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Electron transfer mechanisms at electrode/dye/electrolyte interfaces have been modeled with the assumption that the solvent molecules can penetrate the adsorbed dye layer. However, the rates of electron transfer (k) from TiO2electrodes to adsorbed dyes anchored on the electrode in solution do not show dependency on the dielectric property (Pekar factor) of solvents, implying fewer solvent molecules in the dense dye layer. Here, we measure the transient absorption of the oxidized dyes on TiO2with different adsorption densities and solvents without an electrolyte and find that for low adsorption electrodes, the kvalues scale mostly with the Pekar factor. The results and absorption spectra of oxidized dyes suggest that dyes in densely adsorbed layers are surrounded by a few solvent molecules, thereby decreasing the effective Pekar factor to nearly zero. These findings imply the structure of the dyes can also influence the dielectric environment around the adsorbed molecules and consequently affect the value of k. Thus, this opens a way to control separately the reduction rates of oxidized dyes by electrons in the electrode and redox species in solution, while the results may necessitate the modification of the present electric double-layer models for the dye-adsorbed electrodes.

Electron transfer kinetics between donor and acceptor molecules in electrolytes has been described by Marcus theory using reorganization energy (λ), electronic coupling (H), and free energy difference (ΔG°). In solution, the molecules can collide freely, while collision occurs only at the exposed area of the molecules when the donors or the acceptors are anchored onto an electrode, altering the values of λ and H. To date, these structural effects of electrode-bound molecules have not been considered in detail. To study geometrical effects, we fabricate TiO2electrodes with nine different donor-(π-bridge)-acceptor type molecules and measure the kinetics of electron transfer from five different Co complexes in electrolytes. For densely adsorbed electrodes, the molecules with larger donor moieties have faster reduction kinetics and the kinetics are independent of the length of the π-bridge. When the amount of the adsorbed molecules is reduced, the kinetics become faster and the kinetics depend on the π-bridge length. These phenomena can be partially correlated to the increased exposed area of the molecules to the electrolyte. By fitting the data, we obtain lower λ values for lower dye-loading conditions, which is not expected if only the effect of solvent molecules is considered. Obtained Hvalues with various geometries suggest that it is important not only to increase the exposed area but also to expose the point giving high Hvalues to increase the kinetics. One example found is designing molecules with small molecular orbitals to increase Hvalues, though this would also give large λ values.

The initiation of the nonribosomal peptide synthetase (NRPS) assembly of the bisintercalator natural product thiocoraline involves key enzymatic steps for AMP activation and carrier protein loading of the starter unit 3-hydroxyquinaldic acid (3HQA). Gene cluster data combined with protein sequence homology analysis originally led us to propose that TioJ could be responsible for the AMP activation step, whereas TioO could act as the thiolation (T) domain, facilitating the transfer of 3HQA to the next NRPS module, TioR. Herein, we confirmed the involvement of TioJ in thiocoraline biosynthesis by tioJknockout and in vitroactivation of 3HQA studies. However, we demonstrated that TioJ-activated 3HQA is not loaded onto the T domain TioO, as originally believed, but instead onto a fatty acid synthase (FAS) acyl carrier protein (ACP) domain FabC, which is located outside of the thiocoraline gene cluster. We showed a strong interaction between TioJ and FabC. By generating TioJ point mutants mimicking the active site of highly homologous enzymes activating different molecules, we showed that the identity of the substrate activated by adenylation domains such as TioJ is not determined by only the active site residues that directly interact with the substrate. The insights gained from these enzymatic transformations are valuable in the efforts toward deciphering the complete biosynthetic pathway of thiocoraline and bisintercalators in general.

Enhancing electron transfer (ET) kinetics between densely packed, electrode surface-attached dye molecules and redox couples in solution is a key challenge toward more efficient solar to electrical energy conversion. In densely packed dye layers required for efficient light harvesting, the access of the redox mediator to the π-conjugated backbone of the dyes is restricted, and therefore, ET is blocked. Attaching large pendant groups to the end of the dyes facing the electrolytes has been proposed as one of the strategies for enhancing ET. The pendant group would be the first interaction area for the redox mediator designed to give higher electronic coupling. ET can also be enhanced by confining the electron density in smaller dyes, providing strong electron coupling sites. Because of these countering effects of exposed surface area of the orbitals versus high electron probability density, it is currently not known to what extent attaching large pendant groups can enhance ET rates or, in other words, how to design molecules with increased electronic coupling. Herein, we measured the ET kinetics of over 20 dye molecules with the same thiophene backbone and with pendant units having varied size, electron-donating ability, and conjugation. The electronic structure of the dyes and their redox potential was calculated using density functional theory following a thermodynamic cycle. Conformational search of the lowest energy structures together with calculating the contribution of solvation free energies to the redox potential were found to be the key to get redox potential values closely matching the values measured by cyclic voltammetry in solution. To determine the enhancement in ET rates due to changes to the molecular structure, the ET rates of dyes with the smallest pendant groups (3 dyes) in combination with two Co-bipyridyl electron donors were measured, and the data points were fitted to nonadiabatic Marcus theory. Then, the enhancement factor (EF) for the dyes with extended pendant groups is defined by the ratio between the measured rate of the dyes with extended pendant groups and the value on the fitted curve above at the same driving force. The ET rates were enhanced by up to six times for dyes with larger pendant moieties, with clear trends between EFs and the size of the molecular surface area. Furthermore, a key correlation between the EF and the spatial delocalization index (SDI) of the spin density of the oxidized dyes has been uncovered, suggesting the SDI is an appropriate index to evaluate dyes for ET. EF of the largest pendant group dye PX99 was not as large as expected, mainly because the spin density did not spread uniformly to the large pendant group. Further dye designs should focus on concentrating the spin density more evenly on extended donor moieties by exploring stronger push–pull dye designs and the effect of end groups of the pendant units.