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Papers by tony khoury
Journal of The American Chemical Society, 2008
The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(... more The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b′]porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:7,8-b′′]porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]porphyrinato}silver(II). The first one-electron oxidation and first one-electron reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap, determined by the difference in the first oxidation and first reduction potentials, decreases by introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors, something not previously observed in other metalloporphyrin complexes. The second one-electron oxidation and second one-electron reduction of the compounds both occur at the porphyrin macrocycle to produce Ag(III) porphyrin π-radical cations and Ag(I) porphyrin π-radical anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a positive direction as compared to the same electrode reactions of the porphyrin without the fused quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical anions by π-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at the -pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors.
A focused ion beam (FIB) is used to process 2-D self-assembled photonic porphyrin film flats to f... more A focused ion beam (FIB) is used to process 2-D self-assembled photonic porphyrin film flats to fabricate couplers in 2-D porphyrin slabs. These self-assembled structures have an initial root mean squared (rms) values for surface roughness < 0.5 nm as measured by atomic force microscopy. Under appropriate FIB processing and cutting conditions, the rms value for surface roughness falls to < 0.4 nm.
New Journal of Physics, 2009
The precise structure of monolayers of catalytic manganese porphyrins at the interface of an Au(1... more The precise structure of monolayers of catalytic manganese porphyrins at the interface of an Au(111)-surface and an n-tetradecane liquid has been determined in a liquid-cell scanning tunneling microscope (STM). Before the addition of the manganese porphyrins, an ordered monolayer of lamellae of n-tetradecane molecules on the Au(111) surface could be imaged. It was found that only domain boundaries in this monolayer were correlated to the underlying gold surface, but not the orientation of the n-tetradecane lamellae and the molecules within them. Both the reconstruction of the Au(111) surface and the ordering of the first layer of n-tetradecane can direct the ordering of the manganese porphyrins that are subsequently added to the liquid phase.
Upconversion is a promising technique for significantly enhancing the efficiency of photovoltaic ... more Upconversion is a promising technique for significantly enhancing the efficiency of photovoltaic cells. Molecular systems provide an environment in which long lived triplet states can be exploited to achieve high upconversion efficiencies under solar illumination. We report on the investigation of bi-molecular triplet-triplet annihilation upconversion (TTA-UC) in a Palladium (II) tetrakisquinoxalino porphyrin (PQ4Pd)/rubrene solution. These molecules were studied in solution using UV/VIS spectroscopy to determine their stability in air over a period of weeks. Transient absorption spectroscopy (TAS) was used to directly measure the lifetime of triplet states within these mixtures and hence determine the photoinduced kinetics of the system. The lifetime of porphyrin triplets was reduced from 92.4 μs in pristine PQ4Pd to 2.4 μs in the presence of rubrene. From this change, the rate constant associated with triplet energy transfer (kTET) was calculated as 3.38 × 108M-1s-1. Additionally, a reduction in the absorption of 530 nm light (the ground state rubrene absorption peak) was observed, while the mixture was pumped at the absorption peak of the porphyrin (670 nm). This change became apparent nearly 6 μs after the laser pulse, showing energy transfer from the porphyrin to the rubrene, and allowing further insight into the kinetics of the mechanism.
