Photoexcited Porphyrin Research Articles

Hydrogen‐Bond‐Enhanced Photoreforming of Biomass Furans over a Urea‐Incorporated Cu(II) Porphyrin Framework

AbstractSolar‐driven upgrading of biomass‐derived 5‐hydroxylmethylfurfural (HMF) to 2,5‐furandicarboxylic acid (FDCA) holds great promise for sustainable production of bio‐plastics and resins. However, the process is limited by poor selectivity and sluggish kinetics due to the vertical coordination of HMF at relatively strong metal sites. Here, we purposely developed a Cu(II) porphyrin framework featuring side‐chain incorporated urea linkages, denoted as TBUPP−Cu MOF, to render HMF a weak hydrogen bond at the urea site and flat adsorption via π–π stacking with the benzene moiety. The unique configuration promotes the approaching of −CHO of HMF to the photoexcited porphyrin ring towards kinetically and thermodynamically favourable intermediate formation and subsequent desorption. The charge localisation and orbital energy alignment enable the selective activation of O2 over the porphyrin to generate ⋅O2− and 1O2 instead of highly oxidative H2O2 and ⋅OH via spin‐flip electron transfer, which drive the ambient oxidation of proximal −CHO. The effective utilisation of redox species and circumvented over‐oxidation facilitate a FDCA selectivity of >90 % with a high turnover number of 193 molHMF molCu−1. The facile purification of high‐purity FDCA and zero‐waste recycling of intermediates and durable catalyst feature TBUPP−Cu MOF a promising photo‐oxidation platform towards net‐zero biorefining and organic transformations.

Hydrogen-Bond-Enhanced Photoreforming of Biomass Furans over aUrea-Incorporated Cu(II) Porphyrin Framework.

Solar-driven upgrading of biomass-derived 5-hydroxylmethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) holds great promise for sustainable production of bio-plastics and resins. However, the process is limited by poor selectivity and sluggish kinetics due to the vertical coordination of HMF at relatively strong metal sites. Here, we purposely developed a Cu(II) porphyrin framework featuring side-chain incorporated urea linkages, denoted as TBUPP-Cu MOF, to render HMF a weak hydrogen bond at the urea site and flat adsorption via π-π stacking with the benzyl moiety. The unique configuration promotes the approaching of -CHO of HMF to the photoexcited porphyrin ring towards kinetically and thermodynamically favourable intermediate formation and subsequent desorption. The charge localization and orbital energy alignment enable the selective activation of O2 over the porphyrin to generate ·O2- and 1O2 instead of highly oxidative H2O2 and ·OH via spin-flip electron transfer, which drive the ambient oxidation of proximal -CHO. The effective utilisation of redox species and circumvented over-oxidation facilitate a FDCA selectivity of >90% with a high turnover number of 193 molHMF molCu-1. The facile purification of high-purity FDCA and zero-waste recycling of intermediates and durable catalyst feature TBUPP-Cu MOF a promising photo-oxidation platform towards net-zero biorefining and organic transformations.

Time-resolved infra-red studies of photo-excited porphyrins in the presence of nucleic acids and in HeLa tumour cells: insights into binding site and electron transfer dynamics.

