abstract: Photosystem I (PSI) is a multi-subunit, pigment-protein complex that catalyzes light-driven electron transfer (ET) in its bi-branched reaction center (RC). Recently it was suggested that the initial charge separation (CS) event can take place independently within each ec2/ec3 chlorophyll pair. In order to improve our understanding of this phenomenon, we have generated new mutations in the PsaA and PsaB subunits near the electron transfer cofactor 2 (ec2 chlorophyll). PsaA-Asn604 accepts a hydrogen bond from the water molecule that is the axial ligand of ec2B and the case is similar for PsaB-Asn591 and ec2A. The second set of targeted sites was PsaA-Ala684 and PsaB-Ala664, whose methyl groups are present near ec2A and ec2B, respectively. We generated a number of mutants by targeting the selected protein residues. These mutations were expected to alter the energetics of the primary charge separation event. The PsaA-A684N mutants exhibited increased ET on the B-branch as compared to the A-branch in both in vivo and in vitro conditions. The transient electron paramagnetic resonance (EPR) spectroscopy revealed the formation of increased B-side radical pair (RP) at ambient and cryogenic temperatures. The ultrafast transient absorption spectroscopy and fluorescence decay measurement of the PsaA-A684N and PsaB-A664N showed a slight deceleration of energy trapping. Thus making mutations near ec2 on each branch resulted into modulation of the charge separation process. In the second set of mutants, where ec2 cofactor was target by substitution of PsaA-Asn604 or PsaB-Asn591 to other amino acids, a drop in energy trapping was observed. The quantum yield of CS decreases in Asn to Leu and His mutants on the respective branch. The P700 triplet state was not observed at room and cryogenic temperature for these mutants, nor was a rapid decay of P700+ in the nanosecond timescale, indicating that the mutations do not cause a blockage of electron transfer from the ec3 Chl. Time-resolved fluorescence results showed a decrease in the lifetime of the energy trapping. We interpret this decrease in lifetime as a new channel of excitation energy decay, in which the untrapped energy dissipates as heat through a fast internal conversion process. Thus, a variety of spectroscopic measurements of PSI with point mutations near the ec2 cofactor further support that the ec2 cofactor is involved in energy trapping process. / Dissertation/Thesis / Doctoral Dissertation Biochemistry 2014

abstract: The first chapter reviews three decades of artificial photosynthetic research conducted by the A. Moore, T. Moore, and D. Gust research group. Several carotenoid (Car) and tetrapyrrole containing molecules were synthesized and investigated for excitation energy transfer (EET), photoregulation, and photoprotective functions. These artificial photosynthetic compounds mimicked known processes and investigated proposed mechanisms in natural systems. This research leads to a greater understanding of photosynthesis and design concepts for organic based solar energy conversion devices. The second and third chapters analyze the triplet energy transfer in carotenoid containing dyads. Transient absorption, time-resolved FTIR and resonance Raman spectra revealed that in a 4-amide linked carotenophthalocyanine dyads the Car triplet state is shared across the larger conjugated system, which is similar to protein complexes in oxygenic photosynthetic organisms. In a carotenopurpurin dyad (CarPur) a methylene ester covalent bond prevents the purpurin (Pur) from influencing the Car triplet based on the transient absorption, time-resolved FTIR and resonance Raman spectra. Thus CarPur resembles the antenna proteins from anoxygenic photosynthetic bacteria. Additional examples of carotenoporphyrin dyads further demonstrates the need for orbital overlap for ultrafast triplet energy transfer and the formations of possible intramolecular charge transfer state. The fourth chapter studies a 4-amino phenyl carotenophthalocyanine and its model compounds using high temporal resolution transient absorption spectroscopy techniques. EET from the Car second excited (S2) state to the phthalocyanine (Pc) was determined to be 37% and a coupled hot ground state (S*)/Pc excited state spectrum was observed. Excitation of the tetrapyrrole portion of the dyad did not yield any kinetic differences, but there was an S* signal during the excited states of the dyad. This demonstrates the EET and photoregulating properties of this artificial photosynthetic compound are similar to those of natural photosynthesis. The last chapter covers the synthesis of silicon Pc (SiPc) dyes and the methods for attaching them to gold nanoparticles and flat gold surfaces. SiPc attached to patterned gold surfaces had unperturbed fluorescence, however the selectivity for the gold was low, so alternative materials are under investigation to improve the dye's selectivity for the gold surface. / Dissertation/Thesis / Ph.D. Chemistry 2014

