Upconversion studies typically use a laser as the source of low-energy photons. A lower cost, lower power, and fully tunable system was used here. The Fluoromax-4 spectrofluorometer incorporates a 150-W xenon lamp and illuminates a 1 cm3 volume in a standard cuvette. The 1 cm x 1cm excitation beam was reduced to 1 mm x 1 mm, increasing the power density by 100. The cuvette was mounted upon a translation stage so that the excitation beam could skim the inside surface of the cuvette. This minimized self-absorption of the output. Finally, an optical filter was included between the excitation monochromator and the cuvette to ensure that the solution was not exposed to undesired wavelengths. The instrument was tested with a known sensitizer/emitter system and upconversion was detected. Subsequently, a new pair of compounds was studied but the [Ru(deab)3]2+/BPEA solution did not exhibit upconversion in the modified instrument.

No description available.

No description available.

Intramolecular singlet-singlet (SSET) and triplet-triplet (TTET) energy transfer have been studied in two cyclic octapeptides, 1A and 2A, and their open chain analogs, 1B and 2B. The peptides are constructed by a solid phase synthetic technique from enantiomerically pure amino acids with alternating chirality. Cyclic peptides with this arrangement of amino acids preferentially adopt flat, disk-like conformations where the peptide side chains lie on the outside of the ensemble. In 1A, benzophenone and naphthalene chromophores are incorporated as 4-benzoyl-L-phenylalanine and 2-naphtyl-L-alanine at positions 1 and 5 in the peptide sequence while in 2A, these chromophores occupy positions 1 and 3. Molecular modeling studies indicate that the interchromophore separation is larger in 1A than in 2A. This difference in separation is apparent from the observation of TTET energy transfer in 2A, which is consistent with the short range nature of TTET. Low temperature phosphorescence results indicate that intramolecular TTET is efficient in 2A and 2B and occurs with a rate of kTTET > 9.4x103 s-1. Intramolecular SSET occurs efficiently within these cyclic and open chain peptides. 1A undergoes intramolecular SSET from the naphthalene chromophore to the benzophenone chromophore with kSSET > 3.7x107 s-1, while in 2A with kSSET >3.0x107 s-1. Results obtained by modeling, UV-Visible spectroscopy, fluorescence and phosphorescence spectroscopies and transient absorption experiments are described.

Photon energy upconversion (UC), a process that can convert two or more photons with low energy to a single photon of higher energy, has the potential for overcoming the thermodynamic efficiency limits of sunlight-powered devices and processes. An attractive route to lowering the incident power density for UC lies in harnessing energy transfer through triplet-triplet annihilation (TTA). To maximize energy migration in multicomponent TTA-assisted UC systems, triplet exciton diffusivity of the chromophores within an inert medium is of paramount importance, especially in a solid-state matrix for practical device integration. In this thesis, low-threshold sensitized UC systems were fabricated and demonstrated by a photo-induced interfacial polymerization within a coaxial-flow microfluidic channel and in combination with nanostructured optical semiconductors. Dual-phase structured uniform UC capsules allow for the highly efficient bimolecular interactions required for TTA-based upconversion, as well as mechanical strength for integrity and stability. Through controlled interfacial photopolymerization, diffusive energy transfer-driven photoluminescence in a bi-molecular UC system was explored with concomitant tuning of the capsule properties. We believe that this core-shell structure has significance not only for enabling promising applications in photovoltaic devices and photochromic displays, but also for providing a useful platform for photocatalytic and photosensor units. Furthermore, for improving photon upconverted emission, a photonic crystal was integrated as an optical structure consisting of monodisperse inorganic colloidal nanoparticles and polymer resin. The constructively enhanced reflected light allows for the reuse of solar photons over a broad spectrum, resulting in an increase in the power conversion efficiency of a dye-sensitized solar cell as much as 15-20 %.

