Non-covalent bridging of bithiophenes through chalcogen bonding grips

In this work, chalcogen-functionalized dithiophenes, equipped on both extremities with chalcogen-bonding recognition heterocycles, have been prepared following two synthetic pathways. The insertion of the chalcogenazolo[5,4-β ]pyridine allows the control of the organization at the solid state. X-Ray diffraction analysis of the single crystals, showed that the Te-doped derivatives give the most persistant assemblies, with the molecules arranging at solid-state in wire-like polymeric structures through Te … N interactions. As expected, the introduction of the Se and Te atoms, dramatically decreases the emission properties, with the Te-bearing congeners being virtually non emissive.


Introduction
During the last decades, academic research experienced a strong surge of interest in the field of molecular 1 and polymeric 2 organic semiconducting materials to engineer flexible and printed electronics. 3The advantage of using organic materials rather than inorganic counterparts resides in the fact that one can tailor the molecular HOMO-LUMO gap by organic functionalization of the piconjugated scaffold.Changing the length 4 and the nature 5 of the carbon-based framework, as well as altering its composition by replacing selected carbon atoms with isostructural analogues (known as heteroatom-doping), 6 embody some of the main approaches used to tailor the photophysical properties of molecular materials.Moreover, depending on the structural modification, different molecular arrangements can be obtained in the solid state, and materials with poor organisation could give devices with reduced performances. 7Therefore, growing attention has been given to the use of non-covalent interactions to master molecular arrangements at the solid state.
Along with the fine tuning of the photophysical properties, 8 the use of heavy chalcogen atoms (Se and Te) in small molecules could allow the control of the molecular organization through chalcogenbonding interactions (EBIs). 9The chalcogen bond belongs to the family of Secondary Bonding Interactions (SBIs). 10A SBI include electrostatic (described as σ-hole), orbital mixing and van der Waals contributions.The orbital contribution is typically described as n 2 (Y)→σ*(E-X) donation (X-E•••Y), where non-bonding electrons of the electron-donating atom Y interact with the empty antibonding σ * X−E orbital located on E atom (with E being a central polarizable atom and X its substituent).EBIs have found significant application in crystal engineering, 11 where macrocyclic 12 and wire-like 13 assemblies could be easily form.Our group provided its contribution to the field when we prepared Te-and Sebearing molecular modules able to form supramolecular polymers at the solid state through single Y•••E interactions. 14Aiming at improving the persistency of the recognition, we have developed the chalcogenazolo [5,4-β]pyridine (CGP) scaffold, 15 providing an important contribution to the family of supramolecular synthons acting as both chalcogen bond donors and acceptors. 16This ambivalent nature allows to observe non-covalent dimerization at the solid state, through the formation of double EBIs arranged into a six-membered ring. 17In view of integrating the EBI as tool for controlling the solid-state organization of conducting oligothiophenes in organic electronics, 18 in this work we describe our initial efforts to master the self-assembly of the shortest units, namely the dithiophene.In particular, we report on the functionalisation of dithienyl modules that, terminating with chalcogenazole moieties, establish EBIs at the solid state, forming controlled assemblies (Scheme 1).In analogy with results previously described, 14, 18h one would anticipate that the S and N atoms of the thiophene and azole rings engage in intramolecular EBIs, forcing the chalcogenazolo and thiophenyl units to adopt a locked flat conformation.18h It is also expected that the presence of a heavy chalcogen atom should dramatically influence the photophysical properties of the dithienyl derivatives.At the retro-synthetic planning level, we considered the insertion of Please do not adjust margins the chalcogen-containing moiety in two steps, the dehydrative cyclization reaction to chalcogenazoles, and the amide disconnection.

Synthetic procedures
The synthesis started with the preparation of 2,2'-bithiophene 1.While the Suzuki-Miyaura coupling reaction between 2bromothiophene with thiophen-2-ylboronic acid led to 1 in 40% of yield, Kumada cross-coupling of thiophen-2-ylmagnesium bromide with 2-bromothiophene in the presence of NiCl2(dppp) gave the desired product in 71% of yield.Molecule 1 was then capped with carboxylic acid groups by treating the corresponding solution in Et2O with n-BuLi at -78 °C, followed by the addition of gaseous CO2 (Scheme 2).Following synthetic routes from the literature, 19 we prepared dichalcogenide 4Se and 4Te from 2-bromoaniline 3.In parallel, pyridyl analogues 6Se and 6Te were prepared in low yields starting from 3-amino-2-bromopyridine (Scheme 3). 15The syntheses of derivatives 7X-E were accomplished by reductive cleavage of the dichalcogenide bond in the presence of NaBH4, followed by the alkylation of the chalcogenide intermediate with MeI in good to excellent yields (Scheme 3).Dicarboxamide derivatives 8X-E were obtained through amide bond formation upon reaction with 2Cl, which was obtained from 2OH after treatment with SOCl2.
Dehydrative cyclization to give bis-selenazoles and -tellurazoles 9X-E was performed using POCl3 and NEt3 as a base, according to the synthetic protocol developed in our group (Scheme 3a-f). 14It has to be noted that difficulties in purifying bisamide 8C-Te led us to perform the final step with the crude material.Precisely, the intermediate monocarboxamides were identified in all crude mixtures from the amidation reactions, suggesting that the poor solubility of the intermediates could prevent the full conversion into final products 9X-E.Consequently, we attempted a different synthetic path, using Te-containing amines 7X-Te reacting with the acyl chloride of the commercially available 5bromothiophene-2-carboxylic acid, freshly generated after treatment with SOCl2.Amides 10X-Te were converted into the corresponding cyclized derivatives 11X-Te, following the same protocol used for 8X-E.Finally, a one-pot borylation/Suzuki-Miyaura coupling (BSC) was performed, converting brominated compounds 11X-Te in situ into the boronate derivatives, by treatment with B2Pin2 and KOAc in the presence of the Pd catalyst.Although we proved the chemical compatibility of the tellurazole derivatives with the Pd-catalysed reaction conditions, the overall yields for products 9X-Te did not overcome those reached for the direct bis-functionalization of 2. All unknown molecules were characterized by 1 H-NMR spectroscopy, IR, m.p. and HR-mass spectroscopy (see Supporting Information).
Unfortunately, 13 C-NMR spectra could not be reported for the entire set of compounds because of poor solubility, likely due to their tendency to aggregate in solution through EBIs.

Solid-state arrangement
The strong tendency for aggregation, limited the growth of crystals suitable for X-Ray diffraction analysis.Despite the numerous attempts, amorphous powders were obtained for both pyridylbased molecules 9N-Se and 9N-Te.We obtained X-ray crystal structures for benzochalcogenazoles 9C-Se and 9C-Te.In addition, the solid-state arrangement of monofunctionalized thiophenes 11X-Te was also unravelled from crystals obtained by slow evaporation of a CHCl3 solution.Figures 1 and 2 display the ORTEP representation and the crystal packing for each molecule.
Crystals of the solvate of 9C-Se were obtained by slow evaporation (Figures 1a-1d).As predicted (see above), the Se atoms adopt a syntype conformation due to the formation of intramolecular S•••N EBIs.Interestingly, in this crystal packing it is the solvent molecules that, bridging neighbouring molecular unit of 9C-Se through concurring hydrogen and chalcogen bonds (Figure 1b As far as molecule 9 is concerned, very small crystals were obtained by slow cooling from a hot p-xylene solution (Figures 1e-i).Unlike crystals for 9C-Se , the solvent is not included in the solid, and the molecular arrangement is likely to be predominately driven by Te with a different role in the EBI.Namely, one takes part in an EBI as a chalcogen bonding donor (having the Te atom implicated), whereas the other acts as EBI acceptor through the N atom (Figure 1g).This peculiar recognition pattern forces the molecules to adopt a nonplanar arrangement, and a braided double wire-like architecture is formed (Figure 1i).
Moving to the crystals of monomeric derivatives 11X-Te, one can notice the presence of shorter chalcogen bonds, likely due to the presence of the electron-withdrawing Br atom.The Br atom also actively takes part to the development of the supramolecular assembly through the formation of halogen-bonding interactions.Both crystals grew by slow evaporation of a CHCl3 solution, but the data quality of 11C-Te is poorer than that obtained with tellurazolopyridine analogue 11N-Te; the structure is, indeed, disordered, thus the major component of the disorder has been used for the following discussion (for the actual crystal structure, see SI).
In the case of 11C-Te (Figures 2a-c), two monomers form the asymmetric unit of the crystal structure, each of them interacting with a neighbouring molecule through two SBIs.One side of each molecule sees both the σ-holes of the Te atoms involved as SBI donors, whereas the opposite side takes up the nitrogen and sulfur atoms as SBI acceptors (Figure 2b

Formation of macroscopic morphologies
The formation of macroscopic morphologies for 9C-Te was also investigated by Atomic Force Microscopy (AFM) after drop-cast deposition on mica surfaces.Homogeneous coverage of the surface with 9 was observed, but a zoomed picture reveals the presence of several elongated and layered "islands", likely due to a disordered aggregation of amorphous material (Figure 3b).The same surface was then examined after solvent vapour annealing (SVA): as shown in Figure 3c, the morphology outcome is drastically changed, with the entire surface covered by needle-like aggregates (Figures 3d-3e).The distribution of the nanostructures is reminiscent of the braided arrangement observed in the crystal structure, having the needle-like objects disposed according to a zig-zag pattern on the surface.

