Stereoselective two-carbon ring expansion of allylic amines via electronic control of palladium-promoted equilibria.

General methodologies enabling the two-carbon homologation of pyrrolidine and piperidine systems have yet to be developed. Herein we report that palladium-catalysed allylic amine rearrangements enable efficient two-carbon ring expansion of 2-alkenyl pyrrolidine and piperidines to their azepane and azocane counterparts. Conditions are mild, tolerant of a range of functional groups and the process can occur with high enantioretention. The products formed undergo a range of orthogonal transformations, making them ideal scaffolds for the creation of compound libraries.

Dyes for guest-host liquid crystal applications: a general approach to the rapid computational assessment of useful molecular designs.

A set of experimental dichroic order parameters ranging from ca. +0.66 to -0.22 was obtained by recording polarized UV-visible absorption spectra from aligned samples of fifteen different guest anthraquinone and azo dyes in the nematic host 4-cyano-4'-pentylbiphenyl (5CB). DFT-optimised structures were calculated for between 1 and 16 conformers/tautomers of each dye, and their relative energies, UV-visible absorption wavelengths, oscillator strengths, transition dipole moments, molecular surface tensors and quadrupole tensors were obtained and used in subsequent calculations. A simple approach provided calculated UV-visible absorption spectra of the dyes that gave a qualitative match with the experimental spectra, and the calculated peak positions showed a linear correlation with the experimental values across the full visible range of ca. 350-700 nm. A short-range, shape-based, mean-field orienting potential based on the calculated surface tensors was combined with the calculated transition dipole moment vectors to give calculated dichroic ratios of the dyes that showed a linear correlation across the full range of experimental values. A modification of this mean-field orienting potential to include a long-range, electrostatic component based on the calculated quadrupole tensors gave a slightly improved linear correlation but a slightly worse overall match to the experimental values. The results show that short-range, shape-based interactions dominate the orienting potential for the systems studied here, with the inclusion of long-range quadrupole interactions providing a slightly better model for only some of the dyes. Overall, the use of a mean-field approach allied with molecular properties that can be calculated with relative ease and low computational expense has provided calculated peak positions and dichroic ratios that show good matches and correlations with experimental data from a variety of dye structures without the need to input any experimental data from the dyes. Hence, this method may provide a general and rapid approach to predicting the optical properties of dyes in liquid crystal hosts, enabling candidate dye structures to be screened prior to synthesis.

Iridium(iii) complexes of 1,2,4-triazines as potential bioorthogonal reagents: metal coordination facilitates luminogenic reaction with strained cyclooctynes.

In this paper we describe unprecedented Ir(iii) complexes of 5-(2-pyridyl)-1,2,4-triazine and their reactivity towards the strained cyclooctyne BCN. The coordination of a 1,2,4-triazine ring to an iridium(iii) ion drastically increases the speed of the reaction, showing the second order rate constant of 8 M-1 s-1, the record value to date for a triazine-BCN reaction.

Shape segregation in molecular organisation: a combined X-ray scattering and molecular dynamics study of smectic liquid crystals.

Temperature-dependent X-ray scattering studies have been carried out on 4-undecyloxy-4'-cyanobiphenyl (11OCB) and 4-(12,12-dimethyltridecyloxy)-4'-cyanobiphenyl (t-Bu-11OCB) in the smectic A phase, from which their layer spacings and translational order parameters were obtained. Marked differences between the layer structures of the two compounds were demonstrated, showing that the addition of the t-Bu group results in a higher translational order parameter and wider layer spacing for t-Bu-11OCB than 11OCB. Fully atomistic MD simulations of both compounds run for >1000 ns demonstrated the spontaneous formation of smectic mesophases from isotropic starting geometries, and experimental trends in order parameters and absolute layer spacings were shown to be replicated well. Further analysis showed that both the aromatic head-groups and the alkyl tail-groups exhibit interdigitation in the simulated smectic phases of both compounds, and the difference in the layer structures between 11OCB and t-Bu-11OCB could be attributed mainly to a shape segregation effect arising from the addition of the bulky t-Bu end-group to the alkyl chain.

Photoswitching of Dihydroazulene Derivatives in Liquid-Crystalline Host Systems.

