Thomas P. Vaid

A porphyrin with a C═C unit at its center.

The molecule (C═C)TTP (TTP = tetra-p-tolylporphyrin) and the triflate salt of its dication, [(C═C)TTP][OTf](2), have been synthesized and characterized. NMR spectroscopy, nucleus-independent chemical shift calculations, and the crystal structure of (C═C)TTP indicate that (C═C)TTP is antiaromatic and (C═C)TTP(2+) is aromatic.

Cadmium and zinc thiolate and selenolate metal–organic frameworks

Metal-organic frameworks based on metal-sulfur or metal-selenium bonds are relatively rare; herein we describe the synthesis and structural characterization of several examples, including, for example, [Cd(en)3][Cd(SC6H4S)2], which contains the anionic two-dimensional square-grid network [Cd(SC6H4S)2]n(2n-).

Thermodynamic and electronic properties of tunable II–VI and IV–VI semiconductor based metal–organic frameworks from computational chemistry

Optoelectric control of metal–organic frameworks would open up a new area of applications for hybrid materials. This article reports the calculated thermodynamic and electronic properties of a family of M3(C6X6) metal–organic frameworks (M = Mg, Ca, Zn, Cd, Hg, Ge, Sn, Pb; X = O, S, Se, Te). Herein, we present a systematic approach for studying families of hybrid compounds, and describe extended tunability of their electronic and enthalpic properties through compositional control. It was shown that the formation enthalpy is dictated by the stability of the ligand, and the band gap is tunable depending on both metal and chalcogenide selection. Five compounds were found to be candidate semiconductors as they combine thermodynamic stability with band gaps in the visible range of the electromagnetic spectrum.

Synthesis, Characterization, and Calculated Electronic Structure of the Crystalline Metal−Organic Polymers (Hg(SC 6 H 4 S)(en)) n and (Pb(SC 6 H 4 S)(dien)) n

The reaction of Hg(OAc)(2) with 1,4-benzenedithiol in ethylenediamine at 80 °C yields [Hg(SC(6)H(4)S)(en)](n), while the reaction of Pb(OAc)(2) with 1,4-benzenedithiol in diethylenetriamine at 130 °C yields [Pb(SC(6)H(4)S)(dien)](n). Both products are crystalline materials, and structure determination by synchrotron X-ray powder diffraction revealed that both are essentially one-dimensional metal-organic polymers with -M-SC(6)H(4)S- repeat units. Diffuse reflectance UV-visible spectroscopy indicates band gaps of 2.89 eV for [Hg(SC(6)H(4)S)(en)](n) and 2.54 eV for [Pb(SC(6)H(4)S)(dien)](n), while density functional theory (DFT) band structure calculations yielded band gaps of 2.24 and 2.10 eV, respectively. The two compounds are both infinite polymers of metal atoms linked by 1,4-benzenedithiolate, the prototypical molecule for single-molecule conductivity studies, yet neither compound has significant electrical conductivity as a pressed pellet. In the case of [Pb(SC(6)H(4)S)(dien)](n) calculations indicate fairly flat bands and therefore low carrier mobilities, while the conduction band of [Hg(SC(6)H(4)S)(en)](n) does have moderate dispersion and a calculated electron effective mass of 0.29·m(e). Hybridization of the empty Hg 6s orbital with SC(6)H(4)S orbitals in the conduction band leads to the band dispersion, and suggests that similar hybrid materials with smaller band gaps will be good semiconductors.

Doping of an organic molecular semiconductor by substitutional cocrystallization with a molecular n-dopant

Dopants for organic molecular semiconductors that yield immobile dopant ions are necessary for the creation of stable molecular semiconductor p–n junctions, the basis for almost all traditional inorganic semiconductor devices. We present evidence for the substitutional cocrystallization of tris(4-nitrophenyl)methyl radical (1) with small amounts of tris[4-(dimethylamino)phenyl]methyl radical (2), resulting in n-doped 1 with immobile 2+ counterions. Cyclic voltammetry indicates that electron transfer from 2 to 1 is favored by 0.51 eV. The powder X-ray diffraction patterns of pure 1 and 1 doped with 2 are very similar, indicating substitutional cocrystallization. The electrical conductivity of doped 1 increases with increasing concentration of 2, and the conductivity is constant over time. Variable-temperature conductivity measurements of 1 doped with 2% and 5% 2 indicate that the activation energy of conduction is 0.32 eV at both dopant concentrations.

Computational screening of structural and compositional factors for electrically conductive coordination polymers

The combination of organic and inorganic chemical building blocks to form metal-organic frameworks (MOFs) offers opportunities for producing functional materials suitable for energy generation, storage and conversion. However, such applications rely on robust electron transport and the design of conductive hybrid materials is still in its infancy. Here we apply density functional theory to assess the important structural and compositional factors for forming conducting MOFs. We focus on 1D metal-organic polymers as a model system and assess the choice of organic, inorganic and linking units. The results demonstrate that electronic communication is sensitive to the energy and symmetry of the frontier orbitals associated with the organic and inorganic building blocks and offers guidance on how to optimise electrical conduction in hybrid materials.

Electronic structure and photophysics of (CC)tetra-p-tolylporphyrin2+

The electronic structure and photophysical properties of (C=C)TTP(2+) (TTP = tetra-p-tolylporphyrin) were scrutinized by using quantum mechanical calculations and transient absorption spectroscopic measurements. When compared to a metalloporphyrin, the presence of the C=C unit in (C=C)TTP(2+) causes a splitting of the degenerate LUMO and a large decrease in the HOMO-LUMO gap, while the 2+ charge and tolyl groups lead to additional charge-transfer-like transitions in the visible absorption spectrum. The small HOMO-LUMO gap and ruffled structure lead to a very short excited-state lifetime of 10 ± 0.3 ps.

Synthesis of Protected Benzenepolyselenols

Previously unknown benzenepolyselenols have been synthesized and isolated in their acetyl-protected form. The two molecules 1,3,5-tris(acetylseleno)benzene and 1,2,4,5-tetrakis(acetylseleno)benzene were synthesized by the reductive dealkylation in Na/NH(3) of 1,3,5-tris(tert-butylseleno)benzene and 1,2,4,5-tetrakis(tert-butylseleno)benzene, respectively. Hexakis(tert-butylseleno)benzene was also synthesized and structurally characterized by single-crystal X-ray diffraction, but it was not possible to isolate hexakis(acetylseleno)benzene. The synthetic methodology is likely to be useful in the synthesis of other areneselenols.