@phdthesis{Kaiser2014,
  author    = {Kaiser, Conrad},
  title     = {Donor-Bridge-Acceptor Systems with Varying Bridge Units for the Investigation of Intramolecular and Intermolecular Electron Transfer Processes},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-97614},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {Within this study, the influence of the energetics of the bridge unit on electron transfer (ET) in an electrode-bridge-donor system was investigated in a monolayer environment. This was realized by specifically designing molecules containing ferrocene carboxylic ester donors and hydroquinone derivatives as bridge units and by using a gold electrode as acceptor. The energetics of the hydroquinone derivatives was adjusted by synthetically varying its substituents with the intention of changing the ET speed and mechanisms. Thereby the choice of the substituents was based on the literature known half-wave potentials of similar solvated hydroquinone derivatives and successively confirming them by conducting cyclic voltammetry on the actual bridge units synthesized. Then, a synthetic pathway, which accommodated the limited stability of the integrated terminal ferrocene carbon acid ester, was developed and successfully employed. This was followed by developing a procedure for preparing very dense and highly ordered monolayers from the target molecules on self-made gold microelectrodes. For the electrochemical investigations, several electrolyte solutions were tested until one, which ensured low susceptibility of the characterization setup towards slight changes of the electrode arrangement and measurement parameters while ensuring sufficient stability of the monolayers, was found. Furthermore, a new, commercially available potentiostat was established for the impedance measurements, which reduced the stress on the monolayers during the electrochemical characterizations in comparison to the equipment used in many former studies. Regarding the determination of the ET rates, the data analysis protocol for the impedance measurements developed by Creager et al. was slightly adapted to allow analysis of the investigated monolayers despite their non-ideal behavior. In addition, the influence of changes to the electrical parameters of the impedance scans was investigated to minimize the error in the acquired data. The electrochemical analysis of the monolayers by conducting cyclic voltammetry on MA, MB and MC prepared from A, B and C confirmed the accomplishment of near ideal surface coverage and exceptionally high order. The surface coverages of MB and MC were, probably due to the space filled by the substituents on their bridge units, slightly lower than those of MA. Furthermore, the shape of the redox waves of the ferrocene carboxylic acid redox center in the voltammogram of MA showed a broadening and a shift towards higher potentials, which was assigned to electrostatic interference of oxidized terminal redox centers due to the especially dense packing. However, in the voltammogram of MB, no sharp redox waves of the bridge units, as predicted by the analysis of preliminary monolayers of the same type with low surface coverage, were present. This was attributed to the different and varying microenvironment of the bridge units deeply embedded within high-density monolayers. In detail, the different degree of shielding of each individual bridge unit from counter ions and solvent molecules probably resulted in the half wave potential being shifted to varying higher potentials, thus preventing the formation of sharp redox waves. In addition, electrostatic effects of oxidized bridge units could have enhanced this effect. This leads to the conclusion that the half-wave potentials of fully solvated bridge units determined by the cyclic voltammetry are not suited to predict the energetics of the oxidized bridge states embedded within the prepared high density monolayers. Finally, the monolayers were successfully analyzed by impedance spectroscopy, which showed that the ET rate of MA is slightly higher than that of MB, and both are higher than that of MC. All of the values were, according to literature, in the expected region considering the length and degree of conjugation of the backbone. However, this picture is relativized when considering the targeted