Photoswitches and dyes in the liquid-crystalline nematic phase have the potential for use in a wide range of applications. A large order parameter is desirable to maximize the change in properties induced by an external stimulus. A set of photochromic and nonphotochromic dyes were investigated for these applications. It was found that a bent-shaped 7-substituted dihydroazulene (DHA) photoswitch exhibited liquid-crystalline properties. Further investigation demonstrated that this material actually followed two distinct reaction pathways on heating, to a deactivated form by a 1,5-sigmatropic shift and to a linear 6-substituted DHA. In addition, elimination of hydrogen cyanide from the photoactive DHA gave both bent and linear azulene dyes. In a nematic host that has no absorbance around 350 nm, it was found that only the linear DHA derivative has nematic properties; however, both 6- and 7-substituted DHAs were found to have large order parameters. In the nematic host, ring opening of either DHA to the corresponding vinylheptafulvene resulted in a decrease in dichroic order parameter and an unusually fast back-reaction to a mixture of both DHAs. Likewise, only the linear azulene derivative showed mesomorphic properties. In the same nematic host, large order parameters were also observed for these dyes.

Principal molecular axis and transition dipole moment orientations in liquid crystal systems: an assessment based on studies of guest anthraquinone dyes in a nematic host.

An assessment of five different definitions of the principal molecular axis along which molecules align in a nematic liquid crystal system has been made by analysing fully atomistic molecular dynamics (MD) simulations of a set of anthraquinone dyes in the cyanobiphenyl-based nematic host mixture E7. Principal molecular axes of the dyes defined by minimum moment of inertia, minimum circumference, minimum area, maximum aspect ratio, and surface tensor models were tested, and the surface tensor model was found to give the best description. Analyses of MD simulations of E7 alone showed that the surface tensor model also gave a good description of the principal molecular axes of the host molecules, suggesting that this model may be applicable more generally. Calculated dichroic order parameters of the guest-host systems were obtained by combining the surface tensor analysis with fixed transition dipole moment (TDM) orientations from time-dependent density functional theory (TD-DFT) calculations on optimised structures of the dyes, and the trend between the dyes generally matched the trend in the experimental values. Additional analyses of the guest-host simulations identified the range of conformers explored by the flexible chromophores within the dyes, and TD-DFT calculations on corresponding model structures showed that this flexibility has a significant effect on the TDM orientations within the molecular frames. Calculated dichroic order parameters that included the effects of this flexibility gave a significantly improved match with the experimental values for the more flexible dyes. Overall, the surface tensor model has been shown to provide a rationale for the experimental alignment trends that is based on molecular shape, and molecular flexibility within the chromophores has been shown to be significant for the guest-host systems: the computational approaches reported here may be used as a general aid in the predictive design of dyes with appropriate molecular shapes and flexibilities for guest-host applications.

Experimental and molecular dynamics studies of anthraquinone dyes in a nematic liquid-crystal host: a rationale for observed alignment trends.

Five anthraquinone dyes with bis(4-propylphenyl) substituents, connected via sulfide or amine linking groups at the 1,5-positions or directly at the 2,6-positions, have been studied as guests in the nematic liquid crystal host, E7. Polarized UV-visible absorption spectra of aligned samples were used to obtain experimental dichroic order parameters, which exhibit values in the range 0.51-0.74. Fully atomistic MD simulations of these guest-host systems were carried out, generally using default parameters but using new force constants derived here for the dyes containing flexible phenyl-sulfide and phenyl-amine linking groups. An analysis of the alignment of the dye molecules in these simulations provides calculated molecular order parameters, which are combined with calculated order parameters for the alignment of the transition dipole moments within the dyes, reported previously, to give calculated dichroic order parameters. The trend in the calculated dichroic order parameters between the dyes shows a good match with the trend in the experimental values, enabling the observed variation to be rationalised primarily by changes in the alignment of the calculated transition dipole moments within the dyes; the calculated molecular order parameters show a relatively small variation between the dyes. The results indicate that this computational approach may be used generally to rationalise trends in the alignment of guest molecules in liquid crystal hosts, suggesting that it may also be able to provide a predictive aid in the design of guest dyes.

Dyes in Liquid Crystals: Experimental and Computational Studies of a Guest-Host System Based on a Combined DFT and MD Approach.

Practical applications of guest-host liquid crystal systems are critically dependent on the alignment of the guest species within the liquid crystal host. UV/Vis absorption spectroscopy shows that the 1,5-dihydroxy-2,6-bis-(4-propylphenyl)-9,10-anthraquinone dye aligns within the E7 nematic host, giving an experimental dichroic ratio of 9.40 and dye order parameter of 0.74. This alignment was modelled by using a combination of density functional theory (DFT) and molecular dynamics (MD) computational approaches that do not require the input of experimental data. Time-dependent DFT calculations show that the electronic transition dipole moment is highly aligned with the long molecular axis of the dye. Fully atomistic MD simulations show that the long axis of the dye is less highly aligned within the E7 host, indicating that this contribution limits the overall dye alignment and, thereby, the potential practical applications of this particular system. Importantly, this study demonstrates an experimental and combined DFT and MD computational approach that may be applied generally to guest-host systems, providing a potential route to their rational design.