Light Harvesting Molecular Assemblies in the Design of Highly Luminescent Sol-Gel Derived Glasses
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LIGHT HARVESTING MOLECULAR ASSEMBLIES IN THE DESIGN OF HIGHLY LXLbETfESCENT SOL-GEL DERIVED GLASSES Joel I. Dulebohn,* Bdatrice Van Vlierber e,* Kris A. Berglund,* Ronald B. Lessard,§ Jeong-a Yu,§ and Daniel G. Noce; *Department of Chemical and Agricultural Engineering, IDepartment of Chemistry, and the Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824
ABSTRACT Sol-gel derived molecular composites exhibiting intense luminescence, induced from efficient energy transduction processes, have been prepared. The composites are comprised of an Eu 3 +C 2.2.1 cryptate complex or native Eu 3 + ion embedded in sol-gel derived titania glass films. The titania glasses contain interconnected porous networks that permit the diffusion of exogenous substrates, such as the salts of benzoic and 4-tert-butylbenzoic acids, through the film. Interaction of the substrate with the embedded lanthanide complex is indicated by enhanced luminescence from the lanthanide ion. The carboxylic acid salts whose electronic excited states are produced upon capture of incident photons, undergo facile transfer of their electronic energy to the lanthanide ion. By monitoring europium ion luminescence, the diffusion constants of the benzoate and 4-tertbutylbenzoate salts have been measured. Although the diffusion of the 4-tertbutylbenzoate is slower than that of benzoate, the overall higher sensitivity of the former is consistent with hydrophobic guest-host interactions. These new molecular composites relying on the immobilization of an absorption-energytransfer-emission molecular assembly in porous, optically transparent ceramic glasses may be useful in the design of practical sensing devices.
INTRODUCTION The design of many optical sensors is predicated on the development of suitable matrix supports containing chromophores or lumophores whose chemical or physical properties are modified in the presence of exogenous substrate molecules. The successful realization of optically integrated sensors necessarily requires the production of matrices that allow a target molecule to contact the sensing molecule while retaining their optical transparency. One approach is to use a porous matrix wherein the pores are small relative to the wavelength of light used. Fabrication of the matrix at low temperature is advantageous because thermal degradation pathways of chromophores and/or lumophores is circumvented. The ability to process porous glasses at or near room temperature, which embed a variety of molecules with specially tailored optical and chemical properties, provides for the design of novel molecular composites for chemical and optical sensors. The high porosities of sol-gel derived glasses [1,2], which have often been considered a drawback [3], permit molecules to be brought in contact with embedded sensing molecular assembly by diffusion of exogenous substrate through the glass. In fact the glass can actually be thought of as a membrane composed of interconnected pores. The opportunity to prepare films of small t
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