Porphyrins as Sensitizers and Electron Donors

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Chapter 4 Wide Rim Ferrocene Appended Bis-Porphyrins Prepared by Palladium Coupling Reactions.

 Photoinduced Multistep Electron Transfer in Porphyrin-Fullerene Triads

Porphyrins and fullerenes have been shown to be excellent chromophores for the construction of covalent and supramolecular dyads for photoinduced charge separation. These covalently and supramolecularly coupled porphyrin-fullerene dyads however, do not have charge separated state lifetimes long enough for use in light harvesting devices. A number of research groups have prepared triads and tetrads of their corresponding dyads through addition of secondary donors or secondary acceptors.67,70,199,200 This enables the system to undergo a multistep electron transfer process, spatially separating the charge separated state and increasing the lifetimes of the charge separated state. Multistep electron transfer is found in both photosystems I and II, to achieve longer lived charge separated state lifetimes for generation of ATP.11,201
The groups of Imahori and D‟Souza have respectively prepared covalent and supramolecular triads based on a donor-sensitizer-acceptor model.70,202 In this model the electron hole is moved by the transfer of an electron from a secondary donor to the sensitizer, and the electron charge remains on the acceptor. With the addition of a secondary electron donor such as ferrocene, dipyrrins, triphenylamines or metallated porphyrins to the original porphyrin fullerene dyads, the electron hole can be moved by the transfer of an electron from the secondary donor to the porphyrin. While this electron transfer process results in a loss of energy within the system, the resulting distantly separated radical ion pair attenuates the electronic coupling significantly, thereby prolonging the lifetime of the final charge-separated state. Imahori et al. has prepared a ferrocene (Fc), free base porphyrin (P), fullerene (C60) triad 1.13 and zinc porphyrin (ZnP), free base porphyrin (P) fullerene (C60) triad 1.14 by covalently connecting the chromophores by amide bonds.67,72,77,78 In the triad 1.13, photoexcitation of porphyrin, results in a primary electron transfer from the porphyrin to the fullerene, generating a charge separated state Fc-P•+-C60 The triad then undergoes a secondary electron transfer process from the ferrocene to the porphyrin to generate the Fc•+- P- C60- charge separated state.the spatial distance separating the radical ion pair results in a significantly longer lifetime of charge separated state of up to 16 μs. Upon excitation of 1.14, the zinc porphyrin (ZnP) transfers its singlet energy to the energetically lower lying free base porphyrin (P).72,77 This energy transfer is then followed by sequential electron transfer from the generated singlet excited state of the free base porphyrin to the fullerene to yield ZnP-P•+-C60 followed by asubsequent electron transfer from the ZnP to the H2P•+ to yield ZnP•+-H2P-C60 with a lifetime of 21μs.D‟Souza has reported a supramolecularly assembled ferrocene-porphyrin-fullerene triad 1.22,92 by axial co-ordination of an imadazole functionalized fullerene to a ferrocene functionalized zinc porphyrin. Transient absorption studies have shown that photoexcitation of the zinc porphyrin was followed by electron transfer to the fullerene to yield Fc-ZnP•+-ImC60 This was followed by transfer of an electron from the ferrocene to the zinc porphyrin resulting in
Fc•+-ZnP-ImC60 with a charge separated state lifetime of 10 ns.