A key component of natural photosynthesis are the antenna chromophores (chlorophylls and carotenoids) that capture solar energy and direct it towards the reaction centers of photosystems I and II. Highlighted by highly-ordered crystal structures and synthetic tunability via crystal engineering, metal–organic frameworks (MOFs) have the potential to mimic the natural photosynthetic systems in terms of the efficiency and directionality of energy transfer. Owing to their larger surface areas, MOFs have large absorption cross sections, which amplifies the rate of photon collection. Furthermore, MOFs can be constructed using analogues of chlorophyll and carotenoids that can participate in long-range energy transfer. Herein, we aimed to design photoactive MOFs that can execute one of the critical steps involved in photosynthesis - photon collection and subsequent energy transfer. The influence of spatial arrangement of chromophores on the efficiency and directionality of excitation energy transfer (EET) was investigated in a series of mixed-ligand pyrene- and porphyrin-based MOFs. Due to the significant overlap between the emission spectrum of 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy) and the absorption spectrum of meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP), the co-assembly of these two ligands in a MOF should enable facile energy transfer. Bearing this in mind, three TBAPy-based MOFs with markedly different network topologies (ROD-7, NU-901, and NU-1000) were chosen and a small number of TCPP units were incorporated in their backbone. To gain insight into the photophysical properties of mixed-ligand MOFs, we conducted time-resolved and steady-state fluorescence measurements on them. Stern-Volmer analysis was performed on the fluorescence lifetime data of mixed-ligand MOFs to determine the quenching constants. The quenching constant values for ROD-7, NU-901, NU-1000, and TBAPy solution were found to be 15.03 ± 0.82 M-1, 10.25 ± 0.99 M-1, 8.16 ± 0.41 M-1, and 3.35 ± 0.30 respectively. In addition, the ratio of the fluorescence intensities of TCPP and TBAPy was used to calculate the EET efficiencies in each of the three MOFs. EET efficiencies were in the following order: ROD-7 > NU-901 > NU-1000 > TBAPy-solution. Based on the trends observed for quenching constants and EET efficiencies, two conclusions were drawn: (1) the ligand-to-ligand energy transfer mechanism in MOFs outperforms the diffusion-controlled mechanism in solution phase, (2) energy transfer in MOFs is influenced by their structural parameters and spectral overlap integrals. The enhanced EET efficiency in ROD-7 is attributed to shorter interchromophoric distance, larger orientation factor, and larger spectral overlap integral. Directionality of energy transfer in these MOFs was assessed by calculating excitonic couplings between neighboring TBAPy linkers using the atomic transition charges approach. Rate constants of EET (kEET) along different directions were determined from the excitonic couplings. Based on the kEET values, ROD-7 is expected to demonstrate highly anisotropic EET along the stacking direction. In order to explore the mechanistic aspects of EET in porphyrin-based MOFs, we studied the energy transfer characteristics of PCN-223, a zirconium-based MOF containing TCPP ligands. After performing structural characterization, the photophysical properties of PCN-223 and free TCPP were investigated using steady state and time-resolved spectroscopy. pH-dependent fluorescence quenching experiments were performed on both the MOF and ligand. Stern-Volmer analysis of quenching data revealed that the quenching rate constants for PCN-223 and TCPP were 8.06 ± 1011 M-1s-1 and 2.71 ± 1010 M-1s-1 respectively. The quenching rate constant for PCN-223 is, therefore, an order of magnitude larger than that for TCPP. Additionally, PCN-223 demonstrated a substantially higher extent of quenching (q = 93%) as compared to free TCPP solution (q = 51%), at similar concentrations of quencher. The higher extent of quenching in MOF is attributed to energy transfer from neutral TCPP linkers to N-protonated TCPP linkers. Using the Förster energy transfer model, the rate constant of EET in PCN-223 was calculated. The magnitude of rate constant was in good agreement with the kEET values reported for other porphyrin-based MOFs. Nanosecond transient absorption measurements on PCN-223 revealed the presence of a long-lived triplet state (extending beyond 200 μs) that exhibits the characteristic features of a TCPP-based triplet state. The lifetime of MOF is shorter than that of free ligand, which may be attributed to triplet-triplet energy transfer in the MOF. Lastly, femtosecond transient absorption spectroscopy was employed to study the ultrafast photophysical processes taking place in TCPP and PCN-223. Kinetic analysis of the femtosecond transient absorption data of TCPP and PCN-223 showed the presence of three distinct time components that correspond to: (a) solvent-induced