Metalloporphyrin aggregation is critical for triplet-triplet annihilation (TTA) to occur. In order to maximize the efficiency of TTA, to use as a mechanism of photon upconversion in dye-sensitized solar cells, it is important to understand the phenomenon of absorber aggregation. The aggregation of ZnTPP in polymer films was investigated by fluorescence anisotropy and total internal reflection fluorescence microscopy (TIRFM). Single molecule spectroscopy (SMS) and spectromicroscopy were the techniques used to study single molecules and multimolecular aggregates of ZnTPP/ZnTPPS in polymer films/adsorbed on glass substrates.<p> Fluorescence anisotropy measurements consistently showed depolarized emission from films most concentrated with ZnTPP. This observation was likely a result of energy transfer in and among porphyrin aggregates. Fluorescence intensity decays were also obtained and they illustrated a pattern of decreased fluorescence lifetime (i.e. faster decays) as the concentration of porphyrin in the film increased. These results are consistent with the formation of aggregates, and their increased presence in more concentrated films. The formation of these aggregates quenches the fluorophores fluorescence, resulting in the observed shorter fluorescence lifetimes. <p> TIRFM was performed to study the structure of these polymer films doped with ZnTPP. It was determined that these films consisted of discrete domains and thus lacked homogeneity, and the presence of aggregates was clearly visible. Time-resolved TIRFM measurements were also performed but no interesting results were collected.<p> SMS and spectromicroscopy were the final techniques employed to study porphyrin aggregation. Preliminary measurements were performed with polymer films doped with ZnTPP, and the single step decay time trajectories collected indicated that single molecules were being studied. Furthermore, emission spectra of these molecules were collected and they were similar to those obtained for a bulk measurement, but the bands were slightly shifted in comparison. These measurements were repeated with ZnTPPS adsorbed to glass substrates. Two different patterns of decay trajectories were measured: (i) single step decays corresponding to single ZnTPPS molecules and (ii) multi step/complex decays representative of multimolecular aggregates. Emission spectra were also collected for the multimolecular aggregates, and they were consistent with those of an ensemble measurement but slightly blue-shifted. Such a shift is common when studying aggregates on such a highly polar surface. Thus, these results demonstrate that ZnTPPS aggregates form even at concentrations as low as 10-8 M, and can be studied using SMS despite their weak fluorescence emission.

Metalloporphyrin aggregation is critical for triplet-triplet annihilation (TTA) to occur. In order to maximize the efficiency of TTA, to use as a mechanism of photon upconversion in dye-sensitized solar cells, it is important to understand the phenomenon of absorber aggregation. The aggregation of ZnTPP in polymer films was investigated by fluorescence anisotropy and total internal reflection fluorescence microscopy (TIRFM). Single molecule spectroscopy (SMS) and spectromicroscopy were the techniques used to study single molecules and multimolecular aggregates of ZnTPP/ZnTPPS in polymer films/adsorbed on glass substrates.<p> Fluorescence anisotropy measurements consistently showed depolarized emission from films most concentrated with ZnTPP. This observation was likely a result of energy transfer in and among porphyrin aggregates. Fluorescence intensity decays were also obtained and they illustrated a pattern of decreased fluorescence lifetime (i.e. faster decays) as the concentration of porphyrin in the film increased. These results are consistent with the formation of aggregates, and their increased presence in more concentrated films. The formation of these aggregates quenches the fluorophores fluorescence, resulting in the observed shorter fluorescence lifetimes. <p> TIRFM was performed to study the structure of these polymer films doped with ZnTPP. It was determined that these films consisted of discrete domains and thus lacked homogeneity, and the presence of aggregates was clearly visible. Time-resolved TIRFM measurements were also performed but no interesting results were collected.<p> SMS and spectromicroscopy were the final techniques employed to study porphyrin aggregation. Preliminary measurements were performed with polymer films doped with ZnTPP, and the single step decay time trajectories collected indicated that single molecules were being studied. Furthermore, emission spectra of these molecules were collected and they were similar to those obtained for a bulk measurement, but the bands were slightly shifted in comparison. These measurements were repeated with ZnTPPS adsorbed to glass substrates. Two different patterns of decay trajectories were measured: (i) single step decays corresponding to single ZnTPPS molecules and (ii) multi step/complex decays representative of multimolecular aggregates. Emission spectra were also collected for the multimolecular aggregates, and they were consistent with those of an ensemble measurement but slightly blue-shifted. Such a shift is common when studying aggregates on such a highly polar surface. Thus, these results demonstrate that ZnTPPS aggregates form even at concentrations as low as 10-8 M, and can be studied using SMS despite their weak fluorescence emission.

