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Laura Gagliardi, Dan Frisbie: Ab initio modeling of electronic transport properties of π-conjugated organic oligomers in molecular junctions

Disruption of π−conjugation by control of dihedral angle and insertion of a saturated spacer moiety

Disruption of π−conjugation by (A) control of dihedral angle; (B) insertion of a saturated spacer moiety

In this seed grant, the aim is to employ electronic structure calculations to aid understanding of the electronic transport properties of p-conjugated organic semiconductor molecules. In particular, calculations by Gagliardi will be used to interpret measurements by Frisbie on conjugated molecular wires connected between metal electrodes. Two approaches will be pursued to tailor conjugation length and explore transport. In the first approach, the dihedral angle φ of a biphenyl subunit that is inserted into the wire at one or more locations will be systematically varied, Figure 3. φ can be controlled from 0-90° by using different sterically hindered monomers, and angles above 40° are generally considered to break the conjugation of a π-system. Hopping transport in wires of the same length but with different values of φ will be measured.6-10 Temperature dependent measurements will yield the hopping activation energy also as a function of φ, delineating the role of this important steric parameter on conduction. We expect that φ will control hopping conductivity by controlling barriers for hopping and also by controlling the number of hops.

Gagliardi will calculate the equilibrium geometry and the electronic structure of the wires as a function of φ, the position of the biphenyl group, the number of biphenyl groups, and overall charge state (e.g. number of charges per wire). Her calculations will also address the role of intermolecular interactions in the junction as well as the metalmolecule contacts. These calculations will be employed to interpret the experimental data from Frisbie’s group and investigate a possible trend between φ and the properties of the wires. In the second approach, saturated monomers will be introduced into the wire architecture, such as cyclohexyl groups, Figure 3. As in the dihedral angle strategy, these saturated groups will be used to break the conjugation at a single point in the wire, or at multiple points, with controlled spacings. Again, wire hopping conductivity and activation energy will be measured as a function of the wire length and number and spacing of the conjugation blocking cyclohexyl groups. In parallel, Gagliardi will compute the geometry of neutral and charged wires and the average conjugation length as a function of the number and position of the nonconjugated cyclohexyl groups within individual wires. Intermolecular interactions will also be included.

Together, the theoretical and experimental investigations will address a number of fundamental questions including: (i) does the conductivity decrease linearly with the number of conjugation blocking linkages? (ii) how does the conductivity depend on the separation distance between the blocking linkages, and can this information be used to estimate the carrier delocalization length? (iii) likewise, does the hopping activation energy also scale with the number or spacing of the conjugation blocking units? The answers to these questions will help build a physical picture of the extent of carrier delocalization and thus the number of hops required for charges to traverse the chain length. In general, the tunability of the wire structure facilitates a number of interesting, complementarytheoretical and experimental tudies to understand the role of π-conjugation length on wire conduction.

Funded by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013

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