Excited-state dynamics of alizarin-sensitized TiO2 nanoparticles from resonance Raman spectroscopy

Resonance Raman spectra of alizarin-sensitized TiO2 nanoparticles have been obtained at excitation wavelengths throughout the 488-nm charge transfer absorption band. The resonance Raman spectrum of the alizarin-sensitized TiO2 nanoparticle is significantly different from the spectrum of free alizarin, consistent with a chemisorption-type interaction. This interaction is probably chelation of surface titanium ions by the hydroxy groups of alizarin, supported by the observed enhancement of bridging C–O modes at 1326 cm−1. In contrast to resonance Raman intensity analysis of homogeneous electron transfer where vibrations of both the donor and acceptor are observed, self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using the time-dependent wave packet propagation formalism show mode-specific reorganization along alizarin vibrations exclusively; no resonance-enhanced vibrations attributable to the TiO2 moiety are observed. Therefore, the total resonance Raman-derived reorganization energy is only 0.04 eV, significantly smaller than the observed outer-sphere reorganization energy of 0.2 eV for this system and inner-sphere reorganization energies measured for other molecular systems. The discrepancy is ascribed to a significant environmental component to the outer-sphere reorganization energy arising from rapid dephasing of surface TiO2 units involved in adsorption by strongly coupled interior bath vibrations.