vibrational reorganization of excitation energy, (b) vibrational cooling, and (c) fluorescence. Materials that allow control over the directionality of energy transfer are highly desirable. Core-shell nanocomposites have recently emerged as promising candidates for achieving long-distance, directional energy transfer. For our project, we aim to employ UiO-67-on-PCN‐222 composites as model systems to explore the possibility of achieving directional energy transfer in MOF-based core-shell structures. The core–shell composites were synthesized by following a previously published procedure. Appropriate amounts of Ruthenium(II) tris(5,5′-dicarboxy-2,2′-bipyridine), RuDCBPY, were doped in the shell layer to produce a series of Ru-UiO-67-on-PCN‐222 composites with varying RuDCBPY loadings (CS-1, CS-2, and CS-3). The RuDCBPY-doped core–shell composites were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) imaging, Nitrogen adsorption-desorption isotherms, and diffuse reflectance spectroscopy. Efforts are currently underway to quantify RuDCBPY loadings in CS-1, CS-2, and CS-3. After completing structural characterization, the photophysical properties of CS-1, CS-2, and CS-3 will be investigated with the help of time-resolved and steady-state fluorescence spectroscopy. / Doctor of Philosophy / The pigment−protein complexes in natural photosynthetic units (also known as light harvesting antennas) efficiently capture solar energy and transfer this energy to reaction centers that carry out water splitting reactions. The collective chromophoric behavior of antennas can be replicated by metal-organic frameworks (MOFs). MOFs are crystalline, self-assembled materials composed of metal clusters connected by organic molecules. In this dissertation, we study the factors that govern the energy transfer and light harvesting capabilities of MOFs. In chapter 2, we examined the role of 3D structure of MOFs in energy transfer. In chapter 3, we investigated the influence of pH and temperature on the photophysical properties of MOFs. In chapter 4, we explored the possibility of energy transfer in novel MOF-on-MOF composites. This work is intended to pave the way for the construction of highly efficient MOF-based materials that can serve as the light harvesting and energy-transfer components in solar energy conversion devices.

No description available.

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.

Electronic Excitation Energy Transfer is an important intermolecular photophysical process that can affect the excited state lifetime of a chromophore. A molecule in an electronically excited state can return to the ground state by radiative as well as non-radiative processes. During the excited state lifetime, if the chromophore (energy donor) finds a suitable species (energy acceptor) nearby with resonant energy levels, it can transfer the excitation energy to that species and return to the ground state. This process is called Electronic Excitation Energy Transfer. When the energy donor is fluorescent, the process is called Fluorescence Resonance Energy Transfer (FRET) [1]. FRET is a non-radiative process that affects the fluorescence intensity as well as the excited state lifetime of the donor. It occurs due to the electrostatic coulombic interaction between the transition charge densities of the donor and the acceptor. The rate of energy transfer can be evaluated using the Fermi golden rule of quantum mechanics [2]. When the donor and the acceptor are separated by distances that are much larger in comparison with the sizes of the donor and the acceptor, the interaction between them can be thought of as that between their transition dipoles. In such a case, the interaction between the donor and the acceptor is dipolar and the rate of energy transfer has an R−6 dependence, where R is the distance between the donor and the acceptor [3]. This dependence has first been suggested theoretically by Forster in 1947 [4] followed by the experimental verification by Stryer and Haugland [5]. Since then the process has been used as a spectroscopic ruler to study the conformational dynamics of biopolymers like DNA, RNA, proteins etc [6]. A variety of dye molecules have been explored for donors and acceptors in FRET and the range of distances that can be measured using FRET involving dyes is in the range 1 − 10 nm. When the distances between the donor and the acceptor are not much larger in comparison with their sizes, the dipolar approximation to the interaction is not a very good approximation, thereby leading to deviations from the traditional R-6 dependence. Such non-R-6dependencies are found for polymers, quantum wells, quantum wires etc [7–9]. The interest in such dependencies is due to the need for developing nanoscopic rulers that can measure distances well beyond 10 nm. The objective of our work has been to study energy transfer from fluorophores to various kinds of acceptors that have extended charge densities and understand the distance dependence