energetic alignment of the bridge units. According to the predicted very small energy gap between the oxidized states of the donor and the bridge unit in MB, a domination of the hopping mechanism should have led to a several orders of magnitude higher ET rate than in MA and MC. That this was not the case was attributed to the underestimation of the energy of the oxidized bridge states by utilizing cyclic voltammetry of the fully solvated bridge units (see above). According to the small differences of the ET rates the superexchange process was assumed to be the dominating mechanism not only in MA and MC but also in MB. However, even when shifted, the predicted energetic order of the oxidized bridge states should have led to a moderately decreasing ET rate from MB over MA to MC. The reason for the actual ET rate in MA being slightly higher than in MB might be found in the electrostatic interference of the terminal redox centers in MA (see above). In conclusion, the targeted model systems were prepared and the ET rates were successfully determined. However, the problems concerning the relative energetic positioning of the involved states within the dense monolayers prevented the specific alteration of the speed and mechanism of the ET. The reason for this can be probably found in the high density and order of the monolayers prepared within this work, which hamper the intrusion of the components of the electrolyte solutions. This various degree of stabilization for the individual bridge units by counter ions and solvent molecules leads to the energy of the oxidized bridge states being splitted and shifted towards higher potentials with respect to fully solvated bridge units. This effect might be further enhanced by electrostatics of neighboring already oxidized bridge states. All this makes the predetermination of the energetics of the embedded bridge units extremely difficult. On one hand, this behavior can be considered an obstacle and could probably be circumvented by designing molecules with bulky anchor groups and rigid molecular backbones, which would ensure perpendicular arrangement to the surface and full exposure of the bridge and terminal redox centers to the solvent molecules and counter ions. On the other hand, monolayers which completely embed integral redox centers might open up the opportunity to study the effects of microenvironments similar to those in solid state materials. Regarding mixed valence compounds, the present study focuses on bistriarylamine radical cation F∙+, which contains the [3.3]paracyclophane bridge unit. The results were compared to the, except for the bridge units, identical literature known compounds G∙+ and N∙+ with [2.2]paracyclophane and p-xylene bridges respectively. This led to the conclusion that slightly different bridge units can induce substantial changes to the internal reorganization energy. This is especially noteworthy since it is usually believed that structural adaption limited to the redox centers taking part in the charge transfer dominates the internal reorganization energy. Furthermore, the application of the two-state Mulliken-Hush approach shows that compounds F∙+ and G∙+ have near identical couplings and similar thermal barriers. Confirmation of the latter finding as well as near identical thermal electron transfer rates for both compounds were provided via a cooperation project by Grampp et al. in which these values were directly extracted from temperature dependent electron paramagnetic resonance measurements. These results are quite unexpected since the "through-space" distances of the stacked pi-systems in the paracyclophane bridges differ significantly. They are well within the sum of the van der Waals radii in G∙+ and barely within them in compound F∙+. In addition, these findings weaken the common assumption of the ethylene bridges in G∙+ substantially adding to the electronic coupling, since then, in F∙+, due to its propylene linkers, the coupling should be substantially reduced. Finally, relying on the fact that the electronic couplings are only three times higher and the thermal electron transfer rates are only one order of magnitude higher for N∙+ than for compounds F∙+ and G∙+ shows that intermolecular electron transfer in solid state materials can remain efficient, if the interacting