The research towards more efficient organic molecules for second-order nonlinear optical (NLO) applications has
resulted in substantial improvements of the molecular nonlinear polarizabilities. Different strategies for increasing NLO
responses at the molecular level in ionic chromophores are reviewed. However, only a small subset of the highly
efficient non-centrosymmetric molecules also forms non-centrosymmetric crystals at the bulk level. Examples of
success in achieving polar crystals of molecular salts are presented. Such crystals with a high number density of aligned
dipolar chromophores are promising materials for efficient second-order NLO applications, such as terahertz generation
via difference frequency mixing.
In recent years there has been significant interest in the ability to switch the second-order nonlinear optical (NLO)
response at the molecular level. A compound can be considered as an NLO switch when the response can be turned to
an 'on 'and 'off' state. Several switching schemes at the molecular level have been envisioned. Earlier schemes used
isomerisation and tautomerisation, causing changes in the nature and/or degree of conjugation between electron donor
and acceptor. An alternative approach is based on lowering the electron donating capacity of the electron donor or the
withdrawing capacity of the acceptor group. Here we will present results based on protonation/deprotonation and
oxidation/reduction of the donor group.
A variety of pyridinium and benzothiazolium-substituted conjugated donor-π-acceptor dipoles are studied by hyper-
Rayleigh scattering (HRS) in solution. We have investigated their potential as strong electron acceptor moiety and a
complete series has been synthesized and characterized. An elongation of the conjugated π-system for each molecule is
made and characterized to see the influence of the elongation of the conjugated π-system. A leveling off of the
hyperpolarizability β is already observed after three subsequent double bonds.
Modulating the electronic, magnetic and optical properties at the molecular level using an external trigger has been
extended to the field of nonlinear optics. The switching molecule is designed to have a complexation site or a redox
active unit which can guarantee electronic communication over the whole conjugated system. The alteration of the
nonlinear property is then induced by the modulation of the internal charge-transfer due to the response of the molecule
to the external stimulus. This can be achieved by cation binding, protonation/deprotonation or electrochemically.
Second-order nonlinear optical properties of organic and inorganic materials have been widely investigated. However, it becomes more important to be able to switch the molecules from an 'on' state to an 'off' state, or vise versa. Several switching schemes can be thought of. We will focus our attention to alter the donor properties of metalorganic compounds. Therefore, we will oxidize the metal center of the compound chemically. Since it is more interesting to be able to electrochemically switch the metal center, we developed a combined electrochemistry/hyper-Rayleigh scattering cell to perform in-situ electrochemical switching of the hyper-Rayleigh response.
Sum rules have been shown to impose a fundamental limit on the of nonlinear-optical susceptibility. All of the measured values of the hyperpolarizability and second hyperpolarizability over the last 25+ years, be it on- or off-resonance, fall a factor of 103/2 below these limits. Not only is this result scientifically puzzling on a fundamental level; but, has implications on the kinds of practical devices that can be made. In this work, we discuss molecular engineering techniques that aim to break this bottleneck.
A series of complex salts in which trans-bis[1,2-phenylenebis(dimethylarsine)]chlororuthenium(II) electron donor groups are connected to pyridyl or pyridinium electron acceptors has been prepared. These chromophores exhibit intense, visible metal-to-ligand charge-transfer (MLCT) absorptions and reversible Ru(III/II) (and also in some cases ligand-based) redox processes. Stark (electroabsorption) spectroscopic studies have been used to determine dipole moment changes for the MLCT excitations. Static first hyperpolarizabilities have been calculated according to the two-state model, allowing the derivation of structure-activity correlations for the molecular quadratic nonlinear optical responses.
A series of 2,2':4,4'':4',4'''-quaterpyridinium ligands bearing different N-substituents (methyl, Me; phenyl, Ph; 4-acetylphenyl, AcPh; 2-pyrimidyl, Pym; 3,5-bis-methoxycarbonyl-phenyl, MCP) have been prepared. These compounds feature powerful electron acceptor moieties and hence have been combined with different electron-rich metal centres to give complexes suitable for quadratic nonlinear optical properties. A series of dipolar ruthenium(II) ammine complex salts of the form [RuII(NH3)4LA][PF6]4 and octupolar complex salts of the form [MII(LA)3][PF6]8 (M = Ru(II)/Fe(II), LA = a 2,2':4,4'':4',4'''-quaterpyridinium ligand) have been prepared. These compounds exhibit multiple intense, low energy metal-to-ligand charge-transfer (MLCT) absorptions in the visible region and reversible M(III/II) redox processes. First hyperpolarizabilities have been determined directly by using hyper-Rayleigh scattering and indirectly via calculation from Stark (electroabsorption) spectroscopic data.
A number of novel salts of dipolar cations containing dimethylamino electron donor and pyridinium electron acceptor groups have been prepared and studied by using various physical techniques including hyper-Rayleigh scattering and Stark spectroscopy. In addition to showing very large molecular static first hyperpolarizabilities, several of these salts exhibit pronounced bulk quadratic nonlinear optical effects. Related compounds in which the electron donor is a ruthenium(II) ammine centre have also been investigated, allowing interesting comparisons to be made between transition metal-containing and purely organic nonlinear optical chromophores.
A series of novel metal-organic in-plane complexes trans- (formula available in paper) have been investigated using the hyper-Rayleigh scattering technique. Like other in-plane complexes reported recently they exhibit very large and tunable static first hyperpolarizabilities which are associated with intense visible metal-to-ligand charge-transfer excitations. Moreover, a good correlation was found between the hyperpolarizabilities and the electrochemical properties of the complexes. Furthermore, the effect of the oxidation state of the metal upon the molecular optical nonlinearity has been investigated. Chemical oxidation of the metal is proven to be an excellent tool to reversibly switch the molecular first hyperpolarizabilities of the trans- substituted ruthenium complexes.
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