Solid-State Dilution of Dihydroxybenzophenones with 4,13-Diaza-18-crown-6 for Photocrystallographic Studies

This work forms part of the ongoing drive toward identifying and developing suitable light sensitive substances for photocrystallographic studies. In order to investigate the solid-state dilution of the photoactive dihydroxybenzophenone, X-ray crystal structures of three dihydroxybenzophenone·4,13-diaza-18-crown-6 co-crystals are reported and analyzed. The dihydroxybenzophenone molecules within the co-crystal are compared to those observed in homomolecular dihydroxybenzophenone crystals in terms of their intermolecular contacts, bond geometry and conformation. Molecular volumes and void spaces were calculated using Voronoi–Dirichlet polyhedra and Hirshfeld surface-based space partitioning, demonstrating new ways to represent potential reaction cavities around a photoactive molecule and calculate their packing efficiency. In each case the conditions of solid-state dilution were met. The molecular conformations of the homomolecular environments are retained to varying degrees in the analogous co-crystals. Results show that the co-crystals studied are potentially suitable for photocrystallography. In particular, 2,4-dihydroxybenzophenone molecules in 4,13-diaza-18-crown-6·2­(2,4-dihydroxybenzophenone) co-crystals exhibit high structural similarity to their homomolecular analogues suggesting its photochemical properties could be common to both environments

Photoconversion Bonding Mechanism in Ruthenium Sulfur Dioxide Linkage Photoisomers Revealed by in Situ Diffraction

Three new ruthenium–sulfur dioxide linkage photoisomeric complexes in the [Ru­(NH₃)₄(SO₂)<b>X</b>]­Cl₂·H₂O family (<b>X</b> = pyridine (<b>1</b>); 3-chloropyridine (<b>2</b>); 4-chloropyridine (<b>3</b>)) have been developed in order to examine the effects of the <i>trans</i>-ligand on the nature of the photo-induced SO₂ coordination to the ruthenium ion. Solid-state metastable η¹-O-bound (MS1) and η²-side S,O-bound (MS2) photoisomers are crystallographically resolved by probing a light-induced crystal with in situ diffraction. This so-called photocrystallography reveals the highest known photoconversion fraction of 58(3)% (in <b>1</b>) for any solid-state SO₂ linkage photoisomer. The decay of this MS1 into the MS2 state was modeled via first-order kinetics with a non-zero asymptote. Furthermore, the MS2 decay kinetics of the three compounds were examined according to their systematically varying <i>trans</i>-ligand <b>X</b>; this offers the first experimental evidence that the MS2 state is primarily stabilized by donation from the S–O_bound electrons into the Ru dσ-orbital rather than π-backbonding as previously envisaged. This has important consequences for the optoelectronic application of these materials since this establishes, for the first time, a design protocol that will enable one to control their photoconversion levels

Photoconversion Bonding Mechanism in Ruthenium Sulfur Dioxide Linkage Photoisomers Revealed by in Situ Diffraction

Three new ruthenium–sulfur dioxide linkage photoisomeric complexes in the [Ru­(NH₃)₄(SO₂)<b>X</b>]­Cl₂·H₂O family (<b>X</b> = pyridine (<b>1</b>); 3-chloropyridine (<b>2</b>); 4-chloropyridine (<b>3</b>)) have been developed in order to examine the effects of the <i>trans</i>-ligand on the nature of the photo-induced SO₂ coordination to the ruthenium ion. Solid-state metastable η¹-O-bound (MS1) and η²-side S,O-bound (MS2) photoisomers are crystallographically resolved by probing a light-induced crystal with in situ diffraction. This so-called photocrystallography reveals the highest known photoconversion fraction of 58(3)% (in <b>1</b>) for any solid-state SO₂ linkage photoisomer. The decay of this MS1 into the MS2 state was modeled via first-order kinetics with a non-zero asymptote. Furthermore, the MS2 decay kinetics of the three compounds were examined according to their systematically varying <i>trans</i>-ligand <b>X</b>; this offers the first experimental evidence that the MS2 state is primarily stabilized by donation from the S–O_bound electrons into the Ru dσ-orbital rather than π-backbonding as previously envisaged. This has important consequences for the optoelectronic application of these materials since this establishes, for the first time, a design protocol that will enable one to control their photoconversion levels

Remote Substituent Effects on the Structures and Stabilities of PE π‑Stabilized Diphosphatetrylenes (R₂P)₂E (E = Ge, Sn)

