Spectroscopic Properties of a Self-Assembled Zinc Porphyrin Tetramer II. Time-Resolved Fluorescence Spectroscopy

Stepwise Oxidative C-C Coupling and/or C-N Fusion of Zn(II) meso-Pyridin-2-ylthio-porphyrins

The synthesis and characterization of zinc(II) meso-pyridin-2-ylthio-porphyrins are presented in this manuscript. The (electro)chemical oxidation of [5-(pyridin-2-ylthio)-10,20-bis(p-tolyl)-15-phenylporphyrinato] zinc(II) or [5,15-bis(pyridin-2-ylthio)-10,20-bis(p-tolyl)porphyrinato] zinc(II) leads to the formation of one or two C-N bond(s) by intramolecular nucleophilic attack of the peripheral thiopyridinyl fragment(s) on the neighboring β-pyrrolic position(s) (C-N fusion reaction). In addition, the chemical oxidation of [5-(pyridin-2-ylthio)-10,20-bis(p-tolyl)porphyrinato] zinc(II), i.e., bearing one free meso position, mainly affords the meso,meso-dimer. Further stepwise electrochemical oxidation selectively produces the mono and bis C-N fused meso,meso-dimer. The resulting pyridinium derivatives exhibit important changes in their physicochemical properties (NMR, UV-vis, CV) as compared to their initial unfused precursors. Also, the X-ray crystallographic structures of three unfused monomers, one unfused meso,meso-dimer, and two C-N fused monomers are presented.

A Light-Harvesting/Charge-Separation Model with Energy Gradient Made of Assemblies of meta-Pyridyl Zinc Porphyrins

Self-assembly of porphyrins is a fascinating topic, not only for mimicking chlorophyll assemblies in photosynthetic organisms, but also for the potential of creating molecular-level devices. Herein, zinc porphyrin derivatives bearing a meta-pyridyl group at the meso position were prepared and their assemblies studied in chloroform. Among the porphyrins studied, one with a carbamoylpyridyl moiety gave a distinct ¹ H NMR spectrum in CDCl₃ , which allowed the supramolecular structure in solution to be probed in detail. Ring-current-induced chemical-shift changes in the ¹ H NMR spectrum, together with vapor-pressure osmometry and diffusion-ordered NMR spectroscopy, among other evidence, suggested that the porphyrin molecules form a trimer with a triangular cone structure. Incorporation of a directly linked porphyrin-ferrocene dyad with the same assembling properties in the assemblies led to a rare example of a light-harvesting/charge-separation system in which an energy gradient is incorporated and reductive quenching occurs.

Study of the in vivo antiviral activity against TMV treated with novel 1-(t-butyl)-5-amino-4-pyrazole derivatives containing a 1,3,4-oxadiazole sulfide moiety

A series of new 1-tert-butyl-5-amino-4-pyrazole bioxadiazole sulfide derivatives containing a 1,3,4-oxadiazole moiety were designed and synthesized. The bioactivity results showed that some title compounds exhibited excellent protective activity against TMV and certain insecticidal activity. Among the tested compounds, the EC₅₀ values of 5d, 5j, 5k and 5l were 165.8, 163.2, 159.7 and 193.1 mg/L, respectively, which are better than the EC₅₀ value of ningnanmycin (271.3 mg/L). The chlorophyll contents and the defense enzyme activities of the tobacco leaves after treatment with 5j were significantly increased, which indicated that this series of title compounds may induce the systemic acquired resistance of host to defend against diseases. Further in vivo protective activity research on 5j using TMV with a GFP gene tag found that it can effectively inhibit the spread of TMV in inoculated tobacco. A morphological study with TEM revealed that title compound 5h can cause a distinct break of the rod-shaped TMV. Moreover, the insecticidal activity revealed that the fatality rates of 5a, 5b and 5m against aphidoidea were 85%, 83% and 87%, respectively, which indicated that the title compounds can effectively block the common carrier of plant viruses, thereby effectively reducing the TMV infection risk of tobacco. This series of synergistic effects provide key information for the research and development of antiviral agents.

