Influence Of Isomerism On Two-photon Absorptions Of Organic Molecules: A Theoretical Study

Since the laser was invented in1960, nonlinear optics has quickly developed as a newbranch of modern optics. At present, nonlinear optical materials have been widely used inmany high-tech fields, such as laser technology, optical communication, data storage etc..Organic molecules have attracted great attention in the last two decades, because they can bedesigned at molecular level to achieve the best nonlinear optical responses. Now, theoreticalstudies on nonlinear optical properties of molecular materials are mainly based on quantumchemical methods at ab-initio level. Quantum chemical calculations not only can explain andpredict various nonlinear optical properties, but also, according to the theoretically establishedstructure-to-property relationships, can design new molecular materials and provide guidelines forexperimental syntheses. The experimental measurements and practical applications on nonlinearoptical properties of molecular materials are generally performed in high condensed solventenvironments and the intermolecular interaction can have an important effect on optical properties,therefore, one has to consider the solvent and aggregation effects in the theoretical research.Isomerism is a common phenomenon in chemistry, and the previous investigations indicate thatmolecular isomerism can strongly affect linear and nonlinear optical properties. Recently, theinfluence of isomerism on two-photon absorption properties has been discussed in some papers.The related research needs a further and systematic investigation.The thesis studies rotational isomerism and cis-trans isomerism effects on nonlinear opticalproperties based on the first principles utilizing quantum chemical methods, analyzes therelationships between molecular structural parameters and optical properties in detail. Inaddition, the solvent and aggregation effects have also been discussed. The main contents andresults are shown in brief as follows.I. Influence of rotational isomerism on two-photon absorption properties of FTC moleculeThe rotational isomerism effects on the TPA properties of FTC molecule have been studiedemploying the quadratic response theory at the B3LYP level. With different initial conformations, eight low-energy rotamers are obtained, and their TPA cross sections arecalculated. The results indicate that the rotational isomerism has an important influence on theTPA intensity, for a certain rotamer, a large enhancement of the cross sections is observed; Inthe longer wavelength region, the rotational isomerism also can induce the absorption peakposition to shift a lot.II. Effects of rotational isomerism and bond length alternation on optical spectra of FTCmolecule in solutionThe one-photon absorption spectra of FTC rotamers in different solvent environments havebeen calculated utilizing the time-dependent density functional theory in combination with thepolarizable continuum model. It is found that the difference of the maximum absorptionwavelengths of the rotamers comes to be nearly30nm both in vacuum and in solvents. Theorder of energy and the corresponding Boltzmann distribution populations of the rotamers arechanged a lot in different solvents. Base on the structures optimized by Hartree-Fock method,the Boltzmann averaged absorption spectra have been calculated and the results is consistentwith the experiment value very well. It has led to the conclusion that for the investigatedmolecular systems, the bond length alternation has an important effect on the optical spectraof the molecule.III. Isomerism effects on two-photon absorption properties of DR1moleculeThe geometrical optimizations of DR1molecule are carried out using different quantumchemical methods respectively, and the TPA cross sections of these structures are calculated.The results indicate that rotating the single bond on the conjugated chain can strongly affectthe TPA. The TPA of DR1molecule in the near-infrared light region could be characterized bythe two configurations of cis and trans. Then, their TPA cross sections are calculated and thespectral line of half and half is simulated. It is shown that the two configurations have quitedifferent TPA properties including the peak positions and intensities. In addition, theaggregation effects on TPA properties of DR1molecule have been investigated employingquantum chemical methods in combination with molecular dynamics simulations. It is foundthat the aggregation of DR1molecules usually prevent the charge transfer process, therebydecreasing the TPA intensity.The thesis contains eight chapters in total. The first chapter gives a brief introduction of the nonlinear optics and the advantages of molecular materials, then introduces the basic conceptsand main features of two-photon absorption, as well as material species, at last, summarizesthe research background of isomerism effects on TPA; The second chapter introduces thequantum chemical basic methods, including Hartree-Fock methods and density functionaltheory; In the third chapter, the optical properties of organic molecules, including two-photonabsorption and one-photon absorption, are firstly introduced, and the response theory method isalso described; The fourth chapter introduces the isomerism of organic molecules and its classes,furthermore, the principles of rotational isomerism and cis-trans isomerism are presented; Inchapter five, the influence of rotational isomerism on TPA properties of FTC molecule isinvestigated; In chapter six, the effects of rotational isomerism and bond length alternation onoptical spectra of FTC molecule in solution are further studied; In chapter seven, isomerismeffects on TPA properties of DR1molecule is investigated in detail; The last chapter gives thesummary and prospect.