A Theoretical Study On Two-photon Absorption Properties Of Squaraine Derivatives

Nonlinear optics (NLO) is regarded as a completely new branch of optics since the adventof laser in early1960, where the relationship between action and response is nonlinear. Alongwith the rapid development of the laser technology, it has become one of the most active andimportant field in optics. In the past fifty years, NLO has been already extensively studiedand put forward applications in many fields, such as laser technology, information processingand storage technologies, optical communications, and so on. In comparison with traditionalinorganic nonlinear crystals, organic molecular materials show many advantages includinglarger nonlinear optical coefficient, lower dielectric constant, ultrafast response, high damagethreshold, lower switch energy of polymer device and flexible design for special functionalmaterials with chemical synthetic techniques. Design and synthesis of novel organicfunctional materials has thus been a very important research field. Squaraine derivatives are aclass of chromophore with an electron-accepting core linked with two electron-donatingsubstitutes, forming a special D-A-D structure, in which largely charge-separated excited statemay be generated upon photo-excitation. In recent years, squaraine dyes as good nonlinearmaterials have gained much attention in both theoretical and experimental research. Atpresent, quantum chemical methods have been widely used to study nonlinear opticalproperties because they can predict the optical properties of molecules accurately and giveuseful guidelines for synthesis. Most of the experiments and technical applications usuallyperform in various solutions, and different types of intermolecular interaction could result inaggregation when the chromophores are present at high concentration. So the solvent andaggregation effects on NLO properties should be concerned. In order to investigate theseeffects on NLO response, the molecular dynamics simulation technique combined withquantum chemical calculations has been put forward. So far, using this combined method tostudy the solvent and aggregation effects on NLO properties of organic materials is still littlementioned in the literatures and much work needs to be done in the future.The thesis investigated the effects of hydrogen bond on nonlinear optical properties of squaraine derivatives systematically employing molecular dynamics simulation and quantumchemical theory. The aggregation and solvent effects are considered. The relationships ofstructure to property are analyzed and discussed at length. The main contents and conclusionsare shown as follows.I. Influence of hydrogen bond on two-photon absorption properties of SQ moleculeThe one-and two-photon absorption properties and charge-transfer process of SQ moleculehave been calculated and the influence of intramolecular hydrogen bond on optical propertiesof SQ has been studied at the hybrid density functional theory B3LYP level. The one-andtwo-photon absorption properties of the SQ dimers and the super-molecules which consist ofthe SQ and solvent molecules have been computed and the effects of intermolecular hydrogenbond on optical properties of the system have been investigated. It is demonstrated that theexistence of intramolecular hydrogen bonds can promote the intramolecular charge-transfer,thereby enhancing the strength of the two-photon absorption significantly. However, theaggregation induced by intermolecular hydrogen bond affects the intramolecularcharge-transfer. Most of the dimers have less two-photon absorption cross section with respectto the corresponding monomer. The hydrogen bonds between the solute and the solventmolecules have a great impact on one-and two-photon absorption peak intensity and position.II. Aggregation effects on two-photon absorption properties of OHSQ moleculeThe aggregation behavior of OHSQ molecules in solution has been examined by moleculardynamics simulation. Based on the simulations, the one-and two-photon absorptionproperties of monomers and dimers are calculated by quantum chemical methods. The effectsof hydrogen bond and aggregation on optical properties of the system have been investigated.The results indicate that the intramolecular hydrogen bonds can enhance the intramolecularcharge-transfer, which leads M1to have strong two-photon absorption. Dimers and monomerspossess strong absorption in the same wavelength range. On one hand, the intermolecularaggregation broadens absorption band; and on the other hand, it enhances or decreasestwo-photon absorption intensities. Usually, changing the form of aggregates by variousmethods can improve the nonlinear absorption properties of molecular materials. According toour calculations, for obtaining enhanced two-photon absorption properties, one should try tomake "face to face" aggregation form as D1structure which linked by two hydrogen bonds. There are six chapters in this thesis. The first chapter gives a brief introduction to theessence, the development and the applications of nonlinear optical phenomenon, thenintroduces the basic concepts of two-photon absorption, the features, the development statusand the classification of two-photon absorption molecular materials. In the second chapter,the calculation methods of the optical properties of organic molecular materials areintroduced, including the time-dependent perturbation theory and the response theory. Thedensity functional theory and the molecular dynamics simulation are mainly introduced inthe third chapter. In chapter four, the influence of hydrogen bond on two-photon absorptionproperties of SQ molecule is investigated by the quadratic response theory with B3LYPfunctional. In the fifth chapter, the aggregation effects on two-photon absorption propertiesof OHSQ molecule are studied using the quantum chemical calculations and moleculardynamics simulation. The summary and prospect are given in the last chapter.