posted on 2007-09-20, 00:00authored byThomas Cardolaccia, Alison M. Funston, M. Erkan Kose, Julia M. Keller, John R. Miller, Kirk S. Schanze
The ion radicals of two series of platinum acetylide oligomers have been subjected to study by electrochemical
and pulse radiolysis/transient absorption methods. One series of oligomers, Ptn, has the general structure
PhC⋮C[Pt(PBu3)2C⋮C(1,4-Ph)C⋮C]nPt(PBu3)2C⋮CPh (where x = 0−4, Ph = phenyl and
1,4-Ph = 1,4-phenylene). The second series of oligomers, Pt4Tn, contain a thiophene oligomer core,
C⋮C(2,5-Th)nC⋮C (where n = 1−3 and 2,5-Th = 2,5-thienylene), capped on both ends with
Pt(PBu3)2C⋮C(1,4-Ph)C⋮CPt(PBu3)2C⋮CPh segments. Electrochemical studies reveal that all
of the oligomers feature reversible or quasi-reversible one-electron oxidation at potentials less than 1 V versus
SCE. These oxidations are assigned to the formation of radical cations on the platinum acetylide chains. For
the longer oligomers multiple, reversible one-electron waves are observed at potentials less than 1 V, indicating
that multiple positive polarons can be produced on the oligomers. Pulse-radiolysis/transient absorption
spectroscopy has been used to study the spectra and dynamics of the cation and anion radical states of the
oligomers in dichloroethane and tetrahydrofuran solutions, respectively. All of the ion radicals exhibit two
allowed absorption bands: one in the visible region and the second in the near-infrared region. The ion
radical spectra shift with oligomer length, suggesting that the polarons are delocalized to some extent on the
platinum acetylide chains. Analysis of the electrochemical and pulse radiolysis data combined with the density
functional theory calculations on model ion radicals provides insight into the electronic structure of the positive
and negative ion radical states of the oligomers. A key conclusion of the work is that the polaron states are
concentrated on relatively short oligomer segments.