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.

Theory-Guided Design of Organic Electro-Optic Materials and Devices

“…Materials can also be prepared by sequential synthesis/self-assembly techniques and by crystal growth including from solution, the melt and the vapor phase [97][98][99][100][101][102][103][104][105][106][107]; however, these approaches have not been as widely adapted to the fabrication of prototype devices as have electric field poling methods.…”

Morphology of sulphur-terminated compound deposits condensed on different substrates in vacuum

“…Wet methods are mainly used for deposits manufactoring. Production of the oriented organic films by deposition in vacuum using hydrogen bonding between molecules was reported in [6]. The same STOC were used there.…”

Studies on Structural, Spectral, and Optical Properties of Organic Nonlinear Optical Single Crystal: 2-Amino-4,6-dimethylpyrimidinium p-Hydroxybenzoate

“…Organic nonlinear optical materials are formed based on NLO chromophores, which should be oriented in noncentrosymmetric way in order to exhibit macroscopic secondorder nonlinear optical response. There are several different strategies resulting in macroscopic (2) organic materials, including ferroelectric liquid crystals [9], self-assembled films [10], Langmuir-Blodgett films [11], and poled polymer films [12], as well as single crystals [13]. Among them, polar organic crystals are of special interest, since large NLO effect can be achieved due to the high density of chromophores in the crystals, as well as the most stable packing of chromophores in macroscopic materials, resulting in a superior photochemical and thermal stability as compared with soft category materials [14].…”