Towards double-functionalized small diamondoids: selective electronic band-gap tuning

Role of the solvent polarity on the optical and electronic characteristics of 1-iodoadamantane

The natural absorbance caused by the chromophore and chemical behavior of 1-iodoadamantane is highly influenced by the polarity of different solvent environments. This gives rise to the solvatochromatic shifts in the optical absorption and electronic structure and the experimentally measured UV-vis absorption spectra show significant solvatochromic shifts with respect to the solvent polarity. The absorption shift for both σ to σ*and n to σ* electronic transitions are more dominant in polar solvents than in nonpolar solvents. To obtain a better understanding of the impact of solvent polarity on the 1-iodoadamantane at the molecular level, computational calculations were carried out through implicit solvation. According to this, changes in the HOMO and LUMO energies and electron density distributions of various solvent continuums demonstrate the influence of solvent polarity on the HOMO and LUMO energy levels of the chemical system. This also shows an increment in the HOMO-LUMO gap with respect to the polarity of the solvent.

Effects of oxidative adsorbates and cluster formation on the electronic structure of nanodiamonds

Nanodiamonds (NDs) are modern high-potential materials relevant for applications in biomedicine, photocatalysis, and various other fields. Their electronic surface properties, especially in the liquid phase, are key to their function in the applications, but we show that they are sensitively modified by their interactions with the environment. Two important interaction modes are those with oxidative aqueous adsorbates as well as ND self-aggregation towards the formation of ND clusters. For planar diamond surfaces it is known that the electron density migrates from the diamond towards oxidative adsorbates, which is known as transfer doping. Here, we quantify this effect for highly curved NDs of varying sizes (35-147 C atoms) and surface terminations (H, OH, F), focusing on their interactions with the most abundant aqueous oxidative adsorbates (H₃ O⁺ , O₂ , O₃ ). We prove that the concept of transfer doping stays valid for the case of the high-curvature NDs and can be tuned via the ND's specific properties. Secondly, we investigate the electronic structures of clusters of NDs which are known to form in particular in aqueous dispersions. Upon cluster formation, we find that the optical gaps of the structures are significantly reduced, which explains why different experimental values were obtained for the optical gap of the same structures, and the cluster's LUMO shapes resemble atom-type orbitals, as in the case of isolated spherical NDs. Our findings have implications for ND applications as photocatalysts or electronic devices, where the specific electronic properties are key to the functionality of the ND material.

Bond order effects on the optoelectronic properties of oxygen/sulfur functionalized adamantanes

The objective of this work, is to study adamantanes and to tune their bandgap, since pure adamantane is considered as an insulator due to its high bandgap energy. For this, we doped adamantane with oxygen and sulfur atoms, thus obtaining 730 different structures with double bonds and 730 different structures with single bonds, for a total of 1460 structures, and compared their properties. Among all, 31 molecules were selected that best represented the reduced bandgap behavior. The calculations with greater precision in its results were made using the Local Density Approximation (LDA), in the Density-Functional Theory (DFT) formalism, with PWC functional and TNP basis set. The electronic and optical properties were analyzed, by calculating the energy gap and absorption spectrum. Importantly, we observed that molecules doped with sulfur atoms (double bonds) had their energy gap reduced significantly compared to molecules doped with sulfur and/or oxygen atom with single bonds and pristine adamantane. It was found that in the absorption spectrum, the sulfur-doped structures had their spectrum shifted to the visible region, a fact that becomes relevant for potential dyes and optoelectronic applications. From the seven selected functionalized adamantanes (ADD-04, ADD-05, ADD-07, ADD-19, ADD-20, ADD-41, and ADD-48), any of these could be used as a dye. However, the ADD-20 molecule in particular, which presented optical absorption near (RGB) primary colors, could indicate a potential quantum dot material for application in developing screens of various electronic devices.

