CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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dye gave a pulse width limited injection on TiO 2 . This could be due to the fact the studies
were conducted on bulk TiO 2 . Furthermore, studies on size quantized ZnO with coumarin
dyes by Murakoshi et al [3.28] revealed that the injection time based on their fluorescence
study at femtosecond resolution was multiexponential, with lifetimes from fs-ns, while
injection into TiO 2 occurred in a time of 150 fs. They attributed it to the solvent effect on the
interfacial dynamics as they had used a methanol solution for ZnO and aqueous solution for
TiO 2 . In the present case this could not be the reason as we have observed earlier [3.29] that
the injection dynamics using a surface modified normal TiO 2 (synthesized by sol-gel
procedure) in toluene and found a pulse width limited injected for the bulk TiO 2 -alizarin
system. Therefore ruling out all the other causes that could give a multiexponential injection
we conclude that in the present case it is due to discreteness in the conduction band.
Furthermore, the injection times are large i.e 3 orders of magnitude larger than that of the
bulk samples therefore this cannot be explained on the basis of change of solvent.
The literature study on multiexponential injection as mentioned earlier have all been
carried out on weak binding dyes. Like Ru-bipyridyl complex N3 and C-343. On the contrary
the dye used in the present case is strong binding dye where injection is believed to proceed
via adiabatic process [3.4]. Most electron injection event in literature studies could be
explained on the basis of classical Marcus relation [3.30]. This equation is strictly valid only
for nonadiabatic system, where the injection of the electron from the dye excited state
proceeds into the CB via electron tunneling since the interaction between the dye and the
semiconductor is weak, whereas in case of an adiabatic electron transfer process the electron
transfer proceeds in the same Born-Oppenheimer surface as it moves from the dye to the CB
of the semiconductor. The difference in the two majorly lies in the fact that since in