pi-systems stay within the sum of van der Waals radii of their carbons. Concerning the donor-acceptor dyads, the current investigation centers on triarylamine-cyclophane-naphtalene diimide (TAA-CP-NDI) compounds which display almost complete photoinduced charge separation. Furthermore, their singlet charge separated states show lifetimes of hundreds of nanoseconds, which is rarely found in such simple dyads. In the present case they can be attributed to the particular amount of electronic coupling V (on the order of 100 cm^-1), which is brought about by incorporation of the smallest model systems for pi-stacks, the CPs, together with the nodes on the NDI lowest unoccupied molecular orbital, which electronically decouples the central NDI from its nitrogen substituents. In agreement with studies of [2.2]- and [3.3]paracyclophane bridged mixed valence compounds (see above), the cycolphane bridged dyads show very similar electronic coupling when dealing with ground state processes like charge recombination. However, when investigating excited state processes, like charge separation in the TAA-CP-NDI dyads, one has to bear in mind that the CP orbitals are involved in the formation of intermediate states that likely possess charge transfer character. In this case, the [2.2]paracyclophane bridge obviously induces a stronger coupling than the [3.3]paracyclophane. Another interesting property of the dyads studied here is the substantial population of the triplet charge separated (CS) state of ca. one third regarding both CS states, which is brought about by singlet-triplet interconversion from the singlet CS state. Thus, the triplet CS state with a lifetime of several microseconds acts as a kind of buffer for the CS state before recombining to the ground state and, thus, leads to distinctly prolonged overall lifetimes of the charge separated states. Thus it can be concluded that the intersystem crossing and charge recombination (CR) processes of the CS states are governed by a delicate balance of a large electronic coupling V and a large exchange interaction 2J (both with regard to systems containing a through-space pathway). The latter appears to be induced by second order interaction with a local triplet state lying close in energy to the CS state. This balance results in slow CR- and singlet-triplet- interconversion rates, which differ only by one order of magnitude. Compared to the many NDI containing dyads studied so far, these features of the dyads studied here are, to the best of our knowledge, unique. Especially the combination of high quantum yield of charge separation, long lifetimes and high energy of the charge separated state make the investigated systems interesting for practical applications. Furthermore, the presented unraveling of the underlying mechanisms is of substantial value for the future design of dyads for practical applications regarding the implementation and adjustment of these favorable properties.},
  subject      = {Elektronentransfer},
  language  = {en}
}
@phdthesis{Mueller2011,
  author    = {M{\"u}ller, Christian},
  title     = {Physical Properties of Chromophore Functionalized Gold Nanoparticles},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-57657},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {n this work the synthesis and analysis of chromophore functionalized spherical gold nanoparticles is presented. The optical, electrochemical and spectroelectrochemical properties of these hybrid materials are furthermore studied. The work therefore is divided into two parts. The first part deals with triarylamine and PCTM-radical functionalized gold nanoparticles. The focus thereby was on the synthesis and on the investigations of chromophore-chromophore interactions and gold core-chromophore interactions. The chromopores, especially triarylamines, were attached to the gold core via different bridging units and were studied with optical and electrochemical methods. The purity and dimensions of the nanoparticles was determined by 1H-NMR spectroscopy, diffusion ordered NMR spectroscopy (DOSY), TGA, XPS and STEM. Furthermore a cyclic voltammetry technique was used to determine the composition of the particles via the Randles-Sevcik equation. An analysis of these parameters led to a model of a sea urchin-shaped nanoparticle. Optical