Electrical sensors for chemical processes: Capacitive sensing of photoinduced intramolecular charge transfer reactions and universal bioaffinity sensor based on conductance variation through nanopores

Development of two types of electrical sensors for detecting photochemical and biochemical processes are described. The photoinduced transient displacement current technique (PTDC) is used for studying light induced charge transfer processes in solutions and ionic conductance sensors based on nanoporous alumina membranes are applied for studying DNA binding.; Both, theoretical and experimental development of PTDC technique is provided. In theory, a revised treatment for deducing dipole moments of solute molecules from the light induced electric polarization change is given, that allows application to arbitrarily shaped molecules. A simplified approach for data evaluation is suggested and compared with the analytical solution for an ellipsoidal cavity and with a numerical solution for two spherical ions.; The theoretical results are applied to porphynoid-fullerene dyads and carotenoid (C) porphyrin (P) fullerene (C60) triad. For the dyads, optical and PTDC study demonstrate efficient intramolecular electron transfer from porphyrin to fullerene with the lifetime of the charge transfer state on the order of a few nanoseconds in low polarity solvents. Solvent polarity effect on the yield of charge separation and the conformations in the charge transfer state are investigated in detail. Excitation of a carotenoid (C) porphyrin (P) fullerene (C60) molecular triad yields, via a sequential two-step photoinduced electron transfer process, a charge-separated state, C•+-P-C60•-, with a lifetime of 340 ns in 2-methyltetrahydrofuran solution. PTDC investigation of the dipole moment of the charge-separated state in tetrahydrofuran and 2-methyltetrahydrofuran confirm the formation of a giant dipole with a moment in excess of 150 D, corresponding to separated charges located on the fullerene and carotene moieties of the triad. Detailed comparison corroborates the hypothesis of insignificant conformational changes in the molecule upon charge transfer.; We also show that stable radicals, such as molecular oxygen and TEMPO, can inhibit or accelerate back electron transfer in the charge transfer state by enhancing intersystem crossing of the singlet radical ion pair into its triplet state. The phenomenon is demonstrated on a series of porphynoid-fullerene dyads. The discrimination is linked with the energy of the charge separated state relative to that of the locally excited triplet states. Due to the spin statistics, the reverse intersystem crossing is less efficient, allowing use of oxygen and other paramagnetic species for impeding charge recombination in various electron transfer systems. (Abstract shortened by UMI.)...