Cationic porphyrins based on the 5,10,15,20-meso-(tetrakis-4-N-methylpyridyl) core (TMPyP4) have been studied extensively over many years due to their strong interactions with a variety of nucleic acid structures, and their potential use as photodynamic therapeutic agents and telomerase inhibitors. In this paper, the interactions of metal-free TMPyP4 and Pt(II)TMPyP4 with guanine-containing nucleic acids are studied for the first time using time-resolved infrared spectroscopy (TRIR). In D2O solution (where the metal-free form exists as D2TMPyP4) both compounds yielded similar TRIR spectra (between 1450-1750 cm-1) following pulsed laser excitation in their Soret B-absorption bands. Density functional theory calculations reveal that vibrations centred on the methylpyridinium groups are responsible for the dominant feature at ca. 1640 cm-1. TRIR spectra of D2TMPyP4 or PtTMPyP4 in the presence of guanosine 5'-monophosphate (GMP), double-stranded {d(GC)5}2 or {d(CGCAAATTTGCG)}2 contain negative-going signals, 'bleaches', indicative of binding close to guanine. TRIR signals for D2TMPyP4 or PtTMPyP bound to the quadruplex-forming cMYC sequence {d(TAGGGAGGG)}2T indicate that binding occurs on the stacked guanines. For D2TMPyP4 bound to guanine-containing systems, the TRIR signal at ca. 1640 cm-1 decays on the picosecond timescale, consistent with electron transfer from guanine to the singlet excited state of D2TMPyP4, although IR marker bands for the reduced porphyrin/oxidised guanine were not observed. When PtTMPyP is incorporated into HeLa tumour cells, TRIR studies show protein binding with time-dependent ps/ns changes in the amide absorptions demonstrating TRIR's potential for studying light-activated molecular processes not only with nucleic acids in solution but also in biological cells.

Effect of the second chromophore energy gap on photo-induced electron injection in di-chromophoric porphyrin-sensitized solar cells.

This work investigates the effect of the second chromophore energy gap on charge generation in porphyrin-based di-chromophoric dye-sensitized solar cells (DSSCs). Three di-chromophoric porphyrin dyes (PorY, PorO and PorR) containing three organic chromophores with decreasing frontier orbital energy offsets, including a carbazole-triphenylamine chromophore (yellow, Y), a carbazole fused-thiophene chromophore (orange, O) or a carbazole-thiophene benzothiadiazole thiophene chromophore (red, R), were investigated using optical and electrochemical methods, steady-state photoluminescence and photovoltaic device characterization. Energy transfer from the organic chromophore to the porphyrin was suggested in PorY and PorO as the main charge generation mechanism in DSSCs using these di-chromophoric dyes. On the other hand, electron transfer from the photo-excited porphyrin to the organic chromophore as a competing pathway leading to the loss of photocurrent is suggested for PorR-sensitized solar cells. The latter pathway leading to a loss of photocurrent is due to the lower lying lowest unoccupied molecular orbital of the additional organic chromophore (R) and suggests the limitation of the current di-chromophoric approach to increase the overall efficiency of DSSCs.

Electron Injection Dynamics from Photoexcited Porphyrin Dyes into SnO2and TiO2Nanoparticles

The photoexcited electron injection dynamics of free-base and metallo-derivatives of tris(pentafluorophenyl)porphyrins bound to TiO2 and SnO2 nanoparticle surfaces have been investigated using time-resolved terahertz spectroscopy (TRTS). The metallo-derivatives include Zn(II), Cu(II), Ni(II), and Pd(II). For the TiO2–porphyrin assemblies, electron injection from the photoexcited dye to the semiconductor occurs only when using the zinc derivative as the sensitizer because it is the only dye studied in this report with long-lived excited states higher in energy than the TiO2 conduction band edge. All of the dyes, however, have excited-state energies above the SnO2 conduction band edge, and the electron injection rates vary widely from 0.4 to 200 ps depending on the sensitizer. For the SnO2–porphyrin assemblies, electron injection is strongly influenced by competition with alternate deactivation routes that are accessible following Soret band excitation. These results offer thermodynamic and kinetic considerations for designing improved high-potential porphyrin photoanodes with applications to solar-powered water oxidation.

Recent Advances in Photoinduced Electron Transfer Processes of Fullerene-Based Molecular Assemblies and Nanocomposites

Photosensitized electron-transfer processes of fullerenes hybridized with electron donating or other electron accepting molecules have been surveyed in this review on the basis of the recent results reported mainly from our laboratories. Fullerenes act as photo-sensitizing electron acceptors with respect to a wide variety of electron donors; in addition, fullerenes in the ground state also act as good electron acceptors in the presence of light-absorbing electron donors such as porphyrins. With single-wall carbon nanotubes (SWCNTs), the photoexcited fullerenes act as electron acceptor. In the case of triple fullerene/porphyrin/SWCNT architectures, the photoexcited porphyrins act as electron donors toward the fullerene and SWCNT. These mechanisms are rationalized with the molecular orbital considerations performed for these huge supramolecules. For the confirmation of the electron transfer processes, transient absorption methods have been used, in addition to time-resolved fluorescence spectral measurements. The kinetic data obtained in solution are found to be quite useful to predict the efficiencies of photovoltaic cells.

