| Nitro is a strong electron-withdrawing group,and the nitro substitution makes the absorption spectrum of the compound redshift.Upon excitation to the lowest excited singlet state(S1),nitro-polycyclic aromatic hydrocarbons(NPAHs)undergo two relaxation pathways.One is the generation of nitrogen oxide and aryloxyl radicals,and the other is the intersystem crossing(ISC)to the lowest excited triplet state(T1).The nitro group significantly increases the spin-orbit coupling,resulting in an increase in ISC efficiency and a high yield for the T1 state of the NPAHs compound.The lifetime of the T1 state is on the order of microseconds,allowing for intermolecular reactions like proton transfer and electron transfer.In this dissertation,the model molecules 8-hydroxy-5-nitroquinoline(NO2-QN-OH),2-nitrofluorene(2NF),2,7-dinitrofluorene(DNF)and 4-hydroxy-4’-nitrobiphenyl(HO-Bp-NO2)were selected to investigate the reactivity of their T1 state in terms of electron transfer,proton transfer and hydrogen abstraction reactions in organic solvents,as well as the effect of solvent properties on these reactivities.The transient absorption spectroscopy and time-resolved resonance Raman spectroscopy were used to acquire characteristic spectra and kinetic information of the short-lived intermediates.With the help of theoretical calculations to analyze the structure of the molecules,the corresponding reaction mechanisms were proposed.The following are the specific contents and outcomes of the investigation:1)NO2-QN-OH is widely used as an antimicrobial,anti-inflammatory and anticancer agent.When NO2-QN-OH was photo-excited,the ISC occurred in 0.8 ps to form the Tn state,followed by the internal conversion in 8.5 ps to produce the lowest excited triplet state 3(NO2-QN-OH).In acetonitrile solution,3(NO2-QN-OH)undergoes an intramolecular proton transfer,resulting in the formation of the tautomeric 3(NO2-QNH-O).To our knowledge,this was the first time that the intramolecular excited state proton transfer of 8-hydroxyquinoline-like molecules was observed in a polar aprotic solvent.In acetonitrile solutions with traces of water,3(NO2-QN-OH)dissociated to form the triplet state anion 3(NO2-QN-O),which can be quenched by oxygen to form NO2-QN-O-.In addition,the hydrogen-bonded complex formed by NO2-QN-OH and water molecules in the ground state was photo-excited,dissociated in the S1 state,and then produced NO2-QN-O-via a non-radiative process.According to above findings,drugs should be protected from light during their delivery in order to avoid side effects caused by photosensitivity reactions.And before reaching their effective position,drugs should avoid contact with water to prevent the generation of the anion NO2-QN-O-,which reduce antibacterial activity.2)2NF and DNF are widespread,chemically stable,and poorly biodegradable pollutants in the atmosphere.However,the T1 state has a microsecond lifetime and the photo-induced hydrogen addition reduction is its primary reaction pathway.In nonpolar,polar aprotic,and protic solvents,the T1 state of 2NF and DNF(32NF,3DNF)is capable of undergoing electron transfer reactions with electron donors.The Rehm-Weller equation was used to fit the relationship between the rate constants of electron transfer reactions(logk)and the oxidation potential of the electron donor(Eox(D)).The results show that the reduction potentials of 32NF and 3DNF were the same,at 1.54～1.58 V,and were unaffected by the solvent properties,whereas their recombination energies in different solvents were 7.02～17.29 and 9.08～11.35 kJ mol-1 respectively,and were significantly influenced by the solvent properties.It is noteworthy that the second-order reaction rate constants of 32NF and 3DNF with 1,4-cyclohexadiene in different solvents were 107～108 M-1s-1,indicating that their electronic configurations contain some(n,π*)character.The Kamlet-Taft expression was used to describe the relationship between absorption area ratio,the second-order rate constant of the hydrogen abstraction reaction,the recombination energy and solvent properties,respectively.The results demonstrate that the hydrogen-donating ability of the solvents plays an important role in the aforementioned properties,indicating that the nitro group in 32NF and 3DNF molecules has a strong ability to accept hydrogen bonds.Due to the difference in molecular symmetry between 32NF and 3DNF,the effect of the solvent’s hydrogen-bond donating ability on the absorption maximum is significantly different for the two triplets.The above findings imply that the presence of water and organic substances with a strong ability to donate hydrogen such as alcohols and phenols in the atmosphere,can increase the reactivity and conversion efficiency of 32NF and 3DNF.3)Since nitro is a strong electron withdrawing group,the acidity of 3(HO-Bp-NO2)is much greater than that of the ground state.In addition,the long lifetime of the triplet state allows 3(HO-Bp-NO2)to undergo proton transfer with weak base.In dichloromethane,3(HO-Bp-NO2)with TBA formed the excited hydrogen-bonded complex 3(TBA:…HO-Bp-NO2),which was followed by a rapid proton transfer to form 3(TBAH+…-O-Bp-NO2)and then the hydrogen bond was broken to form 3(-O-Bp-NO2).The characteristic absorption of the hydrogen-bonded complex 3(TBAH+…-O-Bp-NO2)was directly observed in dichloromethane solution but not in polar solvents.This result reveals that the polarity of the solvent influences the stability of hydrogen-bonded complexes during excited-state proton transfer.The identification of hydrogen-bond complexes in weakly polar solvents provides an important kinetic reference and spectral evidence for the subsequent study of the excited state proton transfer mechanism of aromatic hydroxyl compounds. |
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