Synthesis and photophysics of a linear non-covalently linked porphyrin-fullerene dyad

The synthesis and characterization of a new pyridinofullerene ligand capable of forming axially symmetric complexes with ZnTPP is reported; molecular modelling studies, 1H NMR, UV-Vis spectroscopy and fluorescence quenching data support formation of a strong complex between the new ligand and ZnTPP.     

Synthesis and photophysics of porphyrin-fullerene donor-acceptor dyads with conformationally flexible linkers

The synthesis and photophysics of a series of porphyrin-fullerene (P-C₆₀) dyads in which the two chromophores are linked by conformationally flexible polyether chains is reported. Molecular modeling indicates the two moieties adopt a stacked conformation in which the two chromophores are in close proximity. Photoexcitation of the free base dyads in polar solvents such as tetrahydrofuran and benzonitrile, causes electron transfer (ET) to generate charge-separated radical pair (CSRP) states, which were directly detected using transient absorption (TA) techniques. In nonpolar solvents such as toluene, where CSRP states were not directly detected, fullerene triplet state states were formed, according to TA studies as well as singlet oxygen sensitization measurements. The low value of the quantum efficiency for sensitized formation of singlet molecular oxygen [O₂(¹Δ_g)] in toluene and chloroform indicates that singlet energy transduction to give H₂P-¹C₆₀*, followed by intersystem crossing to H₂P-³C₆₀* and energy transfer to ³O₂, is not the operative mechanism. Rather, a mechanism is proposed involving ET to give CSRP states followed by exergonic charge recombination to eventually generate fullerene triplets. Such a mechanism has been demonstrated experimentally for structurally related P-C₆₀ dyads. For the corresponding ZnP-C₆₀ dyads with flexible linkers, only photoinduced ET to generate long-lived CSRP states is observed. Photoinduced charge separation in these dyad systems is extremely rapid, consistent with a through space rather than through-bond mechanism. Charge recombination is up to three orders of magnitude slower, indicating this process occurs in the inverted region of the Marcus curve that relates ET rates to the thermodynamic driving force. These observations once again demonstrate the advantages of incorporating fullerenes as electron acceptor components in photosynthetic model systems.     

Supramolecular open-framework based on 1-D iron phosphate-diphosphate chains assembled through hydrogen bonding

Fe(H₂PO₄)(H₂P₂O₇)·C₅H₅N, a new iron(III) phosphate with an open-framework has been synthesized hydrothermally using pyridine as organic template. The crystal structure was solved ab initio using conventional powder X-ray diffraction data. The unit cell is orthorhombic, a=9.5075(2), b=10.1079(1), c=13.3195(2) Å, space group P2₁2₁2₁, Z=4. The structure consists of FeO₆ octahedra joined by H₂PO₄ and H₂P₂O₇ groups forming linear chains interconnected by hydrogen bonding to give rise to a supramolecular framework enclosing tunnels in which the pyridine molecules reside.     

Spectral, electrochemical, and photophysical studies of a magnesium porphyrin-fullerene dyad

A covalently linked magnesium porphyrin-fullerene (MgPo-C60) dyad was synthesized and its spectral, electrochemical, molecular orbital, and photophysical properties were investigated and the results were compared to the earlier reported zinc porphyrin-fullerene (ZnPo-C60) dyad. The ab initio B3LYP/3-21G(*) computed geometry and electronic structure of the dyad predicted that the HOMO and LUMO are mainly localized on the MgP and C60 units, respectively. In o-dichlorobenzene containing 0.1 M (n-Bu)4NClO4, the synthesized dyad exhibited six one-electron reversible redox reactions within the potential window of the solvent. The oxidation and reduction potentials of the MgP and C60 units indicate stabilization of the charge-separated state. The emission, monitored by both steady-state and time-resolved techniques, revealed efficient quenching of the singlet excited state of the MgP and C60 units. The quenching pathway of the singlet excited MgP moiety involved energy transfer to the appended C60 moiety, generating the singlet excited C60 moiety, from which subsequent charge-separation occurred. The charge recombination rates, k(CR), evaluated from nanosecond transient absorption studies, were found to be 2-3 orders of magnitude smaller than the charge separation rate, k(CS). In o-dichlorobenzene, the lifetime of the radical ion-pair, MgPo*+-C60*-, was found to be 520 ns which is longer than that of ZnPo*+-C60*- indicating better charge stabilization in MgPo-C60. Additional prolongation of the lifetime of MgPo*+-C60*- was achieved by coordinating nitrogenous axial ligands. The solvent effect in controlling the rates of forward and reverse electron transfer is also investigated.     

Ab initio description of photoabsorption and electron transfer in a doubly-linked porphyrin-fullerene dyad

Structure, photoabsorption and excited states of two representative conformations obtained from molecular dynamics (MD) simulations of a doubly-linked porphyrin-fullerene dyad DHD6ee are studied by using both DFT and wavefunction based methods. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the relaxation of the excited state are of special interest. The results obtained with LDA, GGA, and hybrid functionals (SVWN, PBE, and B3LYP, respectively) are analyzed with emphasis on the performance of used functionals as well as from the point of view of their comparison with wavefunction based methods (CCS, CIS(D), and CC2). Characteristics of the MD structures are retained in DFT optimization. The relative orientation of porphyrin and fullerene is significantly influencing the MO energies, the charge transfer (CT) in the ground state of the dyad and the excitation of ground state CT complex (g-CTC). At the same time, the excitation to the locally excited state of porphyrin is only little influenced by the orientation or cc distance. TD-DFT underestimates the excitation energy of the CT state, however for some cases (with relatively short donor-acceptor separations), the use of a hybrid functional like B3LYP alleviates the problem. Wavefunction based methods and CC2 in particular appear to overestimate the CT excitation energies but the inclusion of proper solvation models can significantly improve the results.     

Synthesis of a porphyrin-fullerene pinwheel

We disclose the synthesis of a porphyrin-fullerene pinwheel that was subsequently observed by scanning tunneling microscopy. The molecule was designed to further our understanding of fullerene-surface interactions, directional control, and surface-rolling versus pivoting capabilities of this class of nanomachines. The inner porphyrin provides the square planar configuration that might lead to realization of the pinwheel spiraling motion on surfaces.     

Intramolecular hydrogen bonding plays a crucial role in the photophysics and photochemistry of the GFP chromophore

In commonly studied GFP chromophore analogues such as 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (PHBDI), the dominant photoinduced processes are cis-trans isomerization and subsequent S(1) → S(0) decay via a conical intersection characterized by a highly twisted double bond. The recently synthesized 2-hydroxy-substituted isomer (OHBDI) shows an entirely different photochemical behavior experimentally, since it mainly undergoes ultrafast intramolecular excited-state proton transfer, followed by S(1) → S(0) decay and ground-state reverse hydrogen transfer. We have chosen 4-(2-hydroxybenzylidene)-1H-imidazol-5(4H)-one (OHBI) to model the gas-phase photodynamics of such 2-hydroxy-substituted chromophores. We first use various electronic structure methods (DFT, TDDFT, CC2, DFT/MRCI, OM2/MRCI) to explore the S(0) and S(1) potential energy surfaces of OHBI and to locate the relevant minima, transition state, and minimum-energy conical intersection. These static calculations suggest the following decay mechanism: upon photoexcitation to the S(1) state, an ultrafast adiabatic charge-transfer induced excited-state intramolecular proton transfer (ESIPT) occurs that leads to the S(1) minimum-energy structure. Nearby, there is a S(1)/S(0) minimum-energy conical intersection that allows for an efficient nonadiabatic S(1) → S(0) internal conversion, which is followed by a fast ground-state reverse hydrogen transfer (GSHT). This mechanism is verified by semiempirical OM2/MRCI surface-hopping dynamics simulations, in which the successive ESIPT-GSTH processes are observed, but without cis-trans isomerization (which is a minor path experimentally with less than 5% yield). These gas-phase simulations of OHBI give an estimated first-order decay time of 476 fs for the S(1) state, which is larger but of the same order as the experimental values measured for OHBDI in solution: 270 fs in CH(3)CN and 230 fs in CH(2)Cl(2). The differences between the photoinduced processes of the 2- and 4-hydroxy-substituted chromophores are attributed to the presence or absence of intramolecular hydrogen bonding between the two rings.     

