C. Scott Hartley

ortho-Phenylenes: unusual conjugated oligomers with a surprisingly long effective conjugation length.

ortho-Phenylenes represent a fundamental but relatively unexplored class of conjugated molecular architecture. We have developed a robust synthetic approach to monodisperse o-phenylene oligomers which we have demonstrated by synthesizing a homologous series up to the dodecamer. The o-phenylenes exhibit complex conformational behavior but are biased toward a specific 2-fold-symmetric conformation which we believe corresponds to a stacked helix. Surprisingly, the series exhibits long-range delocalization, as measured by bathochromic shifts in UV/vis spectra. Although the overall magnitude of the shifts is modest (but comparable to some other classes of conjugated materials), the effective conjugation length of the series is approximately eight repeat units. The oligomers also exhibit an unusual hypsochromic shift in their fluorescence spectra with increasing length. The origin of these trends is discussed in the context of conformational analysis and DFT calculations of the frontier molecular orbitals for the series.

Excited-state behavior of ortho-phenylenes.

The excited-state properties of unsubstituted ortho-phenylene oligomers have been studied using TD-DFT. Calculations of vertical transitions at the helical ground-state geometries are in good qualitative agreement with the experimental UV-vis spectra. In the excited state, the spring-like compounds compress; for the longer oligomers, this compression is localized at one end of the oligomer. This behavior explains the unusual experimentally observed hypsochromic shifts in fluorescence spectra with increasing oligomer length.

Conformational Analysis of o-Phenylenes: Helical Oligomers with Frayed Ends

The o-phenylenes represent a fundamental class of conjugated polymers that, unlike the isomeric p-phenylenes, should exhibit rich conformational behavior. Recently, we reported the synthesis and characterization of a series of o-phenylene oligomers featuring unusual electronic properties, including surprisingly long-range delocalization as measured by UV-vis spectroscopy and hypsochromic shifts in fluorescence maxima with increasing length. To rationalize these properties, we hypothesized that the oligomers predominantly assume a stacked helical conformation in solution. This assertion, however, was supported by only indirect evidence. Here we present a thorough investigation of the conformational behavior of this series of o-phenylenes by dynamic NMR spectroscopy and computational chemistry. EXSY experiments, in combination with other two-dimensional NMR techniques, provided full (1)H chemical shift assignments for at least the two most prevalent conformers for each member of the series (hexamer to dodecamer). GIAO density functional theory calculations were then used to relate the NMR data to specific molecular geometries. We have found that the o-phenylenes do indeed assume stacked helical conformations with disorder occurring at the ends. Thus, the o-phenylene motif appears to have great potential as a means to organize arenes into predictable three-dimensional arrangements. Our results also illustrate the power of (1)H NMR GIAO predictions in the solution-phase conformational analysis of oligomers, particularly those with a high density of aromatic subunits.

Triphenylene-Fused Porphyrins

Triphenylene has been successfully fused to the porphyrin periphery through a convenient oxidative ring-closure reaction. Bistriphenylene-fused porphyrins and a dibenzo[fg,op]tetracene-fused porphyrin have also been obtained using a similar approach. These π-extended porphyrins exhibited broadened and bathochromic shifted UV-vis absorptions.

Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants: amplification of the electroclinic coefficient using achiral additives

The atropisomeric compound (R)-2,2′,6,6′-tetramethyl-3,3′-dinitro-4,4′-bis[(4-nonyloxybenzoyl)oxy]biphenyl ((R)-1) was doped in the achiral liquid crystal hosts 2-(4-butoxyphenyl)-5-octyloxypyrimidine (2-PhP) and 4-(4′-heptyl[1,1′-biphen]-4-yl)-1-hexylcyclohexanecarbonitrile (NCB76), and electroclinic coefficients ec were measured as a function of the dopant mole fraction x1 in the chiral SmA* phase at T − TC = +5 K. The extrapolated ec values of 3.07 and 2.28 deg µm V−1 are comparable to some of the highest ec values reported for neat SmA* materials. The electroclinic coefficient of a 4 mol% mixture of (R)-1 in 2-PhP is amplified by achiral 2-phenylpyrimidine additives (5 mol%) that are longer than 2-PhP; in the best case, ec is amplified by a factor of 3.2 with 5-(tetradecyloxy)-2-(4-(tetradecyloxy)phenyl)pyrimidine (3g), which is almost twice as long as 2-PhP. However, no amplification is observed in a 4 mol% mixture of (R)-1 in NCB76 using the same series of additives. A correlation between ec values and the temperature range of the SmA* phase suggests that the amplification of ec with increasing length of the additive 3 in the (R)-1/2-PhP mixture is due primarily to a decrease in the tilt susceptibility coefficient α as the second-order SmA*–SmC* phase transition moves away from the tricritical point. Measurements of smectic layer spacing as a function of T − TC by small-angle X-ray scattering are consistent with this explanation. The results show that the variation in reduced layer spacing dA/dC with T − TC for the pure host 2-PhP fits to a square-root law, which indicates that the second-order SmA–C transition is nearly tricritical. On the other hand, the corresponding variation in dA/dC with T − TC for a 5 mol% mixture of 3g in 2-PhP fits to a linear relation, which indicates that the second-order SmA–C transition approaches typical mean-field behavior.

