The Calculated Investigation On Nonlinear Optical Switchable Effects Of Organic Open-shell Systems Containing Nitronyl-nitroxide Radical

In recent years, great improvement has been obtained in the synthesis of organic radicalsystems and their NLO properties. Nakano. M et al. made a lot of investigations in this aspect,and reported the influence of spin multiplicities and diradical character and so forth on theNLO properties for organic radical systems. The results of these investigations show that, theNLO responses of the organic radical systems are excellent, and the singlet diradicalmolecules with larger and middle diradical character have good NLO properties. This workemploying the quantum chemistry calculation method with the FF method calculate the NLOproperties of NN radical systems and the tuning of the NLO responses and electronicstructures by redox reactions and photoisomerization effects. The results show that:(1) The DFT calculations combined with analytical third derivatives methods werecarried out on PN to calculate the NLO properties by changing substituents. The resultsindicate that the first hyperpolarizability values of PN are sensitive to the substituents, andsystem4a containing two para-hydroxy phenyls branches with a2D structure possesses thelargestβ totvalue (83.134×10-30esu) of all. According to the TD-PBE1PBE calculations, thefirst hyperpolarizability values are largely related to the transition energies, and low transitionenergy can enhance the NLO responses. Moreover, the redox-switchable second-order NLOresponses of the PN have also been studied. The results show that the NLO responses of PNare enlarged by one-electron oxidation. For instance, theβ totvalue of triplet oxidized state2b’ is20.75times larger than that of its neutral states2a due to its con-planar structure, forwhich there is an approximation of two-states swithching. Through the analysis of FMOs, itcan be obtained that oxidization significantly affects the CT pattern and reduces theHOMO-LUMO gap.(2) The NLO properties and photoisomerization effects on the NLO responses ofΛ-shaped DAE derivartves1a(b)-4a(b) are calculated using DFT method. The results showsthere is a good consistency between the optimized values of3a and the crystal datas of itsderivartves3a’. The changes of structures for open-shell molecules3a(b) and4a(b) incluencethe spin interaction, and it is the strongest for4b. There is a great difference between thegeometries of open-ring and closed-ring molecules, and the open-ring molecules are morestable. The analysis of NBO shows that, compared with open-ring forms, the differencebetween the electron-deficient and electron-rich centers of closed-ring molecules is larger, andits tendency of the charge transfer is improved. It is obtained from the FMO analysis that, thechanges of the conjugated bridges has little influences on the distribution of the charge.During the open-ring molecules, the overlapping of the FMO of1a(2a) can be seen, which isgood for the charge transfer, and itsβ totvalue is larger. While3a(4a) has no overlapping,and itsβ totvalue is smaller. Among the closed-ring molecules, the BLA value for2b is thelargest and its aromaticity is bad, then the first hyperpolarizability is the smallest (139.62a.u).The better the NLO responses for the open-ring molecules are, the stronger the switchable effects for them. Theβ totvalues for1b-3b are reduced significantly and about16times assmall as their open-ring forms. Theβ totvalue for4b is4times larger than4a. All themolecules can be NLO switching models. TDDFT calculation elucidate that, the changes oftheβ totvalues between open-ring and closed-ring molecules comfort to the two-level model,the smaller the excitation energy, the larger theβ totvalue.