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J. Phys. Chem. C 2009, 113, 5834–5842
Intramolecular Electron Transfer Reactions Observed for Dawson-Type Polyoxometalates Covalently Linked to Porphyrin Residues Anthony Harriman,*,† Kristopher J. Elliott,† Mohammed A. H. Alamiry,† Loı¨c Le Pleux,‡ Marjorie Se´verac,‡ Yann Pellegrin,‡ Errol Blart,‡ Ce´line Fosse,§ Caroline Cannizzo,| Ce´dric R. Mayer,* ,⊥ and Fabrice Odobel*,‡ Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle UniVersity, Newcastle upon Tyne, NE1 7RU, United Kingdom, UniVersite´ de Nantes, CNRS, Chimie et Interdisciplinarite´: Synthe`se, Analyse, Mode´lisation, UMR CNRS No. 6230, 2, rue de la Houssinie`re - BP 92208 - 44322 Nantes Cedex 3, France, Ecole Nationale Supe´rieure de Chimie de Paris, Laboratoire de Spectrome´trie de Masse, 1 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France, SerVice de Spectrome´trie de Masse, Ecole Nationale Supe´rieure de Chimie Paris, 24 Rue Lhomond, 75231 Paris Cedex 05, France, Institut LaVoisier de Versailles, UMR-CNRS 8180, UniVersite´ de Versailles St-Quentin, Baˆtiment LaVoisier 45 aVenue des Etats-Unis, 78035 Versailles Cedex, France ReceiVed: January 21, 2009; ReVised Manuscript ReceiVed: February 12, 2009
Three photoactive, multicomponent supermolecules have been synthesized and characterized whereby a porphyrin unit is covalently linked to a Dawson-type heteropolyphosphotungstate (POM). The connection has been made via a Huisgen reaction, which gives good yields in all cases, and modified to provide linkages that vary in their degree of internal flexibility. Fluorescence from the porphyrin unit is quenched by the appended POM, for which the efficiency increases with increasing flexibility of the linker. Except for the most rigid connection, fluorescence decay profiles are nonexponential and are interpreted in terms of multiple families of conformers that differ in their ability to undergo light-induced electron transfer. The distribution of ground-state conformers was examined by high-pressure emission spectroscopy. Cyclic voltammetry and spectro-electrochemical studies provide quantitative data for the thermodynamic driving forces and spectral data for the redox products. In all cases, the first-excited singlet state resident on the porphyrin is capable of transferring an electron to the POM. The rate of electron transfer is very slow for the corresponding triplet state of the porphyrin. Photolysis of the porphyrin in the presence of triethanolamine, present as a sacrificial electron donor, leads to formation of the porphyrin π-radical anion. This latter species is able to reduce the POM, but the rate of reaction is remarkably slow. Here, bimolecular electron transfer competes effectively with the intramolecular route, confirming that the triazole linker is a poor conduit for electrons. It was not possible, under these conditions, to load the POM with more than a single electron. The one-electron reduced form of the POM transfers an electron to the singlet-excited-state of the porphyrin so as to form a relatively long-lived charge-shift state. Introduction One of the major scientific challenges to be faced during the early part of this century concerns the design and optimization of molecular catalysts able to couple the inherently one-electron photochemistry to the multielectron chemistry associated with fuel production or CO2 reduction.1 Such new materials are essential for the efficient and sustained capture of solar energy in the form of useful chemical reagents. So far, few materials can compete with naturally occurring enzymes, such as the hydrogenases,2 or colloidal dispersions of noble metals.3 The latter colloids are able to accumulate electronic charge4 and reduce water to H2 but suffer badly from problems of selectivity.5 In searching for alternative materials able to store several electrons, attention has turned to the polyoxometallates (POMs).6 * To whom correspondence should be addressed. E-mail:
[email protected];
[email protected]; cmayer@ chimie.uvsq.fr. † Newcastle University. ‡ Universite´ de Nantes. § Ecole Nationale Supe´rieure de Chimie de Paris. | Ecole Nationale Supe´rieure de Chimie Paris. ⊥ Universite´ de Versailles St-Quentin.
Figure 1. Structures of the POM precursors 1-3 used in this study.