Chemistry-a European Journal, 2009
The site of electron-transfer reduction of AuPQ+ (PQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)... more The site of electron-transfer reduction of AuPQ+ (PQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quino-xalino[2, 3−b′]porphyrin) and AuQPQ+ (QPQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]porphyrin) is changed from the AuIII center to the quinoxaline part of the PQ macrocycle in the presence of Sc3+ in benzonitrile because of strong binding of Sc3+ to the two nitrogen atoms of the quinoxaline moiety. Strong binding of Sc3+ to the corresponding nitrogen atoms on the quinoxaline unit of ZnPQ also occurs for the neutral form. The effects of Sc3+ on the photodynamics of an electron donor–acceptor compound containing a linked ZnII and AuIII porphyrin ([ZnPQ–AuPQ]PF6) have been examined by femto- and nanosecond laser flash photolysis measurements. The observed transient absorption bands at 630 and 670 nm after laser pulse irradiation in the absence of Sc3+ in benzonitrile are assigned to the charge-shifted (CS) state (ZnPQ.+–AuPQ). The CS state decays through back electron transfer (BET) to the ground state rather than to the triplet excited state. The BET rate was determined from the disappearance of the absorption band due to the CS state. The decay of the CS state obeys first-order kinetics. The CS lifetime was determined to be 250 ps in benzonitrile. Addition of Sc3+ to a solution of ZnPQ–AuPQ+ in benzonitrile caused a drastic lengthening of the CS lifetime that was determined to be 430 ns, a value 1700 times longer than the 250 ps lifetime measured in the absence of Sc3+. Such remarkable prolongation of the CS lifetime in the presence of Sc3+ results from a change in the site of electron transfer from the AuIII center to the quinoxaline part of the PQ macrocycle when Sc3+ binds to the quinoxaline moiety, which decelerate BET due to a large reorganization energy of electron transfer. The change in the site of electron transfer was confirmed by ESR measurements, redox potentials, and UV/Vis spectra of the singly reduced products.
Journal of Materials Chemistry, 2010
Self-assembled microrods {based on 5-nitro-10,15,20-trialkylporphyrins [(C n H 2n+1 ) 3 -NO 2 P]}... more Self-assembled microrods {based on 5-nitro-10,15,20-trialkylporphyrins [(C n H 2n+1 ) 3 -NO 2 P]} and microplates {based on 5, 10,15,20-tetraheptylporphyrin [(C 7 H 15 ) 4 -P]} are fabricated and characterised using optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). The length of the alkyl chains and the deposition surface are found to influence the optical properties and microrod self-assembly. When the deposition surface is silica (a-quartz), 5-nitrotrialkylporphyrins, (C 5 H 11 ) 3 -NO 2 P, (C 7 H 15 ) 3 -NO 2 P and (C 11 H 23 ) 3 -NO 2 P all form microrods of 0.7-0.8 micron diameter; the average length of the microrods varies from 170 microns for (C 5 H 11 ) 3 -NO 2 P to about 11 microns for (C 7 H 15 ) 3 -NO 2 P and (C 11 H 23 ) 3 -NO 2 P, whereas (C 19 H 39 ) 3 -NO 2 P with much longer alkyl chains only gives powders. Controlling the precipitation is crucial in preventing the disordered aggregation of assembled layers observed in the bulk. Very interestingly, the microrods formed from (C 7 H 15 ) 3 -NO 2 P show marked iridescent character. When (C 7 H 15 ) 3 -NO 2 P is deposited on silicon, however, longer curved microrods which do not show iridescence are produced. Single crystal X-ray crystallography of (C 7 H 15 ) 3 -NO 2 P reveals the packing of the bulk material which explains the packing topology of the layers observed by AFM but not the iridescence. The observed structural colour of the (C 7 H 15 ) 3 -NO 2 P microrods is explained by staggering of the layers to produce a corrugated surface with a period of 125 nm, as measured by AFM.
Self-assembled porphyrin wires are fabricated using various nitroporphyrin molecular units. 5-nit... more Self-assembled porphyrin wires are fabricated using various nitroporphyrin molecular units. 5-nitro-10,15,20-triheptylporphyrin is found to give the greatest optical transparency and straights wires. A focused ion beam (FIB) is used to demonstrate milling and shaping of the wires.
Journal of Physics: Conference Series, 2009
Following experimental determination of kinetic parameters governing the up-conversion of light b... more Following experimental determination of kinetic parameters governing the up-conversion of light by triplet-triplet annihilation (TTA-UC), we develop a kinetic model to determine the conditions required for efficient up-conversion. We discuss the assumptions underpinning statistical arguments for an upper limit to TTA-UC and argue that no such limit exists.
