title: Self-assembly from the gas-phase: design and implementation of small-molecule chromophore precursors with large nonlinear optical responses.
abstract: Efficiently organizing molecular nonlinear optical (NLO) chromophores having large first-order hyperpolarizabilities (beta) into acentric microstructures for electro-optic (EO) applications represents a significant materials synthesis and processing challenge, in part due to interchromophore dipolar interactions that promote centrosymmetric organization. Here we report the computational modeling, synthesis, and characterization of a series of eight heteroaromatic organic chromophores, designed to self-organize from the vapor phase via directed hydrogen-bond networks, into acentric thin films. Introduction of alpha,omega-donor-acceptor hydrogen-bonding substituents along the molecular long axes tunes properties such as hyperpolarizability, volatility, thermal stability, film-forming properties, and macroscopic NLO response (chi((2))). DFT-level molecular modeling, INDO/S optical property analysis, and sum-overstates computation indicate that molecular-core fluorination and hydrogen-bond donor incorporation can increase beta(vec) up to 40x versus that of typical fluorine-free chromophores. Furthermore, inclusion of sterically induced biphenyl conjugative decoupling between chromophore pi-donor substituents and the hydrogen-bonding donor sites increases beta by approximately 50%. Experimental thin-film second harmonic generation (SHG) spectroscopy confirms these trends in calculated responses, with chi((2)) increasing 7.5x upon chromophore core fluorination and 15x with hydrogen-bonding donor substitution, thereby achieving macroscopic responses as high as 302 pm/V at omega(o) = 1064 nm. In addition to response trends, cluster calculations also reveal linear additivity in beta(vec) with catenation for all benzoic acid-containing chromophores up to longitudinally aligned trimers. Linear scaling of SHG response with film thickness is observed for benzoic acid-containing chromophores up to 1.0 microm film thickness.
authors.affiliation: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA.
authors.forename: David
authors.initials: D
authors.lastname: Frattarelli
authors.forename: Michele
authors.initials: M
authors.lastname: Schiavo
authors.forename: Antonio
authors.initials: A
authors.lastname: Facchetti
authors.forename: Mark A
authors.initials: MA
authors.lastname: Ratner
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks