High-Resolution Optical Studies on C-Phycocyanin via Photochemical Hole Burning

We have shown that both the native C-phycocyanin and its corresponding free biline chromophore undergo reversible, low-temperature photochemistry. We attribute this photochemistry to reversible proton-transfer processes and utilize the observed photoreaction for photochemical hole burning (PHB). Using narrow-band PHB experiments, we have been able to perform high-resolution optical studies and show that the protein-chromophore assembly forms a very rigid structure. The results lead to the conclusion that the light-induced proton transfer occurs most probably in the triplet state

Femtosecond real-time probing of reactions. IX. Hydrogen-atom transfer

The real-time dynamics of hydrogen-atom-transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular ''proton'' transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl-group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen-transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of approximately 7200 cm-1. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O-H-O reaction coordinate. The vibrational periods (2pi/omega) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low-frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational-energy redistribution dynamics at longer times are important to the hydrogen-bond dissociation and to the nonradiative decay of the hydrogen-transferred form

Proceedings of the Thirteenth International Conference on Time-Resolved Vibrational Spectroscopy

The thirteenth meeting in a long-standing series of “Time-Resolved Vibrational Spectroscopy” (TRVS) conferences was held May 19th to 25th at the Kardinal Döpfner Haus in Freising, Germany, organized by the two Munich Universities - Ludwig-Maximilians-Universität and Technische Universität München. This international conference continues the illustrious tradition of the original in 1982, which took place in Lake Placid, NY. The series of meetings was initiated by leading, world-renowned experts in the field of ultrafast laser spectroscopy, and is still guided by its founder, Prof. George Atkinson (University of Arizona and Science and Technology Advisor to the Secretary of State). In its current format, the conference contributes to traditional areas of time resolved vibrational spectroscopies including infrared, Raman and related laser methods. It combines them with the most recent developments to gain new information for research and novel technical applications. The scientific program addressed basic science, applied research and advancing novel commercial applications. The thirteenth conference on Time Resolved Vibrational Spectroscopy promoted science in the areas of physics, chemistry and biology with a strong focus on biochemistry and material science. Vibrational spectra are molecule- and bond-specific. Thus, time-resolved vibrational studies provide detailed structural and kinetic information about primary dynamical processes on the picometer length scale. From this perspective, the goal of achieving a complete understanding of complex chemical and physical processes on the molecular level is well pursued by the recent progress in experimental and theoretical vibrational studies. These proceedings collect research papers presented at the TRVS XIII in Freising, German

Infrared Laser Driven Quantum Dynamics of Double Proton Transfer Reactions and Collective Carbonyl Vibrations

Laser control of ultrafast double proton transfer is investigated for a two-dimensional model for asymmetrically substituted porphycene by means of an infrared pump-dump scheme. Based on the orientation of the transition dipole moments the tautomerization control may lead to an estimated change in the Förster transfer coupling between chromophors of molecular wires of about 60%. The excitation of the degenerate E1 carbonyl stretching vibrations in dimanganese decacarbonyl using circularly polarized laser is shown to trigger wave packet circulation in the subspace of these two modes. A 2D-dissociative PES for CO group is discussed

