We have synthesized a novel 9,9′-spirobifluorene-cored donor-acceptor (D-A) bichromophore system in which the electron-donating (D) moieties are triphenylamine (TPA) and carbazole (CBZ) groups and the electron-withdrawing (A) moieties are 1,3,4-oxadiazole (OXD) groups. The electron-deficient OXD groups efficiently blocked the radical cations delocalization between the two terminal TPA groups, rendering the electropolymerization of the TPA groups feasible. The resulting polymer could be cross-linked further at higher oxidation potentials through electrodimerization occurring at the C3 and C6 positions of the CBZ group. The polymer film obtained exhibited reversible electrochemical oxidation, accompanied by strong color changes with high coloration efficiency and contrast ratio, which could be switched through potential modulation.

The synthesis and photophysical, electrochemical, and spectroelectrochemical characterization of a novel donor-acceptor (D-A) bichromophore system composed of two D-A segments linking through a spiro center are reported. The electron-donating (D) moieties are triphenylamine (TPA) groups, whereas the electron-withdrawing (A) moieties are cyano groups. The particular "spiro" configuration that perpendicularly bonds the D-A chromophores by a tetrahedral carbon, impedes orbital interactions between the branches. Thus, the two TPA substituents act independently, rendering an efficient electropolymerization process feasible. The polymer film obtained showed reversible electrochemical oxidation accompanied by strong color changes with high coloration efficiency and contrast ratio, which can be switched by potential modulation. The remarkable electrochromic behavior of the film is clearly interpreted on the basis of spectroelectrochemical studies. A plausible polymerization mechanism involved with the TPA dimerization reaction is proposed for the electropolymerization process.

Carotenoids (Car) act as ''wires'' that discharge unwanted electrons in the reaction center of higher plants. One step in this ''side-path'' electron conduction is thought to be mediated by Car oxidation. We have carried out direct measurements of the conductance of single-Car molecules under potential control in a membrane-mimicking environment, and we found that when Car are oxidized conductance is enhanced and the electronic decay constant (␤) is decreased. However, the neutral molecule may already be conductive enough to account for observed electron transfer rates.carotenoid ͉ molecular electronics ͉ photosynthesis ͉ potential control ͉ single molecule P hotosynthetic systems are natural photoelectronic devices, integrating electronic and photonic elements in a protein scaffold. Understanding the role played by the different components is important both for understanding photosynthetic processes as well as for using them in artificial molecular electronic devices. Carotenoids (Car) in photosynthesis contribute to light harvesting, structural stabilization, and protection from photooxidation. In photosystem II reaction centers, Car participate in the ''side-path'' electron donation reactions for reduction of P 680 ϩ as part of the photoprotective system. The role of the Car is thought to be that of an electron carrier (1, 2). It is believed that ␤-carotene in photosystem II is oxidized under illumination (1, 2). To investigate whether Car oxidation is a prerequisite for electron transport in photosystem II photoprotection, we measured the conductivity and the length-dependence of the tunneling rate for single-Car molecules under potential control. Measuring electron transport under potential control allows us to simulate the redox environment of the Car under natural conditions and to relate our electronic measurements to its function in vivo.Single (neutral) Car polyenes are much more conductive than alkanes of equivalent length because of their conjugated -electron system (3-5). We and others (6-9) have shown that the conductivity of redox-active single molecules can be regulated by changes in the molecule's oxidation state. Here, we have extended our experimental methods to work in a water-and oxygen-free environment that mimics the conditions found in cell membranes. In this way, we hope to understand one of the roles of Car in photosynthesis and to explore their role as gatable electronic components. Finding such components remains a challenge to date (10).Single-molecule electron transport measurements under potential control were carried out by following the method developed by Xu and Tao (6, 11), for which a scanning tunneling microscope (STM) is used with a partially insulated probe to enable measurements in a conductive electrolyte, which is required for maintaining potential control. A scheme of the experimental setup is shown in Fig. 1 (see also the supporting information, which is published on the PNAS web site). Photosystem II is a transmembrane protein complex, so the surrounding environment o...

A new dye, SSD1, featuring two donor/acceptor chromophores aligned in a spiro configuration with two anchoring groups separated at a distance of 10.05 A (closely matching the distance between the adsorption sites of the anatase TiO(2) surface) undergoes efficient dye adherence on TiO(2) films. A dye-sensitized solar cell incorporating SSD1 exhibited a short-circuit current of 8.9 mA cm(-2), an open-circuit voltage of 0.63 V, a fill factor of 0.67, and a power conversion efficiency of 3.75%.