It has been shown that POMs are good photo-oxidants,7-10 being able to oxidize a wide range of substrates under UV illumination, and function as effective reduction catalysts.11-13 The oneelectron reduced forms are stable in the absence of O2 and are both photo- and electrochromic.14,15 Very recently, it was shown that a tetraruthenium-substituted POM was an efficient electrocatalyst for water oxidation to dioxygen.16-18 Furthermore, POMs can be functionalized by surface attachment of organic moieties so as to form attractive hybrid organic-inorganic
10.1021/jp900643m CCC: $40.75 2009 American Chemical Society Published on Web 03/13/2009
Polyoxometalate-Porphyrin Dyads
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5835
CHART 1: Molecular Formulas of the Three Porphyrin-Functionalized POMs Studied in This Work (M ) Zn)a
a
The abbreviations are used throughout the text, and the numerals refer to the synthetic schemes.
architectures that might display advanced functions.19-22 In this way, it becomes possible to utilize the high electron affinity of the POM for catalytic purposes while using the organic residue as the photon collector. Several examples of hybrid materials have been reported whereby the conjugate is assembled via electrostatic interactions;23-28 a particularly notable system has a Dawson-type polyanion bound to ruthenium(II) tris(2,2′bipyridine) dications and is capable of light-induced electron transfer under visible excitation.29 There are surprisingly few examples of covalently attached dyads based on POMs. For example, hexamolybdate clusters have been covalently grafted on an oligophenylene ethynylene polymer by Peng and coworkers.30 Under these conditions, fluorescence from the polymer was quenched significantly, most likely via photoinduced electron transfer to the associated POM. Quite recently, Ruhlmann and co-workers reported the preparation of a metallosupramolecular system consisting of a POM sandwiched between two ruthenium(II) or zinc(II) porphyrins via axial ligation with a pyridinyl ligand.31 Such molecular systems are important additions to the field. Likewise, Odobel et al. have described POM derivatives equipped with two perylene diimide residues and reported on their photophysical properties.32 In an attempt to expand this field, we now focus attention on a new set of functionalized POMs where a metallo-porphyrin is covalently linked via a short tether (Chart 1). The linkage is formed through a copper-catalyzed Huisgen reaction, which allows a Dawson-type POM to be fitted with alkyne or azido appendages and subsequent attachment of the porphyrin. Metallo-porphyrins have been used widely in artificial photosynthetic systems33 and are known to possess favorable optical and electrochemical properties. An important feature of our synthetic strategy is that the linkage does not promote electronic coupling between the terminal reactants. In fact, despite the growing use
of the Huisgen reaction to couple together donor and acceptor species,34-37 it is far from clear if the resultant 1,3-triazole is an effective electronic conduit. As a consequence, an important objective of this study is to examine how well electrons tunnel along such units. Results and Discussion Synthetic Considerations. For the synthesis of polyoxometalates covalently linked to organic dyes, we aimed for a synthetic route that could be extended to various types of dyes, offering high yield under mild reaction conditions, with few steps and involving minimal purification procedures. Surprisingly, there are few synthetic strategies to attach organic moieties to polyoxometalates via robust covalent connections. One interesting and efficient approach, developed by Peng and co-workers,38,39 is based on the arylimido linkage and permits grafting various organic units to hexamolybdate clusters. Thouvenot and co-workers40 have developed a different strategy in which the polyoxometalates are functionalized by organophosphoryl or organosilyl units. However, until now very few functional units have been attached to POMs by way of such methods. More recently, Lacôte and co-workers22,41,42 reported the monofunctionalization of a Wells-Dawson polyoxotungstate through an organotin tether and its subsequent postfunctionalization by amidation via the copper catalyzed Huisgen 1,3dipolar cycloaddition reaction. Synthesis of the new porphyrin-based POMs 12-14 started with the utilization of polyoxometalates 1-3 as precursors (Figure 1). In the current work, we have extended the chemistry of the previously reported32 azido-functionalized POMs 1-2 and prepared the new derivative 3, which contains two terminal alkyne groups. This latter compound complements the pos-
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SCHEME 1: Outlined Preparation of the Bis(phenyl-ethynyl)-POM 3
SCHEME 2: Synthetic Route to 16 and 24a
a Reagents and conditions: (a) propionic acid, 130 °C, 1 h, 16%; (b) NaOH, toluene, reflux, 5 h, 76%; (c) Zn(OAc)2, CH2Cl2, MeOH, reflux, 1.5 h, 100%; (d) DMF, CuSO4 · 5H2O, ascorbic acid, 2-azidoethyl diethyl phosphonate, 50 °C, 15 h, 99%.