of the rate of energy transfer [10]. We use the Fermi golden rule as the starting point and develop analytical models for evaluating the rate as a function of the distance between the donor and the acceptor. We study the process of energy transfer from fluorescent dye molecules that serve as energy donors to a variety of energy acceptors namely, graphene, doped graphene, single-walled carbon nanotubes and metal nanoparticles. We also study transfer from fluorophores to a semiconducting sheet and a semiconducting tube of electronic charge density. There have been experimental studies in the literature of the fluorescence quenching of dyes near single-walled carbon nanotubes [11–13]. But, there are no studies of the distance dependence of rate. Single-walled carbon nanotubes can be thought of as rolled up sheets of graphene. However, interestingly, there were no reports of fluorescence quenching by graphene at the time when we thought of this possibility. Therefore, we first study the process of energy transfer from a fluorophore, which is kept at a distance z above a layer of graphene to the electronic energy levels of graphene. We find that the long range behavior of the rate has an z -4 dependence on the distance [14, 15]. From our study of transfer from pyrene to graphene, we find that fluorescence quenching can be experimentally observed up to a distance of ~ 30 nm, which is quite large in comparison with the traditional FRET limit (10 nm). Recent experiments that have been performed after our theory was reported have in fact observed the fluorescence quenching of dyes near graphene. Further, the process has been found to be very useful in fabricating devices based on graphene [16], in eliminating fluorescence signals in resonance Raman spectroscopy [17] and in visualizing graphene based sheets using fluorescence quenching microscopy [18]. The process has also been found to be useful in quantitative DNA analysis [19, 20]. We study the transfer of an amount of energy hΩ from a dye molecule to doped graphene [21]. We consider the shift of the Fermi level from the K-point into the conduction band of graphene as a result of doping and evaluate the rate of transfer. We find a crossover of the distance dependence of the rate from z -4 to exponential as the Fermi level is increasingly shifted into the conduction band, with the crossover occurring at a shift of the Fermi level by an amount hΩ/2. We study the process of transfer of excitation energy from a fluorophore kept at a distance d away from the surface of a carbon nanotube to the electronic energy levels of the nanotube. We find both exponential and d−5 behavior of the rate [22]. For the case of metallic nanotubes, when the emission energy of the fluorophore is less than a threshold, the dependence is exponential. Otherwise, it is d−5 . For the case of semiconducting nanotubes, we find that the rate follows an exponential dependence if the amount of energy that is transferred can cause only the excitonic transition of the tube. However, if any other band gap transition is allowed, the rate follows a d−5 dependence. For the case of transfer from pyrene to a (6, 4) nanotube, we find that energy transfer is appreciable up to a distance of ~ 17 nm. We then study the process of energy transfer from a fluorophore to a semiconducting sheet of electronic charge density [10]. We find that the rate has an z-4 dependence. For the case of transfer to a semiconducting tube, we find that the rate has a d -5dependence. The dependencies are in agreement with those obtained for graphene and carbon nanotubes respectively. This shows that the asymptotic distance dependencies are a consequence of the dimensionality of the transition charge densities and are robust. Strouse et al. [23, 24] have studied the process of energy transfer from the dye fluorescein to a 1.4 nm diameter gold nanoparticle. Double-stranded DNA molecules of various lengths were used to fix the distances between the donor and the acceptor. The rate was found to have a d-4distance dependence. They refer to this process as Nanoparticle Surface Energy Transfer (NSET) and the range of distances that can be measured using NSET is more than double that of the traditional FRET experiments. However, theoretical studies that consider the transfer to the plasmonic modes of the nanoparticle find a predominant R-6 dependence [25]. We study the process of energy transfer from the dye fluorescein to a 1.4 nm diameter gold nanoparticle considering the excitation of plasmons as well as electron-hole pairs of the nanoparticle [26]. We find that the rate follows the usual Forster type R−6 distance dependence at large distances. But, at short distances, there are contributions of the form R−-n with n > 6. This is due to the quadrupolar and octupolar modes of excitation of the nanoparticle, the rates corresponding to which have R-8 and R−-10 dependencies respectively. Recent calculations using DFT also find similar deviations at short distances [27].

