TY - BOOK
T1 - Breaking the Cycle
T2 - Ring-opening Dynamics of Cyclic Molecules in the Excited State
AU - Skov, Anders Bo
PY - 2019
Y1 - 2019
N2 - In nature, photoinduced non-adiabatic processes in cyclic molecules sustain and protect life, and understanding how to control photophysics and photochemistry on the femtoto- picosecond timescale will greatly benet the development of solar energy harvesting, photodynamic therapy and sustainable chemistry, to name a few. In this thesis, nonadiabatic processes occuring in cyclic molecules is the unifying topic, and specically the concept of ring-openings is investigated from dierent perspectives. The work is carried out using time-resolved spectroscopy, supported by quantum chemical calculations. In the rst part, control over the ring-opening and -closing reactions of pericyclic reactions is sought. First, the dihydroazulene photoswitch is studied as a candidate for solar energy harvesting. It is found that when ring-opening is coupled with loss of aromaticity, improved energy storage is achieved at the cost of reduced ring-opening eciency. Next, on the ground state, two pathways for closing to dihydroazulene are revealed, with solvent and structure providing the handles by which stereochemistry and ring-closure rate are controlled, aording control of energy-storage and release in the photoswitch. Then, the ring-opening dynamics of a protecting group for reactive dienes, sulfolene, used in the production of several important pharmaceuticals, is studied. It is found that ring-opening occurs eciently through a barrierless pathway along the potential energy surface, allowing for a new, sustainable, way of deprotection. Schematic overview of the types of reactions studied in the thesis.In the second part, control over the intersystem crossing rate is pursued. Here, thiophene and its dimer, bithiophene, are studied as model systems for bulk heterojunction solar cell components. It is found that thiophene undergoes ultrafast ring-opening from the singlet state, while bithiophene instead undergoes intersystem crossing on the same timescale, with the rate depending on the conformation. By tuning the conformation we may thus control the intersystem crossing rate, and, in theory, the eciency of ringopening. In the nal part, ring-opening by localization of dynamics is studied. First in a cyclic disulde, a model system for proteins, it is found that excitation using 200 nm light excites a manifold of Rydberg states, with the dynamics localized mainly in the disulde bond. The disulde bond is broken in less than 100 fs after excitation, with no reformation visible. Next, the preservation of coherence after internal conversion from the S2 to the S1 state in two cyclic amines is investigated, revealing preservation of localized dynamics only in the more symmetric species. The coherence likely originates from the non-destructive amine planarization mode, and so no ring-opening dynamics are observed in this case.
AB - In nature, photoinduced non-adiabatic processes in cyclic molecules sustain and protect life, and understanding how to control photophysics and photochemistry on the femtoto- picosecond timescale will greatly benet the development of solar energy harvesting, photodynamic therapy and sustainable chemistry, to name a few. In this thesis, nonadiabatic processes occuring in cyclic molecules is the unifying topic, and specically the concept of ring-openings is investigated from dierent perspectives. The work is carried out using time-resolved spectroscopy, supported by quantum chemical calculations. In the rst part, control over the ring-opening and -closing reactions of pericyclic reactions is sought. First, the dihydroazulene photoswitch is studied as a candidate for solar energy harvesting. It is found that when ring-opening is coupled with loss of aromaticity, improved energy storage is achieved at the cost of reduced ring-opening eciency. Next, on the ground state, two pathways for closing to dihydroazulene are revealed, with solvent and structure providing the handles by which stereochemistry and ring-closure rate are controlled, aording control of energy-storage and release in the photoswitch. Then, the ring-opening dynamics of a protecting group for reactive dienes, sulfolene, used in the production of several important pharmaceuticals, is studied. It is found that ring-opening occurs eciently through a barrierless pathway along the potential energy surface, allowing for a new, sustainable, way of deprotection. Schematic overview of the types of reactions studied in the thesis.In the second part, control over the intersystem crossing rate is pursued. Here, thiophene and its dimer, bithiophene, are studied as model systems for bulk heterojunction solar cell components. It is found that thiophene undergoes ultrafast ring-opening from the singlet state, while bithiophene instead undergoes intersystem crossing on the same timescale, with the rate depending on the conformation. By tuning the conformation we may thus control the intersystem crossing rate, and, in theory, the eciency of ringopening. In the nal part, ring-opening by localization of dynamics is studied. First in a cyclic disulde, a model system for proteins, it is found that excitation using 200 nm light excites a manifold of Rydberg states, with the dynamics localized mainly in the disulde bond. The disulde bond is broken in less than 100 fs after excitation, with no reformation visible. Next, the preservation of coherence after internal conversion from the S2 to the S1 state in two cyclic amines is investigated, revealing preservation of localized dynamics only in the more symmetric species. The coherence likely originates from the non-destructive amine planarization mode, and so no ring-opening dynamics are observed in this case.
UR - https://soeg.kb.dk/permalink/45KBDK_KGL/1ed7rpq/alma99123207869505763
M3 - Ph.D. thesis
BT - Breaking the Cycle
PB - Department of Chemistry, Faculty of Science, University of Copenhagen
ER -