Preparation and study of the photophysical properties of carbon nanotubes functionalized with metal containing polymers

Abstract

Supramolecular architectures involving carbon nanotubes (CNTs) linked to photosensitizers possess distinguishing electronic properties suitable for optoelectronic and light-induced applications. In this work, light harvesting donor-acceptor systems based on photosensitizing metal containing polymers and CNTs were synthesized. The dynamics of photo-induced electron transfer process in the donor-acceptor system, which are crucial for device application, were investigated by femtosecond time-resolved transient spectroscopy. Different series of ruthenium-containing homopolymers and block copolymers were synthesized by reversible addition-fragmentation chain transfer polymerization. The block copolymers were used to disperse multi-walled carbon nanotubes (MWCNTs) with pyrene groups as the anchoring units and ruthenium complexes as the photosensitizers. The electron transfer processes between ruthenium containing diblock/triblock copolymers and MWCNTs were probed by ultrafast transient absorption spectroscopy respectively. The lifetime of the triplet metal-to-ligand charge transfer (3MLCT) excited states of diblock copolymers in the hybrids is composed of a fast component (122.5 ps) and a slow component (2233 ps) while the triblock copolymers/MWCNTs shows the lifetime of 383 ps. Comparison with the results observed in pure polymers, the lifetime of 3MLCT states of these two systems significantly decreased due to the rapid electron injection process from the 3MLCT excited state of ruthenium complexes to MWCNTs. The different injection rate between the diblock and triblock copolymers/MWCNTs may have relationship with the different morphologies of diblock and triblock copolymers on MWCNTs surface, which were elucidated and predicted by molecular dynamics simulations. These results are contributed to the synthesis of block polymers self-assembling on MWCNTs surface to create light harvesting nanohybrids in governing the efficiency of electron transfer and subsequent photochemical processes. Regioregular poly(3-alkylthiophene)s with ruthenium complexes side chains were synthesized using Grignard metathesis polymerization and postpolymerization modification. The conjugated polymers were used to disperse and functionalize single-walled carbon nanotubes (SWCNTs) by warping SWCNTs with polythiophene backbone. Polythiophene with functionalized side chains was able to disperse and stabilize SWCNTs and has potential applications in sorting small diameter SWCNTs due to selective wrapping SWCNTs with diameter around 1 nm. It can also be used to disperse various diameter sized SWCNTs. The selective dispersion allows the improvement in light energy conversion efficiency and device performance because the electronic properties of SWCNTs are strongly dependent on the diameter. The photosensitizing polymers preferred to wrap on SWCNTs with small diameters in a helically wrapped configuration and tended to form linearly aligned configuration on SWCNTs with larger diameters. The electron transfer dynamics in the polymers/SWCNTs were investigated by ultrafast transient absorption spectroscopy. It was shown that electron transfer process occurred from the ruthenium complexes in polymers to SWCNTs within 114 ps after excitation, which was much faster than the relaxation of the 3MLCT excited state (1.2 ns) of pure polymers. The fundamental photo-induced electron transfer studies show the important role of photosensitizer and CNTs in the donor-acceptor systems and provide a basis for designing efficient light harvesting systems for solar energy conversion or other light sensing applications.