measurements of the particles revealed an anisotropic absorption behavior of the triarylamine units due to gold core-chromophore interaction. However this behavior depends strongly on the relative orientation of the transition dipole moment of the chromophore to the gold surface and the distance of the chromophore to the surface. Hence, the anisotropic behavior was exclusively detected in the spectra of the Au-Tara1 particles. The short and rigid pi-conjugated bridging unit thereby facilitates this gold core-chromophore interaction. It was shown from electrochemical investigations that the triarylamine units can be chemically reversibly oxidized to the triarylamine monoradical cation. Furthermore, the measurements revealed a strong interligand triarylamine-triarylamine interaction which was only seen for the Au-Tara1 particles. The long pi-conjugated bridging units of the Au-Tara2 and Au-Tara3 particles as well as the aliphatic bridging unit of Au-Tara4 prevent any detectable interligand interactions. One may conclude that both the gold core-chromophore and the interligand triarylamine-triarylamine interaction depend on the length and the rigidity of the bridging unit. The electron transfer behavior of the triarylamine units adsorbed onto the gold core was additionally studied via spectroelectrochemical (SEC) measurements which are able to reveal weaker interactions. The investigations of Au-Tara1 and Au-Tara2 revealed a significant strong coupling between neighboring triarylamine units which is due to through-space intervalence interactions. This behavior was not detected for Au-Tara3 or for Au-Tara4. The SEC analysis also revealed that these observed interligand interactions depend on the length and the rigidity of the bridging unit. Thus, the systematic variation of the bridging unit gave a basic insight in the optical and electrochemical properties of triarylamines, located in the vicinity of a gold nanoparticle. The second part of this work aimed at the synthesis of new molecules, denoted as SERS-markers, for immuno SERS applications. For this purpose, the SERS-markers were designed to have a Raman-active unit and a thiol group for chemisorptions to Au/Ag nanoshells. In cooperation with the group of Schl{\"u}cker (University of Osnabr{\"u}ck) the SERS-markers were absorbed onto Au/Ag nanoshells, denoted as SERS-labels, and characterized. The SERS spectra of the SERS-labels exhibited intense and characteristic SERS-signals for each marker. For immuno SERS investigations SEMA3 was functionalized with a hydrophilic end unit. This marker was adsorbed onto an Au/Ag nanoshell and encapsulated with silica. An anti-p63 antibody was bound to the silica surface in order to generate a SERS-labeled antibody for the detection of the tumor suppressor p63 in benign prostate. Immuno-SERS imaging of prostate tissue incubated with SERS-labeled anti-p63 antibodies demonstrated the selective detection of p63 in the basal epithelium. The results show the potential of the method for the detection of several biomolecules in a multiplexing SERS experiment.},
  subject      = {Gold},
  language  = {en}
}
@phdthesis{Kriegisch2005,
  author    = {Kriegisch, Volker},
  title     = {Electron transfer processes between organic redox centres and electrodes via active bridges in self-assembled monolayers},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-15892},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2005},
  abstract  = {Cyclovoltammetrische Messungen der Ferrocenalkylthiole 1 - 3 belegen, dass homogene, gemischte Monolagen aus redoxaktiven Verbindungen und redoxinaktiven Alkylthiolen gebildet werden. Die von Creager et al. bestimmten ET Raten der Ferrocenalkylthiole 1 - 3 konnten hierbei verifiziert werden. Wie erwartet erfolgt eine Abnahme der ET Geschwindigkeit bei einer Kettenverl{\"a}ngerung des Alkylspacers von 2 nach 3. Eine unterschiedliche Konnektivit{\"a}t zwischen Redoxzentrum und Alkylspacer, z. B. die Einf{\"u}hrung einer Carbonyl-Funktion im Falle von 1, unter Beibehaltung der Kettenl{\"a}nge zeigt keinen bemerkbaren Einfluß auf den ET. Trotzt vergleichbaren Abstands der aromatischen Ferrocenthiole 4 und 5 zu der C8-Alkyl-Verbindung 2 zwischen Redoxzentrum und Elektrode, weisen diese aufgrund ihrer starken Konjugation sehr hohe ET Geschwindigkeiten