The Second-order Nonlinear Optical Property Of The Transition Metal Ru(â…¡/â…¢) Complexes With Terpyridyl

Organometallic complexes have become a hotspot in the field of nonlinear optical （NLO）molecular materials due to the diversities of structure and property. Recently, such complexesare more applied to the study of switchable NLO materials. For an effective switching, themolecule must be stable in two （or more） states that offer very different NLO effects. Theswitchable NLO behaviors of molecules could be obtained by oxidation/reduction,protonation/deprotonation, photoisomerization, phototautomerization and another chemicalmodification. As for the organometallic complexes, the preferred scheme is based on theoxidation/reduction of central metal or organic ligand and the protonation/deprotonation ofligand.Ru（Ⅱ/Ⅲ） complexes with terpyridine （tpy） are promising to become the candidates withwonderful NLO properties. Because such complexes exhibit not only the excellentelectrochemical and optical properties but also the high binding affinity of tpy with transitionmetals and the facile functionalization of the4′-position of tpy. The NLO properties ofRu（Ⅱ/Ⅲ） complexes with tpy have been studied by using quantum chemical calculations, andthe relationship between molecular structure and NLO properties have been discussed. Theobtained results are as follows:（1） The effects of substituents and deprotonation on the second-order NLO properties ofterpyridyl Ru（Ⅱ） complexes with benzimidazole have been discussed according to thecalculations at the level DFT CAM-B3LYP/6-31G*（LANL2DZ basic set on Ru atom）.Results indicate that the introduction of electron-withdrawing and electron-donatingsubstituents enhances the first hyperpolarizabilities of the studied systems in various degrees:1852a.u.（X=H）<2841a.u.（X=PhNO2）<3889a.u.（X=PhCHO）<6588a.u.（X=PhCH3）<21549a.u.（X=PhNMe2）. According to the TD-DFT calculated results, the smaller increase inβvecof systems with electron-withdrawing group could be attributed to the additionalmetal-to-ligand charge transfer （MLCT） transition, which is vectorially opposite to thedominant intraligand charge transfer （ILCT） transition. The deprotonation of benzimidazoleleads to an efficient second-order NLO switching. For systems with electron-donating groups,the deprotonation not only reduces the vector magnitude of βvecbut also changes the vectordirection. The βvecvalue of the mono-deprotonated system concluding NMe2is-432a.u., andthe magnitude is about50and11times as small as that of its diprotonated （21549a.u.） andfully deprotonated （-4809a.u.） ones.（2） The relationship between redox states and second-order NLO effects for donorsubstituted terpyridyl Ru（Ⅱ） complexes in different protonatation has been discussedaccording to the calculations at the level DFT CAM-B3LYP/6-31G*（LANL2DZ basic set onRu atom）, where donor substituents point to H, PhNMe2and monopyrrolo-TTF. Resultsindicate that the βvecvalue can be efficiently tuned in oxidized process and theredox-switchable NLO effects will come true. For systems with diprotonated form, the systemwith stronger donor substituent monopyrrolo-TTF is more suitable for redox-switchable NLO molecular materials due to the oxidization of substituent itself: the βvecvalue ofone-electron-oxidized species decreases to17×10^-30esu while the βvecvalue oftwo-electron-oxidized species in singlet spin state increases to3068×10^-30esu, which aregreatly different from580×10^-30esu of their reduced parent. For systems withmono-deprotonated form, however, the oxidization of monopyrrolo-TTF substituent itself nolonger bring about effective redox switching. Instead, the oxidization of the deprotonatedbenzimidazole anion could promote the charge transfer transition from x-axis, enhance the βxcomponent, and then increase the βvecof systems with H and PhNMe2. The alteration ofcharge transfer from x-axis more significantly affects the βvecof systems with fullydeprotonated form, and finally, the βvecvalue of the oxidized system with H increases to itsparent’s13.2times. TD-DFT calculations indicate that the first excited state displays anindispensable role for charge transfer transition from x-axis. The above conclusions will offerimportant basis for designing effective redox-switchable NLO molecular materials.