Towards Harnessing Solar Energy with Iron N-Heterocyclic Carbene Complexes

Negative impact of human activity-related emission of greenhouse gases on Earth's ecosphere has been known for decades, but has recently come into spotlight with unprecedented intensity. This is due to increasingly clear understanding that the global warming could soon spiral out of control. It is therefore essential to diminish the use of fossil fuels and switch to renewable energy sources. Despite extensive progress in solar cells technology, it is still very immature and further substantial improvements (lower cost, higher efficiency) are needed in order for it to compete with fossil fuels.The aim of the current study was to advance towards substituting currently dominating ruthenium for cheap and abundant iron as the metal atom in dye-sensitized solar cells (DSSC). The goal was to design, synthesize and characterize a series of iron-based organic complexes with a predefined set of characteristics for potential application as photosensitizers.Chapter 1 describes design and synthesis of a number of compounds, belonging to the homoleptic iron N-heterocyclic carbene family and investigation of their photophysical properties. The desired performance - efficient electron injection - was proven to occur, when one of the compounds was functionalized with an anchoring group and immobilized onto the semiconductor.Chapter 2 details how a series of heteroleptic Fe N-heterocyclic carbene complexes were synthesized and spectroscopically characterized. The production of the corresponding DSSC materials is currently in progress.In chapter 3 the influence of the complex geometry around the metal center is discussed. Higher octahedricity complex demonstrated promising photophysical behaviour, however further optimization is required in order to improve its excited state properties.Chapter 4 deals with tris N-heterocyclic carbene ligands, which were shown to be highly efficient in stabilizing the oxidation state 3+ of the central atom in bis-tridentate Fe complexes. One compound, containing tripodal anionic boron ligand with three N-heterocyclic carbenes was found to be photoluminescent at room temperature with a highest reported to date quantum yield

Access to new sulfolene derivatives via formylation of thiolan-3-one

New tetrahydrothiophene derivatives, which are demonstrated to be reactive towards various nucleophiles and provide access to [b]- and [c]-fused sulfolenes are described. Application of heterocyclic sulfides as o-quinodimethane precursors is documented. (C) 2013 Elsevier Ltd. All rights reserved

HERFD-XANES probes of electronic structures of ironII/IIIcarbene complexes

Iron centeredN-heterocyclic carbene (Fe-NHC) complexes have shown long-lived excited states with charge transfer character useful for light harvesting applications. Understanding the nature of the metal-ligand bond is of fundamental importance to rationally tailor the properties of transition metal complexes. The high-energy-resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) has been used to probe the valence orbitals of three carbene complexes, [FeII(bpy)(btz)2](PF6)2(bpy = 2,2′-bipyridine, btz = 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene)), [FeIII(btz)3](PF6)3, and [FeIII(phtmeimb)2]PF6(phtmeimb = [phenyl(tris(3-methylimidazol-2-ylidene))borate]−). The multiconfigurational restrict active space (RAS) approach has been used to simulate the metal K pre-edge X-ray absorption spectroscopy of these carbene complexes, and have reproduced the metal K pre-edge spectral features in terms of relative intensity and peak positions. The evident intensity difference between the FeIIand the other two FeIIIcomplexes has been elucidated with different intensity mechanisms in the transition. The smaller splitting between the t2gand egcharacter peak for [FeIII(btz)3](PF6)3has been observed in the experimental measurements and been reproduced in the RAS calculations. The results show how the combination of experimental HERFD-XANES measurements andab initioRAS simulations can give quantitative evaluation of the orbital interactions between metal and ligands for such large and strongly interacting systems and thus allow to understand and predict properties of novel complexes

Electronic structure and excited state properties of iron carbene photosensitizers - A combined X-ray absorption and quantum chemical investigation

