Electronic structure tuning of diamondoids through functionalization

Infrared spectrum of the 1-cyanoadamantane cation: evidence of hydrogen transfer and cage-opening upon ionization

The radical cations of diamondoids are important intermediates in their functionalization reactions and are also candidates as carriers for astronomical absorption and emission features. Although neutral diamondoids have been studied extensively, information regarding their radical cations is largely lacking, particularly for functionalized diamondoid derivatives. Herein, we characterize the structure of the 1-cyanoadamantane radical cation (C10H15CN+, AdCN+) using infrared photodissociation (IRPD) spectroscopy of mass selected AdCN+N2 clusters in the XH stretch range (2400-3500 cm-1) and dispersion-corrected density functional theory calculations (B3LYP-D3BJ/cc-pVTZ). A group of three distinct CH stretch bands are observed in the 2800-3000 cm-1 range, in addition to a highly redshifted absorption at 2580 cm-1 attributed to the acidic CH proton predicted by calculations. An unexpected broad absorption peaking at 3320 cm-1 is also detected and assigned to an NH stretch mode based on its width and frequency. Calculations indicate that hydrogen atom transfer (HAT) from the adamantyl cage (C10H15, Ady) to the N atom of the CN group yields lower energy structures, with an open-cage isomer exhibiting such hydrogen transfer being the global minimum on the potential energy surface. The energy barriers involved in the formation of this open-cage isomer are also lower than those calculated for generation of the analogous open-cage 1-amantadine cation isomer which has previously been identified by IRPD. The combined consideration of IRPD spectra and calculations indicates a major population of the nascent canonical closed-cage isomer and a smaller population of the global minimum isomer featuring both cage-opening and hydrogen transfer.

Microhydration of the adamantane cation: intracluster proton transfer to solvent in [Ad(H2O)n=1-5]+ for n ≥ 3

Radical cations of diamondoids are important intermediates in their functionalization reactions in polar solvents. To explore the role of the solvent at the molecular level, we characterize herein microhydrated radical cation clusters of the parent molecule of the diamondoid family, adamantane (C₁₀H₁₆, Ad), by infrared photodissociation (IRPD) spectroscopy of mass-selected [Ad(H₂O)_n=1-5]⁺ clusters. IRPD spectra of the cation ground electronic state recorded in the CH/OH stretch and fingerprint ranges reveal the first steps of this fundamental H-substitution reaction at the molecular level. Analysis of size-dependent frequency shifts with dispersion-corrected density functional theory calculations (B3LYP-D3/cc-pVTZ) provides detailed information about the acidity of the proton of Ad⁺ as a function of the degree of hydration, the structure of the hydration shell, and the strengths of the CH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. For n = 1, H₂O strongly activates the acidic C-H bond of Ad⁺ by acting as a proton acceptor in a strong CH⋯O ionic H-bond with cation-dipole configuration. For n = 2, the proton is almost equally shared between the adamantyl radical (C₁₀H₁₅, Ady) and the (H₂O)₂ dimer in a strong C⋯H⋯O ionic H-bond. For n ≥ 3, the proton is completely transferred to the H-bonded hydration network. The threshold for this size-dependent intracluster proton transfer to solvent is consistent with the proton affinities of Ady and (H₂O)_n and confirmed by collision-induced dissociation experiments. Comparison with other related microhydrated cations reveals that the acidity of the CH proton of Ad⁺ is in the range of strongly acidic phenol⁺ but lower than for cationic linear alkanes such as pentane⁺. Significantly, the presented IRPD spectra of microhydrated Ad⁺ provide the first spectroscopic molecular-level insight of the chemical reactivity and reaction mechanism of the important class of transient diamondoid radical cations in aqueous solution.

Opening of the Diamondoid Cage upon Ionization Probed by Infrared Spectra of the Amantadine Cation Solvated by Ar, N2 , and H2 O

