Im ersten Teil dieser Arbeit wurden die Synthesen und photophysikalischen Eigenschaften mehrerer Donor-Akzeptor-substituierter Übergangsmetallkomplexe des Typs [(C^N)2Ir(N^N)][PF6] vorgestellt. Das Ir(III) wurde mit Lochleitern wie Carbazol (CZ) und Triarylamin (TAA), die über eine Methyl- und Ethylbrücke mit dem cyclometallierten C^N-Liganden Phenylpyrazol (ppz) und Phenylpyridin (ppy) verbunden waren, verknüpft. Zweizähnige neutrale N^N- und P^P-Liganden wie 2,2’-Bipyridyl (bpy), 3,4,7,8-Tetramethyl-1,10-phenanthrolin (tmp) und cis-1,2-Bis(diphenylphosphino)ethylen (bdppe) wurden als Akzeptoren ausgewählt.Um die Eigenschaften in Vergleich setzen zu können, wurden die entsprechenden Referenzverbindungen ohne Lochleiter hergestellt. Alle Carbazol-Komplexe, bis auf die bdppe-Verbindungen, zeigen Emission und transiente Absorption ähnlich denen ihrer Referenzverbindungen, was sie für Anwendungen in der OLED-Forschung interessant macht. Untersuchungen an LECs (lichtemittierende elektrochemische Zellen) zeigen eine rotverschobene Lumineszenz. Die Triarylamin-Komplexe zeigen keine Emission bei RT, allerdings weisen diese eine intensive, blauverschobene und langlebige Lumineszenz bei 77 K in einer festen Matrix auf. Die transienten Absorptionsspektren unterscheiden sich stark von denen der Referenzverbindungen. Sie weisen charakteristische Merkmale von den Spektren der isolierten Radikalanionen und Radikalkationen auf, was durch spektroelektrochemische Messungen bewiesen wurde. Daraus kann geschlossen werden, dass es sich bei dem angeregten Zustand um einen ladungsgetrennten (CS) Zustand handelt, wo die positive Ladung am Triarylamin-Donor und die negative Ladung am N^N-Akzeptor sitzt. Die Abklingzeiten der angeregten Zustände verlaufen biexponentiell, was ein Hinweis auf das Vorhandensein zweier angeregter Zustände, dem 1CS und 3CS Zustand, ist. Um dieses Verhalten zu untersuchen, wurden verschieden substituierte bpy-Komplexe synthetisiert und analysiert. Temperaturabhängige Messungen der transienten Absorption zeigten, dass alle Ratenkonstanten temperaturunabhängig sind, mit Ausnahme des OMe-substituierten Komplexes. Die Gleichgewichtskonstante K = k1 / k2 ist nahezu eins für alle Komplexe. Bei dem OMe-Komplex sinkt sie mit steigender Temperatur. Eine Auftragung der Ratenkonstanten gegen die Energiedifferenzen, die durch cyclovoltammetrische Messungen erhalten wurden, zeigte, dass alle Konstanten mit steigender Donorstärke am bpy-Liganden abfallen. DFT-Rechnungen an der OMe-Verbindung sind noch in Arbeit. Im zweiten Teil dieser Arbeit wurden neutrale Ir(III) und Pt(II) Komplexe des Typs [(O^O)Ir(N^N)2] und [(O^O)Pt(N^N)] vorgestellt. Hier wurde TAA mit Acetylacetonat (acac) direkt, oder über eine CH2-Brücke verbunden, um den Einfluss der Art der Verbrückung auf die photophysikalischen Eigenschaften zu beobachten. Als zweizähniger N^N-Ligand wurde 2,2’-Bipyridyl (bpy) gewählt. Auch hier wurden alle entsprechenden Referenzverbindungen ohne Triarylamin als Vergleich hergestellt. Zudem wurde der homoleptische fac Ir(N^N)3 Komplex mit Triarylamin, welches über eine Methyl- und Ethylbrücke an Phenylpyrazol angeknüpft wurde, synthetisiert. Die Synthese des Ir(III)-Komplexes mit TAA substituiertem acac-Liganden verknüpft über eine CH2-Brücke konnte nicht hergestellt werden. Alle neutralen TAA-substituierten -diketonato Pt(II) und Ir(III) Komplexe zeigen keine Lumineszenz, außer dem Pt(II)-Komplex mit CH2-Gruppe. Dieser zeigt angeregte Zustände die in guter Übereinstimmung mit den Emissionslebenszeiten bei RT sind. Diese sind ähnlich denen der Referenz, was auf einen 3Pt(N^N)(O^O)- Zustand schließen lässt. Die Komplexe ohne CH2-Brücke zeigen zudem keine transiente Absorption was auf ein Charge-Transfer-Quenching aufgrund der direkten Verknüpfung zwischen Donor und Akzeptor zurückzuführen sein könnte. Der homoleptische fac Ir(N^N)3 Komplex weist weder Emission bei RT, noch transiente Absorption auf. Bei 77 K ist eine stark strukturierte Emission mit einer Abklingzeit von 14 s zu beobachten. Verglichen mit dem literaturbekannten Vergleichskomplex ist die Emission auf die Bevölkerung eines 3Ir(ppz)3-Zustandes zurückzuführen. Unsere Ergebnisse sind grundlegend für die Synthese weiterer Verbindungen mit stärkeren Akzeptoren, wie z. B. Naphthalenimid, um längerlebige ladungsgetrennte Zustände zu erhalten. Diese könnten