Optical Properties of Single- and Double-Functionalized Small Diamondoids

The rational control of the electronic and optical properties of small functionalized diamond-like molecules, the diamondoids, is the focus of this work. Specifically, we investigate the single- and double- functionalization of the lower diamondoids, adamantane, diamantane, and triamantane with -NH₂ and -SH groups and extend the study to N-heterocyclic carbene (NHC) functionalization. On the basis of electronic structure calculations, we predict a significant change in the optical properties of these functionalized diamondoids. Our computations reveal that -NH₂ functionalized diamondoids show UV photoluminescence similar to ideal diamondoids while -SH substituted diamondoids hinder the UV photoluminescence due to the labile nature of the S-H bond in the first excited state. This study also unveils that the UV photoluminescence nature of -NH₂ diamondoids is quenched upon additional functionalization with the -SH group. The double-functionalized derivative can, thus, serve as a sensitive probe for biomolecule binding and sensing environmental changes. The preserved intrinsic properties of the NHC and the ideal diamondoid in NHC-functionalized-diamondoids suggests its utilization in diamondoid-based self-assembled monolayers (SAM), whose UV-photoluminescent signal would be determined entirely by the functionalized diamondoids. Our study aims to pave the path for tuning the properties of diamondoids through a selective choice of the type and number of functional groups. This will aid the realization of optoelectronic devices involving, for example, large-area SAM layers or diamondoid-functionalized electrodes.

Tuning the HOMO-LUMO Energy Gap of Small Diamondoids Using Inverse Molecular Design

Functionalized diamondoids show great potential as building blocks for various new optoelectronic applications. However, until now, only simple mono and double substitutions were investigated. In this work, we considered up to 10 and 6 sites for functionalization of the two smallest diamondoids, adamantane and diamantane, respectively, in search for diamondoid derivatives with a minimal and maximal HOMO-LUMO energy gap. To this end, the energy gap was optimized systematically using an inverse molecular design methodology based on the best-first search algorithm combined with a Monte Carlo component to escape local optima. Adamantane derivatives were found with HOMO-LUMO gaps ranging from 2.42 to 10.63 eV, with 9.45 eV being the energy gap of pure adamantane. For diamantane, similar values were obtained. The structures with the lowest HOMO-LUMO gaps showed apparent push-pull character. The push character is mainly formed by sulfur or nitrogen dopants and thiol groups, whereas the pull character is predominantly determined by the presence of electron-withdrawing nitro or carbonyl groups assisted by amino and hydroxyl groups via the formation of intramolecular hydrogen bonds. In contrast, maximal HOMO-LUMO gaps were obtained by introducing numerous electronegative groups.

Diamondoid-based molecular junctions: a computational study

In this work, we deal with the computational investigation of diamondoid-based molecular conductance junctions and their electronic transport properties. A small diamondoid is placed between the two gold electrodes of the nanogap and is covalently bonded to the gold electrodes through two different molecules, a thiol group and a N-heterocyclic carbene molecule. We have shown that the thiol linker is more efficient as it introduces additional electron paths for transport at lower energies. The influence of doping the diamondoid on the properties of the molecular junctions has been investigated. We find that using a nitrogen atom to dope the diamondoids leads to a considerable increase of the zero bias conductance. For the N-doped system we show an asymmetric feature of the I-V curve of the junctions resulting in rectification within a very small range of bias voltages. The rectifying nature is the result of the characteristic shift in the bias-dependent highest occupied molecular orbital state. In all cases, the efficiency of the device is manifested and is discussed in view of novel nanotechnological applications.

Benchmark investigation of diamondoid-functionalized electrodes for nanopore DNA sequencing

Small diamond-like particles, diamondoids, have been shown to effectively functionalize gold electrodes in order to sense DNA units passing between the nanopore-embedded electrodes. In this work, we present a comparative study of Au(111) electrodes functionalized with different derivatives of lower diamondoids. Focus is put on the electronic and transport properties of such electrodes for different DNA nucleotides placed within the electrode gap. The functionalization promotes a specific binding to DNA leading to different properties for the system, which provides a tool set to systematically improve the signal-to-noise ratio of the electronic measurements across the electrodes. Using quantum transport calculations, we compare the effectiveness of the different functionalized electrodes in distinguishing the four DNA nucleotides. Our results point to the most effective diamondoid functionalization of gold electrodes in view of biosensing applications.

Carbene-mediated self-assembly of diamondoids on metal surfaces

N-heterocyclic carbenes (NHC)s are emerging as an alternative class of molecules to thiol-based self-assembled monolayers (SAMs), making carbene-based SAMs much more stable under harsh environmental conditions. In this work, we have functionalized tiny diamondoids using NHCs in order to prepare highly stable carbene-mediated diamondoid SAMs on metal substrates. Using quantum-mechanical simulations and two different configurations for the carbene-functionalized diamondoids attached on gold, silver, and platinum surfaces we were able to study in detail these materials. Specifically, we focus on the binding characteristics, stability, and adsorption of the NHC-mediated diamondoid SAMs on the metal surfaces. A preferential binding to platinum surfaces was found, while a modulation of the work function in all cases was clear. The surface morphology of all NHC-based diamondoid SAMs was revealed through simulated STM images, which show characteristic features for each surface.