Radical cations of diamondoids, a fundamental class of very stable cyclic hydrocarbon molecules, play an important role in their functionalization reactions and the chemistry of the interstellar medium. Herein, we characterize the structure, energy, and intermolecular interaction of clusters of the amantadine radical cation (Ama⁺, 1‐aminoadamantane) with solvent molecules of different interaction strength by infrared photodissociation (IRPD) spectroscopy of mass‐selected Ama⁺L_n clusters, with L=Ar (n≤3) and L=N₂ and H₂O (n=1), and dispersion‐corrected density functional theory calculations (B3LYP−D3/cc‐pVTZ). Three isomers of Ama⁺ generated by electron ionization are identified by the vibrational properties of their rather different NH₂ groups. The ligands bind preferentially to the acidic NH₂ protons, and the strength of the NH…L ionic H‐bonds are probed by the solvation‐induced red‐shifts in the NH stretch modes. The three Ama⁺ isomers include the most abundant canonical cage isomer (I) produced by vertical ionization, which is separated by appreciable barriers from two bicyclic distonic iminium ions obtained from cage‐opening (primary radical II) and subsequent 1,2 H‐shift (tertiary radical III), the latter of which is the global minimum on the Ama⁺ potential energy surface. The effect of solvation on the energetics of the potential energy profile revealed by the calculations is consistent with the observed relative abundance of the three isomers. Comparison to the adamantane cation indicates that substitution of H by the electron‐donating NH₂ group substantially lowers the barriers for the isomerization reaction.

Infrared Spectrum of the Amantadine Cation: Opening of the Diamondoid Cage upon Ionization

Radical cations of diamondoids, a fundamental class of highly stable cycloalkanes, are intermediates in functionalization reactions and possibly present in the interstellar medium. Herein, we characterize the structure of the radical cation of 1-amantadine (1-C₁₀H₁₅NH₂⁺, Ama⁺), the amino derivative of the parent adamantane (C₁₀H₁₆⁺, Ada⁺), by infrared spectroscopy and density functional theory calculations. The structural isomers of Ama⁺ produced by electron ionization are probed by infrared photodissociation of cold Ar-tagged ions. In addition to the canonical nascent Ama⁺ isomer with an intact C₁₀H₁₅ cage, we identify two distonic bicyclic iminium isomers in which the adamantyl cage opens upon ionization, one of which is lower in energy than the cage isomer. The reaction profile with barriers and intermediates for this cage-opening reaction are determined. Comparison with Ada⁺ suggests that this type of ionization-induced cage-opening may be a common feature for diamondoids and important for their reactivity.

Infrared Spectrum of the Adamantane+ -Water Cation: Hydration-Induced C-H Bond Activation and Free Internal Water Rotation

Diamondoid cations are reactive intermediates in their functionalization reactions in polar solution. Hydration is predicted to strongly activate their C-H bonds in initial proton abstraction reactions. To study the effects of microhydration on the properties of diamondoid cations, we characterize herein the prototypical monohydrated adamantane cation (C₁₀ H₁₆ ⁺ -H₂ O, Ad⁺ -W) in its ground electronic state by infrared photodissociation spectroscopy in the CH and OH stretch ranges and dispersion-corrected density functional theory (DFT) calculations. The water (W) ligand binds to the acidic CH group of Jahn-Teller distorted Ad⁺ via a strong CH⋅⋅⋅O ionic H-bond supported by charge-dipole forces. Although W further enhances the acidity of this CH group along with a proton shift toward the solvent, the proton remains with Ad⁺ in the monohydrate. We infer essentially free internal W rotation from rotational fine structure of the ν₃ band of W, resulting from weak angular anisotropy of the Ad⁺ -W potential.

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.

Theoretical investigations on diamondoids (CnHm, n = 10-41): Nomenclature, structural stabilities, and gap distributions

Combining the congruence check and the first-principles calculations, we have systematically investigated the structural stabilities and gap distributions of possible diamondoids (C_nH_m) with the carbon numbers (n) from 10 to 41. A simple method for the nomenclature is proposed, which can be used to distinguish and screen the candidates with high efficiency. Different from previous theoretical studies, the possible diamondoids can be enumerated according to our nomenclature, without any pre-determination from experiments. The structural stabilities and electronic properties have been studied by density functional based tight binding and first-principles methods, where a nearly linear correlation is found between the energy gaps obtained by these two methods. According to the formation energy of structures, we have determined the stable configurations as a function of chemical potential. The maximum and minimum energy gaps are found to be dominated by the shape of diamondoids for clusters with a given number of carbon atoms, while the gap decreases in general as the size increases due to the quantum confinement.