The electronic structure and excited state properties of a series of iron carbene photosensitizers are elucidated through a combination of X-ray absorption measurements and density functional theory calculations. The X-ray absorption spectra are discussed with regard to the unusual bonding environment in these carbene complexes, highlighting the difference between ferrous and ferric carbene complexes. The valence electronic structure of the core excited FeIII-3d5 complex is predicted by calculating the properties of a CoIII-3d6 carbene complex using the Z+1 approximation. Insight is gained into the potential of sigma-donating ligands as strategy to tune properties for light harvesting applications

Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality

A new generation of octahedral iron(ii)-N-heterocyclic carbene (NHC) complexes, employing different tridentate C^N^C ligands, has been designed and synthesized as earth-abundant photosensitizers for dye sensitized solar cells (DSSCs) and related solar energy conversion applications. This work introduces a linearly aligned push-pull design principle that reaches from the ligand having nitrogen-based electron donors, over the Fe(ii) centre, to the ligand having an electron withdrawing carboxylic acid anchor group. A combination of spectroscopy, electrochemistry, and quantum chemical calculations demonstrate the improved molecular excited state properties in terms of a broader absorption spectrum compared to the reference complex, as well as directional charge-transfer displacement of the lowest excited state towards the semiconductor substrate in accordance with the push-pull design. Prototype DSSCs based on one of the new Fe NHC photosensitizers demonstrate a power conversion efficiency exceeding 1% already for a basic DSSC set-up using only the I−/I3−redox mediator and standard operating conditions, outcompeting the corresponding DSSC based on the homoleptic reference complex. Transient photovoltage measurements confirmed that adding the co-sensitizer chenodeoxycholic acid helped in improving the efficiency by increasing the electron lifetime in TiO2. Time-resolved spectroscopy revealed spectral signatures for successful ultrafast (<100 fs) interfacial electron injection from the heteroleptic dyes to TiO2. However, an ultrafast recombination process results in undesirable fast charge recombination from TiO2back to the oxidized dye, leaving only 5-10% of the initially excited dyes available to contribute to a current in the DSSC. On slower timescales, time-resolved spectroscopy also found that the recombination dynamics (longer than 40 μs) were significantly slower than the regeneration of the oxidized dye by the redox mediator (6-8 μs). Therefore it is the ultrafast recombination down to fs-timescales, between the oxidized dye and the injected electron, that remains as one of the main bottlenecks to be targeted for achieving further improved solar energy conversion efficiencies in future work

Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime

Iron’s abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer (CT) states of most Fe complexes are limited by picosecond or sub-picosecond deactivation through low-lying metal centered (MC) states, resulting in inefficient electron transfer reactivity and complete lack of photoluminescence. Here we show that octahedral coordination of Fe(III) by two mono-anionic facial tris-carbene ligands can suppress such deactivation dramatically. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is [phenyl(tris(3-methylimidazol-1-ylidene))borate]-, exhibits strong, visible, room temperature photoluminescence with a 2.0 ns lifetime and 2% quantum yield via spin-allowed transition from a ligand-to-metal charge-transfer (2 LMCT) state to the ground state (2 GS). Reductive and oxidative electron transfer reactions were observed for the2 LMCT state of [Fe(phtmeimb)2]+ in bimolecular quenching studies with methylviologen and diphenylamine

Hot Branching Dynamics in a Light‐Harvesting Iron Carbene Complex Revealed by Ultrafast X‐ray Emission Spectroscopy

Iron N‐heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub‐ps X‐ray spectroscopy study of an Fe$^{II}$NHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot $^3$MLCT state, from the initially excited $^1$MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the $^3$MC state, in competition with vibrational relaxation and cooling to the relaxed $^3$MLCT state. The relaxed $^3$MLCT state then decays much more slowly (7.6 ps) to the $^3$MC state. The $^3$MC state is rapidly (2.2 ps) deactivated to the ground state. The $^5$MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the $^3$MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition‐metal complexes for similar ultrafast decays to optimize photochemical performance

