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A film of [N,N‘-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine]manganese(III) chloride, 1, adsorbed onto an n-type CdSe single-crystal substrate acts as a stereoselective transducer for chiral analytes, coupling the complexation chemistry of the film to the band gap photoluminescence (PL) intensity of the underlying semiconductor. Exposure of the uncoated semiconductor to phenylpropylene oxide (PPO) and styrene oxide (StO) vapor results in a small PL enhancement relative to a vacuum reference level that is the same within experimental error for the four PPO and for the two StO stereoisomers. In contrast, exposure of the coated semiconductor to PPO and StO vapor substantially enhances the CdSe PL intensity relative to its intensity under vacuum conditions, and the optical response is stereoselective, with the PL enhancements and equilibrium adsorption constants dependent on the chirality of both the adsorbate and film. Use of a S,S-1 film on CdSe gives larger PL enhancements and equilibrium binding constants (estimated using the Langmuir adsorption isotherm model) for S,S,-PPO, R,S-PPO, and R-StO than for the enantiomer of each of these epoxides. When the R,R-1 film is employed on CdSe, the expected enantiomeric relationship is observed, with R,R-PPO, S,R-PPO, and S-StO yielding larger PL enhancements and equilibrium binding constants. Binding constants for the preferred film−analyte interactions are in the range of 103 to 104 atm-1. The PL enhancements can be fit to a dead-layer model, except at short wavelengths where evidence for photodissociation of the epoxide from the film is obtained, and maximum reductions in depletion width caused by epoxide−film adduct formation are estimated to range from ∼200 to 800 Å. The PL response can in principle serve as the basis for an on-line chemical sensor for chiral analytes.

Semiconductors provide a unique perspective on inorganic photochemistry. The electronic structure of common semiconductors permits a coupling of optical and electrical phenomena (1). As a consequence, semiconductors have found widespread use in many common electrooptical devices, including light-emitting diodes (LEDs), diode lasers, and solar cells. In the case of LEDs and diode lasers, electrical excitation produces a highly emissive excited state of the solid; in contrast, in solar cells, photoexcitation can produce an electrical output.Interfaces derived from semiconducting solids afford opportunities for chemical control of their electro-optical properties. The intent of this article is to describe the construction and operation of chemical sensors based on the photoluminescence (PL) of semiconducting solids. We and others have shown that adduct formation involving the binding of ambient molecules to the semiconductor surface can lead to reversible PL changes that can be used as the basis for on-line sensor structures (2, 3). Because these devices link surface coordination chemistry to the electrical and excited-state properties of the semiconductor substrates, they provide a rich collection of applications of inorganic photochemistry. Physical and Electronic StructureOf the common inorganic semiconductors, the emissive II-VI compounds CdS and CdSe [CdS(e)] have proven to be particularly versatile sensor substrates. These solids have the wurtzite crystal structure, illustrated in Figure 1, which comprises hexagonal closepacked chalcogen atoms with half of the tetrahedral holes filled by cadmium atoms (4). Thus, each type of atom is tetrahedrally coordinated exclusively by atoms of the other type. Figure 1 also reveals the polar nature of the CdS(e) crystal: the (0001) face at the top of the figure consists exclusively of Cd atoms, and the opposing (0001) face exclusively of chalcogen atoms (5). Most studies of adduct formation onto single-crystal CdS(e) samples have employed the more highly emissive (0001) face, which is more accurately described as a Cd-rich