Testing Molecular Wires: A Photophysical and Quantum Chemical Assay

ISBN-13: 9783642147395 / Angielski / Twarda / 2010 / 182 str.

Mateusz Wielopolski

Opis

One of the major challenges in current chemistry is to ?nd molecules able to move charges rapidly and ef?ciently from, for example, one terminus to another one under the control of an external electrical, electrochemical or photochemical stimulus. Nature has provided impressive examples of how these goals are achieved. The photosynthetic reaction center protein, for instance, rapidly moves electrons with near unity quantum ef?ciency across a lipid bilayer membrane using several redox cofactors, and thus, serves as a model for developing biomimetic analogues for applications in ?elds such as photovoltaic devices, molecular electronics and photonic materials. In this context, p-conjugated oligomeric molecular assemblies are of particular interest because they provide ef?cient electronic couplings between electroactive units - donor and acceptor termini - and display wire-like behavior. In order to make a molecule able to behave as an ideal molecular wire different requirements need to be ful?lled: i) matching between the donor (acceptor) and bridge energy levels, ii) a good electronic coupling between the electron donor and acceptor units via the bridge orbitals, and iii) a small attenuation factor. Among the many different p-conjugated oligomers, oligo(p-phenylenevin- enes) (oPPV), have emerged as a particularly promising model system that helps to comprehend/rationalize the basic features of polymeric poly(p-phenyle- vinylenes) and also as a versatile building block for novel materials with che- cally tailored properties.

Spis treści

I. Introduction and Motivation 1 1. Introduction to Molecular Electronics 2 1.1. Present Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Limitations of Present Technology . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. Motivation - Focusing on Molecular Wires 9 II. Theoretical Concepts 11 3. Concepts of Photoinduced Electron and Energy Transfer Processes Across Molecular Bridges 12 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2. Electron TransferMechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.1. Superexchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2.2. Charge Hopping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2.3. Interplay ofMechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.3. Electronic Energy Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.3.1. Coulombic Energy Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.3.2. Exchange Energy Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4. Molecule-Assisted Transport of Charges and Energy Across Donor-Wire- Acceptor Junctions 30 4.1. Mechanisms of Charge Transfer throughMolecularWires . . . . . . . . . . . . 32 4.1.1. Superexchange Charge Transfer inMolecularWires . . . . . . . . . . . . 33 4.1.2. Sequential Charge Transfer inMolecularWires . . . . . . . . . . . . . . 34 4.2. Factors that Determine the Charge TransferMechanism . . . . . . . . . . . . . 36 4.2.1. Electronic Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 4.2.2. EnergyMatching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.3. Specific Aspects of Photoinduced Electron Transfer in Organic ?-conjugated Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 4.3.1. Background . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 40 4.3.2. The ClassicalMarcus Theory . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.3.3. Photoexcitation and Relaxation Processes in Solution . . . . . . . . . . 45 4.3.3.1. Photoabsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4.3.3.2. The Franck Condon Principle and Radiative Transitions . . . . 49 4.3.3.3. The Franck Condon Principle and Radiationless Transitions . 52 4.3.3.4. Relaxation Processes Following Photoexcitation . . . . . . . . 55 4.3.3.5. Characterization by Stationary Spectroscopy . . . . . . . . . . 57 4.3.3.6. Characterization by Time-Resolved Spectroscopy . . . . . . . 58 4.3.3.7. Internal Conversion . . . . . . . . . . . . . . . . . . . . . . . . . 58 4.3.4. Influence of the Solvation on the Electronic Relaxation Dynamics . . . 59 4.3.4.1. Static Solvent Influence . . . . . . . . . . . . . . . . . . . . . . . 60 4.3.4.2. Dynamic Solvent Influence . . . . . . . . . . . . . . . . . . . . . 61 5. Examples of Molecular Wire Systems 63 5.1. Oligo(phenylenevinylene)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 5.2. Oligophenylenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 5.3. Oligo(thiophene) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.4. PhotonicWires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 III.Results and Discussion 70 6. Objective 71 7. Instruments and Methods 79 7.1. Photophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.1.1. Absorption spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.1.2. Steady-state emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.1.3. Time-resolved emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.1.4. Femtosecond transient absorption spectroscopy . . . . . . . . . . . . . 80 7.1.5. Nanosecond laser Flash Photolysis . . . . . . . . . . . . . .

Blurb

The field of molecular electronics and organic photovoltaics is steadily growing. One of the major themes in molecular electronics is the construction, measurement, and understanding of the current-voltage response of an electronic circuit, in which molecules may act as conducting elements. The investigated molecular structures in this thesis have been shown to be suitable for distance-independent charge transport. More precisely, the systems investigated were of particular interest due to their ability to provide efficient electronic coupling between electroactive units, and display wire-like behavior in terms of transferring charges from donors to acceptors. Besides impacting on the field of molecular electronics, the results of this research also has applications in the design and development of light harvesting, photoconversion and catalytic modules. This work is a great asset to the field of charge transport through organic pi-conjugated molecules.