Photoluminescent Properties of Cadmium Selenide in Contact with Solutions and Films of Metalloporphyrins. Evidence for Semiconductor-Mediated Adduct Formation of Oxygen with Metalloporphyrins at Room Temperature

Abstract

The band-edge photoluminescence (PL) intensity of etched n-CdSe single crystals is modulated through adsorption of 17 porphyrins and metalloporphyrins from methylene chloride solution. In nitrogen-saturated solution, the PL intensity is reversibly enhanced through adduct formation with octaethylporphyrin (OEP), tetraphenylporphyrin (TPP), their divalent Co, Cu, and Cd derivatives, and with tetra(pentafluorophenyl)porphyrin (TFP), tetra-(4-tolyl)porphyrin (TTP), and tetra-(4-pyridyl)porphyrin (TPyP); in contrast, the PL intensity is reversibly quenched by exposure to divalent Mg, Ni, and Zn complexes of OEP and TPP. The concentration dependence of the PL changes can be fit to the Langmuir adsorption isotherm model to yield binding constants that range from ∼103 to 105 M-1. Saturating methylene chloride with oxygen rather than nitrogen does not affect the CdSe PL nor does oxygen affect the (metallo)porphyrin absorption spectra in methylene chloride solution. However, many of the metalloporphyrin-induced PL responses and binding constants are substantially affected by the change in dissolved gas ambient, evidencing surface-mediated adduct formation between oxygen and these metalloporphyrins at room temperature. Particularly large effects are observed for the Mg, Ni, and Zn OEP and TPP complexes, which produce PL enhancements in oxygen-saturated solution rather than PL quenching in nitrogen-saturated solution. When a thin film of each metalloporphyrin is deposited onto CdSe substrates by solvent evaporation, oxygen can be detected through reversible PL enhancement (NiTPP and ZnTPP) or quenching (all other metalloporphyrins examined save for CdOEP and CdTPP, which gave no response) relative to a gaseous nitrogen ambient. Binding constants obtained were typically in the range ∼1−10 atm-1. Steric and electronic contributions to these effects are discussed.