Anwendung als Photosynthesiser in Solarzellen und anderen optoelektronischen Bauteilen finden. Zudem sind weiter Untersuchungen an LECs und OLEDs für die Carbazol-Komplexe noch immer von Interesse, um das Ausmaß der Triplett-Triplett-Annihilierung zu quantifizieren. / In the first part of this work we presented the synthesis and photophysical properties of a series of transition metal donor-acceptor Ir(III)complexes of the type [(C^N)2Ir(N^N)][PF6]. The Ir(III) was connected with hole conducting donor-moieties like carbazole (CZ) and triarylamine (TAA) linked via a methylene and ethylene bridge to the cyclometalating C^N ligands phenylpyrazole (ppz) and phenylpyridine (ppy). Bidentate N^N and P^P ligands like 2,2’-bipyridyl (bpy), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp) and cis-1,2-bis(diphenylphosphino)ethylene (bdppe) were used as acceptor units. In order to analyse the influence of the electron density of the bpy ligand, TAA-complexes with acceptor- and donor-substituted bpy acceptor units were synthesised. Therefore, 4,4’-dinitro-2,2’-bipyridyl, 4,4’-dichloro-2,2’-bipyridyl, 4,4’-dimethoxy-2,2’-bipyridyl and 4,4’-dimethylamino-2,2’bipyridyl were used as neutral N^N ligands. In order to compare the photophysical properties, all reference compounds without hole conducting component were syntesised. All the carbazole compounds, except the bdppe complexes, exhibit emission and transient absorption properties similar to their reference compounds that make them interesting for OLED (organic light emitting device) applications. LEC (light emitting electrochemical cell) studies show a red shifted luminescence. The triarylamine compounds do not luminesce at RT but they exhibit an intense, blue-shifted and long-lived luminescence at 77 K in a rigid matrix. The transient absorption spectra differ strongly from that of their reference compounds. The spectra display characteristic features of the spectra of the isolated radical anions and cations supported by spectroelectrochemical measurements. Thus, it can be assumed that the transient states are charge separated (CS) states in which the positive charge is localised at the TAA donor units and the negative charge at the N^N acceptor units. The decays of the transient states are biexponentially what indicates the presence of two transient states, the 1CS and the 3CS state. To understand this behaviour the differently substituted bipyridyl-complexes were synthesised and analysed. Temperature dependent transient absorption measurements showed that all rate constants are indepentend of the temperature, except for the complex with OMe subsituents at the bpy ligand. The equilibrium constant K = k1 / k2 is nearly one for all complexes. For the OMe-compound it decreases with increasing temperature. Plotting the rate constants vs. the free energy differences (determined by cyclovoltammetry measurements) shows that all constants are decreasing with increasing donor strength of the bpy ligand. DFT calculations on the OMe-compound are already in work. In the second part of this work, neutral Ir(III) and Pt(II) complexes of the type [(O^O)Ir(N^N)2] and [(O^O)Pt(N^N)] were introduced. There, TTA was connected directly or via a CH2 bridge to acectylacetonate (acac = O^O) in order to probe the influence of the different kinds of connection on the photophysics of the complexes. As the bidentate N^N ligand 2,2’-bipyridyl (bpy) was chosen. All the corresponding reference compounds without triarylamine were obtained in order to compare with the TAA substituted analoga. Furthermore, the homoleptic fac Ir(N^N)3 complex with triarylamine connected via a methylene and ethylene bridge to phenylpyrazole as introduced in the first part of this work was synthesised. The synthesis of the Ir(III) compound with the TAA substituted acac ligand connected via the CH2 group was not successful. All the neutral triarylamine-substituted -diketonato Pt(II) and Ir(III) complexes do not luminesce at RT, except the Pt(II)-complex with CH2 bridge. This compound shows transient state characteristics that are in good agreement with the luminescence lifetimes at RT and that are similar to the reference compound, what suggests to a 3Pt(N^N)(O^O) state. The complexes without the CH2 bridging unit show no transient signals what may be caused by charge-transfer quenching due to the direct linkage