Substituent effects on the photophysical properties of bis(1,10-phenanthroline)copper(I) and bis(2,2':6'2''-terpyridine)ruthenium(II) complexes

Ann Kiyomi Ichinaga Gushurst, Purdue University

Abstract

The charge-transfer (CT) absorption spectra and the emission spectra of a series of Cu(NN)$\sb2\sp+$ complexes, where NN denotes a chelating, heteroaromatic ligand, have been measured in different solvent systems at room temperature and at 90K. In most cases the absorption spectra can be assigned in terms of effective D$\sb{\rm 2d}$ symmetry. The major bands in the visible region are attributed to z-polarized CT transitions to the $\Psi\sp{\*}$ and $\chi\sp{\*}$ orbitals of the ligand, where the z axis is taken as the line joining the metal and ligand centers. For phenanthroline complexes these transitions are poorly resolved unless there are phenyl groups in the 2,9-positions. The emission results illustrate the fact that bulky ligands are required to avoid solvent-induced quenching of the excited state in donor media via a type of exciplex quenching. Even CH$\sb2$Cl$\sb2$ is a strong enough Lewis base to induce quenching. No emission was detected form bis (1,10-phenanthroline)copper(I) at room temperature or in frozen solution. Comparisons of solution and solid state absorbance data reveal that Cu(dpp)$\sb2\sp+$ (where dpp denotes 2,9-diphenyl-1,10-phenanthroline) and the related copper(I) catenate Cu(cat-30)$\sp+$ adopt low-symmetry structures in solution which mimic those found in the solid state. Spectral changes also occur in solution upon cooling. In CH$\sb2$Cl$\sb2$ at 25$\sp\circ$C the excited-state lifetimes of Cu(cat-30)$\sp+$, Cu(dpp)$\sb2\sp+$ and Cu(cat-27)$\sp+$ are 190, 250 and 280 ns, respectively. The copper(I) catenates are weak photooxidants. The data suggest that reductive quenching is difficult to observe because the excited state is only mildly oxidizing (E$\sb{1/2}$ $\approx$ 0.4 V vs SHE). The temperature dependent luminescence lifetimes for a series of bis(2,2$\sp\prime$:6$\sp\prime$,2$\sp{\prime\prime}$-terpyridine)Ru(II) complexes have been measured. The data shows that phenyl groups on the ligands increase the barrier to quenching of the $\sp3$MLCT state via the $\sp3$d-d states. The barriers range from 1480 cm$\sp{-1}$ for Ru(tpy)$\sb2\sp{2+}$ (tpy denotes 2,2$\sp\prime$:6$\sp\prime$,2$\sp{\prime\prime}$-terpyridine) to 2180 cm$\sp{-1}$ when the terpyridine ligand is 4,4$\sp\prime$-diphenyl-2,2$\sp\prime$:6$\sp\prime$,2$\sp {\prime\prime}$-terpyridine or 4,4$\sp\prime$,4$\sp {\prime\prime}$-triphenyl-2,2$\sp\prime$:6$\sp\prime$,2$\sp {\prime\prime}$-terpyridine. The preparation of phenylated ligands and the corresponding Ru(II) complexes are described.

Degree

Ph.D.

Advisors

McMillin, Purdue University.

Subject Area

Chemistry

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