The present work reports the study of noncoherent photon upconversion (NCPU) via triplet-triplet annihilation (TTA) in polymer systems. This upconversion mechanism has application in photovoltaic devices through the utilization of sub-band gap photons for potentially enhanced power conversion efficiencies. First, homomolecular TTA was studied in zinc tetraphenylporphyrin (ZnTPP) in polymer matrices. Here, ZnTPP acts as both the sensitizer and upconverting emitter as TTA yields an S2 excited porphyrin. Use of poly(methyl methacrylate) (PMMA) as the host polymer demonstrates aggregation-driven upconverted fluorescence (UC) by TTA (TTA-UC). The dye-loading ratio of the precursor solution was varied, controlling the degree of pre-aggregation. Power-dependence studies of the champion film demonstrated that TTA-UC is occurring toward the strong annihilation kinetic limit. A sub-linear dependence of upconverted fluorescence on film thickness was observed in this system. The ZnTPP study was extended to polymers possessing low glass transition temperatures, representing molecular diffusion-driven TTA-UC. Upconverted fluorescence was not observed in ZnTPP in a polyurethane (PU) matrix, likely due to coordination of the PU to the axial position of the Zn2+ ion. Low intensity NCPU via homomolecular TTA was observed in ZnTPP in a poly(ethylene glycol) (PEG) matrix, but the kinetic limit was not determined due to film photodegradation. Dye-loading studies revealed that porphyrin self-quenching was evident at low dye concentrations. Likely reasons for the low upconverted fluorescence intensities realized are this self-quenching and the possibility of PEG coordination to the Zn2+ ion, though it is believed self-quenching is the dominant parasitic effect. Strategies to determine the effect and extent of polymer coordination to the Zn2+ ion are discussed. The study of polymer-based NCPU is extended to a pair of macromolecules, each containing a single ruthenium tris(bipyridine) (Ru(bpy)3) core and multiple pendant arms, which in turn, each contain eight 9,10-diphenylanthracene (DPA) moieties. A power-dependence study of NCPU in this system is reported, and TTA-UC in the weak annihilation kinetic limit was observed. Upconverted fluorescence quantum yields vary linearly with excitation power in both polymers, consistent with the observed kinetic limit. Stern-Volmer experiments have compared the quenching of Ru(bpy)3 phosphorescence (Ph) by monomeric and polymeric DPA. These data show an enhancement in quenching rate constant for the DPA polymer (pDPA). Kinetic analysis of the Ru-DPA polymers has revealed that the energy scheme realized in this system is intrachain TTET from Ru(bpy)3 core to DPA emitter followed by interchain TTA between excited DPA moieties. Low intensity upconverted fluorescence is observed in Ru-DPA containing thin films. Based on the results presented, the requirements of future photophysically-active polymers are discussed with regards to meaningful application in photovoltaics.

De nouveaux nano-objets anisotropes fluorescents sont obtenus par l’assemblage d’acènes spécifiquement conçus. Dans des cristaux, nano-rubans et nanoparticules anisotropes de 2,3-dialkyldiphenylanthracènes, les efficacités et la polarisation de l’émission bleue sont remarquables. La couleur de l’émission est accordée par le dopage avec des émetteurs verts et oranges (di- et tétra-phényltétracènes). La microscopie confocale de fluorescence permet d’étudier les cinétiques des états excités et des transferts d’énergie photo-induits, ainsi que la dispersion et les orientations des émetteurs. Pour la première fois, l’influence de la largeur de nano-rubans sur la cinétique d’annihilations triplet-triplet de tétracènes est mise en évidence. La microscopie révèle également le polymorphisme inhabituel d’un dérivé diéthynylphényl-anthracène. Ce travail ouvre des perspectives pour le développement et l’étude de processus fondamentaux de nano-matériaux luminescents. / New fluorescent anisotropic nano-objects are obtained by the assembly of specifically designed acenes. In crystals, nano-ribbons and anisotropic nanoparticles of 2,3-dialkyldiphenylanthracenes, the efficiencies and the polarization of the blue emission is remarkable. The color of the emission is tuned by doping with green and orange emitters (di-and tetra-phenyltetracenes). Confocal fluorescence microscopy is used to study the kinetics of excited states and photo-induced energy transfers, as well as the dispersion and orientation of the emitters. For the first time, the influence of the width of the nano-ribbons on the kinetics of tetracene triplet-triplet annihilations is highlighted. Microscopy also reveals the unusual polymorphism of a diethynylphenyl anthracene derivative. This work opens perspectives for the development and study of fundamental processes of luminescent nano-materials.

A cinética de fotopolimerização do monômero metacrilato de metila (MMA) foi investigada usando o fotoiniciador tioxantona (TX) e o co-iniciador trietilamina (TEA), a fim de elucidar o mecanismo de fotopolimerização. A dilatometria foi usada como técnica para a determinação das cinéticas de polimerização. Determinaram-se os rendimentos quânticos de polimerização. Os estados excitados foram estudados por fluorescência estática para determinar as constantes de supressão de fluorescência pela amina e pelo monômero. Medidas de fotólise por pulso de laser permitiram obter o espectro de absorção triplete-triplete da tioxantona, bem como a constante de supressão do estado triplete. Os resultados mostraram que a supressão ocorre por um processo colisional no estado singlete e que os radicais eficientes na fotoiniciação são originados da amina TEA. A massa molecular média dos polímeros obtidos foi determinada por medidas de cromatografia de permeação em gel (GPC). Em uma segunda etapa, foi investigada a polimerização fotoiniciada por fotoiniciadores usados em resinas dentárias, canforquinona (CQ), 1,2-fenilpropanodiona (PPD), Lucirin e Irgacure. A velocidade de polimerização foi acompanhada por dilatometria usando excitação acima de 348 nm. Obteve-se os rendimentos quânticos de polimerização e os polímeros foram caracterizados por GPC. A maior constante de velocidade de polimerização obtida foi para o Irgacure e o maior rendimento quântico de polimerização obtido foi para o Lucirin. As diferenças na eficiência da polimerização são atribuídas à formação de radicais e à capacidade de gerar radicais a partir da absorção da luz incidente. Por fim, investigou-se os espectros de absorção triplete-triplete (TTA) da TX em diferentes solventes usando a técnica de fotólise por pulso de laser. Observou-se a formação de três bandas, duas referentes ao triplete da TX (uma em 300 nm e outra em 600 nm) e em 400 nm, observou-se a formação do radical cetila da TX. A abstração de hidrogênio do solvente pela TX pode ocorrer em 340 nm em solventes com constante de abstração de hidrogênio (kabstr) alto. Esse fator pode ser observado nos solventes 2-propanol e etanol. O substituinte na TX mostrou, através do parâmetro de Hammett do substituinte que com o aumento da eletronegatividade há um deslocamento do máximo de absorção do transiente para comprimentos de onda menores. / The photopolymerization of the monomer methyl methacrylate (MMA) using thioxanthone (TX) as photoinitiator and triethylamine (TEA) as co-initiator, was investigated in order to explain the photopolymerization mechanism. Dilatometry was used to determine the kinetics of polymerization. The polymerization quantum yields were determined. The excited states were studied using static fluorescence in order to determine the fluorescence rate quenching by the amine and the monomer. The triplet-triplet absorption (TTA) spectrum of thioxanthone and the rate constant of quenching of the triplet state were obtained using flash photolysis technique. The results showed that the singlet state quenching occurs by a colisional process and the efficient radicals in the photoinitiation process are originated from the amine TEA. The molar mass of the obtained polymers was determined by gel permeation chromatography (GPC). The polymerization photoinitiated by the photoinitiators used in dental resins, Camphorquinone (CQ), 1,2- Phenylpropanodione (PPD), Lucirine and Irgacure was investigated in a second step. The rate of polymerization was studied by dilatometry with excitation above 348 nm. The polymerization quantum yields were obtained and the polymers were characterized by GPC. The larger rate constant of polymerization was found for Irgacure and the larger polymerization quantum yield was obtained for Lucirine. The differences in polymerization efficiencies can be tracked to the formation of radicals and the capability to produce the radicals after incident light absorption. Finally, the TTA of TX was studied in different solvents using the flash photolysis technique. The spectrum presented three bands, two due to the transient of TX (at 300 and 600 nm), and one at 400 nm assigned to the cetyl radical. The abstraction of a solvent hydrogen by TX is observed at 340 nm in solvents with high hydrogen abstraction constant (kabstr). This fact can be observed in 2-propanol and ethanol. An increase in the solvent polarity stabilizes the electronic pp* states and unstabilizes the np* states. The substitution in the TX molecule produced a shift to lower wavelength in the maximum absorption of transient with the increase of the electronegativity, as indicated by the dependence of the Hammett parameter.