Electrogenerated chemiluminescence (ECL) studies were performed on several interesting compounds. A series of BODIPY derivatives was examined to understand the structural effects on the electrochemical, spectroscopic, and ECL behavior. Stable electrochemistry and high fluorescence in the green to the red regions were observed. PB, MCPB, DCPB and PM580 produced intense ECL, strong enough to be seen with the naked eye in a lighted room. Unlike MCPB and DCPB, PB produced the multiple ECL peaks. Totally blocked BODIPY compound showed the improvement of fluorescence and ECL quantum yield due to the stability of radicals. Strong signal of EPR data during the oxidative electrolysis was obtained by simultaneous electrochemical-electron paramagnetic resonance technique with home-made cell. Several new antrhacene derivatives such as a variety of 2- and 4-fold anthracene-functionalized tetraarylbimesityls and a series of 9-naphthylanthracene based dimer and trimer were studied. They showed one wave on the oxidation and reduction because of a sequence, two or more electron transfers during the annihilation of the radical ions. Depended on the structure, some of them exhibited excimer formation on ECL spectra. Azide-BTA compound which consists of two triphenylamine and 2,1,3-benzothiadiazole groups at the ends bridged by a fluorene moiety was synthesized and examined. The compound is a newly synthesized D-A-[pi]-A-D molecule which had reversibility upon electrochemical oxidation and reduction, and also showed intense red fluorescence and stable red ECL emission. Using a simple reprecipitation method, well-dispersed and spherical organic nanoparticles of Azide-BTA and 9-naphthylanthracene based dimer were prepared in an aqueous solution. Controlling the preparation condition, the size of nanoparticles can be minimized to 15 nm. Especially we prepared the organic nanoparticles of 9-naphthylanthracene based dimer dispersed in organic solvent, MeCN, one of the preferred solvents for electrochemical studies and ECL. / text

Au cours des vingt dernières années, les nanomatériaux (caractérisés par des dimensions de l’ordre de 10 à 100 nm) ont attiré une attention croissante de par leurs propriétés optoélectroniques uniques. C’est tout particulièrement vrai pour les nanomatériaux inorganiques, tels que les quantum dots, les nanoparticules métalliques ou encore les nanoparticules à base de silice. Par contre, l’étude des nanomatériaux obtenus à partir de molécules organiques est un domaine d’intérêt beaucoup plus récent. Cette thèse présente une étude détaillée de nanoparticules organiques fluorescentes originales et des nanostructures organiques préparées à partir de deux ou trois types de chromophores distincts (nanocomposites binaires et ternaires). En particulier, l'attention est focalisée sur la préparation, la caractérisation et la stabilisation de ces nouveaux nanomatériaux, ainsi que leurs applications dans le domaine biologique et de l’opto-electronique. / During the past two decades, increasing research attention has been devoted to nanomaterials (materials in the range of 10-100 nm) because of their unique optoelectronic properties. In particular, inorganic nanomaterials, such as quantum dots, metal-based nanoparticles and silica nanoparticles, have been investigated extensively. Instead, nanomaterials based on organic molecules are been subject of research only since very recent years. This thesis presents an extensive study of novel fluorescent organic nanoparticles and fluorescent organic binary and ternary nanoassemblies. In particular the attention is focused on the preparation and characterization of organic nanoparticles and new nanocomposites obtained from different types of small organic chromophores, their stabilization and the use of these materials for biological and optoelectronics applications.

