Quinoidal Diindenothienoacenes: Synthesis and Properties of New Functional Organic Materials

We report the preparation and characterization of a new class of quinoidal thienoacenes. The synthetic route is efficient, high-yielding and scalable with the potential for further functionalization. Single crystal X-ray diffraction reveals that, as size increases, the molecules pack in progressively closer 1D arrangements. The title compounds are shown to have amphoteric redox behaviour by cyclic voltammetry. The anion radicals are studied by EPR spectrometry and by computations. The electron-accepting nature, NIR absorption and the low-lying LUMO energies (ca. −4.0 eV) allude to potential use in materials applications.


X-ray Crystallography
General. Diffraction intensities for DI1T-TIPSE, DI2T, DI1T-TESE, 9 and 13 were collected at 100(2) K and for DI3T at 150(2) K on a Bruker Apex2 CCD diffractometer with a micro-focus IµS source using CuKα radiation λ= 1.54178 Å or a sealed X-ray tube with a triumph monochromator, MoKα radiation λ= 0.71073 Å (9 only). Absorption corrections were applied by SADABS. 3 Structures were solved by direct methods and Fourier techniques and refined on F 2 using full matrix least-squares procedures. All non-H atoms were refined with anisotropic thermal parameters. All H atoms were refined in calculated positions in a rigid group

Cyclic Voltammetry
All electrochemical experiments were conducted in a traditional 3-electrode geometry using a Solartron 1287 potentiostat. Electrolyte solutions (0.1 M) were prepared from HPLC-grade CH 2 Cl 2 and anhydrous Bu 4 NBF 4 , and the solutions were freeze-pump-thaw degassed (3x) prior to analysis. Cyclic voltammetry was conducted under a nitrogen atmosphere. The working electrode was a glassy carbon electrode (3-mm diameter), with a Pt-coil counter electrode and Ag wire pseudo reference. The ferrocene/ ferrocenium (Fc/Fc + ) couple was used as an internal standard following each experiment. Potential values were re-referenced to SCE using a value of 0.46 (V vs. SCE) for the Fc/Fc + couple in CH 2 Cl 2 . When necessary, potentials were re-referenced to NHE using SCE = -0.24 (V vs. NHE). LUMO and HOMO levels were approximated using SCE = -4.68 eV vs. vacuum. 5 Cyclic voltammetry experiments were conducted at sweep rates of 50 (reported), 75, 100 and 125 mV s -1 . All scan rates show quasi-reversible kinetics with no alteration of peak splitting with scan rate. E 1/2 values were calculated assuming E 1/2 ≈ E o '=(E anodic + E cathodic )/2 based on these observations for reversible couples; for irreversible couples the E o ' value is estimated as the potential at peak current. The E a,c peak splitting of the Fc/Fc + couple was similar to that of the analyte (~100 mV). The anodic peak current increases linearly with the square root of the scan rate in the range 50 to 125 mV s -1 , indicating a diffusion-controlled process. Analyte concentrations were ca. 1-5 mM.

Electronic Paramagnetic Resonance
Experimental details. An apparatus (Fig. S6) was constructed from borosilicate glass and dried in a 100 °C oven. The apparatus was then cooled to rt under nitrogen and approximately 0.05 mg of DI[n]T was collected on a melting point capillary that was open on both ends and deposited at point A. Potassium metal was added at point B and then opening C was sealed with an oxygen/natural gas torch. Vacuum was pulled (ca. 10 -6 torr) and K metal was sublimed with a Bunsen burner, resulting in a metal mirror inside D. The apparatus was then sealed at point E.
Dry THF (approx. 1 mL) from a NaK still was directly distilled through the vacuum system to A and the apparatus was sealed at point F. Controlled exposure to the potassium mirror resulted in formation of DI[n]T radical anion, from which the EPR spectra in Figs. S5-S7 were obtained.
The EPR spectra were collected on a Bruker EMX-080 spectrometer.

EPR Computational Details.
To determine the hyperfine coupling constants for the hydrogen and silicon nuclei coupled with the anion radical, the EPR spectra were simulated with the EasySpin 6 package utilizing MATLAB code. 7 DFT calculations were performed for the gas phase molecules using Gaussian09 Revision C.01 8 and the results were used to assign the HFCC and carbon spin density locations (Table S1). These computations were carried out at the UB3W91/6-311++G(2df,2pd)//UCAM-B3LYP/6-31++G(d,p) level of theory.