Porphyrin Appended Ferrocene Calixarene BisPorphyrin

The calixarene linked bis-porphyrin host 1.36 with C60 has been shown to produce lifetimes for the charge separated state ranging from 910 to 1480 ps.122,127,128 As with other dyads the charge separated state lifetime is too short to produce a photovoltaic device with high efficiency. A calixarene linked bis-porphyrin functionalized by direct attachment of ferrocene to the porphyrins has been synthesized by Lyons et al. which, when complexed with C60, forms a supramolecular porphyrin-ferrocene-fullerene triad.165 The porphyrin used for the bisporphyrin was prepared using a Suzuki coupling reaction with 4-ferrocene-phenyl boronic acid and 4-acetamidophenyl boronic acid, to form an A2BC type porphyrin, functionalized with ferrocene the 15-position. Binding constants of 4.1 with C60 and C70 were determined by UV visible titrations in toluene and are comparable to the tolyl functionalized bis-porphyrin 2.1. Binding constants of 4.1 and Zn4.1 in toluene are 1.70 x 104 and 1.06 x 104 M-1 respectively for C60 and 1.49 x 105and 7.0 x 104 M-1 for C70.Both the free base and the zinc derivatives of the ferrocene appended hosts 4.1 and Zn4.1 display UV-visible and fluorescence spectra which were identical in shape and position to those of the individual free-base and zinc derivative of meso-tetraphenyl porphyrins.203 The fluorescence intensities, however, were about 25-30 times lower than the analogous ferrocene free hosts 1.36. The porphyrin quenching has been attributed to energy transfer from the lowest singlet excited states centered on the porphyrin moieties, to the low lying triplet of ferrocene.204 Charge separated state life times of the triad were not significantly increased. Instead of acting as an electron donor transferring an electron to the porphyrin, the active role of the ferrocene groups in the photoinduced process was observed as deactivation of the porphyrin singlet levels through photoinduced energy transfer. This has been accredited to the ferrocene groups being located too close to the porphyrins and is further reinforced by the strong quenching of the porphyrin fluorescence in the supramolecular adducts compared to the free 4.1 and Zn4.1 bis-porphyrin hosts with appended ferrocene.

Aims and Strategy

This chapter describes the synthesis of calixarene bis-porphyrins functionalized with ferrocene groups on the wide rim of the calixarene through palladium catalyzed reactions. The remote distance of the ferrocene groups from the porphyrin sensitizers is expected to promote electron transfer and retard energy transfer. Irradiation of the bis-porphyrin-C60 complex excites the porphyrin to a higher singlet energy state The porphyrin can then undergo an electron transfer to the fullerene to generate a charge separated state with the porphyrin radical cation and fullerene radical anion. From there the ferrocene can transfer an electron through to the porphyrin to increase the spatial distance between the two charges and therefore increase the lifetime of the charge separated state. Two different palladium catalyzed reactions have been employed for the functionalization of calixarenes on the wide rim with ferrocene. The first reaction type is Sonogashira coupling reaction,141 which has been used to prepare bis-porphyrins 4.2 and 4.3. Bis-porphyrins 4.2 and 4.3 have been prepared from ethynyl ferrocene and para-ethynyl phenyl ferrocene respectively. Addition of a phenyl groups increases the distance between the ferrocene groups and porphyrins by approximately four angstroms and are of interest in terms of what effect the different spatial distances between the chromophores will have on the lifetimes of the charge separated state. The second reaction used for preparation of wide rim ferrocene functionalized bis-porphyrins was Suzuki coupling.138 The initial synthetic route for the preparation of the ferrocene functionalized bis-porphyrin 4.4, was with a dialkylated bromo-calixarene analogous to the iodo calixarene used into the Sonogashira coupling. The Suzuki coupling proved difficult however and all attempts at coupling ferrocene to the wide rim of the dialkylated calixarene failed. This is briefly discussed in the synthetic section of this chapter. It was decided that a second alkylation of the remaining hydroxyl groups would be employed as these groups can interfere with wide rim chemistry. While a second alkylation of the calixarene decreases the association of the bis-porphyrin host with fullerenes due to a change in the calixarene conformation, it provides the opportunity to modify the order of alkylation and in turn the final position of the ferrocene in the functionalized bis-porphyrin. Two reaction pathways for alkylation of the calixarene have been developed to for Suzuki coupling ferrocene to the wide rim of the calixarene. These pathways differ in the order of alkylation with ethyl bromoacetate and iodobutane. Bis-porphyrin 4.5 was prepared by alkylation of the calixarene with ethyl bromo acetate, followed by bromination of the wide rim of the calixarene with bromine para to the phenol. A second alkylation of the phenol para to the bromo groups with iodobutane and coupling of ferrocene via a Suzuki reaction ultimately results in the ferrocene groups being appended to the aryl rings of the calixarene para to the nbutyl chains. In bis-porphyrin 4.6, the order of alkylation is reversed and the first alkylation of the calixarene is with n-iodobutane. Subsequent bromination para to the phenol hydroxyl groups with bromine and then secondary alkylation with ethyl bromo acetate and Suzuki coupling of ferrocene ultimately results in the ferrocene groups being appended to the aryl rings of the calixarene para to the porphyrin amides. It is hoped that the associated changes in the location of ferrocene groups on the wide rim of the calixarene may lead to different lifetimes of the multistep charge separated state. The amino porphyrin used in the synthesis of 4.2, 4.3, 4.5 and 4.6 were 5-(4-aminophenyl)-15- tolyl -10,20-bis(3,5-di-tert-butylphenyl) porphyrin 2.15. Porphyrin 2.15 provides a number of close contacts to bound fullerenes via CH-π interactions from the tert-butyl methyl groups which increase the porphyrin–fullerene association in comparison to tetra-phenyl porphyrins. The tolyl methyl observed as a singlet at approximately 2.38 ppm provide a good 1H NMR fingerprint for the identification of the bis-porphyrin, integrating for six protons. In 4.5 and 4.6 n-iodobutane was chosen as the reagent for the second alkylation, as the longer chain butyl prevents the aryl rings of the calixarene from interconverting to different conformations. Computational modeling of bis-porphyrins has been employed to investigate if the changes in geometry off the calixarene scaffold upon appending ferrocene to the wide rim. Binding constant measurements have been carried out using UV-visible titrations in toluene and acetonitrile/toluene (1:1) Estimation of the electronic coupling between the porphyrin and fullerene has been determined from charge transfer transitions. Fluorescence spectra have been recorded for the ferrocene appended hosts to investigate if there is any fluorescence quenching of the porphyrin from the ferrocene.

