Man Shing Wong

Novel electro-optic molecular cocrystals with ideal chromophoric orientation and large second-order optical nonlinearities

We show, for the first time to our knowledge, that highly nonlinear optically active as well as orientationally optimized crystalline materials for second-order nonlinear optics can be prepared by a proper design of short hydrogen-bonded molecular aggregates derived from a specially developed merocyanine dye. One of the cocrystals is shown to be a very useful and highly efficient crystalline material for electro-optic devices because of the perfect chromophoric orientation, the large electro-optic coefficient r111=30±3 pm/V at λ=1535 nm, and the greatly improved physical properties such as higher melting point and crystal properties such as crystallinity. In addition, we demonstrate that one can vary or tune the linear and nonlinear optical properties of this cocrystal without modifying the chromophoric orientation in the crystal lattice by changing the crystal growth conditions.

Nonlinear optical organic co-crystals of merocyanine dyes and phenolic derivatives with short hydrogen bonds

Abstract We investigated the co-crystallization of merocyanine dyes M ( M : R–N + C 5 H 4 –CHCH–C 6 H 4 O − , M1 : R=CH 3 , M2 : R=HO–CH 2 –CH 2 –) with phenolic and aniline derivatives and show that the short or very short hydrogen bond between the two phenolic oxygen atoms which lead to a self-assembly of the M dyes and phenol derivatives is the key steering force for the co-crystallization process. The co-crystal formation was studied by melting point determination, by second-harmonic generation using the Kurtz and Perry powder test, and X-ray structural analysis. We present detailed results on the growth, polymorphism, and nonlinear optical properties of co-crystals of derivatives of M1 ( M2 ) and m -nitrophenol (mNP) and co-crystals of M2 and methyl 2,4-dihydroxybenzoate (MDB). Three phases of the co-crystal M2 ·mNP and three phases of the co-crystal M2 ·MDB were found. Both M2 ·MDB(I) and (II) have the same crystal structure (space group symmetry Cc determined by X-ray diffraction), but show different linear and nonlinear optical properties. This unusual property in the co-crystals of M2 ·MDB(I) and (II) may be caused by a different proton location of the short hydrogen bond (O–H–O) in the aggregation between M2 and MDB which could not be resolved by X-ray diffraction.

Crystal engineering based on short hydrogen bonds; cocrystallization of a highly nonlinear optical merocyanine dye with nitrophenol derivatives

Short hydrogen bonds [RO–H–Oca. 2.46(1)A, RH–Oca. 1.2(1)A] in cocrystals of merocyanine dyes and nitrophenol derivatives are an interesting design element for crystal engineering in both framework and application-oriented considerations due to their high bonding strength and high degree of flexibility.

Hydrogen bonded lambda-shaped packing motif based on 4-nitrophenylhydrazones: a promising design tool for engineering acentric crystals

Two newly developed 4-nitrophenylhydrazone derivatives, 4-diethylaminobenzaldehyde 4-nitrophenylhydrazone, DEANPH, and 5-bromothiophene-2-carbaldehyde 4-nitrophenylhydrazone, BTNPH, which exhibit very strong second harmonic signals that are three orders of magnitude greater than that of the urea standard were investigated by single crystal X-ray analysis. The structural comparison with three other strongly optically nonlinear hydrazones revealed that the hydrogen bond-directed lambda-shaped supramolecular assemblies are the key and fundamental bases of all these non-centrosymmetric crystal packings. The high occurrence within this class of acentric crystals is likely due to this hydrogen bonded lambda-shaped packing motif. Since the chromophores are preferentially arranged in a lambda-shape, the acentric crystals are generally optimized for second-order nonlinear optical effects such as second harmonic generation.

Strong hydrogen bonds as a design element for developing new non‐linear optical crystals: Co‐crystals of merocyanine dyes and phenol derivatives

In this work we show that with strong hydrogen bonds, including a ‘very strong’ hydrogen bond and/or strong hydrogen-bonding networks, highly non-linear optically active chromophores, merocyanine dyes and phenol derivatives, are assembled as salt or semi-salt crystals, leading to improvement in crystallinity and interesting non-linear optical properties. Our approach is discussed in connection with four co-crystal structures.

A highly efficient organic second-order nonlinear optical crystal based on a donor-acceptor substituted 4-nitrophenylhydrazone

We present a novel organic crystal, 5-(methylthio)-thiophenecarboxaldehyde-4-nitrophenylhydrazone, with a large effective phase-matchable nonlinear optical coefficient, deff=29±3 pm/V at λ=1318 nm. Single crystals of dimensions up to 25×5×5 mm3 were grown. The large off-diagonal nonlinear optical coefficients can be explained with the two-dimensional geometry of the chromophores in the crystal lattice. Our crystal is ideally suited for optic parametric oscillation in the wavelength range from 1000 to 2500 nm.

Engineering of polar molecular crystals with optimized chromophoric orientation for nonlinear optics

Abstract We review here two new directions in the area of molecular crystal engineering for nonlinear optics where the alignment, tuning and optimization of the acentric orientation of highly extended, dipolar chromophores in the crystalline state will be described. The use of the non-rod-shaped framework-hydrazone skeleton-as a π-conjugation core, due to a high preference for a lambda-shaped packing motif is a very useful tool to acentrically align and optimize chromophoric orientation in the solid state for nonlinear optical effects such as second harmonic generation. The supramolecular synthetic approach based on the short hydrogen bond induced self-assembling of molecular aggregates shows a high tendency to induce an acentric packing and a higher degree of design feasibility in tuning and optimizing the chromophoric orientation within the crystalline lattice. In addition, co-crystals obtained by this approach often show greatly improved crystal and physical properties relative to its starting components.

Stilbazolium based zwitterionic chromophores for electro-optic polymers

Abstract A series of stilbazolium based zwitterionic chromophores was synthesized. They have very large dipole moment (up to μ = 13x10-29 Cm) and high molecular hyperpolarizability (up to β = 135x10-40 m4V-1). Guest-host polymer thin films of the best molecules were prepared and poled by corona discharge. Values of the nonlinear optical coefficient (d31 = 0.7 pm/V. at γ = 1.545 nm) at molecular concentrations of 5x1019 m-3, comparable to that of Disperse Red 1, were obtained. The good molecular characteristics and macroscopic performance of these chromophores combined with their good thermal stability make them promising candidates for side-chain electro-optic polymers.

Highly polar molecular crystals for electro-optic applications

Abstract We report on a series of new acentric 4-nitrophenylhydrazone derivatives which bear more than one hydroxy or methoxy electron donating group whose large molecular nonlinear optical properties were characterized with semi-empirical calculations and hyper-Rayleigh scattering. The second harmonic powder test results of several compounds showed large signals, two to three orders of magnitude larger than urea, which make these materials potentially interesting for nonlinear optics. Crystals of one of these new molecules (3,4-dihydroxybenzaldehyde-4-nitrophenylhydrazone, 3,4-DHNPH) are very promising for electro-optics. The properties are discussed in detail and a comparison with some of the best existing crystals for electro-optic applications is given