. Samples were contained in a l-cmpath length sealed quartzcuvette. Raman photons werecollected using a 180' backscattering geometry. All spectra reported here were the result of the coaddition of 4000 interferograms. Spectra were corrected by subtracting solvent contributions. Typical concentrations were ca. 5 X 10-4 to l t 3 M in various solvents.
3. Results and Discussion a. Electronic Absorption Spectra of Lanthanide Monomers. The I(acac)Gd111(Pc2-)lspectrum is typical of those of the wellknown metalated phthalocyanine monomers (see Figure 1). Its oxidized form in 1,ZDCE presents characteristic bands at 412, 504, 718, and 830 nm. The extra bands observed at 596,630, 656, and 690 nm are reminiscent of the Q bands of the free-base phthalocyanine. Their presence is due to a partial demetalation of the gadolinium derivative which leads to the free base phthalocyanine having a D2h symmetry. The monooxidized phthalocyanines easily aggregate in solution. The spectrum of a freshly oxidized compound IGdlIIPc(acac)l+is compared to one kept for 24 h. One can observe a new broad absorption ranging from 540 to 800 nm with an ill-defined maximum around 640 nm, on the top of which is superimposed the absorption of the demetalated neutral species. This broad absorption is the signature of strongly aggregated phthalocyanines, e.g., tetrasulfonated phthalocyanines in water.IO These data are in agreement with the results previously reported by Stillman et al.ll for the oxidized MgPc formed in situ in various solvents and with various oxidants. They also noticed the same trend toward aggregation and found in some cases similar extra absorption bands attributed to the demetalated species. b. ElectronicAbsorption Spectra of Lanthanide Double-Decker Sandwich Complexes. Figure 2 shows the absorption spectra of ITPP(CeIV)Pc(in its neutral, oxidized, and reduced forms. The ground-state absorption spectrum of the neutral complex displays features common to both ICeIV(TPP)2I4and the phthalocyanine derivatives ICeIv(Pc)215and ISnIv(Pc)2l.l2 By analogy with ICeIV(TPP)$, the absorption centered at 403 nm is attributed to the porphyrin Soret band, which is very weak in the mixed-ligand complex. The same comparison is established with (CeIv(Pc)21 and (SnIv(Pc)21,and the two visible and near-infrared absorptions over 540-650 and 700-900 nm are assigned to the phthalocyanine Q bands in the dimer. The first visible band (540-650 nm) probably contains a contribution of the porphyrin Q bands which should appear around 540-630 nm in the double-decker sandwich complex. As reported for )Ce"f(TPP)21and ICeIV(OEP)21derivat i v e ~(OEP ~ = octaethylporphyrin), a very weak absorption is observed a t wavelengths longer than 900 nm, which tails in the near-infrared up to 1600 nm.
Hole Localization or Delocalization?
A
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