Molecular simulations for the conformational assessment of a porphyrin-fullerene dyad in different environments

Conformational space of a porphyrin-fullerene dyad with the donor and acceptor connected by a relatively flexible linker is studied by molecular dynamics simulations in both non-polar and polar solvents, as well as in vacuum. The most probable conformations obtained from the vacuum MD simulations were optimized with semi-empirical (SE) and density functional theory (DFT) methods and the extent of the structural changes is assessed. The computational results indicate the co-existence of different conformers in both polar and nonpolar solvents showing agreement with experimental results. The most probable vacuum conformations at 300 K are similar to the ones at 0 K, while the structures most often observed in the solvents show less compact conformations. Optimization with SE and DFT calculations leads to structures, which represent relatively well the folded conformations in solvent, which validates the electronic structure calculations relevant to describing photoinduced electron-transfer in H2P-O34-C60.     

Photoinduced electron transfer in a Watson-Crick base-paired, 2-aminopurine:uracil-C60 hydrogen bonding conjugate

A fluorescent reporter molecule, 2-aminopurine was self-assembled via Watson-Crick base-pairing to a uracil appended fullerene to form a donor-acceptor conjugate; efficient photoinduced charge separation was confirmed by time-resolved emission and transient absorption spectral studies.     

Effect of anion ligation on electron transfer of double-linked zinc porphyrin-fullerene dyad

The interaction between metalloporphyrins and their axial ligands plays an important role in the electron transfer (ET) processes in which the excited porphyrin participates. An efficient photoinduced ET reaction of a double-linked zinc(II) porphyrin-fullerene dyad was demonstrated in ionic environment. The chloride ion of tetrabutylammonium chloride (TBACl) electrolyte solution ligates the zinc porphyrin moiety in the dyad which results in a red shift of the absorption bands and lowers the energy of the charge-separated state by about 0.26 eV as compared to the nonligated dyad. Excitation of the porphyrin chromophore results in ET from porphyrin to fullerene in a moderately polar solvent, anisole. In nonionic and nonligating ionic environments, the ET reaction occurs through an intermediate state, an intramolecular exciplex, which has emission in the near-infrared region of the spectrum. This emission is not observed directly for the dyad in TBACl/anisole solution, but evidence of the exciplex intermediate was seen in the time-resolved measurements. The lower energy of the charge-separated state in the ligated environment explains the different ET reaction rates determined in the spectroscopic studies: the charge recombination process of the ligated dyad is about 5 times faster than that of the nonligated one.     

Charge-transfer emission of compact porphyrin-fullerene dyad analyzed by Marcus theory of electron-transfer

A porphyrin-fullerene dyad, which is characterized by a close proximity of the porphyrin donor and the fullerene acceptor, was found to undergo a photoinduced electron transfer both in solutions and in solid films. Near-infrared charge-transfer (CT) emission was observed and analyzed in frame of the semi-classical Marcus electron-transfer theory yielding values for the reaction free energy, -deltaG degrees = 1.75 eV, the internal reorganization energy, lambdav = 0.05 eV, and the donor-acceptor vibrational energy, hv(v) = 0.14 eV, both in solution and in solid film. The influence of the environment on the CT properties of the dyad is described by a single parameter, the outer-sphere reorganization energy, lambdas, which varies from 0.05 eV in non-polar solvents and films to 0.13 eV in solvents of moderate polarity. At low temperatures (T< 200 K), the CT emission consists of distinct bands shifted from each other by value hv(v). This is the first direct observation of the vibrational frequencies of a porphyrin-fullerene donor-acceptor system.     

Matrix polycondensation through hydrogen bonding interaction

Polycondensation of diesters having hydroxyl or pyridine groups was carried out with hexamethylenediamine (HMD) in the presence of (vinyl alcohol/vinyl acetate) copolymers as a matrix polymer. Apparent rates of the polycondensation of dimethyl tartrate (DMT) with HMD increased with increasing contents of PVA units in the copolymers and a strong entanglement between growing polyamide chains and PVA copolymers took place through the adsorption of HMD and DMT on the matrix copolymers. 2,6-Dimethyl pyridine dicarboxylate (2,6-DMP) reacted with HMD in the presence of the PVA copolymers or polysaccharide, while the rate enhancement effect of the matrix polymers was not significantly observed, as in the case of DMT. The effect of the matrix polymers on the polycondensation was discussed in terms of hydrogen bonding interactions.     

