Synthesis and Temperature-Dependence of Hydrogen-Terminated Silicon Clusters

Chalcogen-Expanded Unsaturated Silicon Clusters: Thia-, Selena-, and Tellurasiliconoids

Reactions of silylenes with heavier chalcogens (E) typically result in Si=E double bonds or their π-addition products. In contrast, the oxidation of a silylene-functionalized unsaturated silicon cluster (siliconoid) with Group 16 elements selectively yields cluster expanded siliconoids Si7 E (E=S, Se, Te) fully preserving the unsaturated nature of the cluster scaffold as evident from the NMR signatures of the products. Mechanistic considerations by DFT calculations suggest the intermediacy of a Si6 siliconoid with exohedral Si=E functionality. The reaction thus may serve as model system for the oxidation of surface-bonded silylenes at Si(100) by chalcogens and their diffusion into the silicon bulk.

Nickel-assisted complete cleavage of CO by a silylene/siliconoid hybrid under formation of an Si[double bond, length as m-dash]C enol ether bridge

Reaction of a silylene-functionalized Si6 siliconoid with CO in the presence of catalytic quantities of a nickel(0) complex results in the complete cleavage of the CO triple bond, but preserves the Si6 scaffold with an exohedrally incorporated Si[double bond, length as m-dash]C enol ether bridge. The uncompromised cluster core emphasizes the role of the so-called benzpolarene motif as the energetic silicon pendants of benzene in carbon chemistry.

Indirect and Direct Grafting of Transition Metals to Siliconoids

Unsaturated charge‐neutral silicon clusters (siliconoids) are important as gas‐phase intermediates between molecules and the elemental bulk. With stable zirconocene‐ and hafnocene‐substituted derivatives, we here report the first examples containing directly bonded transition‐metal fragments that are readily accessible from the ligato‐lithiated Si₆ siliconoid (1Li) and Cp₂MCl₂ (M=Zr, Hf). Charge‐neutral siliconoid ligands with pending tetrylene functionality were prepared by the reaction of amidinato chloro tetrylenes [PhC(NtBu)₂]ECl (E=Si, Ge, Sn) with 1Li, thus confirming the principal compatibility of such low‐valent functionalities with the unsaturated Si₆ cluster scaffold. The pronounced donor properties of the tetrylene/siliconoid hybrids allow for their coordination to the Fe(CO)₄ fragment.

Site-selective functionalization of Si6R6 siliconoids

The recent progress in the synthesis of partially substituted neutral silicon clusters (siliconoids) revealed unique structures and electronic anisotropies that are reminiscent of bulk and nano surfaces of silicon. Here, we report the selective 2-lithiation of the global minimum Si6R6 siliconoid at a different vertex than in the previously reported isomeric 4-lithiated derivative (R = 2,4,6- i Pr3C6H2). In order to enable an intuitive distinction of the vertices of the global minimum Si6R6 scaffold (which can be considered the silicon analogue of benzene in terms of thermodynamic stability), we introduce a novel nomenclature in analogy to the ortho-meta-para nomenclature of disubstituted benzenes. By treatment of the 2-lithiated Si6 cluster with Me3SiCl, SiCl4 H3B·SMe2, (Me2N)2PCl as well as with carboxylic acid chlorides RCOCl (R = t Bu, Ph) various 2-functionalized Si6 clusters were obtained and characterized in solution and - in most cases - the solid state. The structural and spectroscopic effect of the position of the newly introduced functional group is discussed by comparison to the corresponding 4-functionalized derivatives.

The Structure of [HSi9 ]3- in the Solid State and Its Unexpected Highly Dynamic Behavior in Solution

We report on the first unambiguous detection of the elusive [HSi₉ ]^3- anion in solutions of liquid ammonia by various ²⁹ Si and ¹ H NMR experiments including chemical exchange saturation transfer (CEST). The characteristic multiplicity patterns of both the ²⁹ Si and ¹ H resonances together with CEST and a partially reduced ¹ H,²⁹ Si coupling constant indicate the presence of a highly dynamic Si₈ entity and a Si-H moiety with slow proton hopping. Theoretical calculations corroborate both reorganization of Si₈ on the picosecond timescale via low vibrational modes and proton hopping. In addition, in a single-crystal X-ray study of (K(DB[18]crown-6))(K([2.2.2]crypt))₂ [HSi₉ ]⋅8.5 NH₃ , the H atom was unequivocally localized at one vertex of the basal square of the monocapped square-antiprismatic cluster. Thus experimental studies and theoretical considerations provide unprecedented insight into both the structure and the dynamic behavior of these cluster anions, which hitherto had been considered to be rigid.

