Revisiting Hafner's Azapentalenes: The Chemistry of 1,3-Bis(dimethylamino)-2-azapentalene.

Stable azaheterocyclic derivatives of pentalene have been reported by the group of Hafner in the 1970s. However, these structures remained of low interest until recently, when they started to be investigated in the context of organic light-emitting diodes' (OLEDs') development. Herein, we revisit the synthesis of stable azapentalene derivative 1,3-bis(dimethylamino)-2-azapentalene and further explore its properties both computationally and experimentally. Beyond the reproduction and optimization of some previously reported transformations, such as formylation and amine substitution, the available scope of reactions was expanded with azo-coupling, selective halogenations, and cross-coupling reactions.

An Exploration of Substituent Effects on the Photophysical Properties of Monobenzopentalenes.

Monobenzopentalenes have received moderate attention compared to dibenzopentalenes, yet their accessibility as stable, non-symmetric structures with diverse substituents could be interesting for materials applications, including molecular photonics. Recently, monobenzopentalene was considered computationally as a potential chromophore for singlet fission (SF) photovoltaics. To advance this compound class towards photonics applications, the excited state energetics must be characterized, computationally and experimentally. In this report we synthesized a series of stable substituted monobenzopentalenes and provided the first experimental exploration of their photophysical properties. Structural and opto-electronic characterization revealed that all derivatives showed 1H NMR shifts in the olefinic region, bond length alternation in the pentalene unit, low-intensity absorptions reflecting the ground-state antiaromatic character and in turn the symmetry forbidden HOMO-to-LUMO transitions of ~2 eV and redox amphotericity. This was also supported by computed aromaticity indices (NICS, ACID, HOMA). Accordingly, substituents did not affect the fulfilment of the energetic criterion of SF, as the computed excited-state energy levels satisfied the required E(S1)/E(T1)>2 relationship. Further spectroscopic measurements revealed a concentration dependent quenching of the excited state and population of the S2 state on the nanosecond timescale, providing initial evidence for unusual photophysics and an alternative entry point for singlet fission with monobenzopentalenes.

Stable Monoareno-pentalenes with Two Olefinic Protons.

A novel class of stable monoareno-pentalenes is introduced that have an olefinic proton on each five-membered ring of the pentalene subunit. Their synthesis was accomplished via a regioselective carbopalladation cascade reaction between ortho-arylacetyleno gem-dibromoolefins and TIPS-acetylene. These molecules could be experimental probes of magnetic (anti)aromaticity effects.

Photoswitching of Local (Anti)Aromaticity in Biphenylene-Based Diarylethene Molecular Switches.

Photoinduced tuning of (anti)aromaticity and associated molecular properties is currently in the focus of attention for both tailoring photochemical reactivity and designing new materials. Here, we report on the synthesis and spectroscopic characterization of diarylethene-based molecular switches embedded in a biphenylene structure composed of rings with different levels of local (anti)aromaticity. We show that it is possible to modulate and control the (anti)aromatic character of each ring through reversible photoswitching of the aryl units of the system between open and closed forms. Remarkably, it is shown that the irreversible formation of an annulated bis(dihydro-thiopyran) side-product that hampers the photoswitching can be efficiently suppressed when the aryl core formed by thienyl groups in one switch is replaced by thiazolyl groups in another.

Unsymmetrical Thienopentalenes: Synthesis, Optoelectronic Properties, and (Anti)aromaticity Analysis.

The synthesis and properties of a series of unsymmetrical thienopentalenes are explored, including both monoareno and diareno derivatives. For the synthesis of monoareno pentalenes, a carbopalladation cascade reaction between alkynes and gem-dibromoolefins was applied. Diareno pentalene derivatives were accessed via gold-catalyzed cyclization of diynes. Thiophene was fused to pentalene in two different geometries via its 2,3 and 3,4 bonds. 2,3-Fusion resulted in increased antiaromaticity of the pentalene unit compared to the 3,4-fusion both in the monoareno and diareno framework. Monothienopentalenes that contained the destabilizing 2,3-fusion could not be isolated. For diareno derivatives, the aromatic character of the different aryl groups fused to the pentalene was not independent. Destabilizing fusion on one side resulted in alleviated aromaticity on the other side and vice versa. The synthesized molecules were characterized experimentally by 1H NMR and UV-vis spectroscopies, cyclic voltammetry, and X-ray crystallography, and their aromatic character was assessed using magnetic (NICS and ACID) and electronic indices (MCI and FLU).

Construction and Properties of Donor-Acceptor Stenhouse Adducts on Gold Surfaces.

