CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...
CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...
CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...
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98<br />
alizarin has extended -electron system and therefore, smaller excitation energy. Due to the<br />
presence of electronegative quinine oxygens lowers the absolute value of the ground and<br />
excited states. As a result the optical activity for the charge-transfer excitations in the TiO 2 -<br />
bound alizarin is significantly smaller than in catechol system, even though the<br />
chromophore-semiconductor coupling is exactly the same. As suggested by Duncan and<br />
Prezhdo [3.36] that the π* orbital of alizarin, spread over the entire molecule and is much<br />
more extended the π * orbital of catechol and therefore mixed less with Ti orbitals and as a<br />
result it offers weakly optically active charge-transfer transition bound to Ti through<br />
catecholate moiety. So it clearly realized that electron injection process in alizarin/TiO 2<br />
cannot be all adiabatic process. It has been realized that to demonstrate the theoretical<br />
prediction it is very important to create a perfect platform where one can observe electron<br />
injection process in alirarin/TiO2 system can be mixture of both adiabatic and nonadiabatic<br />
process. In the present investigation we have observed that the electron transfer process is<br />
multi-exponential and there are longer injection components corresponding to times of 17 ps<br />
and 50ps in addition to the ultrafast component (Table 3.1). Therefore the electron transfer<br />
event in the present case purely suggests that electron injection in alizarin/TiO 2 cannot be all<br />
adiabatic as suggested by Duncan and Prezhdo in their recent review. According to the<br />
optical absorption in UV-vis measurements and the size of the particles as measured in high<br />
resolution TEM suggest that the present system is a finite one which could in fact lead to a<br />
decrease in the density of the acceptor states on the TiO 2 surface and this could give rise to<br />
multi-exponential injection. This fact itself is significant in the sense that if one can control<br />
the injection into different acceptor levels within the conduction band. The loss of energy via<br />
the emission of phonons once the electron is injected into the conduction band gap be