| The goal of this thesis is to explore the photochemical reaction mechanism of two types of photochromic system,i.e.,diarylethene and spiropyran,aiming to provide theoretical reference for experimental design,synthesis and application of photochromic compounds and molecular switches.In the first part,we carried out density function theory(DFT),time-dependent DFT(TD-DFT),multireference complete active space self-consistent field(CASSCF)and second-order perturbation theory(CASPT2)to study the photo-induced ring closure of three diarylethenes compounds as well as the subsequent rearrangements.In the second part,we unveil the photochemical reaction mechanism of spiropyran by using the CASSCF and CASPT2 methods.The results are expected to shed light on the deep understanding of photochemical reaction and to provide theoretical bases for the design of novel molecular switches.The main contents include the following two parts:1.We carried out DFT/TD-DET,CASSCF and CASPT2 to study the reaction mechanism of photo-induced ring closure and rearrangement of three photochromic diarylethene compounds,namely,2-phenylcyclopent-3-[3-2,5-dimethylthiophen]-2-enone,2-phenylcyclopent-3-[2-3,5-dimethylthiophen]-2-enone,3-phenylcyclopent-2-[3-2,5-dimethylthiophen]-2-enone(hereafter named as compounds a,b and c,respectively).The influences of different positions of carbonyl group and S atom are emphasized.The results show that when the S atom is located on the outside of the molecule,the energy of the product after the ring closure is～41.9 kJ/mol higher than that with an inner S atom,thus suggesting that the photocyclization process may be more difficult than the other two.After analyzing the excited-state potential energy curves(PECs)of the three compounds,it is found that when the carbonyl group is on the same side(near the benzene ring),photocyclization of compound b with an outer-S is more difficult to occur than a with an inner-S.When S atoms are on the same side,the PES compound c with carbonyl group near the thiophene side is less slopped than with a carbonyl group near the benzene ring.Than is,the order of the ring tendency of the photo-induced reaction is a>c>b.By comparing the barriers of the rate-determining steps in the rearrangement processes,it can be found that the tendency of the rearrangement processes is consistent with that of the photocyclization reactions.2.The photochromic reaction from spiropyran(SP)to(MC)has received more and more attention in recent years.Taking the spiropyran(1’-methylspiro-6-nitro-[2H-1-benzopyran-2,2’-indoline],named as NO2-BIPS)compounds as the object of study,we studied the photochemical reaction mechanism of two different initial structures of the molecule by adopting the CASPT2/CASSCF methodologies.In this thesis,we mainly discuss the introduction of nitro group and using-CH3 to replace the hydrogen atoms on N with(1’-methylspiro-[2H-1-benzopyran-2,2’-indoline,also known as BIPS).The results show that the strong electron-withdrawing groups have a significant effect on the electronic structure and reaction of spiropyran.At the distance between the carbon and oxygen atoms of 2.3～2.9 A,the ground-state(S0)and excited-state(Si)PESs calculated at the CASSCF level become relatively approximate,while after the CASPT2 energy correction,the gap between S0 and S1 is relatively large,which results in an inefficient non-radioactive transition process.Afterwards,it was discovered that the hydrogen-out-of-plane(HOOP)mode of hydrogen on the bridge carbon atom may lead to non-adiabatic relaxation of a strong interaction region between S1 and S0 states.Compared with BIPS,the energy gap(～100 kJ/mol)of NO2-BIPS in the HOOP region is greater than that of BIPS(～80 kJ/mol),therefore the coupling between Si and S0 is weaker than that of BIPS,which cannot afford an efficient internal conversion,instead,it is advantageous for the ring-opening reaction to continue to the product side.The finds here successfully rationalize the high SP to MC quantum yield of NO2-BIPS. |
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