Rosette Nanotubes: Supramolecular Scaffolds for Organic Optoelectronics

Abstract

Supramolecular rosette nanotubes (RNTs) are obtained by the hierarchical self-assembly of mono and twin G^C building blocks. The mono G^C motif is a fused bicyclic ring of guanine and cytosine with six self-complementary sets of hydrogen bonding sites, while the twin G^C motif is a covalently linked dimer of two mono G^C motifs. The modification of G^C motifs with various functional groups offers an attractive pathway to build surface-functionalized RNTs for different applications. To explore the potential applications of RNTs in the field of organic optoelectronics, three porphyrin- and three oligothiophene-functionalized G^C modules were synthesized. The solubility, self-assembly ability and optical properties of these building blocks were tuned by chemical modification. The porphyrin-mono G^C module G^C-Por 1 formed long RNTs in MeNO2 with a moderate solubility. The porphyrin-twin G^C modules (G^C)2-Por 2 and (G^C)2-Por 3 both displayed good solubility and self-assembly ability in the mixed solvent of 1,2-DCB and MeOH. The porphyrin groups on the RNTs were identified as J-type aggregates in all cases. In comparison, the terthiophene-twin G^C module (G^C)2-3T did not form well-dispersed nanostructures in most organic solvents due to the poor solubility. The sexithiophene-mono G^C module G^C-6T displayed a good solubility in 1,2-DCB and DCM but the self-assembly ability was found to be poor. The sexithiophene-twin G^C module (G^C)2-6T displayed a good solubility in nonpolar solvents and formed well-dispersed long RNTs. The oligothiophene units on the RNTs were identified as H-type aggregates upon the formation of RNTs. These porphyrin- and oligothiophene-functionalized G^C RNTs were characterized by SEM, TEM, AFM and STM. The diameters of the individual RNTs were measured to be in the range of 4–8 nm, which were in good agreement with the values from the molecular modeling simulations. The length of the RNTs varied from a few hundreds of nanometers to several micrometers, which was controlled by different self-assembly conditions. At high concentrations, these RNTs formed interconnected networks alone or with PC61BM in the mixed blends. In all the blended thin films of porphyrin-functionalized RNTs:PC61BM, the fluorescence emissions of porphyrin groups were sufficiently quenched. The phase-separated nanoscale morphology and sufficient photoinduced electron transfer in the blended thin films are highly desired when spin-casting the active layers of bulk-heterojunction organic photovoltaic (OPV) devices. The HOMO and LUMO energy levels of the RNTs of G^C-Por 1, (G^C)2-Por 2, (G^C)2-Por 3 and (G^C)2-6T were characterized by UPS and UV-Vis. The energy-level alignments of all these materials and PC61BM indicate they are potential electron donor-acceptor pairs for OPVs. The I–V properties and conductivity of the thin films of these functional RNTs were measured and were found to display significant improvements in conductivity compared to the nonconductive unassembled counterparts. The conductivity of the RNTs is comparable to those of the conducting polymers. These RNTs may contribute to the repertoire of electron donor materials in solution-processed OPVs and organic semiconductors.