SCHEME 3: Synthetic Route to 11a
a
Reagents and conditions: (a) trifluoroacetic acid, NaNO2, NaN3, 0 °C, 1 h, 100%; (b) Zn(OAc)2, CH2Cl2, MeOH, reflux, 2 h, 99%.
sibilities offered by 1-2 since it opens up the possibility for postfunctionalization of Dawson-type polyoxometalates with azido derivatives (using Huisgen 1,3-dipolar cycloaddition reactions)32,42,43 or with aryl halides (using Sonogashira crosscoupling reaction).39,44 Polyoxometalate 3 was obtained in 76% yield by reacting K10[R2-P2W17O61] · 20H2O with an excess of 4-ethynylphenylphosphonic acid 445 (Scheme 1). To graft the porphyrins to the above functionalized POMs 1-3, we initially prepared two porphyrin derivatives bearing a
terminal alkyne (7) or an azide (11) group (Schemes 2 and 3). The synthetic pathway used to obtain porphyrin 7 and the corresponding reference compound 8 is shown in Scheme 2. Here, the hydroxy-methylbutynyl porphyrin 5 was prepared in 16% yield by condensation of 3,5-di-tert-butylbenzaldehyde and 4-(3-hydroxy-3-methylbut-1-yn-1-yl)benzaldehyde with pyrrole using the Alder method.46 Removal of the hydroxypropane protecting group with sodium hydroxide and subsequent metalation with zinc(II) acetate afforded porphyrin 7 in good yield.
Polyoxometalate-Porphyrin Dyads
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5837 TABLE 1: Half-Wave Potentials (V vs SCE) for the Various Processes Identified for the Porphyrin-POM Dyads in a CH2Cl2-Containing Background Electrolyte compound ZnP ZnP-PSI ZnP-PP ZnP-PA FbP FbP-PSI FbP-PA
P2+/P+
P+/P
1.04 1.00 1.01 1.03 1.07 1.08 1.08
0.71 0.70 0.72 0.71 0.95 0.93 0.95
POM/ POM-
POM-/ POM2-
-0.68 -0.18 -0.30
-1.04 -0.75 -0.82
-0.67 -0.28
-1.00 -0.74
P/P-
P-/P2-
-1.35 -1.33 -1.35 -1.29 -1.05 -1.04 -1.07
-1.58 -1.60 -1.55 -1.55 -1.29 -1.30 -1.30
Figure 2. Spectro-electrochemical study of the stepwise reduction of the ZnP reference compound in DMF solution. Reduction was carried out at -1.5 V vs SCE. The insert shows the before (black) and after (gray) spectra recorded for oxidation of the ZnP at 0.75 V vs SCE.
