Discovery of S···C≡N Intramolecular Bonding in a Thiophenylcyanoacrylate-Based Dye: Realizing Charge Transfer Pathways and Dye···TiO2 Anchoring Characteristics for Dye-Sensitized Solar Cells.

Donor-π-acceptor dyes containing thiophenyl π-conjugated units and cyanoacrylate acceptor groups are among the best-performing organic chromophores used in dye-sensitized solar cell (DSC) applications. Yet, the molecular origins of their high photovoltaic output have remained unclear until now. This synchrotron-based X-ray diffraction study elucidates these origins for the high-performance thiophenylcyanoacrylate-based dye MK-2 (7.7% DSC device efficiency) and its molecular building block, MK-44. The crystal structures of MK-2 and MK-44 are both determined, while a high-resolution charge-density mapping of the smaller molecule was also possible, enabling the nature of its bonding to be detailed. A strong S···C≡N intramolecular interaction is discovered, which bears a bond critical point, thus proving that this interaction should be formally classified as a chemical bond. A topological analysis of the π-conjugated portion of MK-44 shows that this S···C≡N bonding underpins the highly efficient intramolecular charge transfer (ICT) in thiophenylcyanoacrylate dyes. This manifests as two bipartite ICT pathways bearing carboxylate and nitrile end points. In turn, these pathways dictate a preferred COO/CN anchoring mode for the dye as it adsorbs onto TiO2 surfaces, to form the dye···TiO2 interface that constitutes the DSC working electrode. These results corroborate a recent proposal that all cyanoacrylate groups anchor onto TiO2 in this COO/CN binding configuration. Conformational analysis of the MK-44 and MK-2 crystal structures reveals that this S···C≡N bonding will persist in MK-2. Accordingly, this newly discovered bond affords a rational explanation for the attractive photovoltaic properties of MK-2. More generally, this study provides the first unequivocal evidence for an S···C≡N interaction, confirming previous speculative assignments of such interactions in other compounds.

Concerted mitigation of O···H and C(π)···H interactions prospects sixfold gain in optical nonlinearity of ionic stilbazolium derivatives.

DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) is the most commercially successful organic nonlinear optical (NLO) material for frequency-doubling, integrated optics, and THz wave applications. Its success is predicated on its high optical nonlinearity with concurrent sufficient thermal stability. Many chemical derivatives of DAST have therefore been developed to optimize their properties; yet, to date, none have surpassed the overall superiority of DAST for NLO photonic applications. This is perhaps because DAST is an ionic salt wherein its NLO-active cation is influenced by multiple types of subtle intermolecular forces that are hard to quantify, thus, making difficult the molecular engineering of better functioning DAST derivatives. Here, we establish a model parameter, ηinter, that isolates the influence of intermolecular interactions on second-order optical nonlinearity in DAST and its derivatives, using second-harmonic generation (SHG) as a qualifier; by systematically mapping intercorrelations of all possible pairs of intermolecular interactions to ηinter, we uncover a relationship between concerted intermolecular interactions and SHG output. This correlation reveals that a sixfold gain in the intrinsic second-order NLO performance of DAST is possible, by eliminating the identified interactions. This prediction offers the first opportunity to systematically design next-generation DAST-based photonic device nanotechnology to realize such a prospect.

Molecular origins of optoelectronic properties in coumarin dyes: toward designer solar cell and laser applications.

Coumarin derivatives are used in a wide range of applications, such as dye-sensitized solar cells (DSCs) and dye lasers, and have therefore attracted considerable research interest. In order to understand the molecular origins of their optoelectronic properties, molecular structures for 29 coumarin laser dyes are statistically analyzed. To this end, data for 25 compounds were taken from the Cambridge Structural Database and compared with data for four new crystal structures of coumarin laser dyes [Coumarin 487 (C(19)H(23)NO(2)), Coumarin 498 (C(16)H(17)NO(4)S), Coumarin 510 (C(20)H(18)N(2)O(2)), and Coumarin 525 (C(22)H(18)N(2)O(3))], which are reported herein. The competing contributions of different resonance states to the bond lengths of the 4- and 7-substituted coumarin laser dyes are computed based on the harmonic oscillator stabilization energy model. Consequently, a positive correlation between the contribution of the para-quinoidal resonance state and the UV-vis peak absorption wavelength of these coumarins is revealed. Furthermore, the perturbations of optoelectronic properties, owing to chemical substituents in these coumarin laser dyes, are analyzed: it is found that their UV-vis peak absorption and lasing wavelengths experience a red shift, as the electron-donating strength of the 7-position substituent increases and/or the electron-withdrawing strength of the 3- or 4-position substituent rises; this conclusion is corroborated by quantum-chemical calculations. It is also revealed that the closer the relevant substituents align with the direction of the intramolecular charge transfer (ICT), the larger the spectral shifts and the higher the molar extinction coefficients of coumarin laser dyes. These findings are important for understanding the ICT mechanism in coumarins. Meanwhile, all structure-property correlations revealed herein will enable knowledge-based molecular design of coumarins for dye lasers and DSC applications.

Molecular origins of commercial laser dye functionality in azacoumarins and 2-quinolones: LD 425, LD 489 and LD 473.

The molecular structures of three compounds, LD 425 (C(13)H(14)N(2)O(3)) (1), LD 489 (C(15)H(15)F(3)N(2)O(2)) (2) and LD 473 (C(17)H(19)F(3)N(2)O) (3), are determined by single-crystal X-ray diffraction (XRD) at 180 K. Azacoumarins (1) and (2) possess para-quinoidal bond-length patterns in their benzene rings due to intramolecular charge transfer (ICT) from these rings to the adjoining rings. In contrast, substitution of O with N within the coumarin heterocycle, to form a 2-quinolone, results in the suppression of this ICT effect. Instead, charge transfer within the heterocycle is shown to become more pronounced. Resonance theory is employed to discuss these bond pattern differences and characteristic spectral blue shifts in relation to their coumarin analogues. The application of this theory offers an intuitive understanding of the structure-property relationships in azacoumarins and 2-quinolones which is further supported by quantum chemical calculations. Such an understanding is important for recognizing ICT mechanisms in these compounds which can then be used to facilitate the molecular design of new laser dyes with the desired spectral shifts.

Bis{µ-2-[(pyridin-2-yl)imino-meth-yl]phenolato}bis-[(2-formyl-phenolato)copper(II)].

The asymmetric unit of the title compound, [Cu(2)(C(12)H(9)N(2)O)(2)(C(7)H(5)O(2))(2)], contains two independent (2-formyl-phen-olato){2-[(pyridin-2-yl)imino-meth-yl]phenolato}copper(II) mol-ecules that form pseudocentrosymmetric dimers via inter-actions between the Cu and pyridyl N atoms of independent monomers. The square-planar geometry of the Cu atoms in the monomer thus becomes square-based pyramidal in the dimer. The crystal studied was an inversion twin, with unequal populations of 0.353 (17) and 0.647 (17).

Tetra-µ-acetato-κO:O'-bis-[(2-amino-3,5-dichloro-pyridine-κN)copper(II)](Cu-Cu).

The title binuclear Cu(II) complex, [Cu(2)(CH(3)CO(2))(4)(C(5)H(4)Cl(2)N(2))(2)], is disposed about a crystallographic inversion center, located at the mid-point of the Cu-Cu connecting line. The Cu⋯Cu distance is 2.6600 (6) Šand each metal atom exhibits a Jahn-Teller-distorted octa-hedral geometry.