Organic solar cells have the potential to become a low-cost photovoltaic technology. One approach to further increase the device efficiency aimsvto cover the near-infrared region of the sunvspectrum. However, suitable absorber materials are rare. This thesis focuses on the material class of aza-bodipy and dibenzo-aza-bodipy as near-infrared absorber materials for organic solar cells. Besides the synthesis of novel thiophene-substituted aza-bodipys, azadiisoindomethenes were prepared by the addition of Grignard reagents to phthalodinitrile an subsequent reduction with formamide. Starting from these azadiisoindomethenes as precursors, complexes with borondifluoride, boroncatechole and transition metals were synthesized. The optical and electrochemical properties of all compounds prepared were investigated by experimental and theoretical methods. The (dibenzo-)aza-bodipys are characterized by their electronic structure, comprising a central electron acceptor core and peripheral electron donor units. The substituents at the donor units offer a stronger impact on the HOMO energy than on the LUMO energy. Electron donating substituents at the donor units result in an overall decreased HOMO-LUMO gap. This allows to redshift the absorption maximum up to 800 nm. The corresponding dibenzo-analogues already demonstrate a bathochromic shift of the absorption compared to the (non-annulated) aza-bodipys. Yet, the central acceptor is weakened and a further redshift by substituents is less distinct. The compounds can be thermally evaporated in high vacuum. The required thermal stability is increased in some cases by boroncatechol compared to borondifluoride complexes, without significant influence on the optical and electrochemical properties. Besides the characterization of the molecular properties, promising materials were evaluated in thin fifilms and solar cell devices. The charge carrier mobility in the measured compounds were found to be between 10E-6 and 10E-4 cm2V-1s-1. The charge transport parameters were calculated on the basis of obtained single crystal structures. It was found that a high charge carrier mobility may be attributed to a better molecular overlap and a short intermolecular distance in the corresponding solid state structure. Selected materials were characterized in organic solar cells. In solution processed devices, the dibenzo-aza-bodipys reached efficiencies of 1.6 % and 2.1 %, as donor materials in combination with PC61BM and PC71BM as acceptor. The main limiting factor in these devices turned out to be the low fill factor of 30 %. From a series of vacuum processed devices with aza-bodipys and dibenzo-aza-bodipys, increased voltages were obtained with decreasing HOMO energy of the bodipy derivatives. A suitable near-infrared absorbing dibenzo-aza-bodipy exhibited a contribution to the photocurrent from 750 - 950 nm. / Die organische Photovoltaik hat das Potential eine kostengünstige Solarzellentechnologie zu werden. Ein Ansatz die Effizienz weiter zu steigern besteht darin den aktiven Spektralbereich in den nahen Infrarotbereich zu erweitern. Bisher gibt es jedoch nur wenige geeignete Materialien. In dieser Arbeit werden Verbindungen aus der Materialklasse der Aza-Bodipy und Dibenzo-Aza-Bodipy als Absorbermaterialien für den nahen Infrarotbereich zur Verwendung in organischen Solarzellen untersucht. Neben der Synthese von neuen Thiophen-substituierten Aza-Bodipys wurden Azadiisoindomethine durch die Addition von Grignardverbindungen an Phthalodinitril und anschließender Reduktion mit Formamid dargestellt. Ausgehend von den Azadiisoindomethinen sind neue Bordifluorid, Borbrenzcatechin und Übergangsmetallkomplexe synthetisiert worden. Alle Substanzen sind mit experimentellen und theoretischen Methoden auf ihre optischen und elektrochemischen Eigenschaften hin untersucht