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

Delocalization-to-Localization Charge Transition in Diferrocenyl-Oligothienylene-Vinylene Molecular Wires as a Function of the Size by Raman Spectroscopy

In going from short to large size thienylene-vinylene diferrocenyl cations, the transition from a charge delocalized to a localized state is addressed by resonance Raman spectroscopy and supported by theoretical model chemistry. The shorter members, dimer and tetramer, display conjugated structures near the cyanine limit of bond length equalization as a result of the strong interferrocene charge resonance, producing a full charge <i>delocalized</i> mixed valence system. In the longest octamer, charge resonance vanishes and the cation is <i>localized</i> at the bridge center (the mixed valence property disappears). The hexamer is at the <i>delocalized</i>-to-<i>localized</i> turning point. Solvent and variable-temperature Raman measurements highlight this borderline property. A detailed structure–property correlation of bond length alternation data and Raman frequencies is proposed to account for the whole set of spectroscopic properties, with emphasis on the changes observed with the size of the molecular wire

EDOT-Based Copolymers with Pendant Anthraquinone Units: Analysis of Their Optoelectronic Properties within the Double-Cable Context

Here we present the synthesis and optical and structural characterization of a new series of alternating donor–acceptor copolymers based on 3,4-ethylenedioxythiophene (EDOT). The donor consists of phenylene, alkylated fluorene, or diethynyl-1,4-phenylene as the aryl units copolymerized with the EDOT group with and without a 9,10-anthraquinone (AQ) side chain as the acceptor group, namely, as <b>PEX</b>-<b>AQ</b> and <b>PEX</b>, respectively. The changes in the electronic absorption/emission spectra and redox properties are analyzed in detail as a function of (i) the composition of the copolymers and (ii) the inclusion of the pendant 9,10-anthraquinone. Density functional theory calculations are used to interpret the experimental results. Special emphasis is placed on the flexibility of the phenylene-based copolymer spine which favors the tendency toward aggregation and confers interesting solvathocromic, thermochromic, and sonochromic properties when applying external stimuli. This is explained by means of an order–disorder conformational transformation (driven by π–π stacking) in solution upon ultrasonication, temperature changes, and the nature of the solvents. A comparison of the transient triplet–triplet absorption spectra of the phenylene-based <b>PEB</b> copolymer in the unaggregated and aggregated forms reveals the formation of accessible triplet excited states whose nature is discussed in connection with the molecular flexibility of the copolymer backbone. These studies highlight the relevance of the precise control of the polymer architecture (donor–acceptor strategy in <b>PEX</b> versus double-cable concept in <b>PEX</b>-<b>AQ</b>) for suitable tunability of the optical and electronic properties of processable conjugated polymers for organic electronics

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