L’utilisation de nano-objets luminescents en milieu biologique est devenue très répandue, notamment en vue d’applications biomédical est elles que l’imagerie, la thérapie et le diagnostic. Jusqu’à récemment, les principaux travaux réalisés dans ce domaine concernaient les nanoparticules de silice dopées ou fonctionnalisées avec des molécules organiques, les nanoparticules d’or et les nanoparticules semi-conductrices (quantum dots, i.e., QDs). Toutefois, un certain nombre de limitations demeurent pour les applications dans le domaine du vivant, en lien notamment avec des problèmes de stabilité, de biocompatibilité et de toxicité ou encore de biodégradabilité. En parallèle,un certain nombre de molécules organiques fluorescentes non-toxiques ont été utilisées comme sondes fluorescentes en milieu biologique, mais leur brillance demeure limitée. L’idée directrice de la thèse est de concevoir et synthétiser de nouveaux chromophores organiques présentant une émission modulable (du visible au proche infrarouge) et adaptés à la préparation de nanoparticules organiques fluorescentes (FONs) combinant à la fois une brillance extrêmement élevée, une excellente stabilité colloïdale et une photostabilité adaptée à leur utilisation en imagerie in vitro et in vivo. De tels nanoobjets ultra-brillants pourraient alors représenter une alternative très intéressante aux nanoparticules actuellement les plus utilisées en imagerie de fluorescence du vivant (QDs). Le manuscrit décrit la synthèse et les propriétés de plusieurs classes de molécules fluorescentes spécifiquement conçues pour former des telles FONS par auto-assemblage dans l’eau. La préparation de ces FONs est présentée et leurs propriétés étudiées et discutées. Enfin des applications concrètes en bio-imagerie sont présentées. / Nowadays the use of bright luminescent nano-objects in biological environment is a topic that is gaining more and more importance, especially for biomedical applications such as imaging, the rapyand diagnostic. So far, numerous studies have been conducted with gold nanoparticles, silica nanoparticles (doped or functionalized with organic molecules), as well as semiconductor nanoparticles (quantum dots, i.e., QDs). However, most of these nanoparticles suffer from drawbacks (in terms of stability, biocompatibility, eco-toxicity or degradability). On the other hand, several nontoxic fluorescent molecular probes have been widely used, but most of the time their brightness remain modest in biological environments compared to QDs. Our idea is to engineer new organicchromophores with tunable emission wavelength (from visible to near infrared) for further preparation of organic fluorescent nanoparticles (so called FONs) that display giant one-photon and two-photonbrightness, as well as good colloidal and chemical stability, and suitable photostability for in vitro andin vivo imaging. As such, these FONs would represent interesting alternatives to QDs for use in bioimaging. This manuscript describes the synthesis and characterization of new classes of fluorescent molecules specifically engineered as building blocks for the fast preparation of such nanoparticles byself-aggregation in water. The FONs were fully characterized from both morphological and photophysical points of view and further used in bioimaging.

During the past two decades, increasing research attention has been devoted to nanomaterials (materials in the range of 10-100 nm) because of their unique optoelectronic properties. In particular, inorganic nanomaterials, such as quantum dots, metal-based nanoparticles and silica nanoparticles, have been investigated extensively. Instead, nanomaterials based on organic molecules are been subject of research only since very recent years. This thesis presents an extensive study of novel fluorescent organic nanoparticles and fluorescent organic binary and ternary nanoassemblies. In particular the attention is focused on the preparation and characterization of organic nanoparticles and new nanocomposites obtained from different types of small organic chromophores, their stabilization and the use of these materials for biological and optoelectronics applications.