worden. Die elektronische Struktur der (Dibenzo-)Aza-Bodipys ist charakterisiert durch periphere Elektronendonoreinheiten um einen zentralen Elektronenakzeptor. Die langwelligste Absorptionsbande kann in beiden Systemen durch Elektronen schiebende Gruppen an den Donoreinheiten bathochrom, auf über 800 nm verschoben werden. Die Ursache liegt in einem stärkeren Einfluss der Substituenten auf das HOMO als auf das LUMO und einem damit einhergehenden stärkeren Anstieg der HOMO-Energie woraus eine verkleinerte HOMO-LUMO Lücke resultiert. Die Dibenzo-Aza-Bodipys zeichnen sich durch eine rotverschobene Absorption gegenüber den (nicht benzannulierten) Aza-Bodipys aus. Jedoch ist der Akzeptor in den Dibenzo-Aza-Bodipys abgeschwächt, so dass die Rotverschiebung durch die selben Substituenten weniger stark ausgeprägt ist und die Energieniveaus tendenziell höher liegen. Die Verbindungen lassen sich thermisch im Vakuum verdampfen. Die für das Verdampfen wichtige thermische Stabilität, kann durch Austausch von Bordifluorid mit Borbrenzcatechol erhöht werden, ohne die optischen und elektronischen Eigenschaften wesentlich zu beeinflussen. Neben der Charakterisierung der molekularen Eigenschaften, sind einige Verbindungen im Dünnfifilm auf ihre elektrischen Eigenschaften und in Solarzellen untersucht worden. Die Ladungsträgerbeweglichkeit liegt bei den gemessenen Verbindungen zwischen 10E-6 und 10E-4 cm2V-1s-1. Durch Berechnung der Ladungstransportparameter auf Basis erhaltener Kristallstrukturen ist eine höhere Beweglichkeit auf eine günstigere Packung und einen geringeren intermolekularen Abstand zurückgeführt worden. Ausgewählte Verbindungen sind als Donormaterialien in organischen Solarzellen charakterisiert worden. Aus Lösungsmittel prozessierte Solarzellen mit Dibenzo-Aza-Bodipys erreichen eine Effifizienz von 1.6 % mit PC61BM, und 2.1 % mit PC71BM als Akzeptor. Der Effizienz limitierende Faktor ist hierbei der niedrige Füllfaktor von ca. 30 %. In vakuumprozessierten Solarzellen mit planarem Dono-Akzeptor-Übergang von Aza-Bodipys und Dibenzo-Aza-Bodipys hat sich gezeigt, dass die erhaltene Spannung mit abnehmender HOMO Energie der Materialien gesteigert wird. Ein geeignetes Dibenzo-Aza-Bodipy Material ist mit einen Beitrag zum Photostrom im nahen Infrarotbereich, von 750 - 950 nm, gezeigt worden.

La coopération fonctionnelle entre le cytochrome P450 3A (CYP3A) et la P-glycoprotéine (P-gp) décrite dans la littérature est un facteur déterminant contribuant à la variabilité de la biodisponibilité de la plupart des médicaments administrés per os. Le CYP3A et la P-gp sont des protéines membranaires dont les localisations respectives communément attribuées sont la membrane du réticulum endoplasmique pour le CYP3A et la membrane plasmique pour la P-gp. Le but de notre étude était une description intégrée au niveau cellulaire du fonctionnement de ces protéines membranaires de détoxication des médicaments et de leurs métabolites, grâce à la détermination de la localisation cellulaire de ces protéines par immunomarquage et par suivi fonctionnel de composés fluorescents, à l'aide de techniques de microscopie à épifluorescence. Le matériel utilisé comprenait des coupes de foie de rat et humain, des hépatocytes en culture primaire ou des lignées cellulaires de fibroblastes de Hamster chinois D/ADX surexprimant la P-gp. Nous avons ainsi démontré que le CYP3A se localise exclusivement au niveau du réticulum endoplasmique, alors que la P-gp est retrouvée au pôle apical des hépatocytes, au niveau des canalicules biliaires et sur la membrane plasmique des fibroblastes. Une colocalisation membranaire des composés et enzymes, synonyme d'une grande efficacité de détoxication, n'a pas été mise en évidence dans les hépatocytes, à cause de difficultés techniques et d'une trop faible fluorescence des substrats utilisés.

