Synthetic Principles for Bandgap Control in Linear pi-Conjugated Systems.

Property tuning in selected examples of D–π–A molecules has been discussed and summarized in this review article. The tuning and structure–property relationships have been demonstrated on the particular A, π and D parts of the push–pull molecule. Special emphasis has been put on the tuning of the FMO levels and optical properties. Further prospective applications of the given chromophore have also been considered.

https://pubs.rsc.org/en/content/articlepdf/2014/RA/C4RA11264D

Fundamental aspects of property tuning in push-pull molecules†

The linear and nonlinear optical properties of donor−acceptor π-electron chromophores having conjugated bridges varying in aromaticity and electron excessivity are investigated using the INDO/1 semiempirical Hamiltonian. NLO response properties of such chromophores are more sensitive to the electron excessivity of the bridges than to bridge aromaticity. Electron-excessive/deficient heterocyclic bridges have lesser/greater tendency to deplete the electron density from the donor/acceptor groups and thus increase their donor/acceptor ability. Thus, electron-excessive heterocyclic bridges act as auxiliary donors while electron-deficient heterocyclic bridges act as auxiliary acceptors, significantly enhancing NLO response. Interposing charged five-membered auxiliary donor and acceptor fragments between strong donor or acceptor groups results in large computed second-order NLO response.

Large molecular hyperpolarizabilities. Quantitative analysis of aromaticity and auxiliary donor-acceptor effects

Recent progress in second-order nonlinear optical polymers and dendrimers

Polymeric electro-optic materials have recently been developed that, when fabricated into devices such as Mach-Zehnder interferometers, permit drive ( Vπ) voltages of less than 1 V to be realized at the telecommunications wavelength of 1.3 and 1.55 microns. Operation of polymeric electro-optic modulators to frequencies (bandwidths) of greater than 100 GHz has been demonstrated. The total insertion loss of polymeric electro-optic modulators has been reduced to values as low as 5 dB, which is competitive with values obtained for lithium niobate modulators and is much lower than that obtained for gallium arsenide electro-absorptive modulators. Polymeric electro-optic modulators can be operated for long periods of time at temperatures on the order of 100 °C. Techniques have been developed for seamlessly integrating polymeric electro-optic circuitry with passive low loss optical circuitry (e.g., silica long-haul transmission fiber and medium-range fluoropolymer fibers) and with very large scale integration (VLSI) semiconductor electronics. These advances have created a considerable interest in the commercialization of polymeric electrooptic materials. Polymeric electro-optic materials are now being evaluated for applications such as phased array radar, satellite and fiber telecommunications, cable television (CATV), optical gyroscopes, electronic counter measure (ECM) systems, backplane interconnects for high-speed computers, ultrafast (100 Gbit/s) analog-to-digital (A/D) converters, land mine detection, radio frequency (rf) photonics, and spatial light modulators. This review discusses the structure-function relationships that had to be defined and the synthesis and processing advances achieved before materials appropriate for commercialization could be produced. Topics discussed include the design and synthesis of chromophores that simultaneously exhibit large molecular hyperpolarizability, low optical absorption, processability (e.g., solubility in various processing media), and the prerequisite (thermal, chemical, electrochemical, and photochemical) stability to survive conditions encountered in the fabrication and operation of polymeric electro-optic devices. Chromophore-chromophore intermolecular electrostatic interactions have been shown to be the most serious problem impeding the optimization of electro-optic activity in organic materials. The quantitative theoretical treatment of such interactions by equilibrium and Monte Carlo statistical mechanical methods is discussed. Rules for designing chromophores with shapes leading to maximum obtainable electro-optic activity are discussed and the synthetic realization of such structures is reviewed. The role of electrostatic interactions in influencing the choice of processing conditions is also discussed. A number of processing steps, including spin casting, electric-field poling, lattice hardening, fabrication of buried channel waveguides (including deposition of cladding layers), electrode deposition and connection with electronic circuitry, and integration of active and passive optical circuitry, are required. Each of these steps can affect device performance (e.g., influence optical loss, electro-optic activity, and stability). The systematic optimization of each of these steps is reviewed. Finally, device design and operation are reviewed and speculation on the future of the field is expressed.

Nonlinear Optical Polymeric Materials: From Chromophore Design to Commercial Applications

It is shown experimentally that the ketene dithioacetal group is very effective as an electron-donating group in the design and synthesis of thermally stable and efficient second-order nonlinear optical chromophores.

Ketene dithioacetal as a π-electron donor in second-order nonlinear optical chromophores

Functionalized Fused Thiophenes: A New Class of Thermally Stable and Efficient Second-Order Nonlinear Optical Chromophores

Based on the ‘unconventional’ dithiolydinemethyl type donors and tricyanovinyl acceptor; a new series of highly efficient (βµ= 940–2400 × 10–48 esu) and thermally stable (275–315 °C) thiophene derived nonlinear optical chromophores have been developed.

Optimization of thermal stability and second-order nonlinear optical properties of thiophene derived chromophores

Novel Mitsunobu and tricyanovinylation reactions have been employed to incorporate efficient nonlinear optical chromophores as side-chains into aromatic polyimides resulting in high electro-optic activity and long-term alignment stability at 100 °C for more than 500 h.

Varanasi Pushkara Rao

Papers

Ketene dithioacetal as a π-electron donor in second-order nonlinear optical chromophores

Functionalized Fused Thiophenes: A New Class of Thermally Stable and Efficient Second-Order Nonlinear Optical Chromophores

Optimization of thermal stability and second-order nonlinear optical properties of thiophene derived chromophores

Design and synthesis of thermally stable side-chain polyimides for second-order nonlinear optical applications

Achieving excellent tradeoffs among optical, chemical and thermal properties in second-order nonlinear optical chromophores