Les nanoparticules inorganiques luminescentes ont suscité un intérêt croissant au cours des dernières décennies, notamment pour leur application en imagerie biologique. Un certain nombre d’entre elles présentent toutefois des limitations telles que toxicité, absence de biodégradabilité, faible brillance, clignotements…. Dans cette optique, les nanoparticules fluorescentes à base de petites molécules organiques (FONs) offrent une solution alternative prometteuse aux nanoparticules inorganiques pour l'imagerie biologique. Le principal défi réside dans l'élaboration des nanoparticules organiques possédant une brillance élevée, une bonne stabilité dans l'eau (y compris en milieu biologique), une bonne biocompatibilité ainsi qu'une émission accordable dans le visible et au-delà dans le proche infrarouge (pour une détection plus aisée en milieu diffusant). Dans cette optique, nous avons utilisé une stratégie basée sur l’utilisation de chromophores dipolaires de type "push pull" « adaptés ». Au cours du travail, la synthèse de séries de chromophores homologues bâtis sur le même système conjugué et ayant en commun un groupe donneur de type triphénylamine (destiné à préserver les propriétés de luminescence) présentant ou non des motifs encombrants positionnés a été réalisée. Les nanoparticules correspondantes ont été préparées selon un protocole classique, simple et rapide à mettre en oeuvre (précipitation). L’étude des propriétés photophysiques des nanoparticules organiques fluorescentes ainsi obtenues a été réalisée et mise en perspective avec celles des chromophores en solution dans des solvants organiques de polarité variable. Une étude systématique de l’évolution dans le temps des propriétés optiques des nanoparticules organiques a été réalisée permettant de mettre en lumière des relations entre la structure des sous-unités chromophoriques et la stabilité colloïdale et « optique » des nanoparticules. Ces études ont permis d’identifier des nanoparticules émettant dans le proche infrarouge extrêmement brillantes et présentant une stabilité colloïdale remarquable dans l’eau, une photostabilité accrue et une très bonne biocompatibilité. De ce fait, ces nanoparticules ont pu être utilisées avec succès dans l'imagerie biologique des cellules et le suivi (tracking) à l'échelle de la particule unique, démontrant l'intérêt de la démarche d'ingénierie mise en oeuvre. / During the last decades, luminescent inorganic nanoparticles have attracted a large interest in different fields including biological imaging. However, a number of them have drawbacks such as toxicity and absence of biodegradability. Recently, molecular-based fluorescent organic nanoparticles (FONs) have emerged as a promising alternative to inorganic nanoparticles for bioimaging. The main challenge lies in the elaboration of organic nanoparticles that combine large brightness, good colloidal stability in biological environments) and biocompatibility as well as NIR emission (to allow improved detection in thick tissues). To achieve this objective, we have implemented a molecular engineering strategy based on dedicated polar and polarizable "push pull" chromophore built from a triphenylamine donor moiety and a specific pi-conjugated system. The corresponding nanoparticles were readily prepared by the reprecipitation method. In the present manuscript, the synthesis of the chromophores and the preparation and characterization of the organic fluorescent nanoparticles is described. A comprehensive investigation of their photophysical properties and study of their colloidal stability is presented allowing to derive structure-property relationships. The implemented study led to innovative NIR-emitting nanoparticles combining large brightness (superior to those of QDs and NIR-emitting organic dyes), remarkable colloid stability and suitable photostability. These nanoparticles have been successfully used for single particle tracking and imaging in cells, while no toxic effect was observed.

The aim of this diploma thesis is to study the optical properties of new -conjugated molecules based on 1,4-di(4'-N, N-diphenylaminostyryl)benzene and their potential application in advanced imaging techniques of biological specimens, the so-called multiphoton microscopy. The thesis focuses mainly on the characterization of their optical properties and the determination of their two-photon absorption cross-section using a unique laser equipment. Furthermore, a suitable methodology for the preparation of nanoparticles from these molecules, their characterization and stability are also developed. The thesis also investigates the influence of structure on the optical properties of these molecules. The relation between the length of the conjugated system and the presence of substituents on the backbone has been shown for the optical properties of the molecules in the solvent, the position and shape of the two-photon absorption spectrum and the value of two-photon absorption cross-section. It has been shown that this substance retains its unique fluorescent properties even in the form of nanoparticles and therefore appears to be a suitable candidate for the observation of biological specimens using multiphoton fluorescence microscopy. The work contributes to the knowledge base for the design of the chemical structure of molecules with desired properties.