By use of the usual conditions (CuSO4 · 5H2O, ascorbic acid in a polar solvent),43,47 the Huisgen reaction between the 2-azidoethyl phosphonate48 and porphyrin 7 resulted in quantitative formation of the reference compound 8. The azido porphyrin 11 was prepared from the known 5-(4aminophenyl)-10,15,20-(3,5-di-tert-butylphenyl)porphyrin 949 by conversion to the diazonium salt and subsequent reaction with an excess of sodium azide (Scheme 3).50 Zinc(II) cations were inserted into the porphyrin cavity of 11 using the usual methods. The reaction of zinc porphyrin 7 with POMs 1 and 2, using the classical conditions of click chemistry,43,47 gave the functionalized materials ZnP-PSI and ZnP-PA in good to high yield. Furthermore, the bisethynyl-substituted POM 3 was coupled to the azido porphyrin 11 under the same conditions and afforded ZnP-PP, also in quantitative yield. During the copper-catalyzed Huisgen reaction, possible trans-metalation of zinc porphyrin by copper cannot be fully excluded, but if it occurs it must be to a very limited extent since this side reaction was not observed during the preparation of the reference porphyrin 8 obtained with the same reaction conditions. To extend the series of available photoactive molecules, the central zinc(II) cations were removed under acidic conditions so as to form the corresponding free-base porphyrins (FbP) attached to the functionalized POM. It should be noted that the three linkages differ primarily in their relative flexibilities; viz ZnP-PA is highly flexible and the two redox-active units can approach each other, ZnP-PSI is semirigid, while ZnP-PP is essentially rigid (Chart 1). Electrochemistry. The new multicomponent systems, and their reference compounds, were examined by cyclic voltammetry in deoxygenated dichloromethane with P(Ph)4BF4 as supporting electrolyte and with a standard calomel electrode (SCE) as reference (Table 1). Identification of the various electrochemical steps was made by comparison to the reference compound. One-electron oxidation of the zinc(II) porphyrin (ZnP) unit was easily resolved, and the derived half-wave potential (E1/2) was found to be around 0.7 V vs SCE in each case. There is no obvious effect of the linkage on the value, and the peak followed quasireversible behavior for each system. The first oxidation step corresponds to formation of the ZnP π-radical cation, which is stable under these conditions. There is a second oxidation step, also showing quasireversible behavior, that corresponds to formation of the ZnP π-dication.51,52 Here, the corresponding E1/2 value is around 1 V vs SCE and remains independent of the nature of the linkage. There is no obvious indication that the two porphyrins appended to the POM
Figure 3. Spectro-electrochemical study for one- (red curve) and twoelectron (blue curve) reduction of ZnP-PA in DMF solution. Reduction was carried out at -0.4 and -0.8 V vs SCE, respectively. The insert shows an enlargement of the Q-band region.
Figure 4. Absorption and fluorescence spectra recorded for ZnP-PSI in DMF solution. The insert shows the phosphorescence spectrum recorded at 77 K.
communicate with each other. There are two well-defined reduction steps associated with the ZnP moiety, leading to formation of the ZnP π-radical anion and the π-dianion, respectively.53,54 These steps are quasireversible and the oneelectron, half-wave potentials are independent of the type of linker used to attach the porphyrin to the POM (Table 1). Two additional one-electron reduction steps are seen for the POM.55-57 Those POMs substituted with the silyl anchoring group display a more cathodic first-reduction step than those with the phosphoryl contact; compare the E1/2 values of ca. -0.7 V vs SCE for ZnP-PSI with that of ca. -0.3 V vs SCE for ZnP-PA and -0.2 V for ZnP-PP. Thus, the ease of reduction of the POM shows a clear dependence on the nature of the linkage or more specifically on the type of anchoring group used for the attachment.58 Replacing ZnP with FbP has no effect on the
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TABLE 2: Summary of the Photophysical Properties Recorded for the Various Multicomponent Systems in Deoxygenated DMF at Room Temperature compound ZnP ZnP-PSI ZnP-PA ZnP-PP FbP FbP-PA
λMAX/nm 429, 428, 429, 429, 421, 421,
561, 560, 561, 561, 517, 517,
601 601 601 601 649 649
λFLU/nm 608, 607, 608, 609, 652, 652,
662 658 661 661 720 720
electrochemistry of the POM but makes each oxidation step more difficult by ca. 0.24 eV while favoring reduction of the FbP by the same amount.59 Since a major objective of this work relates to exploring the electron-transfer chemistry of these new functionalized POMs, consideration was given to the thermodynamic driving forces for light-induced charge transfer from the excited state of the ZnP unit to the appended POM. The main results are given in the Supporting Information in the form of energy-level diagrams constructed on the basis of the emission spectroscopy that will be described below (Figure S4 of Supporting Information). Basically, light-induced electron transfer from the first-excited singlet state (S1) localized on the ZnP moiety to the POM is highly favorable in all cases. The change in Gibbs