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.

Ultrasensitive Calorimetric Measurements of the Electronic Heat Capacity of Graphene.

Heat capacity is an invaluable quantity in condensed matter physics and yet has been completely inaccessible in two-dimensional (2D) van der Waals (vdW) materials, owing to their ultrafast thermal relaxation times and the lack of suitable nanoscale thermometers. Here, we demonstrate a novel thermal relaxation calorimetry scheme that allows the first measurements of the electronic heat capacity of graphene. It is enabled by combining a radio frequency Johnson noise thermometer, which can measure the electronic temperature with a sensitivity of ∼20 mK/Hz1/2, and a photomixed optical heater that modulates Te with a frequency of up to Ω = 0.2 THz. This allows record sensitive measurements of the electronic heat capacity Ce < 10 -19 J/K and the fastest measurement of electronic thermal relaxation time τe < 10 -12 s yet achieved by a calorimeter. These features advance heat capacity metrology into the realm of nanoscale and low-dimensional systems and provide an avenue for the investigation of their thermodynamic quantities.

Magic-Angle Bilayer Graphene Nanocalorimeters: Toward Broadband, Energy-Resolving Single Photon Detection.

Because of the ultralow photon energies at mid-infrared and terahertz frequencies, in these bands photodetectors are notoriously underdeveloped, and broadband single photon detectors (SPDs) are nonexistent. Advanced SPDs exploit thermal effects in nanostructured superconductors, and their performance is currently limited to the more energetic near-infrared photons due to their high electronic heat capacity. Here, we demonstrate a superconducting magic-angle bilayer graphene (MAG) device that is theoretically capable of detecting single photons of ultralow energies by utilizing its record-low heat capacity and sharp superconducting transition. We theoretically quantify its calorimetric photoresponse and estimate its detection limits. This device allows the detection of ultrabroad range single photons from the visible to sub-terahertz with a response time around 4 ns and energy resolution better than 1 THz. These attributes position MAG as an exceptional material for long-wavelength single photon sensing, which could revolutionize such disparate fields as quantum information processing and radio astronomy.

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.

Optically Imaged Striped Domains of Nonequilibrium Electronic and Nuclear Spins in a Fractional Quantum Hall Liquid.

Using photoluminescence microscopy enhanced by magnetic resonance, we visualize in real space both electron and nuclear polarization occurring in nonequilibrium fraction quantum Hall (FQH) liquids. We observe stripelike domain regions comprising FQH excited states which discretely form when the FQH liquid is excited by a source-drain current. These regions are deformable and give rise to bidirectionally polarized nuclear spins as spin-resolved electrons flow across their boundaries.

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.

Structure and reactivity of thiazolium azo dyes: UV-visible, resonance Raman, NMR, and computational studies of the reaction mechanism in alkaline solution.

UV-visible absorption, resonance Raman, and (1)H NMR spectroscopy, allied with density functional theory (DFT) calculations, have been used to study the structure, bonding, and alkaline hydrolysis mechanism of the cationic thiazloium azo dye, 2-[2-[4-(diethylamino)phenyl]diazenyl]-3-methyl-thiazolium (1a), along with a series of six related dyes with different 4-dialkylamino groups and/or other phenyl ring substituents (2a-c, 3a-c) and the related isothiazolium azo dye, 5-[2-[4-(dimethylamino)phenyl]diazenyl]-2-methyl-isothiazolium (4). These diazahemicyanine dyes are calculated to have a similar low-energy structure that is cis, trans at the (iso)thiazolium-azo group, and for which the calculated Raman spectra provide a good match with the experimental data; the calculations on these structures are used to assign and discuss the transitions giving rise to the experimental spectra, and to consider the bonding and its variation between the dyes. UV-visible, Raman, and NMR spectra recorded from minutes to several weeks after raising the pH of an aqueous solution of 1a to ca. 11.5 show that the dominant initial step in the reaction is loss of diethylamine to produce a quinonimine (ca. hours), with subsequent reactions occurring on longer time scales (ca. days to weeks); kinetic analyses give a rate constant of 2.6 × 10(-2) dm(3) mol(-1) s(-1) for reaction of 1a with OH(-). UV-visible spectra recorded on raising the pH of the other dyes in solution show similar changes that are attributed to the same general reaction mechanism, but with different rate constants for which the dependence on structure is discussed.

Resonance Raman and UV-visible spectroscopy of black dyes on textiles.