The two-photon absorption (2PA) phenomenon has attracted attention from various fields ranging from chemistry and biology to optics and engineering. Two of the common NLO applications in which organic materials have been used are three-dimensional (3D) fluorescence imaging and optical power limiting. Two-photon absorbing materials are, therefore, in great demand to meet the needs of emerging technologies. Organic molecules show great promise to meet this need as they can be customized through molecular engineering, and as the development of two-photon materials that suit practical application intensifies, so does research to meet this need. However, there remains some uncertainty in the particulars of design criteria for molecules with large 2PA cross sections at desired wavelengths, as such research to understand structure-property relationships is matter of significant importance. As a result, the full potential of 2PA materials has not been fully exploited. Several strategies to enhance the magnitude and tune the wavelength of 2PA have been reported for ?-conjugated organic molecules. On this account, we have designed novel fluorophores using the fluorene moiety and modified it to tune the properties of the compounds. Chapter 2 of this dissertation reports the successful application of fluorene-based compounds in photocatalysis; a process that involves the decomposition of organic compounds into environmentally friendly carbon dioxide and water attesting to the photostability of the fluorene moiety. A facile organic nanoparticle preparation method is reported in chapter 3 using the reprecipitation method, whose surface was then modified using a naturally occurring surfactant, Lecithin, and were then successfully used in fluorescence cell imaging. Chapter 4 reports the design and synthesis of a fluorene-based compound using an acceptor, s-indacene-1, 3, 5, 7(2H, 6H)-tetra one, or Janus Dione, a moiety that is relatively new and that has not been fully exploited despite its very attractive features. Owing to the hydrophobicity of this compound, notwithstanding its unprecedented 2PA cross section, it was not applicable in fluorescence cell imaging but provided the tenets for the design of related derivative. This limitation was circumvented in the concluding chapter by tuning the compound's hydrophilicity. The hydrophilic Janus dione probe was then used as envisioned for cell imaging as the dual prerequisites for fluorescence imaging probes; large 2PA cross sections and high fluorescence quantum yields were met.

Les performances des techniques de bioimagerie et de biodétection peuvent être améliorées grâce aux nanoparticules fluorescentes (NPs) permettant un transfert d’énergie résonante de type Förster (FRET) efficace. Le but de mon projet de thèse est le développement de NPs polymériques brillantes et ultrastables encapsulant des fluorophores, capables de produire un FRET au-delà du rayon de Förster. Il a été montré que les groupements encombrés sont essentiels pour minimiser l’auto-extinction et le blanchiment des fluorophores encapsulés. Par ailleurs, la matrice polymérique joue un rôle crucial dans le contrôle de l’effet collaboratif entre fluorophores du au transfert d’énergie d’excitation. Puis, en utilisant cet effet collaboratif entre fluorophores, nous avons conçu des NPs présentant une photocommutation efficace, ainsi qu'un phénomène de "light harvesting" très important. Enfin, de très petites NPs avec un FRET efficace à leur surface ont été élaborées et appliquées pour la détection ultra-sensible de protéines. Les résultats obtenus fournissent de nouvelles perspectives dans le développement des nanoparticules brillantes avec un transfert d'énergie efficace, ainsi que des nano-sondes pour la détection de molécules uniques. / Performance of biosensing and bioimaging techniques can be improved by fluorescent nanoparticles (NPs) capable of efficient Förster resonance energy transfer (FRET). The aim of my PhD project is to develop bright and photostable dye-loaded polymer NPs capable to undergo efficient FRET beyond the Förster radius. We showed that bulky groups are essential for minimizing self-quenching and bleaching of encapsulated dyes. Moreover, polymer matrix plays a crucial role in controlling the inter-fluorophore communication by excitation energy transfer. Then, by exploiting communication of dyes, we designed NPs exhibiting efficient photoswitching as well as giant light-harvesting. Finally, very small NPs with efficient FRET to their surface were developed and applied for ultra-sensitive molecule detection of proteins. The obtained results provide new insights in the development of bright nanoparticles with efficient energy transfer as well as nano-probes for single-molecule detection.

