New 1,3-dithiol-2-ylidene donor-π-acceptor chromophores with intramolecular charge-transfer properties, and related donor-π-donor molecules:: synthesis, electrochemistry, X-ray crystal structures, non-linear optical properties and theoretical calculations

New donor-pi-acceptor chromophores 5a-c, 7a-c, 8a-c, 13a-c and 14b-c have been synthesised: substituted 1,3-dithiole derivatives span a range of donor abilities [viz. 4,5-dimethyl > 4,5-bis(methylsulfanyl) > 4,5-bis(methoxycarbonyl)], a conjugated ethylenic or oligoethylenic linker varies the central pi-electron unit tone, two and four conjugated double bonds) and dicyanomethylene and N-cyanoimine groups are the acceptor units. Extended tetrathiafulvalene derivatives 15a-c have also been synthesised. The electronic absorption spectra of compounds 5, 7, 8 and 14 reveal a broad low-energy intramolecular charge-transfer band [lambda(max)(MeCN) 354-533 nm] which shifts bathochromically with increasing donor strength of the dithiole ring, and with increasing length of the conjugative pathway. The solution redox properties of 5, 7, 8 and 14, studied by cyclic voltammetry, reveal a reversible one-electron oxidation wave, attributed to the formation of the radical cation of the 1,3-dithiol-2-ylidene moiety, and an irreversible one-electron reduction to form the radical anion located on the dicyanomethylene or N-cyanoimine groups. For the bis(dithiole) donors 15a-c a single two-electron redox wave is observed. The non-linear optical (NLO) properties of 7a-c and 14b have been determined using the EFISH technique: moderate NLO properties are observed for compounds 7a-c [mu.beta(0) 85-112 x 10(-48) esu] whereas for the more extensively conjugated compound 14b, the value is increased to mu.beta(0) 1170 x 10(-48) esu. The molecular structure and electronic properties of the unsubstituted (R = H) compounds 7, 8, 1 8 and 15 have been calculated within the density functional theory approach. The minimum energy conformation corresponds to the trans orientation of the side chain for 7, and the cis orientation for 8, in agreement with X-ray crystal structures and solution NMR data. The redox properties and electronic spectra are discussed on the basis of molecular orbital energies and topologies. The X-ray crystal structures of compounds 7b, 8b and 15b are reported. In 7b the 1,3-dithiol-2-ylidene and dicyanopropene systems are planar and form an angle of 3 degrees between each other; the 'butadiene' unit has a trans configuration. Molecule 8b is planar (with the exception of the methyl groups) and the imine nitrogen atom is in a syn orientation towards the dithiole group, forming a short intramolecular S ... N contact [2.719(6) Angstrom]. For both 7b and 8b pi-delocalisation is observed within the central conjugated spacer unit. Molecule 15b has a predominantly planar structure, with the central tetraene unit in an all-trans configuration.