Michael A. J. Rodgers

Fluorescence from pyrene solubilized in aqueous micelles. A model for quenching by inorganic ions

Abstract When pyrene is solubilized within cetyl trimethyl ammonium bromide micelles the lifetime of its fluorescent state is very dependent upon the concentration of iodide ions within the surrounding aqueous phase. Quantitative examination of the kinetic data throws doubt on the existing model for quenching which requires pyrene to diffuse from within the hydrocarbon core to the peripheral region where encounter and quenching can occur. An alternative model is proposed which argues that the water-lipid interface penetrates deep into the micellar body close to the included pyrene such that quencher ions can encounter pyrene without the latter having to move appreciably. The observed effects of ionic strength add support to the new model.

Kinetic and Thermodynamic Character of Reducing Species Produced on Pulse Radiolysis of Acetonitrile

During nanosecond pulse radiolysis of liquid acetonitrile a transient optical absorption was observed in the spectral range 500 to 1500 nm. The extreme reactivity of this species towards solutes capable of accepting electrons allowed its assignment as a reducing entity. In the purest liquid prepared this species had a half-life of some 900 ns. Variable temperature studies showed that this reducing species, having an intense maximum near 1450 nm at + 62°C, was drastically reduced in absorption intensity at –40°C, where the decay character changed and a weak maximum was observed near 550 nm. It was concluded that the two species are different chemical forms of a reducing entity joined by an equilibrium. Tentative identification of the reversible change according to (CH3CN)–+ CH3CN ⇌(CH3CN)–2 with the monomeric anion as the infrared absorbing entity was based on thermodynamic considerations and the effects of adding polar liquids. Quantitative examination of the data allowed a value of –34.9 kJ mol–1 to be extracted for the binding energy of the anion dimer. Measurements of the yields of pyrene and trans-stilbene radical anion formed after pulse radiolysis of appropriate solutions led to the evaluation of G(reducing entity)= 1.03. In addition it was found that O– radicals, formed by reaction between (CH3CN)– and N2O, reacted with acetonitrile via both addition and abstraction routes. Further, in solutions containing tetracyanobenzene (TCNB) and O2, capture of the negative entity by O2 was followed by electron transfer from O–2 to TCNB.

Formation kinetics of the pyrene dimer cation observed by pulse radiolysis

Abstract The pyrene dimer cation is formed from the monomeric species in liquid acetone at −75°C with a rate constant close to the diffusion value.

Evidence for a triplet exciplex state from pulsed laser photolysis

Abstract Pulsed laser photoexcitation studies of the system pyrene and diethyl aniline have been carried out in an acetonitriletoluene mixed solvent system of varying composition. The photo-processes occuring over the first few nanoseconds have been observed which show that the amine induces the pyrene * (S 1 ) to undergo efficient intersystem crossing, and that this process is the only one which produces any pyrene * (T 1 ) in the first 40 ns after excitation. Subsequent changes depend on the solvent composition. In neat toluene no further pyrene * (T 1 ) is formed: inthe presence of CH 3 CN, additional pyrene * (T 1 ) arises in a process which is thought to be dissociation of a triplet exciplex with a neutral lifetime of 47 ns. A mechanism for the triplet exciplex formation is discussed in terms of geminate ion separation and recombination.

Primary processes in acetone radiolysis

Acetone molecular ions, produced as primary species during electron beam irradiation of liquid acetone have been observed to exist in two kinetically distinguishable forms: (i) free ions which have escaped the spur and which decay by independent routes, and (ii) spur-ions which are confined by Coulombic interactions and decay in a geminate manner following a t–½ expression.Added solutes, shown to be reactive towards either free positive or free negative ions also prevent the formation of excited states of acetone excited molecules in a manner which shows that geminate recombination of ion-pairs is the process responsible for their formation.

Observation of the anthracene excimer during nanosecond pulse radiolysis of benzene solutions

Abstract A new absorption band occurring in the near infrared region during pulse radiolysis of solutions of anthracene in benzene is attributed to an electronic transition of the anthracene excimer state. The half-life of the species is

Production and decay of the F– centre in KCl studied by pulse radiolysis

The thermal decay of F– centres has been studied by pulse radiolysis in pure KCl crystals. At room temperature ∼75% of the F– centres decay exponentially with a mean life of 190 µs. The other 25 % decay by a non-first order process (perhaps by reaction with unstable hole centres).As the temperature is lowered, the fraction of non-first order process becomes smaller, and is zero below 270 K. The rate constant for the first order process decreased with temperature; the activation energy was 0.60 ± 0.04 eV.The lifetime of 190 µs was found to be independent fo both the integrated total dose given to the crystal and the dose per pulse. This implies that the F– decay does not involve reaction with any other radiation-produced species, either short- or long-lived. A new mechanism for F– decay is put forward, involving thermal ionisation as the initial step, followed by F2 centre formation.

Decay of the F– centre in some alkali halides

The production and subsequent decay of the F– centre in NaF, NaCI, KBr and KI has been studied by pulse radiolysis. The centre decays by a mixture of thermal and photolytic processes, with temperature-independent photolysis dominant at low termperature. The thermal decay exhibited frist order kinetic behaviour in all cases, with a rate constant which was independent of both the dose per pulse and the previous radiation history of the sample: this is used as evidence that decay is via thermal ionisation rather than the vacancy capture process postulated previously.The temperature dependence of the rate constant leads to the following values for the Arrhenius activation energy: NaF: 1.5 ± 0.15, NaCl: 1.10 ± 0.04, KBr: 0.43 ± 0.03, KI: 0.47 ± 0.02 eV.

Photobleaching of the F– centre in KCl

Photobleaching of F– centres in KCl has been studied between 100 and 200 K. Under conditions of constant incident light intensity the photobleaching efficiency was constant and independent of temperature, indicating that the quantum yield for photodestruction of F– is constant, and probably equal to unity at all temperatures.Previous studies of F– photolysis have shown that at low temperatures (⩽ 80 K) the process F–→ 2F occurs. The quantum yield for F centre formation falls with increasing temperature, to become virtually zero at 200 K; it has been suggested that the reformation of F–(F–→ F–) is a competing process at higher temperatures.The present results are inconsistent with the above mechanism; it is necessary to postulate a high temperature process that involves neither F– nor F centres as an end product. This high temperature process is probably formation of F2 aggregate centres, since F2 centres also result from the thermal decay of F– centres at temperatures above 240 K.

Photophysical processes in the molecular complexes of 1,2,4,5-tetracyanobenzene with aromatic donors

The photophysical parameters of the (TCNB)–mesitylene molecular complex were investigated using a 1 MW nitrogen gas laser. Increasing amounts of CH3CN cause a decrease in the quantum yield and lifetime of the EDA fluorescent state, which was attributed to a decrease in the radiative rate constant and an increase in the non-radiative rate constants. The yield of TCNB–benzene triplet state, as a function of CH3CN, was monitored by direct infrared observation and by measuring the yield of anthracene triplet generated as a result of energy transfer. The reduction in the EDA triplet state yield as CH3CN increased, was shown to be due to the decrease in the EDA singlet state lifetime. No triplet state was monitored for the TCNB–mesitylene system. The change in the S1→ S[graphic omitted]0 transition with CH3CN was postulated to be caused by solvent destabilization of the ground Franck–Condon state S[graphic omitted]0.