In this article, we review a dynamic covalent gel system developed as a high temperature well construction fluid. The key gel/fluid phase changes and related materials properties are addressable via the constitutional and coordination dynamics of the equilibrium and non-equilibrium molecular species comprising the material. The interplay between these species and external stimuli leads to material adaptability. Specifically, the introduction of metal ions into a non-equilibrium hemiaminal gel reverts this phase into a non-equilibrium liquid. When heated, this liquid transforms itself catalytically into the thermodynamically favoured closed-ring polyhexahydrotriazine (PHT) gel product. The temperature stability of different PHT gel formulations is evaluated as a function of the inclusion of various salts. It is possible to revert this thermodynamic PHT gel back into a liquid. This pH dependent transformation depends on the R groups linking the hexahydrotriazines (HTs) to one another. While polyethylene glycol (PEG) based PHT gels revert to liquids with water and mild protonation conditions, in comparison, polypropylene glycol (PPG) based gels require stronger acid conditions with heat, or a different more nucleophilically driven ring-opening mechanism by, for example, phosphines. The covalent dynamic chemistry in this chemical system gives way to many possible applications in addition to the high temperature solution-gelation (sol-gels) for which it has been primarily designed.

Three new [Ru(bpy)2X]+ complex ions, where bpy represents bipyridyl ligand and X denotes pyridyl diazolate or pyrazinyl diazolate coordination site, have been computationally designed and synthesized as pH-sensitive molecules. The choice of pyridyl and pyrazinyl moieties allows for the nitrogen content to vary, whereas the influence of the protonation site is quantified by using 1,2-diazolate and 1,3-diazolate derivatives. The absorption and emission properties of the deprotonated and protonated complex ions were characterized by UV-vis and photoluminescence spectroscopy as well as by time-dependent density functional theory. Protonation causes (1) a strong blue shift in the lowest energy 3MLCT → S0 emission wavelengths, (2) a substantial increase in the emission intensity, and (3) a change in the character of the corresponding 3MLCT emitting states. The blue shift in the emission wavelength becomes less pronounced when the nitrogen content in the X-ligand increases and when going from 1,2- to 1,3-diazolate derivatives. The contrast in the emission intensity of the protonated/deprotonated forms is the highest for the complex ion, containing a 2-pyridyl derivative of the 1,2-diazolate. The complex ions are suggested as potential pH-responsive materials based on change in the color and intensity of the emitted radiation. The broad impact of the research demonstrates that the modification of the nitrogen content and position within the protonable ligands is an effective approach for modulation of the pH-optosensing properties of Ru-polypyridyl complexes.

In the current manuscript we discuss the response of dynamic metallogels that display reversion to the liquid state when exposed to phosphines. The metallogels are formed through the condensation of formaldehyde and poly(alkyloxide) amines in polar aprotic solvents. The gel formation can be catalyzed with trivalent metals (Al(III and Fe(III)) with concomitant enhanced dynamism (gelation/degelation). When various phosphines are introduced, the metallogel is irreversibly liquefied. This process adds a new vector for controlling the bulk properties of this class of materials. Here, we explore the mechanism in detail for the reaction of tris(carboxyethyl) phosphine with N, N, N-triethoxylethyl-1,3,5-hexahydro-1,3,5-triazine (HEHT, 1) a stable derivative of the active hexahydrotriazine (HT) core in dimethylformamide in the presence or absence of Al(III). Additionally, density functional theory is used on the model N, N, N-trimethyl system (MHT, 2) to estimate reaction parameters and predict nuclear magnetic resonance spectra.

