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Anthony Harriman

Publications and source records attributed to Anthony Harriman.

At least 19 recordsLinked to original sources
PubMed2006

Ultrafast intersystem crossing in 9,10-anthraquinones and intramolecular charge separation in an anthraquinone-based dyad.

Femtosecond transient absorption spectroscopy was employed to determine quantitatively the ultrafast S1-T1 intersystem crossing in a 2-substituted 9,10-anthraquinone derivative (3), kisc = 2.5 x 10(12) s-1. Notwithstanding this rapid process, photoexcitation of dyad 1 is followed by competition between intersystem crossing and intramolecular charge separation, the latter leading to a short-lived (2 ps) singlet charge-transfer (CT) state. The local triplet state itself undergoes slower charge separation to populate a relatively long-lived (130 ns) triplet CT state. An earlier report about the formation of an extremely long-lived CT state (> 900 micros) in 1 was found to be erroneous and was related to the sacrificial photo-oxidation of the dimethylsulfoxide solvent used in that study. Finally, some important criteria have been formulated for future experimental validation of "unusually long-lived" CT states.

Journal Article↗
PubMed2006

Photophysical properties of ruthenium(II) tris(2,2'-bipyridine) complexes bearing conjugated thiophene appendages.

A small series of ruthenium(II) tris(2,2'-bipyridine) complexes has been synthesized in which ethynylated thiophene residues are attached to one of the 2,2'-bipyridine ligands. The photophysical properties depend on the conjugation length of the thiophene-based ligand, and in each case, dual emission is observed. The two emitting states reside in thermal equilibrium at ambient temperature and can be resolved by emission spectral curve-fitting routines. This allows the properties of the two states to be evaluated in both fluid butyronitrile solution and a transparent KBr disk. It is concluded that both emitting states are of metal-to-ligand charge-transfer (MLCT) character, and despite the presence of conjugated thiophene residues, there is no indication for a low-lying pi,pi*-triplet state that promotes nonradiative decay of the excited-state manifold. A key feature of these systems is that the conjugation length imposed by the thiophene-based ligand helps to control the rate constants for both radiative and nonradiative decay from the two MLCT triplet states.

Journal Article↗
PubMed2006

Rapid energy transfer in cascade-type bodipy dyes.

Three new molecular dyads, comprising a bora-3a,4a-diaza-s-indacene (Bodipy) dye linked to two aromatic polycycles via the boron center, have been synthesized and fully characterized. The polycyclic compounds are either pyrene or perylene, or a mixture of both. Whereas the absorption spectral profiles contain important contributions from each of the subunits, fluorescence occurs exclusively from the Bodipy fragment. Intramolecular excitation energy transfer is extremely efficient in each case, even though spectral overlap integrals for the pyrene-based system are modest. Although these polycycles are sterically congested, molecular dynamics simulations indicate that they are in dynamic motion, and this hinders proper computation of the orientation factors for Förster-type energy transfer. These new dyes, especially the mixed polycycle system, greatly extend the range of excitation wavelengths that can be used for fluorescence microscopy.

Journal Article↗
PubMed2006

An apparent angle dependence for the nonradiative deactivation of excited triplet states of sterically constrained, binuclear ruthenium(II) bis(2,2':6',2' '-terpyridine) complexes.

The photophysical properties are reported for a series of binuclear ruthenium(II) bis(2,2':6',2"-terpyridine) complexes built around a geometrically constrained, biphenyl-based bridge. The luminescence quantum yield and lifetime increase progressively with decreasing temperature, but the derived rate constant for nonradiative decay of the lowest-energy triplet state depends on the length of a tethering strap attached at the 2,2'-positions of the biphenyl unit. Since the length of the strap determines the dihedral angle for the central C-C bond, the rate of nonradiative decay shows a pronounced dependence on angle. The minimum rate of nonradiative decay occurs when the dihedral angle is 90 degrees, but there is a maximum in the rate when the dihedral angle is about 45 degrees. This effect does not appear to be related to the extent of electron delocalization at the triplet level but can be explained in terms of variable coupling with a low-frequency vibrational mode associated with the strapped biphenyl unit.

Journal Article↗
PubMed2006

Competing through-space and through-bond, intramolecular triplet-energy transfer in a supposedly rigid ruthenium(II) tris(2,2'-bipyridine)--fullerene molecular dyad.