Le pyrrole est un hétérocycle souvent présent dans les produits naturels et les composés biologiquement actifs. Son étude intensive en synthèse organique est donc d'un grand intérêt. Ainsi, la première réaction acido-catalysée de pyrroles sur des nitrones, reportée par notre équipe, a conduit selon les conditions expérimentales employées, soit aux 2,2-bis(pyrrolyl)alcanes symétriques, soit aux N-benzylhydroxylamines pyrroliques de façon sélective. Les N-hydroxylamines pyrroliques sont des composés intéressants et ont été le point de départ des deux études méthodologiques développées dans ce manuscrit. Les bis(pyrrolyl)alcanes non symétriques ont pu être synthétisés efficacement et avec une bonne diversité, en une étape, par réaction entre des N-hydroxylamines pyrroliques et des pyrroles. La méthode a aussi permis la préparation de ter(pyrrolyl)alcanes symétriques et non symétriques. Une application de cette méthodologie a été la préparation de marqueurs de fluorescence F-Bodipy® non symétriques. Ces derniers ont été obtenus pour la première fois et avec de bons rendements, directement à partir de 2,2'-bis(pyrrolyl)alcanes non symétriques. De plus, un F-Bodipy® présentant une chiralité axiale a pu être synthétisé. Nous avons également étudié l'accès aux acides a-N-hydroxyaminés, analogues N-hydroxylés des acides a-aminés. Une réaction hautement diastéréosélective entre le pyrrole et une nitrone cyclique chirale a donné accès à une hydroxylamine pyrrolique. Une séquence en trois étapes a permis d'obtenir un a-N-hydroxyamino ester avec un bon rendement et un bon excès énantiomère de 80%. La méthode a ensuite été appliquée à d'autres hétéroaromatiques pyrroliques, indoliques et furaniques. Aucun acide a-N-hydroxaminé comportant l'un de ces hétérocycles n'était décrit précédemment dans la littérature. Les rendements globaux obtenus sont bons (36-62%) et les excès énantiomères peuvent atteindre des valeurs ≥ 98%.

Au cours de ces travaux de thèse, la synthèse de BODIPYs émettant dans le rouge et le proche infrarouge a été réalisée. Un premier travail d'extension de la conjugaison électronique (soit par introduction de groupements aromatiques sur le corps du BODIPY, soit par dimérisation) a permis d'atteindre les basses énergies du spectre électromagnétique. La validation d'une méthode de synthèse de BODIPYs à l'aide d'orthoesters a permis une mise en oeuvre aisée et une nette amélioration de leurs rendements. Parmi les fluorophores obtenus, les dithiényl-BODIPYs émettant dans la fenêtre thérapeutique (650-900 nm) présentent des propriétés optiques et électrochimiques intéressantes. Ils ont été utilisés pour des applications dans le domaine biologique. Après leur solubilisation dans les milieux aqueux par introduction de groupements ammoniums, la détection d'un acide aminé, la cystéine, a pu être réalisée par spectroscopies d'absorption et de fluorescence mettant en évidence un système ratiométrique. De plus, le marquage d'une protéine modèle, le sérum d'albumine bovine, permet d'envisager l'utilisation de ce type de BODIPYs pour l'imagerie biologique grâce à un rendement quantique de fluorescence élevé en milieu physiologique.

Les travaux réalisés dans le cadre de la thèse avaient pour but la synthèse de nouveaux fluorophores basés sur les unités BODIPYs et hydroxyphényl-s-triazines. Pour être utilisées comme sondes efficaces dans des applications en chimie et en biologie, les chromophores doivent présenter une forte brillance et des rendements quantiques de fluorescence élevés, une modulation aisée de ses propriétés optiques ainsi qu'une bonne stabilité chimique et photochimique. Une stratégie originale et efficace a été développée pour l'obtention de dérivés d'o-hydroxyphényl-s-triazine. Ces derniers présentent des déplacements de Stokes anormalement élevés ayant pour origine un transfert de proton intramoléculaire ultrarapide à l'état excité (ESIPT). L'extension du système π-conjugué sur les différentes plateformes de départ a été réalisée par couplages pallado-catalysés et/ou condensation de Knoevenagel, permettant ainsi d'accéder à une large gamme spectrale en absorption et en émission. Le développement d'une nouvelle voie de synthèse à permis l'obtention d'oligo-BODIPYs par couplage oxydant réalisé en associant PIFA et un acide de Lewis. Toutes les propriétés photophysiques des nouvelles sondes ont été étudiées et ont fait l'objet de discussions détaillées.

