Expanding the Chemical Space of Tetracyanobuta‐1,3‐diene (TCBD) through a Cyano‐Diels‐Alder Reaction: Synthesis, Structure, and Physicochemical Properties of an Anthryl‐fused‐TCBD Derivative

Abstract Tetracyanobuta‐1,3‐diene (TCBD) is a powerful and versatile electron‐acceptor moiety widely used for the preparation of electroactive conjugates. While many reports addressing its electron‐accepting capability have appeared in the literature, significantly scarcer are those dealing with its chemical modification, a relevant topic which allows to broaden the chemical space of this interesting functional unit. Here, we report on the first example of a high‐yielding cyano‐Diels‐Alder (CDA) reaction between TCBD, that is, where a nitrile group acts as a dienophile, and an anthryl moiety, that is, acting as a diene. The resulting anthryl‐fused‐TCBD derivative, which structure was unambiguously identified by X‐ray diffraction, shows high thermal stability, remarkable electron‐accepting capability, and interesting electronic ground‐ and excited‐state features, as characterized by a thorough theoretical, electrochemical, and photophysical investigation. Moreover, a detailed kinetic analysis of the intramolecular CDA reaction transforming the anthryl‐TCBD‐based reactant into the anthryl‐fused‐TCBD product was carried out at different temperatures.


Materials and Methods
Chemicals and solvents were purchased from commercial suppliers (Aldrich, Fluka, Strem, Acros and Fischer) and used without further purification. All dry solvents were freshly distilled under argon over an appropriate drying agent before use. Column chromatography was carried solvents were measured at room temperature by using a PerkinElmer Lambda 2 double beam spectrometer. The data were recorded with a slit width of 2 nm and a scan rate of 480 nm/min.
Spectroelectrochemistry was performed with a Cary 5000 double-beam spectrometer (Varian, USA) at room temperature. The measurements were conducted in a homebuilt three-electrode thin-layer cell with a path length of 1 mm. The three-electrode setup consists of a platinum mesh working electrode, a platinum wire counter electrode and a silver wire reference electrode.
The measurements were carried out in THF (purged with Ar for 5 min) with 0.2 M NBu4PF6 as the supporting electrolyte. A single cyclic voltammogram of the sample (scan rate: 0.1 V/s) was recorded first to identify the proper oxidation and reduction potential (vs. Ag wire) to apply for the spectroelectrochemical measurements.
fs-TA spectra were acquired with the HELIOS (0 to 5500 ps) from Ultrafast Systems. The laser source is the Clark MXR CPA2101 Ti:sapphire amplifier (775 nm, 1 kHz, 150 fs pulse width).
The excitation pulses of 550 and 430 nm were generated with a noncollinear optical parameter (NOPA, Clark MXR) and small spectral width was ensured by using a bandpass filter (FWHM 10 nm). A white light supercontinuum is generated by focusing a fraction of the fundamental 775 nm onto a 2 mm (Vis: ~420-760 nm) or 1 cm (NIR: 790-1400 nm) sapphire disk. All the measurements were conducted in 2 mm quartz cuvettes at ambient condition with continuous stirring. The optical density of solution was kept at around 0.3 at the excitation wavelength.
The power of the pump beam was kept at 200 nJ. The stability of samples were ensured by recording the UV-vis absorption spectrum before and after the transient absorption experiment.
Global analysis of the transient absorption data was performed with the GloTarAn software.

Quantum mechanical calculations
Theoretical calculations were performed at DFT level using the Gaussian16 software package. [2] Optimization of the ground state in every system have been done at B3LYP [3] /6-31+G(d,p) level of theory. UV-vis absorption spectra were computed through DFT timedependent extension (TD-DFT) [4] at CAM-B3LYP [5] /6-31+G(d,p) level of theory. To take care of dispersion energy correction Grimme's D3 dispersion correction [6] has been implemented in all the calculations. In addition, the implicit solvent effect was included by the conductor version of the polarizable continuum model (C-PCM). [5] The crystallographic structures of 1 and 2 were considered as starting point for the QM calculations.

Kinetic model for the 2→1 transformation
The fitting procedure of the rate constants for the 2→1 transformation was reached through the solution in time of the first order reaction close to equilibrium: [7] where k is the forward rate constant and k' is the backward rate constant. The dataset used in the fitting has been retrieved from the variation of the absorption peaks (i.e., 324, 378 and 575 nm) through time that, through Lambert-Beer equation, can be related to the variation of the concentration of the reactant and the product in time. These datasets have been produced at five different temperatures (i.e., 333, 343, 348, 353 and 363 K). The forward rate constants have been estimated averaging the fitted ones at different wavelengths at each temperature. The activation energy for the reaction 2→1 has been calculated as the slope of the curve produced by the Arrhenius plot ln(k(T)) vs 1/T of the previously computed forward rate constants. Compound 3 was prepared with a modified procedure from literature: [8] 9-Bromoanthracene (257 mg, 1 mmol), CuI (10 mg, 0.05 mmol, 5 mol%) and Pd              at the TCBD unit (i.e., red centroid region). The arrow shows the distance between the two centroids, whose magnitude is 4.05 Å. Computed electron and hole centroids as defined in [ [9] ].      Table S3.5. Bond lengths for the multicyano moiety (i.e., TCBD or fused-TCBD) for neutral and mono-reduced species of 2 and 1. The notation used for the bonds refers to those reported in Figure S3.11.   Potentials are referred to E1/2 of the Fc + /Fc redox couple.  This could be ascribed to a (partial) retro-CDA reaction considering that the spectra recorded at longer time show features similar to that of 2 upon reduction ( Fig. S4.4b). Note: both reduction potentials at −0.98 and −1.10 V aim at the first electron reduction process.   6.