trans-2 addition pattern to power charge transfer in dendronized metalloporphyrin C60 conjugates

Tuning electron transfer in supramolecular nano-architectures made of fullerenes and porphyrins

The current work focuses on self-assembled nano-architectures in which metal-ligand coordination between a zinc tetraphenyl-porphyrin (ZnP) and a zinc tetrakis(4-((1,3-dithiol-2-ylidene)methyl)phenyl)-porphyrin (ZnP-TDP), as electron donors, and functionalized fullerenes (C60) featuring different conjugated pyridine substituents as electron acceptors have been designed and investigated. Stoichiometric ratios and binding constants were derived from absorption and fluorescence measurements. Important insight into the free-energy change of charge separation and recombination was obtained from differential pulse voltammetry studies. Compelling evidence for energy transfer, charge separation and recombination was obtained from femtosecond and nanosecond transient-absorption measurements in a wide temperature range. Intramolecular energy transfer is found to take place from TDP to ZnP followed by intramolecular charge transfer from ZnP to C60. Semiempirical and density-functional theory calculations were used to help understand the excited-state deactivation mechanisms.

Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn(III)(TBP8Cz)] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn(III)(TBP8Cz) to give Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] with protonation constants of 9.0 × 10(6) and 4.7 × 10(3) M(-1), respectively. The protonated sites of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn(III)(OTf)(TBP8Cz(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O2 to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] in the presence of O2 revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn(III)(OTf)(TBP8Cz(H)) reacted with O2 with a diffusion-limited rate constant to produce the putative Mn(IV)(O2(•-))(OTf)(TBP8Cz(H)), whereas the tripseptet excited state of [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] exhibited no reactivity toward O2. In the presence of HOTf, Mn(V)(O)(TBP8Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn(III)(OTf)(TBP8Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d12, suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn(III)(OTf)(TBP8Cz(H)), acts as an efficient photocatalyst for the oxidation of HMB by O2 to the alcohol.

Synthesis and Atropisomerism of Cascaded Tetraphenylporphyrin-[60]Fullerene Hybrids

Flexible, linked dendritic tetraphenylporphyrin (TPP)-fullerene hybrids were synthesized. They were designed to gain insight into and mimic the primary events in the natural photosynthetic reaction center. These multiporphyrin moieties are based on a light-harvesting concept. Moreover, they incorporate multiple redox components aligned along a redox gradient. Newkome-type dendrons were added to these TPP-fullerene hybrids. In principle they can mediate pH-dependent water solubility, which, however, could not be observed in this case. A protecting-group strategy using tert-butyldiphenylsilyl groups allows convergent synthesis of the dendritic compounds. The dendritic multiporphyrins were synthesized separately and can be used as individual building blocks. Atropisomerism was observed in the dendritic compounds, and single atropisomers could be assigned to the corresponding peaks of a characteristic pattern in the NMR spectra. Deprotection of the Newkome-type dendrons was shown to be feasible under mild conditions that leave the redox gradient intact.

Investigation of photoinduced electron transfer on TiO2 nanowire arrays/porphyrin composite via scanning electrochemical microscopy

A photo-induced electron transfer (PET) system was constructed by the combination of vertically aligned single-crystal TiO₂ nanowire arrays and porphyrin to investigate the mechanism of the charge transfer process in artificial photosynthesis.

Molecular wires – impact of π-conjugation and implementation of molecular bottlenecks

We highlight recent progress in the field of electron transport through molecular bridges as integrative parts of electron donor–bridge–acceptor conjugates.

Photocatalytic oxygenation of 10-methyl-9,10-dihydroacridine by O₂ with manganese porphyrins

Photocatalytic oxygenation of 10-methyl-9,10-dihydroacridine (AcrH₂) by dioxygen (O₂) with a manganese porphyrin [(P)Mn^III: 5,10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphinatomanganese(III) hydroxide [(TMP)Mn^III(OH)] (1) or 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinatomanganese(III) acetate [(TPFPP)Mn^III(CH₃COO)] (2)] occurred to yield 10-methyl-(9,10H)-acridone (Acr=O) in an oxygen-saturated benzonitrile (PhCN) solution under visible light irradiation. The photocatalytic reactivity of (P)Mn^III in the presence of O₂ is in proportion to concentrations of AcrH₂ or O₂ with the maximum turnover numbers of 17 and 6 for 1 and 2, respectively. The quantum yield with 1 was determined to be 0.14%. Deuterium kinetic isotope effects (KIEs) were observed with KIE = 22 for 1 and KIE = 6 for 2, indicating that hydrogen-atom transfer from AcrH₂ is involved in the rate-determining step of the photocatalytic reaction. Femtosecond transient absorption measurements are consistent with photoexcitation of (P)Mn^III, resulting in intersystem crossing from a tripquintet excited state to a tripseptet excited state. A mechanism is proposed where the tripseptet excited state reacts with O₂ to produce a putative (P)Mn^IV superoxo complex. Hydrogen-atom transfer from AcrH₂ to (P)Mn^IV(O₂^•–) generating a hydroperoxo complex (P)Mn^IV(OOH) and AcrH^• is likely the rate-determining step, in competition with back electron transfer to regenerate the ground state (P)Mn^III and O₂. The subsequent reductive O–O bond cleavage by AcrH^• may occur rapidly inside of the reaction cage to produce (P)Mn^V(O) and AcrH(OH), followed by the oxidation of AcrH(OH) by (P)Mn^V(O) to yield Acr=O with regeneration of (P)Mn^III.