Methoden der transienten Vierwellenmischung mit Femtosekunden-Zeitauflösung werden angewendet, um die Phasen- und Energierelaxation optisch selektiv erzeugter Anregungszustände im Lichtsammelkomplex II höherer Pflanzen (LHC II) sowie im Halbleiter Cadmiumsulfid (CdS) bei verschiedenen Temperaturen zu untersuchen. Für den LHC II werden die Ergebnisse der Messungen des zeitaufgelösten und integrierten Zweipuls-Photonenechos mit Resultaten aus Pump-Test-Experimenten verglichen, um unter Einbeziehung von Literaturdaten Rückschlüsse über den Charakter der phasenzerstörenden Prozesse zu ziehen und Zusammenhänge zu Strukturdaten des Komplexes aufzudecken. Die vorliegende Arbeit liefert erstmals einen systematischen Überblick über die totalen Phasenrelaxationszeiten T2 im Bereich der Qy-Bande des LHC II von 640 bis 685 nm bei 5 K. Das bei 5 K beobachtete Photonenechosignal am LHC II zeigt in Abhängigkeit von der Verzögerung der beiden Anregungsimpulse ein multiexponentielles Abklingen, das auf die Überlagerung der Einflüsse mehrerer Relaxationsprozesse zurückgeführt wird. Dabei lassen sich drei charakteristische Bereiche der Phasenrelaxationszeit unterscheiden, die verschiedenen phasenzerstörenden Prozessen zugeordnet werden. Ein Vergleich mit Resultaten aus Pump-Test-Experimenten führt zu der Schlußfolgerung, daß die Phasenrelaxation im LHC II bei 5 K für Wellenlängen £ 675 nm im wesentlichen durch den Energietransfer auf einer sub-ps Zeitskala bestimmt wird. Für Wellenlängen > 675 nm steigt die Phasenrelaxationszeit stark an und wird insbesondere im Bereich der tiefsten Anregungszustände um 680 nm durch reine Phasenzerstörung dominiert. Ab 20 K setzt bei dieser Wellenlänge ein zusätzlicher phasenzerstörender Prozeß ein, der mit steigender Temperatur zu einem mäßigen linearen Anstieg der Phasenrelaxationsrate (T2)-1 führt. Die Ursache ist vermutlich ein Aufwärts-Energietransfer. Im Bereich der Chlorophyll a-Absorption vernichten außerdem (physiologisch irrelevante) Multiexzitoneneffekte die Kohärenz der angeregten Zustände, verursacht durch die hohe Anregungsintensität. Zusammenfassend kann festgestellt werden, daß die Erhaltung der Kohärenz für die Funktionalität des LHC II eine untergeordnete Rolle spielt. Die wesentlichen Prozesse sind der schnelle räumliche Energietransfer und die Energierelaxation auf das Niveau des primären Elektrondonators P680 im Reaktionszentrum. Am Halbleiter CdS wird erstmals ein mittels Zwei-Photonen-Absorption angeregtes Photonenecho beschrieben, das in Abhängigkeit von der Wellenlänge charakteristische Quantenbeats mit einer Periode von 700 bis 800 fs zeigt. Das stark gedämpfte periodische Echosignal tritt sowohl für positive als auch für negative Verzögerungszeiten t der Anregungsimpulse auf, wobei die Abklingzeit für t>0 mit 170±10 fs doppelt so groß ist wie für t / Transient four-wave-mixing experiments with femtosecond resolution are performed in order to investigate phase and energy relaxation processes of optically excited states in the light harvesting complex II of higher plants (LHC II) and in the semiconductor cadmium sulfide (CdS) at different wavelengths and temperatures. Extensive studies of the time resolved and integrated two-pulse photon echo on LHC II are combined with pump-probe experiments. Results of both methods together with literature data are used to characterize the nature of dephasing processes and to reveal connections with structural data of the complex. This study gives the first systematic survey of total dephasing times T2 in the spectral region of the Qy-absorption band of LHC II from 640 to 685 nm at 5 K. In the case of LHC II, the photon echo signal at 5 K monitored as a function of delay between both excitation pulses shows a multi-exponential decay which is attributed to the superposition of several relaxation processes. Three characteristic dephasing time domains can be distinguished, ascribed to different dephasing processes. Comparing photon echo and pump-probe results it can be concluded that dephasing in LHC II at 5 K and for wavelengths £ 675 nm is dominated by the fast excitation energy transfer on a sub-ps time scale. At wavelengths > 675 nm the total dephasing time increases drastically. The loss of coherence of the lowest excited states around 680 nm at 5 K is mainly determined by pure dephasing. An additional dephasing process, probably uphill energy transfer, occurs at temperatures higher than 20 K leading to a moderate linear rise of the dephasing rate (T2)-1 with increasing