Title

Electrochemistry of crown ether fullerenes, intermolecular signal communication, and electron transport at electrodes modified with 4,4'-bipyridinium dications

Date of Award

2004

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Committee Member

Francisco M. Raymo, Committee Chair

Second Committee Member

Luis Echegoyen, Committee Member

Abstract

In the first chapter, the electrochemistry of mono and bis-crown ether fullerene conjugates of C70 in the absence and presence of different cations is discussed. The addition of K+ causes shifts of up to 80 mV in the reduction potentials of the mono-crown ether conjugates. In the case of the bis-crown ether conjugates, cation complexation can cause shifts of up to 170 mV. Studies with a reference compound, in which the crown ether is anchored by only one malonate group, clearly demonstrate that the cation must be tightly held close to the fullerene core in order to observe significant shifts in the redox potentials. For all the crown ether conjugates, the anodic shifts in the redox potentials are attributed to through-space electrostatic interactions between the fullerene core and the cation.Chapter two focuses on the chemical communication between two independent molecular components. One of them is a photoactive merocyanine that switches to a spiropyran, releasing a proton when stimulated with visible light. The other is a 4,4'-pyridylpyridinium monocation that captures the released proton, producing a 4,4'-bipyridinium dication. Under the irradiation conditions employed, the transformation requires approximately 15 minutes to reach the photostationary state. In the dark, the ensemble of communicating molecules re-equilibrates to the original state in approximately 5 days. These processes can be monitored following the photoinduced enhancement and thermal decay of the current for the reduction of the dications to the corresponding radical cations. A binary logic analysis of the signal transduction operated by the communicating molecules reveals the characteristic behavior of sequential logic operators, which are the basic components of digital memories.In the last chapter, the self-assembly and electron transport of redox-active multiple layers on gold electrodes is discussed. The building block of these multiple layers is a bipyrdinium bisthiol, which has thiol groups at its two ends and bipyridinium dications at its core. Based upon studies with other bipyridinium derivatives, at least one thiol group is necessary for the self-assembly to occur. Furthermore, only derivatives with two bipyridinium centers can form multiple layers on gold. Additionally, the second thiol group of the building block enhances the overall stability of the multiple layer assembly, presumably as a result of interlayer disulfide linkages.Electron transport through the multiple layer assemblies exhibits diffusion-like behavior. Values of 2.9 x 10-8 mol cm-2 s-1/2 and 3.7 x 10-8 mol cm-2 s-1/2 were determined for DA1/2C using chronoamperometry and impedance spectroscopy, respectively. Furthermore, at appropriate surface coverage values, these multiple layers can mediate the transfer of electrons from the electrode to a redox probe solution, but prevent electron transfer in the opposite direction.In agreement with other studies on positively charged films, redox-active anions can be incorporated into the multiple layer assemblies. Similar to the behavior of the bipyridinium centers, electron transport through the anions also exhibits diffusion-like behavior. In addition, after the incorporation of the anions into the assembly, electron transfer between the electrode and the redox probe in solution becomes bidirectional.

Keywords

Chemistry, Physical

Link to Full Text

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