A rare P–E π interaction between the lone pair of a planar P center and the vacant p orbital at the Ge or Sn center provides efficient stabilization for P-substituted tetrylenes (R₂P)₂E (E = Ge, Sn) and enables isolation of the first example of a compound with a crystallographically authenticated PSn bond. Subtle changes in the electronic properties of the bulky aryl substituents in these compounds change the preference for planar versus pyramidal P centers in the solid state; however, variable-temperature NMR spectroscopy indicates that in solution these species are subject to a dynamic equilibrium, which interconverts the planar and pyramidal P centers. Consistent with this, density functional theory studies suggest that there is only a small energy difference between the planar and pyramidal forms of these compounds and reveal a small singlet–triplet energy separation, suggesting potentially interesting reactivities

Molecular Origins of Optoelectronic Properties in Coumarins 343, 314T, 445, and 522B

The relationships between the structure and laser dye properties of four coumarin derivatives are investigated to assist in knowledge-based molecular design of coumarins for various optoelectronic applications. Four new crystal structures of coumarins 343, 314T, 445, and 522B are determined at 120 K and analyzed via the empirical harmonic–oscillator–stabilization–energy and bond-length–alternation models, based on resonance theory. Results from these analyses are used to rationalize the optoelectronic properties of these coumarins, such as their UV–vis peak absorption wavelength, molar extinction coefficient, and fluorescence quantum efficiency. The specific molecular structural features of these four coumarins and the effects on their optoelectronic properties are further examined via a comparison with other similar coumarin derivatives, including coumarins 314, 500, and 522. These findings are corroborated by density functional theory (DFT) and time-dependent DFT calculations. The structure–property correlations revealed herein provide a foundation for the molecular engineering of coumarins with “dial-up” optoelectronic properties to suit a given device application

Molecular and Supramolecular Origins of Optical Nonlinearity in <i>N</i>‑Methylurea

The delicate balance between solid-state intermolecular interactions and electron-donating methyl-group influences in <i>N</i>-methylurea (NMU) is shown to distinguish its nonlinear optical properties, relative to those of urea, a standard reference material for second harmonic generation (SHG). The solid-state intermolecular interactions in NMU are identified using neutron diffraction data, showing that hydrogen bonding generates an extensive 3D supramolecular network of NMU molecules with secondary and tertiary nonbonded contacts helping to hold this network in a closely packed form. The undulating “urea tape” motif within this network renders an overall packing arrangement that is less SHG-favorable than that of urea, which exhibits a more head-to-tail molecular alignment. The primary, secondary, and tertiary nonbonded contacts are classified using graph-sets, Hirshfeld surfaces, and fingerprint plots. H···H contacts in NMU contribute to the overall Hirshfeld surface area much more than in urea, forming at the expense of O···H interactions. However, SHG-contributing electronic effects of the methyl group in NMU provide some compensation to these hydrogen-bonding influences. This methyl group is also shown to librate, which could augment SHG. Our experimental results offer a direct response to previous density functional theory calculations on NMU and urea, corroborating their predictions as well as enabling a better relationship between the molecular and bulk optical nonlinearity of NMU. To that end, crystal engineering options are discussed with a view to balancing these seemingly conflicting structural attributes, so that one can produce an SHG-active form of NMU that is superior to urea

Relating Electron Donor and Carboxylic Acid Anchoring Substitution Effects in Azo Dyes to Dye-Sensitized Solar Cell Performance

The relationship between the molecular structures of a series of azo dyes and their operational performance when applied to dye-sensitized solar cells (DSSCs) is probed via experimental and computational analysis. Seven azo dyes, with three different donating groups (dimethylamino, diethylamino, and dipropylamino) and carboxylic acid anchoring positions (<i>ortho</i>-, <i>meta</i>-, and <i>para</i>-substituted phenyl rings) are studied. Single-crystal X-ray diffraction is employed in order to analyze the effects of conformation and quantify the contribution of quinoidal resonance forms to the intramolecular charge transfer (ICT), which controls their intrinsic photovoltaic potential from an electronic standpoint. Harmonic oscillator stabilization energy (HOSE) calculations indicate that the <i>para</i>- and <i>ortho</i>-azo dyes exhibit potential for DSSC application. However, from a geometrical standpoint, the crystal structure data, proton nuclear magnetic resonance spectroscopy (¹H NMR), and density functional theory (DFT) all indicate that intramolecular hydrogen bonds form in <i>ortho</i>-dyes within both solid and solution states, impeding their intrinsic ICT-based photovoltaic potential, and offering insights into the photostability of azo dyes and the dye···TiO₂ anchoring mechanism in DSSCs. Donor effects are manifested in the packing mode and molecular planarity revealed by X-ray crystallography and in the UV/vis absorption spectra. DFT and time-dependent density functional theory (TDDFT) were performed to understand the electronic and optical properties of these azo dyes; these calculations compare well with experimental findings. Operational tests of DSSCs, functionalized by these azo dyes, show that the carboxylic acid anchoring position plays a crucial role in DSSC performance, while donating groups offer a much less obvious effect on the overall DSSC device efficiency

