Study On The Photochemical Reaction Mechanism Of Organic Conjugated Molecules Containing Polar Covalent Double Bonds

The dissertation is aimed to provide mechanistic understanding on the photochemical reaction of conjugated organic molecules with polar covalent double bond,as well as to provide references to design and synthesize more efficient aggregation-induced emissive compounds and light-driven molecular motors.Time-Dependent Density Functional(TDDFT)and multireference Complete Active Space Self-Consistent Field(CASSCF)method were applied to study the mechanism of fluorescence quenching in methanol solution and fluorescence enhancement in crystal for 4-diethylamino-2-benzylidene malonic acid dimethyl ester(BIM).In addition,the working mechanism of a light-driven BN-stilbene rotary motor,which is constructed by relacing the central C=C bond of stilbene rotary motor with a polar B=N bond,were carried out;by comparing with its parent motor(stilbene),the advantages and drawbacks of BN-stibene motor were found,which will give theoretical support for experimentally design of molecular motor as well as improving their performance.The main contents include the following two parts:1.We report a QM(including TD-DFT and CASSCF)and ONIOM(QM:MIM)study on the fluorescence quenching in methanol solution and fluorescence enhancement in crystal for a styrene derivative,namely 4-diethylamino-2-benzylidene malonic acid dimethyl ester(BIM)that possesses push-pull structure and AIE properties.The results show that in methanol solution the weakening of ethylenic C=C bond after photoexcitation initiates a barrierless relaxation via one-bond rotation around it,until the reactive molecule reaches a low energy intermediate with strong charge-transfer character,then a S1/S0 conical intersection optimized near the charge-transfer intermediate is responsible for the fluorescence quenching in the dilute solution.The existences of charge-transfer intermediate as well as the conical intersection in the vicinity,which has not been observed in other symmetric(or less polar)phenylethylenebased luminophores,are the major features of BIM in solution.While in crystalline phase,the excited-state deactivation channels via torsional motions,either via one-bond rotation or via hula-twist mechanism,are restricted by steric hindrance and electrostatic repulsion from surrounding molecules,and thus fluorescence is enhanced.2.A model compound of light-driven BN-stilbene rotary motor,constructed from a stilbene rotary motor synthesized by Feringa et al as templated and by relacing it central C=C bond with a polar B=N bond,were designed.CASPT2//CASSCF calculations show the purposed BN-stilbene motor has four ground-state helical conformers that are both geometrically and energetically similar to its parent compound stilbene;the analysese of bond length and dipole moment of BN-stilbene confirms the expectation of the polar covalent double-bond character in B=N bond.Based on the revealed working mechanism of the BN-stilbene molecular motor as well as the comparison with stilbene,it is found that BN-stilbene has obvious advantages to be used as molecular rotary motor:the geometry and energy of S1/S0-CI are closer to that of the excited stable intermediate,which in turn improves the the unidirectivity in the rotation process.Meanwhile,there are still some weaknesses,for instance,the possibility of photoisomerization decreases as the strong coupling interaction among the ground state and excited state exists in the start point of trans-stable?cis-unstable photoisomerization.Therefore,further structure modifications are needed to improve its applicability as light-driven molecular rotary motor.