Excited state design is an efficient approach toward new applications in molecular electronics spanning solar cells, artificial photosynthesis and biomedical diagnostics. Ruthenium-(II)-polypiridyl based complexes are an example of molecular building blocks with tunable single and dual wavelength emission that can be controlled by excited state engineering via selective ligand modification. Here we investigate three new heteroleptic [Ru(bpy)(2)X](+) complex ions, where X represents pyridinyl or pyrazinyl derivatives of diazolates, providing tunable emission in the visible and infrared region. The dual emission is shown to arise from the presence of two excited states consisting of a triplet metal-to-ligand charge transfer state localized on a bipyridine ligand, (MLCT)-M-3 (bpy), and a state that either is entirely localized on the X ligand or is partially delocalized also spanning part of the bipyridine ligands, (MLCT)-M-3(X). By a suitable choice of the X ligand, emission from (MLCT)-M-3(bpy) and (MLCT)-M-3 (X) states can be rationally varied between 743 and 865 nm and from 555 to 679 nm, respectively. An increase in the nitrogen content of the six membered ring of the X ligand results in a blue shift of the (MLCT)-M-3(bpy) emission but a red shift for the (MLCT)-M-3 (X) emission. The wavelength difference between (MLCT)-M-3(bpy) and (MLCT)-M-3 (X) emissions can be tuned from 84 to 310 nm and is proportional to the difference in LUMO energies (reduction potentials) of the isolated ligands. Our study provides key information toward new routes for the design of optically active dual wavelength molecular emitters.

The present book chapter is devoted to metal (M) homoleptic complexes with pyridyl–triazolate ligands (L) and their importance for the molecular design of optical and conducting materials. It summarizes our recently published, as well as new DFT/TD-DFT results on the role of the ligand, metal and oxidation state for tuning of molecular and supramolecular properties in ML 2 complexes. The intermolecular interactions are taken into account by comparison between monomer (ML 2 ) and dimer organometallic complexes (ML 2 … ML 2 ). The ligand effect is demonstrated in Pt(II)-complexes by modifying the electronic structure of the pyridyl-triazolate ligands. The role of the metal is estimated in the case of Pd(II) and Pt(II)-complexes with the same pyridyl-triazolate ligand. The effect of the oxidation state is revealed by simulations on Pt(II)-dimers in mixed valence [ML 2 … ML 2 ] +• and doubly oxidized [ML 2 … ML 2 ] 2+ states. The obtained structure-property relationships are interpreted with respect to available experimental data and are used for the formulation of molecular design strategies.

Dynamic polymers assembled through hemiaminal and aminal functionalities reversibly fragment upon binding to trivalent metals. Gels produced with these dynamic polymers are broken down to liquids after the addition of metal salts. Nuclear magnetic resonance spectroscopy studies and density functional theory calculations of intermediates reveal that the presence of these metals causes shifts in the energetic landscape of the intermediates in the condensation pathway to render stable nonequilibrium products. These species remain stable in the liquid phase at room temperature but convert to gels upon heating. With thermal activation, the fragmented ligands transform catalytically into closed-ring hexahydrotriazine products, which are macroscopically observable as new gels with distinct physical properties. The interplay between equilibrium and nonequilibrium gels and liquids and the ligands responsible for these transformations has been observed rheologically, giving controlled gel times dictated by the thermodynamics and kinetics of the system. This constitutionally dynamic macromolecular system offers the possibility of harnessing an equilibrium/nonequilibrium system in tandem with its inherent self-healing properties and triggered release functionality.

Electron conjugation stabilizes unsaturated systems and diminishes the differences among bond distances. Experimentally, Kistiakowsky and coworkers first measured and noticed the difference between the hydrogenation heats of carbon-carbon double bonds in conjugated systems. For instance, the hydrogenation heat of butadiene is 57.1 kcal mol-1, which is less than two times that of the hydrogenation heat of 1-butene (30.3 kcal mol-1), and the difference (3.5 kcal mol-1) is the extra stabilization due to the resonance between two double bonds in the former, and is referred to as the experimental resonance energy. Following Kistiakowsky's definition, Rogers et al. studied the stepwise hydrogenation of 1,3-butadiyne and concluded that there is no conjugation stabilization in this molecule. This claim received objections instantly, but Rogers and coworkers further showed the destabilizing conjugation in 2,3-butanedione and cyanogen. Within resonance theory, the conjugation energy is derived "by subtracting the actual energy of the molecule in question from that of the most stable contributing structure." The notable difference between the experimental and theoretical resonance energies lies in that the former needs other real reference molecules while the latter does not. Here we propose and validate a new concept, intramolecular multi-bond strain, which refers to the repulsion among π bonds. The π-π repulsion, which is contributed to by both Pauli exchange and electrostatic interaction, is quantified with the B4H2 model system (16.9 kcal mol-1), and is compared with the σ-σ repulsion in B2H4 (7.7 kcal mol-1). The significance of the π-π repulsion can be demonstrated by the much longer carbon-nitrogen bond in nitrobenzene (1.486 Å) than in aniline (1.407 Å), the very long and weak nitrogen-nitrogen bond (1.756 Å) in dinitrogen tetroxide, and the instability of long polyynes. This new concept successfully reconciles the discrepancy between experimental and theoretical conjugation energies. However, we maintain that by definition, electron conjugation must be stabilizing.

