Photoluminescence and charge-transfer complexes of calixarenes grafted on TiO2 nanoparticles

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 uniformly throughout the excitation spectrum when compared with the material before calixarene grafting, and these effects become stronger as calixarene upper rim substituents become more electron-withdrawing. The single-step synthesis protocols described here electronically couple calixarenes with surfaces of oxide semiconductors, leading to sensitization of TiO2 for absorption in the visible region and provide a systematic method for controlling and understanding surface dipole-mediated electron-transfer phenomena relevant. to the photocatalytic and optoelectronic properities of TiO2.