Proton-Coupled Electron Transfer for Long-Lived Charge Separation and Photocatalytic Water Splitting

The current dissertation covers a fundamental chemical process of proton coupled electron transfer (PCET) which involves simultaneous transfer of both proton and electron in the chemical system. Ultrafast spectroscopy is a valuable tool that provides insight into the process of PCET. Femtosecond and nanosecond transient absorption (TA) experiments, as well as time-resolved infrared (TRIR) spectroscopy, are widely used to monitor electron and proton transfer processes as they allow one to investigate of both the electron and the proton transfer processes. To perform high quality experiments on the subpicosecond/subnanosecond timescale we designed new data acquisition software. The variety of controls allows the user to monitor different acquisition parameters. The graphs display the transient spectrum at current delay position, decay at the certain wavelength, and overall dynamics of the processes which occurs after excitation by colored map. Several subprograms were designed to tune the white light continuum and overlap between the pump and the probe beam. Using this software and equipment we investigated photophysical properties of 4-substituted naphthalimides (NI). The compounds chosen for the study can be split into two groups: NI and chloronaphthalimide (ClNI) which have high oxidation potentials, weak fluorescence and upon excitation form nonpolar ππ* excited state, and other three NI which forms long-lived ICT states and have lower oxidation potentials and higher fluorescence quantum yields. After detailed investigation only 4-methylthionaphthalimide (MeSNI) was chosen as a model compound for PCET study. The appropriate pyridine (NO2CNpy) was chosen since it can be considered as strong acceptor and can undergo reversible 1e- reduction with formation of the strong absorbing radical-anion. Due to weak basic properties it does not form the salt with NI derivatives; however it still can undergo formation of hydrogen-bonded complex. Using NMR titration the binding constant was calculated. Since it is very small the 100% complexation can never be achieved which makes analysis of the excited state dynamics complicated. The visible pump-probe experiment with MeSNI, MeSNIMe, NO2CNpy, and py combined with spectroelectrochemical studies confirmed the photoinduced electron transfer and proton coupled electron transfer for MeSNI. To investigate flavionum salt derivatives that serve as compound for photocatalytic water splitting we synthesized N(5)-ethyl flavonium perchlorate, N(5)-ethyl flavonium pseudobase, N(5)-isopropyl flavinium perchlorate, N(5)-isopropyl flavonium pseudobase and their methoxy derivatives. We used two synthetic approaches publised previously. The first which involves reduction with lithium aluminum hydride reduces number of steps and provides better yields while the second one which based on monoalkylation of the aniline derivatives allows introduction of variety of substituents. MeLF and EtFl+ from the steady-state absorption and emission spectroscopy experiments demonstrate the similar features since they have similar electronic structures according to the calculations. The long wavelength absorption for the N(5)-ethyl flavonium perchlorate is explained by stabilization of LUMO level. This is more pronounced in case of isopropyl group. From transient spectroscopy experiments EtFl+ demonstrates a shorter excited state lifetime compared to that of MeLF, due to faster thermal deactivation in EtFl+ dictated by the energy gap law. The hydroxy and methoxy forms have different photophysical properties compared to the salt and methyllumiflavin since the conjugation is decreased. This leads to the hypsochromic shift of absorption and emission maxima, decreased lifetime, and low fluorescence quantum yield. From visible pump-probe experiment no significant differenc..