Polaron Induced Conductance Switching in Conjugated Oligophenylene: A First-Principles Analysis

For π-conjugated systems, polaron formation has a major impact on their optoelectronic properties. In fact, for such systems, an exquisite interplay between electron delocalization and the steric effect determines their ground state properties. However, an excess charge (positive or negative) injection causes structural reorientation because of extended conjugation. Herein, we investigate the effect of such an excess charge in an individual polyphenylene on its quantum conductance behavior. By combining the DFT and NEGF formalisms, we characterize both structural and electronic changes occurring upon electron and hole injection. We demonstrate that for both the cationic and anionic radicals, the excess charge is observed to be localized, inducing a partial planarization of the molecule and forming cationic and anionic polarons, respectively. The calculated low-bias conductance values determine the polaronic effect and could be implemented for easy determination and measurement of polaron formation. In fact, cationic and anionic polarons induce a large degree of conductance switching, involving a decrease and increase of conductance, respectively.

In Situ Tuning of the Charge-Carrier Polarity in Imidazole-Linked Single-Molecule Junctions

Manipulating the nature of the charge carriers at the single-molecule level is one of the major challenges of molecular electronics. Using first-principles quantum transport calculations, we have investigated the electronic transport properties of imidazole-linked single-molecule junctions and identified the hydrogen atom bonded to the pyrrole-like nitrogen in imidazole as a switch to tune the polarity of the charge carriers. Our calculations show that the chemical nature of the imidazole anchors is dramatically altered by dehydrogenation, which changes the dominant charge carriers from electrons to holes. It is also revealed that upon dehydrogenation the interfacial Au-N bonds are modified from donor-acceptor-like to covalent, along with a significant promotion of the low-bias conductance and the junction stability. At variance with other traditional methods that always require drastic modifications of the junction structure, our findings provide a promising approach to tailor in situ the polarity of charge carriers in molecular electronic devices.

Comparison of charge transport and opto-electronic properties of pyrene and anthracene derivatives for OLED applications

In this paper, three organic semiconductors such as 9-[(5-nitropyridin-2-aminoethyl)iminiomethyl]-anthracene (a), N'-((pyren-4-yl)methylene)isonicotinohydrazide (b), and novel organic semiconductor N-(2-((pyren-4-yl)methyleneamino)ethyl)-5-nitropyridin-2-amine (c) were prepared. Their structures were assessed using NMR and elemental analysis techniques. While compound (a) and compound (c) have the same wing unit ([(5-nitropyridin-2-aminoethyl) iminiomethyl]), compounds (b) and (c) have the same core unit (5-nitropyridin-2-amine). Based on TD-DFT and Marcus theories, we have explored the effects of molecular structure on the opto-electronic properties for OLED applications. Our results show that wing units of molecules impact more on the opto-electronics properties than on core units. The compounds (a) and (c) with the same wing unit have exhibited quite similar behaviors in terms of both structural and opto-electronic parameters. However, a similar situation has not been observed for compounds (b) and (c) with the same core unit. In conclusion, our results indicate that compounds (a) and (c) exhibit obvious advantages for OLEDs in terms of calculated opto-electronic and charge transport properties such as better absorption and emission parameters, lower energy gaps and reorganisation energies and higher charge mobility.

Guidelines and diagnostics for charge carrier tuning in thiophene-based wires

Reported experimental trends in charge carrier tuning in single molecule junctions of oligothiophene-based wires are rationalized by means of frontier molecular orbital theory.