Electron-transfer rates and electronic coupling factors for ferrocene groups attached to gold electrodes via oligo(phenylethynyl) “molecular wire” bridges of variable length and structure are reported. Attachment to gold was achieved via thiol groups at the end of the bridge opposite the ferrocene. Bridge structures were designed to promote strong coupling between gold and ferrocene, thereby promoting rapid electron transport over long distances. The effects of bridge length and of substituents on the phenyl rings in the bridge were addressed. Bridges containing between three and six phenylethynyl units were studied, and a “beta” value of 0.36 Å-1 describing the exponential distance dependence of bridge-mediated electron-transfer rates was obtained. The effect on the rates of adding two propoxy groups onto one of the phenyl rings in the bridge was examined and found to be minimal. The standard electron-transfer rate constant of 350 s-1 obtained for the adsorbate with the longest bridge (six phenylethynyl units, corresponding to an electron-transfer distance of approximately 43 Å) corresponds to an electronic coupling factor between ferrocene and gold of approximately 0.7 cm-1. The extrapolated rate constants at very short distances were nearly the same for the conjugated bridge series and for a related monolayer series in which ferrocene groups were linked to gold via aliphatic bridges. The extrapolated rate constants at short distance also agree with a calculated rate constant for the limiting case of adiabatic electron transfer at an electrode.

Reported here are two types of curved π-molecules that are π-isoelectronic to planar hexabenzocoronene (HBC) but are forced out of planarity either by an embedded seven-membered ring or by atom crowding at the fjord region. Embedding a heptagon in HBC leads to a novel saddle-shaped molecule 1, whose π-backbone is slightly less curved than the previously reported [7]circulene in terms of the average Gauss curvature, but surprisingly much more rigid than [7]circulene. Overcrowded fjord regions in novel derivatives of hexabenzoperylene (HBP) 2a,b lead to both chiral twisted and antifolded conformers. The successful synthesis of 1 and 2a,b is related to introducing alkoxyl groups to unprecedented positions of hexaphenylbenzenes. It is found that the red twisted isomer of 2b isomerizes at elevated temperature to the yellow anti-folded conformer. This finding along with the study on the thermodynamics and kinetics of the thermal isomerization has improved the early understandings on the conformation of HBP. In the crystals, 1 lacks π-π interactions between neighboring molecules, while twisted-2a exhibits both face-to-face and edge-to-face π-π interactions. Twisted-2b is found to function as a p-type semiconductor in thin film transistors, but the thin films of 1 appear insulating presumably due to lacking π-π interactions. By exploring three different types of curvatures in 1 and the two isomers of 2b, this study has revealed that the curvature of π-face plays a role in determining the frontier molecular orbital energy levels and π-π interactions and thus needs to be considered when one designs new organic semiconductors.

This study presents a new type of negatively curved nanographene (C H ) that contains an unprecedented pattern of heptagons. A tert-butylated derivative of C H was successfully synthesized using tetrabenzodipleiadiene as a key building block. This synthesis involved a ring expansion reaction as a key step to form the seven-membered rings in the framework of tetrabenzodipleiadiene. The single-crystal structure reveals a saddle-shaped molecule with a highly bent naphthalene moiety at the center of the polycyclic backbone. As found from the DFT calculations, this aromatic saddle is flexible at room temperature and has a saddle-shaped geometry as the dominant conformation. The DFT calculations along with experimental results show that the attachment of t-butyl groups to the central tetrabenzodipleiadiene moiety of nanographene C H can stabilize the saddle conformation and make this nanographene less flexible.

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