Study of electron transfer processes in platinum containing conjugated polymers/carbon nanotube and ruthenium complex/zinc oxide hybrids

Abstract

Single-wall carbon nanotubes have drawn tremendous attentions due to the unique electrical, mechanical, thermal and optical properties. In this work, two new platinum containing conjugated polymers Poly-F8-Pt(CNC) and Poly-BT6-Pt(CNC) were designed and synthesized to disperse SWNTs. The excited state electron transfer properties of the polymers and the polymer/SWNT hybrids were studied by transient absorption spectroscopy. For Poly-F8-Pt(CNC), after photoexcitation at 400 nm, the electron transfer from the singlet excited states of the polymer backbone to the singlet excited state of platinum complex occurred in less than 100 fs. The intersystem crossing process takes place from the singlet to the triplet excited state of Pt complex within 400 ps. After that, the triplet excited state decayed with a time constant of 189 ns. For Poly-BT6-Pt(CNC), after photoexcitation at 400 nm, the electron transfer from singlet excited states of the polymer backbone to singlet excited state of platinum complex part in less than 100 fs. The singlet excited state of platinum complex part undergoes intersystem crossing to the triplet excited state of complex in 15.8 ps. It was also observed that part of the Pt complex triplet excitons back transfer to the triplet excited state of the conjugated polymer backbone with a time constant of 273 ps. Both the triplet excited states of the platinum complex and the polymer backbone decay with a time constant of 217 ns. The observed triplet excitons back transfer results have significant implications for the design of novel triplet sensitized solar cells. For Poly-F8-Pt(CNC)/SWNTs hybrid, the electron transfer from Poly-F8-Pt(CNC) singlet excited state to the SWNTs occurred in 52 ps, and the electrons and polarons recombined in 445 ps. For Poly-BT6-Pt(CNC)/SWNTs hybrid, the electron injected from SWNTs to Poly-BT6-Pt(CNC) in 4.7 ps. Then the electrons and polarons recombined in 315 ps. The transient absorption results suggest that Poly-F8-Pt(CNC) acted as a p-type polymer, which donates electrons to SWNTs. However, Poly-BT6-Pt(CNC) acted as an n-type polymer, which accepts electrons from SWNTs. Dye-sensitized solar cells (DSSCs) have drawn much attentions due to the high versatility and low cost. In this work, ruthenium complex with diazonium group as an anchoring unit was synthesized and covalently functionalized to the surface of ZnO by UV irradiation. XPS, EDX, and TEM studies confirmed the presence of covalent linkage between the ruthenium complex and ZnO substrate. The charge generation and the transport dynamics of both the model ruthenium complex and ruthenium complex-ZnO hybrid were monitored by ultrafast transient absorption(TA) spectroscopy. Surprisingly, no TA band of the excited state was observed both in the growth and decay processes of the hybrid, which is probably due to the direct electron injection from the complex to the conduction band of ZnO. This is probably due to the strong electronic coupling between ruthenium complex and ZnO by diazonium salt functionalization. The time constants for the ground states bleaching of the Ru(tpy)(tpy-Ph)-ZnO are 8.1 ps (34 %) and 167 ps (66 %), both of which are shorter than those for the Ru(tpy)(tpy-Ph) 21.7 ps (36 %) and 360 ps (64 %). This further suggests the strong electronic coupling between ruthenium complex and ZnO. Such light harvesting system has the potential in improving the efficiency of the dyes in DSSC.