xx, 318 p. ： ill. (some col.) / Highly conjugated, carbon rich molecules are of great interest due to their unique optoelectronic properties. These molecules are now recognized as suitable materials for advanced materials applications such as light-emitting diodes, photovoltaics, and thin film transistors. Of particular interest is the indenofluorene (IF) scaffold, a 6-5-6-5-6-fused ring system that holds a striking topological similarity to pentacene, a very successful electron donating organic semiconductor. Also of interest is another class of compounds referred to as tetrakis(arylethynyl)benzenes, TAEBs, which are cruciform-shaped molecules that possess numerous pathways for electronic and photonic transfer and are amenable to a host of substitution patterns. Chapter I examines the history of the IF scaffold and its development over the last century through the use of literature examples relating to both its synthesis and potential use as an emerging class of electron accepting materials. Chapter II introduces the feasibility of stable, fully conjugated IFs. Two examples of 5,6,11,12-tetraethynyl-indeno[1,2- b ]fluorenes are synthesized where their structural and optoelectronic properties are explored. Chapter III further explores the IF scaffold and outlines the synthesis of a series of 6,12-diethynylindeno[1,2-b ]fluorenes in conjunction with detailed computational, structural, and photophysical studies. Chapter IV discusses the synthesis and characterization of a series of 6,12-diarylindeno[1,2- b ]fluorenes and examines their structural and optoelectronic properties. Chapter V describes a series of donor/acceptor-functionalized TAEBs that incorporate the 4,4-difluoro-4-bora-3a,4a,-diaza-s -indacene moiety, better known as BODIPY, as the acceptor unit. Additionally, two TAEB molecules and three structurally related bis(arylethynyl)benzene (BAEB) isomers where only acceptors are used to evaluate the effectiveness of the donor group are synthesized. This dissertation includes both previously published and unpublished co-authored material. / Committee in charge: Professor David R. Tyler， Chairperson； Professor Michael M. Haley， Advisor； Professor Victoria J. DeRose， Member； Professor Shih-Yuan Liu， Member； Professor Scott D. Bridgham， Outside Member

Lors de ces trois années de thèse, le développement de nouvelles molécules luminescentes, reposant sur la stabilisation d’un squelette dibenzoBODIPY a été réalisé. Dans un premier temps, la modulation des propriétés photophysiques a été effectuée (par introduction de groupement aromatique et/ou donneur soit lors de la formation du dibenzoBODIPY, soit par couplage croisé). Dans un second temps, nous nous sommes intéressés à la synthèse de dibenzoBODIPYs dissymétriques. La dissymétrie des composés a été réalisée soit par condensation mixte de dicétone soit par mono-couplage des dibenzoBODIPYs. L’utilisation de ces derniers a permis d’obtenir des dibenzoBODIPYs chiraux (soit par formation de liaison C-B, soit par formation de liaison B-O). La mono-fonctionnalisation intramoléculaire par le groupement phénol formé in situ est la première à ce jour. Enfin la formation de nouveau BODIPYs β-fusionnés par un hétérocycle a été entrepris. Nous avons synthétisé le premier BODIPY β-fusionné par un benzofurane. / During this PhD, the development of new luminescent molecules based on the stabilization of a dibenzoBODIPY scaffold was performed. Firstly, the modulation of the photophysical properties was carried out by the introduction of an aromatic and/or a donor group, either during the formation of dibenzoBODIPY or by cross-coupling. Secondly, we were interested in the synthesis of asymmetrical dibenzoBODIPYs. The dissymmetry of compounds was done either by mixed condensation of a diketone or by dibenzoBODIPYs mono-coupling. The use of the latters yielded chiral dibenzoBODIPYs either by C-B bond formation, or by B-O bond formation. Intramolecular mono-functionalization phenol formed in situ has never been reported so far. Finally, the synthesis of β‑fused BODIPYs with a heterocycle was undertaken. We synthesized the first β‑fused BODIPYs by a benzofuran

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.