Computational Modeling of Bis-Porphyrins

In order to investigate spatial distances between the ferrocene groups and the porphyrin sensitizers, as well as any significant differences in geometry of the bis-porphyrins, computational modeling of the host-guest complexes 4.2, 4.3, 4.5 and 4.6 with C60 were employed using the two layer ONIOM method described in Chapter Two. The calixarene, the amide linkers groups as well as the appended ferrocenes were modeled in the high layer using DFT with the B3LYP hybrid functional and a 6-31G(d) basis set. The porphyrins and C60 were modeled in the low layer with molecular mechanics which adequately describes the porphyrinfullerene interaction. Nickel porphyrins were used in the computational modeling to help maintain planarity of the porphyrin versus unmetallated porphyrins.

Modeling of Bis-Porphyrins 4.2 and 4.3 with C60

Both 4.2 and 4.3 display a pinched cone conformation similar to the non-ferrocene substituted bis-porphyrin 2.1 due to the narrow rim hydrogen bonding motif. Hydrogen bonding distances are consistent between both hosts ranging from 1.89 to 1.92 Å. Hydrogen bonding between the amide NH and the ether oxygen show distances of 2.33 and 2.40 Å for 4.2 and 4.3. The interplanar angles between the phenol rings are relatively unchanged for the both hosts 4.2 and 4.3 with angles of approximately 78°. The porphyrin amide functionalized rings do vary slightly, with angles of 22.6° and 24.7° for 4.2 and 4.3 respectively. The porphyrins are tilted slightly towards each other due to CH-π interactions between the methyl groups of one tertbutyl phenyl and a tert-butyl phenyl ring on the second porphyrin. The interplanar porphyrin angles and center to center porphyrin distances for 4.2 are 73.8° and 9.71 Å, and for 4.3 are 73.8 ° and 9.71 Å. The C60 is arranged with 6:6 ring junctions centered over the porphyrin at distances of 2.87-3.15 Å, several CH-π interactions from ortho-protons on the 10 and 20 phenyl substituents and the fullerene and a number of CH-π interactions between the methyl groups of the tert-butyl groups and the fullerene. The key differences between the triads are the distance between the ferrocene secondary donors and the porphyrin sensitizers. The center to center distance of the ferrocene to the porphyrin in 4.2 is 16.02-16.15 Å .With the addition of the phenyl ring the center to center distance in 4.3 extends to 19.65-19.99 Å. Optimized structures for 4.2 and 4.3 are shown in Figure 12.6. A table of key structural characteristics for hosts 4.2 and 4.3 as well as the nonferrocene functionalized host 2.1 are given in Table 12.1