free energy accompanying this event varies from -0.68 eV for ZnP-PSI to -1.16 eV for ZnP-PP. Charge recombination to reform the ground-state is highly exoergonic, involving the dissipation of between 0.9 and 1.4 eV according to the nature of the surface attachment. Similarly, there are modest thermodynamic driving forces for electron transfer from the S1 state associated with the corresponding FbP to the POM, with ∆G0 ranging from -0.31 to -0.73 eV. Turning attention to the lowest-energy triplet state (T1) localized on the ZnP unit, where the triplet energy is obtained by low-temperature phosphorescence spectroscopy, it appears that the driving force for electron transfer is decreased substantially. Thus, ∆G0 for the triplet state varies from only -0.18 eV for ZnP-PSI to -0.66 eV for ZnP-PP. The situation is more critical for the T1 state associated with the FbP where ∆G0 ranges from +0.17 eV for FbP-PSI to -0.22 eV for FbP-PA. Spectro-electrochemical studies have been reported for reduction of the POMs, and it is well-known that the one-electron reduced forms are blue colored,60 with the absorption spectral profile stretching across the far-red region and into the nearIR.61 Similar experiments were performed with the ZnP reference compound. Thus, controlled oxidation of the ZnP unit in N,N-dimethylformamide (DMF) results in progressive conversion of the porphyrin to the corresponding π-radical cation (Figure 2). This latter species absorbs weakly to the red of the ZnP Soret band and in the far red region. Stepwise reduction of the ZnP results in formation of the corresponding π-radical anion (Figure 2). Again, the electrochemical step is accompanied by loss of the intense Soret band characteristic of the ZnP unit and by the appearance of weak absorption bands around 440 nm and at long wavelength. Both the π-radical cation and the π-radical anion are stable with respect to disproportionation and persist in solution in the absence of molecular oxygen. It might be noted that DMF coordinates weakly to the central cation for all the ZnP systems studied here (see Supporting Information). Reduction of the POM occurs at slightly cathodic potentials and this has a pronounced effect on the absorption spectrum of the ZnP, as shown for ZnP-PA in Figure 3. Addition of one electron to the POM causes a loss of intensity and a 9-nm red shift for the Soret band and a 14-nm red shift for the Q bands.
ΦF
τS/ns
τT/µs
0.046 0.009 0.008 0.037 0.140 0.040
2.2 1.4 (42%), 0.40 (23%) 2.0 (29%), 0.10 (23%) 2.0 10.2 7.0 (37%), 2.9 (10%)
130 105 110 100 150 148
As the second electron is added to the POM, the Soret band recovers its intensity but undergoes a further red shift of 3 nm while the Q bands are red-shifted by an additional 5 nm. These spectral effects could be due to electronic factors or might arise from replacement of the weakly coordinated DMF molecule with a more potent coordinating ligand. The changes were not observed when reduction was carried out in the presence of pyridine, which coordinates strongly to the zinc(II) cation.62 These electrochromic effects are fully reversible but occur only for the flexible linkage. The most reasonable explanation of the effect is that the ZnP unit is attracted toward the surface of the reduced POM, but access is blocked by axially coordinated ligands like pyridine. The spectral changes then result from an electronic effect. In all cases, after the POM has received two electrons further reduction leads to formation of the ZnP π-radical anion. Photophysics. The absorption spectra recorded for the new functionalized POMs are dominated by the porphyrin unit, the POM itself contributing essentially nothing to the spectral profile in the visible and near-UV regions, and are independent of the nature of the linkage. Thus, for ZnP-PSI the absorption profile exhibits the expected Soret and Q-band maxima (λMAX) at 430 nm and 560/600 nm, respectively (Figure 4). Spectra recorded for the other systems are superimposable. Fluorescence from the ZnP S1 is easily identified in each case, with well-resolved maxima (λFLU), while phosphorescence from the T1 state can be seen in an optical glass at 77 K (Figure 4). These emission spectra are unchanged compared to those recorded for the reference compound and remain independent of the type of linker used to form the dyad. Similar behavior was found for the corresponding FbP-based systems (see Supporting Information). It was noticeable, however, that the fluorescence quantum yield (ΦF) was decreased relative to the reference porphyrin and dependent on the nature of the linkage. The results are collected in Table 2, and it can be seen that whereas ΦF for ZnP-PP remains comparable to that recorded for the reference compound the value derived for ZnP-PA is decreased by a factor of ca. 6-fold. It might be mentioned that the ΦF value measured for the reference ZnP is in good accord with expectations based on literature data.63,64 Similar behavior was noted for the FbP-based systems (Table 2). As such, it is clear that the appended POM quenches fluorescence from the porphyrin S1, with the rate being controlled by the flexibility of the linkage. The rigid linker does not favor fluorescence quenching but this is promoted by the more flexible tether. A possible explanation for such behavior is that quenching demands orbital contact, or at least close proximity of the reactants. Furthermore, since fluorescence quenching is more pronounced for ZnP relative to FbP, where the thermodynamic driving force is larger, we might anticipate that an electrontransfer mechanism is involved. Time-resolved fluorescence studies carried out in DMF at room temperature indicate that the ZnP reference compound 8 possesses an excited-state lifetime (τS) of 2.2 ns, this being in