Synthesis and photophysics of a novel photocatalyst for hydrogen production based on a tetrapyridoacridine bridging ligand

Molecular photocatalysts allow for selectively tuning their function on a molecular level based on an in-depth understanding of their chemical and photophysical properties. This contribution reports the synthesis and photophysical characterization of the novel molecular photocatalyst [(tbbpy)₂Ru(tpac)PdCl₂]²⁺RutpacPd (with tpac = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2?,3?-j]acridine) and its mononuclear building block. Furthermore, detailed photocatalytic activity measurements of RutpacPd are presented. The introduction of the tpac-ligand into the molecular framework offers a potential route to reduce the impact of water as compared to the well-studied class of RutpphzPd (with tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2?,3?-j]phenazine) complexes. The distinct impact of water on the electron-transfer processes in tpphz-ligands stems from the possibility of water to form hydrogen bonds to the phenazine nitrogen atoms and will potentially reduced when replacing the phenazine by the acridine unit. The effect of this structural variation on the catalytic properties and the underlying ultrafast intramolecular charge transfer behavior will be discussed in detail.     

Self-assembly of a pi-electronic amphiphile consisting of a zinc porphyrin-fullerene dyad: formation of micro-vesicles with a high stability

An amphiphilic zinc porphyrin-fullerene dyad appended with triethyleneglycol chains in aqueous media forms uniformly-sized multilamellar vesicles with a mean diameter of 100 nm that are thermally stable and robust against membrane lysis with surfactants.     

Metal-free organocatalysis through explicit hydrogen bonding interactions

The metal (ion)-free catalysis of organic reactions is a contemporary challenge that is just being taken up by chemists. Hence, this field is in its infancy and is briefly reviewed here, along with some rough guidelines and concepts for further catalyst development. Catalysis through explicit hydrogen bonding interactions offers attractive alternatives to metal (ion)-catalyzed reactions by combining supramolecular recognition with chemical transformations in an environmentally benign fashion. Although the catalytic rate accelerations relative to uncatalyzed reactions are often considerably less than for the metal (ion)-catalyzed variants, this need not be a disadvantage. Also, owing to weaker enthalpic binding interactions, product inhibition is rarely a problem and hydrogen bond additives are truly catalytic, even in water.     

Guest recognition of a tetrapyridinohemicarcerand through hydrogen bonding and constrictive binding interactions

Tetrapyridinohemicarcerand 2 having four hydrogen-bonding acceptors of inward-directing pyridyl units was synthesized and their binding properties for a variety of organic guest molecules have been investigated. Tetrapyridinohemicarcerand 2 formed kinetically stable complexes with various sulfonic acids via intermolecular -SO₃H-pyridyl hydrogen bonding and constrictive binding interactions in C₂D₂Cl₄ at 25 °C. But carboxylic acids or alcohols cannot be a stable guest at the same conditions. Tetrapyridinohemicarcerand 2 also binds various disubstituted benzenes. Especially 1,4-diiodobenzene forms stable hemicarceplex 1,4-diiodobenzene@2, which seems to be stabilized by constrictive binding as well as by -C-H?I interactions between dioxymethylene of 2 and iodo group of guest.     

Stabilization of fulleropyrrolidine N-oxides through intrarotaxane hydrogen bonding

The chemical stabilization of labile fulleropyrrolidine N-oxides is achieved by encapsulation through intrarotaxane hydrogen bonding.     

2-Aminopyridinium ions activate nitroalkenes through hydrogen bonding

2-Aminopyridinium ions were found to activate nitroalkenes toward conjugate addition of heteroaromatic compounds with low catalyst loadings and the Diels-Alder reaction with the periselectivity opposite to that observed with metal Lewis acids, raising the possibility that a 2-aminopyridinium core might be a promising catalaphore for the asymmetric catalyst design.