INFLUENCE OF VISCOUS MEDIA ON CHARGE TRANSFER REACTIONS

The influence of viscous H_O-Dextran media on two charge transfer reactions has been investigated using laser photolysis coupled with pico and nanosecond time resolved absorption spectroscopy. In the first case, a charge-transfer reaction from a solute (potassium ferrocyanide) to the solvent was studied and the electron solvation dynamics was followed A solvation time delay, depending on the polymer concentration, is observed which indicates that electrons remain quasi-free for longer periods in these media. In the second case, the quenching process of the triplet state of an excited metallo porphyrin(ZnP4+) by an electron acceptor, methylviologen chloride (MV 2+, 2 CI-) is studied. The measured triplet quenching rate constant values decrease with increasing dextran concentration, but remain higher than those calculated, taking into account a simple viscosity effect. This result is explained in terms of a competition between a decay of the diffusion rate constants of (ZnP4+)* and MV 2+ due to a viscosity effect and an enhanced cage effect around the ions (ZnP 4+* MV 2+) Once formed, this last effect favours the charge transfer reaction.

The influence of viscous H_O-Dextran media on two charge transfer reactions has been investigated using laser photolysis coupled with pico and nanosecond time resolved absorption spectroscopy. In the first case, a charge-transfer reaction from a solute (potassium ferrocyanide) to the solvent was studied and the electron solvation dynamics was followed A solvation time delay, depending on the polymer concentration, is observed which indicates that electrons remain quasi-free for longer periods in these media. In the second case, the quenching process of the triplet state of an excited metallo porphyrin(ZnP4+) by an electron acceptor, methylviologen chloride (MV 2+, 2 CI-) is studied. The measured triplet quenching rate constant values decrease with increasing dextran concentration, but remain higher than those calculated, taking into account a simple viscosity effect. This result is explained in terms of a competition between a decay INTRODUCTION The hydrodynamic properties of aqueous solutions of dextran, a polymer of biological interest, are interesting in many respects. It is known, for example, that the dextran solutions may be used as a blood plasma susbstitute and it is therefore of a great interest to understand the influence of such biological structured media on chemical reactions in comparison with organized media such as micelles and polyelectrolytes. Free and solvated electrons, or excited molecules are the species usually generated upon the irradiation of a given medium, and their reducing or oxidizing properties are often used to induce chemical reactions. The influence of the local environment, especially in structured media can strongly influence the physical properties and reactivity of such species. In the present work, we report the study of the influence of viscous aqueous dextran media on two charge transfer reactions leading in the first case to the formation of solvated electrons, and in the second case to organic ions with a great reducing power.

Part. I
The mechanism of electron injection, localization and solvation in non polar and polar media have been extensively investigated. It    The absorbance rise time of solvated electrons.
The value for an aqueous solution of potassium ferrocyanide in the absence of dextran was 28 + 2 ps. Since the formation time of hydrated electrons is known to be very fast (less than 0,3 ps) this result is quite surprising.
Taking into account the frequency dependent retardation of a light pulse through different media for the exciting and analyzing beam, cannot account for this large discrepancy. A similar result has been found by Miyasaka et al 2 for neat H20 and has been interpreted in terms of a multiphotonic H20 excitation process withi'n the pulse width. Since their experimental conditions (pulse width 20 ps fwhm and 0,5 0,7 mJ outpout power focussed into a spot of 2 mm diameter) are very similar to ours, one can not rule out at formation of "perturbed" water molecules attached to the polymer. A consequence of this is that the induced rotation of the water dipole monomers outside the cluster becomes more and more difficult with increasing dextran concentration, and it takes a longer time for the electron to deepen its trap and become solvated. This interpretation is also consistent with the break observed in the dielectric properties of water molecules for dextran-concentration above 43 % weight, which has been interpreted in terms of an it;crease in the "perturbed" water concentration at 8-9 the expense of the freely rotating molecules A 450 fs electron hydration risetime has been reported in anionic aqueous micelles and interpreted in terms of mi-7 croviscosity change of the electron boundary layer Hence it would seem that the presence of structured media such as micelles cannot induce so large a microviscosity change as found for polymers.  (Table I)   In the solid state, with Ru (bpy)32+ and MV 2+ adsorbed on cellulose, the MV + species produced upon irradiation was found to be especially stable even in the presence of oxygen 7 In rigid media, the possibility of hindering the motion of the oxidised and reduced species seemed to decrease the probability of the back reaction.
Thus, the aqueous solutions of dextran for which the viscosity can be varied from 0,01 to 50 poises, from a liquid to a gel-like phase, appear to be suitable media for this kind of study.
In the present work, we report the effect of high viscosity and structuration of the medium upon the quenching process of the triplet state of an excited metalloporphyrin (ZnP 4+) by MV2+, and the charge separation process following the charge transfer reaction. The laser induced processes are followed by nanosecond time resolved absorption spectroscopy.
The nanosecond absorption spectroscopy set up has been described elsewhere 8 Briefly, the 532 nm exciting laser pulse (from a Nd YAG) has a 6 ns f.woh.m, duration and its energy output (up to 600 mJ) was varied from 5 to 7 mJ in this study. The    It is seen that kq remains constant for 0 to 6 dextran % by weight but decreases above this. This decrease might be explained in terms of the k diff rate constant depen-dence upon the dextran concentration, due to the viscosity of the solutions becoming very high with increasing dextran concentration.
With the aim of checking this, a theoretical value of kq was calculated, taking into account a simple viscosity effect (table II)  The plot of kq (calc) as a function of dextran concentration, shows a large decrease of k (calc.) compared with q the experimental findings. We attribute this discrepancy to a competition between a decay of the diffusion rate-constant arising from a viscosity effect (macroscopic property of the medium) and an enhanced cage effect around the ion pair (Tznp4+, MV 2+) due to microscopic environment, the latter effect slowing down the dissociation reaction and thus favoring the charge transfer reaction.
In the light of the present study, it is seen that the use of very viscous and structured media, is able to enhance the charge transfer process and leads to longer lived + MV species. These results are promising especially when one knows that the reduction of water to H 2 cannot be carried out without the presence of a sacrificial electron donor. A steady state photolysis study of these media is under investigation in order to understand the efficiency of such a system in the solar energy conversion process.