Ulrich Gubler

Elongated push–pull diphenylpolyenes for nonlinear optics: molecular engineering of quadratic and cubic optical nonlinearities via tuning of intramolecular charge transfer

Abstract Push–pull polyenes are of particular interest for nonlinear optics (NLO) as well as model compounds for long-distance intramolecular charge transfer (ICT). In order to tune the ICT phenomenon and control the linear and nonlinear optical properties, we have synthesized and investigated several series of soluble push–pull diphenylpolyenes of increasing length and having various donor (D) and acceptor (A) end groups. Their linear and NLO properties have been studied by performing electro-optical absorption measurements (EOAM) and third-harmonic generation (THG) experiments in solution. Each push–pull molecule exhibits an intense ICT absorption band in the visible characterized by an increase in dipole moment upon excitation (Δ μ ). By lengthening the polyenic chain, linear increases in excited-state dipoles are achieved while the ground-state dipole is maintained constant. The polyenic chain thus allows for long-distance intramolecular charge transfer. In addition, tuning of the magnitude of the ICT phenomenon and of the nonlinear responses is achieved by varying the donor and acceptor end groups: increasing the D/A strength leads to concomitant enhancements of Δ μ , quadratic ( β ) and cubic ( γ ) hyperpolarizabilities values. Giant Δ μ values (up to more than 30 D) and enhanced non-resonant quadratic and cubic NLO molecular responses (i.e. β (0)=500×10 −30 esu and γ (0)=8000×10 −36 esu) have been attained while maintaining suitable solubility, stability and transparency.

Molecular design for third-order nonlinear optics

We discuss recent developments in the design of molecules for applications in third-order nonlinear optics with emphasis on all-optical signal processing and two-photon absorption. We especially concentrate on functional substitution patterns and conjugation length expansion. We show that low molecular symmetry with regard to the conjugation path of the delocalized electrons was found to be a good guideline towards linearly conjugated molecules with large second-order hyperpolarizabilities γ. We show that this guideline is also valid for two-dimensionally conjugated systems and that the observed effects can be explained by the symmetry of the electronic wavefunctions. We present scaling laws and critical conjugation lengths of rod-like molecules with electrons delocalized over a one-dimensional path and show that the exponent tends to be constant for various polymers in the transparency range and that the values presented here are of similar magnitude for various organic materials systems. Finally, we discuss different materials systems with regard to the figures of merit relevant for all-optical signal processing.

Pt‐Tetraethynylethene Molecular Scaffolding: Synthesis and Characterization of a Novel Class of Organometallic Molecular Rods

The series of monodisperse Pt-bridged TEE oligomers 3a-f was prepared by oxidative Glaser-Hay oligomerization of monomer 7 under end-capping conditions. These novel molecular rods extend in length from 3.3 nm (monomeric 3a) to 12.1 nm (hexameric 3 f). Their isolation was achieved by high performance gel permeation chromatography (GPC), and their purification was best monitored by analytical GPC in combination with matrix-assisted laser-desorption-ionization mass spectrometry (MALDI-TOF MS). The mass spectra of each oligomer revealed the molecular ion or its sodium complex as parent ion together with a clean, highly characteristic fragmentation pattern. Delayed addition of the end-capping reagent PhC(triple bond)CH to the oligomerization mixture afforded polymer 10 with an average of approximately 32 repeat units and a remarkably narrow molecular weight distribution (Mw/Mn=1.06), which is indicative of a living polymerization process. UV/Vis spectral data as well as measurements of the second hyperpolarizability gamma by third harmonic generation (THG) revealed a nearly complete lack of pi-electron delocalization along the oligomeric backbone. The Pt atoms act as true insulating centers, and the Pt-C(sp) bonds hardly possess any pi character. The synthesis of the molecular rods 3a-f provides another demonstration of the power of oxidative acetylenic homocouplings for the preparation of unusual nanoarchitecture.

Monodisperse Poly(triacetylene) Oligomers Extending from Monomer to Hexadecamer: Joint Experimental and Theoretical Investigation of Physical Properties

A series of monodisperse Et3-Si-end-capped poly(triacetylene) (PTA) oligomers ranging from monomer to hexadecamer was prepared by a fast and efficient statistical deprotection-oxidative Hay oligomerization protocol. The PTA oligomers exhibit an increasingly deep-yellow color with lengthening of the pi-conjugated backbone, feature excellent solubility in aprotic solvents, and exhibit melting points up to > 22 degrees C for the hexadecameric rod. This new dramatically extended oligo(enediyne) series now enables to investigate the evolution of the physico-chemical effects in PTAs beyond the linear 1/n versus property regime into the higher oligomer region where saturation becomes apparent. We report the results of joint experimental and theoretical studies, including analysis of the 13C NMR spectra, evaluation of the linear (UV/ Vis) and nonlinear [third-harmonic generation (THG) and degenerate four-wave mixing (DFWM)] optical properties, and characterization of the redox properties with cyclic and steady-state voltammetry. Up to the hexadecameric rod, an increasingly facile one-electron reduction step is observed, showing at the stage of the dodecamer, a leveling off tendency from the linear correlation between the inverse number of monomer units and the first reduction potential. The effective conjugation length (ECL) determined by means of UV/Vis spectroscopy revealed a pi-electron-delocalization length of about n = 10 monomeric units, which corroborates well with the oligomeric length for which in the 13C NMR spectrum C(sp2) and C(sp) resonances start to overlap. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) measurements revealed for the second-order hyperpolarizability gamma a power law increase gammma-alpha-n(a) for oligomers up to the octamer with exponential factors a= 2.46+/-0.10 and a=2.64+/-0.20, respectively, followed by a smooth saturation around n = 10 repeating units. The power law coefficient a calculated with the help of the valence effective Hamiltonian (VEH) method combined to a sum-over-states (SOS) formalism corroborates well with the values found by both THG and DFWM experiments. Up to the PTA heptamer, INDO (intermediate neglect of differential overlap)-calculated gas-phase ionization potentials and electron affinities obey a linear relationship as a function of the inverse number of monomer units displaying a strong electron-hole symmetry. The onset of saturation for the electron affinity is calculated to occur around the octamer, in accordance with experimentally obtained results from electrochemical measurements.