Photo-excitation energy transfer between a titanium(iv)–porphyrin complex and oxygen molecule

The Ti-TPyP reagent, i.e. an acidic aqueous solution of oxo[5,10,15,20-tetra(4-pyridyl)-porphyrinato]titanium(IV) complex, denoted as TiO(tpyp), was developed as a highly sensitive and specific reagent for detecting hydrogen peroxide. In our previous paper, the photosensitizing effect of the TiO(tpyp) complex on the detection of hydrogen peroxide using this reagent was explored based on the experimental findings. In the present paper, computational analysis was carried out for the molecular level elucidation of the energy transfer mechanism between the photo-excited TiO(tpyp) complex and triplet oxygen to form singlet oxygen under aqueous acidic conditions. The protonated form of the TiO(tpyp) complex, i.e. [TiO(tpypH4)]4+ complex, was found to be the entity responsible for the energy transfer to triplet oxygen. The interaction sites between the [TiO(tpypH4)]4+ complex with triplet oxygen were then discussed based on their molecular orbital characteristics and the potential energy changes accompanying the approach of triplet oxygen molecule Σ3O2 to the [TiO(tpypH4)]4+ complex. The obtained results show that the C–N portion of the porphyrin ring plays a predominant role in the energy transfer from the photo-excited porphyrin to Σ3O2. Σ3O2 is then converted to Δ1O2 through Σ1O2. The Δ1O2 molecule reaches its stable state at the distance of 3.21 Å from the C–N portion of the porphyrin ring with its stabilization energy of 0.99 kcal mol−1. The stabilization energy for the Δ1O2 molecule thus formed is small, implying no stable bond formation between the Δ1O2 molecule and the [TiO(tpypH4)]4+ complex and that it moves away from the C–N portion immediately after the energy transfer from the photo-excited porphyrin.

Photosensitized electron transfer processes of nanocarbons applicable to solar cells

Photosensitized electron-transfer processes of nanocarbon materials hybridized with electron donating or electron accepting molecules have been surveyed in this tutorial review on the basis of the recent results reported mainly from our laboratories. As nano-carbon materials, fullerenes and single wall carbon nanotubes (SWCNTs) have been employed. Fullerenes act as photo-sensitizing electron acceptors with respect to a wide variety of electron donors; in addition, the fullerenes act as good ground state electron acceptors in the presence of light-absorbing electron donors such as porphyrins and phthalocyanines. In the case of SWCNTs, their ground states act as electron acceptor and electron donors, depending on the photosensitizers. For example, with respect to the photoexcited porphyrins and phthalocyanines, SWCNTs usually act as electron acceptors, whereas for the photoexcited fullerenes, SWCNTs act as electron donors. The diameter sorted semi-conductive SWCNTs have been used to verify the size-dependent electron transfer rates. For the confirmation of the electron transfer processes, the transient absorption methods have been widely used, in addition to the time-resolved fluorescence spectral measurements. The kinetic data thus obtained in solution are found to be quite useful to predict the efficiencies of photovoltaic cells constructed on semiconductor nanoparticle modified electrodes and their photocatalytic processes.

Effect of photo-oxidation of on the determination of and using the - reagent.

