Investigating light-induced processes in covalent dye-catalyst assemblies for hydrogen production

The light-induced processes occurring in two dye-catalyst assemblies for light-driven hydrogen production were investigated by ultrafast transient absorption spectroscopy. These dyads consist of a push-pull organic dye based on a cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) bridge, covalently linked to two different H2-evolving cobalt catalysts. Whatever the nature of the latter, photoinduced intramolecular electron transfer from the excited state of the dye to the catalytic center was never observed. Instead, and in sharp contrast to the reference dye, a fast intersystem crossing (ISC) populates a long-lived triplet excited state, which in turn non-radiatively decays to the ground state. This study thus shows how the interplay of different structures in a dye-catalyst assembly can lead to unexpected excited state behavior and might open up new possibilities in the area of organic triplet sensitizers. More importantly, a reductive quenching mechanism with an external electron donor must be considered to drive hydrogen production with these dye-catalyst assemblies. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Synthèse et caractérisation de complexes hétéroleptiques de cuivre(I) pour la conversion de l'énergie solaire

This thesis describes the synthesis and characterization of new heteroleptic copper(I) complexes with diimine and diphosphine ligands for solar energy conversion. The complexes were synthesized with the HETPHEN approach, and were fully characterized by physicochemical and photophysical techniques.At first, a study of the properties of heteroleptic structures was carried out, considering in particular the effect of the steric hindrance of the coordination sphere. Two new ligands containing an extended phenanthroline core, and bromo groups, are then described together with the corresponding complexes. The improvement of the visible absorption properties was achieved with chromophoric push-pull styrylbipyridine ligands. This strategy allowed to obtain a series of complexes with impressively high absorption coefficients.In chapter 4, the use of heteroleptic copper(I) diimine complexes to build rod-like molecular arrays for photoinduced electron transfer was developed. Two Cu-NDI (naphthalene diimide) dyads were synthesized and characterized. Transient absorption spectroscopy evidenced photoinduced electron transfers. A secondary electron donor (ferrocene) was then included, to provide a triad with an increased charge-separated state lifetime of 30 ns in acetonitrile. The last chapter focuses on the design of complexes containing an anchoring function, for the use as sensitizers in dye-sensitized solar cells (DSSCs). Two series of dyes were prepared, one of them showing very promising photovoltaic performances (2.4% under 1000 W/m2).Cette thèse décrit la synthèse et la caractérisation de complexes hétéroleptiques de cuivre(I) avec des ligands diimine et diphosphine pour la conversion de l’énergie solaire. Les molécules ont été synthétisées en utilisant l’approche HETPHEN, et ont fait l’objet de caractérisations physicochimiques et photophysiques.En premier lieu, la synthèse et l’étude de nouveaux complexes de cuivre hétéroleptiques ont été menées en considérant en particulier l’effet de l’encombrement stérique de la sphère de coordination. Deux ligands originaux contenant un noyau phénanthroline étendu, et des fonctions réactives bromo, sont ensuite décrits ainsi que les complexes correspondants. Une augmentation des coefficients d’absorption dans le visible a été obtenue grâce à l’utilisation de ligands push-pull¬ de type styrylbipyridine.Dans le chapitre 4, l’utilisation des complexes hétéroleptiques de cuivre(I) diimine pour le transfert de charge photoinduit dans des assemblés linéaires a été développée. Deux dyades Cu-NBI (naphthalène bisimide) ont été synthétisées et caractérisées. La spectroscopie d’absorption transitoire a mis en évidence des transferts d’électron photoinduits. Un donneur d’électrons secondaire (ferrocène) a ensuite été rajouté, pour former une triade qui conduit à un temps de vie de l’état à charges séparées de 30 ns dans l’acétonitrile. Le dernier chapitre porte sur la conception de sensibilisateurs dans des cellules solaires à colorant (DSSCs). Deux séries de complexes de cuivre(I) ont été préparées et caractérisées. L’une des deux a montré des performances photovoltaïques très prometteuses (2,4% sous 1000 W/m2)

Fluoroalkyl and fluoroaryl fullerenes, polycyclic aromatic hydrocarbons, and copper(I) complexes: synthesis, structure, electrochemical, photophysical, and device properties

Includes bibliographical references.2020 Summer.In many fields of research, ranging from materials chemistry to medicinal chemistry, understanding the structural, electrochemical, and photophysical properties of materials is essential to establishing trends and predicting usefulness and future performance. This work has focused on the impact of strongly electron withdrawing perfluoroaryl and perfluoroalkyl substituents on the properties of fullerenes, polycyclic aromatic hydrocarbons (PAHs), hetero-PAHs, and copper(I) complexes with the goal of establishing and understanding the fundamental reasons for any observed trends. In Chapter 1, the first successful example of vacuum-deposited organic photovoltaic cells (OPVs) based on a fullerene derivative and a small-molecule donor is reported. A series of thermally robust fluorous fullerene acceptors with experimental gas-phase electron affinities ranging from 2.8 to 3.3 eV are paired with new dicyanovinyl thiophene-based molecular donors to enable direct comparison of their performance in planar and bulk heterojunction architectures in cells fabricated by vacuum deposition. Unprecedented insights into the role of the acceptor intrinsic molecular and electronic structures are obtained, which are not obscured by solvent and additive effects as in the typical solution-processed fullerene-based OPVs. Additionally, the fullerene derivative, C60CF2, was utilized in vacuum-deposited organic field effect transistors (OFETs), and it was shown to have superior device lifetime compared to C60 based OFETs. In Chapter 2, a new synthesis of 9,10-bis(perfluorobenzyl)anthracene, a promising blue organic light emitting diode (OLED) material is reported. The yield was improved from 7% to 17%, while the separations conditions were improved to only require one stage of HPLC. In Chapters 3 and 4, the trifluoromethylation of two hetero-PAHs, phenanthroline and phenanthridine, is discussed. The structure, solid-state packing, and electronic properties of the products are examined. Previously unknown structure-property relationships were established between the electronic properties and the position of CF3 groups. Additionally, the synthesis and excited-state dynamics for a series of homoleptic copper(I) phenanthroline complexes with 2, 3, and 4 trifluoromethyl groups are reported. Surprisingly, the observed time-resolved dynamics and emission trend is that addition of trifluoromethyl groups past two decreases the excited state lifetime and increases excited-state distortion

Ultrafast photophysics of high-spin metal complexes

Transition metal complexes (TMCs) show unusual optical, magnetic and chemical properties in comparison to purely organic molecules. These properties make them suitable for several applications. Organic light emitting diodes (OLEDs), catalysis, anti-cancer drugs, molecular switches and new magnetic materials are all areas where TMCs are in current use or within the scope of future applications. It is important to study their photophysical properties since they are of great importance in all these applications. To study the photophysical dynamics of TMCs, an ultrafast broadband transient absorption (TA) spectrometer was set up. For that, two spectrometers were build using charge coupled device(CCD) cameras as detector. Further, a white light continuum was required in order to measure broadband spectra. White light generation was induced in CaF2, which needs continuous movement in order to avoid damage on the crystalline CaF2 disk itself. In this thesis, the photophysics of three groups of TMCs were studied by several optical techniques including ultrafast TA spectroscopy. First, three manganese complexes - two of them showing single molecule magnet (SMM) behaviour - were studied. Manganese complexes are known to have fascinating magnetic and catalytic properties. Their usage as SMMs and catalytic centre in chlorophyll makes them an important class of TMCs. To study the photophysical properties of Mn SMMs, a comparison of Mn complexes with one, three and six Mn ions in one molecule was performed. Two out of the three samples were Mn(III)-based SMMs, which are [Mn(III)3O(Et − sao)3(β−pic)3(ClO4)] or Mn3 and [Mn(III)6O2(Et−sao)6(O2CP h(Me)2)2(EtOH)6] or Mn6. The third one was the Mn(acac)3 (acac ≡ acetylacetonate anion) complex. As a result from the ultrafast TA, it was found that a vibrational wave packet is formed upon photoexcitation of all three complexes. The results show new possibilities for non-thermal control of the magnetisation in SMMs and open up new molecular design challenges to control the wave packet motion in the excited state. Iron complexes, especially iron in oxidation state +2 with six electrons in the d-orbitals, are well-studied due to a spin-crossover phenomenon that can be triggered by optical stimulation. In this thesis, the focus is on the complex [F e3O(Ac)6(H2O)3] (F e3) with three iron ions in mixed oxidation states. One ion is in oxidation state +2 while the other two are in oxidation state +3 (Fe(III)Fe(III)Fe(II)). This molecule is of interest due to possible long-lived charge transfer states and spin dynamics due to the mixed oxidation states. Ultrafast transient absorption spectroscopy was performed of the Fe complex in solution at room-temperature exciting either at 400 nm or 520 nm and a long-lived excited-state absorption (ESA) signal was observed. The broad ESA band is comprised of several un-resolved bands, showing shoulders at 405 nm, 440 nm and 530 nm. From the transient absorption results, three decay constants of τ1 = 360 ± 30 fs, τ2 = 5.3 ± 0.6 ps, τ3 = 65 ± 5 ps and a long-lived state (τ4 > 500 ps) were extracted by a global multi-exponential fit over the full wavelength range (340 nm to 690 nm). The comparison of the TA results with two pump wavelengths indicates that the lowest excited state is populated on a sub 120 fs time scale. Calculations on the coupled cluster level of theory, performed by collaborators, showed that this state has a mixture of both charge-transfer and ligand-field/d-orbital character. Third, the ultrafast photophysics of a terbium phthalocyanine double-decker complex (TbPc2) were investigated. The interaction of the ligands via the terbium linker and the luminescence properties are studied. Static UV/Vis absorption and luminescence spectroscopy were performed. The typical Q- and B-bands associated with excitations in the first and second excited singlet states of the phthalocyanine ligands were found. In the luminescence spectrum after excitation in the UV, signatures of the typical emission for a Tb(III) ion in the green part of the spectrum were found, overlapped with emission from the ligands. The ultrafast TA difference spectra are dominated by ground state bleach bands, which overlap with ESA. The dynamics show a long-lived state, which is important for the luminescence of the Tb3+ ion. The energy transfer pathway was tentatively assigned to a direct excitation of the emissive Tb3+ via the B-band levels of the radical phthalocyanine

Using coherence to enhance function in chemical and biophysical systems

Coherence phenomena arise from interference, or the addition, of wave-like amplitudes with fixed phase differences. Although coherence has been shown to yield transformative ways for improving function, advances have been confined to pristine matter and coherence was considered fragile. However, recent evidence of coherence in chemical and biological systems suggests that the phenomena are robust and can survive in the face of disorder and noise. Here we survey the state of recent discoveries, present viewpoints that suggest that coherence can be used in complex chemical systems, and discuss the role of coherence as a design element in realizing function

The Accuracy of Electron Density from Theory : Calculating Electric Field Gradients and X-ray Scattering for Molecules and Solutions

