Crystalline transition metal-based sensors for the optical detection of oxygen

Abstract

Oxygen is essential to life, motivating the development of optical oxygen sensors. Current sensing methods rely on either electrochemical measurements or an optical response from solutions or polymer based materials. These techniques often suffer from support degradation, oxygen consumption, and response plots that are difficult to interpret. A promising alternative to current optical sensing technologies are porous crystalline solids, as these solids provide a consistent emission site along with the necessary open space to facilitate diffusion of molecular oxygen through the material. This thesis explores neutral, mono-, di-, and tricationic coordination complexes for use as solid-state devices to detect oxygen. This series of transition metal complexes was synthesized, and the packing motifs studied by single crystal X-ray diffraction. After investigating the crystalline packing, photophysical properties, and sensitivity of emission toward oxygen, it was determined that the mono- and dicationic complexes are the most promising materials based on their sensitivity and long term stability. In Chapter 1, a brief introduction to oxygen sensing and the goals of this project are given. In Chapter 2, the synthesis and characterization of a series of neutral zinc(II) compounds are reported. One specific compound, Zn(terpy-*)Br₂ (where terpy-* = 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine) showed promise for detecting small oxygen concentrations. However, stability testing determined the compound was not stable toward LED illumination and oxygen exposure. In Chapter 3, a monocationic family of compounds based on the [Cu(P^P)(N^N)]<super>+</super> core (where P^P is a bidentate phosphine and N^N is a bidentate amine) was explored. After the sensing ability of the compounds was determined, their stability was rigorously tested. The studies demonstrate that compounds with tfpb<super>-</super> (where tfpb<super>-</super> = tetrakis[bis-3,5-trifluoromethyl(phenylborate)]) counterions were typically more stable and more sensitive to changes in oxygen than their BF₄<super>-</super> and pfpb<super>-</super> (pfpb<super>-</super> = tetrakis-(pentafluorophenyl)borate) counterparts. Additionally, a mechanism for the degradation of these sensors is proposed. In Chapter 4, a zinc(II) dicationic polypyridine complex was tested and compared to a previously reported and analogous ruthenium(II) compound. Based on the knowledge that similarly shaped molecules tend to pack in a similar way, it was hypothesized that crystalline materials of both the Ru(II) and Zn(II) compounds would pack similarly and therefore sense oxygen in a similar manner. Even though the compounds crystallize in a similar manner and the trends in oxygen sensitivity are the same, the <italic>K_SV</italic> parameters were not as similar as was predicted. The difference in sensing ability is due to a different quenching mechanism in the Ru(II) and Zn(II) complexes. Nonetheless, the Zn(II) family of compounds is a reliable and inexpensive solid-state oxygen sensing material. Finally in Chapter 5, tricationic, 4'-tolyl-2,2':6',2''-terpyridine (tol-terpy) iridium(III) compounds were synthesized and tested for oxygen sensitivity. While none of these materials were sensitive toward changes in oxygen concentration, three new crystal structures of iridium(III) bis-tol-terpy compounds have been reported along with the structure of one neutral mono-tol-terpy complex. These four structures represent a significant contribution to the crystallographic database as only seven structures of this class of compound had been reported before this work.