Photoluminescence and charge-transfer complexes of calixarenes grafted on TiO ₂ nanoparticles

Calix[4]arenes and thiacalix[4]arenes, cyclic tetramers of phenol, are synthesized with para position (upper rim) feri-butyl, Br, and NO ₂ groups and grafted covalently onto surfaces of TiO ₂ 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 TiO ₂ absorb only UV light, but calixarene - TiO ₂ 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 TiO ₂ 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-TiO ₂ 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 TiO ₂ 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 properties of TiO ₂.