Charge-recombination luminescence

This chapter discusses the intenelation between mechanical properties, molecular mobility and chemical reactivity of curing epoxy-amine thermosets, illustrated by examples of how the charge recombination luminescence (CRL), heat-capacity and rate constants of chemical reactions are influenced by gelation and vitrification during isothermal cure. A comparison of dynamic mechanical, CRL and modulated temperature DSC data shows that vitrification is accompanied by an increase in CRL and a decrease in heat-capacity, and that the heat-capacity and CRL continue to change after the viscoelastic properties have levelled out. It is also shown how the rate constant of an intermolecular secondary amine reaction, measured by near infirared spectroscopy, is sensitive to gelation, whereas the intramolecular rate constant instead is sensitive to vitrification. 

In this chapter the interrelation between mechanical properties, molecular mobility and chemical reactivity is discussed. Examples of how the changes in charge recombination luminescence, heat capacity and rate constants of chemical reactions can be related to the evolution of viscoelastic properties and the transitions encountered during isothermal cure of thermosetting materials are given. The possible application of the experimental techniques involved to in-situ cure process monitoring is also reviewed. 

Charge recombination luminescence was measured in a set-up described in detail elsewhere (75). Stoichiometric mixtures of DGEBF and DDM in aluminium pans were taken to the cure tenq)erature at 15 G/min and cured isothermally under nitrogen in a chamber covered by a quartz window. The sanq>le was intermittently irradiated with a Kulzer Duralex UV-3(X) fibre optic wand for 60 s. After each irradiation the shutter of the photomultiplier was opened with a delay of 5 s, and the initial intensity of emitted light, 7o, was recorded. 

Figure 3. Charge-recombination luminescence intensity during isothermal cure of DGEBF/DDM at three diffoent temperatures.

By analyzing the spectra of the low-temperature recombination luminescence of GaP crystals under conditions of stationary excitation, Tomas et al. [63] have determined the distances of electron tunneling. The spectra of this luminescence at 1.6 K consist of a large number of intensive narrow lines. The appearance of these lines is due to the fact that the energy of the quantum hv emitted as a result of electron tunneling between the charged donor and the acceptor depends on the distance, R, between the reagents... 

Upon light excitation of dark-adapted PS II, the primary charge separation takes place, forming P-680 and Pheo. This probably happens in a small number of picoseconds. Electron transfer from Pheo to occurs in a few hundred picoseconds, stabilizing the separated charges [112]. If is already reduced the (P-680 Pheo ) radical pair can still be formed, although perhaps with a low quantum yield (see Ref. 145), but now it lasts for a few nanoseconds [142] and gives rise to some recombination luminescence or, at low temperature, populates the triplet state of P-680 [141], which itself decays with a of around 1 ms [166]. 

The first observation of luminescence associated with a charge recombination process came in the early part of the 20th century in experiments involving oxidative electrolysis of aqueous halide solution at mercury anodes [1]. The report describes the observation of colored flames from the mercury pool surface following formation of a coating of mercury halide on the electrode surface during electrolysis and the authors relate this to flame emission of mercury halides. 

Weller and Zachariasse thoroughly investigated exciplex formation and luminescence for donor acceptor systems in THF [18]. A particularly interesting result from their work came from an examination of the temperature dependence of radiative charge recombination between 9,10-dimethylanthracene anion (DMA") and TPTA+ in THF [19]. They found that both exciplex emission and fluorescence from DMA were observed in solution at low temperature (ca. —50°C). As the solution temperature is raised, the excimer emission decreases in relative intensity, and at room temperature the emission is nearly completely DMA fluorescence. The monomer-to-exciplex emission intensity ratio as a function of temperature follows Arrhenius kinetic behavior and yields an activation barrier that is nearly the same as the energy gap between the exciplex and the DMA states. Thus, their model consisted of reaction of the solvent-separated ions to form an intimate emissive ion pair which could dissociate to yield the singlet anthracene derivative. 

The thrust of this chapter has been to provide a brief overview of this fascinating area, from the essential theoretical framework provided by electron-transfer theory to the array of potential novel device applications. From early studies of radiative charge recombination, experimental approaches developed greater sophistication and the range of chemical reaction types expanded. The ability to generate luminescence by electrochemical excitation presents a rich array of potential device applications and the future of research in this area is certainly bright. 

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