Peter J. Thistlethwaite

Direct observation of photoautomerism kinetics in 7-quinolinol by picosecond spectroscopy

Abstract The kinetics of enol—keto (lactim—lactam) photoautomerism of 7-quinolinol in methanol have been studied by picosecond fluorescence spcctroscopy using a laser—streak camera system. On excitation of the enol ground slate, approximately half of the excited enol tautomcrizes to the keto form. An upper limit to the tautomerization rate constant of 4.8 × 109 s−1 has been determined.

Solvation effects in the phototautomerization of 7-quinolinol

Abstract Some aspects of the enol → keto phototautomerization of 7-quinolinol in methanol have been re-examined. The results show that phototautomerization only occurs in suitably solvated enol tautomers.

The fluorescence behaviour of methyl and phenyl salicylate

Abstract Fluorcsccnce lifetimes tor the 450 nm emission of methyl and phenyl salicylate in various solvents have been measured. Qucnching studics on the 340 nm fluorescence of these molecules point to the existence of three distinct ground state conformers.

Kinetic evidence for excited state proton transfer in salicylamide

Abstract The fluorescence decay of aqueous salicylamide has been studied by picosecond spectroscopy, using a streak camera. Analysis of the complex decay observed suggests that the fluorescence spectrum consists of two overlapping components. The results have been interpreted in terms of an excited state proton transfer mechanism.

Pressure-area isotherms and fluorescence behaviour of 12-(1-pyrenyl)dodecanoic acid at the air-aqueous solution interface

Abstract Surface pressure-area (π- A ) isotherms of 12-(1-pyrenyl)dodecanoic acid (PDA), alone, and in mixed films with eicosanol, on pure water and basic subphases, were measured concomitantly with the emission spectra of the film at various stages of compression. On a pure water subphase certain portions of the π- A curves could be clearly correlated with the formation of aggregates, primarily composed by PDA molecules. The main driving force for aggregate formation we attribute to hydrogen bonding between the carboxylic moieties of the PDA. Adjustment of the subphase pH such that the carboxylic groups are ionised leads to a large reduction in the degree of PDA aggregation which is seen by a distinct change in the profile of the π- A curves and more clearly by a dramatic reduction in the amount of pyrene excimer emission from the film.

The formation and structure of aggregates in hemicyanine monolayers

Absorption spectra of a hemicyanine monolayer on a sodium sulfate subphase were recorded as a function of surface pressure. Upon compression, the intensity of the monomer band is reduced and two new bands are observed. The results from principal factor analysis of the spectra and the clear isosbestic points indicate that the two bands are due to exciton splitting from an aggregate structure with two molecules per unit cell. By comparison with spectra obtained previously, the influence of counterions and the dilution of the dye monolayer on the formation and structure of the aggregates is investigated. The addition of sulfate counterions to the subphase raises the threshold surface concentration for the formation of aggregates, and produces an aggregate structure in which one long molecular axis is oriented closer to the surface normal. Dilution of the dye monolayer results in a small change to the aggregate structure. Furthermore, if it is assumed that the dye-diluent monolayer is inhomogeneous, dilution has no significant effect on the surface concentration dependence of the formation of aggregates.

Two-dimensional diffusion of amphiphiles in phospholipid monolayers at the air-water interface.

Steady-state and time-resolved fluorescence spectroscopy has been used to examine lateral diffusion in dipalmitoyl-L-alpha-phosphatidylcholine (DPPC) and dimyristoyl-L-alpha-phosphatidylcholine (DMPC) monolayers at the air-water interface, by studying the fluorescence quenching of a pyrene-labeled phospholipid (pyrene-DPPE) by two amphiphilic quenchers. Steady-state fluorescence measurements revealed pyrene-DPPE to be homogeneously distributed in the DMPC lipid matrix for all measured surface pressures and only in the liquid-expanded (LE) phase of the DPPC monolayer. Time-resolved fluorescence decays for pyrene-DPPE in DMPC and DPPC (LE phase) in the absence of quencher were best described by a single-exponential function, also suggesting a homogeneous distribution of pyrene-DPPE within the monolayer films. Addition of quencher to the monolayer film produced nonexponential decay behavior, which is adequately described by the continuum theory of diffusion-controlled quenching in a two-dimensional environment. Steady-state fluorescence measurements yielded lateral diffusion coefficients significantly larger than those obtained from time-resolved data. The difference in these values was ascribed to the influence of static quenching in the case of the steady-state measurements. The lateral diffusion coefficients obtained in the DMPC monolayers were found to decrease with increasing surface pressure, reflecting a decrease in monolayer fluidity with compression.

Excited-state relaxation in 1-amino-8-naphthalene sulphonate

Abstract Flourescence quantum yields and lifetimes have been measured for the isomeric species, 1-amino-8-naphthalene sulphonate and 2-amino-1-naphthalene sulphonate, in a range of solvents. The former shows a change in both radiative and non-radiative rate with solvent, while in the latter, the radiative rate is constant. The results suggest that, for 1-amino-naphthalene sulphonate, relaxation from the Franck-Condon state involves an increase in charge-transfer character.

The temperature dependence of the fluorescence lifetime of pinacyanol iodide

Abstract The fluorescence decay time of pinacyanol (1,1′-diethyl-2,2′-carbocyanine) iodide has been measured in the temperature range 95–225 K. The results indicate that two separate internal-conversion processes are operative. One of these involves torsional relaxation and is viscosity dependent.

Chiral and achiral linear coordination polymers from aldaric acids

The structures of a series of 1D coordination polymers in which divalent centres are bridged by saccharate or mucate anions are presented. In each structure the coordination environment of the metal centres is completed by a 1,10-phenanthroline or 2,2′-bipyridine ligand. Hydrogen bonding between infinite parallel chains leads to a 2D network in each structure with the aromatic groups extending above and below this plane. These groups serve as pillars between the hydrogen-bonded sheets. π–π interactions occur between aromatic groups on neighbouring parallel sheets. In most cases narrow channels filled with disordered water molecules are formed between the aromatic groups on the sides and the hydrogen-bonded sheets that lie above and below. CO2 sorption studies on two compounds are also reported.