Self-assembled Triads Research Articles

Singlet oxygen generation by nanoassemblies containing porphyrin macrocycles: Steric and screening effects, energy transfer and competing processes

In this semi-review paper, we discuss some principal aspects that should be taken into account upon quantitative analysis of the interaction with molecular oxygen O2 and the singlet oxygen (1O2 or [Formula: see text] generation by various tetrapyrrolic photosensitizers (including monomers, chemical dimers, triads, pentads and organic-inorganic nanoassemblies). In all cases, the direct experimental measurements of 1O2 emission at [Formula: see text] = 1.27 μ need to be corrected to the solvent influence (refractive indexes, molecular polarizability) on the rate constant of the radiative transition [Formula: see text] in a 1O2 molecule. For porphyrin and chlorin chemical dimers, T1 states quenching by O2 followed by 1O2 generation depends on donor-acceptor interactions between the dimer halves, and extra-ligation effects as well as the spacer flexibility. In the case of self-assembled triads and pentads containing Zn-porphyrin dimers and pyridyl substituted porphyrin free bases (H2P) as extra-ligands, quenching rate constants of H2P T1 states by O2 are smaller compared to those found for individual monomeric H2P molecules which is explained by the spatial screening influence of a strongly quenched Zn-porphyrin dimer in multiporphyrin complexes. Finally, at 293 K for nanoassemblies of two types (semiconductor quantum dots CdSe/ZnS + coordinative linked tetra-pyridyl porphyrins, and semiconductor negatively charged quantum dots AgInS/ZnS + positively charged porphyrin molecules) it was quantitatively shown that non-radiative energy transfer FRET quantum dot [Formula: see text] porphyrin (competing with electron tunneling under conditions of quantum confinement) is only responsible for the singlet oxygen 1O2 generation by nanoassemblies.

Highly chemoselective and fast practical visible photoreduction of nitroaromatic compounds to aromatic amines and amides using a self-assembled triad TiO2-TEOA-NC (LMCT/EDA) complex system

The triad TiO2-TEOA-NC (LMCT/EDA) complex system was self-assembled and used as a heterosupramolecular nano device in fast practical reduction of nitro compounds under visible light irradiation.

Light-Driven Redox Activation of CO2- and H2-Activating Complexes in a Self-Assembled Triad.

We report a self-assembled triad for artificial photosynthesis composed of a chromophore, carbon-dioxide reduction catalyst, and hydrogen-oxidation complex, which is designed to operate without conventional sacrificial redox equivalents. Excitation of the zinc-porphyrin chromophore of the triad results in ultrafast charge transfer between a tungsten-alkylidyne donor and a rhenium diimine tricarbonyl acceptor, producing a charge-separated state that persists on the time scale of tens of nanoseconds and is thermodynamically capable of the primary dihydrogen and carbon dioxide binding steps for initiating the reverse water-gas shift reaction. The charge-transfer behavior of this system was probed using transient absorption spectroscopy in the visible, near-infrared, and mid-infrared spectral regions. The behavior of the triad was compared with that of the zinc-porphyrin-rhenium-diimide dyad; the triad was found to have a significantly longer charge-separated lifetime than other previously reported porphyrin-rhenium diimine compounds.

Modulation of Energy Transfer into Sequential Electron Transfer upon Axial Coordination of Tetrathiafulvalene in an Aluminum(III) Porphyrin-Free-Base Porphyrin Dyad.

