In print at Nature Chemistry (2016): http://dx.doi.org/10.1038/nchem.2505 AbstractFunctionalization of atomically thin nanomaterials enables the tailoring of their chemical, optical, and electronic properties. Exfoliated black phosphorus -a layered two-dimensional semiconductor exhibiting favorable charge carrier mobility, tunable bandgap, and highly anisotropic properties -is chemically reactive and degrades rapidly in ambient conditions. In contrast, here we show that covalent aryl diazonium functionalization suppresses the chemical degradation of exfoliated black phosphorus even following weeks of ambient exposure. This chemical modification scheme spontaneously forms phosphorus-carbon bonds, has a reaction rate sensitive to the aryl diazonium substituent, and alters the electronic properties of exfoliated black phosphorus, ultimately yielding a strong, tunable p-type doping that simultaneously improves field-effect transistor mobility and on/off current ratio. This chemical functionalization pathway controllably modifies the properties of exfoliated black phosphorus, thus improving its prospects for nanoelectronic applications.

We have used multiconfigurational (MC) and multireference (MR) methods (CASSCF, CASPT2, and MRCI) to study d–d transitions and other optical excitations for octahedral [M(H2O)6] n+ clusters (M = Ti, V, Mn, Cr, Fe, Co, Ni, Cu) as models of hematite and other transition-metal oxides of interest in solar fuels. For [Fe(H2O)6]3+, all calculations substantially overestimate the d–d transition energies (∼3.0 versus ∼1.5 eV) compared to what has been experimentally assigned. This problem occurs even though theory accurately describes (1) the lowest d–d transition energy in the atomic ion Fe3+ (∼4.4 eV), (2) the t2g–eg splitting (∼1.4 eV) in [Fe(H2O)6]3+, and (3) the ligand-to-metal charge transfer (LMCT) energy in [Fe(H2O)6]3+. Indeed, the results for Fe3+ and the t2g–eg splitting suggest that the lowest d–d excitation energy in the hexa-aqua complex should be ∼3 eV (or slightly below because of Jahn–Teller stabilization), as we find. Possible origins for the d–d discrepancy are examined, including Fe2+ and low-spin Fe3+ impurities. For the [M(H2O)6] n+ clusters not involving Fe(III), our MR calculations show reasonable correlation (mostly within 0.5 eV) with experiments for the d–d transitions, including consistent trends for the intensities of spin-allowed and spin-forbidden transitions. Our calculations also greatly complement experimental data because (1) experimental results for some species are insufficient or even scarce, (2) some of the experimental peaks were not observed directly but were inferred, and (3) the nature or existence of some shoulder peaks and weak peaks is uncertain. Our MR calculations have also been used to study convergence of the results with choice of active space, including the importance of the “double shell” effect in which there are 10 active d orbitals per transition-metal atom rather than 5. The results show that the larger active space does not significantly change the excitation energy, although it lowers the absolute energies for complexes with high 3d occupations. This indicates that reasonable accuracy can be achieved using MR methods in studies of transition-metal oxide clusters using minimal active spaces. This study establishes fundamental principles for the further modeling of larger cluster models of pure and doped hematite and other metal oxides.

Octahedral monomeric and dimeric iron oxide clusters represent the basic units in many iron oxide and oxide-hydroxide minerals. In this paper, we provide a detailed theoretical analysis of the structural and optical properties of the most important of these clusters in a vacuum and in an aqueous environment. An evaluation of various computational methods was performed on the experimentally well-known monomer [Fe(H 2 O) 6 ] 3+ , and it is found that all methods provide similar and reliable structures. Most density functional theory (DFT) methods reasonably reproduce the spin-forbidden sextet−quartet d−d transition energy, which also resembles the lowest transition energies in many infinite octahedral iron oxide systems. On the other hand, Hartree−Fock (HF) and MP2 methods significantly overestimate this energy. The ligand-to-metal charge transfer (LMCT) energy is highly sensitive to the method employed, with the closest agreement with experiment provided by the BHandHLYP functional. Thermodynamic property calculations suggest that dimerization reactions starting from [Fe(H 2 O) 6 ] 3+ are highly exothermic in a vacuum. In contrast, these reactions have insignificant energy changes in solution, though the singly μoxo bridged dimer is slightly favored. The electrostatic repulsion between two charged monomers hinders their close contact. The singly μ-oxo bridged dimer suffers less from this because of its maximal Fe−Fe distance, which is consistent with the existence of stable crystal structures for this dimer. A comparison between the calculated structures and experimental results suggests that several dimer species coexist in solution. The calculated ferromagnetic and antiferromagnetic states of the dimers are found to have comparable energies and structures. While the singly μ-oxo and doubly μ-hydroxo bridged dimers have spin states that are well separated in energy, the spin states in the triply μ-hydroxo bridged dimer pack closely. The single excitation d−d transition in the dimer structure is comparable in energy to the d−d transition in the monomer, while the double excitation d−d transition, i.e., simultaneous excitation of two iron centers, has a higher excitation energy that is 1.6−2.6 times the single excitation energy but below the LMCT energy. This means that doubly excited states can be populated during the non-radiative relaxation of iron oxide clusters following initial photoexcitation of the LMCT state.

Single crystal structures have been determined for the three cofacial, oxygen-bridged, silicon phthalocyanine oligomers, [((CH(3))(3)SiO)(2)(CH(3))SiO](SiPcO)(2-4)[Si(CH(3))(OSi(CH(3))(3))(2)], and for the corresponding monomer. The data for the oligomers give structural parameters for a matching set of three cofacial, oxygen-bridged silicon phthalocyanine oligomers for the first time. The staggering angles between the six adjacent cofacial ring pairs in the three oligomers are not in a random distribution nor in a cluster at the intuitively expected angle of 45° but rather are in two clusters, one at an angle of 15° and the other at an angle of 41°. These two clusters lead to the conclusion that long, directional interactions (LDI) exist between the adjacent ring pairs. An understanding of these interactions is provided by atoms-in-molecules (AIM) and reduced-density-gradient (RDG) studies. A survey of the staggering angles in other single-atom-bridged, cofacial phthalocyanine oligomers provides further evidence for the existence of LDI between cofacial phthalocyanine ring pairs in single-atom-bridged phthalocyanine oligomers.

Si-E bondings in hexacoordinate silicon phthalocyanine were analyzed using bond order (BO), energy partition, atoms in molecules (AIM), electron localization function (ELF), and localized orbital locator (LOL). Bond models were proposed to explain differences between hexacoordinate and tetracoordinate Si-E bondings. Aromaticity of silicon phthalocyanine was investigated using nucleus-independent chemical shift (NICS), harmonic oscillator model of aromaticity (HOMA), conceptual density functional theory (DFT), ring critical point (RCP) descriptors, and delocalization index (DI). Structure, energy, bonding, and aromaticity of tetracoordinate silicon phthalocyanine were studied and compared with hexacoordinate one.