Der Gegenstand der vorliegenden Arbeit ist die Untersuchung von photoinduzierten Energietransferprozessen (EET) und Elektronentransferprozessen (ET) in Modellsystemen, die als potentiell geeignet für eine Nutzung in der artifiziellen Photosynthese angesehen werden. Den beiden wesentlichen Zugängen zur Architektur artifizieller Photosynthese-Systeme entsprechend wurden vergleichend kovalente und sich selbst organisierende Systeme untersucht. In beiden Zugängen wurden ähnliche chemische Komponenten als optisch aktive Moleküle eingesetzt, insbesondere Phthalocyanine mit einem Silizium-Zentralatom (SiPc). Durch eine Kombination von stationären und zeitaufgelösten optisch-spektroskopischen Methoden konnten die lichtinduzierten ET- und EET-Prozesse identifiziert und quantifiziert werden. Im ersten Teil der Arbeit wurden mehrere kovalent gebundene Triaden und eine Pentade untersucht. In allen Systemen finden sehr effiziente ET und EET Prozesse statt. Es wurde gezeigt, dass das Lösungsmittel großen Einfluss auf die photophysikalischen Eigenschaften der Systeme ausübt. Die Lebensdauer des ladungsseparierten Zustandes variiert von 1,7 ns in Toluol bis 30 ps in DMF. Im zweiten Teil der Arbeit wurde erstmals gezeigt, dass sich in wässriger Lösung ein supramolekularer Komplex, bestehend aus einem Beta-Cyklodextrin (CD), einem konjugierten Subphthylocyanin (SubPc), einem Porphyrin (Por) und einem SiPc bilden kann. Letzteres wurde über unterschiedliche Ketten an zwei CDs kovalent gebunden. Die Selbstorganisation wird über hydrophobe Wechselwirkungen vermittelt und die Bildung der Komplexe ist sehr effizient. Nach selektiver Anregung von SubPc finden sequenzielle ET- und EET-Prozesse von SubPc zu SiPc statt. Das Por spielt die Rolle einer energetischen und elektronischen Brücke und ermöglicht die ET und EET-Prozesse von SubPc zu SiPc. Die Ladungsrekombination in den Grundzustand geschieht innerhalb von 1,7 ns. / The main objective of the present thesis was to conduct investigations of photo-induced electron transfer (ET) and excitation energy transfer (EET) processes in model compounds that are considered potentially appropriate for use in artificial photosynthesis. Two approaches have been used to construct the artificial photosynthetic systems, namely covalent and supramolecular approach. In both systems similar optically active molecules have been employed, particularly silicon-based phthalocyanines (SiPc). A comparative study between the covalently-linked and self-assembled systems had been conducted. For these purposes, thorough spectroscopic measurements in the UV/Vis range had been performed on these conjugates. A combination of steady-state and time-resolved experiments allowed an identification and quantification of the photo-induced ET and EET processes. In the first part of the work several covalently bound triads and a pentad bearing a central SiPc unit were studied. In all systems highly efficient ET and EET processes take place. It was found that the solvent exerts great influence on the photophysical properties of the systems. The lifetime of the charge-separated state varied from 1.7 ns (toluene) down to 30 ps (DMF). In the second part of the thesis, for the first time the formation of ternary supramolecular complexes consisting of a beta-cyclodextrin (CD), a conjugated subphthalocyanine (SubPc), a porphyrin (Por) and a series of SiPcs substituted axially with two CDs via different spacers was shown. These components are held in water by host-guest interactions and the formation of these host-guest complexes was found to be very efficient. Upon excitation of the SubPc-part of the complex sequential ET and EET processes from SubPc to SiPc take place. The Por dye acts as a transfer bridge enabling these processes. The probability of ET is controlled by the linker between CD and SiPc. Charge recombination to the ground state occurs within 1.7 ns.

