Linda de la Garza

Photoinduced electron transfer in tetrathiafulvalene-porphyrin-fullerene molecular triads

The two molecular triads 1a and 1b consisting of a porphyrin (P) covalently linked to a fullerene (C60) electron acceptor and tetrathiafulvalene (TTF) electron-donor moiety were synthesized, and their photochemical properties were determined by transient absorption and emission techniques. Excitation of the free-base-porphyrin moiety of the TTF−P2 H−C60 triad 1a in tetrahydro-2-methylfuran solution yields the porphyrin first excited singlet state TTF−1P2 H−C60, which undergoes photoinduced electron transfer with a time constant of 25 ps to give TTF−P2 H.+−C60.−. This intermediate charge-separated state has a lifetime of 230 ps, decaying mainly by a charge-shift reaction to yield a final state, TTF.+−P2 H−C60.−. The final state has a lifetime of 660 ns, is formed with an overall yield of 92%, and preserves ca. 1.0 eV of the 1.9 eV inherent in the porphyrin excited state. Similar behavior is observed for the zinc analog 1b. The TTF-PZn.+−C60.− state is formed by ultrafast electron transfer from the porphyrinatozinc excited singlet state with a time constant of 1.5 ps. The final TTF.+−PZn−C60.− state is generated with a yield of 16%, and also has a lifetime of 660 ns. Although charge recombination to yield a triplet has been observed in related donor-acceptor systems, the TTF.+−P−C60.− states recombine to the ground state, because the molecule lacks low-energy triplet states. This structural feature leads to a longer lifetime for the final charge-separated state, during which the stored energy could be harvested for solar-energy conversion or molecular optoelectronic applications.

Photoinduced electron transfer in π-extended tetrathiafulvalene–porphyrin–fullerene triad molecules

Two molecular triads consisting of a porphyrin (P) covalently linked to a fullerene electron acceptor (C60) and a π-extended tetrathiafulvalene electron donor (TTF) have been synthesized. Time resolved spectroscopic investigations of the triad featuring a free base porphyrin moiety (TTF–P2H–C60) show that in 2-methyltetrahydrofuran solution, excitation of the porphyrin leads to formation of a TTF–P2H˙+–C60˙− charge-separated state in 25 ps. Electron transfer from the TTF generates a final TTF˙+–P2H–C60˙−state with an overall yield of 0.87. This species decays to the ground state in 1.07 µs. Similar experiments on the zinc analog, TTF–PZn–C60, show formation of TTF–PZn˙+–C60˙− in 1.5 ps, followed by generation of TTF˙+–PZn–C60˙− with a yield of 0.09. This charge-separated state also decays to the ground state in 1.07 µs. Comparison of these results with those for previously reported triads with different donor moieties reveals differences in electron transfer rate constants that can be qualitatively understood in the framework of the Marcus–Hush electron transfer formalism.

Drain current control in a hybrid molecular/MOSFET device

Abstract We have developed a hybrid molecular/MOSFET, which is sensitive to the presence of a molecular layer attached to its surface. The application of the molecular layer was investigated by observing changes in the threshold current of the device. A significant shift in the threshold voltage was attributed to an increase in the electron charge density in the MOSFET channel, resulting from an increase in the positive fixed charge at the native oxide surface. A numerical simulation supports this conclusion. It is speculated that the molecules protonate the surface of the SiO 2 due to the higher acidity of the molecular groups compared to that of the native oxide. To assess the validity of this hypothesis a series of molecules with similar structure but with different acidities (p K a values) were investigated. Preliminary results showing the systematic variation of Δ V th and p K a are presented.

LIGHT-INDUCED CHARGE SEPARATION ACROSS BIO-INORGANIC INTERFACE

Rational design of hybrid biomolecule — nanoparticulate semiconductor conjugates enables coupling of functionality of biomolecules with the capability of semiconductors for solar energy capture, that can have potential application in energy conversion, sensing and catalysis. The particular challenge is to obtain efficient charge separation analogous to the natural photosynthesis process. The synthesis of axially anisotropic TiO2 nano-objects such as tubes, rods and bricks, as well as spherical and faceted nanoparticles has been developed in our laboratory. Depending on their size and shape, these nanostructures exhibit different domains of crystallinity, surface areas and aspect ratios. Moreover, in order to accommodate for high curvature in nanoscale regime, the surfaces of TiO2 nano-objects reconstructs resulting in changes in the coordination of surface Ti atoms from octahedral (D2d) to square pyramidal structures (C4v). The formation of these coordinatively unsaturated Ti atoms, thus depends strongly on the size and shape of nanocrystallites and affects trapping and reactivity of photogenerated charges. We have exploited these coordinatively unsaturated Ti atoms to coupe electron-donating (such as dopamine) and electron-accepting (pyrroloquinoline quinone) conductive linkers that allow wiring of biomolecules and proteins resulting in enhanced charge separation which increases the yield of ensuing chemical transformations.