between donor and acceptor unit. The homoleptic fac Ir(N^N)3 complex exhibits no emission at RT and no transient signals. At 77 K it shows a highly structured emission with 14 s lifetime. Compared to the literature-known reference compound this emission is caused by the population of a 3Ir(ppz)3 state. Our findings are important for designing complexes with stronger acceptor units (i.e. naphthaleneimide) for long CS states lifetimes to be used as photosynthesisers in solar cells and other optoelectronic devices. Besides, LEC and OLED studies on the carbazole complexes are still of interest to analyse the degree of triplet-triplet-annihiliation in these devices.

Metal-Organic Frameworks (MOFs), a class of porous materials realized via reticular construction of a plethora of organic linkers and metal nodes, have emerged as excellent candidates for light-harvesting compositions (LHC), photo or electrocatalysis. This is due to their ability to organize chromophores and metal nodes with desired functionalities, and remarkable porosity that allows efficient mass transfer of reactants and electrolytes. Recent studies have shown intriguing delocalized excited state of the orderly organized pigments in MOFs. Furthermore, the accessible pores/channels allow it to host complementary optical/redox active species within the frameworks by means of de novo or postsynthetic functionalization, as a manner for MOF compositions to integrate functionalities beyond photosensitizer, such as photo/electrocatalytic sites. In such multi-component assemblies, profound understanding of charge transfer and separation process is crucial to make the designed LHC efficient. Therefore, we could adopt chromophoric MOFs as a scaffold to systematically investigate photoinduced charge transfer by installing judiciously selected redox moieties into MOFs, whose unique electronic properties could define distinct electronic interplay with MOFs. From an aspect of further applications, photo-generated electrons can be utilized more efficiently by an external electric field applied on MOF films, which prolongs the charge-separation lifetime. For this purpose, sufficient electrical conductivity is necessary to allow charges delivered across the MOF film. Considering a large energy mismatch between the majority of traditional metal nodes including metal oxo clusters and carboxylic based struts, charge transport is defined by a slow hopping process, which hinders the harvesting of relatively short-lived separated charges. Hence, developing neoteric linkage chemistry is critically needed to overcome the charge-transport challenge.Keeping these points in mind, the scope of this dissertation mainly focuses on unraveling the fundamental principles of photoinduced charge transfer and separation, ground-state charge transport boosted by nontraditional coordination chemistry and incorporation of complementary redox species, and their substantial correlation with MOF-based photocatalysis, electrocatalysis and photoelectrocatalysis. The first chapter lays the foundational knowledge regarding generic properties (chemical and physical) of MOFs, and adopted typical postsynthetic functionalization method, namely, solvent-assisted ligand incorporation (SALI), and other physical processes including photoinduced charge and energy transfer among components within MOFs, and mechanism of electron transport, that has so far been understood, in MOFs driven by an external electric field and commonly used approaches to measure that. Chapter two and three reveal the rule to control photoinduced charge transfer in MOF compositions prepared by the installation of a series of zinc porphyrins possessing gradient excited-state and frontier-orbital energy that can define distinct charge-transfer driving force into the mesopore of a photosensitizing MOF, NU-1000. These compositions show potential for their utilization as artificial light-harvesting assemblies. Chapter four highlights new design for solid porous CO2 reduction catalysts realized by introducing cobalt phthalocyanine into NU-1000. Importantly, we interpreted the catalytic activity from the aspect of charge transport efficiency, by comparing with catalysts constituted by NU-1000 and different molecular catalysts. To harvest the photo-generated electrons, an external electric field can be applied on MOF films deposited