Polyoxometalate-Porphyrin Dyads SCHEME 4: Light-Induced Electron Transfer from the Excited Singlet State of the ZnP-Based Chromophore to the POM, According to the Different Families of Conformations Present in Slow Equilibrium
good agreement with values found for related ZnP derivatives.63,64 The rigidly linked system ZnP-PP gave a τS value that was only slightly decreased relative to the standard, thereby confirming the relative ΦF values. In both cases, the decay profiles were well explained in terms of monoexponential components. For both ZnP-PA and ZnP-PSI the fluorescence decay profiles could not be analyzed in terms of single exponential components but gave reasonable fits to dual-exponential kinetics (Table 2). Even so, in order to account for the observed ΦF values it is necessary to allow for a substantial fraction of the dyad existing in a nonfluorescent form. For ZnP-PSI, for example, the entire fluorescence properties require that the compound exists in at least three distinct conformations that differ according to their ability to undergo quenching. About 50% of the ensemble must reside in a conformation where quenching is highly favorable; here the τS must be less than 50 ps or so. Of the remainder, half exists in a conformation that allows efficient quenching of the ZnP S1 state, but the remainder is present in a conformation for which fluorescence quenching is difficult. Similar behavior is found for FbP-PSI (Table 2). It is tempting to assign these conformers on the basis of their separation distance between the redox-active terminals, but there is no direct evidence for this situation. However, comparison of the behavior observed for the three ZnP-based systems supports such a hypothesis, and we will return to this point below. Thus, the fluorescence quenching effects can be summarized as in Scheme 4. Pulsed laser excitation of the ZnP reference compound in deoxygenated DMF gives rise to the triplet excited state, which has a characteristic transient differential absorption spectrum.65 The triplet state decays via first-order kinetics, at low laser intensity, with a lifetime (τT) of 130 µs. This lifetime is shortened in the presence of molecular oxygen or iodoethane. For the FbP reference compound, τT has a value of 150 µs under the same conditions (Table 2). The triplet state is formed also for the various ZnP-loaded POMs, although the yield depends markedly on the nature of the linker. In fact, the triplet is formed in high yield for ZnP-PP but can barely be detected for ZnPPSI, where extensive fluorescence quenching takes place. For the POM-bearing systems, τT is shortened slightly for the ZnPbased systems but not for the FbP-based case. In this latter system, electron transfer from T1 to the POM is thermodynamically unfavorable. Surprisingly, rather similar τT values are found
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5839 SCHEME 5: Representation of the Intra- and Intermolecular Electron Transfer Steps Undertaken by the Triplet State of the ZnP Unit for ZnP-PP
for the three ZnP-based dyads, despite the fact that the driving force for electron transfer varies among the compounds. The triplet state will be formed only for conformations that do not promote efficient fluorescence quenching, and on the relevant time scale, it is not possible to rule out bimolecular electron transfer. Indeed, increasing the concentration of dyad does lead to a shortening of τT, at least in the case of ZnP-PP, and systematic variations in concentration allowed determination of the bimolecular (k ) 1.6 × 108 M-1 s-1) and unimolecular (k ) 6 × 103 s-1) rate constants for electron transfer from T1 to the POM (see Supporting Information). This situation indicates that the linker is a poor conductor of electronic charge and electron transfer from the T1 state is primarily by way of bimolecular processes (Scheme 5); for this system, the intramolecular rate constant for electron transfer from S1 is 2 × 107 s-1. A further conclusion to be drawn from this work is that the different conformations implied from the fluorescence quenching studies interconvert on a surprisingly slow time scale. High-Pressure Studies. To better understand the nature of the supposed conformational heterogeneity for the flexible linkers, the fluorescence properties were examined as a function of applied pressure. Starting with the ZnP reference compound in deoxygenated DMF, it was observed that the fluorescence peak displayed a 5-nm red shift under applied pressure ( 400 nm) of the ZnP reference compound in deoxygenated DMF containing water (5% v/v) and triethanolamine (0.2 M). The insert shows an enlargement of the Q-band region.