Physical Chemistry Chemical Physics, 2010
Photochemical upconversion is performed, whereby emitter triplet states are produced through trip... more Photochemical upconversion is performed, whereby emitter triplet states are produced through triplet energy transfer from sensitizer molecules excited with low energy photons. The triplet emitter molecules undergo triplet-triplet annihilation to yield excited singlet states which emit upconverted fluorescence. Experiments comparing the 560 nm prompt fluorescence when rubrene emitter molecules are excited directly, using 525 nm laser pulses, to the delayed, upconverted fluorescence when the porphyrin sensitizer molecules are excited with 670 nm laser pulses reveal annihilation efficiencies to produce excited singlet emitters in excess of 20%. Conservative measurements reveal a 25% annihilation efficiency, while a direct comparison between the prompt and delayed fluorescence yield suggests a value as high as 33%. Due to fluorescence quenching, the photon upconversion efficiencies are lower, at 16%.
Journal of Materials Chemistry, 2010
Hexa-peri-hexabenzocoronene (HBC) is a discotic polycyclic aromatic hydrocarbon with good charge ... more Hexa-peri-hexabenzocoronene (HBC) is a discotic polycyclic aromatic hydrocarbon with good charge transport characteristics while the light harvesting property of porphyrins is well-known. In order to take advantage of the properties of both materials, a porphyrin-hexa-peri-hexabenzocoroneneporphyrin triad 6 was synthesised for the first time. In photoluminescence studies, the emission from the higher energy HBC unit was completely quenched by the porphyrin unit indicating efficient energy transfer. This novel hybrid material 6 was tested in bulk heterojunction photovoltaic devices reaching a power conversion efficiency of 1.2%.
Journal of Physics: Conference Series, 2009
Page 1. Efficient up-conversion by triplet-triplet annihilation This article has been downloaded ... more Page 1. Efficient up-conversion by triplet-triplet annihilation This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2009 J. Phys.: Conf. Ser. 185 012002 (http://iopscience.iop.org/1742-6596/185/1/012002) ...
Journal of Physical Chemistry A, 2008
Through-porphyrin electronic communication is investigated using "linear-type" and "corner-type" ... more Through-porphyrin electronic communication is investigated using "linear-type" and "corner-type" bis-(quinoxalino)porphyrins in free-base form and their Zn II , Cu II , Ni II , and Pd II derivatives. These compounds are porphyrins with quinoxalines fused on opposite or adjacent , '-pyrrolic positions; they were synthesized from 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-porphyrin-2,3,12,13-and -2,3,7,8-tetraone, respectively, by reaction with 1,2-phenylenediamine. The degree of electron spin delocalization into the fused rings in the π-radical anions of the free-base and metal(II) bisquinoxalinoporphyrins was elucidated by electrochemistry, UV-vis absorption, and electron spin resonance (ESR) spectra of the singly reduced species and density functional theory calculations. Hyperfine splitting patterns in the ESR spectra of the π-radical anions show that symmetric molecules have delocalized electron spin, indicating that significant inter-quinoxaline interactions are mediated through the central porphyrin unit, these interactions being sufficient to guarantee throughmolecule conduction. However, when molecular symmetry is broken by tautomeric exchange of the inner nitrogen hydrogens in the free-base porphyrin with a corner-type quinoxaline substitution pattern, the π-radical anion becomes confined so that one quinoxaline group is omitted from spin delocalization. This indicates the appearance of a unidirectional barrier to through-molecule conduction, suggesting a new motif for chemically controlled rectification.
Journal of Physical Chemistry Letters, 2010
Upconversion (UC) via triplet-triplet annihilation (TTA) is a promising concept to improve the en... more Upconversion (UC) via triplet-triplet annihilation (TTA) is a promising concept to improve the energy conversion efficiency of solar cells by harvesting photons below the energy threshold. Here, we present a kinetic study of the delayed fluorescence induced by TTA to explore the maximum efficiency of this process. In our model system we find that more than 60% of the triplet molecules that decay by TTA produce emitters in their first excited singlet state, so that the observed TTA effiency exceeds 40% at the point of the highest triplet emitter concentration. This result thoroughly disproves any spin-statistical limitation for the annihilation efficiency and thus has crucial consequences for the applicability of an upconvertor based on TTA, which are discussed.
New Journal of Chemistry, 2009
ABSTRACT
Chemical Communications, 2007
New Journal of Chemistry, 2008
... Maxwell J. Crossley*, Craig S. Sheehan, Tony Khoury, Jeffery R. Reimers and Paul J. Sintic. S... more ... Maxwell J. Crossley*, Craig S. Sheehan, Tony Khoury, Jeffery R. Reimers and Paul J. Sintic. School of Chemistry, The University of Sydney, NSW ... 5 and quinoxalino[2,3-b]porphyrins 1315 with nitrogen dioxide gave a mixture of β-pyrrolic functionalised nitro-porphyrin isomers ...