Axially assembled aluminum(III) porphyrin based dyads and triads have been constructed to investigate the factors that govern the energy and electron transfer processes in a perpendicular direction to the porphyrin plane. In the aluminum(III) porphyrin-free-base porphyrin (AlPor-Ph-H2Por) dyad, the AlPor occupies the basal plane, while the free-base porphyrin (H2Por) with electron withdrawing groups resides in the axial position through a benzoate spacer. The NMR, UV-visible absorption, and steady-state fluorescence studies confirm that the coordination of pyridine appended tetrathiafulvalene (TTF) derivative (TTF-py or TTF-Ph-py) to the dyad in noncoordinating solvents afford vertically arranged supramolecular self-assembled triads (TTF-py→AlPor-Ph-H2Por and TTF-Ph-py→AlPor-Ph-H2Por). Time-resolved studies revealed that the AlPor in dyad and triads undergoes photoinduced energy and/or electron transfer processes. Interestingly, the energy and electron donating/accepting nature of AlPor can be modulated by changing the solvent polarity or by stimulating a new competing process using a TTF molecule. In modest polar solvents (dichloromethane and o-dichlorobenzene), excitation of AlPor leads singlet-singlet energy transfer from the excited singlet state of AlPor ((1)AlPor*) to H2Por with a moderate rate constant (k(EnT)) of 1.78 × 10(8) s(-1). In contrast, excitation of AlPor in the triad results in ultrafast electron transfer from TTF to (1)AlPor* with a rate constant (k(ET)) of 8.33 × 10(9)-1.25 × 10(10) s(-1), which outcompetes the energy transfer from (1)AlPor* to H2Por and yields the primary radical pair TTF(+•)-AlPor(-•)-H2Por. A subsequent electron shift to H2Por generates a spatially well-separated TTF(+•)-AlPor-H2Por(-•) radical pair.

Photoinduced electron transfer in a supramolecular triad produced by porphyrin anion-induced electron transfer from tetrathiafulvalene calix[4]pyrrole to Li(+)@C60.

Binding of a porphyrin carboxylate anion () to tetrathiafulvalene calix[4]pyrrole (TTF-C4P) results in electron transfer from TTF-C4P to Li(+)@C60 to produce the charge-separated state (1/TTF-C4P˙(+)/Li(+)@C60˙(-)) in benzonitrile. Upon photoexcitation of , photoinduced electron transfer from the triplet excited state of to TTF-C4P˙(+) occurs to produce the higher energy charge-separated state (˙(+)/TTF-C4P/Li(+)@C60˙(-)), which decays to the ground state with a lifetime of 4.8 μs.

First demonstration of two-step FRET in a synthetic supramolecular assembly

We report a trichromophoric pseudorotaxane (3NPC) that demonstrates a two-step FRET-based relay comprising of primary donor–primary acceptor/secondary donor–secondary acceptor pairs. To achieve this, naphthalene (1) and coumarin (3) were used as the primary donor and secondary acceptor, respectively; while pyrene (2 or T) unit, having spectral overlap with both 1 and 3, was used as the primary acceptor as well as the secondary donor. Results of time resolved and steady-state emission studies reveal that sequential resonance energy transfer (FRET 1 → 2 → 3) and direct resonance energy transfer (FRET 1 → 3) processes are operational simultaneously. In the sequential energy transfer process (1 → 2 → 3) of the triad system, pyrene acts as a transmitter and makes the whole supramolecular assembly behave as a linear energy transferring wire. Sensitised emission of the coumarin moiety, i.e. the secondary and eventual final acceptor, offers the possibility of evaluating the performance of this linear trichromophoric system by one and two-step FRET mechanisms. Formation of an interwoven complex between the host (NCC24O8) and the guest (T) components, utilizing various non-bonding interactions (intercomponent [N+–H⋯O], [C–H⋯O] and π–π stacking interactions) has been confirmed by 1H NMR studies in solution and a single crystal X-structural study in the solid state. The formation constant (Kf = 2.31 × 103 M−1) for the [2]pseudorotaxane was evaluated using isothermal titration calorimetric studies. The complex gives rise to the first crystal structure of a self-assembled [2]pseudorotaxane that demonstrates sequential two-step FRET based energy transfer in a self-assembled triad that is held in an assembly through non-covalent interactions, such as hydrogen bonding and π–π stacking.