DFT/TD-DFT investigation has been performed on pyridyl triazolatoplatinum(II) complexes with a systematic variation of the donor/acceptor properties of the ligand in order to illuminate its effect on the metallophilic intermolecular interaction in ground and excited states. The X electronic properties of the pyridyl triazolate ligand were modified by the pyridine substituent: N(CH3)(2), H, CHO, or CHC(CN)(2). The simulations reveal that the donor/ acceptor strength of the substituent has a strong impact on the metallophilic interaction in the excited state and affects the emission properties at the supramolecular level. The theoretically derived structure property relationships are corroborated by experimental data. Finally, it is proposed that the modification of the pi-electronic character of the substituent (ligand field) can be applied in the molecular design of smart luminescent materials with light -driven metallophilic interactions.

The fabrication of highly functional materials for practical devices requires a deep understanding of the association between morphological and structural properties and applications. A controlled hydrothermal method to produce single crystal ZnO hexagonal nanodisks, nanorings, and nanoroses using a mixed solution of zinc sulfate (ZnSO4) and hexamethylenetetramine (HMTA) without the need of catalysts, substrates, or templates at low temperature (75 °C) is introduced. Metal-semiconductor-metal (MSM) ultraviolet (UV) detectors were fabricated based on individual and multiple single-crystal zinc oxide (ZnO) hexagonal nanodisks. High quality single crystal individual nanodisk devices were fabricated with inkjet-printed silver electrodes. The detectors fabricated show record photoresponsivity (3300 A/W) and external quantum efficiency (1.2 × 10(4)), which we attribute to the absence of grain boundaries in the single crystal ZnO nanodisk and the polarity of its exposed surface.

The structural weights of the canonical resonance contributors used in the Two-state valence-bond charge-transfer model, neutral (N, R1) and ionic (VB-CT, R2), to the ground states and excited states of a series of linear dipolar intramolecular charge-transfer chromophores containing a buta-1,3-dien-1,4-diyl bridge have been computed by using the block-localized wavefunction (BLW) method at the B3LYP/6-311+G(d) level to provide the first quantitative assessment of this simple model. Ground- and excited-state analysis reveals surprisingly low ground-state structural weights for the VB-CT resonance form using either this Two-state model or an expanded Ten-state model. The VB-CT state is found to be more prominent in the excited state. Individual resonance forms were structurally optimized to understand the origins of the bond length alternation (BLA) of the bridging unit. Using a Wheland energy-based weighting scheme, the weighted average of the optimized bond lengths with the Two-state model was unable to reproduce the BLA features with values 0.04 to 0.02 Å too large compared to the fully delocalized (FD) structure (BLW: ca. -0.13 to -0.07 Å, FD: ca. -0.09 to -0.05 Å). Instead, an expanded Ten-state model fit the BLA values of the FD structure to within only 0.001 Å of FD.

The photoluminescence spectra of a series of 5-substituted pyridyl-1,2,3-triazolato Pt(II) homoleptic complexes show weak emission tunability (ranging from λ=397-408 nm) in dilute (10(-6) M) ethanolic solutions at the monomer level and strong tunability in concentrated solutions (10(-4) M) and thin films (ranging from λ=487-625 nm) from dimeric excited states (excimers). The results of density functional calculations (PBE0) attribute this "turn-on" sensitivity and intensity in the excimer to strong Pt-Pt metallophilic interactions and a change in the excited-state character from singlet metal-to-ligand charge transfer ((1)MLCT) to singlet metal-metal-to-ligand charge transfer ((1)MMLCT) emissions in agreement with lifetime measurements.

