Study On The Properties And Responsive Mechanisms Of Two-photon Fluorescent Molecular Probes

The birth of laser promotes the rapid development of nonlinear optics and makes it tobe an important branch of modern optics. Moreover, there is a growing concern aboutnonlinear optics. Exploring new nonlinear optical materials have received extensiveattention in recent years, owing to their potential applications in many fields, such as laserfrequency doubling, laser frequency mixing, integrated optics, parametric amplificationand oscillation, beam steering, optical communication, image multiplication andtransformation, beam distortion removing, complete optical connecting, optical limitingand threshold monitoring, optical computer and other fields. Meanwhile, organicnonlinear optical materials have received great interest due to they have many advantagesover the inorganic materials, including wide response wave band, well flexibility, highoptical damage threshold, easy combination and modification as well as low cost. Inrecent years, organic two-photon materials are expected to have potential applications inmany fields, especially in two-photon fluorescent microscopy. Synthesis and study oforganic molecules with large two-photon absorption (TPA) cross sections andupconversion fluorescent intensity will greatly promote the applications of two-photonfluorescent microscopy imaging technology in biological systems. Certainly, utilizationsof the two-photon fluorescent microscopy imaging rely on development of efficient TPAmaterials. Study of two-photon fluorescent probe is a nascent research fields, showing agreat development potential and broad application prospects.The thesis studies the linear and nonlinear optical properties of various newlysynthesized organic molecules, as well as the responsive mechanisms of two-photonfluorescent probes by utilizing quantum chemical computional methods at the theab-initio level. The research contains five sections: the first section studies the solventeffects of terpyridine-based chromophore; the second section discusses the structure-property relationships of two-photon molecular materials; the third sectionstudies on the protonation-induced modulation of one-and two-photon absorptionproperties for quadripolar dyes; the forth section studies the responsive mechanism oftwo-photon fluorescent probes; the fifth section studies the responsive mechanism ofoctupolar molecules as pH probes. The main contents and results are as follows.1. Solvent effects of terpyridine-based chromophoreAt ab-initio level, the time-dependent hybrid density functional theory (TD-DFT)and response theory have been applied to study the solvent effect on the geometrical andelctronic structure as well as one-and two-photon absorption properties of2,2’:6’,2’’-terpyridine-based chromophore L. It is found that solvatochromic shift ofcharge-transfer (CT) appears nonmonotonic behavior with regard to the polarity of solvent.Besides, the one-photon absorption (OPA) strengths are enlarged slightly and are moredependent on the optical dielectric constant. The TPA cross sections are enhanced whenconsidering solvent effect compared to that in gas phase and show an increasing trendwith the increase of solvent polarity. All the theoretical results are in reasonable agreementwith experimental data.2. Structure-property relationships of two-photon molecular materialsAt ab-initio level, various models and methods have been applied to study the one-and two-photon absorption properties of a series of one-dimensional molecules withD-π-A and A’-D-A styles, as well as multi-branched molecules. In addition, we alsodiscuss the effect of isomerism on one-and two-photon absorption properties. The studydemonstrates that the TPA activities are closely related to molecular factors such as theconjugation length, the strength to push and pull electrons of terminal groups, coplanarity,and the molecular dimensionality. While the π center properties have little effect on theTPA properties.3. Protonation-induced modulation of one-and two-photon absorption properties forquadripolar dyesThe TD-DFT in combination with analytical response theory is used to explore theOPA and emission as well as TPA properties of quadripolar compounds with hydroxyl, amino or pyridyl end groups upon (de)protonation process. Both their OPA and emissionas well as TPA properties show obvious changes upon passing from their neutral to theirdoubly (de)protonated state. The intramolecular charge-transfer (ICT) analysisdemonstrates the variation of the electronic properties of the terminal groups by the(de)protonation process. The computational results are in reasonable agreement with theexperimental ones.4. Responsive mechanism of two-photon fluorescent probesThe response theory has been applied to investigate the one-and TPA as well asemission properties of a series of potential zinc ion and pH sensitive materials containing2-(2’-hydroxyphenyl)benzoxazole (HPBO) end groups. Special emphasis is placed on theevolution of the optical properties upon combining with zinc ions or deprotonation.Calculated results indicate that, upon combining with zinc ions or deprotonation, theseHPBO derivatives show drastic changes in their OPA, emission, and TPA properties.Moreover, the mechanism of the probes is analyzed to be the intramolecular chargetransfer. These compounds are thus proved to be excellent candidates for two-photonfluorescent zinc and pH probes.5. Responsive mechanism of octupolar molecules as pH probes(1) Protonation effects on one-and two-photon absorption properties of octupolarchromophore TA. The protonation effects on one-and two-photon absorption properties ofan octupolar molecule TA with1,3,5-triazine core and pyrrole electron-donatingend-groups have been studied at hybrid density functional theory level. A computationalscheme is developed to simulate a proton attached to an atom. The numerical results showthat large changes in both one-and two-photon absorption properties are observed whenthe compound is transformed from neutral to threefold protonated states. When thecompound is protonated, more charge transfer states appear and the absorption band has ared-shift. Furthermore, the TPA cross section is largely enhanced. The theoreticalcalculations demonstrate the protonation effect on promoting the intramolecular chargetransfer strength. The results present qualitative agreement with the experimentalobservations. A TPA switch with the compound TA based on the protonation effect is proposed.(2) Theoretical investigation of nonlinear optical properties upon (de)protonation of aseries of octupolar molecules as pH sensors. Quantum chemical calculations of structural,linear, and nonlinear optical properties are carried out for a series of octupolar pH sensors.The finite fields method and the response theory method are used to compute the firsthyperpolarizabilities and the OPA and emission, as well as TPA properties, respectively.The effects of the (de)protonation, terminal groups and the central core, location of thepyridyl nitrogen, and conjugation bridges are discussed in detail. Special emphasis isplaced on the evolution of the optical properties upon (de)protonation.(De)protonationleads to red shifts of the OPA, emission, and TPA spectra, and greatly enhances the firsthyperpolarizabilities and TPA cross sections, which are owing to the variation of the ICTin the (de)protonation process. The maximum increase for the first hyperpolarizabilitycomponent and TPA cross section is up to26and6fold upon deprotonation, respectively.In addition, study on the structure-property relationships is thoroughly investigated. Boththe introduction of CH3to the central benzene ring and the para position of the pyridylnitrogen are of benefit to the enhancement of nonlinear responses. Moreover, replacingthe triple bonds by double bonds in the conjugation bridge makes the optical propertiesdepend on the dihedral angles between the central plane and the terminal rings. Besides,our calculated results match well with the experimental data.There are nine chapters in this thesis. The first chapter gives a brief introduction tothe nonlinear optical phenomenon and the development process of nonliear optics andtwo-photon molecular materials. The basic theories, including the ab-initio method, theHF method, the B-O approximation, the DFT theory, and time-dependent perturbationtheory, are introduced in the second chapter. Chaper three introduces the methods tocalculate the TPA cross sections, including the sum-over states method, the few statesmethod, the finite fields method, and the response theory method. The forth chapterstudies the solvent effects of terpyridine-based chromophore. The fifth chapter discussesthe structure-property relationships of two-photon molecular materials. The sixth chapterstudies on the protonation-induced modulation of one-and two-photon absorption properties for quadripolar dyes. The seventh chapter discusses the responsive mechanismof two-photon fluorescent probes. The eigth chapter studies the responsive mechanism ofoctupolar molecules as pH probes. The summary and prospect are presented in the lastchapter.