Photoluminescence and Charge-Transfer Complexes of Calixarenes Grafted on TiO2 Nanoparticles

Abstract

Calix[4]arenes and thiacalix[4]arenes, cyclic tetramers of phenol, are synthesized with para position (upper rim) tert-butyl, Br, and NO2 groups and grafted covalently onto surfaces of TiO2 nanoparticles up to a geometrical maximum surface density of 0.30 nm-2. Grafted calixarenes are hydrolytically stable and are shown to exist in their ‘cone' conformation from comparison with model materials synthesized by grafting preformed calixarene−Ti complexes. Individually, protonated calixarenes and TiO2 absorb only UV light, but calixarene−TiO2 hybrid organic−inorganic materials absorb light at significantly lower energies in the visible range (>2.2 eV, <560 nm), reflecting ligand-to-metal charge transfer (LMCT) between calixarene and Ti centers on surfaces of TiO2 nanoparticles. These absorption energies do not depend on the identity and electron-withdrawing properties of upper rim groups in calixarenes. However, the steady-state photoluminescence emission of the calixarene−TiO2 hybrid material is weakened unif...