on transparent electrodes under photoexcitation, for which sufficient electrical conductivity is a must. Therefore, in chapter five, a new semiconducting coordination polymer framework was developed by employing a novel carbodithioate group for the linkage with nickel(II) that extends in three dimensions, which shows enhanced, electrical conductivity (i.e. 10-6 – 10-7 S cm-1) in contrast to traditional carboxylate-based MOFs due to a more delocalized electronic feature of the carbodithioate-nickel cluster. More importantly, its unique electronic properties, especially a long-lived charge-separation state captured by transient-absorption technique, could alleviate the compromise between electrical conductivity and charge separation (resulted from bandgap) of light-harvesting material. We then extend this binding group to chromium(III), as introduced in chapter 6, leading to a paramagnetic 3D coordination polymer with metallic conductivity as opposed to its nickel counterpart.

The general theme of this dissertation concerns how the locality of an anionic moiety, be it a weakly coordinating anion or an anionic ligand, affect the spectroscopic and structural properties of organotransition metal complexes. Probing the columbic interactions between traditional and novel weakly coordinating anions with transition metal complexes, enables synthetic chemists to select anions that can improve catalytic transformations, impart stability of reactive intermediates, or develop new mechanistic insights. Additionally, presented herein is the manifestation of a new class of luminescent copper complexes which bear a weakly coordinating anionic N-heterocyclic carbene ligand.Firstly, a qualitative scale of coordinating ability is prepared by pairing traditional anions and weakly-coordinating anions with [Pd(IPr)(C(O)C9H6N)]+. NMR, IR, Computation, %Vbur, and X-ray crystallographic techniques are used to study the solution and solid-state interactions of these salts. During this study, a novel anion, denoted IMP- is prepared where two B(C6F5) groups are bridged by a phenyl imidazole core. Ultimately, it was found that sterics dictate coordinating ability observed by NMR and %Vbur, while IR and computation show the electronic effects of anion coordination. Continuing our understanding of the interplay between cation and anion, anionic Au(I) complexes are synthesized and paired with the same palladium cation in our first investigation. The framework of these Au(I) anions features a weakly coordinating N-heterocyclic carbene ligand that bears a borate moiety of the NHC backbone. Facile dissociation of a dimethyl sulfide ligand with metal alkoxide/phenoxides/thiophenoxides affords sodium or potassium salts. With these anions in hand, ion pairs are isolated in polar solvents and in the solid state. Au anions reside in the outer sphere of the palladium cation; like that of weakly coordinating anions such as BArF4-. Lastly, Luminescent group 11 organometallic complexes featuring N-heterocyclic carbene (NHC) ligands offer a swath of applications; catalytic transformations in organic chemistry to inorganic material uses in light emitting technologies. Conventional complexes are of the type NHC-M-X, where M is Cu, Ag, or Au and X represents anionic ligands that are often prone to hydrolysis. In this dissertation, Cu(I) complexes featuring this N- heterocyclic carbene ligand bearing a weakly coordinating anionic substituent (WCA-NHC) are prepared. (WCA-NHC)-M-L are air and moisture stable and differ from conventional NHC-M-X in that the metal can be supported by 2 datively-bound ligands. Initial computation reveals a change in dipole of (WCA-NHC)-Cu-PR3 charge transfer compared to that of reported NHC-M-X. By exchanging triphenylphosphine for diphenyl-2-pyridyl phosphine, we can change the emission wavelength by about 200 nm. / Chemistry

