Excited state interactions of dicationic porphyrins with DNA and the photochemistry of 4'-aryl substituted platinum(II) terpyridines

Stephanie Anne Bejune, Purdue University

Abstract

Porphyrin and platinum(II) terpyridine complexes are of great interest for their ability to interact with DNA and act as photosensitizers. Both systems exhibit emissive electronic excited states and can therefore serve as probes of their local environment. This work describes the synthesis and DNA binding studies of trans-5,15-bis-(4-(N-methylpyridiniumyl))porphyrin (H2DMPyP4) and its zinc derivative. The mode of DNA binding has been determined through absorption, emission, and circular dichroism spectroscopy as well as viscosity measurements. It is concluded that minimal steric requirements allow the free porphyrin to be a non-specific intercalator. Intercalation is favored with the zinc porphyrin as long as the DNA substrate is rigid enough to facilitate loss of the axial ligand. In addition, evidence of a cooperative binding interaction wherein two porphyrins intercalate into near-neighbor sites is presented. Viscosity measurements and gel electrophoresis demonstrate that trans-5,15-bis-(3-(N-methylpyridiniumyl))porphyrinatozinc(II) (Zn(II)DMPyP3) intercalates into random sequence DNA substrates. Attempts to insert copper into meso-tetrakis-(4-(N-methylpyridiniumyl))porphyrin (H2TMPyP4) and H2DMPyP3 are described. Preliminary excited-state lifetimes of the Cu(II)TMPyP4-DNA adducts are reported. This work also describes the effect of 4′-aryl substitution on the photophysics of platinum(II) terpyridine complexes. The parent complex is not emissive in fluid solution, but aromatic substitution at the 4 ′ position of the trpy ligand leads to long-lived excited states due to mixing between metal- and ligand-centered states. Upon substitution at the 4′-position of terpyridine with pyrene, extensive mixing in the excited state leads to a very long lifetime and unexpected dual emission in deoxygenated solution. Studies with [Pt(4′-(2-naphthyl)-trpy)Cl] + further elucidate the role of delocalization in the excited state photophysics.

Degree

Ph.D.

Advisors

McMillin, Purdue University.

Subject Area

Inorganic chemistry|Biochemistry|Chemistry

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