Synthesis and characterization of novel platinum acetylide oligomers

Abstract

To learn about excited state geometry in biphenyl-containing platinum acetylides, we synthesized a series of compounds that have biphenyl ligands. The ligands consisted of biphenyl(<b>I</b>), the hindered 2'-methyl biphenyl(<b>III</b>) and planar fluorenyl(<b>IV</b>) groups. We also synthesized a "half" complex(<b>II</b>) consisting of one ligand attached to the platinum atom. The optical properties of these compounds were measured by ground state absorption, phosphorescence, ultrafast transient absorption and nanosecond transient absorption spectroscopy. DFT calculations were performed to determine the ground state and triplet state geometries and the lowest triplet energy. TDDFT calculations were performed to determine singlet excited state energies. Compared to the reference compound <b>I</b>, ground state spectra show a blue shift in <b>II</b> and <b>III</b> and red shift in <b>IV</b>, showing the singlet energy is sensitive to conjugation and biphenyl twist angle. Comparison of the phosphorescence spectra of <b>I</b> and <b>II</b> shows the triplet exciton is confined to one ligand. The time behavior of the ultrafast excited state absorption spectrum of <b>I</b> shows a red shift within 1 ps from the initial spectrum. This behavior is not seen in <b>IV</b>. The different behavior suggests formation of the triplet state of <b>I</b> is accompanied by conversion from a non-planar to a planar conformation while <b>IV</b> retains a planar conformation.