Brian Frank

Silicon Organic Hybrid Technology—A Platform for Practical Nonlinear Optics

A cost-effective route to build electrically as well as optically controlled modulators in silicon photonics is reviewed. The technology enables modulation at bit rates beyond 100 Gbit/s. This platform relies on the well-established silicon-based complementary metal-oxide-semiconductor processing technology for fabricating silicon-on-insulator (SOI) waveguides, while an organic cladding layer adds the required nonlinearity. The strength of this hybrid technology is discussed, and two key devices in communications are exemplarily regarded in more detail. The first device demonstrates demultiplexing of a 120 Gbit/s signal by means of four-wave mixing in a slot-waveguide that has been filled with a highly nonlinear chi(3)-organic material. The second device is a 100 Gbit/s/1 V electrooptic modulator based on a slow-light SOI photonic crystal covered with a chi(2) -nonlinear organic material.

One-electron-reduced and -oxidized stages of donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes of different molecular architectures.

A series of monomeric and oligomeric donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of pi-electron delocalization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied pi systems, limited pi-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron delocalization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.

Donor-substituted diphenylacetylene derivatives act as electron donors and acceptors.

Despite the predominant electron donor character of p-phenylenediamine, our studies on extended p-phenylenediamine derivatives show that they can not only be chemically oxidized, giving well-known Wurster-type radical cations, but also be chemically reduced, giving radical anions. Making use of EPR/ENDOR spectroscopy and supported by DFT calculations, we were able to reveal the extent of π-electron delocalization in the paramagnetic species and to shed light onto the geometry and bond lengths. While for the radical anions spin was found to be mostly delocalized into the π-system, the radical cations can be described as essentially N-centered. Furthermore, we performed electrochemical characterizations using cyclic voltammetry to gain insight into the thermodynamics of the redox processes. The photophysical properties of the parent extended p-phenylenediamine were investigated by absorption, emission, and excitation spectroscopy. The fluorescence quantum yield and the excited-state lifetime of the neutral precursors in hexane and acetonitrile were determined to establish elementary differences originating from solvent effects.