Anita C. Jones

Europium complexes with high total photoluminescence quantum yields in solution and in PMMA

We have prepared complexes of formula [Eu(beta-diketonate)(3)(DPEPO)] and shown quantitative excited-state energy transfer from the ligands combined with efficient Ln luminescence leading to exceptionally-high total photoluminescence quantum yield of up to 80% in solution and in PMMA.

Luminescent, enantiopure, phenylatopyridine iridium-based coordination capsules

The first molecular capsule based on an [Ir(ppy)(2)](+) unit (ppy = 2-phenylatopyridine) has been prepared. Following the development of a method to resolve rac-[(Ir(ppy)(2)Cl)(2)] into its enantiopure forms, homochiral Ir(6)L(4) octahedra where obtained with the tritopic 1,3,5-tricyanobenzene. Solution studies and X-ray diffraction show that these capsules encapsulate four of the six associated counteranions and that these can be exchanged for other anionic guests. Initial photophysical studies have shown that an ensemble of weakly coordinating ligands can lead to luminescence not present in comparable mononuclear systems.

Photonic crystal fibres for chemical sensing and photochemistry

In this review, we introduce photonic crystal fibre as a novel optofluidic microdevice that can be employed as both a versatile chemical sensor and a highly efficient microreactor. We demonstrate that it provides an excellent platform in which light and chemical samples can strongly interact for quantitative spectroscopic analysis or photoactivation purposes. The use of photonic crystal fibre in photochemistry and sensing is discussed and recent results on gas and liquid sensing as well as on photochemical and catalytic reactions are reviewed. These developments demonstrate that the tight light confinement, enhanced light-matter interaction and reduced sample volume offered by photonic crystal fibre make it useful in a wide range of chemical applications.

Characterization and reduction of reabsorption losses in luminescent solar concentrators

The effects of excitation wavelength on the optical properties (emission spectrum and quantum yield) of a luminescent solar concentrator (LSC) containing a fluorescent organic dye (Lumogen F Rot 305) are studied. Excitation at wavelengths on the long-wavelength edge of the absorption spectrum of the dye results in redshifted emission, but the quantum yield remains constant at 100%. The origin of this effect and its consequences are discussed. The extent of the long-wavelength tail of the absorption spectrum of the dye is determined and the importance in reabsorption losses is shown. The optical efficiencies and photon transport probabilities of LSCs containing either an organic dye or a rare-earth lanthanide complex are compared using ray-tracing simulations and experiment. The optical efficiency is shown to depend strongly on the Stokes shift of the fluorophore. The lanthanide complex, which has a very large Stokes shift, exhibits a higher optical efficiency than the dye (64% cf. 50%), despite its lower quantum yield (86% cf. 100%).

Time-resolved fluorescence of 2-aminopurine as a probe of base flipping in M.HhaI–DNA complexes

DNA base flipping is an important mechanism in molecular enzymology, but its study is limited by the lack of an accessible and reliable diagnostic technique. A series of crystalline complexes of a DNA methyltransferase, M.HhaI, and its cognate DNA, in which a fluorescent nucleobase analogue, 2-aminopurine (AP), occupies defined positions with respect the target flipped base, have been prepared and their structures determined at higher than 2 Å resolution. From time-resolved fluorescence measurements of these single crystals, we have established that the fluorescence decay function of AP shows a pronounced, characteristic response to base flipping: the loss of the very short (∼100 ps) decay component and the large increase in the amplitude of the long (∼10 ns) component. When AP is positioned at sites other than the target site, this response is not seen. Most significantly, we have shown that the same clear response is apparent when M.HhaI complexes with DNA in solution, giving an unambiguous signal of base flipping. Analysis of the AP fluorescence decay function reveals conformational heterogeneity in the DNA–enzyme complexes that cannot be discerned from the present X-ray structures.

Quantitative mapping of aqueous microfluidic temperature with sub-degree resolution using fluorescence lifetime imaging microscopy

The use of a water-soluble, thermo-responsive polymer as a highly sensitive fluorescence-lifetime probe of microfluidic temperature is demonstrated. The fluorescence lifetime of poly(N-isopropylacrylamide) labelled with a benzofurazan fluorophore is shown to have a steep dependence on temperature around the polymer phase transition and the photophysical origin of this response is established. The use of this unusual fluorescent probe in conjunction with fluorescence lifetime imaging microscopy (FLIM) enables the spatial variation of temperature in a microfluidic device to be mapped, on the micron scale, with a resolution of less than 0.1 degrees C. This represents an increase in temperature resolution of an order of magnitude over that achieved previously by FLIM of temperature-sensitive dyes.