ortho-Phenylene oligomers with terminal push–pull substitution

ortho-Phenylenes are an emerging class of helical oligomers and polymers. We have synthesized a series of push-pull-substituted o-phenylene oligomers (dimethylamino/nitro) up to the octamer. Conformational analysis of the hexamer using a combination of low-temperature NMR spectroscopy and ab initio predictions of (1)H NMR chemical shifts indicates that, like other o-phenylenes, they exist as compact helices in solution. However, the substituents are found to have a significant effect on their conformational behavior: the nitro-functionalized terminus is 3-fold more likely to twist out of the helix. Protonation of the dimethylamino group favors the helical conformer. UV/vis spectroscopy indicates that the direct charge-transfer interaction between the push-pull substituents attenuates quickly compared to other conjugated systems, with no significant charge-transfer band for oligomers longer than the trimer. On protonation of the dimethylamino group, significant bathochromic shifts with increasing oligomer length are observed: the effective conjugation length is 9 repeat units, more than twice that of the parent oligomer. This behavior may be rationalized through examination of the frontier molecular orbitals of these compounds, which exhibit greater delocalization after protonation, as shown by DFT calculations.

Covalent ladder formation becomes kinetically trapped beyond four rungs

Scrambling experiments suggest that the self-assembly of 2D ladders via imine metathesis is kinetically trapped at four or more rungs. Consequently, ladders containing five or more rungs cannot be synthesized in high yield under the conditions used, as misaligned out-of-register byproducts cannot self-correct.

Graphene synthesis: Nanoribbons from the bottom-up

The organic synthesis of graphene nanostructures requires exceptionally efficient chemistry and is made more challenging by difficulties in characterization and processing. Now, solution-dispersible graphene nanoribbons have been synthesized on the gram scale.

Formation of smectic phases in binary liquid crystal mixtures with a huge length ratio

Summary A system of two liquid-crystalline phenylpyrimidines differing strongly in molecular length was studied. The phase diagram of these two chemically similar mesogens, with a length ratio of 2, was investigated, and detailed X-ray diffraction and electrooptical measurements were performed. The phase diagram revealed a destabilization of the nematic phase, which is present in the pure short compound, while the smectic state was stabilized. The short compound forms smectic A and smectic C phases, whereas the longer compound forms a broad smectic C phase and a narrow higher-ordered smectic phase. Nevertheless, in the mixtures, the smectic C phase is destabilized and disappears rapidly, whereas smectic A is the only stable phase observed over a broad concentration range. In addition, the smectic translational order parameters as well as the tilt angles of the mixtures are reduced. The higher-ordered smectic phase of the longer mesogen was identified as a smectic F phase.

Systematic Variation of Length Ratio and the Formation of Smectic A and Smectic C Phases

The phase diagrams of four binary mixtures of chemically similar smectogenic mesogens differing only in molecular length are investigated. In these bidisperse systems the length ratio varies systematically. The phase diagrams show the stabilization of the smectic A and the destabilization of the smectic C phase with increasing length ratio as a general trend. Detailed small-angle X-ray diffraction and electro-optic measurements revealed a decrease in smectic translational order and a continuous reduction of the tilt angle with increasing length difference. These surprising results are of general interest for the understanding of the structure and dynamics of smectic phases. The remarkably strong impact of the length difference on the smectic layer structure and the phase behavior is discussed from a mechanistic point of view taking into account sterical interactions. For the observed structural changes in these bidisperse smectics we propose pronounced out-of-layer fluctuations with increasing length difference as driving force, causing neighboring molecules within nearest layer into a smectic A-like packing.