Journal of The American Chemical Society, 2009
The fabrication of porphyrin thin films derived from dichloro [5,10,15,20-tetra(heptyl)porphyrina... more The fabrication of porphyrin thin films derived from dichloro [5,10,15,20-tetra(heptyl)porphyrinato]tin(IV) [Cl-Sn(THP)-Cl] in the holes of photonic crystal fibers over 90 cm in length is described. Evanescent field spectroscopy (EFS) is used to investigate the interfacial properties of the films, with the high surface optical intensity and the long path length combining to produce significant absorption. By comparison with results obtained for similar films formed from Cl-Sn(THP)-Cl inside fused-silica cuvettes and on glass slides, the film is shown to be chemisorbed as a surface Si-O-Sn(THP)-X (X ) Cl or OH) species. In addition to the usual porphyrin Q and Soret bands, new absorptions in the in-fiber films are observed by EFS at 445 nm and between 660-930 nm. The 660-930 nm band is interpreted as a porphyrin to silicon charge-transfer transition and postulated to arise following chemisorption at mechanical-strain induced defect sites on the silica surface. Such defect sites are caused by the optical fiber production process and are less prevalent on other glass surfaces. EFS within optical fibers therefore offers new ways for understanding interface phenomena such as surface adsorbates on glass. Such understanding will benefit all devices that exploit interface phenomena, both in optical fibers and other integrated waveguide forms. They may be directly exploited to create ultrasensitive molecular detectors and could yield novel photonic devices.
Nanotechnology, 2009
Self-assembled supramolecular structures such as optical wires, films and 2D slabs offer a new ge... more Self-assembled supramolecular structures such as optical wires, films and 2D slabs offer a new generation of electronic and optical devices. In particular, self-assembled porphyrin devices, including those integrated onto silica and silicon platforms, open new opportunities in photonic applications spanning molecular biosensing, photovoltaics and telecommunications. All reports to date, however, largely highlight the potential but have not established a clear pathway to the actual implementation of more complex device prototypes. In this paper, we propose and demonstrate the use of a focused ion beam (FIB) to process and fabricate devices in porphyrin-based supramolecular structures. These self-assembled structures have an initial root mean squared (rms) values for surface roughness of <0.5 nm as measured by atomic force microscopy. Under appropriate FIB processing and cutting conditions, the rms value for surface roughness falls to <0.4 nm, comparable with some of the best optical flatnesses obtained within, for example, structured optical fibres and integrated optical waveguides. The milling rate of the porphyrin structures was estimated to be ∼70% of that of silica. The versatility of a FIB as a tool for rapid processing and fabricating 1D and 2D photonic waveguide structures within supramolecular self-assembled platforms is demonstrated by fabricating a 2D coupler, setting the groundwork for true optical device engineering and integration using these new organic systems.
We have investigated a photochemical up-conversion system comprising a molecular mixture of a pal... more We have investigated a photochemical up-conversion system comprising a molecular mixture of a palladium porphyrin to harvest light, and a polycyclic aromatic hydrocarbon to emit light. The energy of harvested photons is stored as molecular triplet states which then annihilate to bring about up-converted fluorescence. The limiting efficiency of such triplet-triplet annihilation up-conversion has been believed to be 11% for some time. However, by rigorously investigating the kinetics of delayed fluorescence following pulsed excitation, we demonstrate instantaneous annihilation efficiencies exceeding 40%, and limiting efficiencies for the current system of ~60%. We attribute the high efficiencies obtained to the electronic structure of the emitting molecule, which exhibits an exceptionally high T2 molecular state. We utilize the kinetic data obtained to model an up-converting layer irradiated with broadband sunlight, finding that ~3% efficiencies can be obtained with the current system, with this improving dramatically upon optimization of various parameters.
Journal of Physical Chemistry A, 2007
We report on a study of singlet-singlet annihilation kinetics in a series of Zn(II)-porphyrin-app... more 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.