The research presented in the dissertation deals with the synthesis, characterization, photophysical, electrochemical, and pump probe studies of porphyrin-fullerene based donor-acceptor conjugates. The first chapter provides insights into the introduction of the thesis, which explains the events that occur in natural photosynthesis and the mimicking process of an artificial photosynthesis based on natural photosynthesis, works done in covalently and non-covalently linked donor acceptor systems, and the penetration of the literature related to the long-lived charge-separated states donor-acceptor conjugates. The second chapter details the physical methods employed to monitor the various photochemical processes in the donor-acceptor moiety. The third chapter focusses on designing and synthesizing a platinum porphyrin-fullerene dyad used for long-lived charged-separated state. The formation of a high-energy, long-lived radical ion pair by electron transfer from the triplet excited state is orchestrated in the dyad. The porphyrin ring is modified with three triphenylamine which act as secondary electron donors. The spin state of the electrons leading to the formation of long-lived charge-separated state is demonstrated by time-resolved optical and EPR spectroscopy. The fourth chapter studies metal ligand axial coordination. Two porphyrins were self-assembled via metal-ligand axial coordination of phenyl imidazole functionalized fulleropyrrolidine. A 1:2 complex formation with ImC60 was observed in the case of (TTP)Co, while for (TPA)4PCo only a 1:1 complex was possible. Spectroelectrochemistry revealed the formation of Co (III) porphyrin cation instead of Co (II) porphyrin radical cation during the oxidation of phenyl imidazole coordinated cobalt porphyrin. Using computational and electrochemical results, an energy level diagram was constructed to visualize the various photochemical events. Using femtosecond transient absorption spectroscopy, it was possible to observe the energy transfer and charge-separation process. The fifth chapter deals with the singlet oxygen generation of platinum and palladium porphyrins. In this chapter, a series of meso-substituted porphyrins are synthesized and metalated by platinum and pallidum porphyrins and characterized by several methods. The ability of both platinum and palladium porphyrins reveals higher electrochemical redox gaps as compared to their free base porphyrins. Both platinum and palladium porphyrins can generate singlet oxygen and probe by monitoring the photoluminescence of 1O2 at 1270 nm. The study highlights the importance of different meso-substituents in triplet porphyrin sensitizers that can estimate the singlet oxygen quantum yield, which is useful for photodynamic therapy, chemical synthesis, and other applications.

The photosynthetic process has the unique ability to capture energy from sunlight and accumulate that energy in sugars and starch. This thesis deals with the light driven part of photosynthesis. The aim has been to investigate how the light-absorbing protein complexes Photosystem I (PS I) and Photosystem II (PS II), react upon illumination of light with lower energy (far-red light; 700-850 nm) than the absorption peak at respective primary donor, P700 and P680.  The results were unexpected. At 295 K, we showed that both PS I and PS II were able to perform photochemistry with light up to 130 nm above its respective primary donor absorption maxima. As such, it was found that the primary donors’ action spectra extended approximately 80 nm further out into the red-region of the spectrum than previously reported.  The ability to perform photochemistry with far-red light was conserved at cryogenic temperatures (< 77 K) in both photosystems. By performing EPR measurements on various photosystem preparations, under different illumination conditions the origin of the effect was localized to their respective reaction center. It is also likely that underlying mechanism is analogous for PS I and PS II, given the similarities in spatial coordination of the reaction center pigments. For PS II, the results obtained allowed us to suggest a model involving a previously unknown electron transfer pathway. This model is based upon the conclusion that the primary cation from primary charge separation induced by far-red light resides primarily on ChlD1 in P680. This is in contrast to the cation being located on PD1, as has been suggested as for visible light illumination. The property to drive photochemistry with far-red wavelengths implies a hither to unknown absorption band, probably originating from the pigments that compose P700 and P680. The results presented here might clarify how the pigments inside P680 are coupled and also how the complex charge separation processes within the first picoseconds that initiate photosynthetic reactions occur.

