Synthesis of Dendronized Polymers on the Au(111) Surface

Dendronized polymers (DPs) consist of a linear polymeric backbone with dendritic side chains. Fine-tuning of the functional groups in the side chains enriches the structural versatility of the DPs and imparts a variety of novel physical properties. Herein, the first on-surface synthesis of DPs is achieved via the postfunctionalization of polymers on Au(111), in which the surface-confinement-induced planar conformation and chiral configurations were unambiguously characterized. While the dendronized monomer was synthesized in situ on Au(111), the subsequent polymerization afforded only short, cross-linked DP chains owing to multiple side reactions. The postfunctionalization approach selectively produced brominated polyphenylene backbone moieties by the deiodination polymerization of 4-bromo-4″-iodo-5'-(4-iodophenyl)-1,1':3',1″-terphenyl on Au(111), which smoothly underwent divergent cross-coupling reactions with two different isocyanides to form two types of DPs as individual long chains.

Electronic Structure Tunability by Periodic meta-Ligand Spacing in One-Dimensional Organic Semiconductors

Designing molecular organic semiconductors with distinct frontier orbitals is key for the development of devices with desirable properties. Generating defined organic nanostructures with atomic precision can be accomplished by on-surface synthesis. We use this "dry" chemistry to introduce topological variations in a conjugated poly( para-phenylene) chain in the form of meta-junctions. As evidenced by STM and LEED, we produce a macroscopically ordered, monolayer thin zigzag chain film on a vicinal silver crystal. These cross-conjugated nanostructures are expected to display altered electronic properties, which are now unraveled by highly complementary experimental techniques (ARPES and STS) and theoretical calculations (DFT and EPWE). We find that meta-junctions dominate the weakly dispersive band structure, while the band gap is tunable by altering the linear segment's length. These periodic topology effects induce significant loss of the electronic coupling between neighboring linear segments leading to partial electron confinement in the form of weakly coupled quantum dots. Such periodic quantum interference effects determine the overall semiconducting character and functionality of the chains.

Constructive quantum interference in a bis-copper six-porphyrin nanoring

The exchange interaction, J, between two spin centres is a convenient measure of through bond electronic communication. Here, we investigate quantum interference phenomena in a bis-copper six-porphyrin nanoring by electron paramagnetic resonance spectroscopy via measurement of the exchange coupling between the copper centres. Using an analytical expression accounting for both dipolar and exchange coupling to simulate the time traces obtained in a double electron electron resonance experiment, we demonstrate that J can be quantified to high precision even in the presence of significant through-space coupling. We show that the exchange coupling between two spin centres is increased by a factor of 4.5 in the ring structure with two parallel coupling paths as compared to an otherwise identical system with just one coupling path, which is a clear signature of constructive quantum interference.

Quantum interference in charge transport is attracting interest with applications in nanoelectronics and quantum computing. Here, the authors present a method for quantifying electronic transmission through molecules, and demonstrate constructive quantum interference in a molecule with two identical, parallel coupling paths.

Charge transport in a liquid crystalline triphenylene polymer monolayer at air-solid interface

We have prepared a monolayer of a novel liquid crystalline polymer derived from 2,6-dihydroxy-3,7,10,11-tetraalkoxy-triphenylene (PHAT) at an air-water interface and transferred it onto freshly cleaved mica as well as gold coated mica substrates by the Langmuir-Blodgett (L-B) technique. The atomic force microscope (AFM) images of these L-B films show a uniform coverage with a thickness of 1.5 nm. Electrical conductivity measurements were carried out on the PHAT monolayer deposited on the gold coated mica substrate using a current sensing AFM (CSAFM). The gold substrate-PHAT monolayer-cantilever tip of CSAFM forms a metal-insulator-metal (M-I-M) junction. The CSAFM yields a non-linear current-voltage (I-V) curve for the M-I-M junction. The analysis of the I-V characteristics of the M-I-M junction indicated that the charge transport in the liquid crystalline polymer monolayer is by the direct tunneling mechanism. The barrier height for the PHAT monolayer was estimated to be 1.22 ± 0.02 eV.

Enhancing the photoluminescence emission of conjugated MEH-PPV by light processing

We show here that treatment of thin films of conjugated polymers by illumination with light leads to an increase of the intensity of their photoluminescence by up to 42%. The corresponding enhancement of absorbance was much less pronounced. We explain this significant enhancement of photoluminescence by a planarization of the conjugated polymer chains induced by photoexcitations even below the glass transition temperature, possibly due to an increased conjugation length. Interestingly, the photoluminescence remains at the enhanced level for more than 71 h after treatment of the films by illumination with light, likely due to the fact that below the glass transition temperature no restoring force could return the conjugated chains into their initial conformational state.