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Absorption and emission spectroscopy
The effect of the Se and Te atoms on the photophysical properties of the dithiophene framework was evaluated by UV/Vis absorption and emission spectroscopy, and the key photophysical data are summarized in Table 1.In general, all compounds show low molar absorption coefficients. 19Analysing bischalcogenazoles 9C-Se and 9C-Te and bischalcogenazolopyridine derivatives 9N-Se and 9N-Te separately, batochromic shifts were noticed when passing from the Se-to the Te-bearing congeners.
A broadening of the vibrational structure becomes more evident as the mass of the chalcogen atom increases, as proven by the poorly structured band observed for 9N-Te (Figure 4). 19In terms of emission, weak emissive properties (fluorescence quantum yields: 0.03-0.08)were noticed in solution only for the Se-containing compounds at rt. Considering the presence of the heavy-atom Te atom, we conjectured that derivatives 9 and (induced by the presence of the heaviest Te atom) could possess strong phosphorescent emission. 20Unfortunately, steady state measurements demonstrated a pronounced fluorescence quenching descending from Se to Te.In parallel, no phosphorescence was detected for any of the derivatives at rt, nor in a 77 K glassy matrix. 19

Preparation of dicarboxamides 8
A two-necked flask was loaded with 102 mg (0.4 mmol) of 2 under N2.The solid was dissolved in 2.4 mL of SOCl2, then the system was stirred at reflux overnight.Once the solvent was removed under vacuum, the resulting acyl chloride derivative was dissolved in 1 mL of CHCl3 and DMAP (0.02 mmol) was added.This was followed by the addition of a solution of 0.18 mL of NEt3 (1.28 mmol) and 0.88 mmol of the relevant substrate 7X-E in 1.4 mL of CHCl3.The reaction was stirred at reflux for 24 hours, water added, and the solution extracted with CHCl3.The combined organic extracts were washed with brine, dried over Na2SO4, filtered and the solvents removed under reduced pressure.The crude materials were purified by silica gel chromatography.

Fig. 1 .Fig. 2 .
Fig. 1.On the left a) ORTEP representation of a single molecule of 9C-Se; b) evidence of SBIs and H-bonds, distances are expressed in Å; c) van der Waals (vdW) radii representation of the crystal packing according to a ribbon organization; d) vdW representation of the crystal packing with a zoom to highlight the intermolecular π-π distances (expressed in Å).Solvent of crystallization: CHCl3.Space group: P-1.On the right e) ORTEP representation of a single molecule of 9C-Te; f) crystal unit of 9C-Te with chalcogen bond highlighted, distances are expressed in Å; g) vdW representation of the crystal packing with a zoom to highlight the different chalc ogen bonds involving both the sides of the same molecule; i) vdW radii representation of the crystal packing according to a braided double wire-like organization.Solvent of crystallization: p-xylene.Space group: P 21/c

Fig. 3 .
Fig. 3. Top: AFM images of the surface prepared by drop coating of a solution of 9C-Te in CHCl3, a) AFM image of the whole surface (125 x 125 μm); b) AFM zoomed image of the surface (10 x 10 μm), showing layered islands (highlighted by black squares).Bottom: AFM images of the surface after solvent vapour annealing treatment, c) AFM image of the whole surface (125 x 125 μm); d) AFM zoomed image showing in detail one of the needle-like objects (20 x 20 μm); e) Additional AFM zoomed image showing one of the extremities of the needle-like objects (3 x 3 μm).
To a suspension of corresponding dicarboxamides 8C-Se or 8 (0.04 mmol) in 3.8 mL of 1,4-dioxane under N2, 66 μL of NEt3 (0.48 mmol) and a solution of POCl3 (0.16 mmol) in 0.2 mL of 1,4dioxane were added.The reaction was stirred at reflux overnight, diluted with CHCl3 and washed with a saturated solution of NaHCO3.The aqueous phase was extracted with CHCl3, and the combined organic extracts washed with brine, dried over MgSO4, filtered and the solvents removed under reduced pressure.The crude materials were purified by silica gel chromatography.Preparation of CGP-containing bithiophenes 9N-Se and 9N-TeTo a suspension of corresponding dicarboxamides 8N-Se or 8N-Te (0.04 mmol) in 3.8 mL of 1,4-dioxane under N2, 0.13 mL of NEt3 (0.96 mmol) and a solution of POCl3 (0.32 mmol) in 0.2 mL of 1,4dioxane were added.The reaction was stirred at reflux overnight, diluted with CHCl3 and washed with a saturated solution of NaHCO3.The aqueous phase was extracted with CHCl3, the combined organic extracts washed with brine, dried over Na2SO4, filtered and the solvents removed under reduced pressure.The crude materials were purified by silica gel chromatography.