X-ray structure determinations on four Diels-Alder adducts derived from the reactions of cyano- and ester-substituted alkenes with anthracene and 9,10-dimethylanthracene have shown the bonds formed in the adduction to be particularly long. Their lengths range from 1.58 to 1.62 Å, some of the longest known for Diels-Alder adducts. Formation of the four adducts is detectably reversible at ambient temperature and is associated with free energies of reaction ranging from -2.5 to -40.6 kJ  mol(-1). The solution equilibria have been experimentally characterised by NMR spectroscopy. Density-functional-theory calculations at the MPW1K/6-31+G(d,p) level with PCM solvation agree with experiment with average errors of 6 kJ  mol(-1) in free energies of reaction and structural agreement in adduct bond lengths of 0.013 Å. To understand more fully the cause of the reversibility and its relationship to the long adduct bond lengths, natural-bond-orbital (NBO) analysis was applied to quantify donor-acceptor interactions within the molecules. Both electron donation into the σ*-anti-bonding orbital of the adduct bond and electron withdrawal from the σ-bonding orbital are found to be responsible for this bond elongation.

Transparent, highly percolated networks of regio-regular poly(3-hexylthiophene) (rr-P3HT)-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) are deposited, and the charge transfer processes of these nanohybrids are studied using spectroscopic and electrical measurements. The data disclose hole doping of s-SWNTs by the polymer, challenging the prevalent electron-doping hypothesis. Through controlled fabrication, high- to low-density nanohybrid networks are achieved, with low-density hybrid carbon nanotube networks tested as hole transport layers (HTLs) for bulk heterojunction (BHJ) organic photovoltaics (OPV). OPVs incorporating these rr-P3HT/s-SWNT networks as the HTL demonstrate the best large area (70 mm(2)) carbon nanotube incorporated organic solar cells to date with a power conversion efficiency of 7.6%. This signifies the strong capability of nanohybrids as an efficient hole extraction layer, and we believe that dense nanohybrid networks have the potential to replace expensive and material scarce inorganic transparent electrodes in large area electronics toward the realization of low-cost flexible electronics.

We report the synthesis and physical study of a series of 1,1-dicyano-4-[4-(diethylamino)phenyl]buta-1,3-dienes in which the number and position of additional CN substituents along the 1,1-dicyanobuta-1,3-dienyl fragment is systematically varied. While X-ray analysis provided unambiguous information about molecular geometries in the crystal, UV/Vis and electrochemical measurements, by cyclic voltammetry (CV) and rotating disk voltammetry (RDV), revealed that introduction of additional cyano groups in the C2- and C4-positions most affected the optical properties of these molecules in solution, in terms of intramolecular charge-transfer absorption energy and intensity. A comparison with structurally related chromophores indicates that the shift of the anilino donor from position 2/3 to 4 along the butadiene scaffold results in a remarkable bathochromic shift of the ICT absorption maxima, mainly due to the higher planarity in the present series. These findings are further corroborated by density functional theory calculations. Preliminary nonlinear optical (NLO) measurements confirm the promise of the new push-pull chromophores as third-order nonlinear-optical molecular materials.

Proacetylenic: An aniline-based donor–acceptor-substituted butatriene exhibits not only cumulene-like dimerization to give a [4]radialene (λmax=756 nm), but also acetylene-like (proacetylenic) reactivity towards tetracyanoethene, affording via [2+2] cycloaddition–cycloreversion a NIR-absorbing zwitterionic chromophore (λmax=825 nm). The cationic charge on the imminium-type nitrogen in the zwitterion is evidenced by host–guest complexation with a tetraphosphonate cavitand. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

We report the synthesis and physical study of a series of 1,1-dicyano-4-[4-(diethylamino)phenyl]buta-1,3-dienes in which the number and position of additional CN substituents along the 1,1-dicyanobuta-1,3-dienyl fragment is systematically varied. While X-ray analysis provided unambiguous information about molecular geometries in the crystal, UV/Vis and electrochemical measurements, by cyclic voltammetry (CV) and rotating disk voltammetry (RDV), revealed that introduction of additional cyano groups in the C2- and C4-positions most affected the optical properties of these molecules in solution, in terms of intramolecular charge-transfer absorption energy and intensity. A comparison with structurally related chromophores indicates that the shift of the anilino donor from position 2/3 to 4 along the butadiene scaffold results in a remarkable bathochromic shift of the ICT absorption maxima, mainly due to the higher planarity in the present series. These findings are further corroborated by density functional theory calculations. Preliminary nonlinear optical (NLO) measurements confirm the promise of the new push-pull chromophores as third-order nonlinear-optical molecular materials.