Substituent Effects That Control Conjugated Oligomer Conformation through Non-covalent Interactions.

Although understanding the conformations and arrangements of conjugated materials as solids is key to their prospective applications, predictive power over these structural factors remains elusive. In this work, substituent effects tune non-covalent interactions between side-chain fluorinated benzyl esters and main-chain terminal arenes, in turn controlling the conformations and interchromophore aggregation of three-ring phenylene-ethynylenes (PEs). Cofacial fluoroarene-arene (ArF-ArH) interactions cause twisting in the PE backbone, interrupting intramolecular conjugation as well as blocking chromophore aggregation, both of which prevent the typically observed bathochromic shift observed upon transitioning PEs from solution to solid. This work highlights two structural factors that determine whether the ArF-ArH interactions, and the resulting twisted, unaggregated chromophores, occur in these solids: (i) the electron-releasing characteristic of substituents on ArH, with more electron-releasing character favoring ArF-ArH interactions, and (ii) the fluorination pattern of the ArF ring, with 2,3,4,5,6-pentafluorophenyl favoring ArF-ArH interactions over 2,4,6-trifluorophenyl. These trends indicate that considerations of electrostatic complementarity, whether through a polar-π or substituent-substituent mechanism, can serve as an effective design principle in controlling the interaction strengths, and therefore the optoelectronic properties, of these molecules as solids.

Combining electronic and steric effects for highly stable unsymmetric pentacenes.

This paper describes the reactivity of unsymmetrically substituted pentacenes to photochemical oxidation. Acenes in general, and pentacenes in particular, are a key family of compounds for a variety of organic electronics applications. The instability of many pentacene derivatives, particularly to oxidation, is an important restriction in their applicability. Several substitution strategies for decreasing the reactivity of pentacene exist, but these almost always involve symmetrically substituted derivatives, restricting the chemical space of structures from which to choose. In this paper, we demonstrate that combining electronic and steric effects yields highly stable unsymmetrically substituted pentacenes.

Phosphine substitution and linkage isomerization in cyclopentadienylruthenium bis(triphenylphosphine)thiocyanide and selenocyanide, CpRu(PPh3)2NCS and CpRu(PPh3)2SeCN.

The reaction between CpRu(PPh3)2NCS (1a) and PMePh2 yields CpRu(PPh3)(PMePh2)NCS (2a) while CpRu(PPh3)(PMePh2)Cl reacts with SCN- to form the S-bonded isomer, CpRu(PPh3)(PMePh2)SCN (2b). Compound 1a and the linkage isomers of 2 were characterized by X-ray crystallography. The kinetics of the reaction between 1a and PMePh2 under pseudo-first order conditions in THF and in fluorobenzene to form 2a are consistent with a dissociative interchange mechanism. Activation parameters for the reaction are: ΔH† = 15.7 ± 0.6 kcal mol-1 and ΔS† = -35 ± 2 cal mol-1 K-1 in THF vs. ΔH† = 24.8 ± 1.2 kcal mol-1 and ΔS† = -6 ± 4 cal mol-1 K-1 in C6H5F. In the presence of added SCN-, the rate of phosphine substitution is unchanged but a mixture of 2a and 2b is observed. The selenocyanate derivative, CpRu(PPh3)2SeCN (3b), crystallizes as the Se-bonded linkage isomer. Compound 3b reacts with PMePh2 under pseudo-first order conditions in fluorobenzene to form CpRu(PPh3)(PMePh2)SeCN (4b) at a much faster rate than 1a with activation parameters: ΔH† = 30.9 ± 4.8 kcal mol-1 and ΔS† = 22.4 ± 15.9 cal mol-1 K-1 with no evidence for linkage isomerization to the N-bonded products.

Reversible mechanofluorochromism of aniline-terminated phenylene ethynylenes.

Seven three-ring phenylene-ethynylene (PE) structural analogs, differing only in the lengths of alkyl chains on terminal aniline substituents, show 50-62 nm bathochromic shifts in emission maxima in response to mechanical force (mechanofluorochromism, MC). These shifts are fully reversible with heat or solvent fuming. Shearing of these solids yields a transition from green-emitting crystalline phases to orange-emitting amorphous phases as established by differential scanning calorimetry and X-ray diffraction. Molecules with shorter alkyl chain lengths required higher temperatures to recover the hypsochromically shifted crystalline phases after grinding, while the recovery with chain lengths longer than butyl occurred at room temperature. In addition to this structure-dependent thermochromism, these compounds retain their MC properties in polymer hosts to various extents. The crystalline phases of these materials have PE chromophores that are twisted due to non-covalent perfluoroarene-arene (ArF-ArH) interactions involving perfluorophenyl pendants and the terminal rings of the PE chromophore, resulting in interrupted conjugation and an absence of chromophore aggregation. The MC behavior of an analog without the perfluoroarene rings is severely attenuated. This work demonstrates the general utility of twisted PEs as stimuli-responsive moieties and reveals clear structure-property relationships regarding the effects of alkyl chain length on these materials.

Two-dimensional bricklayer arrangements of tolans using halogen bonding interactions.

Diphenylacetylene (tolan) derivatives with self-complementary aryl halides and halogen bond-accepting nitriles form 2D bricklayer packing motifs when halogen bonding occurs. When halogen bonding is absent, as occurred with fluorinated aryl bromides, the molecules adopt other packing motifs. These results suggest halogen bonding is potentially useful for producing rarely observed 2D bricklayer motifs in organic semiconductors.

Nickel(II) cyclidenes with appended ethylpyridine receptor centers as molecular tweezers for dicarboxylic acids.

A series of 14-, 15-, and 16-membered nickel(II) cyclidene macrocycles appended with 2-aminoethyl(2-pyridine) receptors I-III, respectively, were prepared and characterized by X-ray crystallography and NMR techniques. The 14- and 15-membered macrocycles I and II exist in a planar or extended Z-configuration, whereas the 16-membered macrocycle III was saddle shaped and had two asymmetric configurations in the unit cell (IIIa in a "capped" configuration and IIIb in an "open" configuration). Variable-temperature (1)H NMR studies of III in CD(3)CN were conducted (25-65 degrees C), and at room temperature, the interconversion between capping and uncapping is slow on the NMR time scale, resulting in a broad spectrum, whereas at 65 degrees C, interconversion was fast. (1)H NMR binding studies indicated I-III bind unsaturated dicarboxylic acids in a 1:1 stoichiometry with binding constants approaching 400 M(-)(1) in CD(3)CN, and the binding strength was dependent on the shape of the macrocyclic cyclidene platforms, whereas monocarboxylic acids were not bound. Generally, the planar 14-membered cyclidene I bound diacids the weakest and the 16-membered cyclidene III bound diacids the strongest. The presence of nuclear Overhauser effect spectrometry cross peaks in a 20 mM solution of 1:1 II-maleic acid indicates that the binding mode is ditopic with the guest being encapsulated by the aminoethylpyridine arms above the macrocyclic framework.