Journal of The American Chemical Society, 2008
The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(... more The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b′]porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:7,8-b′′]porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]porphyrinato}silver(II). The first one-electron oxidation and first one-electron reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap, determined by the difference in the first oxidation and first reduction potentials, decreases by introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors, something not previously observed in other metalloporphyrin complexes. The second one-electron oxidation and second one-electron reduction of the compounds both occur at the porphyrin macrocycle to produce Ag(III) porphyrin π-radical cations and Ag(I) porphyrin π-radical anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a positive direction as compared to the same electrode reactions of the porphyrin without the fused quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical anions by π-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at the -pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors.
A focused ion beam (FIB) is used to process 2-D self-assembled photonic porphyrin film flats to f... more A focused ion beam (FIB) is used to process 2-D self-assembled photonic porphyrin film flats to fabricate couplers in 2-D porphyrin slabs. These self-assembled structures have an initial root mean squared (rms) values for surface roughness < 0.5 nm as measured by atomic force microscopy. Under appropriate FIB processing and cutting conditions, the rms value for surface roughness falls to < 0.4 nm.
New Journal of Physics, 2009
The precise structure of monolayers of catalytic manganese porphyrins at the interface of an Au(1... more The precise structure of monolayers of catalytic manganese porphyrins at the interface of an Au(111)-surface and an n-tetradecane liquid has been determined in a liquid-cell scanning tunneling microscope (STM). Before the addition of the manganese porphyrins, an ordered monolayer of lamellae of n-tetradecane molecules on the Au(111) surface could be imaged. It was found that only domain boundaries in this monolayer were correlated to the underlying gold surface, but not the orientation of the n-tetradecane lamellae and the molecules within them. Both the reconstruction of the Au(111) surface and the ordering of the first layer of n-tetradecane can direct the ordering of the manganese porphyrins that are subsequently added to the liquid phase.
Upconversion is a promising technique for significantly enhancing the efficiency of photovoltaic ... more Upconversion is a promising technique for significantly enhancing the efficiency of photovoltaic cells. Molecular systems provide an environment in which long lived triplet states can be exploited to achieve high upconversion efficiencies under solar illumination. We report on the investigation of bi-molecular triplet-triplet annihilation upconversion (TTA-UC) in a Palladium (II) tetrakisquinoxalino porphyrin (PQ4Pd)/rubrene solution. These molecules were studied in solution using UV/VIS spectroscopy to determine their stability in air over a period of weeks. Transient absorption spectroscopy (TAS) was used to directly measure the lifetime of triplet states within these mixtures and hence determine the photoinduced kinetics of the system. The lifetime of porphyrin triplets was reduced from 92.4 μs in pristine PQ4Pd to 2.4 μs in the presence of rubrene. From this change, the rate constant associated with triplet energy transfer (kTET) was calculated as 3.38 × 108M-1s-1. Additionally, a reduction in the absorption of 530 nm light (the ground state rubrene absorption peak) was observed, while the mixture was pumped at the absorption peak of the porphyrin (670 nm). This change became apparent nearly 6 μs after the laser pulse, showing energy transfer from the porphyrin to the rubrene, and allowing further insight into the kinetics of the mechanism.