A ditopic ruthenium(II) tris(2,2'-bipyridyl)-based fullerene conjugate has been synthesized so as to separate the photoactive terminals by way of a short ethynylene spacer group that is expected to act as a rigid rod. Intramolecular triplet-energy transfer from the metal complex to the fullerene is quantitative at all temperatures and there is no indication for competing electron transfer. Temperature dependence studies indicate two pathways for triplet-energy transfer. An activationless route dominates at low temperature and is attributed to through-bond electron exchange that takes place via super-exchange interactions. The triplet energy of the bridging unit lies well above that of the metal complex. An activated process is switched-on at high temperatures and is believed to involve through-space electron exchange within closed conformations. Molecular dynamics simulations predict that, in addition to an extended conformation, the linker can distort in such a way that the terminals come into orbital contact. In fact, the resultant closed conformation possesses an idealised geometry for fast electron exchange.

Journal Article↗
PubMed2006

Synthesis and photophysical properties of borondipyrromethene dyes bearing aryl substituents at the boron center.

Several borondipyrromethene (Bodipy) dyes bearing an aryl nucleus linked directly to the boron center have been prepared under mild conditions. The choice of Grignard or lithio organo-metallic reagents allows the isolation of B(F)(aryl) or B(aryl)2 derivatives; where aryl refers to phenyl, anisyl, naphthyl, or pyrenyl fragments. A single crystal, X-ray structure determination for the bis-anisyl compound shows that the sp3 hybridized boron center remains pseudo-tetrahedral and that the B-C bond distances are 1.615 and 1.636 A. All compounds are electrode active but replacement of the fluorine atoms by aryl fragments renders the Bodipy unit more easily oxidized by 100 mV in the B(F)(aryl) and 180 mV in the B(aryl)2 compounds whereas reduction is made more difficult by a comparable amount. Strong fluorescence is observed from the Bodipy fluorophore present in each of the new dyes, with the radiative rate constant being independent of the nature of the aryl substituent. The fluorescence quantum yields are solvent dependent and, at least in some cases (aryl = anisyl or pyrenyl), nonradiative decay from the first-excited singlet state is strongly activated. There is no indication, however, for population of a charge-transfer state, in which the aryl substituent acts as donor and the Bodipy fragment functions as acceptor, that is strongly coupled to the ground state. Instead, it is conjectured that nonradiative decay involves a conformational change driven by the solvophobic effect. Thus, the rate of nonradiative decay in any given solvent increases with increasing surface accessibility (or molar volume) of the aryl substituent. Intramolecular energy transfer from pyrene or naphthalene residues to Bodipy is quantitative.

Boron Compounds↗
PubMed2006

Electron transfer in self-assembled orthogonal structures.

Two new molecular dyads, comprising pyrromethene (bodipy) and 2,2':6',2"-terpyridine (terpy) subunits, have been synthesized and fully characterized. Absorption and fluorescence spectral profiles are dominated by contributions from the bodipy unit. Zinc(II) cations bind to the vacant terpy ligand to form both 1:1 and 1:2 (cation:ligand) complexes, as evidenced by X-ray structural data, NMR and spectrophotometric titrations. Attachment of the cations is accompanied by a substantial decrease in fluorescence from the bodipy chromophore due to intramolecular electron transfer across the orthogonal structure. At low temperature, nuclear tunneling occurs and the rate of electron transfer is essentially activationless. However, activated electron transfer is seen at higher temperatures and allows calculation of the corresponding reorganization energy and electronic coupling matrix element. In both cases, charge recombination is faster than charge separation.

Boron Compounds↗
PubMed2006

A strategy for the synthesis of metal bis(2,2':6',2' '-terpyridine)-terminated molecular dyads having controlled torsion angles at the central biphenyl linker.

The synthesis of a series of binuclear complexes comprising bis(2,2':6',2' '-terpyridine)ruthenium(II) and -osmium(II) centers connected via a geometrically constrained 4,4'-biphenyl bridge is described. These compounds have been prepared by a "synthesis-at-metal" approach as well as by the conventional method of synthesizing the ligand and subsequently attaching the metal center. A computational investigation into the behavior of the biphenyl-based bridges has been used to provide lowest-energy conformations and to estimate the degree of internal fluctuation about the mean torsion angle. It is shown that the length of the constraining strap determines both the torsion angle and the internal flexibility, with longer straps twisting the biphenyl group so as to relax stereochemical interactions between the linking oxygen atoms. Longer straps can be formed from poly(ethylene glycol) residues that provide an additional binding site for small cations. Electrospray mass spectrometry carried out on solutions of these crown ether-like bridges confirmed that Li+, Na+, and K+ ions bind in the form of 1:1 complexes. This range of compounds should permit rational examination of how the torsion angle affects the rate of through-bond electron transfer, electron exchange, and charge shift.