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: Non-photochemical quenching (NPQ) is a photoprotective regulatory mechanism essential to the robustness of the photosynthetic apparatus of green plants. Energy flow within the low-light adapted reaction centers is dynamically optimized to match the continuously fluctuating light conditions found in nature. Activated by compartmentalized decreases in pH resulting from photosynthetic activity during periods of elevated photon flux, NPQ induces rapid thermal dissipation of excess excitation energy that would otherwise overwhelm the apparatus’s ability to consume it. Consequently, the frequency of charge separation decreases and the formation of potentially deleterious, high-energy intermediates slows, thereby reducing the threat of photodamage by disallowing their accumulation. Herein is described the synthesis and photophysical analysis of a molecular triad that mimics the effects of NPQ on charge separation within the photosynthetic reaction centers. Steady-state absorption and emission, time-resolved fluorescence, and transient absorption spectroscopies were used to demonstrate reversible quenching of the first singlet excited state affecting the quantum yield of charge separation by approximately one order of magnitude. As in the natural system, the populations of unquenched and quenched states and, therefore, the overall yields of charge separation were found to be dependent upon acid concentration. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2015

The efficiency of photoconversion systems, such as organic photovoltaic (OPV) cells, is largely controlled by a series of fundamental photophysical processes occurring at the interface before carrier collection. A profound understanding of ultrafast interfacial charge transfer (CT), charge separation (CS), and charge recombination (CR) is the key determinant to improving the overall performances of photovoltaic devices. The discussion in this dissertation primarily focuses on the relevant parameters that are involved in photon absorption, exciton separation, carrier transport, carrier recombination and carrier collection in organic photovoltaic devices. A combination of steady-state and femtosecond broadband transient spectroscopies was used to investigate the photoinduced charge carrier dynamics in various donor-acceptor systems. Furthermore, this study was extended to investigate some important factors that influence charge transfer in donor-acceptor systems, such as the morphology, energy band alignment, electronic properties and chemical structure. Interestingly, clear correlations among the steady-state measurements, time-resolved spectroscopy results, grain alignment of the electron transporting layer (ETL), carrier mobility, and device performance are found. In this thesis, we explored the significant impacts of ultrafast charge separation and charge recombination at donor/acceptor (D/A) interfaces on the performance of a conjugated polymer PTB7-Th device with three fullerene acceptors: PC71BM, PC61BM and IC60BA. Time-resolved laser spectroscopy and high-resolution electron microscopy can illustrate the basis for fabricating solar cell devices with improved performances. In addition, we studied the effects of the incorporation of heavy metals into π-conjugated chromophores on electron transfer by monitoring the triplet state lifetime of the oligomer using transient absorption spectroscopy, as understanding the mechanisms controlling intersystem crossing and photoinduced electron transfer dynamics is required to improve the device performance of solar cells. Here, we evaluated the effects of incorporating Pt(II) on intersystem crossing and photoinduced electron transfer by comparing and analyzing the photoexcited dynamics of DPP-Pt(II)(acac) and metal-free DPP with different acceptors such as TCNE, TMPyP, and TPyP.