Chemistry-a European Journal, 2009
The site of electron-transfer reduction of AuPQ+ (PQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)... more The site of electron-transfer reduction of AuPQ+ (PQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quino-xalino[2, 3−b′]porphyrin) and AuQPQ+ (QPQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]porphyrin) is changed from the AuIII center to the quinoxaline part of the PQ macrocycle in the presence of Sc3+ in benzonitrile because of strong binding of Sc3+ to the two nitrogen atoms of the quinoxaline moiety. Strong binding of Sc3+ to the corresponding nitrogen atoms on the quinoxaline unit of ZnPQ also occurs for the neutral form. The effects of Sc3+ on the photodynamics of an electron donor–acceptor compound containing a linked ZnII and AuIII porphyrin ([ZnPQ–AuPQ]PF6) have been examined by femto- and nanosecond laser flash photolysis measurements. The observed transient absorption bands at 630 and 670 nm after laser pulse irradiation in the absence of Sc3+ in benzonitrile are assigned to the charge-shifted (CS) state (ZnPQ.+–AuPQ). The CS state decays through back electron transfer (BET) to the ground state rather than to the triplet excited state. The BET rate was determined from the disappearance of the absorption band due to the CS state. The decay of the CS state obeys first-order kinetics. The CS lifetime was determined to be 250 ps in benzonitrile. Addition of Sc3+ to a solution of ZnPQ–AuPQ+ in benzonitrile caused a drastic lengthening of the CS lifetime that was determined to be 430 ns, a value 1700 times longer than the 250 ps lifetime measured in the absence of Sc3+. Such remarkable prolongation of the CS lifetime in the presence of Sc3+ results from a change in the site of electron transfer from the AuIII center to the quinoxaline part of the PQ macrocycle when Sc3+ binds to the quinoxaline moiety, which decelerate BET due to a large reorganization energy of electron transfer. The change in the site of electron transfer was confirmed by ESR measurements, redox potentials, and UV/Vis spectra of the singly reduced products.
Journal of Materials Chemistry, 2010
Self-assembled microrods {based on 5-nitro-10,15,20-trialkylporphyrins [(C n H 2n+1 ) 3 -NO 2 P]}... more Self-assembled microrods {based on 5-nitro-10,15,20-trialkylporphyrins [(C n H 2n+1 ) 3 -NO 2 P]} and microplates {based on 5, 10,15,20-tetraheptylporphyrin [(C 7 H 15 ) 4 -P]} are fabricated and characterised using optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). The length of the alkyl chains and the deposition surface are found to influence the optical properties and microrod self-assembly. When the deposition surface is silica (a-quartz), 5-nitrotrialkylporphyrins, (C 5 H 11 ) 3 -NO 2 P, (C 7 H 15 ) 3 -NO 2 P and (C 11 H 23 ) 3 -NO 2 P all form microrods of 0.7-0.8 micron diameter; the average length of the microrods varies from 170 microns for (C 5 H 11 ) 3 -NO 2 P to about 11 microns for (C 7 H 15 ) 3 -NO 2 P and (C 11 H 23 ) 3 -NO 2 P, whereas (C 19 H 39 ) 3 -NO 2 P with much longer alkyl chains only gives powders. Controlling the precipitation is crucial in preventing the disordered aggregation of assembled layers observed in the bulk. Very interestingly, the microrods formed from (C 7 H 15 ) 3 -NO 2 P show marked iridescent character. When (C 7 H 15 ) 3 -NO 2 P is deposited on silicon, however, longer curved microrods which do not show iridescence are produced. Single crystal X-ray crystallography of (C 7 H 15 ) 3 -NO 2 P reveals the packing of the bulk material which explains the packing topology of the layers observed by AFM but not the iridescence. The observed structural colour of the (C 7 H 15 ) 3 -NO 2 P microrods is explained by staggering of the layers to produce a corrugated surface with a period of 125 nm, as measured by AFM.
Self-assembled porphyrin wires are fabricated using various nitroporphyrin molecular units. 5-nit... more Self-assembled porphyrin wires are fabricated using various nitroporphyrin molecular units. 5-nitro-10,15,20-triheptylporphyrin is found to give the greatest optical transparency and straights wires. A focused ion beam (FIB) is used to demonstrate milling and shaping of the wires.
Journal of Physics: Conference Series, 2009
Following experimental determination of kinetic parameters governing the up-conversion of light b... more Following experimental determination of kinetic parameters governing the up-conversion of light by triplet-triplet annihilation (TTA-UC), we develop a kinetic model to determine the conditions required for efficient up-conversion. We discuss the assumptions underpinning statistical arguments for an upper limit to TTA-UC and argue that no such limit exists.
Physical Chemistry Chemical Physics, 2010
Photochemical upconversion is performed, whereby emitter triplet states are produced through trip... more Photochemical upconversion is performed, whereby emitter triplet states are produced through triplet energy transfer from sensitizer molecules excited with low energy photons. The triplet emitter molecules undergo triplet-triplet annihilation to yield excited singlet states which emit upconverted fluorescence. Experiments comparing the 560 nm prompt fluorescence when rubrene emitter molecules are excited directly, using 525 nm laser pulses, to the delayed, upconverted fluorescence when the porphyrin sensitizer molecules are excited with 670 nm laser pulses reveal annihilation efficiencies to produce excited singlet emitters in excess of 20%. Conservative measurements reveal a 25% annihilation efficiency, while a direct comparison between the prompt and delayed fluorescence yield suggests a value as high as 33%. Due to fluorescence quenching, the photon upconversion efficiencies are lower, at 16%.
