Aggregation-Induced Enhanced Emission (AIEE) from <i>N</i>,<i>N</i>‑Octyl-7,7′-diazaisoindigo-Based Organogel

A new kind of low molecular-mass organic gelator (LMOG) π-electron-deficient <i>N</i>,<i>N</i>-octyl-7,7′-diazaisoindigo (<b>1</b>) with aggregation-induced enhanced emission (AIEE) phenomenon is described. This organogel is capable of self-assembling through intermolecular H-bonding and π–π interactions between diazaisoindigo molecules. Its rheological properties, X-ray diffraction pattern, optical properties and theoretical calculations were investigated. The AIEE effect is exhibited in fluorescence during the formation of the supramolecular organogel, which persisted in the xerogel state, and the spectral red-shifts suggest the formation of <i>J</i>-type aggregates during the gelation process via π–π interactions in microbelts or 3D networks. Fluorescence lifetime and quantum yield significantly increase from dilute solution to the aggregate state. From a theoretical perspective, the effect of the aggregation of <b>1</b> on the photophysical properties was also studied by means of the density functional theory (DFT). In this sense, the lowest energy electronic transitions were calculated for both the single molecule and different size aggregates in order to predict spectral shifts. In addition, the geometry and molecular properties of the excited state were analyzed in different material states

Dispersion Overwhelms Charge Transfer in Determining the Magnitude of the First Hyperpolarizability in Triindole Octupoles

A comprehensive series of systematically functionalized <i>C</i>₃-symmetric hexakis (<i>para</i>-substituted) triindoles has been studied for its linear and second-order nonlinear optical properties. The carbazole-derived triindole central core is electron rich and electron donating. The peripheric substitution pattern, resulting in a donor–donor or a donor–acceptor charge transfer, is reflected in the UV–vis absorption spectra, where a lower energy charge-transfer band is observed for the donor–acceptor pattern. On-resonance all compounds exhibit a strongly enhanced second-order nonlinear optical response, critically dependent on the particular wavelength but showing no clear correlation with the charge-transfer character imparted by the peripheral substituents. Nonetheless, extremely large values are obtained: we measured the highest value ever reported for octupolar compounds in transparent conditions on-resonance. Off-resonance significantly smaller values are found, which are very similar for all compounds and show no correlation with the charge-transfer character as well. Both observations have been unambiguously confirmed by (linear and nonlinear) spectroelectrochemistry on a donor–donor structure, effectively transforming this to an acceptor–donor structure (by oxidizing the donor triindole core to an electron-accepting triindole-based cation radical). The strong wavelength dependence of the first hyperpolarizability values around resonance is clearly shown to be overwhelmed by dispersion effects and not to be determined by the charge-transfer pattern in these octupolar materials. This finding provides insight for independent tuning of the linear absorptive properties, determined by the charge-transfer pattern, and the second-order nonlinear polarizability, not determined by this pattern but strongly dispersive

Symmetry Lowering in Triindoles: Impact on the Electronic and Photophysical Properties

The electronic and photophysical properties of 6,11-dihydro-5<i>H</i>-diindolo­[2,3-<i>a</i>:2′,3′-<i>c</i>]­carbazole, an asymmetric cyclic dehydrotrimer of indole, have been explored and compared to its symmetric analogue, 10,15-dihydro-5<i>H</i>-diindolo­[3,2-<i>a</i>:3′,2′-<i>c</i>]­carbazole (triindole), a well-known high hole mobility semiconductor. To this purpose, we use a joint experimental and theoretical approach that combines absorption and emission spectroscopies, cyclic voltammetry, and spectroelectrochemistry with DFT calculations. Lowering the symmetry of the triindole platform causes a red-shift of the absorption edge and emission maxima and improved the fluorescence quantum yield. Cyclic voltammetry and spectroelectrochemistry reveal the reversible nature of the two observable oxidation processes in the alkylated asymmetric triindoles together with an increase in the stabillity of their oxidized species. On the other hand, the insertion of alkyl groups on the nitrogen atoms results in a further fluorescence enhancement although larger reorganization energies are found. DFT and time-dependent (TD-DFT) calculations successfully support the experimental data and aid in the understanding of the tuning of the physicochemical properties of the triindole platform upon symmetry lowering toward their incorporation in electronic devices

Simple Donor–Acceptor Luminogen Based on an Azaindole Derivative as Solid-State Emitter for Organic Light-Emitting Devices

A new luminescent azaindole derivative has been designed and synthesized to be implemented in a light-emitting device. The molecular geometry and frontier orbital energy have been investigated by using density functional theory. This newly synthesized molecule emits strongly both in solution and solid state. The fluorescence emission in THF upon addition of an increasing amount of H₂O has been measured also, displaying an increase of the fluorescence signal which is associated with an aggregation-induced emission phenomenon. Double-layer light-emitting devices including an active layer of the novel emissive material were successfully fabricated. The red-shift of the electroluminescence spectra with respect to the photoluminiscence emission was attributed to the presence of different types of complexes in the emissive material (EM) layer, which are preferentially formed between the nearest-neighbor hole–electron pairs of the electron injection and the EM layers (electroplex)

