Weak Intermolecular Interactions in the Crystal Structures of Molecules with Tetrahedral Symmetry: Diamondoid Nets and Other Motifs

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: Motif-forming characteristics of weak intermolecular interactions (Br‚‚‚‚Br, CtCH‚‚‚‚CtC, and CtCBr‚‚‚‚CtC) were examined in the solid state structures, determined by single-crystal X-ray diffraction, of tetraphenylmethane and 1,3,5,7-tetraphenyladamantane derivatives substituted at the four vertices with bromo, ethynyl, bromoethynyl, and 1,3-diethynyl groups. The crystals of the bromo-and ethynyl-substituted tetraphenylmethane derivatives exhibited triply interwoven diamondoid lattices sustained by weak interactions between bromo and ethynyl groups. It was observed that when bromo-and ethynyl groups are interchanged in these compounds, their solid-state structures did not change significantly. The crystals of tetrakis(4-bromoethynyphenyl)methane, as well as 1,4-and 1,3,5-bromoethynylbenzene, were sustained by rare, T-shaped CtCBr‚‚‚‚CtC contacts, but a comparison between these solid-state structures showed significant differences. All terminal 1,3-diynes were unstable compounds that decomposed rapidly. In general, it was observed that tetrahedral building blocks with a larger tetrahedral core, such as adamantane, formed disordered structures or fragile, highly solvated crystals

Light-harvesting and electronic contacting capabilities of Ru(II) Ipa rod and star complexes-first principles predictions

Homoleptic ([Ru(L)(3)](2+)) and heteroleptic (Ru(NCS)(2)(bpy)(L)) complexes carrying rigid anchor-cum-spacer ligands (L), comprised of ethynylene-phenylene spacer groups and a designated anchor group, including the isophthalic group (Ipa), have been studied computationally using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The results of the calculations are related to recently measured photophysical properties of selected complexes, and provide theoretical insight into the capabilities of the complexes to mediate interfacial electronic interactions. Opportunities to use the rigid spacer motifs to differentiate structural control and interfacial electronic interactions in heterogeneous systems are discussed, including predictions of promising strategies to enhance long-range interfacial electronic contacting capabilities using star complexes with new anchor group motifs

Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group Position

Four Zn(II)-tetra(carboxyphenyl)porphyrins in soln. and bound to metal oxide (TiO2, ZnO, and ZrO2) nanoparticle films were studied to det. the effect of the spacer length and anchoring group position (para or meta) on their binding geometry and photoelectrochem. and photophys. properties. The properties of three types of anchoring groups (COOH and COONHEt3) for four Zn(II)-porphyrins (Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin (p-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-(4-carboxyphenyl)phenyl)porphyrin (m-ZnTCP2P), and Zn(II)-5,10,15,20-tetra(3-ethynyl(4-carboxyphenyl)phenyl)porphyrin (m-ZnTC(PEP)P)) were compared. In m-ZnTCPP, m-ZnTCP2P, and m-ZnTC(PEP)P the four anchoring groups are in the meta position on the meso-Ph rings of the porphyrin macrocycle, thus favoring a planar binding mode to the metal oxide surfaces. The three meta-substituted porphyrin salts have rigid spacer units of increasing length (Ph (P), biphenyl (P2), and diphenylethynyl (PEP)) between the porphyrin ring and the carboxy anchoring groups, thus raising the macrocycle from the metal oxide surface. All porphyrins studied here, when bound to TiO2 and ZnO, exhibited quenching of the fluorescence emission, consistent with electron injection into the conduction band of the semiconductor. Steady-state UV-vis and fluorescence studies of p-ZnTCPP on insulating ZrO2 showed evidence of aggregation and exciton coupling. This was not obsd. in any of the meta-substituted porphyrins. The photoelectrochem. properties (IPCE, Voc, and Isc) of the porphyrins bound to TiO2 films in solar cells have been measured and rationalized with respect to the sensitizer binding geometry and distance from the surface

Large Footprint Pyrene Chromophores Anchored to Planar and Colloidal Metal Oxide Thin Films

