Ground-state And Excited-state Electronic Structures And Chemical Bonding Studies Of Conjugated Molecules Containing BN Units

In the 1990s,Feringa et al successfully designed polyene light-driven molecular rotation motors.Light-driven stilbene rotary motor has achieved applications in driving nano cars and photopharmacology.However,during the photo-isomerization process of stilbene motor with a C=C rotation axis,the energies of the conical intersection point(Si/So-CI)becomes high and deviates from the rotation path,which impairs the unidirectionality of these motors.Hence,to improve the unidirectional rotation of molecular motors,base on the isoelectric equivalences of BN and CC,Liu et al designed a stilbene-based motor with a BN rotary axis by replacing the center C=C rotating axis with B-N unit.It is found that the excited state process of BN-stilbene motor has a great dependence on theoretical methods,which significantly affect the qualitative evaluation and theoretical prediction of these molecular motors.To give better insights of the theoretical methods for designing BN-stilbene molecular motors,we have benchmarked the performance of different density functionals in calculating the vertical transition energies,emission energies and geometries of conjugated molecules with BN units,and rules for designing their chemical properties.To summarize,the related rules for chemical properties of conjugated compounds with B-N polar covalent bonds,and found optimal functionals;on the top of the benchmarking results,we designed a F10-BN-stilbene motor with better performance.The main objectives of this work include:1.To analyze the vertical excitation energies of 12 BN compounds with different conjugation degrees and the emission energies of 9 BN compounds,and their geometries,using 11 popular functionals(B3LYP,PBEO,mPW1PW91,M06,BMK,BH&HLYP,M06-2X,coB97,coB97X,?B97X-D and CAM-B3LYP)and 7 basic sets(cc-pVDZ,ccpVTZ,6-31G(d,p),6-31++G(d,p),6-311G(d,p),6-311++G(d,p)and 6-311G(2df,2pd)).The results show that the number of the basic functions and the dispersion functions play an important role in calculating the vertical transition energies of BN compounds.The errors of vertical transition energies of the 11 functionals follow the order:6311++G(d,p)<6-31++G(d,p)<cc-pVTZ<6-3 11 G(2df,2pd)<6-3 11 G(d,p)<cc-pVDZ<631G(d,p).Also,12 BN compounds are singlet valence excited,and the errors of globalhybrids(B3LYP,PBEO,mPW1PW91 and M06)with exact-like exchange(20-30%)calculating vertical transition energies are less than global-hybrids(BMK,BH&HLYP and M06-2X)with exact-like exchange(>40%)and range-separated hybrids(coB97,?B97X,coB97X-D and CAM-B3LYP)(in Scheme 3-1,see Chapter 3).For 11 functionals,B3LYP is easier to overestimate the BN bond lengths than others and BH&HLYP is easier to underestimate the BN bond lengths.The whole errors of vertical transition energies,emission energies and geometries give small errors with the functionals:(PBEO,mPW1PW91,M06,BH&HLYP,M06-2X,BMK and coB97XD).M06/6-311++G(d,p)with smaller errors were chosen to calculate some fundamental characters of six BNNaphthalenes(BN-Naphs).Whether BN-Naphs replacing C-C or C=C unit with BN unit,BN bonds have double-bonded character.Maximum absorption wavelength and aromaticity of replacing the C-C unit with BN unit in polycyclic aromatic hydrocarbons are larger than replacing the C=C unit with BN unit and vertical transition energies of the former are more likely to be overestimated than the latter with BN unit under the same chemical environment.Vertical transition energies of polycyclic aromatic hydrocarbons with the chemical environment of-HB=NH-are more likely to be overestimated thanB=N-.2.F10-BN-stilbene rotary motor substituted with fluorene has been designed and its photoisomerization mechanism has been studied in preliminary.TDA-BHHLYP/631G(d)and SF-BHHLYP/6-31G(d)results show that the driving force of the F10-BNstilbene motor is larger than both the stilbene and fluorene motors.The S1-S0 energy gap of the fluorene motor is close to stilbene.The ground state thermal isomerization energies barrier is slightly lower than stilbene and fluorene motors.S1/S0 CI of F10-BN-stilbene motor is close to S1-min,when compared with the performance of the first-generation stilbene and second-generation fluorene motors.Therefore,F10-BN-stilbene motor is easy for nonradiative transition and it may be a potential and excellent molecular motor.