Photophysical studies of pi -conjugated oligomers and polymers that incorporate inorganic MLCT chromophores

The use of pi-conjugated polymers in various material devices has encouraged increased research interest in their photophysical properties in recent years. The integration of transition metal chromophores with metal-to-ligand charge transfer (MLCT) excited states into the polymers permits easy variation of the polymer excited-state properties, since the MLCT chromophore redox potentials are tunable via ligand variation. With this design consideration in mind, the photophysics of two different types of metal-organic polymers are presented.;First, a series of PPE-type aryleneethynylene oligomers containing a 2,2'-bipyridyl unit for transition metal coordination are considered. The oligomers have a well-defined repeat structure, unlike polymers previously studied in our group with the same subunits that exhibited unusual photophysical behavior. These oligomers are studied by themselves and coordinated to --ReI (CO)3Cl and --Re I(CO)3(NCCH3)+OTf-- chromophores. Second, a series of poly(3-alkylthiophene) polymers containing varying amounts of 2,2'-bipyridyl subunits are synthesized and coordinated to --RuII(2,2' -bipyridine)22+ and --OsII(2,2 '-bipyridine)22+ chromophores. The photophysical and photothermal results of these two types of molecules are presented and interpreted, paving the way for the design of better polymers for materials applications. Particular attention is paid to the following: (1) The effect of oligomer and polymer coordination to the metal chromophore. Upon coordination of the metal chromophore to the oligomer or polymer, the primary absorption bands red-shift. Furthermore, organic-based fluorescence is quenched, giving rise to an MLCT-based emission. (2) The effect of increasing the oligomer effective conjugation length. When the conjugation length is increased by lengthening the oligomer, results show that anticipated photophysical results are not obtained for the PPE-type oligomers, suggesting conjugation breaks in the oligomer backbone via geometric twisting. (3) The observed excited-state equilibrium of ligand- and metal-based states ( p3,p*↔M 3LCT ). It is clear through ligand manipulation of the transition metal chromophore that both ligand and metal-based excited states are populated upon photoexcitation, and the observed emission and transient absorption photophysics are controlled by this equilibrium. (4) The effect of oligomer and polymer structure on the observed photophysics. Altering the repeat unit length or switching from a "linear" to "bent" pi-backbone structure dramatically alters the observed photophysics.