studies seem to be consistent with the notion of several sets of conformers that do not interconvert quickly. The corresponding pressure dependence for the dyad formed from the more rigid linkage, ZnP-PP, closely resembles that noted for the reference system, ZnP. Steady-State Photolysis. Illumination (λ > 400 nm) of the ZnP reference compound in deoxygenated DMF containing 5% v/v water and triethanolamine (0.2 M) caused reduction of the porphyrin to the corresponding π-radical anion (Figure 6). Reaction did not occur in the absence of either water or triethanolamine (TEOA) and was suppressed by the presence of molecular oxygen. A linear Stern-Volmer plot was obtained for the quenching of the ZnP S1 by TEOA under these conditions, from which the bimolecular quenching rate constant was derived to be 1.3 × 108 M-1 s-1. Under the same conditions,
Harriman et al. the ZnP T1 is quenched with a rate constant of 4 × 105 M-1 s-1. It is well-known that TEOA acts as a sacrificial electron donor,67 whereby one-electron oxidation is followed by rapid proton loss to form a carbon-centered radical that itself is highly reducing.68 Thus, two reducing equivalents arise from one molecule of donor. On prolonged irradiations, the ZnP is converted to the corresponding chlorin, which can be detected by its characteristic absorption spectral band centered at around 625 nm.69 Photolysis of the ZnP-functionalized POMs under the same conditions, where only the ZnP absorbs incident photons, results in formation of the one-electron reduced form of the POM. Even in cases where the two-electron reduced POM might be expected on thermodynamic grounds, reaction was restricted to the addition of one electron. Prolonged illumination does not result in accumulation of the ZnP π-radical anion or indeed the chlorin. Laser flash photolysis of ZnP-PP in deoxygenated DMF containing water (5% v/v) and TEOA (0.2 M) confirmed that the ZnP π-radical anion70 is formed from both S1 and T1. This species decayed slowly with a typical half-life of 1.9 ms in deoxygenated solution at room temperature. At the end of reaction, the one-electron reduced POM was present, but electron transfer did not take place entirely via the intramolecular route. Thus, the lifetime of the ZnP π-radical anion was shortened on increasing the concentration of ZnP-PP while there were indications for bimolecular processes at high concentration of π-radical anion. This is clear indication that the rigid linker does not function as an effective conduit for electron transfer. For the more flexible linkers, intramolecular electron transfer is faster and can be assigned to a mixture of diffusive and through-bond processes, again with the latter step being slow (Scheme 6). To enquire further into the apparent reluctance of the rigid linkage to promote through-bond electron transfer, an experiment was carried out with ZnP-PP after reduction of the POM. Thus, the relative fluorescence yield was recorded for ZnP-PP in deoxygenated DMF following illumination into the Soret band. The sample was housed within the spectro-electrochemical cell, and a modest cathodic potential was applied (-0.50 V vs SCE) such that one-electron reduction of the POM occurred. The fluorescence yield was monitored continuously during electrochemical reduction of the POM, but there was no obvious change, even on exhaustive electrolysis. The relatively low fluorescence intensity inherent to this system prevented accurate analysis but the change in fluorescence yield was certainly less than 10%. Under these conditions, the thermodynamic driving force for electron transfer from the reduced POM to the S1 state localized on the ZnP unit is 1.07 eV, and we can set the upper limit for the rate constant as being