The construction of a donor-acceptor Stenhouse adduct molecular layer on a gold surface is presented. To avoid the incompatibility of the thiol surface-binding group with the donor-acceptor polyene structure of the switch, an interfacial reaction approach was followed. Poly(dopamine)-supported gold nanoparticles on quartz slides were chosen as substrates, which was expected to facilitate both the interfacial reaction and the switching process by providing favorable steric conditions due to the curved particle surface. The reaction between the surface-bound donor half and the CF3-isoxazolone-based acceptor half was proved to be successful by X-ray photoelectron spectroscopy (XPS). However, UV-vis measurements suggested that a closed, cyclopentenone-containing structure of the switch formed on the surface irreversibly. Analysis of the wetting behavior of the surface revealed spontaneous water spreading that could be associated with conformational changes of the closed isomer.

A photoresponsive palladium complex of an azopyridyl-triazole ligand: light-controlled solubility drives catalytic activity in the Suzuki coupling reaction.

Herein, the design and synthesis of a click-derived Pd-complex merged with a photoswitchable azobenzene unit is presented. While in the trans-form of the switch the complex showed limited solubility, the photogenerated cis-form rendered the molecule soluble in polar solvents. This light-controllable solubility was exploited to affect the catalytic activity in the Suzuki coupling reaction. The effect of the substrate and catalyst concentration and light intensity on the proceeding and outcome of the reaction was studied. Dehalogenation of the aryl iodide starting material was found to be a major side reaction; however, its occurrence was dependent on the applied light intensity.

Structure-Property Relationships in Unsymmetric Bis(antiaromatics): Who Wins the Battle between Pentalene and Benzocyclobutadiene?†.

According to the currently accepted structure-property relationships, aceno-pentalenes with an angular shape (fused to the 1,2-bond of the acene) exhibit higher antiaromaticity than those with a linear shape (fused to the 2,3-bond of the acene). To explore and expand the current view, we designed and synthesized molecules where two isomeric, yet, different, 8π antiaromatic subunits, a benzocyclobutadiene (BCB) and a pentalene, are combined into, respectively, an angular and a linear topology via an unsaturated six-membered ring. The antiaromatic character of the molecules is supported experimentally by 1H NMR, UV-vis, and cyclic voltammetry measurements and X-ray crystallography. The experimental results are further confirmed by theoretical studies including the calculation of several aromaticity indices (NICS, ACID, HOMA, FLU, MCI). In the case of the angular molecule, double bond-localization within the connecting six-membered ring resulted in reduced antiaromaticity of both the BCB and pentalene subunits, while the linear structure provided a competitive situation for the two unequal [4n]π subunits. We found that in the latter case the BCB unit alleviated its unfavorable antiaromaticity more efficiently, leaving the pentalene with strong antiaromaticity. Thus, a reversed structure-antiaromaticity relationship when compared to aceno-pentalenes was achieved.

Sunlight-powered kHz rotation of a hemithioindigo-based molecular motor.

Photodriven molecular motors are able to convert light energy into directional motion and hold great promise as miniaturized powering units for future nanomachines. In the current state of the art, considerable efforts have still to be made to increase the efficiency of energy transduction and devise systems that allow operation in ambient and non-damaging conditions with high rates of directional motions. The need for ultraviolet light to induce the motion of virtually all available light-driven motors especially hampers the broad applicability of these systems. We describe here a hemithioindigo-based molecular motor, which is powered exclusively by nondestructive visible light (up to 500 nm) and rotates completely directionally with kHz frequency at 20 °C. This is the fastest directional motion of a synthetic system driven by visible light to date permitting materials and biocompatible irradiation conditions to establish similarly high speeds as natural molecular motors.

Inheritance of longevity evinces no secular trend among members of six New England families born 1650-1874.

This study investigated the historical trend in resemblance between first-degree relatives for age at death. Data from genealogies of six New England families (N = 13,656) were divided into nine 25 year birth cohorts, 1650-1874, to test the hypothesis that familial influence on human longevity has changed during the past 300 years. Heritability (h2 ) for longevity demonstrated no historical trend, whether calculated by regression of offspring's longevity on paternal, maternal, or mid-parental longevity or by intraclass correlations (t) among sibships. Ninety-five percent confidence intervals (C.I.) for h2 (additive genetic variance) were in the range 0.10-0.33 for parent-offspring regressions and 0.16-0.22 based on mean of sibship regressed on mean of parents. Based on sibship t, the 95% C.I. for the upper limit to h2 (which includes variance contributions caused by dominance interactions and common developmental environment as well as additive genetic effects) was 0.33-0.41. In this socially elite sample, the statistical contribution of first-degree relatives to age at death has varied within a historically consistent range over the past 300 years, directly implying a persistent genetic influence on longevity. The magnitude of this influence with respect to additive genetic variance, about 10-30%, may overestimate h2 because of the elite nature of the sample. Nevertheless, these results support a genetic component to lifespan even though the majority of variation in human longevity is not explained by genetic factors.