The efficiency and mechanism of electron- and energy transfer events occurring in both in natural and synthetic donor-acceptor systems depend on their distance, relative orientation, and the nature of the surrounding media. Fundamental knowledge gained from model studies is key in building efficient energy harvesting and optoelectronic devices. Faster charge separation and slower charge recombination in donor-acceptor systems is often sought out. In our continued effort to build donor-acceptor systems using near-IR sensitizers, in the present study, we report ground and excited state charge transfer in newly synthesized, directly linked, tetrads featuring bisdonor (donor = phenothiazine and ferrocene), BF2-chelated azadipyrromethane (azaBODIPY) and C60 entities. The tetrads synthesized using multi-step synthetic procedure revealed strong charge transfer interactions in the ground state involving the donor and azaBODIPY entities. The near-IR emitting azaBODIPY acted as a photosensitizing electron acceptor along with fullerene while the phenothiazine and ferrocene entities acted as electron donors. The triads (bisdonor-azaBODIPY) and tetrads revealed ultrafast photoinduced charge separation leading to D•+-azaBODIPY•–-C60 and D•+-azaBODIPY-C60•– (D = phenothiazine or ferrocene) charge separated states from the femtosecond transient absorption spectral studies in both polar and nonpolar solvent media. The charge separated states populated the triplet excited state of azaBODIPY prior returning to the ground state.

Molecular engineering of donor-acceptor dyads and multimodular systems to control the yield and lifetime of charge separation is one of the key goals of artificial photosynthesis for harvesting sustainably solar energy. The design of the donor-acceptor systems mimic a part of green plants and bacterial photosynthetic processes. The photochemical events in natural photosynthesis involve the capturing and funneling of solar energy by a group of well-organized chromophores referred to as an ‘antenna' system causing an electron transfer into the ‘reaction center,' where an electron transfer processes occur resulting a long-lived charge separated state. Over the last two to three decades, many efforts have been directed by the scientific community designing of multi-modular systems that are capable of capturing most of the useful sunlight and generating charge separated states of prolonged lifetimes with adequate amounts of energy. In this dissertation, we report on the design and synthesis of donor–acceptor conjugates with the goal of modulating the yield and lifetime of their charge separated states and hence, improving the conversion of light energy into chemical potential. In simple donor-acceptor systems, generally, the energy and electron transfer events originate from the singlet excited state of the donor or acceptor and can store the greatest amount of energy but must be fast to out compete intersystem crossing. To address this limitation, we have designed novel donor –acceptor conjugates that use high-energy triplet sensitizers in which electron transfer is initiated from the long lived triplet state of the donor. The triplet photosensitizers used were palladium(II) porphyrin and platinum(II) porphyrin. Heavy metal effect in these porphyrins promoted intersystem crossing and the energies of their excited state was quite high. For the case of palladium (II) porphyrin the energy stored was found to 1.89 eV and that of platinum(II) porphyrin 1.84 eV. In addition to using triplet photosensitizers as donors, we have used donors that are difficult to oxidize and hence producing long lived charge separated states with adequate amount of stored energy. The system that was used for this study is zinc porphyrin with meso-aryl pentafluorophenyl substituents and fullerene, C60 as the acceptor. The presence of fluorine substituents on zinc porphyrin makes it harder to undergo oxidation. When this high potential donor-acceptor system undergoes a photoinduced charge-separation, the estimated energy stored was found to be 1.70 eV, one of the highest reported in literature so far. To further extend the lifetime of the charge separated states generated in this high-potential zinc porphyrin-fullerene dyad a pyridine functionalized tetrathiafulvalene was axially coordinated to the Zn metal producing a supramolecular triad capable of producing long-lived charge separated state. In a subsequent study, a multi-modular donor-acceptor system composed of a porphyrin, fullerene (C60) and a BF2-chelated dipyrromethene (BODIPY) with a supramolecular arrangement in the form of porphyrin-BODIPY-C60, one of the few reported in literature. By selectively exciting BODIPY and ZnP moieties, efficient singlet-singlet energy transfer from 1BODIPY * to ZnP in toluene was observed in the case of the dyad ZnP-BODIPY. However, when ZnP is excited, electron transfer occurred with the formation ZnP.+-BODIPY-C60.- charge separated state persisting for microseconds.