Photocatalytic Hydrogen Evolution by a Synthetic [FeFe] Hydrogenase Mimic Encapsulated in a Porphyrin Cage

The design of a biomimetic and fully base metal photocatalytic system for photocatalytic proton reduction in a homogeneous medium is described. A synthetic pyridylphosphole-appended [FeFe] hydrogenase mimic was encapsulated inside a supramolecular zinc porphyrin-based metal-organic cage structure Fe4 (Zn-L)6 . The binding is driven by the selective pyridine-zinc porphyrin interaction and results in the catalyst being bound strongly inside the hydrophobic cavity of the cage. Excitation of the capsule-forming porphyrin ligands with visible light while probing the IR spectrum confirmed that electron transfer takes place from the excited porphyrin cage to the catalyst residing inside the capsule. Light-driven proton reduction was achieved by irradiation of an acidic solution of the caged catalyst with visible light.

Excitonic dynamics of Chlorophyll-a molecules in chitosan hydrogel scaffold

Non-coherent energy hopping (hopping rate 4.28 ns⁻¹) through excitonically coupled 23° aligned Chl-a molecules within chitosan hydrogel matrix, for an artificial light harvesting system.

Spectroscopic Demonstration of Exciton Dynamics and Excimer Formation in a Sterically Controlled Perylene Bisimide Dimer Aggregate

Although it is commonly known that H-type PBI aggregates give rise to a broad, red-shifted excimer fluorescence with considerably longer fluorescence lifetimes than observed for the monomers, the underlying mechanisms of excimer formation and other relevant exciton dynamics in such π-stacked systems are still far from being understood. In this context, we demonstrate a thorough spectroscopic investigation on the exciton relaxation pathways, including excimer formation, in a perylene-3,4:9,10-bis(dicarboximide) (PBI) dimer aggregate 1 by using time-resolved fluorescence and transient absorption spectroscopy combined with excitation-power and polarization dependence. It was found that the excited dimer formation process followed by structural rearrangement is approximately two times faster than observed within larger PBI aggregates. Excitation-power-dependent transient absorption decay profiles revealed the fully delocalized nature of excitons in the dimer as opposed to larger stacks.

Electron transfer in electron donor-acceptor ensembles containing porphyrins and metalloporphyrins

Recent advances in electron transfer chemistry of electron donor-acceptor ensembles reported containing porphyrins and metalloporphyrins reported in ICPP-2 are reviewed briefly by focusing on the fundamental aspects of electron transfer reactions.

Photophysical properties of self-assembling porphinatozinc and photoinduced electron transfer with fullerenes

Photochemical and photophysical properties of self-assembling 5-(4-pyridyl)-10,15,20-triphenylporphinatozinc ( Znpyp 3) have been studied by steady-state and time-resolved absorption in addition to time-resolved fluorescence spectroscopy. Self-assembling oligomers ( Znpyp 3)n( n = 3 at 0.1 mM) were synthetically generated and assigned to a zigzag chain oligomeric structure, where n is dependant on the concentration and temperature. The lifetimes of the singlet and triplet excited states of ( Znpyp 3)n depend on n and the axial ligation. The rate-constant of ( Znpyp 3)n for the intermolecular T-T annihilation process was smaller than that of monomeric porphyrins. In the photoinduced electron-transfer to fullerenes ( C 60 and C 70), it was revealed that the rate-constants and efficiencies for ( Znpyp 3)n were essentially the same as those of the monomer. In the back electron transfer, the rate-constants of oligomers were smaller than that of the monomeric porphyrin, which suggests hole-delocalization along the porphyrin chain.