Electronic and Optical Properties of Methylated Adamantanes

Recent theoretical work has identified functionalized diamondoids as promising candidates for the tailoring of fluorescent nanomaterials. However, experiments confirming that optical gap tuning can be achieved through functionalization have, up until now, found only systems where fluorescence is quenched. We address this shortcoming by investigating a series of methylated adamantanes. For the first time, a class of functionalized diamondoids is shown to fluoresce in the gas phase. In order to understand the evolution of the optical and electronic structure properties with degree of functionalization, photoelectron spectroscopy was used to map the occupied valence electronic structure, while absorption and fluorescence spectroscopies yielded information about the unoccupied electronic structure and postexcitation relaxation behavior. The resulting spectra were modeled by (time-dependent) density functional theory. These results show that it is possible to overcome fluorescence quenching when functionalizing diamondoids and represent a significant step toward tailoring the electronic structure of these and other semiconductor particles in a manner suitable to applications.

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.

Sandwiches of N-doped diamondoids and benzenevialone pair–cation and cation–pi interaction: a DFT study

Cation–lone pair and cation–pi interactions in the complexes of N-doped dimondoids.

Electron-vibration coupling induced renormalization in the photoemission spectrum of diamondoids

The development of theories and methods devoted to the accurate calculation of the electronic quasi-particle states and levels of molecules, clusters and solids is of prime importance to interpret the experimental data. These quantum systems are often modelled by using the Born–Oppenheimer approximation where the coupling between the electrons and vibrational modes is not fully taken into account, and the electrons are treated as pure quasi-particles. Here, we show that in small diamond cages, called diamondoids, the electron–vibration coupling leads to the breakdown of the electron quasi-particle picture. More importantly, we demonstrate that the strong electron–vibration coupling is essential to properly describe the overall lineshape of the experimental photoemission spectrum. This cannot be obtained by methods within Born–Oppenheimer approximation. Moreover, we deduce a link between the vibronic states found by our many-body perturbation theory approach and the well-known Jahn–Teller effect.

The electron–vibration coupling is essential to describe the photoelectron properties of molecules. Here, the authors show theoretically and experimentally that the electron–vibration coupling is very large in diamonoids, and link the deduced vibronic states to the well-known Jahn–Teller effect.

Vibrationally resolved optical spectra of modified diamondoids obtained from time-dependent correlation function methods

Optical properties of modified diamondoids have been studied theoretically using vibrationally resolved electronic absorption, emission and resonance Raman spectra. A time-dependent correlation function approach has been used for electronic two-state models, comprising a ground state (g) and a bright, excited state (e), the latter determined from linear-response, time-dependent density functional theory (TD-DFT). The harmonic and Condon approximations were adopted. In most cases origin shifts, frequency alteration and Duschinsky rotation in excited states were considered. For other cases where no excited state geometry optimization and normal mode analysis were possible or desired, a short-time approximation was used. The optical properties and spectra have been computed for (i) a set of recently synthesized sp(2)/sp(3) hybrid species with C[double bond, length as m-dash]C double-bond connected saturated diamondoid subunits, (ii) functionalized (mostly by thiol or thione groups) diamondoids and (iii) urotropine and other C-substituted diamondoids. The ultimate goal is to tailor optical and electronic features of diamondoids by electronic blending, functionalization and substitution, based on a molecular-level understanding of the ongoing photophysics.

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

Diamondoids are nanoscale diamond-like cage structures with hydrogen terminations, which can occur in various sizes and with a diverse type of modifications. In this work, we focus on the structural alterations and the effect of doping and functionalization on the electronic properties of diamondoids, from the smallest adamantane to heptamantane. The results are based on quantum mechanical calculations. We perform a self-consistent study, starting with doping the smallest diamondoid, adamantane. Boron, nitrogen, silicon, oxygen, and phosphorus are chosen as dopants at sites which have been previously optimized and are also consistent with the literature. At a next step, an amine- and a thiol- group are separately used to functionalize the adamantane molecule. We mainly focus on a double functionalization of diamondoids up to heptamantane using both these atomic groups. The effect of isomeration in the case of tetramantane is also studied. We discuss the higher efficiency of a double-functionalization compared to doping or a single-functionalization of diamondoids in tuning the electronic properties, such as the electronic band-gap, of modified small diamondoids in view of their novel nanotechnological applications.

Laser-induced fluorescence of free diamondoid molecules

We report on the laser-induced fluorescence of diamondoids in the gas phase. The spectra show well defined vibrational structure, whose complex nature is assigned with the help of TDDFT computations.

