ABSTRACT
The effects of oxygen on the photochemical properties of ruthenium(II) complexes in solution and in polymers are reported. In solution, the complex is actually protected from decomposition by the presence of oxygen as a result of deactivation of the complex by oxygen quenching before it can undergo ligand loss by monomolecular dissociation; however, in polymers, the presence of oxygen increases photochemical decomposition. Singlet molecular oxygen, a product of the oxygen quenching process, may attack the ground state complex or triplet oxygen may directly attack the excited state of the complex. Both mechanisms may be involved in the photodestruction of the complex. The role of oxygen in the photodecomposition was examined by monitoring the photochemical decomposition of various complexes of different singlet oxygen reactivity, as well as absorption and mass spectroscopy studies. It is suggested that in polymers, unlike in solutions, the newly formed reactive singlet oxygen is not able to diffuse away from the complex. The singlet oxygen, trapped in close proximity to the metal complex, has an enhanced opportunity to attack it. This cage effect is supported by studies using tris(1,10-phenanthroline)ruthenium(II) in poly(ethylene glycol) of increasing molecular weight to create an increasingly constraining cage around the complex. Increased poly(ethylene glycol) molecular weight leads to increased oxygen attack of the complex, supporting the cage effect.
ABSTRACT
Luminescence-based, polymer-supported oxygen sensors, particularly those based on platinum group complexes, continue to be of analytical importance. Commercial applications range from the macroscopic (e.g. aerodynamic investigations in wind tunnels, monitoring of oxygen concentration during fermentation, and measurement of biological oxygen demand) to the microscopic (e.g. imaging of oxygen in blood, tissue, cells and other biological samples). Problems hindering the design of improved oxygen sensors include non-linear Stern-Volmer calibration plots and the multi-exponentiality of measured lifetime decays, both of which are attributed primarily to heterogeneity of the sensor molecule in the polymer support matrix. Conventional, confocal and two-photon fluorescence microscopy have proven to be invaluable tools with which the microscale heterogeneity and response of luminescence-based oxygen sensors can be investigated and compared to the macroscopic response. Results obtained for three ruthenium(II) alpha-diimine complexes in polydimethylsiloxane polymer supports indicate the presence of unquenched microcrystals within the polymer matrix that probably degrade oxygen quenching sensitivity and linearity of the Stern-Volmer quenching plot. Two-photon fluorescence microscopy proved most useful for imaging microcrystals within sensor films, and conventional microscopy allowed direct comparison between microscopic and macroscopic sensor response. The implications of the results in the rational design and mass production of luminescence-based oxygen sensors are significant.
ABSTRACT
Oxygen quenching of a series of Os(II) complexes with α-diimine ligands has been studied in a predominantly poly(dimethylsiloxane) (PDMS) polymer and in Gp-163 (an acrylate modified PDMS). Unlike previous Ru(II) complexes used as oxygen sensors, the Os complexes can be excited by readily available, high-intensity, low-cost, red diode lasers at 635, 650, and 670 nm. Variations in the polymer properties have been made in order to delineate the structural features important for satisfactory use of supports for oxygen sensors. A key factor is matching the hydrophobicity of the sensor and support for optimal compatibility and minimizing the size of low oxygen diffusion domains.