temperature. Furthermore, the dephasing in the spectral region of chlorophyll a absorption is affected by (physiologically irrelevant) multi-excitonic effects caused by the high excitation energy. In summary, it can be concluded that the preservation of coherence plays a minor role in the functionality of LHC II. The main processes are the fast spatial excitation energy transfer and the energy relaxation down to the energetic level of the primary electron donor P680 of the reaction center. Investigations of four-wave mixing signals of the semiconductor CdS resulted in the first description of a two-photon excited photon echo in CdS showing characteristic quantum beats with a period of 700 to 800 fs in dependence on wavelength. The strongly damped periodical echo signal is found for both positive and negative delay times t between the excitation pulses. The decay time for t>0 amounts to 170±10 fs and is twice as large as for t

Title: Artificial light-harvesting antenna based on an aggregation of bacteriochlorophyll c with selected pigments Author: Tomáš Malina Department: Department of Chemical Physics and Optics Supervisor of the master thesis: doc. RNDr. Jakub Pšenčík, Ph.D., KCHFO MFF UK Abstract: Solar energy is one of the most important energy sources for all living organisms. The light harvesting takes place in specialised photosynthetic complexes called antennas; they typically contain pigments held by a protein scaffold. Antennas of green bacteria, chlorosomes, are unique in this respect, for they do not need proteins to organise the pigments. The pigments contained in chlorosomes, bacteriochlorophyll (BChl) c, d or e, aggregate spontaneously. This self-aggregation can be used to form an artificial light-harvesting antenna the absorption spectrum of which can be extended by addition of other pigments. Antennas based on aggregation of BChl c with β-carotene and BChl a were prepared by a fast and slow method. The excitation energy transfer efficiency between these pigments was studied. The efficiency of energy transfer from BChl c to BChl a reached up to 95 %, the efficiency of energy transfer from β-carotene to BChl c was lower. An important role of β- carotene in artificial aggregates as well as in chlorosomes is its...

Les nanoparticules polymériques fluorescentes apparaissent comme des outils importants pour l'imagerie en temps réel des processus biologiques au niveau moléculaire et cellulaire. L’objectif de mon projet de doctorat a été d’optimiser les nanoparticules polymériques fluorescentes pour l’imagerie biologique. Premièrement, nous avons pu, en faisant varier la chimie des polymères, obtenir un très bon contrôle de leur taille. Ceci a permis de mettre en évidence l’importance de la taille des NPs pour des applications intracellulaires avec une taille maximale de 23 nm pour une distribution dans tout le cytosol. Deuxièmement, nous avons pu montrer que la simple adsorption d’un amphiphile PEGylé de type Pluronic permet la stabilisation des nanoparticules dans des milieux biologiques. Le nombre de molécules incorporées et leur stabilité ont été étudiés en combinant des techniques de FRET et de FCS. Les meilleures formulations résultent en une stabilité des nanoparticules in vivo, ce qui a permis leur imagerie en tant que particules individuelles dans les vaisseaux sanguins du cerveau de souris. Troisièmement, le transfert d’énergie entre différents fluorophores encapsulés dans les NPs a été étudié et optimisé. / Fluorescent polymeric nanoparticles appear as important tools for real-time imaging of biological processes at the molecular and cellular level. The objective of my PhD project was to optimize fluorescent polymeric nanoparticles for biological imaging. First, by varying the chemistry of the polymers, we have been able to obtain a very good control of their size. This made it possible to highlight the importance of NPs size for intracellular applications with a maximum size of 23 nm for optimal distribution throughout the cytosol. Secondly, we have shown that simple adsorption of a PEGylated amphiphiles pluronic family allows the stabilization of nanoparticles in biological media. The number of incorporated molecules and their stability has been studied by combining FRET and FCS techniques. The best formulations result in nanoparticle stability in vivo, which allowed their imaging as individual particles in the blood vessels of the mouse brain. Third, energy transfer among different fluorophores encapsulated in NPs has been studied and optimized.

