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.

Keywords: anthryl-fused derivative; cyano-Diels-Alder reaction; electron acceptor; photophysics; tetracyanobuta-1,3-diene.

Scheme 1

Synthetic route to anthryl‐fused‐TCBD‐aniline 1. i) Tetracyanoethylene, THF, 40 °C, 16 h; ii) toluene, 70 °C, 12 h. The numbering of the anthryl moiety positions is presented for derivative 3.

Figure 1

a) Side, b) front, and c) top view (with respect to the cyclopentene five‐membered ring) of the X‐ray crystal structure of anthryl‐fused‐TCBD‐aniline 1. Carbon atoms are colored in light gray, nitrogen atoms in light blue, and hydrogen atoms in white. Chloroform molecules of crystallization have been omitted for clarity. Structure details are given in the Supporting Information.

Figure 2

¹H NMR spectra (CDCl₃) of a) anthryl‐TCBD‐aniline 2 and b) anthryl‐fused‐TCBD‐aniline 1. Capital letters refer to protons’ assignment of 1 and 2 in Scheme 1. * denotes residual solvent signals.

Figure 3

UV‐vis absorption spectra of acetonitrile solutions of 2 (blue line) and 1 (red line). Inset: DFT‐calculated spectra of 2 (blue line) and 1 (red line) in acetonitrile.

Figure 4

UV‐vis absorption spectra of a toluene solution of 2 heated at 90 °C recorded at t=0 (blue line) and t=76 min. (red line); intermediate spectra recorded every 2 min (grey lines). The blue and red line spectra correspond to that of anthryl‐TCBD‐aniline and anthryl‐fused‐TCBD‐aniline 2 and 1, respectively. Inset: Variation of the absorbance (ln abs) monitored at 324 (circles), 378 (squares), and 575 nm (triangles) as a function of time.

Figure 5

Cyclic voltammograms of 1 (top) and 2 (bottom), measured at a scan rate of 0.1 V s⁻¹ in a 0.1 M solution of n‐Bu₄NPF₆ in THF. Potentials are referred to E _1/2 of the Fc⁺/Fc redox couple.

Figure 6

a) Femtosecond transient absorption spectra (fs TAS) of 1 (λ _exc=550 nm) with time delays between 0.1 and 100 ps in the visible and near‐infrared region in air‐saturated benzonitrile solution at room temperature. b) Evolution‐associated spectra (EAS) obtained by a global fit of the fs TA data according to a three‐species kinetic model in GloTarAn. c) Selected kinetic traces with the corresponding fit. d) Concentration evolution with time related to each species in b).