Mechanism Study Of The Photodegradation Of Two Typical Antibiotics And The Photoreduction Of CO₂

Photodegradation of antibiotic as an emerging contaminant in water environment and photoreduction of CO2 as a green-house gas are two highly concerned environmental photochemical processes.The prediction of the photodegradation of antibiotics and the contamination control of antibiotics by photochemical method are currently difficult to achieve.High-efficient CO2 photoreduction is not yet implemented.One common problem is the incomprehension of photophysical and photochemical mechanisms.Aimed at a deep understanding of the photophysical and photochemical mechanisms,this dissertation combined theoretical and experimental studies to explore the radiative transitions,non-radiative transitions,and reaction mechanism.Based on these,reaction quantum yield was obtained and rate-limiting step was determined.The following are the main research contents:(1)Through quantum chemistry computation,the photophysical and photochemical processes of sulfapyridine(SPY)were investigated.The results indicate that in the S1 state the rate constant of intersystem crossing(ISC)is lower than that of internal conversion for 5 degrees of magnitude and lower than that of fluorescence emission for 3 degrees of magnitude,which limits SPY to enter the T1 state.The reason is that the spin-orbit coupling constant(Hso)is low.In the triplet state there are two steps of radical reactions.The second step has a high activation energy(20 kcal mol-1)which is the rate limiting step.Based on the rate constants of photophysical and photochemical processes in the photodegradation of SPY,the rate constant of the direct photodegradation is calculated,which is close to the experimentally observed value in the steady state photolysis experiments.(2)Through quantum chemistry computation,the photophysical and photochemical processes of norfloxacin(NOR)were investigated.The results indicate that in the S1 state the rate constant of intersystem crossing(ISC)is lower than that of internal conversion for 7 degrees of magnitude and lower than that of fluorescence emission for 6 degrees of magnitude,which limits NOR to enter the T1 state.The rate constant photochemical process is 7 degrees of magnitude larger than that of ISC,meaning ISC is the rate-limiting step of the entire photophysical and photochemical processes.The low rate constant of ISC results from the low Hso and the bad coupling of vibration between the initial and final state.Ca2+ion can raise the rate constant of ISC by increasing the Hso and enhance the coupling of vibration of the ISC transition.(3)For the CO2 photoreduction,7.5 nm quantum dots of high stability were fabricated using pure sodium copper chlorophyllin.This quantum dot was applied in CO2 photoreduction in water without any sacrificial agent or photosensitizer and CO and O2 were obtained.The catalyst and reagents are treated as a whole-hydrogen bonding complex.The quantum chemistry computational studies of photophysical and photochemical processes based on the hydrogen bonding complexes were conducted.The results indicate that in photophysical process the activation of CO2 to CO2-·by receiving an electron is induced by excited-state hydrogen bonding.In photochemical process,excited-state hydrogen bonding induces the electron and protons transfers to form CO,H2O,and OH·.The carboxyl group and the N atom on the porphyrin framework are the reaction centers.The mechanism is coherent with experiments in reaction order,reaction rate constant,product,and rate-limiting step(ISC process).Through computational method,it was found that Fe3+ can raise the rate constant of ISC by increasing the Hso for 2 degrees of magnitude.The catalytical activation of producing of CO was experimentally promoted by 2 degrees of magnitude after the addition of FeCl3 in the photocatalytic experiments,which was coherent with the calculated result.