Photoluminescent Properties of Cadmium Selenide in Contact with Solutions and Films of Vaska's Complex: Effects of Oxygen and Carbon Monoxide

Abstract

The band-edge photoluminescence (PL) intensity of etched n-CdSe single crystals is reversibly enhanced in the presence of nitrogen-saturated toluene solutions of trans-IrCl(CO)(PPh3)2, Vaska's complex (1), relative to nitrogen-saturated toluene alone. The PL enhancement is indicative of adsorption of a Lewis base onto the n-CdSe crystals. When the toluene solution of 1 is saturated with carbon monoxide, repetition of these experiments produces larger reversible PL enhancements; when the same solution is saturated with oxygen, large reversible quenching is observed relative to toluene. These PL intensity changes are attributed to binding to the surface of 1·CO and 1·O2, the CO and O2 adducts of 1, respectively. The concentration-dependent PL changes are well fit by the Langmuir adsorption isotherm model; responses to 1 and 1·CO are characterized by similar binding constants K of nearly 105 M-1, while K for 1·O2 is 107 M-1. In the solid state, 1 is known to bind CO and O2 reversibly. When the CdSe surface is coated with thin films of 1, evaporated from toluene solution, CO and O2 gases can be detected through reversible enhancement and quenching, respectively, of CdSe PL intensity relative to a gaseous nitrogen ambient; neither gas affects the PL intensity of uncoated CdSe samples appreciably. Films of 1 coated onto NaCl plates or CdSe crystals yield binding constants for CO from IR spectral changes that are experimentally indistinguishable from those obtained by PL measurements, ∼102 M-1. In contrast, the binding constant for O2, determined from IR spectral changes to be ∼102 M-1, is roughly an order of magnitude lower than that estimated from PL changes, evidencing a roughly 10-fold enhancement in binding affinity at the semiconductor−film interface for 1 toward O2 relative to the bulk film environment. Aspects of the coordination chemistry of 1 that may account for these electrooptical effects are discussed.