Resonance Raman and UV-visible diffuse reflectance spectra were recorded from samples of cotton, viscose, polyester, nylon, and acrylic textile swatches dyed black with one of seven single dyes, a mixture of two dyes, or one of seven mixtures of three dyes. The samples generally gave characteristic Raman spectra of the dyes, demonstrating that the technique is applicable for the forensic analysis of dyed black textiles. Survey studies of the widely used dye Reactive Black 5 show that essentially the same Raman spectrum is obtained on bulk sampling from the dye in solution, on viscose, on cotton at different uptakes, and on microscope sampling from the dye in cotton threads and single fibres. The effects of laser irradiation on the Raman bands and emission backgrounds from textile samples with and without dye are also reported.

Reductive reaction mechanisms of the azo dye Orange II in aqueous solution and in cellulose: from radical intermediates to products.

Reductive reaction mechanisms of the azo dye Orange II (Acid Orange 7) in aqueous solution have been studied from radical intermediates through to the final products using a combination of nanosecond time-resolved UV-visible absorption spectroscopy, steady-state photolysis, and HPLC techniques. The dye is reduced by photochemically produced 2-hydroxy-2-propyl radicals at a near-diffusion-controlled rate (k2 = 2.2 x 10(9) dm3 mol(-1) s(-1)) to give the dye radical anion, which then disproportionates (k3 = 2.6 x 10(8) dm3 mol(-1) s(-1)) to re-form the parent dye and a hydrazine. The hydrazine decomposes to 4-aminobenzenesulfonate and a naphthylimine species, which hydrolyses to give 1,2-naphthoquinone; this naphthoquinone and 4-aminobenzenesulfonate react to give a species that reacts further in the presence of air to form an indophenol dye. The reduction of Orange II has also been studied in cellophane, where the rate constant for dye reduction by 2-hydroxy-2-propyl radicals is approximately two orders of magnitude lower than that in aqueous solution.

Ground- and excited-state infrared spectra of an azacrown-substituted [(bpy)Re(CO)3L]+ complex: structure and bonding in ground and excited states and effects of Ba2+ binding.

Ground- and excited-state infrared spectra are reported for a [(bpy)ReI(CO)3L]+ complex (bpy = 2,2'-bipyridine) in which L contains an azacrown ether that is linked to Re via an amidopyridyl group. Ground-state band assignments are made with the aid of spectra from model complexes in which a similar electron-donating dimethylamino group replaces the azacrown, in which an electron-donor group is absent, and from the L ligands, in conjunction with DFT calculations. Picosecond time-resolved IR (TRIR) spectra in the nu(CO) region show bands characteristic of a metal-to-ligand charge-transfer (MLCT) excited state, [(bpy*-)ReII(CO)3L]+, from the complex in which an electron-donor group is absent, whereas those from the azacrown complex show bands of an MLCT state evolving into those characteristic of a ligand-to-ligand charge-transfer (LLCT) excited state, [(bpy*-)ReI(CO)3(L*+)]+, formed upon intramolecular electron transfer. Picosecond TRIR spectra of the azacrown complex in the fingerprint region show strong L ligand bands that indicate that significant charge redistribution occurs within this ligand in the MLCT state and that decay as the LLCT state forms. Picosecond TRIR spectra obtained when Ba2+ was complexed to the azacrown show bands of only an MLCT state at all times up to 2 ns, consistent with the presence of Ba2+ inhibiting electron transfer from the azacrown N atom to form the LLCT state, and the positions of the bands in the fingerprint region provide direct evidence for the proposal that charge redistribution within the L ligand induces Ba2+ release from the azacrown in the MLCT state.

Ground and excited state resonance Raman spectra of an azacrown-substituted [(bpy)Re(CO)3L]+ complex: characterization of excited states, determination of structure and bonding, and observation of metal cation release from the azacrown.