The present thesis entitled, "Molecular expression through fluorescence: Studies in probe design and aggregation" describes very simple bi-functional donor-acceptor poly-aromatic fluorophores that have been shown to possess distinctive properties depending on the context in which they are studied. In a sense, this work is an effort in exemplifying the inherent diversity and power of "molecular expression", with the central theme here being the phenomenon of fluorescence. The work has been divided into four chapters, each having a self-contained introduction. Chapter 1: First instance of metal ion (Zn2+) sensing exclusively at amphiphilic interfaces. (1 -pyrenyl)rnethyl-bis- [(2-pyridyl)methyl]amine (Pybpa), a simple, bi-functional fluorophore was synthesized. Pybpa has the modular design of a photoinduced electron transfer (PET) based analyte sensor. In Pybpa, a photoinduced electron transfer (PET) operates from the pyrenyl nitrogen (PyCH2-iV) to the excited pyrenyl (Py*) chromophore leading to fluorescence quenching. Zn2+ ion binding to the bis-picolyl (bpa) unit of Pybpa stops the PET process and leads to fluorescence enhancement. Thus Pybpa was able to sense Zn2+" in organic solvents. In water, however, Pybpa showed pronounced aggregation and the probe did not sense any metal ion. Surfactant micelles provide hydrophobic regions in water and the dynamic rnicellar assemblies could disrupt Pybpa aggregates. Pybpa monomers solubilized in micelles were responsive to Zn2+ in the low micro molar concentration range. The metal ion sensing on micelles was reflective of the charge of the interface. The sensing is negligible on cationic surface (CTAB), moderate on negatively charged surface (SDS micelles) and is the most efficient on neutral interface provided by TWEEN-20 micelles. With the Pybpa 'sensor*, no sensing is possible in water and hence the sensing is exclusive to the interface. Pybpa doped in membranous aggregates like phosphatidylcholine (PC) lipid bilayers, exists in monomeric form, and was able to sense Zn . The sensing on phosphatidylcholine (PC) bilayer vesicles was found to depend on the fluidity of the membrane. Zn2^ sensing with interfacially bound probe "was extended to a globular protein bovine serum albumin (BSA). BSA, a carrier protein, can bind hydrophobic molecules as well as metal ions like Zn2f. BSA was shown to disrupt Pybpa aggregation and bind Pybpa in a facile manner. BSA bound Pybpa was able to sense externally added Zn2+. Biological sensing of trace amounts of Zn2+ has been considered important since Zn2+ is crucial for eukaryotic systems. This is the first example of such 'exclusive' interfacial sensing of a metal ion. Chapter 2: Towards understanding and modulating self-assembly of pyrenyl bis-picolyl a mine: Organic nanoparticles that show tunable emission. Pybpa was found to aggregate in water in the size range of 80-250 nm. Evidence of aggregation was seen at concentrations as low as 1 |iM. The nanoscopic particles formed were characterized through transmission electron microscopy (TEM) and dynamic light scattering (DLS). Pybpa in water showed dual emission bands, with one band resernhling the emission from 'monomeric' Pybpa (as seen in solutions in organic solvents) and a broad red-shifted emission band (A,max ~ 480 ran) designated as "aggregate/nanoparticle" emission. Distinct excitation spectra for the two emission bands indicate that the bands (the '390 nm' band and the '480 nm' band) originate through distinct excitation/emission channels. The time resolved emission decay for the 'monomer' emission (397 nm) showed a substantial contribution from a long-lived pyrene-like excited state (x = 103.9 ns, 40% relative amplitude). On the other hand, the decay at 475 nm (for the nanoparticle/aggregate emission band) was considerably faster, with no evidence of any pyrene-like long-lived state. The short lifetimes indicated an exciplex nature of the red-shifted emission band, X-,nax~480 nm. The effect of temperature and urea on these aggregates was examined. The nanoparticles formed even in a concentrated urea solution (7.8 M). The aggregates formed in urea were found to be more emissive, indicating a 'looser' aggregate with reduced fluorescence quenching. Similar results were obtained on heating the aggregate. Increasing the concentration of Pybpa in water causes a change in the nature of the colloids formed as exemplified by increase in aggregate size and a decrease in the