Modulated Fluorescence in LB Films Based on DADQs-A Potential Sensing Surface?

Novel fluorescent Langmuir-Blodgett (LB) films have been constructed from three different amphiphilic dicynaoquinodimethanes (DADQs). The DADQs varied in functional group structure, which had an impact on the LB film structure and the fluorescence properties. As the fluorescence of DADQs competes with non-radiative decay (conformational change), the packing and/or free volume in the LB film will influence the average fluorescence lifetime and integrated intensity. The pristine (blank) LB films were then exposed to a selection of non-fluorescent target analytes (some with environmental relevance) and the fluorescence was measured and analyzed relative to the pristine LB film. Exposure of the LB films to selected target analytes results in a modulation of the fluorescence, both with respect to average fluorescence lifetime and integrated intensity. The modulation of the fluorescence is different for different DADQ LB films and can be attributed to restricted non-radiative decays or charge transfer reactions between target analyte and DADQ LB film. The response from the DADQ LB films shows that these systems can be developed into sensing surfaces based on fluorescence measurements.

A study of the dynamics and structure of the dielectric anomaly within the molecular solid TEA(TCNQ)2.

With rising interest in organic-based functional materials, it is important to understand the nature of magnetic and electrical transitions within these types of systems. One intriguing material is triethylammonium bis-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)2) where there is an order-disorder transition at ∼220 K. This work focuses on novel neutron scattering techniques to understand the motion of the TEA cations at this transition and explain why we see the dielectric behaviour and possible ferroelectricity within this type of system. We show that the motion of the methyl groups of the TEA cation is spatially restricted below 220 K, whereas above the dielectric anomaly at 220 K, they are free to re-orientate, which ultimately leads to some rich behaviour that could be further exploited. Lastly, we also study the dynamics at this transition using a variety of additional techniques, helping to provide a consistent picture of the motions of the cations.

No evidence for room temperature ferromagnetism in the high temperature metal-organic material: Ni2TCNQ.

The search for ferromagnetic organic-based compounds has been a particular challenge to both chemists and physicists over the past few decades. The synthesis of the Ni2A, where A is an organic acceptor; tetracyanoethene (TCNE), 3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) or 7,7,8,8-tetracyanoquinodimethane (TCNQ) (Jain et al 2007 Nature 445 291), was reported to be a great advancement with claims that the ferromagnetism persisted to well above room temperature. There were, however some substantial flaws in the methodology associated with the synthesis and physical characterisation. Our work solely studies the Ni2TCNQ compound where we find no evidence for the existence of inherent ferromagnetism within the material that was reported in the original paper. Instead, we find that the magnetism is due to superparamagnetic nickel nanoparticles embedded in an amorphous matrix. It is hoped that our work will also show that one must be careful when using Ni(COD)2 as a precursor in the synthesis of magnetic materials and that the usefulness of the reported synthetic method is extremely limited.

Magnetic ordering of defects in a molecular spin-Peierls system.

With interest in charge transfer compounds growing steadily, it is important to understand all aspects of the underlying physics of these systems, including the properties of the defects and interfaces that are universally present in actual experimental systems. For the study of these defects and their interactions a spin-Peierls (SP) system provides a useful testing ground. This work presents an investigation within the SP phase of potassium TCNQF4 where anomalous features are observed in both the magnetic susceptibility and ESR spectra for temperatures between 60 K and 100 K. Muon spin spectroscopy measurements confirm the presence of these anomalous magnetic features, with low temperature zero-field data exhibiting the damped oscillatory form that is a characteristic signature of static magnetic order. This ordering is most likely due to the interaction between structurally correlated magnetic defects in the system. The critical behaviour of the temperature dependent muon spin rotation frequency indicates that a 2D Ising model is applicable to the magnetic ordering of these defects. We show that these observations can be explained by a simple model in which the magnetic defects are located at stacking faults, which provide them with a 2D structural framework to constrain their interactions.

Temperature dependence of electrical transport in a pressure-sensitive nanocomposite.

