Kinetics And Microscopic Mechanism Of Hydrogen Transfer Reactions In The Lowest Triplet Of Nitronaphthalene Derivatives

The NO₂ group directly bonded to nitronaphthalene derivatives（NNDs）can increase drastically spin orbital coupling so that the lowest excited triplet（T₁）can be generated in a great yield and has long lifetime.Reduction by addition of hydrogen to the T₁states is the major degradation（photolysis）pathway of NNDs.To investigate the effect of intramolecular charge transfer（ICT）character and heteroatoms on the properties and hydrogen transfer reaction activity of NNDs in their T₁ state,in this thesis 4-hydroxy-4’-nitrobiphenyl（HO-Bp-NO₂）,5-nitroquinoline（5NQ）,8-nitroquinoline（8NQ）and 5-bromo-8-nitroisoquinoline（5Br8NIQ）were selected as the research objects.Nanosecond transient absorption spectroscopy was used to acquire characteristic spectra and kinetic information on reaction intermediates.Under the help of time-dependent density functional theory（TD-DFT）the structure of intermediates were identified.We aim to explore the properties and reactivities of NNDs in their T₁ states and reveal the microscopic mechanism of their hydrogen transfer reaction from different hydrogen donors.The main research results are as follows:（1）In the ground state（S₀）,p K_a（HO-Bp-NO₂）=10.7,and the nitro group is hard to be protonated.In the ICT T₁ state,the proton bonded to nitro O has an acidity constant value of p K_a^*=3.1,and the hydroxyl of ³HO-Bp-NO₂H⁺has an acid dissociation constant p K_a^*=2.7.In polar aprotic solvents,a new absorption band at 450 nm has been observed,and it is identified as open shell singlet O-Bp-NO₂H which is the tautomer of HO-Bp-NO₂ and generated through protonation and deprotonation of T₁.The T₁ and S₀ states also can undergo electron coupled proton transfer to generate nitro hydrogen abstracting radical HO-Bp-NO₂H·.Kinetic analysis and theoretical simulations in protic solvents（alcohols）reveal the cluster model of the proton transfer between the T₁ state and alcohol molecules.（2）The relaxation channel of 8NQ in the excited state is affected by the position of N atoms relative to the nitro group,so 8NQ has a low T₁ state yield.In contrast,the T₁ state of protonated8NQ（8NQH⁺）has a lifetime about 80 ns,and it has a similar transient absorption spectrum as that of 1-nitronaphthalene.The hydrogen abstraction reaction with alcohols,chloroform,tetrahydrofuran and other hydrogen donors can generate cationic radicals with the second-order rate reaction constants of 10⁶-10⁷ M^-1s^-1,faster than that of 1-nitronaphthalene.The reaction rate constant of ³8NQH⁺with the hydrogen donor correlates with the ionization potential of the hydrogen donor in a Rehm-Weller-like relationship,and it reveals that electron transfer is the rate-determining step in the process..（3）The transient spectra of T₁ state for 5NQ and 5Br8NIQ are similar to that of1-nitronaphthalene,and the T₁ undergo a one-step hydrogen abstraction reaction with alcohols and electron coupled proton transfer reaction with phenols.There is a small energy gap between T₁（π,π^*）and T₂（n,π^*）for 5NQ,resulting in the rate of one-step hydrogen abstraction reaction about 10⁵ M^-1s^-1,faster than 1-nitronaphthalene.After the T₁ of 5Br8NIQ abstracting hydrogen atom from alcohols and phenols to produce 5Br8NIQH·,the C-Br bond of 5Br8NIQH·can dissociate to generate cationic radical⁺8NIQH·.All these results show that the relative position of the heteroatom N in the ring and nitro group in molecules of nitroquinoline/isoquinoline can change electronic configurations of these T₁ states and affect their excited state attenuation channel.As a result,The T₁ states of nitroquinolines/isoquinolines show different spectral information and reaction mechanism.