Reversed Freeze Quench Method near the Solvent Phase Transition

Abstract

Freeze quenching is a general method for trapping reaction intermediates on a (sub)millisecond time scale. The method relies on a mixing and subsequent rapid freezing of solutions of reactants. If the reaction is limited by diffusion, it may be advantageous to initially mix the reactants under conditions where the reaction does not proceed, e.g., by mixing them at low temperature as solids. The temperature may then be raised close to the melting point of the solvent. Depending on the viscosity of the solvent, the temperature can be raised either by heating or by applying laser pulses of nanosecond length with concomitant conversion of light into heat. A reduction of the dead time and a good control of the reaction speed in comparison to the standard freeze quench technique has been achieved with this method. The feasibility of the method in combination with EPR spectroscopy is verified by examining the important prototypical reductions of benzoquinone and 2,6-dichlorophenolindophenol by ascorbate as representatives for two-step redox reactions. By using light pulses of a laser, the reaction could be driven with rates lowered by 4 orders of magnitude as compared to room temperature reaction rates. This has allowed the observation of previously unobserved radical intermediates: the reduction of DCPIP by ascorbate is found to be strongly pH dependent. It proceeds via two one-electron steps at low pH, whereas at neutral pH, the reduction of DCPIP by ascorbate proceeds in a 1:2 stoichiometry followed by a disproportionation of the ascorbate radicals.

Photosystem I Enhances the Efficiency of a Natural, Gel-Based Dye-Sensitized Solar Cell

“…Having an excess of AscH serves to increase the concentration of DCPIPH 2 species in the cell by continually reducing DCPIP that is not reacted at the cathode. A schematic of the AscH/DCPIP mediator system with PSI is shown in Figure S2. , As reported herein, PSI can increase photocurrent at low mediator concentrations by converting the DCPIPH 2 back to DCPIP near the cathode, yielding a higher rate of reduction and increasing the performance of the cell. The reaction kinetics for the AscH/DCPIP reaction mechanism are pH dependent, prompting a comparison of cell performance at different pH values …”

“…These large increases in the ratio of species are possible because of the ultralow concentration of oxidized species in the AscH/DCPIP system prior to light exposure and the localization of PSI on the cathode. The greater increase in photovoltage for the devices at neutral pH is attributed to D - reacting slower with AscH than DH does at low pH, so the oxidized species generated by PSI are not consumed as rapidly, increasing the ratio of oxidized to reduced species at the cathode. , …”

“…The greater increase in photovoltage for the devices at neutral pH is attributed to Dreacting slower with AscH than DH does at low pH, so the oxidized species generated by PSI are not consumed as rapidly, increasing the ratio of oxidized to reduced species at the cathode. 43,45 Devices at low pH were linked in series to increase the total photovoltage (Figure 3b). The PSI-integrated devices exhibit a linear increase in photovoltage of up to 2.1 V for five devices in series as compared to 1.5 V for the devices without PSI.…”