The role of a diamondoid as a hydrogen donor or acceptor in probing DNA nucleobases

It has been shown that diamondoids can interact with DNA by forming relatively strong hydrogen bonds to DNA units, such as nucleobases. For this interaction to occur the diamondoids must be chemically modified in order to provide donor/acceptor groups for the hydrogen bond. We show here that the exact arrangement of an amine-modified adamantane with respect to a neighboring nucleobase has a significant influence on the strength of the hydrogen bond. Whether the diamondoid acts as a hydrogen donor or acceptor in the hydrogen binding to the nucleobase affects the electronic structure and thereby the electronic band-gaps of the diamondoid-nucleobase complex. In a donor arrangement of the diamondoid close to a nucleobase, the interaction energies are weak, but the electronic band-gaps differ significantly. Exactly the opposite trend is observed in an acceptor arrangement of the diamondoid. In each of these cases the frontier orbitals of the diamondoid and the nucleobase play a different role in the binding. The results are discussed in view of a diamondoid-based biosensing device.

Stable boron nitride diamondoids as nanoscale materials

We predict the stability of diamondoids made up of boron and nitrogen instead of carbon atoms. The results are based on quantum-mechanical calculations within density functional theory (DFT) and show some very distinct features compared to the regular carbon-based diamondoids. These features are evaluated with respect to the energetics and electronic properties of the boron nitride diamondoids as compared to the respective properties of the carbon-based diamondoids. We find that BN-diamondoids are overall more stable than their respective C-diamondoid counterparts. The electronic band-gaps (E(g)) of the former are overall lower than those for the latter nanostructures but do not show a very distinct trend with their size. Contrary to the lower C-diamondoids, the BN-diamondoids are semiconducting and show a depletion of charge on the nitrogen site. Their differences in the distribution of the molecular orbitals, compared to their carbon-based counterparts, offer additional bonding and functionalization possibilities. These tiny BN-based nanostructures could potentially be used as nanobuilding blocks complementing or substituting the C-diamondoids, based on the desired properties. An experimental realization of boron nitride diamondoids remains to show their feasibility.

Type-dependent identification of DNA nucleobases by using diamondoids

The possibility of distinguishing between DNA nucleobases of different sizes is manifested here through quantum-mechanical simulations. By using derivatives of small, modified diamond clusters, known as diamondoids, it is possible to separate the pyrimidines (cytosine and thymine) from the larger purines (adenine and guanine), according to the collective electronic and binding properties of these DNA nucleobases and the diamondoid. The latter acts as a probe with which these properties can be examined in detail. Short single-stranded DNA is built up from single nucleobases to reveal the effect of each DNA unit on the sensing abilities of the diamondoid probe. Several ways of orienting the nucleobases, nucleosides, nucleotides, and short single-stranded DNA are investigated; these lead to quite different electronic properties and may or may not enhance the possibility of separating the DNA nucleobases. For the optimum orientation, that is, one that promotes stronger hydrogen bonding of the diamondoid to the short DNA strand, it is found that the electronic band gaps of a purine strand lie in a completely different range to the band gaps of a pyrimidine strand. This difference can be over 1 eV, which is measurable and shows the potential of using diamondoids and their derivatives in biosensing devices.

Quasiparticle electronic structure and optical absorption of diamond nanoparticles from ab initio many-body perturbation theory

The excited states of small-diameter diamond nanoparticles in the gas phase are studied using the GW method and Bethe-Salpeter equation (BSE) within the ab initio many-body perturbation theory. The calculated ionization potentials and optical gaps are in agreement with experimental results, with the average error about 0.2 eV. The electron affinity is negative and the lowest unoccupied molecular orbital is rather delocalized. Precise determination of the electron affinity requires one to take the off-diagonal matrix elements of the self-energy operator into account in the GW calculation. BSE calculations predict a large exciton binding energy which is an order of magnitude larger than that in the bulk diamond.

Chemically modified diamondoids as biosensors for DNA

Understanding the interaction of biological molecules with materials is essential in view of the novel potential applications arising when these two are combined. To this end, we investigate the interaction of DNA with diamondoids, a broad family of tiny hydrogen-terminated diamond clusters with high technological potential. We model this interaction through quantum-mechanical computer simulations and focus on the hydrogen bonding possibilities of the different DNA nucleobases to the lower amine-modified diamondoids with respect to their relative distance and orientation. Our aim is to promote the binding between these two units, and probe this through the association energy, the electronic structure of the nucleobase-diamondoid system, and the specific role of their frontier orbitals. We discuss the relevance of our results in view of biosensing applications and specifically nanopore sequencing of DNA.