Charge transfer excitations in cofacial fullerene-porphyrin complexes

Porphyrin and fullerene donor-acceptor complexes have been extensively studied for their photo-induced charge transfer characteristics. We present the electronic structure of ground states and a few charge transfer excited states of four cofacial porphyrin-fullerene molecular constructs studied using density functional theory at the all-electron level using large polarized basis sets. The donors are base and Zn-tetraphenyl porphyrins and the acceptor molecules are C(60) and C(70). The complexes reported here are non-bonded with a face-to-face distance between the porphyrin and the fullerene of 2.7 to 3.0 Å. The energies of the low lying excited states including charge transfer states calculated using our recent excited state method are in good agreement with available experimental values. We find that replacing C(60) by C(70) in a given dyad may increase the lowest charge transfer excitation energy by about 0.27 eV. Variation of donor in these complexes has marginal effect on the lowest charge transfer excitation energy. The interfacial dipole moments and lowest charge transfer states are studied as a function of face-to-face distance.

A charge-transfer challenge: combining fullerenes and metalloporphyrins in aqueous environments

A series of truly water-soluble C(60)/porphyrin electron donor-acceptor conjugates has been synthesized to serve as powerful mimics of photosynthetic reaction centers. To this end, the overall water-solubility of the conjugates was achieved by adding hydrophilic dendrimers of different generations to the porphyrin moiety. An important variable is the metal center of the porphyrin; we examined zinc(II), copper(II), cobalt(II), nickel(II), iron(III), and manganese(III). The first insights into electronic communication between the electron donors and the electron acceptors came from electrochemical assays, which clearly indicate that the redox processes centered either on C(60) or the porphyrins are mutually affected. Absorption measurements, however, revealed that the electronic communication in terms of, for example, charge-transfer features, remains spectroscopically invisible. The polar environment that water provides is likely to be a cause of the lack of detection. Despite this, transient absorption measurements confirm that intramolecular charge separation processes in the excited state lead to rapid deactivation of the excited states and, in turn, afford the formation of radical ion pair states in all of the investigated cases. Most importantly, the lifetimes of the radical ion pairs were found to depend strongly on several aspects. The nature of the coordinated metal center and the type of dendrimer have a profound impact on the lifetime. It has been revealed that the nature/electronic configuration of the metal centers is decisive in powering a charge recombination that either reinstates the ground state or any given multiplet excited state. Conversely, the equilibrium of two opposing forces in the dendrimers, that is, the interactions between their hydrophilic regions and the solvent and the electronic communication between their hydrophobic regions and the porphyrin and/or fullerene, is the key to tuning the lifetimes.

Conformationally constrained macrocyclic diporphyrin-fullerene artificial photosynthetic reaction center

Photosynthetic reaction centers convert excitation energy from absorbed sunlight into chemical potential energy in the form of a charge-separated state. The rates of the electron transfer reactions necessary to achieve long-lived, high-energy charge-separated states with high quantum yields are determined in part by precise control of the electronic coupling among the chromophores, donors, and acceptors and of the reaction energetics. Successful artificial photosynthetic reaction centers for solar energy conversion have similar requirements. Control of electronic coupling in particular necessitates chemical linkages between active component moieties that both mediate coupling and restrict conformational mobility so that only spatial arrangements that promote favorable coupling are populated. Toward this end, we report the synthesis, structure, and photochemical properties of an artificial reaction center containing two porphyrin electron donor moieties and a fullerene electron acceptor in a macrocyclic arrangement involving a ring of 42 atoms. The two porphyrins are closely spaced, in an arrangement reminiscent of that of the special pair in bacterial reaction centers. The molecule is produced by an unusual cyclization reaction that yields mainly a product with C(2) symmetry and trans-2 disubstitution at the fullerene. The macrocycle maintains a rigid, highly constrained structure that was determined by UV-vis spectroscopy, NMR, mass spectrometry, and molecular modeling at the semiempirical PM6 and DFT (B3LYP/6-31G**) levels. Transient absorption results for the macrocycle in 2-methyltetrahydrofuran reveal photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene to form a P(•+)-C(60)(•-)-P charge separated state with a time constant of 1.1 ps. Photoinduced electron transfer to the fullerene excited singlet state to form the same charge-separated state has a time constant of 15 ps. The charge-separated state is formed with a quantum yield of essentially unity and has a lifetime of 2.7 ns. The ultrafast charge separation coupled with charge recombination that is over 2000 times slower is consistent with a very rigid molecular structure having a small reorganization energy for electron transfer, relative to related porphyrin-fullerene molecules.