A [(bpy)Re(CO)3L+] complex (bpy = 2,2'-bipyridine) in which L contains a phenyl-azacrown ether that is attached to Re via an amidopyridyl linking group has been studied by steady state and nanosecond time-resolved resonance Raman spectroscopy. Vibrational band assignments have been aided by studies of model complexes in which a similar electron-donating dimethylamino group replaces the azacrown or in which an electron-donor group is absent, and by density functional theory calculations. The ground state resonance Raman spectra show nu(bpy) and nu(CO) bands of the (bpy)Re(CO)3 group when excitation is exclusively in resonance with the Re --> bpy metal-to-ligand charge-transfer (MLCT) transition, whereas L ligand bands are dominant when it is in resonance with the strong intra-ligand charge-transfer (ILCT) transition present for L ligands with electron-donor groups. Transient resonance Raman (RR) spectra obtained on single color (385 nm) pulsed excitation of the complexes in which an electron-donor group is absent show bpy*- bands of the MLCT excited state, whereas those of the complexes with electron-donor groups show both bpy*- bands and a down-shifted nu(CO) band that together are characteristic of an L-to-bpy ligand-to-ligand charge-transfer (LLCT) excited state. Samples in which a metal cation (Li+, Na+, Ca2+, Ba2+) is bound to the azacrown in the ground state show bands from both excited states, consistent with a mechanism in which the LLCT state forms after metal cation release from the MLCT state. Nanosecond time-resolved RR spectra from two-color (355 nm pump, 500 nm probe) experiments on the electron-donor systems show L-ligand bands characteristic of the LLCT state; the same bands are observed from samples in which a metal cation is bound to the azacrown in the ground state, and their time dependence is consistent with the proposed mechanism in which the rate constant for ion release in the MLCT state depends on the identity of the metal cation.

Experimental and computational studies of structure and bonding in parent and reduced forms of the azo dye Orange II.

The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present as > or = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals.

Cation sensors containing a (bpy)Re(CO)3 group linked to an azacrown ether via an alkenyl or alkynyl spacer: synthesis, characterisation, and complexation with metal cations in solution.

Two [(bpy)Re(CO)3L]+ complexes (bpy = 2,2'-bipyridine), where L contains an aza-15-crown-5 ether which is linked to Re via an alkenyl- or alkynyl-pyridine spacer, have been synthesised along with model complexes. Solutions of the complexes in acetonitrile have been studied by UV-Vis absorption spectroscopy, and by 1D and 2D 1H NMR spectroscopy. Strong UV-Vis bands, assigned to intraligand charge-transfer transitions localised at the L ligands, blue shift on protonation of the azacrown nitrogen atom or on complexation of alkali-metal (Li+, Na+ and K+) or alkaline-earth metal (Mg2+, Ca2+ and Ba2+) cations to the azacrown; the magnitude of the blue shift is dependent on the cation, with protonation giving the largest shift of ca. 100 nm. Cation binding constants in the range of log K= 1-4 depend strongly on the identity of the metal cation. Protonation or cation complexation causes downfield shifts in the 1H NMR resonances from most of the azacrown and L ligand protons, and their magnitudes correlate with those of the blue shifts in the UV-Vis bands; shifts in the azacrown 1H NMR resonances report on how the different metal cations interact with the macrocycle. UV-Vis and 1H NMR spectra of the free L ligands enable the effect of the Re centre to be assessed. Together, the data indicate that the alkene spacer gives a more responsive sensor than the alkyne spacer by providing stronger electronic communication across the L ligand.

Photoinduced Ba2+ release and thermal rebinding by an azacrown ether linked by an alkynyl pyridine to a (bpy)Re(CO)3 group.

A novel complex in which an azacrown ether is linked by an alkynyl pyridine to a (bpy)Re(CO)3 group acts as a light-controlled ion switch: Ba2+ bound to the azacrown is released in ca. 10 ns on UV excitation, and rebinds thermally from bulk solution in ca. 1 micros.

Resonance Raman spectroscopy of photolabile transition metal carbonyls: controlled photoalteration with continuous wave lasers to record spectra of reactants or transient species.

A method has been developed that enables resonance Raman spectra of photolabile species in solution to be recorded under conditions where the level of photoalteration is controlled: a low level enables reactant spectra to be recorded, whereas a high level enables the spectra of short-lived transient species to be recorded in real time using continuous-wave (CW) lasers and standard Raman detection equipment. The design includes a sealed flow system, enabling air-sensitive species to be studied under an inert atmosphere. A simple theoretical model has been developed to aid the interpretation of experimental results, and its applicability is demonstrated. Controlled photoalteration and its theory are demonstrated with 413.1-nm excitation of carbonmonoxymyoglobin (MbCO), which generates deoxymyoglobin (deoxy-Mb) on photolysis, and for which the spectra of both species are well established. The methods have also been applied to two air-sensitive, photolabile transition metal carbonyls using 514.5-nm wavelength excitation: for Cp2Mo2(CO)6 (Cp = eta5-C5H5), increasing levels of photoalteration result only in a decrease in the parent band intensities, relative to the solvent bands; for Cp2Fe2(CO)4, increasing levels of photoalteration result in the appearance of additional bands that are assigned to the transient species CpFe(mu-CO)3FeCp, formed following the loss of a CO ligand.