A number of dithioacetate and dithiolate mono- and dianions have been synthesized and characterized through Z-scan measurements, with some showing significant third order nonlinear optical (NLO) behavior. Tetralkylphosphonium cations were utilized in tandem with the nonlinear anions so as to minimize electrostatic interactions within the salt, consequently resulting in the materials being room temperature ionic liquids (RTILs), which have numerous advantages over typical organic-based materials. Anions composed of metal-ligand systems were also tested for NLO behavior as components of novel ionic liquid materials. These RTILs introduce a new class of materials with potential applications in optical limiting and other all-optical devices.
A new formulation of ab initio theory is presented that treats a large molecule in terms of wavefunctions of its constituent molecular subunits (to be called fragments). The method aims to achieve near conventional ab initio accuracy but using a truncated set of fragment orbitals with a consequent drastic reduction of computing time and storage requirement.
A new class of materials, which demonstrates a potentially high third-order nonlinear optical (NLO) response based on highly polarizable anions, is discussed. Two examples of such ions included in this article are trithiocarbonate (CS32-) and aci-nitro-dithioacetate (NO2CHCS22-. The NLO response of any real materials containing these ions will be highly dependent on the nature of the cation present. It is expected that smaller and more localizing cations will have a smaller NLO response. We have performed ab initio finite-field computations of several anions along with some salts of the aci-nitro-dithioacetate anion to illustrate this effect.
Both theoretical and experimental studies in the past have indicated that the charge transport in lightly doped polyacetylene is due primarily to traveling charged solitonic waves along the polymer chain backbone accompanied by hopping from one chain to another. The conductivity in this model is still dictated by a bandgap. The nature of the ground and excited states of the doped system, however, is not fully understood. Previous ab initio calculations on polyenes doped by a single iodine atom have brought out the interesting feature that, while calculations at the Hartree- Fock level lead to the charge-transfer state as the ground state, a correlated calculation, on the other hand, shows it to be an excited state with the ground state showing little charge transfer. Since, however, only polyiodide anions I3-, I5-, etc. are found in solution rather than neutral radicals such as I, I3 etc., inferences based on the calculation employing a single iodine atom are not conclusive. We present here a systematic ab initio study in which the nature of the ground and excited states of polyenes, doped with iodine, are investigated.
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