Journal Article↗
PubMed2006

Intramolecular energy-transfer processes in a bis(porphyrin)-ruthenium(II) bis(2,2':6',2''-terpyridine) molecular array.

A molecular triad has been synthesized comprising two free-base porphyrin terminals linked to a central ruthenium(II) bis(2,2':6',2''-terpyridine) subunit via meso-phenylene groups. Illumination into the ruthenium(II) complex is accompanied by rapid intramolecular energy transfer from the metal-to-ligand, charge-transfer (MLCT) triplet to the lowest-energy pi-pi* triplet state localized on one of the porphyrin subunits. Transfer takes place from a vibrationally excited level which lowers the activation energy. The electronic coupling matrix element for this process is 73 cm(-1). Selective illumination into the lowest-energy singlet excited state (S1) localized on the porphyrin leads to fast singlet-triplet energy transfer that populates the MLCT triplet state with high efficiency. This latter process occurs via Dexter-type electron exchange at room temperature, but the activation energy is high and the reaction is prohibited at low temperature. For this latter process, the electronic coupling matrix element is only 8 cm(-1).

Journal Article↗
PubMed2006

The mechanism of long-range electron exchange in molecular-scale photonic wires.

Several disparate systems have been examined as putative molecular-scale wires able to conduct charge over relatively long distances under illumination. The key feature of such systems concerns the level of electronic coupling between the terminals, which are themselves formed from photoactive metal poly(pyridine) complexes. In particular, a series of linear polynuclear ruthenium(II) tris(2,2'-bipyridine) complexes has been synthesized whereby individual chromophores are separated by 1,4-diethynylenebenzene subunits bearing dialkoxy groups for improved solubility. These arrays contain two, three, four five metal centres. The compounds are reasonably soluble in polar organic solvents and possess optical absorption spectral properties that are dominated by transitions associated with the polytopic ligand. Weak luminescence is observed for each complex in deoxygenated acetonitrile at room temperature that appears to be characteristic of emission from a metal-to-ligand, charge-transfer triplet state. The emission lifetime is essentially independent of temperature, at least over a modest range. There is no indication for interaction between close-lying triplet states and no obvious sign of a low-energy pi,pi* triplet associated with the polytopic ligand. The photophysical properties suggest, however, that the longer arrays are segmented due to intramolecular charge-transfer interactions. The ligands bind zinc(II) cations in solution and thereby affect the absorption and emission spectra.

Journal Article↗
PubMed2005

Intramolecular energy transfer in pyrene-bodipy molecular dyads and triads.

Molecules bearing a 4,4-difluoro-8-(aryl)-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (bodipy) core and 1-pyrenyl-1-phenyl-4-(1-ethynylpyrene), or 1-phenyl-4-[1-ethynyl-(6-ethynylpyrene)pyrene] units were constructed in a step-by-step procedure based on palladium(0)-promoted cross-coupling reactions with the required preconstructed modules. X-ray structures of single crystals reveal a twisted arrangement of the two chromophores. In one case, an almost perfect orthogonal arrangement is found. These dyes are strongly luminescent in solution and display rich electrochemistry in which all redox processes of the bodipy and pyrene fragments are clearly resolved. The absorption spectra indicate that the bodipy and pyrene chromophores are spectrally isolated, thereby inducing a large "virtual" Stokes shift. The latter is realised by efficient transfer of intramolecular excitation energy by the Förster dipole-dipole mechanism. The rate of energy transfer depends on the structure of the dual-dye system and decreases as the centre-to-centre separation increases. The energy transfer efficiency, however, exceeds 90 % in all cases. The linkage of two pyrene residues by an ethyne group leads to a decrease in the energy-transfer efficiency, with the two polycycles acting as a single chromophore. The directly linked bodipy-pyrene dual dye binds to DNA and operates as an efficient solar concentrator when dispersed in plastic.

Boron Compounds↗
PubMed2005

Charge on the move: how electron-transfer dynamics depend on molecular conformation.