Molecular and ionic complexes of pyrrolidinofullerene bearing chelating 3-pyridyl units

Molecular and ionic complexes of cis-2',5'-di(pyridin-3-yl)pyrrolidino[3',4':1,9](C(60)-I(h))[5,6]fullerene DP3FP with chlorobenzene (C(6)H(5)Cl), manganese(II) tetraphenylporphyrin (Mn(II)TPP) and tetrakis(dimethylamino)ethylene (TDAE) have been obtained for the first time. X-ray single crystal structure determination for the crystalline DP3FP·C(6)H(5)Cl (1) solvate proved unambiguously its molecular structure with the cis-arrangement of chelating 3-pyridyl groups. It has been demonstrated that DP3FP easily forms self-assembled photoactive complexes with metallated porphyrins. For example, the formation of a 1 : 1 complex between DP3FP and zinc (II) tetraphenylporphyrin (Zn(II)TPP) in cyclohexane solution (2) was evidenced using absorption spectroscopy. A successful X-ray single crystal structure determination was performed for a self-assembled triad composed of a DP3FP molecule linked with two Mn(II)TPP molecules in {DP3FP·(Mn(II)TPP)(2)}·(C(6)H(4)Cl(2))(3) (3). A strong organic donor TDAE reduces DP3FP to the radical anion state thus forming an ionic complex (TDAE˙(+))·(DP3FP˙(-))·(C(6)H(4)Cl(2))(1.6) (4). Optical, electronic and magnetic properties of 4 were investigated in detail. The performed studies strongly suggest that pyrrolidinofullerene DP3FP can be used as a building block in the design of various organic materials with advanced optoelectronic and/or magnetic properties.

A supramolecular Star Wars Tie Fighter Ship: electron transfer in a self-assembled triad composed of two zinc naphthalocyanines and a fullerene

Photoactive supramolecules composed of electron donor and electron acceptor entities are important for light energy harvesting applications. In the present study, a Star Wars Tie Fighter Ship shaped supramolecular triad was constructed by self-assembling two zinc naphthalocyanines to a fulleropyrrolidine bearing two pyridine entities using an axial coordination approach. Optical absorption and emission studies revealed stable complex formation, and the experimentally determined free-energy change revealed the possibility of electron transfer from singlet excited zinc naphthalocyanine to the fulleropyrrolidine. The picosecond time-resolved emission technique was utilized to evaluate the kinetics of charge separation while nanosecond transient absorption spectral studies provided evidence for electron transfer quenching. The measured charge-separation rate, k CS and quantum yield, Φ CS were found to be 5.7 × 109 s −1 and 0.93 in toluene, respectively, indicating an efficient process within the supramolecular triad. The charge recombination rate (k CR ) of the supramolecular ion-pair calculated from the nanosecond transient absorption technique was found to be 3.5 × 107 s −1 yielding a lifetime for the radical ion-pair (τ RIP ) of about 30 ns. Changing the solvent from the noncoordinating toluene to the coordinating benzonitrile or THF destroyed the supramolecular structure, and under these experimental conditions, only intermolecular electron transfer from the triplet excited zinc naphthalocyanine to fulleropyrrolidine could be observed. Under these conditions, the measured electron transfer rates, k et , T inter , were found to be 2.6 × 107 M −1. s −1 in benzonitrile and 1.2 × 107 M −1. s −1 in THF, respectively.

Excited-state energy-transfer dynamics in self-assembled triads composed of two porphyrins and an intervening Bis(dipyrrinato)metal complex.