Ph. D. Thesis.Electron density is the principle determinant of the characteristic properties of molecules, their structure and dynamics. For this reason it is vital to ascertain accurate densities. One method of predicting electron densities is density functional theory (DFT). It is derived from the Hohenburg-Kohn theorem which states that an exact ground-state energy should yield an exact electron density and vice versa. In reality the exact solution is not fully known as exchange and correlation have to be estimated. Were it known, accurate densities and energies could be calculated at a fraction of ab initio computational cost. However, it is noted that functional methods have deviated from the path to the exact functional due to energetic overfitting. This study used the electric field gradient (EFG) as an electron-density probe to facilitate comparison both to ab initio calculation (CCSD(T)) and microwave spectroscopy for simple transition metal complexes and halogenated aromatic compounds. EFGs improved with increased Hartree-Fock (HF) exchange fraction encountered for higher rungs of Jacob’s Ladder due to selfinteraction error (SIE) reduction. SIE cancellation was uneven between transition metals, halogens and aromatic rings, causing functional-dependent electronegativity. Electron density can also be inferred from X-ray scattering. X-ray free-electron lasers (XFELs) are used to probe molecular structure and dynamics on ultrafast time scales. Solutions contains additional scattering signals other than the desired solute from the solvent and solutesolvent. The solvent term can be extracted experimentally or via molecular dynamics (MD) trajectories. Theory is also the only method of predicting the solute-solvent term independently. The solvent force-field parameters can be derived from experiment or theoretically from DFT calculation. The impact of the chosen force field on the the predicted scattering profiles was evaluated herein, Quantum Bespoke Kit (QUBE) and all-atom Optimised Potentials for Liquid Simulations (OPLS-AA) force-fields were used to assess theoretically- and experimentally-derived parameters respectively for common solvents for the same test solute (I2). Force-field dependence is elucidated for both terms due to differences in non-bonded parameters.There also remains further investigation to better approximate experimental solvent terms. Solute-solvent scattering occurs on comparable scales to the solute scattering. XFELs have also been applied recently to improve understanding metal-to-ligand charge transfer (MLCT) in transition metal complexes and the influence of polar solvents in their structures. QUBE was used to investigate the ground and excited states of [Cu(phen)2]+ and compared to recent classical MD and quantum-mechanical/molecular mechanical (QM/MM) simulations. It performed particularly well in regards to identifying Cu-N bond asymmetry and solvent influence in the ligand-ligand dihedral in the triplet state which were not identified in previous theoretical investigation but in agreement with recent experimental understanding. This improvement was attributed to its use of high-rung DFT in parameterisation (wB97X-D). Overall this investigation evaluate thoroughly the current state of theory in reproducing accurate electron densities, highlighting the importance of reducing DFT SIE to improve density accuracy, which in turn impacts force-field parameter quality, indicating that DFT improvement impacts all branches of theoretical chemistry

Seventh international conference on time-resolved vibrational spectroscopy

The International Conference on Time-Resolved Vibrational Spectroscopy (TRVS) is widely recognized as the major international forum for the discussion of advances in this rapidly growing field. The 1995 conference was the seventh in a series that began at Lake Placid, New York, 1982. Santa Fe, New Mexico, was the site of the Seventh International Conference on Time-Resolved Vibrational Spectroscopy, held from June 11 to 16, 1995. TRVS-7 was attended by 157 participants from 16 countries and 85 institutions, and research ranging across the full breadth of the field of time-resolved vibrational spectroscopy was presented. Advances in both experimental capabilities for time-resolved vibrational measurements and in theoretical descriptions of time-resolved vibrational methods continue to occur, and several sessions of the conference were devoted to discussion of these advances and the associated new directions in TRVS. Continuing the interdisciplinary tradition of the TRVS meetings, applications of time-resolved vibrational methods to problems in physics, biology, materials science, and chemistry comprised a large portion of the papers presented at the conference

N,C,N- und P,C,P-Carbodiphosphorane – Anwendungen als Chelatliganden und quantenchemische Analyse davon abgeleiteter und verwandter OLED-Emitter-Materialien

Innovative Beiträge dieser Dissertation finden sich im Bereich der metallorganischen Synthese neuartiger Carbodiphosphoran-Chelatliganden und ihrer Komplexe, in erstmaligen Untersuchungen zur Anwendung dieser Carbodiphosphorane als Template für OLED-Emitter- Komplexe sowie in der quantenchemischen Analyse der Grenzorbitale im Grundzustand und angeregten Zuständen, somit im Absorptions- und Emissionsverhalten derartiger Verbindungen