polydispersity index. Also seen was a substantial red shift in the 'aggregate emission'. At higher concentrations, the presence of three independent excitation/emission channels was observed. It is likely that a new type of aggregated Pybpa species formed at higher concentration, which emits at longer wavelength (A,rnax~540 nm), In such a scenario, it is possible to tune the emission wavelength by the choice of appropriate wavelength of excitation. Further, there is an opportunity to tailor the emission properties by controlling the aggregation behavior. The modulation of emission is one of the primary goals of research on fluorescent organic nanoparticles. Chapter 3: Photophysical properties of aryl-terpyridines in solution, solid and aggregated state: Unique CT emission from nanoparticles in water. Two aryl terpyridines, 4T-(l-pyrenyl)-2,2l:6'52fl-terpyridine (Pytpy) and 4'-(9-anthryl)-2,2':6',2n-terpyridine (Antpy), where the fluorophoric pyrene or anthracene unit is directly coupled to the terpyridine unit, were synthesized. The aryl terpyridines conjugates can be viewed as donor-acceptor molecules that are conformationally labile, with the possibility of rotation around three single bonds. It was of interest to see as to how conformational effects express themselves in different environments, especially in relation to the possibility of charge separation. Crystal structure data and Serni-empirical AMI calculations revealed a twisted molecular conformation for each of the molecules. Absorption and emission (steady state as well as tirne-resolved) behavior of Pytpy and Antpy in various organic solvents have been presented. The molecules showed only limited conjugation between the two units in the absorption behavior with the degree of conjugation being greater for Pytpy. In the emission behavior, only a single emission band (with a single lifetime) was observed in all organic solvent. Steady state and time resolved fluorescence data suggest the existence of a mixed or coupled, largely 7t—7i* state, with only marginal charge separation. The various photophysical parameters have been determined for the two systems. It appears that in the excited state, the inesomeric interactions show an increase for each of the two aryl-terpyridines, indicating at least a partially planar geometry in the excited state. Some specific solvent effects were observed for the molecules in alcoholic solvents and there was evidence of excited state H-bonding occurs for the aryl terpyridines in polar protic organic solvents, especially methanol. Pytpy and Antpy self-assembled in water over a large concentration range (1-100 |xM) to form spherical nanoparticles in the size range of 150-200 nm, as characterized by TEM and DLS. The absorption spectra for both conjugates showed red shift of the absorption bands in water (-10 nrn) along with significant tailing of the long-wavelength bands. The change in emission behavior in going from solution to the aggregates in water was very dramatic. Multiple, broadened, highly red-shifted emission bands for both Antpy and Pytpy were observed. Quite significantly, a long lifetime component in the emission decay was shown by the conjugates in water as compared to the lifetimes observed in solution. The data points towards a unique CT emission for Antpy and Pytpy aggregates in water. The excitation spectra for the multiple emission bands seen for Pytpy (or Antpy) were observed to be identical. Thus a single ground state population is responsible for emission over the entire range (approximately 420 nin - 600 nm). The existence of multiple emission bands and the large bathochromic shifts are exclusively due to excited state effects in the aggregated state in water. It appears that excited state H-bonding of the tpy N with water helps facilitate the excited state CT. The solid-state behavior of Pytpy and Antpy lias been examined and the emission from the two crystalline solids is very distinct. Antpy emission showed a X,,nax at -430 nm while Pytpy emission peaked at ~ 560 nm. The difference in the solid-state emission behavior exhibited by Pytpy and Antpy is explained through a consideration of the crystal packing for the two molecules. The degree of n-facial stacking was observed to be much greater for Pytpy. The observation of the distinct packing and emission shown by solid Pytpy and Antpy is highly significant if one considers the identical emission shown by the aqueous nanoparticles of the