The synthesis and characterization of various triads composed of a linear array of two zinc porphyrins joined via an intervening bis(dipyrrinato)metal(II) complex are reported. The preparation exploits the facile complexation of dipyrrins with divalent metal ions to give bis(dipyrrinato)metal(II) complexes [abbreviated (dp)(2)M]. Copper(II) and palladium(II) chelates of dipyrrins (available by oxidation of dipyrromethanes) were prepared in 50-80% yield. A one-flask synthesis of bis(dipyrrinato)zinc(II) complexes was developed by oxidation of a dipyrromethane with DDQ or p-chloranil in the presence of Zn(OAc)(2).2H(2)O in THF ( approximately 80% yield). Three routes were developed for preparing porphyrin-dipyrrins: (1). Suzuki coupling of a boronate-substituted zinc porphyrin (ZnP) and bis[5-(4-iodophenyl)dipyrrinato]Pd(II) to give the (ZnP-dp)(2)Pd triad (50% yield), followed by selective demetalation of the (dp)(2)Pd unit by treatment with 1,4-dithiothreitol under neutral conditions (71% yield); (2). oxidation of a porphyrin-dipyrromethane with p-chloranil in the presence of Zn(OAc)(2).2H(2)O followed by chromatography on silica gel (71% yield); and (3). condensation of a dipyrrin-dipyrromethane and a dipyrromethane-dicarbinol under InCl(3) catalysis followed by oxidation with DDQ (10-16% yield). Four triads of form (ZnP-dp)(2)Zn were prepared in 83-97% yield by treatment of a porphyrin-dipyrrin with Zn(OAc)(2).2H(2)O at room temperature. Free base dipyrrins typically absorb at 430-440 nm, while the bis(dipyrrinato)metal complexes absorb at 460-490 nm. The fluorescence spectra/yields and excited-state lifetimes of the (ZnP-dp)(2)Zn triad in toluene show (1). efficient energy transfer from the bis(dipyrrinato)zinc(II) chromophore to the zinc porphyrins (98.5% yield), and (2). little or no quenching of the resulting excited zinc porphyrin relative to the isolated chromophore. Taken together, these results indicate that bis(dipyrrinato)zinc(II) complexes can serve as self-assembling linkers that further function as secondary light-collection elements in porphyrin-based light-harvesting arrays.

Self-assembled supramolecular triad composed of fulleropyrrolidine bearing two pyridine moieties axially coordinated to two zinc porphyrins

Spectroscopic, redox, and photochemical behavior of a self-assembled donor-acceptor triad formed by axial coordination of zinc meso-tetraphenylporphyrin, Zn ( TPP ) with fulleropyrrolidine bearing two pyridine moieties ( C 60( Py )2) was investigated. The UV-visible and 1 H NMR spectral studies revealed supramolecular 1:2 triad formation between C 60( Py )2 and Zn ( TPP ). The determined overall formation constant, K was as large as 1.45 × 104 mol -2 in o-dichlorobenzene (o-DCB). The 1 H NMR studies revealed axial coordination of both the pyridine entities of C 60( Py )2 to the central metal ion of the zinc porphyrins. Electrochemical studies of the isolated self-assembled triad in o-DCB established the molecular stoichiometry of the supramolecular complex possessing one C 60( Py )2 and two Zn ( TPP ) entities. The excited state electron transfer reactions were monitored by both steady-state emission as well as transient absorption techniques. In o-DCB, upon coordination of the pyridine entities of C 60( Py )2 to two Zn ( TPP ), the main quenching pathway involved charge-separation from the singlet excited Zn ( TPP ) to the C 60 moiety. However, in a coordinating solvent like PhCN , intermolecular electron-transfer takes place from the triplet excited state of Zn ( TPP ) to the C 60 moiety.

Interaction of multiporphyrin systems with molecular oxygen in liquid solutions: extra-ligation and screening effects

Steady-state and time-resolved studies indicate that for a sequence of porphyrin or chlorin chemical dimers Zn-cyclodimer→(ZnOEP) 2Ph→(ZnOEP) 2→(ZnHTPP) 2→(ZnOEChl) 2 with relative lowering of excited S 1- and T 1-states, the extra-ligation by pyridine (PYR) does not influence essentially on fluorescence parameters but leads to an increase of T 1-states non-radiative decay (the most pronounced for dimers with higher lying T 1-levels). For pyridinated dimers at 293 K T 1-states quenching by molecular oxygen depends on the spacer flexibility and donor–acceptor interactions with PYR. In self-assembled triads and pentads energy and electron transfer (within a few ps) takes place from Zn-dimers to pyridyl substituted porphyrin extra-ligand, H 2P, followed by the effective population of H 2P T 1-state. For these systems, bimolecular constants of H 2P T 1-states quenching by O 2 decrease by 1.4–1.8 times with respect to those found for individual monomeric porphyrins. This effect is explained by the screening action of a strongly quenched Zn–porphyrin dimer subunit limiting the access of oxygen molecule to the excited extra-ligand.

Temperature and solvent dependent charge transfer in self-organized porphyrin-quinone compounds

Abstract