title: Dipolar Second-Order Nonlinear Optical Chromophores Containing Ferrocene, Octamethylferrocene, and Ruthenocene Donors and Strong π-Acceptors: Crystal Structures and Comparison of π-Donor Strengths.
abstract: Crystal structures have been determined for six dipolar polyene chromophores with metallocenyl - ferrocenyl (Fc), octamethylferrocenyl (Fc″), or ruthenocenyl (Rc) - donors and strong heterocyclic acceptors based on 1,3-diethyl-2-thiobarbituric acid or 3-dicyanomethylidene-2,3-dihydrobenzothiophene-1,1-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 polymorphs, 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 towards the [(η(6)-fulvene)(η(5)-cyclopentadienyl)metal(II)](+) extreme. DFT geometries are in excellent agreement with those determined crystallographically; while the π-donor strengths of the three metallocenyl 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 octamethylferrocenyl is the stronger π-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.
authors.affiliation: Department of Natural Sciences, New Mexico Highlands University, Las Vegas, NM 87701; School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332-0400; Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055.
authors.forename: Tiffany L
authors.initials: TL
authors.lastname: Kinnibrugh
authors.forename: Seyhan
authors.initials: S
authors.lastname: Salman
authors.forename: Yulia A
authors.initials: YA
authors.lastname: Getmanenko
authors.forename: Veaceslav
authors.initials: V
authors.lastname: Coropceanu
authors.forename: William W
authors.initials: WW
authors.lastname: Porter
authors.forename: Tatiana V
authors.initials: TV
authors.lastname: Timofeeva
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Matzger
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Brédas
authors.forename: Seth R
authors.initials: SR
authors.lastname: Marder
authors.forename: Stephen
authors.initials: S
authors.lastname: Barlow