Spectroelectrochemical analysis of bianthrone monolayers

The redox chemistry of quinones and their analogues can be seen in virtually all living organisms and without them, important biological functions would not be possible. They are an essential component in bioenergetics and occur naturally as components in many biochemical molecules. The redox properties of quinone and its analogues, particularly species that display an observable change between their oxidised and reduced species, has also shown potential in the field of molecular electronics and sensors and also provide accessible models for the study of electron transfer. Much work has been done to develop an understanding of such molecules and further study is vital to expand this understanding and encourage the exploitation of these molecules. In work prior to this thesis, focus has been placed on the electrochemical and photochemical analysis of bianthrone in solution. In brief, bianthrone undergoes a conformational change from a puckered form to a twisted form when undergoing a two-electron transfer in solution. This was described as an ECE mechanism. However, no electrochemical studies had been carried out on bianthrone selfassembled monolayers immobilised on electrode surfaces and no evidence existed to suggest that the conformational behaviour of bianthrone in monolayer form would be comparable to its behaviour in solution. Studies into the formation of self-assembled monolayers of bianthrone on mercury electrodes were carried out. General electrochemical properties suggest that bianthrone undergoes a two-electron, two-proton transfer. Bianthrone voltammetry showed evidence of current spikes that appeared in the voltammetry as the surface coverage approached full surface coverage. These spikes were indicative of intermolecular interactions between molecules on the surface. Studies into the dynamics of bianthrone monolayer formation were carried out by comparing a theoretical model, obtained from a paper by Hubbard et al. that described the influence of diffusion of molecules to the surface and the reorganization of molecules on the electrode surface, with surface coverage/time data obtained by cyclic voltammetry. . The diffusion rate constants ranged from 6.0 x 10'11 for 0.09 \xM to 1.5 x 10'12 for 0.3 j^M. The surface reorganization rate constants, K, ranged from 9.31 x 10'7 for 0.09 jiM to 9.8 x 10'8 for 0.3 jiM. A spectroelectrochemical cell in which electrochemistry could be carried out while being incorporated into a Raman microscope was fabricated to examine bianthrone monolayers adsorbed on mercury electrodes. The potential of the cell was held at different points between where the monolayer was fully oxidised and fully reduced according to the cyclic voltammetry obtained and Raman spectra were recorded at these points. The spectra taken at these potentials were compared and contrasted to elucidate any structural differences that occurred in the reduction of the monolayer. This analysis revealed some subtle changes in intensity in the lower wavenumber region of the spectra. However, there were no changes indicative of a large structural change to the monolayer, As a result of this analysis, it is not believed that bianthrone undergoes conformational changes that would induce major structural changes to the monolayer upon reduction. Studies into the formation of bianthrone monolayers on glassy carbon electrodes were carried out. The voltammetry is consistent with a surface adsorbed molecule undergoing a two-electron, two-proton transfer. Unlike adsorption on mercury, current spikes are not observed Studies into the dynamics of monolayer formation on GC were also carried out using the same theoretical model as published by Hubbard et al Values for the surface reorganization rate constant were obtained. This value ranged from 1 26 xlO 7 ± 0 05 cms 1 for 1 |iM to 4 36 ± 0 02 x 10 8 cms 1 for 20 |iM. A spectroelectrochemical cell similar to the mercury electrochemical cell in which electrochemistry could be carried out but that could still be incorporated into a Raman microscope was fabricated for use with a GC electrode Raman spectra of bianthrone monolayers recorded in the absence of an applied potential compared favourably with the spectra of solid bianthrone and spectra obtained in the literature. The potential of the cell was held constant at different points between where the monolayer was oxidised and reduced according to the voltammetry obtained. The spectra recorded at each potential were compared and contrasted to elucidate any structural differences that may have occurred at these potentials as the monolayer is reduced. Some changes in intensity at certain bands were observed, however, there were no large changes in the spectra associated with large structural changes arising from conformational changes in the molecule. From this, it was concluded that conformational change in the molecule did not occur when it was immobilised on the glassy carbon surface. Electrochemistry was carried out on bianthrone in the solid state, on a glassy carbon electrode Evidence of “breaking in” phenomenon was observed from voltammetric studies of the solid layer

Pyrrolidinyl Group as Charge Donor for the Excited State Intramolecular Chargetransferin 3-(4-methoxyphenyl)-1-(4-(pyrrolidin-1-yl) phenyl) Prop-2-en-1-one