1 Chapter 1 Introduction 
1.1 Clean Energy Through Photovolatics
1.2 Photoinduced Electron Transfer in Photosynthesis
1.3 Photoinduced Electron Transfer for Solar Energy Conversion
1.4 Porphyrins
1.5 Porphyrins as Sensitizers and Electron Donors
1.6 Fullerenes
1.7 Fullerenes as Electron Acceptors
1.8 Marcus Theory of Electron Transfer
1.9 Endohedral Metallofullerenes
1.10 Photoinduced Multistep Energy and Electron Transfer
1.11 Supramolecular Bonding for Molecular Assembly
1.12 Self-Assembled Supramolecular Porphyrin-Fullerene Dyads
1.13 Supramolecular Porphyrin-Fullerene Chemistry
1.14 Supramolecular Bis-Porphyrins Hosts for Fullerenes
1.15 Calix[n]arenes
1.16 Wide Rim Calixarene Linked Bis-Porphyrins
1.17 Narrow Rim Calixarene Linked Bis-Porphyrins
1.18 Aims of this Research
2 Chapter 2 Synthesis and Modification of Porphyrins for Calixarene Bis-Porphyrin Hosts
2.1 Introduction
2.2 Aim and Strategy
2.3 Computational Modeling of Bis-Porphryins
2.4 Synthesis of Bis-Porphyrins
2.5 Fullerene Binding studies with Bis-Porphyrins
2.6 Porphyrin-Fullerene Co-Crystallates
2.7 Charge Transfer Bands
2.8 Summary
2.9 Experimental
3 Chapter 3 Tetra-alkylated and Extended Linker Calixarene Bis-Porphyrins
3.1 Introduction
3.2 Aim and Strategy
3.3 Computational Modeling of Bis-Porphyrins
3.4 Synthesis of Bis-Porphyrin
3.5 Fullerene Binding Studies
3.6 Variable Temperature NMR studies
3.7 X-Crystal structures
3.8 Charge Transfer Bands of 3.9 and 3.10 in Toluene.
3.9 Summary
3.10 Experimental
4 Chapter 4 Wide Rim Ferrocene Appended Bis-Porphyrins Prepared by Palladium Coupling Reactions.
4.1 Photoinduced Multistep Electron Transfer in Porphyrin-Fullerene Triads
4.2 Aims and Strategy
4.3 Synthesis of Ferrocene Appended Bis-Porphyrins
4.4 Fullerene Binding Studies with Bis-Porphyrins
4.5 Fluorescence of Bis-Porphyrins
4.6 X-Ray Crystal Structures
4.7 Porphyrin-Fullerene Charge Transfer Bands
4.8 Summary
4.9 Experimental
5 Chapter 5 Wide Rim Appended Ferrocene Bis-Porphyrins Prepared by Amide Coupling
5.1 Introduction
5.2 Aims and Strategy
5.3 Computational Modeling of Bis-Porphyrins 5.6 and 5.7
5.4 Synthesis of Ferrocene Appended Bis-Porphyrins
5.5 Fullerene Binding Studies with Bis-Porphyrins 5.6 and 5.7
5.6 Fluorescence of Bis-Porphyrins 3.10, 5.6 and 5.7
5.7 Charge Transfer Bands
5.8 Summary
5.9 Experimental
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