Dipolar second-order nonlinear optical chromophores containing ferrocene, octamethylferrocene, and ruthenocene donors and strong π-acceptors: crystal structures and comparison of ;r-donor strengths

Crystal structures have been determined for six dipolar polyene chromophores with metaiiocenyi ferrocenyl (Fc), octamethylferroeenyl (Fc ), or ruthenocenyl (Rc)-donors and strong heterocyclic acceptors based on l,3-diethyl-2-thiobarbituric acid or 3-dicyanomethylidene-2,3- dihydrobenzothiophene-l,l-dioxide. In each case, crystals were found to belong to centrosymmetric space groups. For one example, polymer-induced heteronucleation revealed the existence of two additional forms, which were inactive in second-harmonic generation, suggesting that they were also centrosymmetric. The bond-length alternations between the formally double and single bonds of the polyene bridges are reduced compared to simple polyenes, indicating significant contribution from charge-separated resonance structures, although the metallocenes are not significantly distorted toward the [(η ⁶-fulvene)(η ⁵-eyelopentadienyl)metal(II)] ⁺ extreme. DFT geometries are in excellent agreement with those determined crystallographically; while the π-donor strengths of the three metaiiocenyi groups are insufficiently different to result in detectable differences in the crystallographic bond-length alternations, the DFT geometries, as well as DFT calculations of partial charges for atoms, suggest that π-donor strength decreases in the order Fc ≫ Fc > Rc. NMR, IR, and electrochemical evidence also suggests that octamethylferroeenyl is the stronger TT-donor, exhibiting similar π-donor strength to a p-(dialkylamino)phenyl group, while ferrocenyl and ruthenocenyl show very similar π-donor strengths to one another in chromophores of this type.