Printed nanocomposites are of significant application potential in numerous technologies, such as touch-sensitive sensors and surfaces. Here, temperature dependent electrical transport measurements were undertaken on a recently developed screen-printed, multicomponent, nanocomposite ink to develop a detailed understanding of the electrical transport mechanisms. A theoretical model combining contributions from linear percolative conduction and nonlinear conduction attributed to field-assisted quantum tunneling successfully describes the temperature dependent conduction observed.

Ultrafast dynamics and computational studies on diaminodicyanoquinodimethanes (DADQs).

Three diaminodicyanoquinodimethanes, 4-(R(1)R(2)C)-1-[(NC)2C]-C6H4 (R(1),R(2) = H2N, 1; R(1) = 3,5-Me2-4-OCH4H6N-, R(2) = H2N, 2; R(1) = 3,5-Me2-4-OCH4H6N-, R(2) = 4-Me-C5H9N, 3), were investigated using carbon-13 NMR, steady-state, and ultrafast transient absorption and ultrafast fluorescence spectroscopies to unravel the unusual characteristics of this class of chromophores. Computed (GIAO)B3LYP/6-31G* data for the zwitterions 1-3 using necessary solvation (PCM) models were shown to be in excellent agreement with observed structural and carbon-13 NMR data. The ground-state geometries of 1-3 contain a cationic methine group R(1)R(2)C- twisted from the C6H4 ring and an anionic methine group (NC)2C- in plane with the C6H4 ring in solution and solid state. The (13)C chemical shifts of the peak corresponding to the methine carbon at the (NC)2C- group of 1-3 are observed at 32.5-34.7 ppm, which are some 55 ppm upfield compared with the (13)C chemical shift for the methine carbons in TCNQ, 1,4-[(NC)2C]2-C6H4. The decay of the excited state in diaminodicyanoquinodimethanes is fast and dominated by nonradiative processes on the picosecond time scale, which depends on the viscosity of the medium. The dynamics of the excited-state decay is therefore limited by conformational changes through an intramolecular twisting motion. This twisting motion is hindered by friction, which, in turn, also depends on the functional group size of the system. The dominant nonradiative pathways after excitation are due to twisted excited-state conformers according to TD-DFT computations.

Controlling nickel nanoparticle size in an organic/metal-organic matrix through the use of different solvents.

Nickel nanoparticles have been created in an organic-based matrix by the reaction of Ni(COD)2 (COD = 1,5-bis-cyclooctadiene) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4). The size of the nickel nanoparticles can be controlled by the use of different solvents and inclusion of tetrahydrofuran (THF) within the reaction to stabilise the Ni(0) atoms from the Ni(COD)2. Materials are characterised with a combination of X-ray diffraction, electron microscopy and magnetometry and it is found that samples made using a halocarbon solvent resulted in clustered bulk Ni particles (size ≤ 10 nm) with anomalously high superparamagnetic blocking temperatures. Using an isocyanide solvent produces smaller (size ∼ 1 nm), well dispersed particles that show little evidence of superparamagnetic blocking in the range of temperatures investigated (>2 K). In all samples there is another component which dominates the magnetic response at low temperatures and shows an interesting temperature dependent scaling behaviour when plotted as M vs. B/T which we believe is related to the organo-metallic matrix that the particles are trapped within. We propose that the enhanced blocking temperature of particles synthesised using halocarbon solvents can be attributed to inter-particle dipolar interactions and nanoparticle-matrix exchange interactions.

A multi-component nanocomposite screen-printed ink with non-linear touch sensitive electrical conductivity.

Printable electronics is an innovative area of technology with great commercial potential. Here, a screen-printed functional ink, comprising a combination of semiconducting acicular particles, electrically insulating nanoparticles and a base polymer ink, is described that exhibits pronounced pressure sensitive electrical properties for applications in sensing and touch sensitive surfaces. The combination of these components in the as-printed ink yield a complex structure and a large and reproducible touch pressure sensitive resistance range. In contrast to the case for some composite systems, the resistance changes occur down to applied pressures of 13 Pa. Current-voltage measurements at fixed pressures show monotonic non-linear behaviour, which becomes more Ohmic at higher pressures and in all cases shows some hysteresis. The physical basis for conduction, particularly in the low pressure regime, can be described in terms of field assisted quantum mechanical tunnelling.