title: Twisted Thiophene-Based Chromophores with Enhanced Intramolecular Charge Transfer for Cooperative Amplification of Third-Order Optical Nonlinearity.
abstract: Exploiting synergistic cooperation between multiple sources of optical nonlinearity, we report the design, synthesis, and nonlinear optical properties of a series of electron-rich thiophene-containing donor-acceptor chromophores with condensed π-systems and sterically regulated inter-aryl twist angles. These structures couple two key mechanisms underlying optical nonlinearity, namely, (i) intramolecular charge transfer, greatly enhanced by increased electron density and reduced aromaticity at chromophore thiophene rings and (ii) a twisted chromophore geometry, producing a manifold of close-lying excited states and dipole moment changes between ground and excited states that are nearly twice that of untwisted systems. Spectroscopic, electrochemical, and nonlinear Z-scan measurements, combined with quantum chemical calculations, illuminate relationships between molecular structure and mechanisms of enhancement of the nonlinear refractive index. Experiment and calculations together reveal ground-state structures that are strongly responsive to the solvent polarity, leading to substantial negative solvatochromism (Δλ ≈ 10(2) nm) and prevailing zwitterionic/aromatic structures in the solid state and in polar solvents. Ground-to-excited-state energy gaps below 2.0 eV are obtained in condensed π-systems, with lower energy gaps for twisted versus untwisted systems. The real part of the second hyperpolarizability in the twisted structures is much greater than the imaginary part, with the highest twist angle chromophore giving |Re(γ)/Im(γ)| ≈ 100, making such chromophores very promising for all-optical-switching applications.
authors.affiliation: School of Chemistry and Biochemistry, School of Materials Science and Engineering, and Center for Organic Photonics and Electronics, Georgia Institute of Technology , Atlanta, Georgia 30332, United States.
authors.forename: Natasha B
authors.initials: NB
authors.lastname: Teran
authors.forename: Guang S
authors.initials: GS
authors.lastname: He
authors.forename: Alexander
authors.initials: A
authors.lastname: Baev
authors.affiliation: Department of Chemistry and Materials Research Center, Northwestern University , Evanston, Illinois 60208, United States.
authors.forename: Yanrong
authors.initials: Y
authors.lastname: Shi
authors.forename: Mark T
authors.initials: MT
authors.lastname: Swihart
authors.forename: Paras N
authors.initials: PN
authors.lastname: Prasad
authors.affiliation: Department of Chemistry and Materials Research Center, Northwestern University , Evanston, Illinois 60208, United States.
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks
authors.affiliation: School of Chemistry and Biochemistry, School of Materials Science and Engineering, and Center for Organic Photonics and Electronics, Georgia Institute of Technology , Atlanta, Georgia 30332, United States.
authors.forename: John R
authors.initials: JR
authors.lastname: Reynolds