Fluorescence-lifetime imaging and super-resolution microscopies shed light on the directed- and self-assembly of functional porphyrins onto carbon nanotubes and flat surfaces

Functional porphyrins have attracted intense attention due to their remarkably high extinction coefficients in the visible region and potential for optical and energy-related applications. Two new routes to functionalised SWNTs have been established using a bulky Zn-II-porphyrin featuring thiolate groups at the periphery. We probed the optical properties of this zinc(II)-substituted, bulky aryl porphyrin and those of the corresponding new nano-composites with single walled carbon nanotube (SWNTs) and coronene, as a model for graphene. We report hereby on: i) the supramolecular interactions between the pristine SWNTs and Zn-II-porphyrin by virtue of pi-pi stacking, and ii) a novel covalent binding strategy based on the Bingel reaction. The functional porphyrins used acted as dispersing agent for the SWNTs and the resulting nanohybrids showed improved dispersibility in common organic solvents. The synthesized hybrid materials were probed by various characterisation techniques, leading to the prediction that supramolecular polymerisation and host-guest functionalities control the fluorescence emission intensity and fluorescence lifetime properties. For the first time, XPS studies highlighted the differences in covalent versus non-covalent attachments of functional metalloporphyrins to SWNTs. Gas-phase DFT calculations indicated that the Zn-II-porphyrin interacts non-covalently with SWNTs to form a donor-acceptor complex. The covalent attachment of the porphyrin chromophore to the surface of SWNTs affects the absorption and emission properties of the hybrid system to a greater extent than in the case of the supramolecular functionalisation of the SWNTs. This represents a synthetic challenge as well as an opportunity in the design of functional nanohybrids for future sensing and optoelectronic applications.
S.I.P. and S.W.B. thank The Royal Society and STFC for funding. B.Y.M. thanks the University of Bath for a studentship (ORS). J.B.d.l.S. acknowledges support for a European Union Marie Curie Career Integration Grant. Dr. Rory L. Arrowsmith is thanked for the initial stages of the DFT calculations of the ZnII-porphyrin. Dr. Amy Kieran is thanked for initial experiments in porphyrin synthesis. The authors thank EPSRC National Service for computational chemistry software and High Performance Computational facilities at Imperial College London for support and computational facilities. Professors Jeremy K. M. Sanders and Paul Raithy are acknowledged for training, helpful discussions in supramolecular chemistry, and porphyrin supramolecular chemistry, as well as mentorship to S.I.P. and her group. Dr. Gabriele Kociok-Kohn is thanked for assistance with X-ray diffraction. B.J.H. and S.I.P. thank EPSRC for funding from the CDT in Sustainable Chemical Technologies. The S.I.P. group thanks the EPSRC for funding, as a member of the Centre of Graphene Science. The authors thank EPSRC National Service for Mass Spectrometry at Swansea and EPSRC National Service for Crystallography at Southampton for data collection. The authors also acknowledge the ERC for the Consolidator Grant O2SENSE.
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