Sensitization and binding of a large footprint pyrene chromophore to planar (sapphire) and colloidal metal oxide films (TiO2 and ZrO2) is investigated. The model compound combines a 1-pyrenyl-ethynylenephenylene unit with a new adamantane-tripodal linker that binds to the surface. The linker design, combining a large footprint (2 nm2) of the tripodal linker with the meta position of the COOH anchoring groups, was suggested from atomistic models, and it aims to provide improved spacing control. The pyrene chromophore unit provides a probe of sensitizer−sensitizer interactions through its propensity to form excimers, unless neighboring pyrene units are sufficiently spaced (≥3.5 Å). Absorption and fluorescence studies, and a comparison with a pyrene−rigid rod model compound, suggest that the new tripodal anchor group allows spacing control on planar surfaces. On colloidal films, the linker provides spacing control at low surface coverage but sensitizer−sensitizer interactions are still observed on colloidal films at high surface coverage. Implications for the functionalization of metal oxide films in hybrid molecule−metal oxide semiconductor material systems are discussed

Fluorescence Enhancement of Di-<i>p</i>-tolyl Viologen by Complexation in Cucurbit[7]uril

A viologen derivative, 1,1′-di-<i>p</i>-tolyl-(4,4′-bipyridine)-1,1′-diium dichloride (DTV²⁺), was studied in solution and encapsulated in cucurbit[7]­uril (CB7), a macrocyclic host. Upon encapsulation, DTV²⁺ exhibited dramatically enhanced fluorescence. Aqueous solutions of DTV²⁺ were weakly fluorescent (Φ = 0.01, τ < 20 ps), whereas the emission of the DTV²⁺@2CB7 complex was enhanced by 1 order of magnitude (Φ = 0.12, τ = 0.7 ns) and blue-shifted by 35 nm. Similar properties were observed in the presence of NaCl. DTV²⁺ in a poly­(methyl methacrylate) matrix was fluorescent with a spectrum similar to that observed for the complex in solution. ¹H NMR and UV–vis titrations indicated that the DTV²⁺@2CB7 complex is formed in aqueous solutions with complexation constants <i>K</i>₁ = (1.2 ± 0.3) × 10⁴ M^–1 and <i>K</i>₂= (1.0 ± 0.4) × 10⁴ M^–1 in water. Density functional theory and configuration interaction singles calculations suggested that the hindrance of the rotational relaxation of the S₁ state of DTV²⁺ caused by encapsulation within the host or a polymer matrix plays a key role in the observed emission enhancement. The absorption and emission spectra of DTV²⁺@2CB7 in water exhibited a large Stokes shift (ΔSt ∼ 9000 cm^–1) and no fine structure. DTV²⁺ is a good electron acceptor [<i>E</i>°(DTV²⁺/DTV^•+) = −0.30 V vs Ag/AgCl] and a strong photooxidant [<i>E</i>°(DTV*²⁺/DTV^•+) = 0.09 V vs NHE])

Vibrational Spectroscopy on Photoexcited Dye-Sensitized Films via Pump-Degenerate Four-Wave Mixing

Molecular sensitization of semiconductor films is an important technology for energy and environmental applications including solar energy conversion, photocatalytic hydrogen production, and water purification. Dye-sensitized films are also scientifically complex and interesting systems with a long history of research. In most applications, photoinduced heterogeneous electron transfer (HET) at the molecule/semiconductor interface is of critical importance, and while great progress has been made in understanding HET, many open questions remain. Of particular interest is the role of combined electronic and vibrational effects and coherence of the dye during HET. The ultrafast nature of the process, the rapid intramolecular vibrational energy redistribution, and vibrational cooling present complications in the study of vibronic coupling in HET. We present the application of a time domain vibrational spectroscopypump-degenerate four-wave mixing (pump-DFWM)to dye-sensitized solid-state semiconductor films. Pump-DFWM can measure Raman-active vibrational modes that are triggered by excitation of the sample with an actinic pump pulse. Modifications to the instrument for solid-state samples and its application to an anatase TiO₂ film sensitized by a Zn-porphyrin dye are discussed. We show an effective combination of experimental techniques to overcome typical challenges in measuring solid-state samples with laser spectroscopy and observe molecular vibrations following HET in a picosecond time window. The cation spectrum of the dye shows modes that can be assigned to the linker group and a mode that is localized on the Zn-phorphyrin chromophore and that is connected to photoexcitation