Stable unsaturated silicon clusters (siliconoids)

Unsaturated silicon clusters are key intermediates of silicon deposition processes from the gas phase, which is reflected in the recently introduced term "siliconoids" for silicon rich clusters with at least one hemispheroidally coordinated vertex. Unlike the case of metalloid clusters of heavier Group 14 elements, stable homonuclear derivatives containing silicon have become available just recently. This review summarizes the developments since the first report on a stable unsaturated silicon cluster in 2005. The synthesis, structure and reactivity under retention of the unsaturated vertices (i.e. the functionalization) of stable siliconoids is discussed within the broader context of soluble Zintl anions of silicon and metalloid clusters. A structural parameter is introduced as a quantitative measure to characterize the "hemispheroidality" of an unsubstituted vertex of a siliconoid.

Theoretical Study on the Photochemical Behavior of 4-(Dimethylamino)benzonitrile

Ab initio calculations have been performed to examine the photochemical behavior of 4-(dimethylamino)benzenzonitrile (DMABN). The conical intersection between S2 and S1 (S2/S1-CIX), where the internal conversion takes place after the main transition of S0-S2 at the equilibrium geometry in S0, is characterized by a dimethylamino-twisted quinoid structure where aromaticity of the benzene ring is lost. The optimized geometry of the charge transfer (CT) state in S1 has a feature similar to that of S2/S1-CIX but is not energetically stabilized so much. Consequently, electronically excited DMABN with CT character relaxes into the most stable locally excited (LE) state in S1 through a recrossing at S2/S1-CIX in gas phase or nonpolar solvent. In polar solvent, in contrast, the equilibration between LE and CT takes place in S1 so that the CT state is more stable because of electrostatic interaction. The excited states of DMABN derivatives have been also examined. On the basis of the present computational results, a new and simple guiding principle of the emission properties is proposed, where conventional twisted intramolecular CT (TICT) and planar intramolecular CT (PICT) models are properly incorporated.

Intramolecular Charge Transfer in 4-Aminobenzonitriles Does Not Necessarily Need the Twist

In electron donor/acceptor species such as 4-(dimethylamino)benzonitrile (DMABN), the excitation to the S(2) state is followed by internal conversion to the locally excited (LE) state. Dual fluorescence then becomes possible from both the LE and the twisted intramolecular charge-transfer (TICT) states. A detailed mechanism for the ICT of DMABN and 4-aminobenzonitrile (ABN) is presented in this work. The two emitting S(1) species are adiabatically linked along the amino torsion reaction coordinate. However, the S(2)/S(1) CT-LE radiationless decay occurs via an extended conical intersection "seam" that runs almost parallel to this torsional coordinate. At the lowest energy point on this conical intersection seam, the amino group is untwisted; however, the seam is accessible for a large range of torsional angles. Thus, the S(1) LE-TICT equilibration and dual fluorescence will be controlled by (a) the S(1) torsional reaction path and (b) the position along the amino group twist coordinate where the S(2)/S(1) CT-LE radiationless decay occurs. For DMABN, population of LE and TICT can occur because the two species have similar stabilities. However, in ABN, the equilibrium lies in favor of LE, as a TICT state was found at much higher energy with a low reaction barrier toward LE. This explains why dual fluorescence cannot be observed in ABN. The S(1)-->S(0) deactivation channel accessible from the LE state was also studied.