الطقوس وجبروت الرموز: قراءة في الوظائف والدلالات ضمن مجتمع متحوّل

تتناول هذه الدراسة بالتحليل القضايا المتعلقة بالتحولات الاجتماعية التي ترافقها تحولات على مستوى الرمز و الدلالات. و بعد التساؤل عن فحوى هذه العلاقة بين ما هو اجتماعي و ما هو رمزي، و بعد التنظير لقضايا الرمز والشعائر الطقسية تحاول هذه الدراسة الإجابة عن الكيفية التي يواجه بها المجتمع المحلي هذه التحولات. إذ أن الحياة الاجتماعية قد تعلمنت و حركة تقسيم العمل قد عرفت مداها واستقلت أنشطة الحياة المختلفة عن الحياة الدينية، و مع هذه التغيرات، انتقلت الأنشطة الرمزية و الطقوسية إلى مستوى آخر مختلف عما سبق. لكن تمسك بعض الفئات بهذه الطقوس و الشعائر يمنحها تمسكا قويا بالهوية، و يؤدي وظيفة عيادية تجعلهم يفلتون من الرقابة اليومية و من طغيان الثقافة، البرمجيات والأزرار.Cet article aborde les questions relatives aux mutations sociales accompagnées de mutations symboliques et signifiantes. En s’interrogeant sur la nature de la relation qui existe entre ce qui est social et ce qui symbolique, et après théorisation des questions du symbole et du rite, cette étude tente de dévoiler la réaction de la société face à ces mutations.La vie sociale étant mondialisée et le mouvement de répartition des tâches ayant atteint ses limites, les différentes activités de la vie demeurent aujourd’hui indépendantes de la vie religieuse. Car grâce à ce changement, les activités symboliques et rituelles sont passées d’un niveau à un autre niveau. Cependant, l‘attachement de certains groupes à ces rituels, les reliant fortement à l’identité, exerce une fonction clinique qui leur permet de fuir le contrôle quotidien et la tyrannie de la culture, les logiciels et les manipulations digitales.This article tackles questions relating to so social change accompanied by important symbolic mutations. By questioning the nature of the relationship existing between what is social and what is symbolic, and after theorizing questions of symbol and rites, this study tries to reveal social reaction faced with these mutations.Social life being globalized and the repartition of tasks having reached its limits, the different activities of daily life nowadays remain independent from religious life. Therefore thanks to this change, symbolic activities and rituals have passed from one level to another one. Nevertheless the attachment of some groups to these rituals, strongly linked to identity, exerts a clinical function which allows them to flee from daily control and cultural tyranny, from software and digital manipulation.Este artículo aborda cuestiones relativas a las mutaciones sociales acompañadas de mutaciones simbólicas y significantes. Al interrogarnos sobre la naturaleza de la relación que existe entre lo que es social y lo que es simbólico, y tras la teorización de las cuestiones del símbolo y del rito, este estudio intenta desvelar la reacción de la sociedad frente a dichas mutaciones.Siendo mundializada la vida social y el movimiento del repartimiento de las tareas habiendo alcanzado sus limites, las diferentes actividades de la vida resultan hoy día independientes de la vida religiosa. Pues, gracias a este cambio, las actividades simbólicas y rituales han pasado de un nivel a otro nivel. Sin embargo, el apego de ciertos grupos a estos rituales que les ligan fuertemente a la identidad, ejerce una función clínica que les permite huir del control cotidiano y de la tiranía de la cultura, los logiciales y las manipulaciones digitales

Hot branching dynamics in a light‐harvesting iron carbene complex revealed by ultrafast x‐ray emission spectroscopy

Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an FeIINHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot 3MLCT state, from the initially excited 1MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3MC state, in competition with vibrational relaxation and cooling to the relaxed 3MLCT state. The relaxed 3MLCT state then decays much more slowly (7.6 ps) to the 3MC state. The 3MC state is rapidly (2.2 ps) deactivated to the ground state. The 5MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance