Photoswitching activation of a ferrocenyl-stilbene analogue by its covalent grafting to gold.

Until now, surface-deposited stilbenes have been much less studied than other photochromic systems. Here, an asymmetrically substituted styrene incorporating a redox-active ferrocene moiety and a terminal alkyne group has been synthesised to investigate its photoisomerization in solution, and upon the formation of chemisorbed self-assembled monolayers through a carbon-gold bond formation. Charge transport measurements across the monolayers reveal that upon chemical linkage to the gold substrate there is an alteration of the isomerization pathway, which favours the trans to cis conversion, which is not observed in solution. The experimental observations are interpreted based on quantum chemistry calculations.

Robust Organic Radical Molecular Junctions Using Acetylene Terminated Groups for C-Au Bond Formation.

Organic paramagnetic and electroactive molecules are attracting interest as core components of molecular electronic and spintronic devices. Currently, further progress is hindered by the modest stability and reproducibility of the molecule/electrode contact. We report the synthesis of a persistent organic radical bearing one and two terminal alkyne groups to form Au-C σ bonds. The formation and stability of self-assembled monolayers and the electron transport through single-molecule junctions at room temperature have been studied. The combined analysis of both systems demonstrates that this linker forms a robust covalent bond with gold and a better-defined contact when compared to traditional sulfur-based linkers. Density functional theory and quantum transport calculations support the experimental observation highlighting a reduced variability of conductance values for the C-Au based junction. Our findings advance the quest for robustness and reproducibility of devices based on electroactive molecules.

Design of Perchlorotriphenylmethyl (PTM) Radical-Based Compounds for Optoelectronic Applications: The Role of Orbital Delocalization.

Perchlorotriphenylmethyl (PTM) radical-based compounds are widely exploited as molecular switching units. However, their application in optoelectronics is limited by the fact that they exhibit intense absorption bands only in a narrow range of the UV region around 385 nm. Recent experimental works have reported new PTM based compounds which present a broad absorption in the visible region although the origin of this behavior is not fully explained. In this context, Time-Dependent Density Functional Theory (TD-DFT) calculations have been performed to rationalize the optical properties of these compounds. Moreover, a new compound based on PTM disubstituted with bistriazene units has been synthetized and characterized to complete the set of available experimental data on related compounds. The results point to the delocalization of the Highest Occupied Molecular Orbital (HOMO) of the substituents along the PTM core as the origin of the new high absorption bands in the visible region. As a consequence, the absorption of the PTM-based compounds can be tuned via the choice of the nature of the donor substituent, type of connection, and number of substituents.

Allocation of Ambipolar Charges on an Organic Diradical with a Vinylene-Phenylenediyne Bridge.

Two redox and magnetically active perchlorotriphenylmethyl (•PTM) radical units have been connected as end-capping groups to a bis(phenylene)diyne chain through vinylene linkers. Negative and positive charged species have been generated, and the influence of the bridge on their stabilization is discussed. Partial reduction of the electron-withdrawing •PTM radicals results in a class-II mixed-valence system with the negative charge located on the terminal PTM units, proving the efficiency of the conjugated chain for the electron transport between the two terminal sites. Counterintuitively, the oxidation process does not occur along the electron-rich bridge but on the vinylene units. The •PTM radicals play a key role in the stabilization of the cationic species, promoting the generation of quinoidal ring segments.

Exploiting the versatile alkyne-based chemistry for expanding the applications of a stable triphenylmethyl organic radical on surfaces.

The incorporation of terminal alkynes into the chemical structure of persistent organic perchlorotriphenylmethyl (PTM) radicals provides new chemical tools to expand their potential applications. In this work, this is demonstrated by the chemical functionalization of two types of substrates, hydrogenated SiO2-free silicon (Si-H) and gold, and, by exploiting the click chemistry, scarcely used with organic radicals, to synthesise multifunctional systems. On one hand, the one-step functionalization of Si-H allows a light-triggered capacitance switch to be successfully achieved under electrochemical conditions. On the other hand, the click reaction between the alkyne-terminated PTM radical and a ferrocene azide derivative, used here as a model azide system, leads to a multistate electrochemical switch. The successful post-surface modification makes the self-assembled monolayers reported here an appealing platform to synthesise multifunctional systems grafted on surfaces.