The Theoretical Study On Nonlinear Optical Properties Of Organic Radical Systems Containing Nitrogen, Oxygen, Sulfur Atom

Recently, the organic radical systems have broadly studied in the field of nonlinear optical（NLO）. The design of high-performance organic radical nonlinear optical materials becomesone of key points. Nakano et al. deeply and systematically investigated the NLO of organicradical in theory, especially the third order NLO of diradical systems. The last study finds thatthe supermolecular and cyclic radical systems exhibit excellent NLO respond. This arousesenergy into the development of nonlinear optical materials.The NLO properties of organic radical systems containing nitrogen, oxygen, sulfur atomhave been calculated by DFT. And discussing the relationship between conjugated bridge type,molecular conformation, size, redox switch and NLO respond. The research results reveal:（1） We calculated and investigated the polarizabilities （α_s） and the firsthyperpolarizabilities (β_tot) value and stabilities for a series of TTF⁺·-π conjugatedbridge-verdazyl cation diradicals by using the UPBE1PBE method. The introduction ofconjugated bridges to TTF⁺·-verdazyl cation diradicals results in the conjugation of systemsincreasing, and then theα_sandβ_totvalues increasing (except for theβ_totvalue ofsystem2S). The spin multiplicity and conformation both have influence onα_sand theβ_tot.Compared to the singlet of diradical systems, theα_sdecrease while theβ_totincreaseobviously on triplet. Taking the system2for example, the change of conformation has slightlyinfluence on theα_sandβ_totvalue in singlet, while theα_sandβ_totvalues decrease withincreasing of the dihedral angle θ1and θ2in triplet.（2） The static second hyperpolarizability （γ） of aromatic diimide molecules have beenstudied for the first time at DFT level combined with a finite field. This study shows that thesize of the aromatic core can affect the static second hyperpolarizability （γ）. Increasing thenumber of benzenes along the longitudinal axis can effectively improve the γ values,whereas an increase in the number of benzenes along the perpendicular axis does not enhancethe γ values. Furthermore, we discussed the NLO responses of the reduced form radicalanions1^·-,5^·-and6^·- which were obtained by a reversible redox process. The results showthat the γ values of the radical anions are changed by the redox process. For the reducedform radical anion6^·-, the γ value is-1906.71×10^-36esu, and its absolute value is⁷.3timeslarger than that of its neutral parent. An analysis of the BLA values demonstrates that the γvalue is closely related to the conjugation of the aromatic core used in the redox process.