Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis

Abstract

A wide variety of monomeric and oligomeric, donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2+2] cycloaddition between tetracyanoethylene (TNCE) and donor-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes. Reaction yields are often nearly quantitative but can be affected by the electron-donating power and steric demands of the alkyne substituents. The intramolecular charge-transfer (CT) interactions between the donor and TCBD acceptor moieties were comprehensively investigated by X-ray crystallography, electrochemistry, UV-visible spectroscopy, and theoretical calculations. Despite the nonplanarity of the new chromophores, which have a substantial twist between the two dicyanovinyl planes, efficient intramolecular CT interactions are observed, and the crystal structures demonstrate a high quinoid character in strong donor substituents, such as N,N-dimethylanilino (DMA) rings. The maxima of the CT bands shift bathochromically upon reduction of the amount of conjugative coupling between strong donor and acceptor moieties. Each TCBD moiety undergoes two reversible, one-electron reduction steps. Thus, a tri-TCBD derivative with a 1,3,5-trisubstituted benzene core shows six reversible reduction steps within an exceptionally narrow potential range of 1.0 V. The first reduction potential E(red,1) is strongly influenced by the donor substitution: introduction of more donor moieties causes an increasingly twisted TCBD structure, a fact that results in the elevation of the LUMO level and, consequently, a more difficult first reduction. The potentials are also strongly influenced by the nature of the donor residues and the extent of donor-acceptor coupling. A careful comparison of electrochemical data and the correlation with UV-visible spectra made it possible to estimate unknown physical parameters such as the E(red,1) of unsubstituted TCBD (-0.31 V vs Fc+/Fc) as well as the maxima of highly broadened CT bands. Donor-substituted TCBDs are stable molecules and can be sublimed without decomposition. With their high third-order optical nonlinearities, as revealed in preliminary measurements, they should become interesting chromophores for ultra-thin film formation by vapor deposition techniques and have applications in opto-electronic devices.

Third- and high-order nonlinear optical properties of an intramolecular charge-transfer compound

“…A distinct solvatochromic effect of (TCNQ) 2 OPV 3 has been observed in varied solvents, which was a characteristic behavior of the dipolar ICT molecules. [19][20][21][22]44 The major CT band at 677 nm (1.83 eV) in CH 2 Cl 2 red-shis to 687 nm (1.80 eV) in CH 3 CN and 718 nm (1.73 eV) in DMF, and blue-shis to 658 nm (1.88 eV) in THF and 625 nm (1.98 eV) in toluene. The solvent effect of the CT band generally follows the reported tendency of red shi of such ICT compounds when increasing the solvent …”

“…These naturally stable, nonplanar chromophores are ideal platforms for efficient intramolecular charge-transfer (ICT) processes and are regarded as promising nonlinear optical materials. [19][20][21][22][32][33][34][35][36] In this work, we have designed and synthesized an oligo-(phenylenevinylene) (OPV) bridged ICT compound, (TCNQ) 2 -OPV 3 , and studied its third-and h-order optical nonlinearities by a nonlinear optical imaging technique with the phase-object (NIT-PO) at the entry of a 4f coherent imaging system. 37 The method is based on a 4f system with top-hat beams and its theoretical model is simple Fourier optics, which leads to many advantages such as simple optical alignment, very high sensitivity and insensitivity to statistical uc-tuations of the laser beam.…”

Polymeric Chemosensors: A Conventional Platform with New Click Chemistry

“…Diederich and his co-workers systematically investigated the reactivity of alkyne molecules, and they found that aromatic amines, such as N,N-dialkylaniline, are some of the best substituents for the effective activation of the alkyne moiety. 45 It was found by later studies that other donors, such as thiophene, tetrathiafulvalene (TTF), porphyrin, azulene, and some organometallic substances, are also effective for the activation of the alkyne moiety (see Figure 16). 4653 Moreover, several TCNQ and other quinodimethane derivatives, such as 2,3,5,6-tetrafluoro-TCNQ (F 4 -TCNQ), 2,5-dialkoxy-TCNQ, and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), can also be employed as effective acceptors of these reactions (see Figure 16).…”

Conjugation and optoelectronic properties of acetylenic scaffolds and charge-transfer chromophores

“…9), accessible in often quantitative yields in an atom-economic synthesis [85] by formal [2+2] cycloaddition between TCNE and donor-substituted alkynes, followed by electrocyclic ring-opening of the initially formed cyclobutenes [86,87]. The [2+2] cycloaddition of TCNE with electron-rich metal acetylides, followed by retro-electrocyclization to give organometallic TCBD derivatives had actually been described by Bruce and coworkers as early as 1981 [88,89], but the corresponding reaction with electron-rich alkynes had not been systematically explored [90,91].…”

“…10) are particularly remarkable. Thus, the trimeric derivative 27 undergoes six reversible one-electron reduction steps, each centered on a dicyanovinyl moiety, in the narrow potential range between -0.69 and -1.69 V [87] (Fig. 10).…”