Two-Photon Applications of Transition Metal Polypyridyl Complexes

Abstract

Materials that undergo two-photon absorbtion (2PA), the simultaneous absorption of two photons, are finding increasing use in many applications including 3D fluorescence microscopy, 3D data storage, and photodynamic therapy (PDT). For efficient use, a large two-photon cross-section is desired which can arise from centrosymmetric charge transfer in push-pull electron donor-acceptor (D-A) diads. These structural motifs have been applied to the construction of organic-based chromophores yielding materials with remarkably high two-photon absorption cross-sections, yet, few metal based examples have been studied. Hence, this thesis concerns research into enhanced two photon absorption and emission properties of d6 transition metal complexes bearing suitably structurally modified polypyridyl chromophores using a combination of experimental and theoretical data Two polar tolylterpyridyl-stilbene amine ligands, where NR2 = methyl and phenyl (1a and 1b respectively) have been synthesised and coordinated to a range of d6 transition metals as D-π-A-π-D motifs (RuII and IrIII) or D-π-A variations (ReI and PtII). A single crystal X-ray structure of ligand (1b) was obtained. Spectroscopic analysis indicated successful synthesis of all compounds. One photon luminescence spectroscopy indicated ligand centred emissions for all compounds. Unfortunately, 2PA measurements were unsuccessful for the compounds due to their weak emission and likely small cross-sections too low for the equipment available. Attempts at cis-trans isomerisation of the stilbene bond in the ligands 1a and 1b by UV irradiation were successful, and the isomerisation process was monitored by both UV-visible spectroscopy and 1H NMR spectroscopy. Two 5-substituted-1,10-phenanthroline ligands bearing fluorenyl units (7 and 8) were successfully coordinated to IrIII cyclometallated with phenylpyridine (ppy) and benzo-[H]-quinoline (pq). A single crystal X-ray structure was obtained for [Ir(ppy)2(7)][PF6] (7a). The complexes demonstrated strong emission originating from a triplet metal to ligand charge transfer (3MLCT) excited states due to their long lived luminescent lifetimes measured up to 2 μs. Quantum yields were measured up to 22 % and theie triplet oxygen quenching efficiencies were established by their Stern-Volmer quenching constants, KSV that were determined to be ca. 40 bar-1 and 60 bar-1 for the ppy complexes of 7 and 8 respectively. Preliminary in vitro experiments performed with C6 Glioma cells treated with [Ir(ppy)2(7)][PF6] (7a) show efficient sensitization for triplet oxygen (3O2) by two-photon excitation at 740 nm resulting in photodynamic effects which led to localised cell damage and death. This complex also demonstrated relatively high two-photon absorption cross-sections ranging from 50-80 Goeppert-Mayer units (GM) between 750 and 800 nm. Two new ReI complexes have been synthesised utilising the ligands 7 and 8 (7c and 8c). A single crystal X-ray structure was obtained for Re(CO)3(7)Cl. (7c) These complexes also exhibited relatively long luminescence lifetimes of ca. 300 ns originating from a 3MLCT state, but emission quantum yields were much lower than corresponding IrIII complexes. The Stern-Volmer quenching analysis demonstrated much less efficient quenching by 3O2, with KSV values of around 8 bar-1 for both compounds. A ligand containg a fluorenyl linker between two 5-substituted-1,10-phenanthroline units (9) has been utilised as a bridge for IrIII-ReI (9a) and IrIII- RuII (9b) complexes. Emission in these complexes arises from 3MLCT excited states and both exhibit dual component long lived luminescent lifetimes up to 1.3 μs and quantum yields around 22 %. KSV constants were measured at 23 and 41 bar-1 for Ir-Re (9a) and Ir-Ru (9b) respectively. Furthermore, the Ir-Ru complex demonstrated a markedly large two-photon cross-section of 350 GM at 730 nm.

Date of Award	31 Dec 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Louise Natrajan (Supervisor)