The absorption and steady state emission properties of a chemically synthesized chalcone, 3-(4-methoxyphenyl)-1-(4-(pyrrolidin-1-yl) phenyl) prop-2-en-1-one (MPPP) containing asymmetrical donor and acceptor groups has been investigated both experimentally and theoretically. The ground state, MPPP has a significant intramolecular charge transfer (ICT) character and a great sensitivity to the hydrogen bond donating ability of the medium as reflected from the absorption spectra in pure non polar, polar and neutral solvents. On the other hand, its excited singlet state exhibits high ICT characters as manifested by the drastic solvatochromic effects. These results are consistent with the data. The absorption spectra of the compound MPPP undergoes minor changes with increasing polarity of the solvents and the fluorescence spectra experiences a distinct bathochromic shifts in the both position and fluorescence quantum yields, increases reaching a maximum before decrease with increasing the solvent effects. The quantum yields decrease with increase in the solvent polarity. The magnitude of change in the dipole moment was also calculated using Austin Model 1 (AM1). These results suggest that the evidence about the intramolecular charge transfer in the emitting singlet state of this compound. The solvent dependence of quantum yields of MPPP was interpreted on the basis of positive and negative solvatokinetic as well as hydrogen bonding effects. Intramolecular charge (ICT) transfer took place from pyrrolidine nitrogen to α, β unsaturated carbonyl in the ground state

Energy and Charge Transfer in Organic Materials and Its Spectroscopic Signature: An Ab Initio Approach

Energy and charge transfer processes in organic materials have received a tremendous amount of attention in recent years, due to their impact on functionality within a wide range of applications. One prominent example is the field of organic photovoltaics (OPVs), where significant improvements in power conversion efficiency and durability have been achieved over the last decade. Another example is organic scintillators, which have seen a renewed interest due to the constrained supply of helium–3 gas, as well as their ability to discriminate between types of ionizing radiation. Advancement in the design of organic photovoltaic and luminescent materials can be facilitated by molecular level insights into the processes of energy transfer, gained through both experimental observations and theoretical and computational modeling. Thus, this thesis utilizes computational techniques to investigate excited states, and their spectroscopic signatures, in molecular systems that are experimentally relevant for OPVs and organic scintillators. In Chapter II of this thesis, a computational protocol based on density functional theory (DFT) is presented for calculating the dependence of the vibrational frequency of a carbonyl reporter mode on the electronic state of the molecular system, in the context of charge transfer (CT) in organic molecules. This protocol was utilized to study a system consisting of a phenyl–C61–butyric acid methyl ester electron acceptor with a N,N–dimethylaniline donor, in which small frequency shifts of less than 4 cm−1 were observed between the ground state and the CT excited state. A Stark tuning rate of 0.768 cm−1/(MV/cm) was calculated between the vibrational frequency and the electric field. In Chapter III of this thesis, the CT process in a carotenoid–porphyrin–C60 molecular triad was investigated in its two primary conformations (bent/linear) with an explicit tetrahydrofuran solvent via molecular dynamics. Vibrational frequency distributions were calculated for the amide I mode and found to be sensitive to the three electronic states relevant to CT: the Pi–Pi* excited state, the porphyrin-to-C60 CT state, and the carotenoid-to-C60 charge-separated state, with shifts as large as 40–60 cm−1 observed between the CT1 and CT2 states. Rate constants between these states were calculated with a hierarchy of approximations based on the linearized semiclassical method. The CT process was determined to occur via a two-step mechanism, Pi–Pi* -> CT1 -> CT2, where the second step is mediated by the bent-to-linear conformation change. In Chapter IV of this thesis, the role of intersystem crossing (ISC) from S1 to Tn in the pulse-shape discrimination (PSD) ability of single-crystal trans–stilbene was investigated. Time-dependent DFT was used with the newly developed OT– SRSH–PCM method to calculate the excited states, and an equilibrium Fermi’s golden rule approach was employed to calculate transition rate constants. The ISC rates were found to be too slow to compete with prompt fluorescence, and thus do not significantly impact the PSD ability. Deuteration of trans–stilbene was found to have a retarding effect on the ISC rates, with rate constants reduced by as much as 30%. Finally, in Chapter V of this thesis, a novel compute-to-learn pedagogy is presented, in which students design and develop interactive demonstrations of physical chemistry concepts in a peer-led studio environment. The rationale behind the pedagogy and improvements made over the course of three iterations are discussed, as well as an initial assessment of the pedagogy conducted via end-of-semester interviews.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147569/1/klwill_1.pd