Journal of Materials Chemistry, 2010
Hexa-peri-hexabenzocoronene (HBC) is a discotic polycyclic aromatic hydrocarbon with good charge ... more Hexa-peri-hexabenzocoronene (HBC) is a discotic polycyclic aromatic hydrocarbon with good charge transport characteristics while the light harvesting property of porphyrins is well-known. In order to take advantage of the properties of both materials, a porphyrin-hexa-peri-hexabenzocoroneneporphyrin triad 6 was synthesised for the first time. In photoluminescence studies, the emission from the higher energy HBC unit was completely quenched by the porphyrin unit indicating efficient energy transfer. This novel hybrid material 6 was tested in bulk heterojunction photovoltaic devices reaching a power conversion efficiency of 1.2%.
Journal of Physics: Conference Series, 2009
Page 1. Efficient up-conversion by triplet-triplet annihilation This article has been downloaded ... more Page 1. Efficient up-conversion by triplet-triplet annihilation This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2009 J. Phys.: Conf. Ser. 185 012002 (http://iopscience.iop.org/1742-6596/185/1/012002) ...
Journal of Physical Chemistry A, 2008
Through-porphyrin electronic communication is investigated using "linear-type" and "corner-type" ... more Through-porphyrin electronic communication is investigated using "linear-type" and "corner-type" bis-(quinoxalino)porphyrins in free-base form and their Zn II , Cu II , Ni II , and Pd II derivatives. These compounds are porphyrins with quinoxalines fused on opposite or adjacent , '-pyrrolic positions; they were synthesized from 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-porphyrin-2,3,12,13-and -2,3,7,8-tetraone, respectively, by reaction with 1,2-phenylenediamine. The degree of electron spin delocalization into the fused rings in the π-radical anions of the free-base and metal(II) bisquinoxalinoporphyrins was elucidated by electrochemistry, UV-vis absorption, and electron spin resonance (ESR) spectra of the singly reduced species and density functional theory calculations. Hyperfine splitting patterns in the ESR spectra of the π-radical anions show that symmetric molecules have delocalized electron spin, indicating that significant inter-quinoxaline interactions are mediated through the central porphyrin unit, these interactions being sufficient to guarantee throughmolecule conduction. However, when molecular symmetry is broken by tautomeric exchange of the inner nitrogen hydrogens in the free-base porphyrin with a corner-type quinoxaline substitution pattern, the π-radical anion becomes confined so that one quinoxaline group is omitted from spin delocalization. This indicates the appearance of a unidirectional barrier to through-molecule conduction, suggesting a new motif for chemically controlled rectification.
Journal of Physical Chemistry Letters, 2010
Upconversion (UC) via triplet-triplet annihilation (TTA) is a promising concept to improve the en... more Upconversion (UC) via triplet-triplet annihilation (TTA) is a promising concept to improve the energy conversion efficiency of solar cells by harvesting photons below the energy threshold. Here, we present a kinetic study of the delayed fluorescence induced by TTA to explore the maximum efficiency of this process. In our model system we find that more than 60% of the triplet molecules that decay by TTA produce emitters in their first excited singlet state, so that the observed TTA effiency exceeds 40% at the point of the highest triplet emitter concentration. This result thoroughly disproves any spin-statistical limitation for the annihilation efficiency and thus has crucial consequences for the applicability of an upconvertor based on TTA, which are discussed.
New Journal of Chemistry, 2009
ABSTRACT
Chemical Communications, 2007
New Journal of Chemistry, 2008
... Maxwell J. Crossley*, Craig S. Sheehan, Tony Khoury, Jeffery R. Reimers and Paul J. Sintic. S... more ... Maxwell J. Crossley*, Craig S. Sheehan, Tony Khoury, Jeffery R. Reimers and Paul J. Sintic. School of Chemistry, The University of Sydney, NSW ... 5 and quinoxalino[2,3-b]porphyrins 1315 with nitrogen dioxide gave a mixture of β-pyrrolic functionalised nitro-porphyrin isomers ...
