Effects Of Relative Orientation Between Branches On Two-photon Absorption Properties For Multi-branched Molecules

For two-photon process providing3-D resolution and improved penetration of light intoabsorbing materials, the materials with two-photon absorption (TPA) response have numerouspotential applications, such as two-photon fluorescence microscopy,3D optical data storage,optical limiting, photodynamic therapy, and so on.In comparison with inorganic materials, organic material not only has the importantcharacters of high mechanical strength, good processing performance and ease of moleculartailoring, but also show excellent nonlinear optical properties, such as large nonlinear opticalcoefficient, low dielectric constant, high response speed and light damage threshold, etc,which have become hot topic in the nonlinear optical field. In the past decades, to exploitcompounds exhibiting large TPA at desired wavelength, the synthetic work has been focusedon molecules of growing complexity: from simple dipolar to quadrupolar and octupolar todendrimer. It has been reported that for multi-chromophores, the interplay and couplingamong the branches have significant effects on nonlinear optical response. Depending on thenature and strength of the coupling, the TPA intensity can lead to various tends: cooperativeenhancement, addictive or even decreasing behavior. Therefore, the conformational changesof multi-branched molecules as well as the relative orientation between branches will have agreat impact on their two-photon absorption properties. However, there are still few relatedtheoretical investigations at present. More research is required.In1990, Denk et al. from Cornel University applied TPA excitation to the confocal laserscanning microscope, opening up two-photon fluorescence microscopy and imaging this newfield. Due to two-photon fluorescence microscopy with near infrared excitation, dark fieldimaging, avoiding fluorescent bleaching and reducing light-induced toxicity, high transverseand longitudinal resolution, etc, studies of two-photon fluorescent probes have gain more and more attention, but still far less than the number of works on one-photon fluorescence probe.And the relevant theoretical studies on two-photon fluorescent probes are also rare. So, thetheorists are urged to fully understand the mechanic of two-photon fluorescent probes, and tryto design organic molecules with large TPA cross section and strong up-conversionfluorescence, to promote two-photon fluorescence microscope imaging widely used inbiological systems.In this paper, through quantum chemical (QC) methods, firstly, we have studied the one-and two-photon absorption properties of a group of multi-branched molecules; then, using QCmethods in combination with molecular dynamic simulations, we have examined theinfluence of conformational changes of the dimer and the orientation between the branches onTPA properties, and analyzes carefully the relationship between the molecular structureparameters and nonlinear optical properties; finally, we have explored how the two-photonfluorescence probes work in acidic conditions. The main contents and results are outlined inbrief as follows:Ⅰ A theoretical investigation on two-photon absorption properties of multi-branchedmoleculesThe one-and two-photon absorption spectra have been investigated utilizing thetime-dependent density functional combined with the polarizable continuum model. BothB3LYP and HF are applied to optimize the molecular structures of molecules T1-T5. Basedon these optimized molecular structures, we have computed the one-photon absorptionproperties using B3LYP and CAM-B3LYP functional, respectively. It turned out that based onthe optimized structure from B3LYP, CAM-B3LYP is good choice to calculate the one-photonspectra, the calculated spectra only having slight blue-shift. Thus, we have also applied thesame method to study the TPA spectra of molecules T1-T5. The results show that the TPAproperties are not only depended on the degree/direction of intra-branch charge transfer, butalso very closely related to the coupling between the branches. And TPA response ofmulti-branched molecules can be enhanced by increasing the π-conjugated length of theintra-branches or the introduction of some groups modifying the molecular structure. In viewof TPA cross sections measured in experiment using single wavelength, our calculation couldprovide some more accurate information of the spectra. Ⅱ Effects of torsional disorder and position isomerism on two-photon absorptionproperties of polar chromophore dimersWe have examined the influence of structural changes, including the torsional disorder ofintra-and inter-branches of dipolar chromophore dimers inCH2Cl2, on TPA properties byemploying QC methods in combination with molecular dynamic simulations. For the studiedmolecular system, the inter-and intra-branch torsional angles are prone to fluctuate.Molecular dynamics (MD) simulations are used to observe the conformational evolutions ofthe dimer with different initial configuration in CH2Cl2solution, and the evolutions of theinter-branch torsional angle () and intra-branch torsional angle (θ) are obtained. It is foundthat the and θ are changed continually during all the simulations. A set of constrainedgeometries with different inter-and intra-branches torsional angles, and θ, are optimizedand used to calculate two-photon absorption spectra. Our calculations show that the energydifference of the dimers with different torsional angle is fairly small. These demonstrate thatthe changes of torsional angles and θ is possible from the thermodynamic point of view.In order to investigate the effect of torsion angle on TPA properties, we have calculated theTPA spectra of the constrained geometries. The results show that the TPA intensity is reducedgradually with the decrease of the and θ. Predictably, inter-and intra-branch torsionaldisorder would significantly weaken TPA intensity of the dimer, which could explain theexperimental results. To exploit the structural conditions for aggregation-induced TPAenhancement, based on the monomer, we have designed a series of covalent dimers bygrafting two monomers to a benzene ring. Eight probable isomers are fully optimized andused to investigate TPA properties. Our results suggest that the cooperative enhancement canbe achieved when the subunits are in closer proximity with larger interchromophore angle.This work would be helpful to predict the TPA properties of multichromophoric units basedon the properties of the individual chromophores and their mutual arrangement.Ⅲ Protonation effect on one-and two-photon absorption properties of benzimidazole-basedratiometric two-photon fluorescent probesUsing the theory of quardratic response method combined with the polarizable continuum model, we have studied a serial of benzimidazole-based ratiometric two-photon fluorescentprobes in acidic conditions, including the protonation effect on molecular geometric structure,electronic structure, and optical properties. It is found that the protonation process does notalter the molecular geometry, but the charge distribution in benzimidazole group significantlyvaries, The one-and two-photon absorption spectra of protonated probe in comparison withthe probe in neutral PH, are both largely red-shift. And TPA cross section has alsoconsiderably increased. The calculated results are in good agreement with the experiment,which provides theoretical guidance for exploiting good two-photon fluorescent probes in thefuture.The thesis is divided into eight chapters. The first chapter gives a brief introduction of thenonlinear optics and the advantages of organic molecular materials, then introduces the TPAprocess and main features of two-photon absorption, as well as the promising of TPA material,at last, summarizes the research background of our studies; The second chapter introduces thequantum chemical basic methods, including Hartree-Fock methods and density functionaltheory; In the third chapter, firstly, gives the computational formulas of one-and two-photonabsorption properties of organic molecules are introduced, and then, describes the responsetheory method and gives several common solvent models; The fourth chapter introducesmolecular dynamics simulation, including algorithm and the main simulation process, andseveral applications in nonlinear optics; The last three chapter is our work. In chapter five, theone-and two-photon absorption properties of multi-branched molecules is investigated; Inchapter six, the Effects of torsional disorder and position isomerism on two-photon absorptionproperties of polar chromophore dimers are further studied; In chapter seven, Protonationeffect on one-and two-photon absorption properties of benzimidazole-based ratiometrictwo-photon fluorescent probes is investigated in detail; The last chapter gives the summaryand prospect.