High Hole Mobility in Triindole-Based Columnar phases: Removing the Bottleneck of Homogeneous Macroscopic Orientation

We report the synthesis, mesomorphic behavior, and mobility values of a series of highly ordered <i>N</i>-substituted triindole-based columnar liquid crystals. Shortening the length of <i>N</i>-alkylic substituents from <i>N</i>-dodecyl to <i>N</i>-methyl chains results in a drastic approach of the disks within the columns and in an impressive increase in charge carrier mobility. An study of aggregation in solution provide insights into the intermolecular forces responsible of the reduction of the intrastack distance as the size of the <i>N</i>-alkyl chains is decreased and offer evidence of stabilization of the columns by the contribution of cooperative CH−π interactions. The materials presented here exhibit mobility values, even in totally misaligned columnar phases, that may compete with those of the best polycrystalline organic semiconductors, without the need of costly vacuum evaporation processes

Synthesis and Properties of New Dimeric η²‑Diyne Complexes of Cobalt Linked through an Azobenzene Ligand

The reaction between 4,4′-diiodoazobenzene (<b>0</b>) with excess trimethylsilylacetylene or dec-1-yne in the presence of catalytic amounts of PdCl₂(PPh₃)₂ and CuI, Sonogashira coupling conditions, gave rise to the formation of 4,4′-bis­(trimethylsilylethynyl)­azobenzene (<b>1</b>) and 4,4′-bis­(dec-1-ynyl)­azobenzene (<b>2</b>), in good yield, in addition to icosa-9,11-diyne (<b>3</b>) as byproduct in the last case. The analogous reactions from <b>0</b> and the alkynyl-cobalt complexes Co₂(CO)₄(μ-L-L)­(μ₂-η²-SiMe₃C₂)]­(CCH) (L-L = dppm, dmpm) afforded <b>12</b>–<b>14</b>. Complexes <b>4</b>–<b>7</b> and <b>15</b> have been obtained by direct reaction between Co₂(CO)₈ and the organic ligands <b>1</b>–<b>3</b>. In order to increase the stabilization of Co–Co bonds, diphenylphosphinomethane-substituted alkyne-carbonyl complexes <b>8</b>–<b>11</b> were prepared by substitution reaction of carbonyl ligands in the presence of Me₃NO at the Co₂(CO)₆ units from <b>4</b>–<b>7</b>. All products were characterized by analytical and spectroscopic data (IR, ¹H, ¹³C, and ³¹P NMR, and MS). Crystals of <b>9</b> suitable for single-crystal X-ray diffraction were grown, and the molecular structure of this compound is discussed. In this paper we report a comparative electrochemical study of these complexes by means of cyclic and square-wave voltammetry techniques, the <i>trans–cis</i> photoisomerization study of azobenzene compounds, and the thermotropic liquid-crystalline behavior

Nonlinear Optical Thin Film Device from a Chiral Octopolar Phenylacetylene Liquid Crystal

A set of chiral discotic phenylacetylenes have been synthesized by 3-fold Sonogashira coupling between different ethynylbenzenes and triiodobenzenes. The resultant bulk materials are fully characterized by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction. The octopolar nature of the target compounds is studied by UV–vis absorption spectroscopy and hyper-Raleigh scattering in solution. Optimization of the donor–acceptor substitution yields both high hyperpolarizability values and appreciable mesomorphic properties. A simple thin film device for second harmonic generation has been prepared from the nitro-substituted liquid crystalline derivative

Mobility versus Alignment of a Semiconducting π‑Extended Discotic Liquid-Crystalline Triindole

The p-type semiconducting properties of a triphenylene-fused triindole mesogen, have been studied by applying two complementary methods which have different alignment requirements. The attachment of only three flexible alkyl chains to the nitrogen atoms of this π-extended core is sufficient to induce columnar mesomorphism. High hole mobility values (0.65 cm² V^–1 s^–1) have been estimated by space-charge limited current (SCLC) measurements in a diode-like structure which are easily prepared from the melt, rendering this material a good candidate for OPVs and OLEDs devices. The mobility predicted theoretically via a hole-hopping mechanism is in very good agreement with the experimental values determined at the SCLC regime. On the other hand the hole mobility determined on solution processed thin film transistors (OFETs) is significantly lower, which can be rationalized by the high tendency of these large molecules to align on surfaces with their extended π-conjugated core parallel to the substrate as demonstrated by SERS. Despite the differences obtained with the two methods, the acceptable performance found on OFETs fabricated by simple drop-casting processing of such an enlarged aromatic core is remarkable and suggests facile hopping between neighboring molecular columns owing to the large conducting/isolating ratio found in this discotic compound