Structural Induced Control of Energy Transfer within Zn(II)−Porphyrin Dendrimers

We report on a study of singlet-singlet annihilation kinetics in a series of Zn(II)-porphyrin-appended dendrimers, where the energy transfer efficiency is significantly improved by extending the molecular chain that connects the light-harvesting chromophores to the dendrimeric backbone with one additional carbon. For the largest dendrimer having 64 Zn(II)-porphyrins, only approximately 10% of the excitation intensity is needed in order to observe the same extent of annihilation in the dendrimers with the additional carbon in the connecting chain as compared to those without. Complete annihilation, until only one chromophore remains excited, now occurs within subunits of seven chromophores, when half of the chromophores are excited. The improvement of the annihilation efficiency in the largest dendrimer with 64 porphyrins can be explained by the presence of a the two-step delayed annihilation process, involving energy hopping from excited to nonexcited chromophores prior to annihilation. In the smallest dendrimer with only four chromophores, delayed annihilation is not present, since the direct annihilation process is more efficient than the two-step delayed annihilation process. As the dendrimer size increases and the chances of originally exciting two neighboring chromophores decreases, the delayed annihilation process becomes more visible. The additional carbon, added to the connecting chain, results in more favorable chromophore distances and orientations for energy hopping. Hence, the improved energy transfer properties makes the Zn(II)-porphyrin-appended dendrimers with the additional carbon promising candidates as light-harvesting antennas for artificial photosynthesis.

Energy Transfer within Zn-Porphyrin Dendrimers: Study of the Singlet−Singlet Annihilation Kinetics

In this article, we explore energy transfer processes within a series of Zn-porphyrin-appended dendrimers by means of excitation intensity dependent transient absorption measurements. We report singlet-singlet annihilation on two distinct time scales of 18 +/- 5 ps and 130 +/- 10 ps in the dimer and the dendrimers. The two distinct processes reflect the presence of two structural conformer distributions. Analysis of the singlet-singlet annihilation transient kinetics shows that sequential annihilation occurs within subunits up to four Zn-porphyrins in the dendrimers. The onset of the singlet-singlet annihilation process depending on the size of the molecule reveals a difference in the number of communicating Zn-porphyrins. We further report a full characterization of the transient absorption kinetics of the monomer over a spectral range from 450 to 730 nm.

Study of the Isotropic and Anisotropic Fluorescence of Two Oligothiophenes by Femtosecond Time-Resolved Spectroscopy

The excited-state dynamics of two oligothiophenes, 5,5'-dicarboxyhaldehyde 2,2',5',2' '-terthiophene and 5-carboxyhaldehyde 2,2',5',2' '-terthiophene, were studied by time-resolved fluorescence spectroscopy, in the femtosecond regime. The isotropic and anisotropic parameters of their fluorescence were calculated. The angle (alpha) between the absorption and emission molecular dipoles was estimated from the initial fluorescence anisotropy. The effect of the chemical substituents, at the ends of the main chain of the molecule, on the temporal behavior of the fluorescence was investigated. Particularly, the nonsymmetric oligothiophene molecule (containing one aldehyde group) exhibits shorter excited-state isotropic decay time than the symmetric one (containing two aldehyde groups). This is due to the higher value of the emission dipole moment of the nonsymmetric oligothiophene in comparison with that of the symmetric one. Additionally, the two materials have almost the same anisotropic fluorescence parameters, and this is attributed to the same rotational motions in the excited state due to their similar molecular structures.

Porphyrin Boxes Constructed by Homochiral Self-Sorting Assembly: Optical Separation, Exciton Coupling, and Efficient Excitation Energy Migration

meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-meso-linked dimers Dn assemble spontaneously, in noncoordinating solvents such as CHCl3, into tetrameric porphyrin squares Sn and porphyrin boxes Bn, respectively. Interestingly, formation of Bn from Dn proceeds via homochiral self-sorting assembly, which has been verified by optical separations of B1 and B2. Optically pure enantiomers of B1 and B2 display strong Cotton effects in the CD spectra, which reflect the length of the pyridyl arm, thus providing evidence for the exciton coupling between the noncovalent neighboring porphyrin rings. Excitation energy migration processes within Bn have been investigated by steady-state and time-resolved spectroscopic methods in conjunction with polarization anisotropy measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Förster-type incoherent energy hopping model by assuming a number of hopping sites of N = 4 and an exciton coherence length of L = 2. Consequently, the excitation energy hopping rates between the zinc(II) diporphyrin units have been estimated for B1 (48 ps)(-1), B2 (98 +/- 3 ps)(-1), and B3 (361 +/- 6 ps)(-1). Overall, the self-assembled porphyrin boxes Bn serve as a well-defined three-dimensional model for the light-harvesting complex.