Synthesis of phenanthroline-based conjugated polymers and study of their photophysical properties

Abstract

Phenanthroline-based conjugated polymers with donor-acceptor structures, or with terpyridyl and ruthenium(II) bis(terpyridyl) pendant groups were synthesized by palladium-catalyzed coupling reactions. The photophyscial properties of the conjugated polymers were investigated by experimental and computational methods. The applications of these polymers for non-covalent functionalization of carbon nanotubes (CNTs) were also studied. Micro-Raman spectroscopy and femtosecond time-resolved transient absorption (fs-TA) spectroscopy were utilized to investigate the electronic structures of the polymer/CNT hybrids, and the electronic interactions between the conjugated polymers and CNTs. Four phenanthroline derivatives with extensive conjugated structures were synthesized. Selected phenanthroline-based monomers were conjugated to electron-rich dialkynyl monomers to fine tune the electronic properties of the resulting phenanthroline-based poly(arylene ethynylene)s (PAEs). The electronic transitions involved in the PAEs were examined by solvent-dependent UV-vis absorption measurements and DFT calculations, which revealed the aggregation behavior of the PAEs in chloroform/methanol mixture. Molecular dynamics (MD) simulations were performed to study the interactions between selected phenanthroline derivatives and CNTs. The results suggest that the phenanthroline derivatives mainly interact with the CNTs through the planar fused ring moieties. Micro-Raman spectroscopy results show that the PAEs can disperse SWCNTs with larger diameter in the range of 1.1 – 1.6 nm without disrupting the electronic structures. The results demonstrate the phenanthroline-based PAEs are promising CNT dispersants. Two macromolecular assemblies based on poly(2,3-diphenylpyrazino[2,3-f][1,10]phenanthroline-co-thiophene-co-fluorene) scaffold, with terpyridyl and ruthenium(II) bis(terpyridyl) pendant groups were synthesized for non-covalent functionalization of multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). fs-TA spectroscopy was employed to investigate the electronic interactions between the polymers and CNTs. The half-life of the polymer/SWCNT hybrid consisting of the polymer with terpyridyl pendant group is one order of magnitude shorter than that of pure polymer in the picosecond time regime, suggesting that there may be an electron transfer between the polymer and SWCNTs. However, the electronic interaction becomes insignificant after formation of ruthenium(II) complex. These results may provide insight into the design of polymers to optimize the electronic interactions between the polymer and CNTs in the polymer/CNT hybrids for light harvesting applications.