Luminescent materials find numerous applications in recent times and have enriched human lives in several different ways. From display and lighting technologies to security, sensing and biological investigations, luminescent organic compounds have become indispensible and often preferred over their inorganic counterparts. The versatility of organic materials arises from their comparative low costs, ease of fine-tuning, low toxicity and the possibility to develop flexible devices. Even until very recent times, the investigations and usage of organic luminescent materials were mostly limited to solution-state properties. However, with progress of available characterisation techniques and parallel development of their usage in solid-state devices and other applications (e.g. security, forensics, sensing etc.), significantly greater attention has been paid to the development and investigations of solid-state emissive organic materials. In solid-state applications, apart from the molecular properties of any given material, their cumulative i.e. bulk physical properties are of even greater importance. Thus, investigations of structure-property relationships in organic luminescent compounds to understand their molecular and bulk properties are of fundamental interest. In this thesis, NPI (1,8-naphthalimide) and BODIPY (boron-dipyrromethene) dyes were investigated to provide a broad overview of their structure-property correlations. Among commonly encountered organic luminescent materials, NPIs and BODIPYs have emerged as two broad classes of luminescent organic compounds, finding applications as functional luminescent materials in various fields. However, lack of understanding for controlling the cumulative emissive properties of these compounds has limited their usage as active solid-state emitters in various applications. This thesis presents several new insights into the molecular and bulk emissive properties of these two classes of luminescent dyes (NPIs and BODIPYs). The contents of the six chapters contained in this thesis are summarised below. Chapter 1 summarises the available understanding of the basic concepts of photoluminescence and the design strategies to develop solid-state luminescent and AIE (aggregation-induced emission) active materials. This chapter also emphasises in the basic nature of the NPI and BODIPY compounds, their substitution patterns and their inherent characteristics and touches upon the relatively unexplored properties of NPI and BODIPY based materials. The importance and scope of the work reported in the thesis is outlined at the end of the chapter. Chapter 2 describes a detailed investigation of a series of seven (4-oxoaryl substituted) NPI compounds (1-7) providing an insight into the molecular and cumulative photophysical behaviour of these compounds. The low ICT characteristics of the NPIs, coupled with the twisted geometry, facilitated solid-state luminescence in these materials. The solution and solid-state luminescent properties of these compounds can be directly correlated to their structural rigidity, nature of substituents and solid-state intermolecular interactions (e.g. π-π stacking, C-H•••O interactions etc.). The solid-state crystal structures of the NPI siblings are profoundly affected by the pendant substituents. All of the NPIs (1-7) show antiparallel dimeric π-π stacking interactions in the solid-state which can further extend in parallel, alternate, orthogonal or lateral fashion depending on the steric and electronic nature of the C-4′ substituents. Structural investigations including Hirsfeld surface analysis methods reveal that while strongly interacting systems show weak to moderate emission in their condensed states, weakly interacting systems show strong emission yields under the same conditions. The nature of packing and extended structures also affects the emission colors of the NPIs in the solid-state. DFT computational studies were utilized to understand the molecular and cumulative electronic behavior of the NPIs. Apart from the investigation of solid-state luminescence, other functional potentials of these NPIs were also explored. One of the compounds (i.e. 4) shows chemodosimetric response towards aqueous Hg(II) species with a ‘turn-on’ response. Also, depending on the molecular flexibility of the compounds, promising AIEE (aggregation-induced emission enhancement) features were observed in these NPIs. Later (in Chapter 3), we developed a systematic investigation in a series of purely organic NPIs, restricting various parameters, to attain a thorough understanding of such AIEE properties. Chapter 3 describes a detailed experimental and computational study in order gain an insight into the AIE (aggregation-induced emission) and AIEE mechanisms in NPI compounds. Systematic