auf. Die elektronischen Kopplungsfaktoren selbst deuten auf einen nichtadiabtischen ET zwischen Redoxzentrum und Elektrode hin. Wie erwartet kommt es zu einem Anwachsen der Kopplungsfaktoren bei sich verk{\"u}rzender Kettenl{\"a}nge oder bei Einf{\"u}hrung konjugierter Spacersysteme. Zusammenfassend kann gesagt werden, dass Erfahrungen hinsichtlich der Pr{\"a}paration der Monolagen gesammelt, die gemessenen ET Raten f{\"u}r der literaturbekannten Verbindungen 1 - 3 best{\"a}tigt und diese Informationen auf die konjugierten Verbindungen 4 und 5 angewandt werden konnten. Im zweiten Teil wurden die Triarylamin- (29, 32) und Phenothiazinalkylthiole (35) bez{\"u}glich ihres ET Verhaltens in gemischten Monolagen untersucht. Mittels Cyclovoltammetrie konnte gezeigt werden, daß einheitlich geformte, verd{\"u}nnte Monolagen vorliegen. Die ET Raten der Triarylamin- (29, 32) und Phenithiazinalkylthiole (35) sind jedoch um den Faktor 10 bis 100 h{\"o}her als vergleichbare Ferrocenalkylthiole gleicher Kettenl{\"a}nge [1, 2], wohingegen f{\"u}r Monolagen, welche [Ru(bpy)2(pp)]+-Alkythiole enthalten, {\"a}quivalente Werte gefunden wurden [3]. Die ET Geschwindigkeit wird von zwei Parametern beeinflusst: dem elektronischen Kopplungsmatrixelement und der Regorganisationsenergie \&\#61548;\&\#61472; [4]. Die ET Geschwindigkeit in Donor-substituierten Alkylthiolen wird haupts{\"a}chlich durch \&\#61548; beeinflusst und sogar kleine {\"A}nderungen dieser zeigen eine große Auswirkung auf die zu untersuchenden Prozesse. Aus diesem Grund wird eine Zunahme der ET Geschwindigkeit von Ferrocen (hohe Reorganisationsenergie) {\"u}ber die Phenothiazinverbindung 35 und [Ru(bpy)2(pp)]+ zu den Triarylaminchromophoren 29 und 32 (niedrige Reorganisationsenergie) beobachtet. Weiterhin spielt, im Gegensatz zu Beobachtung von Creager et al. an {\"a}quivalenten Ferrocenverbingungen, die Anbindung des Redoxzentrums an den Alkylspacer eine bedeutende Rolle. Im Falle der elektronenreichen Ether-verbr{\"u}ckten Verbindung 29 wird der ET nicht alleine durch \&\#61548;, sondern ebenso durch mesomere Effekte bestimmt. Bei 29 kommt es durch Lokalisation der positiven Ladung nahe der Ether Funktion formal zu einer Kettenverk{\"u}rzung um eine „Methyleneinheit", welche schließlich in h{\"o}heren ET Geschwindigkeiten resultiert. Im dritten Teil dieser Dissertation wurde ein Serie „molekularer Dr{\"a}hte" bestehend aus Methoxy- oder Chlorid-substituierten Triarylamin- und Phenothiazinverbindungen mit unterschiedlichen Br{\"u}ckeneinheiten und Br{\"u}ckenl{\"a}ngen zwischen Redoxzentrum und Ankerfunktion dargestellt und im Hinblick auf ihr ET Verhalten untersucht. Durch cyclovoltammetrische und UV/Vis-spektroskopische Untersuchungen konnte gezeigt werden, dass sowohl die Oxidationspotentiale als auch die energetischen Zust{\"a}nde der Chromophore recht gut durch Einf{\"u}hrung unterschiedlicher Redoxzentren und Br{\"u}ckeneinheiten beeinflusst werden k{\"o}nnen. Trotz erfolgreicher Kontrolle der Dichte der Chromophoreinheiten in den gemischten Monolagen konnte nur f{\"u}r die Verbindungen 49, 52 und 87 mit Nitril-substituierten Br{\"u}ckeneinheiten verl{\"a}ssliche ET Geschwindigkeiten erhalten werden. Bei diesen Chromphoren ist ein Absinken der ET Geschwindigkeit bei zunehmender Dichte der redoxaktiven Molek{\"u}le in den gemischten Monolagen zu beobachten, welche auf eine {\"A}nderung der Adsorptionsgeometrie hindeutet. Bei zunehmender Packungsdichte der Chromophore f{\"u}hrt dies zu einer aufrechteren Stellung der redoxaktiven Spezies. F{\"u}r alle anderen Verbindungen konnten keine Werte aufgrund der zu schnellen ET Geschwindigkeiten ermittelt werden. Konformelle, wie auch die sehr geringe Abstandsabh{\"a}ngigkeit des ET, resultieren in hohen ET Geschwindigkeiten oder auch ung{\"u}nstige HOMO-LUMO Energien bez{\"u}glich des Donors, der Br{\"u}cke und der Elektrode sind Gr{\"u}nde f{\"u}r dieses Verhalten. Die Tatsache, dass Verbindung 49 und 52 beinahe die gleichen Geschwindigkeitskonstanten des ETs unabh{\"a}ngig von der Anzahl der Br{\"u}ckeneinheiten (n = 2, n = 3) besitzen, deutet darauf hin, dass ein Hopping-Prozess stattfindet, bei welchem eine geringere L{\"a}ngenabh{\"a}ngigkeit des ETs als bei eine Superexchange-Mechanismus zu erwarten ist.},
  subject      = {Monoschicht},
  language  = {en}
}