The Ti-TPyP reagent, i.e. an acidic aqueous solution of oxo[5,10,15,20-tetra(4-pyridyl)-porphyrinato]titanium(iv) complex, denoted as TiO(tpyp), was developed as a highly sensitive reagent for determining hydrogen peroxide. The Ti-TPyP reagent exhibits a sharp absorption peak at 432 nm. Following the addition of hydrogen peroxide to the reagent, a new absorption peak appeared at 450 nm. Its absorbance was proportional to the hydrogen peroxide concentration with an apparent molar absorptivity of 1.1 × 105 L mol-1 cm-1. In the determination of hydrogen peroxide using the Ti-TPyP reagent, ascorbic acid concurrently present in the samples caused a positive error in the analytical results. In the present paper, the source of the error was revealed to be due to the production of hydrogen peroxide through the oxidation of ascorbic acid by singlet oxygen arising from the energy transfer between the dissolved oxygen molecule and the photo-excited porphyrin moiety of the TiO(tpyp) complex. The reaction mechanism was proposed for the photo-oxidation of ascorbic acid caused by the TiO(tpyp) complex to produce dehydroascorbic acid and hydrogen peroxide. The effect of ascorbic acid is really a serious problem in the practical uses of the Ti-TPyP reagent in food and biochemical analysis. However, use of ascorbic acid oxidase as a scavenger for ascorbic acid was found to be quite effective to remove its effect in the flow injection analysis of 3-hydroxybutyric acid in human serum.

Reduced Graphene Oxide and Porphyrin. An Interactive Affair in 2-D

Photoexcited cationic 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) undergoes charge-transfer interaction with chemically reduced graphene oxide (RGO). Formation of the ground-state TMPyP-RGO complex in solution is marked by the red-shift of the porphyrin absorption band. This complexation was analyzed by Benesi-Hildebrand plot. Porphyrin fluorescence lifetime reduced from 5 to 1 ns upon complexation with RGO, indicating excited-state interaction between singlet excited porphyrin and RGO. Femtosecond transient absorption measurements carried out with TMPyP adsorbed on RGO film revealed fast decay of the singlet excited state, followed by the formation of a longer-living product with an absorption maximum around 515 nm indicating the formation of a porphyrin radical cation. The ability of TMPyP-RGO to undergo photoinduced charge separation was further confirmed from the photoelectrochemical measurements. TMPyP-RGO coated conducting glass electrodes are capable of generating photocurrent under visible excitation. These results are indicative of the electron transfer between photoexcited porphyrin and RGO. The role of graphene in accepting and shuttling electrons in light-harvesting assemblies is discussed.

Effects of Protonation State on a Tyrosine−Histidine Bioinspired Redox Mediator

The conversion of tyrosine to the corresponding tyrosyl radical in photosystem II (PSII) is an example of proton-coupled electron transfer. Although the tyrosine moiety (Tyr(Z)) is known to function as a redox mediator between the photo-oxidized primary donor (P680(•+)) and the Mn-containing oxygen-evolving complex, the protonation states involved in the course of the reaction remain an active area of investigation. Herein, we report on the optical, structural, and electrochemical properties of tyrosine-histidine constructs, which model the function of their naturally occurring counterparts in PSII. Electrochemical studies show that the phenoxyl/phenol couple of the model is chemically reversible and thermodynamically capable of water oxidation. Studies under acidic and basic conditions provide clear evidence that an ionizable proton controls the electrochemical potential of the tyrosine-histidine mimic and that an exogenous base or acid can be used to generate a low-potential or high-potential mediator, respectively. The phenoxyl/phenoxide couple associated with the low-potential mediator is thermodynamically incapable of water oxidation, whereas the relay associated with the high-potential mediator is thermodynamically incapable of reducing an attached photoexcited porphyrin. These studies provide insight regarding the mechanistic role of the tyrosine-histidine complex in water oxidation and strategies for making use of hydrogen bonds to affect the coupling between proton and electron transfer in artificial photosynthetic systems.

First Cellulose Langmuir‐Blodgett Films towards Photocurrent Generation Systems

AbstractLangmuir‐Blodgett films of a cellulose derivative containing porphyrins, porphyrin‐cellulose, were fabricated in order to construct a cellulose‐based molecular photocurrent generation system. On visible light illumination of the LB monolayer film deposited on an ITO electrode, anodic photocurrents were observed with a quantum yield of 1.6% at an applied potential of 0 V versus SCE, and 3.8–4.6% at 0.2–0.3 V versus SCE. These values indicate that the self‐quenching of the photoexcited porphyrins in the cellulose LB film was suppressed, while porphyrin moieties in the LB film had a densely packed structure. This is because the porphyrins are located at a distance of approximately 1.0 nm along the cellulose backbone.magnified image

