Theoretical Study Of Proton Transfer Strategy Regulating Nonlinear Optical Properties Of D-π-A Framework

Since the advent of laser,nonlinear optics（NLO）has attracted the attention of many scientists because of its extensive application in various fields,in which the design and development of high-performance NLO materials is of great importance.In recent years,more and more materials chemists have focused on the design and study of organic NLO materials with donor-π-acceptor（D-π-A）configuration.Some researchers have reported that intramolecular proton transfer can effectively regulate the conjugation degree of organicπ-conjugated system to affect the electronic structure and photoelectric properties of the system.Therefore,the D（-NH₂）-π-A（-NO₂）geometric configuration is selected in this paper,and dihydrotetraazatetracenes（DHTTs）,4,4’-（hydrazine-1,2-diylidene bis（methanylylidene））bis（3-hydroxybenzoic acid）（HDBB）and phthalocyanine are selected asπ-conjugation bridges respectively.The effect of intramolecular proton transfer on geometric structure and NLO properties is investigated by Density Functional Theory（DFT）.Specific works are as follows:1.Based on the structure of DHTTs,we design four molecules 1-4 according to the location of the proton.The results show that the first hyperpolarizability(β_tot)of the system is affected by the C-N bond length,the aromaticity of the pyrazine rings and the charge transfer.Theβ_totvalues increase gradually from 1 to 4.The first hyperpolarizability density diagram,theβvalues under the condition of sum-over-state and the transition orbit diagram of the main contribution of molecular crucial states verify the rationality of the variation ofβ_totvalues.Interestingly,theβ_tot value of 4 is the sum of that of 2 and 3,which has a certain correlation with the number of proton transfer.Further analysis shows that proton transfer can adjust the conjugation degree ofπ-conjugation bridge,resulting in the corresponding change of the ground state to excited state dipole moment difference（Δμ）,which is the main reason for the change of NLO property of the system.2.Similar to the above,this work design three molecules a-c by replacing two carboxyl groups in HDBB with amino and nitro groups.Comparing their structures,we discover that protons can transfer between O and N,which will also lead to changes in their N-N bond length and charge distribution.It is worth noting that molecule b has the lowest energy,the highestβ_totvalue and b can be obtained from both a and c by proton transfer.Molecule a has a lower energy barrier to b,so it’s easier for a to get to b than c.Electron space range<R²>and unit spherical representation（USR）confirm the trend ofβ_totvalues.According to the two-level formula,it is found that the change of oscillator strength f₀,Δμand transition energyΔE together lead to the change ofβ_tot values.3.The above two works focus on proton transfer on conjugation rings.What about the effect of proton transfer in the conjugation rings on the NLO property of D-π-A system?In this work,we design molecules 1-4 based on phthalocyanine and its proton transfer characteristics.The results show that the bond length alternation（BLA）values,aromaticity and electron distribution have important effects on theβ_tot values under static condition.From 1 to 4,theβ_totvalues increase gradually.Importantly,theβ_tot values of molecule 4 is about 16 times of that of1.The USR diagram proves the correctness of the variation trend ofβ_tot values.The increase ofβ_tot values is mainly caused by the decrease ofΔE.Further studies show that the intracyclic proton transfer also plays an important role in regulating the dynamic first hyperpolarizability of the system.In this paper,a series of organicπ-conjugation bridge models are designed,and the effects of proton transfer on the structure and NLO property of D-π-A system are studied respectively.We hope that this paper can provide theoretical basis for the design and development of high-performance organic NLO materials,and open up new ideas for their experimental synthesis.