AVS 52nd International Symposium
    Electronic Materials and Processing Thursday Sessions
       Session EM1-ThM

Paper EM1-ThM3
Electronic Structure and Charge Addition Effects in Phenylene Ethynylene Oligomers: a Comparison of Pristine versus -NO@sub2@ Substituted Molecules

Thursday, November 3, 2005, 9:00 am, Room 309

Session: Molecular Electronics
Presenter: S.W. Robey, NIST
Authors: S.W. Robey, NIST
C.D. Zangmeister, NIST
R.D. van Zee, NIST
Correspondent: Click to Email
Interest in aromatic systems based on phenylene ethynylene oligomers (OPE) has been spurred by reports of intriguing transport behavior in metal-molecule-metal junctions. There have been reports of the observation of negative differential resistance (NDR) characteristics by substitution of electron-withdrawing -NO@sub2@ groups on the central ring of a three-ring phenylene ethynylene oligomer, whereas this behavior is not observed for the "pristine", unsubstituted molecule. In this contribution, the relevant @pi@ electronic structure near the Fermi level, determined via photoelectron spectroscopy for self-assembled monolayers on gold, is compared and contrasted for three-ring phenylene ethynylene oligomers with and without substitution of -NO@sub2@ groups. Further comparison with -NH@sub2@ substituted oligomers and results of ab inito calculations will also be discussed. In addition, theoretical work has invoked redox-like or polaronic effects to provide an explanation for NDR behavior. We have performed measurements to shed light on this issue by comparison of the influence of additional charge in pristine versus nitro-substituted OPE, added by "doping" with K. For pristine OPE, sequential addition of K leads to a gradual shift of levels to higher binding energy reminiscent of "rigid band" behavior, although some distortion/conformation change is suggested by relative shifts of @pi@ levels. For large charge addition, two new states are observed in the original @pi@ to @pi@* gap. For the nitro-substituted oligomer, a similar spectrum is observed for large K addition, but the behavior as a function charge addition is quite different. After an initial shift by ~ 0.6 eV, the spectrum is nearly pinned for subsequent K addition before a final shift brings it to near coincidence with the pristine oligomer spectrum. These results will be discussed in the context of polaronic/bipolaronic effects and the influence of unoccupied levels related to the -NO@sub2@ substitution.