two molecules and brings to fore the 'nanoparticle effect' in water as compared to a simple concentration effect. It was also demonstrated that it was possible to modulate the aggregation of the terpyridines through additives, like metal ions Chapter 4: Pyrenyl terpyridine as a ratiometric fluorescence probe for sensing order and polarity of membranous aggregates. Pytpy was examined for its utility in probing surfactant aggregates, particularly membranous assemblies. la lipid bilayer vesicles made of phosphatidylcholine (PC) lipids (like dimyristoyl phosphatidylcholine, DMPC or egg-yolk PC) Pytpy showed an emission profile with marked similarity to that shown by the probe in water. Specifically, a broad red-shifted emission with A,maxin. the 500 nm region was observed. In addition, a peak in the -420 nm region was also seen. Fluorescence anisotropy was used to confirm the presence of vesicle-bound probe. Excitation spectra confirmed the presence of two distinct probe populations, om responsible for the '420 nm9 emission and another population responsible for the multiple, red-shifted emission bands. The emission behavior was indicative of aggregation of Pytpy on the vesicle surface and CT effects operating in conjunction with H-bonding. Fluorescence lifetime measurements, carried out at different Is suggest the CT nature of the red-shifted emission. The aggregation of the probe on the bilayer interface was confirmed by concentration and temperature dependence of the emission profile. The role of water in stabilizing this CT emission on bilayer surfaces was shown with use of a surface dehydrating agent polyethylene glycol (PEG). All these results helped build a model for the behavior of Pytpy in water. Pytpy aggregates on bilayer surface and shows a red-shifted CT emission with stabilization by interfacial water. Thus, the Pytpy 'aggregate' has a shallow, water accessible location in the bilayer. In addition to this, there is another Pytpy population responsible for the emission in the 420 nm region, and this second population might have a comparatively deeper location. The wavelength of the CT emission was sensitive to the polarity of the interface as evidenced "by the results obtained with bilayers made of a number of PC lipids. In general, the X™ax of the CTband showed a red shift with increasing polarity. The increase in polarity also caused an increase in the average lifetime of the probe. Pytpy could distinguish between vesicles made of lipids of different head groups. Aggregates made of phosphatidylethanolamine (PE) head group are in general less hydrated than PC lipid assemblies and Pytpy emission reflected this when examined in vesicles made of related lipids (dioleoyl lipids, DOPC and DOPE; dirnyristoyl lipids, DMPC and DMPE). Pytpy emission from PE vesicles was quenched and showed a pronounced blue shift in the emission Xmax vis-a-vis PC bilayers. Thus, dehydration of the interface consistently led to the destabilization of the CT state. Further, Pytpy emission was also responsive to hydration in more complex mixed PC-PE assemblies. Pytpy emission "behavior was also used to probe fluidity in complex "mixed" lipid assemblies- The effect of cholesterol on DMPC bilayers in terms of its known ability to dehydrate the bilayer was reported through a blue-shift Xmax of CT emission band. Further, cholesterol also causes drastic change in the bilayer at concentrations greater than ~ 30 mol%. This change in the bilayer was sensed through a sudden reduction in fluorescence intensity. Also from a careful analysis of Pytpy in various PC and PE vesicles, it emerged that the more fluid aggregates showed larger quantum yields. Thus, Pytpy could simultaneously report on both the polarity and fluidity of lipidic aggregates. Pytpy could also provide information about the order of an assembly. While the probe aggregated in bilayers and other membranous assemblies and showed water assisted CT emission, in more dynamic assemblies like micelles, Pytpy aggregates were not sustained, Pytpy in micelles showed emission spectra very similar to that seen in solutions in aprotic organic solvents. Thus, Pytpy proved to be a very useful ratiometric sensor for vesicle-to-rnicelle transition. Also, it has been possible to study some surfactant-lipid mixed assemblies that show phase separation. Pytpy reported the formation of a 'rigid', bilayer-like phases in mixed assemblies that are called bicelles.

No description available.