Die vorliegende Arbeit untersucht den Anregungsenergie-Transfer in Nano-hybrid-Systemen, welche zweidimensionale Übergangsmetall-Dichalkonid-Schichten (TMDCs) enthalten. Heterostrukturen, welche TMDC-Schichten mit sogenannten nulldimensionalen Systemen kombinieren, werden als wesentlich für die nächste Generation von elektronischen und photonischen Bauelementen angesehen. Trotz dieser großen Bedeutung existieren wenige theoretische Untersuchungen. Insbesondere ist der Anregungsenergie-Transfer in diesen Hybridsystemen nicht umfassend erklärt, und die Behandlung von TMDC-Schichten bezieht sich auf sehr kleine oder periodische Systeme. Daher wird in der Arbeit der Versuch unternommen, existierende Theorien zu verbessern, und es werden Transferprozesse in zwei Typen von Heterostrukturen simuliert. Die berechneten Systeme enthalten tausende von Atomen und kommen damit in den Bereich experimentell untersuchter Strukturen. In dem einen Nanohybrid-System ist eine MoS2-Monoschicht mit einem einzelnen Para-Sexiphenyl-Molekül kombiniert, wogegen im zweiten System ein CdSe-Nanokristall an der MoS2-Mono-schicht plaziert ist. Dabei ermöglicht die Coulomb-Wechselwirkung zwischen Monoschicht und Molekül bzw. Nanokristall den Anregungsenergie-Transfer. In allen untersuchten Heterostrukturen ist die Stärke der Anregungsenergie-Transfer-Kopplung auf den sub-meV-Bereich beschränkt. In diesem Bereich ist der Anregungsenergie-Transfer inkohärent und bestimmt durch Raten, die aus Fermi's Goldener Regel folgen. Auch wird eine Abhängigkeit der Transferrate von der relativen Position des para-Sexiphenyl-Moleküls gefunden. Durch die Analyse der Übergangsladungsdichte des CdSe-Nanokristalls kann aufgezeigt werden, dass die energetisch tiefliegenden Exziton-Niveaus mit ausgeprägtem Dipolcharakter zu einer stärkeren Transferkopplung führen. Die resultierenden Transferzeiten erstrecken sich vom Piko- zum Nanosekunden-Bereich und decken sich mit entsprechend gemessenen Werten. / This thesis explores the excitation energy transfer in two-dimensional transition metal dichalcogenides (TMDCs) based nanohybrid systems. Such heterostructures combining TMDC layers with zero-dimensional materials are considered in next-generation electronics and photonics. However, there exists a shortage of current theoretical work, because the general process of excitation energy transfer in these hybrid systems has rarely been explored and the treatment of TMDCs is limited to a small size. We therefore improve the existing theories and investigate the transfer phenomena in two types of heterostructures. The considered systems contain thousands of atoms close to the experimental system size. In the first nanohybrid system, a MoS2 monolayer is combined with a single para-sexiphenyl molecule. In the second hybrid, a CdSe semiconductor spherical nanocrystal is placed close to the MoS2 monolayer. The MoS2 monolayer is coupled to the para-sexiphenyl molecule or the CdSe spherical nanocrystal via Coulomb interaction, which makes the excitation energy transfer mechanism possible. In our heterostructures, all excitation energy transfer coupling strengths lie in the meV-range or below. Within this limitation, the non-coherent excitation transfer is determined by rate expressions derived from Fermi’s Golden Rule. An effective transfer rate dependency on the relative positions of the para-sexiphenyl molecule is found. For the case of the CdSe spherical nanocrystal , by visualizing the shape of transition charge densities of CdSe excitons, we find that the low-lying exciton levels with more obvious dipole character lead to a stronger transfer coupling. The resultant transfer times range from picoseconds to nanoseconds and coincide with experimental data.