No abstract

Organotransition metal complexes have much to offer as nonlinear optical (NLO) chromophores since they allow large hyperpolarizabilities to be combined with various other physical properties in an unparalleled diversity of molecular structures. This Account summarizes our synthetic chemistry driven investigations in the NLO research field, which have focused primarily on complexes of ruthenium, and to some extent iron. A number of fascinating discoveries are described, including the first demonstration of redox-induced switching of NLO behavior and chromophores which disobey the otherwise universal rule that elongation of polyene systems leads to continual increases in quadratic NLO responses.

Reversible switching of quadratic NLO responses can be achieved in organic and metallo‐organic molecular compounds by application of various external stimuli. Strategies for the molecular engineering of switchable NLO materials are outlined (an example is depicted), and practical demonstrations of the switching of NLO effects, such as SHG, are discussed.

The new salts trans‐4′‐(dimethylamino)‐N‐R‐4‐stilbazolium hexafluorophosphate (R = methyl, Me 1, phenyl, Ph 2, 2,4‐dinitrophenyl, DNPh 3, 2‐pyrimidyl, Pym 4, Scheme 1) have been prepared. Their electronic absorption spectra show intense, visible intramolecular charge‐transfer bands, the energy (Emax) of which decreases in the order R = Me > Ph > DNPh > Pym. This trend arises from the steadily increasing electron deficiency of the pyridinium ring, a phenomenon also observed in cyclic voltammetric and 1H nuclear magnetic resonance (NMR) data. Fluorescence‐free first hyperpolarizability β values of [1–4]PF6 were measured by using femtosecond hyper‐Rayleigh scattering (HRS) with acetonitrile solutions and a 1300 nm laser, and static first hyperpolarizabilities β0 were obtained by application of the two‐state model. The HRS results indicate that the N‐aryl chromophores in [2–4]PF6 have considerably larger β0 values than their N‐methyl counterpart in [1]PF6, with a ca. 10‐fold increase in β0 observed in moving from [1]PF6 to [4]PF6 (25 → 230 × 10–30 esu). Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K allowed the derivation of dipole moment changes Δμ12 (10.9–14.8 D), which have been used to calculate β0 according to the two‐state equation β0 = 3Δμ12(μ12)2/2(Emax)2 (μ12 = transition dipole moment). With the exception of [1]PF6, the Stark‐derived β0 values are in reasonable agreement with those from HRS. However, the increase in β0 in moving from [1]PF6 to [4]PF6 is only 2‐fold for the Stark data (90 → 185 × 10–30 esu). The observed trend of increasing β0 in the order [1]PF6 < [3]PF6 < [2]PF6 < [4]PF6 arises from a combination of decreasing Emax and increasing Δμ12, with only a slight increase in μ12 between [1]PF6 and [4]PF6. It is likely that the β0 values for [3]PF6 are lower than expected due to the steric effect of the ortho‐NO2 group, which causes twisting of the DNPh ring out of the plane of the stilbazolium unit. A single crystal X‐ray structure shows that [2]PF6 crystallizes in the space group Cc, with head‐to‐tail alignment and almost parallel stacking of the pseudo‐planar stilbazolium portions of the cations to form polar sheets within a polar bulk structure. [2]PF6 is essentially isostructural with the related Schiff base salt trans‐4‐[(4‐dimethylaminophenyl)iminomethyl]‐N‐phenylpyridinium hexafluorophosphate ([8]PF6). Second harmonic generation (SHG) studies on [2]PF6 and [8]PF6 using a 1907 nm laser and sieved powdered samples (53–63 μm) afforded efficiencies of 470 and 240 times that of urea, respectively. Under the same conditions, the well‐studied compound [1]p‐MeC6H4SO3 gave an SHG efficiency of 550 times that of urea.

In this paper, we describe the extremely unusual optical properties of Ru(II)-based electron donor-acceptor (D-A) polyene and some closely related chromophores. For three different polyene series, the intense, visible d-->pi* metal-to-ligand charge-transfer bands unexpectedly blue-shift as the number of E-ethylene units (n) increases from 1 to 3, and the static first hyperpolarizabilities beta(0) determined via hyper-Rayleigh scattering and Stark spectroscopy maximize at n = 2, in marked contrast to other known D-A polyenes in which beta(0) increases steadily with n. Time-dependent density-functional theory and finite field calculations verify these empirical trends, which arise from the orbital structures of the complexes. This study illustrates that transition metal-based nonlinear optical chromophores can show very different behavior when compared with their more thoroughly studied purely organic counterparts.

Low-energy metal-to-ligand charge-transfer (MLCT) excitations are associated with the very large molecular nonlinear optical (NLO) coefficients, β, of the complexes [Ru (NH ) (N-R-4,4'-bipyridinium)] (R=methyl, phenyl, or 4-acetylphenyl). Chemical oxidation to the Ru forms causes bleaching of the MLCT absorptions and marked attenuation of the NLO responses. Both effects are completely reversed upon re-reduction, and the extent of the β switching is about 10- to 20-fold.

Quantum sum rules impose limits on the hyperpolarizability, beta. A survey of the largest second-order molecular susceptibilities finds what appears to be a universal gap between the experimental results and the fundamental limits. In this work, we use theory, linear spectroscopy, Raman spectroscopy, and measured values of beta (using hyper-Rayleigh scattering and Stark Spectroscopy) to show that this gap is due to an unfavorable arrangement of excited state energies. The question of whether this result is a universal property of a quantum system or a matter of present paradigms for making molecules is discussed.

In this article, we describe a series of new complex salts in which electron-rich transition-metal centers are coordinated to three electron-accepting N-methyl/aryl-2,2‘:4,4‘ ‘:4‘,4‘ ‘‘-quaterpyridinium ligands. These complexes contain either RuII or FeII ions and have been characterized by using various techniques, including electronic absorption spectroscopy and cyclic voltammetry. Molecular quadratic nonlinear optical (NLO) responses β have been determined by using hyper-Rayleigh scattering at 800 nm and also via Stark (electroabsorption) spectroscopic studies on the intense, visible d → π* metal-to-ligand charge-transfer bands. The latter experiments reveal that these putatively octupolar D 3 chromophores exhibit two substantial components of the β tensor which are associated with transitions to dipolar excited states. Computations involving time-dependent density-functional theory and the finite field method serve to further illuminate the electronic structures and associated linear and NLO properties of the new chromophoric salts.