Aqueous carbon nanohorn–pyrene–porphyrin nanoensembles: Controlling charge-transfer interactions

A combination of π–π stacking and electrostatic interactions was used to integrate an anionic porphyrin (H 2P 2−) with carbon nanohorns (CNHs), mediated by positively charged pyrene (pyr +) units. The resulting water-soluble CNHs–pyr +–H 2P 2− nanoensemble was characterized by optical spectroscopy and electron microscopy techniques. The strong fluorescence emission of H 2P 2− was significantly quenched by CNHs, suggesting effective energy and electron transfer between the photo-excited porphyrin and the extended π-electronic network of nanohorns. Blank experiments validated the supramolecular concept of synergistic electrostatic interactions and signified the importance of CNHs as electron acceptors towards the preparation of some novel donor-acceptor nanosystems.

Metal ion dependent fluorescence quenching in a crown ether bridged porphyrin—fullerene dyad

The fluorescence decay kinetics from a benzonitrile solution of a dibenzo-18-crown-6 ether bridged porphyrin-fullerene dyad has been studied in the presence of a range of metal ions. Dual-exponential fluorescence decay behaviour has been attributed to conformational flexibility of the molecule influencing quenching interactions between the photo-excited porphyrin and fullerene. Additions of sodium, potassium and lithium ions significantly modulate the observed fluorescence decay processes while the larger tetrabutylammonium ion has only a minor affect. The results are discussed in terms of ion inclusion within the crown ether affecting both the bridge conformational properties and donor-acceptor electronic interactions.

Nano-structure of polyimide and poly(isobutyl methacrylate) monolayer films studied by interlayer electron transfer between porphyrin and pyromellitic diimide moieties

Hetero-multilayers composed of porphyrin-containing poly(isobutyl methacrylate) (PiBMA) and polyimide (PI) monolayers were prepared by using the Langmuir-Blodgett technique, and the interlayer electron transfer process from photo-excited porphyrins to pyromellitic diimide moieties in PI layers was observed by fluorescence spectroscopy. The efficiency of the electron transfer showed strong dependence on the number of PI layers, suggesting the presence of defects in the PI plane, which were filled with PI after the successive deposition of PI monolayers. The three-dimensional and two-dimensional active sphere models (Perrin model) well explained the interlayer electron transfer phenomena. We discussed the nano-structures of the multilayer films in terms of the thickness of each layer and the location of hydrophobic porphyrins in the PiBMA monolayer.

Primary photoprocesses in cationic 5,10,15,20-meso-tetrakis(4-N-methylpyridiniumyl)porphyrin and its transition metal complexes bound with nucleic acids

Photophysical properties of meso-tetrakis(4-N-methylpyridiniumyl)porphyrin ( TMpyP 4) and its metallocomplexes M (II) TMpy P4 ( M = Zn , Cu , Ni , Co ) bound to natural DNA and synthetic poly-, oligo- and mononucleotides are considered with a primary emphasis placed upon intermolecular interaction of the photoexcited porphyrins with the nearest environment. Quenching of the fluorescent S 1 (but not triplet T 1) state due to guanine to porphyrin electron transfer is observed for TMpyP 4 intercalated between GC base pairs of the double-strand helixes, whereas in the case of TMpyP 4 complexed with guanosine monophosphate (GMP) both S 1 and T 1 states of the porphyrin are quenched. Furthermore, a dependence of the efficiency of TMpyP 4 triplet state quenching by the dissolved molecular oxygen from air on the porphyrin localization enables one to readily distinguish porphyrin groove binding mode from intercalation. Excited states of the TMpyP 4 complexes with transition metals, in spite of their very short lifetimes, also interact with nucleic acid components by means of an axial ligand binding/release to/from the metal. A possible structure of the five-coordinate excited complex (“exciplex”) formed in case of CuTMpyP 4 groove binding to some single- and double-strand polynucleotides is discussed.

Triplet porphyrins as donors in intramolecular electron transfer and their intermolecular interaction with free radicals

Porphyrins and related compounds are basic moieties which upon photoexcitation produce paramagnetic transients important to many processes in biology, material science and light–energy conversion. This short review demonstrates the application of time-resolved EPR spectroscopy to two processes in which the photoexcited singlet and/or triplet are involved: (1) intramolecular electron transfer in photoexcited donor–acceptor systems embedded in liquid crystals, where the porphyrins are the electron donors attached to different types of acceptors; and (2) intermolecular magnetic interactions between photoexcited porphyrin triplets and free radicals. In both systems the electron spin plays an important part with regards to the route of the magnetic interactions involved.

Effect of methylene chain length on electron transfer and photocurrent generation using viologen-linked porphyrin monolayer modified ITO electrode

The increase of the electron transfer yield from the photoexcited porphyrin to viologen was achieved by forming a Langmuir-Blodgett (LB) film of viologen-linked porphyrin. Four kinds of porphyrin covalently linked to viologen (BuPC n V: n = 3,4,5,8) by methylene chain, in which the center-to-center distance between porphyrin and viologen is less than 1.7 nm, were synthesized, and the molecularly oriented films of BuPC n V ( n = 3,4,5,8) were prepared on indium tin oxide (ITO) electrodes using an LB technique. The effect of methylene chain length both on the quenching of the fluorescence from porphyrin by viologen and on the photocurrent generation was investigated. The fluorescence quenching by viologen increased with the decrease of the methylene chain length, showing that the electron transfer from the photoexcited porphyrin to viologen is easy to take place when the distance between porphyrin and viologen is shorter. Furthermore, the fluorescence was quenched more efficiently by viologen in the LB film compared with a methanol solution, indicating that the fluorescence is quenched both by the bonded viologen and by the neighboring viologen in the LB film of BuPC n V ( n = 3,4,5). Under visible light irradiation, a cathodic photocurrent was generated, which increased with the decrease of the methylene chain length owing to the increase of the electron transfer yield from the photoexcitted porphyrin to viologen. The orientation of BuPC n V ( n = 3,4,5,8) molecules on the ITO electrode was investigated by the effect of bias voltage and dioxygen concentration on the photocurrent. It was shown that the porphyrin moieties are attached to the surface of the ITO electrode and that the photocurrent is ascribable to the photoinduced charge separation in the BuPC n V ( n = 3,4,5) molecules.

Efficient Photoinduced Orthogonal Energy and Electron Transfer Reactions via Phospholipid Membrane-Bound Donors and Acceptors

A three component, liposome-bound photochemical molecular device (PMD) consisting of energy and electron transfer reactions is described. Bilayer membrane surface-associated dyes, 5,10,15,20-tetrakis[4-(trimethylammonio)-phenyl]-21H,2 3H-porphine tetra-p-tosylate salt and N,N[prime]-bis[(3-trimethylammonio)propyl]thiadicarbocya nine tribromide, are the energy donor and acceptor, respectively, in a blue light stimulated energy transfer reaction along the vesicle surface. The electronically excited cyanine is quenched by electron transfer from the phospholipid membrane bound triphenylbenzyl borate anion, which is located in the lipid bilayer interior. The PMD exhibits sequential reactions following electronic excitation with the novel feature that the steps proceed with orthogonal orientation: energy transfer occurs parallel to the membrane surface, and electron transfer occurs perpendicular to the surface. Photobleaching and fluorescence quenching experiments verify the transfer reactions, and Stern-Volmer analysis was used to estimate the reaction rate constants. At the highest concentrations examined of energy and electron acceptor ca. 60% of the photoexcited porphyrins were quenched by energy transfer to the cyanine. 56 refs., 6 figs., 3 tabs.

Vectorial electron transfer from an interfacial photoexcited porphyrin to ground-state fullerene C60 and C70 and from ascorbate to triplet C60 and C70 in a lipid bilayer

Vectorial electron transfer from an interfacial photoexcited porphyrin to ground-state fullerene C60 and C70 and from ascorbate to triplet C60 and C70 in a lipid bilayer