Singlet Excited State Of Porphyrin Research Articles

Photoinduced Electron Transfer Reactions of Water Soluble Porphyrins in Zeolite Environment.

Excited state interactions of zeolite adsorbed porphyrins have been investigated by steady state luminescence quenching technique with certain antioxidants such as reduced glutathione, ascorbic acid and L-cysteine. The zeolite supported porphyrins, meso-tetra (N-methyl-4-pyridyl) porphyrin (H2TMPyP4+) and zinc tetra(N-methyl-4-pyridyl) porphyrin (ZnTMPyP4+) were prepared and characterized by various techniques such as Diffuse Reflectance Spectra (DRS), Scanning Electron Microscope (SEM), powder X-Ray Diffraction (XRD) and BET surface area. The interaction of zeolites with porphyrins are shown to increase the lifetime of the singlet excited state of porphyrins and decays are biphasic in nature. The splitting of the emission band of porphyrins occurs in 1:1 glycerol: water solution due to the changes in the dielectric of the solvation sphere associated with porphyrin. The Stern-Volmer plots of I0/I vs quencher concentration [Q] were linear in the whole range of [Q] used. These studies revealed the effective quenching for zinc porphyrin compared to free base porphyrin. The effect of quenchers and zeolite acidity has also been studied and the quenching rate constant (kq) is found in the order of 109M-1s-1. The quenching reaction obeys Rehm-Weller Equation and is shown to be due to thermodynamically favoured electron transfer from quenchers to the excited singlet state of porphyrins (reductive quenching).

Multiple photosynthetic reaction centers composed of supramolecular assemblies of a zinc porphyrin dendrimer with pyridylnaphthalenediimide

Multiple photosynthetic reaction centers have successfully been constructed using a supramolecular complex of zinc porphyrin dendrimer [D(ZnP)16] with pyridylnaphthalenediimide (PyNIm) in benzonitrile. The apparent formation constant determined from the fluorescence quenching of the singlet excited state of porphyrin moieties by PyNIm is significantly larger than that determined from the UV-vis spectral change. This indicates that efficient energy migration occurs between the porphyrin units of the dendrimer prior to the electron transfer from the singlet excited state of zinc porphyrin to PyNIm. The charge-separated (CS) state has been successfully detected as the transient absorption spectrum in the laser flash photolysis. The CS lifetime of in the supramolecular complex of D(ZnP)16 with PyNIm was determined to be 0.83 ms at 298 K.

Photoinduced Charge Separation in Ion-Paired Porphyrin−Single-Wall Carbon Nanotube Donor−Acceptor Hybrids

Photoinduced electron transfer in self-assembled (via ion-pairing) porphyrin−single-wall carbon nanotube (SWNT) donor−acceptor hybrids has been demonstrated. Toward this, first, SWNTs were solubilized by π−π stacking of pyrene (Pyr) functionalized with either a cationic (alkyl ammonium; Pyr-NH3+) or anionic (butyric acid; Pyr-COO−) terminal groups. The SWNT/Pyr-NH3+ and SWNT/Pyr-COO− hybrids thus obtained were subsequently used to generate donor−acceptor hybrids by ion-pairing with free-base or zinc(II) porphyrins bearing either peripheral anionic (sulfonatophenyl) or cationic (N-methylpyridyl) groups. The SWNT/Pyr-NH3+/(TPPS)M and SWNT/Pyr-COO−/(TMPyP)M (where TPPS = tetrakis(4-sulfonatophenyl)porphyrin, TMPyP = tetrakis(N-methylpyridyl)porphyrin, and M = 2H or Zn(II)) thus obtained were spectrally and electrochemically characterized to arrive at the structure and redox potentials. Both steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited state of porphyrins in the nanohybrids, suggesting either electron or energy transfer from the excited porphyrins to SWNTs. Nanosecond transient absorption spectra revealed the formation of one-electron oxidation of the porphyrins with a simultaneous one-electron reduction of SWNT. The charge separation process is further confirmed with the aid of an electron mediator, methyl or hexyl viologen dication (MV2+ or HV2+) and an electron−hole shifter in the studied solvents, water, or dimethylformamide (DMF). As a consequence of the photoinduced processes, accumulations of MV•+ and HV•+ as a result of the electron-pooling were observed with 50−100% yields in the presence of a sacrificial electron donor, thus demonstrating the importance of the photosensitizer-functionalized SWNT nanohybrids in the photogeneration of redox products.

Multi-Triphenylamine-Substituted Porphyrin-Fullerene Conjugates as Charge Stabilizing “Antenna−Reaction Center” Mimics

A new concept of charge stabilization via delocalization of the pi-cation radical species over the donor macrocycle substituents in a relatively simple donor-acceptor bearing multimodular conjugates is reported. The newly synthesized multimodular systems were composed of three covalently linked triphenylamine entities at the meso position of the porphyrin ring and one fulleropyrrolidine at the fourth meso position. The triphenylamine entities were expected to act as energy transferring antenna units and to enhance the electron donating ability of both free-base and zinc(II) porphyrin derivatives of these pentads. Appreciable electronic interactions between the meso-substituted triphenylamine entities and the porphyrin pi-system were observed, and as a consequence, these moieties acted together as an electron-donor while the fullerene moiety acted as an electron-acceptor in the multimodular conjugates. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier highest occupied molecular orbital (HOMO) over the triphenylamine entities in addition to the porphyrin macrocycle. Free-energy calculations suggested that the light-induced processes from the singlet excited state of porphyrins are exothermic in the investigated multimodular conjugates. The occurrence of photoinduced charge-separation and charge-recombination processes was confirmed by the combination of time-resolved fluorescence and nanosecond transient absorption spectral measurements. Charge-separated states, on the order of a few microseconds, were observed as a result of the delocalization of the pi-cation radical species over the porphyrin macrocycle and the meso-substituted triphenylamine entities. The present study successfully demonstrates a novel approach of charge-stabilization in donor-acceptor multimodular conjugates.

Design and Studies on Supramolecular Ferrocene−Porphyrin−Fullerene Constructs for Generating Long-Lived Charge Separated States

Supramolecular ferrocene-porphyrin-fullerene constructs, in which covalently linked ferrocene-porphyrin-crown ether compounds were self-assembled with alkylammonium cation functionalized fullerenes, have been designed to achieve stepwise electron transfer and hole shift to generate long-lived charge separated states. The adopted crown ether-alkylammonium cation binding strategy resulted in stable conjugates as revealed by computational studies performed by the DFT B3LYP/3-21G(*) method in addition to the binding constants obtained from fluorescence quenching studies. The free-energy changes for charge-separation and charge-recombination were varied by the choice of different metal ions in the porphyrin cavity. Free-energy calculations suggested that the light-induced electron-transfer processes from the singlet excited state of porphyrins to be exothermic in all of the investigated supramolecular dyads and triads. Photoinduced charge-separation and charge-recombination processes have been confirmed by the combination of the time-resolved fluorescence and nanosecond transient absorption spectral measurements. In case of the triads, the charge-recombination processes of the radical anion of the fullerene moiety take place in two steps, viz., a direct charge recombination from the porphyrin cation radical and a slower step involving distant charge recombination from the ferrocene cation moiety. The rates of charge recombination for the second route were found to be an order of magnitude slower than the former route, thus fulfilling the condition for charge migration to generate long-lived charge-separated states in supramolecular systems.

π-Complex formation in electron-transfer reactions of porphyrins

π-Complex formation between porphyrins and their radical cations plays an important role in self-exchange electron transfer between neutral porphyrins and the radical cations, leading to negative activation enthalpies when the stabilization energy of the π-complex is larger than the activation energy for the intracomplex electron transfer in the π-complex. A number of porphyrin molecules are self-organized on three-dimensional gold nanoclusters to form monolayer-protected gold nanoclusters (MPCs) that act as an efficient photocatalyst for the uphill reduction of HV2+ by BNAH to produce 1-benzylnicotinamidinium ion (BNA+) and hexyl viologen radical cation (HV·+). Such three-dimensional architectures of porphyrin MPCs with large surface area allow supramolecular π-complexation between MPCs and HV2+, resulting in fast electron transfer from the singlet excited state of porphyrin to HV2+ on MPCs. The π-π interaction is exploited to develop efficient photovoltaic devices consisting of porphyrin and fullerene assemblies which have an enhanced light-harvesting efficiency throughout the solar spectrum together with a highly efficient conversion of the harvested light into electrical energy.

ChemInform Abstract: Inter‐ and Intramolecular Quenching of the Singlet Excited State of Porphyrins by Ferrocene.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Inter- and intramolecular quenching of the singlet excited state of porphyrins by ferrocene

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTInter- and intramolecular quenching of the singlet excited state of porphyrins by ferroceneRichard Giasson, Eric J. Lee, Xiaohong Zhao, and Mark S. WrightonCite this: J. Phys. Chem. 1993, 97, 11, 2596–2601Publication Date (Print):March 1, 1993Publication History Published online1 May 2002Published inissue 1 March 1993https://doi.org/10.1021/j100113a021RIGHTS & PERMISSIONSArticle Views466Altmetric-Citations61LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (698 KB) Get e-Alerts

Formation of a long-lived intramolecular radical ion pair from the singlet excited state of porphyrin-methylviologens combined with flexible chains

Photoinduced charge separation (CS) and charge recombination (CR) reactions in the singlet excited state of porphyrin (P) attached to two methylviologens (MV2+) by flexible chains have been studied in dimethylsulphoxide and methanol-acetone mixture by means of picosecond and nanosecond laser photolysis methods. An extremely slow CR rate compared to the CS rate has been observed.

Interactions of excited-state porphyrin–quinone in reversed micelles studied by time-resolved fluorescence spectroscopy

Interactions in reversed micelles of benzyldimethyl-n-hexadecylammonium chloride (BHDC) between excited-state porphyrin (ZnTPP) and quinones [duroquinone (DQ) and anthraquinone-2-sulphonate (AQS)] have been investigated by time-resolved spectroscopy. Essentially dynamic quenching is observed when the quencher DQ is solubilised in the organic phase, although this is affected by the distribution of quencher molecules at the micellar interface.When the quencher (AQS) is bound at the interface, the decay of the singlet excited state of porphyrin (ZnTPP) also bound at the interface in the absence of water (ω0=[H2O]/[surf]= 0) shows two components: one with a mean lifetime 〈τ〉≈ 1 ns, which corresponds to unquenched porphyrin, and a much shorter 〈τ〉= 100 ps, which accounts for the decay of quenched porphyrin. For quencher concentrations in the range of [AQS]= 1–4 mmol dm–3 the observed decay data may be interpreted in terms of a static active-sphere quenching model, where only bound quencher molecules within a certain volume around the bound porphyrin molecule lead to quenching. The lifetimes obtained in these studies indicate a pseudounimolecular electron-transfer rate constant for P*i⋯Qi of the order of 1010 s–1.