The discovery of any new technology is usually accompanied by a need for new or improved materials which make that technology useful in practical applications. In the case of two-photon absorption (2PA) this has truly been the case. Since its first demonstration in 1961, there has been an ever increasing quest to understand the relationships between two-photon absorption and the structure of two-photon absorbing materials. This quest has been motivated by the many applications for 2PA which have been reported, including fluorescence bioimaging, 3D microfabrication, 3D optical data storage, upconverted lasing, and photodynamic therapy. The work presented in this dissertation represents another step in the effort to better understand the structure/property relationships of 2PA. In this work a new, squaraine-fluorenesquaraine molecule, proposed through a joint effort of quantum and synthetic chemists, was synthesized and its photophysical properties were measured. The measurements included linear and two-photon photophysical properties, as well as solvatochromic behavior. Quantum calculations were done to aid in understanding those photophysical and solvatochromic properties. A single squaraine dye was also synthesized and used as a model compound to assist in understanding this new structure. In Chapter 1 an introduction to 2PA and several of its applications is given. Chapter 2 gives a background of 2PA structure/property relationships that have been reported to date, based on work done with polymethine dyes. Chapter 3 gives a full account of the synthesis, characterization, and detailed quantum chemical analyses of this new squaraine-fluorenesquaraine molecule and the corresponding model compound squaraine dye. Chapter 4 gives some additional work and suggested future directions.

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This dissertation describes the investigation of the synthesis and characterization of new squaraine-based photonic and electronic materials. In the first part of this thesis, squaraine dyes with large conjugation systems, including extended squaraines consisting of bis(donor)substituted vinylene-heterocycles and bis(indolinylenemethyl)squaraine-based oligomers linking through different π-bridges were designed, synthesized and characterized to exhibit strong two-photon absorption (2PA) for femotosecond and nanosecond optical-power limiting applications in the near-infrared (NIR). One of the dendronized squaraine forms smooth and high optical quality films with large NIR transparency window. In the second part, a series of squaraine- and phthalocyanine-based metal complexes were studied. Those dyes did not show large triplet quantum yield but significantly improved photovoltaic performance compared to the metal-free compounds. In the last part, an effective approach on optimizing bis(indolinylenemethyl)-based squaraine sensitizers with various surface anchor groups and π-linkers, achieved high power conversion efficiencies (PCEs) of 6.7% in liquid dye-sensitized solar cells (DSSCs) and 2.7% in solid-state DSSCs, which stand out all the previous reported squaraine-based sensitizers.

The primary focus of this dissertation is the development of novel fluorescent near-infrared molecules for various applications. In Chapter 1, a compound dU-BZ synthesized via Sonogashira coupling reaction methodology is described. A deoxyuridine building block was introduced to enhance hydrophilic properties and reduce toxicity, while an alkynylated benzothiazolium dye was incorporated for near-IR emission and reduce photodamage and phototoxicity that is characteristic of common fluorphores that are excited by UV or visible light. A 30-fold enhancement of fluorescence intensity of dU-BZ was achieved in a viscous environment. Values of fluorescence quantum yields in 99% glycerol/1% methanol (v/v) of varying temperature from 293 K to 343 K, together with fluorescence quantum yields, radiative and nonradiative rate constants and fluorescence lifetimes in glycerol/methanol solutions of varying viscosities from 4.8 to 950 cP were determined. It was found that both fluorescence quantum yields and fluorescence lifetimes increased with increasing viscosity, which is consistent with results predicted by theory. This suggests that the newly designed compound dU-BZ is capable of functioning as a probe of local microviscosity, and was later confirmed by in vitro bioimaging experiments. In Chapter 2, a new BAPTA (O,O*-bis(2-aminophenyl)ethyleneglycol-N,N,N*,N*-tetra acetic acid) and BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)-based calcium indicator, BAPBO-3, is reported. A new synthetic route was employed to simplify both synthesis and purification, which tend to be low yielding and cumbersome for BAPTA derivatives. Upon excitation, a 1.5-fold increase in fluorescence intensity in buffer containing 39 ?? Ca2+ and a 3-fold increase in fluorescence intensity in buffer containing 1 M Ca2+ was observed; modest but promising fluorescence turn-on enhancements. In Chapter 3, a newly-designed unsymmetrical squaraine dye, SQ3, was synthesized. A one-pot synthesis was employed resulting in a 10% yield, a result that is generally quite favorable for the creation of unsymmetrical squaraines Photophysical and photochemical characterization was conducted in various solvents, and a 678 nm absorption maximum and a 692 nm emission maximum were recorded in DMSO solution with a fluorescence quantum yield of 0.32. In vitro cell studies demonstrated that SQ3 can be used as a near-IR probe for bioimaging.

De nouvelles sondes oligonucléotidiques fluorescentes spécifiques des structures d'acides nucléiques en double brin ont été obtenues. Ces oligonucléotides formant des triples hélices, sont liés par leurs extrémités 5'-ou/et 3' à une série de marqueurs fluorescents de la famille des cyanines monométhines possédant des propriétés intercalantes, par des bras polyméthylène de longueur variable. Dans tous les cas, l'hybridation des sondes avec les cibles en double brin s'accompagne d'une forte augmentation de l'intensité du signal fluorescent émis qui dépend de la structure de la cyanine et des paramètres de sa liaison aux oligonucléotides (longueur du bras de liaison et sa position de fixation sur les oligonucléotides et sur les marqueurs). En présence d'une cible simple brin, les modifications du signal ne sont pas significatives. La présence du marqueur stabilise les triples hélices. Les sondes comportant le thiazole orange, attaché par le noyau benzothiazole via un bras de liaison à huit méthylènes, présente le meilleur compromis en termes de stabilisation, d'intensité de fluorescence et d'augmentation d'intensité de fluorescence lors de l'hybridation.<br />Dans le but d'obtenir de nouvelles sondes utilisables en milieu cellulaire et in vivo, de nouveaux marqueurs fluorescents de type squaraine ont été synthétisés et couplés pour la première fois, à notre connaissance, à des oligonucléotides. Les conjugués correspondant émettent un signal fluorescent détectable au-delà de 650 nm et possèdent des rendements quantiques de fluorescence élevés. La photostabilité des marqueurs et des conjugués, en milieu acide faible et à 37 °C a été fortement augmentée après incorporation d'atomes de fluor sur le squaraine.