Highly polarizable chromophores for nonlinear optics: syntheses, structures and properties of donor-acceptor substituted thiophenes and oligothiophenes

Abstract A series of new chiral donor-acceptor substituted mono- (1a,b), bi- (2a,b,f) and terthiophenes (3) as well as thienyl- and bithienylpyridinium salts 1d-f and 2i-k with various counterions has been synthesized. This class of thiophenes exhibit excellent molecular nonlinear optical properties such as second-order polarizabilities β or second-order hyperpolarizabilities γ. With respect to an application as nonlinear optical materials, some of these compounds have been investigated by Kurtz powder technique as well as by X-ray crystallographic analysis. The second-harmonic generation (SHG) powder efficiencies are measured at 1.064 μm and 1.3 μm with urea as reference. Furthermore, the influence of the chiral donors prolinol and methoxy-methylpyrrolidine in the thiophene derivatives as well as the variation of counterions in the pyridinium salts on the orientation of the chromophores in the crystalline lattice was investigated.

Third-order nonlinear optical properties of in-backbone substituted conjugated polymers

We present an alternative approach to tune the third-order nonlinear optical properties of organic molecules by inserting a functional group into the path of the π-electron conjugation instead of at its ends. We show that this scheme has a beneficial influence on the second-order hyperpolarizability for short molecules, but that in two instances where such molecules were polymerized into longer molecules the overall hyperpolarizability is reduced.

Synthesis and Physical Investigation of Donor–Donor and Acceptor–Acceptor End-Functionalized Monodisperse Poly(triacetylene) Oligomers

Two series of monodisperse, terminally donor-donor [D-D, D=4-(dimethylamino)phenyl] and acceptor-acceptor [A-A, A=4-nitrophenyl] functionalized poly(triacetylene) (PTA) oligomers ranging from monomer to hexamer were synthesized by oxidative Hay oligomerization under end-capping conditions. The corresponding D-D and A-A end-substituted polymers with an average degree of polymerization (DP) of n approximately 18 and n approximately 12, respectively, were also prepared and served as reference points for the corresponding infinitely long polymers. These terminally functionalized PTA oligomers and polymers are yellow- to orange-colored compounds, displaying excellent solubility in aprotic solvents with melting points above 200 degrees C for the hexamers. For the 4-(dimethylamino)phenyl substituted compounds, a consistent first oxidation potential around +0.42 V versus Fc/Fc+ (ferrocene/ferricinium) was observed, whereas the 4-nitrophenyl functionalized systems underwent a reversible reductive two-electron transfer around -1.40V versus Fc/Fc+. The nature of the end-groups has a dramatic influence on the electronic absorption spectra. Saturation of the linear optical properties in the D-D series occurs at significantly shorter chain-length [effective conjugation length (ECL) of n approximately 4 monomer units] than in the A-A substituted or the previously reported Me3Si- and Et3Si-end-capped PTA oligomer series (ECL: n approximately 10 monomer units). Similar observations with respect to the ECL were made by measurement of the Raman-active v(C triple bound C) stretches. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) experiments showed that shorter oligomers of terminally D-D or A-A functionalized PTAs display higher second hyperpolarizabilities gamma than the corresponding R3Si-end-capped series (R=alkyl). Moreover, they disclose a distinct peak of the nonlinearity per monomer unit at intermediate backbone lengths. In THG experiments, the second hyperpolarizabilities for long D/A-functionalized PTA oligomers attained the same saturation values as observed for the corresponding R3Si-end-capped rods. The nonlinearities measured by DFWM of the D-D and A-A substituted PTAs were found to be larger than for the silylated ones, which can be explained by the closeness of the two-photon resonance.

A study on a microring structure for practical devices

We propose a microring structure based on non-identical waveguides for the implementation in practical devices. Our design pays attention to aspects such as fabrication tolerances and fiber coupling efficiency. A detailed study has been carried out on the coupling between the non-identical waveguides in the analyzed structure.

Scaling laws of second-order hyperpolarizabilities in molecular wires

Summary form only given. Whereas principles for the optimization of first-order hyperpolarizabilities /spl beta/ are well established, rules for the second-order hyperpolarizability /spl gamma/ are less well documented, owing to the significantly more complex physics. We have shown by DFWM, THG measurements, and by quantum-chemical calculations that the chain-length dependence of the second-order hyperpolarizability /spl gamma/ of poly(triacetylene) molecular wires follows a power law for short oligomers. This power law saturates smoothly around 60 carbon-carbon bonds, which corresponds to an effective conjugation length of 7.5 nm. A comparison with various other systems with electrons delocalized over a one-dimensional path shows that the exponent a tends to be around 2.5 for various polymers in the transparency range. In the presence of two- or three-photon resonances, the measured exponent a increases. Moreover, the values presented here are of similar magnitude as for other organic material systems.