Siân L. Williams

Picosecond time-resolved spectroscopy of the photocolouration reaction of photochromic naphthoxazine-spiro-indolines

The photochemical formation of the open merocyanine forms of several naphthoxazine-spiro-indolines in different solvents have been studied using both picosecond transient absorption (PTA) and picosecond time-resolved resonance Raman (PTR3) methods. The PTA studies have established the presence of several metastable species in the photochemical formation of the coloured merocyanine form of these photochromic compounds. The primary photochemical step occurs on the sub-ps timescale and is followed by the formation of a cisoid intermediate over the next 6–30 ps. This cisoid species then isomerises to the equilibrated distribution of transoid isomers of the merocyanine form with a lifetime that is dependent upon both solvent viscosity and polarity as well as the nature of the substituents on the naphthalene part of the molecule. However, the rate of this cis → trans isomerisation is unaffected on changing the N-alkyl group on the indoline part of the molecule from a methyl to an isobutyl group. The PTR3 studies have demonstrated that, in butan-1-ol, there are at least three different transient species with characteristic vibrational spectra which evolve with different lifetimes to give the final isomeric distribution over the first few ns of the reaction. In cyclohexane it is likely that the formation of a single species is being probed, which is fully developed after the first 200 ps of the reaction. This single species has a spectrum which is the same as the equilibrated steady-state resonance Raman merocyanine spectrum. It is likely that the evolution in the PTR3 spectra obtained here, in butan-1-ol, results from an equilibration of initially formed transoid merocyanine isomers to give a more stable distribution in this polar hydrogen-bonded solvent. Such an equilibration appears to be unnecessary in a non-polar solvent such as cyclohexane and it is suggested that this is because the transoid isomer, initially formed, is already in its most stable form.

Photophysics of 1-methyllumichrome

Abstract Singlet and triplet excited states properties of 1-methyllumichrome have been studied in a series of non-polar, polar aprotic, and polar protic solvents, and adsorbed to a cellulose matrix. These observations are discussed in terms of the possible solvent-solute interactions. The absorption and emission spectra and the fluorescence lifetimes and quantum yields of 1-methyllumichrome have been measured, along with the transient absorption spectra. The excited state decays are all single-exponential, suggesting a single emitting species present in all cases. The spectroscopic data show that the singlet excited state properties of 1-methyllumichrome depend on the solute-solvent hydrogen interaction. Significant changes in fluorescence quantum yields and fluorescence lifetimes were recorded and explained by variations of the non-radiative decay rate constant. Placing the 1-methyllumichrome in a restricted environment caused pronounced changes in its behaviour under laser flash photolysis. Transient absorption measurements of 1-methyllumichrome in H 2 O+β-CD and in a cellulose matrix, provided the spectra of the radical anion and of the triplet excited state of the 1-methyllumichrome anion, respectively. Singlet oxygen is shown to be photosensitised in high yield, and this observation provides an insight into possible photodegradation pathways mediated by this molecule. In addition to the experiments, the nature of the electronic structure of 1-methyllumichrome has been studied by means of the time-dependent density functional theory.

Photon activated biological adhesives in surgery

Abstract There are a wide variety of adhesives available for use in surgery, ranging from cyanoacrylates for skin closure to fibrin-based mixtures for use as lung sealants. The development of light-activated tissue adhesives, commonly known as solders have been undermined by the uncertainty surrounding the end-point for irradiation. A number of methods have been used to define the correct exposure period, from back scatter of light from the tissue/adhesive interface through to thermal feedback systems. However these approaches have not been widely realised in a clinical environment. This paper describes the development of a photochemical approach to light-activated tissue adhesives providing a bifunctional protection that has been shown to generate rapid and strong tissue bonding whilst eliminating the risk of over exposure to damaging radiation using a molecular switch, which protects the underlying tissue from thermal damage.