Internal pressure and surface tension of bare and hydrogen coated silicon nanoparticles

We present a study of internal pressure and surface tension of bare and hydrogen coated silicon nanoparticles of 2-10 nm diameter as a function of temperature, using molecular dynamics simulations employing a reparametrized Kohen-Tully-Stillinger interatomic potential. The internal pressure was found to increase with decreasing particle size but the density was found to be independent of the particle size. We showed that for covalent bond structures, changes in surface curvature and the associated surface forces were not sufficient to significantly change bond lengths and angles. Thus, the surface tension was also found to be independent of the particle size. Surface tension was found to decrease with increasing particle temperature while the internal pressure did not vary with temperature. The presence of hydrogen on the surface of a particle significantly reduces surface tension (e.g., drops from 0.83 J/m(2) to 0.42 J/m(2) at 1500 K). The computed pressure of bare and coated particles was found to follow the classical Laplace-Young equation.

Time-Resolved Resonance Raman Study of the Triplet States of p-Hydroxyacetophenone and the p-Hydroxyphenacyl Diethyl Phosphate Phototrigger Compound

Pico- and nanosecond time-resolved resonance Raman (TR3) spectroscopy have been utilized to study the dynamics and structure of p-hydroxyacetophenone (HA) and the p-hydroxyphenacyl-caged phototrigger compound p-hydroxyphenacyl diethyl phosphate (HPDP) in acetonitrile solution. Transient intermediates were detected and attributed to the triplet states of HA and HPDP. Nanosecond-TR3 measurements were done for two isotopically substituted HA molecules to help better assign the triplet state carbonyl C=O stretching and the ring related vibrational modes. The dynamics of formation and the spectral characteristics for the triplet states were found to be similar for the HA and HPDP. The temporal evolution at very early picosecond time scale indicates there is rapid intersystem crossing (ISC) conversion and subsequent relaxation of the excess energy of the initially produced energetic triplet state. B3LYP/6-311G** density functional theory (DFT) calculations were done to determine the structures and vibrational frequencies for both the triplet and ground states of HA and HPDP. The calculated spectra reproduce the experimental spectra and the observed isotopic shifts reasonably well and were used to make tentative assignments to all the experimentally observed features. The triplet states were found to have extensive conjugated pipi* nature with a single-bond-like carbonyl CO bond. We briefly compare the triplet structure and formation dynamics of HA and HPDP as well as the conformational changes upon going from the ground state to the triplet state. We discuss our present results in relation to the initial pathway for the p-hydroxyphenacyl photodeprotection process. We also compare and discuss the properties of the HA pipi* triplet state relative to the published results of other aromatic carbonyl compounds.

On the Nature of the Low-Lying Singlet States of 4-(Dimethyl-amino)benzonitrile

4-(N,N-Dimethyl-amino)benzonitrile (DMABN) is a prototype molecule for dual fluorescence. The anomalous emission has been attributed to an intramolecular charge-transfer (ICT) state, and the structure of the latter is still subject to some controversy. We applied a recently developed analytical gradient code for the approximate coupled-cluster singles-and-doubles method CC2 in combination with accurate basis sets to address this problem. Fully optimized excited state structures are presented for the ICT state and the so-called locally excited state, and recent transient IR and Raman measurements on the excited states are interpreted by means of calculated harmonic frequencies. Strong evidence is found for an electronic decoupling of the phenyl and the dimethyl-amino moiety, resulting in a minimum structure for the ICT state with a twisted geometry. In contrast to previous findings, the structure of this state is, at least in the gas phase, not C(2v) symmetric but distorted towards C(s) symmetry. The distortion coordinate is a pyramidalization of the phenyl carbon atom carrying the dimethyl-amino group. The results from the CC2 model are supported by single-point calculations using more elaborate coupled-cluster models (CCSD, CCSDR(3)) and by CASSCF calculations.

Hydrogenated silicon fullerenes: effects of H on the stability of metal-encapsulated silicon clusters

Ab initio calculations of H interaction on Si12M, Si18M2 (M=Cr, Mo, and W), and Zr@Si(16) fullerene (f) show relatively weak binding of H in agreement with experimental results of H free Si12M and Si18M2 clusters. Adsorption of H enhances sp(3) bonding between the Si atoms, weakens the M-Si cage interactions, and leads to distortions in the cages. Si12CrH12 has 4 mu(B) magnetic moment in contrast to zero for Si12Cr. Removal of the M atom leads to stable empty cages of Si12H12, f-Si16H16, and f-Si20H20 with large highest occupied-lowest unoccupied molecular orbital gaps of 2.5-3.0 eV, making them attractive for optoelectronic applications.