Quadratic Nonlinear Optical Response in Partially Charged Donor-Substituted Tetrathiafulvalene: From a Computational Investigation to a Rational Synthetic Feasibility

A computational (ZINDO) investigation is employed to estimate the molecular hyperpolarizability (β) of a [(TTF−NH 2 ) 8 ] (TTF = tetrathiafulvalene) octameric stack at various fractional (0 < ρ < 1) oxidation states of TTF ρ + . While the neutral [(TTF−NH 2 ) 8 ] octamer exhibits a modest (7.6 × 10 - 30 cm 5 esu - 1 ) static β 0 value, in relation to a nearly centrosymmetric electronic structure, the formal (TTF−NH 2 ) 0.25+ oxidation state leads to a giant (β 0 = 3891 × 10 - 30 cm 5 esu - 1 per octamer) NLO response. This effect is related to the appearance of an extremely intense HOMO → LUMO-based low-lying transition, occurring at 1260 nm upon fractional oxidation, thus providing new and unexpected interest in TTF-donor substituted molecules. To illustrate the chemical feasibility of such species, two kinds of derivatives are synthesized: a TTF-dimethylaniline (TTF-DMA) by using in particular a Suzuki cross-coupling reaction between iodo-TTF and p -dimethylaminophenylboronic acid and a TTF-vinyldimethylaniline (TTF-VDMA) by a Wittig-type condensation notably between TTF-carboxaldehyde and p -(dimethylamino)benzyltriphenylphosphonium bromide. Their NLO response, evaluated at 1.064 μm by the Hyper Raleigh Scattering technique, leads to β values around 240 × 10 - 30 cm 5 esu - 1, in both cases. Additionally, these monomeric species are observed to exhibit very strong multiphoton fluorescence in solution after one electron oxidation. Different routes are discussed toward a possible engineering of TTF-donor based chromophores into oligomeric stacks in the solid state.