Electrooxidation of 5-substituted indoles

The electrochemical oxidation of a wide variety of 5-substituted indole monomers at a platinum electrode results in the formation of a redox active film. Electrochemical and spectroscopic evidence is consistent with the redox species in the film being a cyclic trimer. In contrast, the electropolymerisation of 5-aminoindole and 5-hydroxyindole on a platinum electrode does not result in redox active film formation. This is attributed to the adsorption of the monomer onto the metal electrode via the substituent, which inhibits this reaction. However, electropolymerisation of these monomers onto a predeposited film of 5-cyanoindole or 5-nitroindole results in the formation of the cyclic trimer. Electrochemical studies using a rotating-ring disc electrode (RRDE) have confirmed the stoichiometry of the trimerisation reaction and also that the redox active cyclic trimer species shows reversible one electron redox activity. The half-wave potential for the reduction of each of these trimers shows a linear dependence with the Hammett substituent constant, σ+ or σ-, as appropriate, determined for a para-substituted aromatic indicating that the 5-substituent is conjugated into the π-electron system of the trimer. This indicates that judicious choice of substituents allows control of the trimer redox potential.

Optically trapped microsensors for microfluidic temperature measurement by fluorescence lifetime imaging microscopy

The novel combination of optical tweezers and fluorescence lifetime imaging microscopy (FLIM) has been used, in conjunction with specially developed temperature-sensitive fluorescent microprobes, for the non-invasive measurement of temperature in a microfluidic device. This approach retains the capability of FLIM to deliver quantitative mapping of microfluidic temperature without the disadvantageous need to introduce a fluorescent dye that pervades the entire micofluidic system. This is achieved by encapsulating the temperature-sensitive Rhodamine B fluorophore within a microdroplet which can be held and manipulated in the microfluidic flow using optical tweezers. The microdroplet is a double bubble in which an aqueous droplet of the fluorescent dye is surrounded by an oil shell which serves both to contain the fluorophore and to provide the refractive index differential required for optical trapping of the droplet in an external aqueous medium.

The novel use of NMR spectroscopy with in situ laser irradiation to study azo photoisomerisation

Abstract Cis–trans photoisomerisation of four azo dyes has been studied by 1 H nuclear magnetic resonance (NMR) spectroscopy, with in situ laser irradiation of the sample. The laser radiation was coupled into the sample within the NMR magnet via an optical fibre. This enabled the first 1 H NMR measurements to be made of the transient cis isomers of 4-(4-nitrophenylazo)aniline (CI Disperse Orange 3) and 4-[N-ethyl-N-(2-hydroxyethyl)amino]-4′-nitroazobenzene (CI Disperse Red 1). Typical NMR methods, including 1 H 1D, 1 H – 1 H 2D COSY and 1 H – 1 H 2D EXSY/NOESY, were used to assign 1 H resonances of the trans and cis isomers. Ab initio calculations were used to predict the chemical shifts of cis- and trans-azobenzene, in good agreement with the experimental results, and to rationalise the chemical shift changes observed on isomerisation.

2-aminopurine as a fluorescent probe of DNA conformation and the DNA–enzyme interface

Nearly 50 years since its potential as a fluorescent base analogue was first recognized, 2-aminopurine (2AP) continues to be the most widely used fluorescent probe of DNA structure and the perturbation of that structure by interaction with enzymes and other molecules. In this review, we begin by considering the origin of the dramatic and intriguing difference in photophysical properties between 2AP and its structural isomer, adenine; although 2AP differs from the natural base only in the position of the exocyclic amine group, its fluorescence intensity is one thousand times greater. We then discuss the mechanism of interbase quenching of 2AP fluorescence in DNA, which is the basis of its use as a conformational probe but remains imperfectly understood. There are hundreds of examples in the literature of the use of changes in the fluorescence intensity of 2AP as the basis of assays of conformational change; however, in this review we will consider in detail only a few intensity-based studies. Our primary aim is to highlight the use of time-resolved fluorescence measurements, and the interpretation of fluorescence decay parameters, to explore the structure and dynamics of DNA. We discuss the salient features of the fluorescence decay of 2AP when incorporated in DNA and review the use of decay measurements in studying duplexes, single strands and other structures. We survey the use of 2AP as a probe of DNA-enzyme interaction and enzyme-induced distortion, focusing particularly on its use to study base flipping and the enhanced mechanistic insights that can be gained by a detailed analysis of the decay parameters, rather than merely monitoring changes in fluorescence intensity. Finally we reflect on the merits and shortcomings of 2AP and the prospects for its wider adoption as a fluorescence-decay-based probe.