This tutorial review illustrates the many facets whereby the molecular conformation helps to control the rates of through-bond electron transfer. A brief introduction to Marcus theory is given, highlighting the importance of the coupling element and the super-exchange mechanism, before considering the reasons why the coupling element might depend on the molecular geometry. The methods currently available for determination of both the coupling element and the geometry are reviewed and various examples are given for systems where the structure controls the degree of electronic coupling along the molecular axis. The role of the "bridge" connecting the donor and acceptor is emphasized.

Electrons↗
PubMed2005

Charge shift and triplet state formation in the 9-mesityl-10-methylacridinium cation.

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.

Journal Article↗
PubMed2005

Long-lived charge-transfer states in compact donor-acceptor dyads.

Photoinduced electron transfer is a widely applied method to convert photon energy into a useful (electro)chemical potential, both in nature and in artificial devices. There is a continuing effort to develop molecular systems in which the charge-transfer state, populated by photoinduced electron transfer, survives sufficiently long to tap the energy stored in it. In general this has been found to require the construction of rather complex molecular systems, but more recently a few approaches have been reported that allow the use of much more simple and relatively small electron donor-acceptor dyads for this purpose. The most successful examples of such systems seem to be those that apply "electron spin control" to slow down the spontaneous decay of the charge-transfer state, and these are reviewed in this minireview, with a discussion of the underlying principles and a critical evaluation of some of the claims made with regard to using a pronounced "inverted-region effect" as an alternative method to prolong the lifetime of charge-transfer states.

Electron Transport↗
PubMed2005

Photophysical properties of binuclear ruthenium(ii) bis(2,2':6',2''-terpyridine) complexes built around a central 2,2'-bipyrimidine receptor.

A binuclear complex has been synthesized having ruthenium(ii) bis(2,2':6',2''-terpyridine) terminals attached to a central 2,2'-bipyrimidine unit via ethynylene groups. Cyclic voltammetry indicates that the substituted terpyridine is the most easily reduced subunit and the main chromophore involves charge transfer from the metal centre to this ligand. The resultant metal-to-ligand, charge-transfer (MLCT) triplet state is weakly emissive and has a lifetime of 60 ns in deoxygenated solution at room temperature. The luminescence yield and lifetime increase with decreasing temperature in a manner that indicates the lowest-energy MLCT triplet couples to at least two higher-energy triplets. Cations can bind to the central bipyrimidine unit, forming both 1:1 and 1:2 (ligand:metal) complexes as confirmed by electrospray MS analysis. The photophysical properties depend on the number of bound cations and on the nature of the cation. In the specific case of binding zinc(ii) cations, the 1:1 complex has a triplet lifetime of 8.0 ns while that of the 1:2 complex is 1.8 ns. The 1:1 complexes formed with Ba(2+) and Mg(2+) are more luminescent than is the parent compound while the 1:2 complexes are much less luminescent. It is shown that the coordinated cations raise the reduction potential of the central bipyrimidine unit and thereby increase the activation energy for coupling with the metal-centred state. Complexation also introduces a non-emissive intramolecular charge-transfer (ICT) state that couples to the lowest-energy MLCT triplet and provides an additional non-radiative decay route. The triplet state of the 1:2 complex formed with added Zn(2+) cations decays preferentially via this ICT state.

Journal Article↗
PubMed2005

The effect of solvent polarity on the photophysical properties of 4-cyano-(4'-methylthio)diphenylacetylene: a prototypic donor-acceptor system.

The photophysical properties of the target compound are extremely sensitive to changes in solvent polarity since the lowest-energy excited states possess considerable charge-transfer character. Excitation results in a greatly increased dipole moment, with the resultant excited singlet state retaining a lifetime of ca. 1 ns in all solvents. Radiative decay involves coupling between the lowest-energy excited singlet state and both the ground state and an upper excited singlet state. The level of coupling to the upper singlet decreases in non-polar solvents, presumably due to symmetry factors. The radiative rate constant decreases smoothly with increasing solvent polarity function as the molecule acquires an ever increasing dipolar character. Non-radiative decay includes both intersystem crossing and internal conversion, but the former process dominates in polar solvents. The excited singlet state lifetime is very weakly dependent upon temperature in the solid state. However, in polar solutions where the Stokes' shift decreases with decreasing temperature, there is clear evidence for an activated process. This is believed to involve coupling to the upper-lying singlet excited state.

Acetylene↗
PubMed2005

The photophysical properties of a julolidene-based molecular rotor.

The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures.

Electrochemistry↗