structural perturbation was used to fine tune the luminescence properties of three new 1,8-naphthalimides (8-10) in solution and as aggregates. The NPIs (8-10) show blue emission in solution state and the fluorescence quantum yields depend on their molecular rigidity. In concentrated solutions of the NPIs, intermolecular interactions were found to result in quenching of fluorescence. In contrast, upon aggregation (in THF:H2O mixtures), two of the NPIs show aggregation-induced-emission-enhancement (AIEE). The NPIs also show moderately high solid-state emission quantum yields (~10-12.7 %). The AIEE behaviors of the NPIs depend on their molecular rigidity and nature of intermolecular interactions. The NPIs (8-10) show different extents of intermolecular (π-π and C-H•••O) interactions in their solid-state structures depending on their substituents. Detailed photophysical, computational and structural investigations suggest that only an optimal balance of structural flexibility and intermolecular communication is the effective recipe for achieving AIEE characteristics in these NPIs. Chapter 4 presents the design, synthesis and detailed investigations and potential applications of a series of NPI-BODIPY dyads (11-13). The NPI and BODIPY moieties in these dyads are electronically separated by oxoaryl bridges and the compounds only differ structurally with respect to methyl substitutions on the BODIPY fluorophore. The NPI and BODIPY moieties retain their optical features in these molecular dyads (11- 13). Dyads 11-13 show dual emission in solution state originating from the two separate fluorescent units. The variations of the dual emission in these compounds are controlled by the structural flexibility of the systems. The dyads also show significant AIES (Aggregation-Induced-Emission Switching) features upon formation of nano-aggregates in THF-H2O mixtures with visual changes in emission from green to red color. Whereas the flexible and aggregation prone system (i.e. compound 11) shows aggregation-induced enhancement of emission, rigid systems with less favorable intermolecular interactions (i.e. compound 12-13) show aggregation-induced quenching of emission. The emission-intensity vs. the structural-flexibility correlations were found to be reverse in solution and aggregated states. Photophysical and structural investigations suggest that the intermolecular interactions (e.g. π-π stacking etc.) play major role in controlling emission of these compounds in aggregated states. Similar trends were also observed in the solid-state luminescence of these compounds. The applications of the luminescent dyads 11-13 as live-cell imaging dyes was also investigated. Chapter 5 describes investigations of photophysical properties of a series of six BODIPY dyes (14-19) in which there is a systematic alteration of a common -C6H4Si(CH3)3 substituent. Inrelated constitutional isomers, the systematic increment of steric congestion and lowering of molecular symmetry around the BODIPY core result in a steady increment of solution and solid- state fluorescence quantum yields. The increasing fluorescence quantum yields (solution, solid state) with increasing steric congestions show that the molecular free rotation and aggregation-induced fluorescence quenching of BODIPYs can be successfully suppressed by lowering the flexibility of the molecules. Photophysical and DFT investigations reveal that the electronic band gap in any set of these constitutional isomers remain almost similar. However, the crystal structures of the compounds reveal that the solid-state colour and quantum yields of the compounds in solid-state are also related to the nature of intermolecular interactions. Chapter 6 demonstrates the use of DFT computational methods to understand the effect of alkyl groups in governing the basic structural and electronic aspects of BODIPY dyes. As demonstrated in Chapter 4 and Chapter 5, apparently electronically inactive alkyl groups can be of immense importance to control the overall photophysics of BODIPYs. In this context, a systematic strategy su was utilized considering all possible outcomes of constitutionally-isomeric molecules to understand the effects of alkyl groups on the BODIPY molecules. Four different computational methods were employed to ascertain the unanimity of the observed trends associated with the molecular properties. In line with experimental observations, it was found that alkyl substituents in BODIPY dyes situated at 3/5-positions effectively participate in stabilization as well as planarization of such molecules. Screening of all the possible isomeric molecular systems was used to understand the individual properties and overall effects of the typical alkyl substituents in controlling several basic properties of such BODIPY molecules.