Predicting Solar-Cell Dyes for Cosensitization

A major limitation of using organic dyes for dye-sensitized solar cells (DSCs) has been their lack of broad optical absorption. Cosensitization, in which two complementary dyes are incorporated into a DSC, offers a route to combat this problem. Here we construct and implement a design route for materials discovery of new dyes for cosensitization, beginning with a chemically compatible series of existing laser dyes which are without an anchor group necessary for DSC use. We determine the crystal structures for this dye series and use their geometries to establish the DSC molecular design prerequisites aided by density-functional theory and time-dependent density-functional theory calculations. Based on insights gained from these existing dyes, modified sensitizers are computationally designed to include a suitable anchor group. A DSC cosensitization strategy for these modified sensitizers is predicted, using the central features of highest-occupied and lowest-unoccupied molecular orbital positioning, optical absorption properties, intramolecular charge-transfer characteristics, and steric effects as selection criteria. Through this molecular engineering of a series of existing non-DSC dyes, we predict new materials for DSC cosensitization

Air-stable fluorescent primary phosphine complexes of molybdenum and tungsten

<p>Here we report a study on the reactivity of the fluorescent air-stable primary phosphine 8-((4-phosphino)phenyl)-4,4-dimethyl-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene <b>4</b> with [Mo(CO)₆], [W(CO)₆], [Mo(CO)₄(piperidine)] and [W(CO)₄(piperidine)] which yields the mono-phosphine [Mo(CO)₅(<b>4</b>)] <b>5</b>, [W(CO)₅(<b>4</b>)] <b>6</b> and di-phosphine <i>cis</i>-[Mo(CO)₄(<b>4</b>)₂] <i>cis</i>-<b>7</b> and <i>cis</i>-[W(CO)₄(<b>4</b>)₂] <i>cis</i>-<b>9</b> complexes as the predominant products. In addition to the characterization of these complexes by multinuclear NMR and IR spectroscopy, and mass spectrometry, the solid-state structures of <b>6</b> and <i>cis</i>-<b>7</b> have also been determined by single-crystal X-ray diffraction. The photophysical properties of the complexes show that the incorporation of the phosphorus ligand has a limited effect on the absorption and emission profile of the Bodipy core and that they retain similar quantum yields to their parent Bodipy dyes. Small quantities of the by-products <i>trans</i>-[Mo(CO)₄(<b>4</b>)₂] <i>trans</i>-<b>7</b>, <i>cis</i>-[Mo(CO)₄(piperidine)(<b>4</b>)] <i>cis</i>-<b>8</b> and <i>cis</i>-[W(CO)₄(piperidine)(<b>4</b>)] <i>cis</i>-<b>10</b> were also isolated and characterized.</p

Relating Electron Donor and Carboxylic Acid Anchoring Substitution Effects in Azo Dyes to Dye-Sensitized Solar Cell Performance

The relationship between the molecular structures of a series of azo dyes and their operational performance when applied to dye-sensitized solar cells (DSSCs) is probed via experimental and computational analysis. Seven azo dyes, with three different donating groups (dimethylamino, diethylamino, and dipropylamino) and carboxylic acid anchoring positions (<i>ortho</i>-, <i>meta</i>-, and <i>para</i>-substituted phenyl rings) are studied. Single-crystal X-ray diffraction is employed in order to analyze the effects of conformation and quantify the contribution of quinoidal resonance forms to the intramolecular charge transfer (ICT), which controls their intrinsic photovoltaic potential from an electronic standpoint. Harmonic oscillator stabilization energy (HOSE) calculations indicate that the <i>para</i>- and <i>ortho</i>-azo dyes exhibit potential for DSSC application. However, from a geometrical standpoint, the crystal structure data, proton nuclear magnetic resonance spectroscopy (¹H NMR), and density functional theory (DFT) all indicate that intramolecular hydrogen bonds form in <i>ortho</i>-dyes within both solid and solution states, impeding their intrinsic ICT-based photovoltaic potential, and offering insights into the photostability of azo dyes and the dye···TiO₂ anchoring mechanism in DSSCs. Donor effects are manifested in the packing mode and molecular planarity revealed by X-ray crystallography and in the UV/vis absorption spectra. DFT and time-dependent density functional theory (TDDFT) were performed to understand the electronic and optical properties of these azo dyes; these calculations compare well with experimental findings. Operational tests of DSSCs, functionalized by these azo dyes, show that the carboxylic acid anchoring position plays a crucial role in DSSC performance, while donating groups offer a much less obvious effect on the overall DSSC device efficiency