Journal of The American Chemical Society, 2009
The fabrication of porphyrin thin films derived from dichloro [5,10,15,20-tetra(heptyl)porphyrina... more The fabrication of porphyrin thin films derived from dichloro [5,10,15,20-tetra(heptyl)porphyrinato]tin(IV) [Cl-Sn(THP)-Cl] in the holes of photonic crystal fibers over 90 cm in length is described. Evanescent field spectroscopy (EFS) is used to investigate the interfacial properties of the films, with the high surface optical intensity and the long path length combining to produce significant absorption. By comparison with results obtained for similar films formed from Cl-Sn(THP)-Cl inside fused-silica cuvettes and on glass slides, the film is shown to be chemisorbed as a surface Si-O-Sn(THP)-X (X ) Cl or OH) species. In addition to the usual porphyrin Q and Soret bands, new absorptions in the in-fiber films are observed by EFS at 445 nm and between 660-930 nm. The 660-930 nm band is interpreted as a porphyrin to silicon charge-transfer transition and postulated to arise following chemisorption at mechanical-strain induced defect sites on the silica surface. Such defect sites are caused by the optical fiber production process and are less prevalent on other glass surfaces. EFS within optical fibers therefore offers new ways for understanding interface phenomena such as surface adsorbates on glass. Such understanding will benefit all devices that exploit interface phenomena, both in optical fibers and other integrated waveguide forms. They may be directly exploited to create ultrasensitive molecular detectors and could yield novel photonic devices.
Nanotechnology, 2009
Self-assembled supramolecular structures such as optical wires, films and 2D slabs offer a new ge... more Self-assembled supramolecular structures such as optical wires, films and 2D slabs offer a new generation of electronic and optical devices. In particular, self-assembled porphyrin devices, including those integrated onto silica and silicon platforms, open new opportunities in photonic applications spanning molecular biosensing, photovoltaics and telecommunications. All reports to date, however, largely highlight the potential but have not established a clear pathway to the actual implementation of more complex device prototypes. In this paper, we propose and demonstrate the use of a focused ion beam (FIB) to process and fabricate devices in porphyrin-based supramolecular structures. These self-assembled structures have an initial root mean squared (rms) values for surface roughness of <0.5 nm as measured by atomic force microscopy. Under appropriate FIB processing and cutting conditions, the rms value for surface roughness falls to <0.4 nm, comparable with some of the best optical flatnesses obtained within, for example, structured optical fibres and integrated optical waveguides. The milling rate of the porphyrin structures was estimated to be ∼70% of that of silica. The versatility of a FIB as a tool for rapid processing and fabricating 1D and 2D photonic waveguide structures within supramolecular self-assembled platforms is demonstrated by fabricating a 2D coupler, setting the groundwork for true optical device engineering and integration using these new organic systems.
We have investigated a photochemical up-conversion system comprising a molecular mixture of a pal... more We have investigated a photochemical up-conversion system comprising a molecular mixture of a palladium porphyrin to harvest light, and a polycyclic aromatic hydrocarbon to emit light. The energy of harvested photons is stored as molecular triplet states which then annihilate to bring about up-converted fluorescence. The limiting efficiency of such triplet-triplet annihilation up-conversion has been believed to be 11% for some time. However, by rigorously investigating the kinetics of delayed fluorescence following pulsed excitation, we demonstrate instantaneous annihilation efficiencies exceeding 40%, and limiting efficiencies for the current system of ~60%. We attribute the high efficiencies obtained to the electronic structure of the emitting molecule, which exhibits an exceptionally high T2 molecular state. We utilize the kinetic data obtained to model an up-converting layer irradiated with broadband sunlight, finding that ~3% efficiencies can be obtained with the current system, with this improving dramatically upon optimization of various parameters.
Journal of Physical Chemistry A, 2007
We report on a study of singlet-singlet annihilation kinetics in a series of Zn(II)-porphyrin-app... more 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.