Influence Of Donor-Acceptor Torsion Angle On Photochromic Properties Of Visible Light Driven Turn-on Fluorescence Dithienylethenes

A Photochromism refers to the reversible color conversion of molecules that can occur under different light conditions.Among organic photochromic compounds,diarylethenes(DAEs)derivatives have received great attention from researchers for their excellent characteristics such as good thermal stability,fatigue resistance and fast response.The conversion between the two optical isomers of conventional DAEs requires at least one direction triggered by UV light,however,high-energy UV light has disadvantages such as low penetration,non-selective absorption and high damage to cellular tissues,thus all-visible-light-driven DAEs are more attractive for practical applications.In this dissertation,based on the UV-driven "Turn-on" type fluorescent diarylethenes BTTO4 and its derivatives,a larger group of donor fragment is introduced to extend the π-conjugation system of the molecule to realize the all-visiblelight-driven "Turn-on" type fluorescent DAEs.The effect of the torsion angle on the molecular properties of fluorescent DAEs has systematically investigated.The main parts of of this dissertation are as follows:(1)Theoretical studies: A theoretical study of the properties of the ground and excited states of the designed molecules by DFT and TD-DFT methods has been carried out.It is found that the HOMO-LUMO energy gap of the ring-opening isomers of the designed molecules is significantly reduced,which is favorable for cyclization reactions by visible light.(2)Studies on D-A type DAEs.Dibenzothiophene and benzonaphthothiophene groups with large size and have strong electron-giving ability,are directly connected with BTTO4 to form D-A type DAEs(A1-A4)with visible-light-driven "turn-on" fluorescence,and all four molecules show excellent stability.Theoretical calculations gave torsion angles are 47°,26°,56° and 27° for molecules A1-A4,respectively.The variation of torsion angles and the effect on their photophysical properties were investigated for the molecules A1,A2 and A4,where the same dibenzothiophene donor group is attached at different sites and for molecules A2 and A3 where different donor groups attached at the same sites.The results showed that the torsion angle is an important influence factor for regulating the energy bandgap of DAEs.On the other hand,A3 was found to have the best photostability,but a very low fluorescence quantum yield.(3)Studies on D-π-A type DAEs.The D-π-A structure was introduced to improve the fluorescence quantum yield of the molecule,and the large 9,9-dimethylacridine and carbazole groups were linked to BTTO4 through the benzene ring as a bridge,forming the D-π-A type DAEs(B1-B2)with visible light-driven "Turn-on" fluorescence.Theoretical calculations showed that both B1 and B2 have very large torsion angles of90° and 52°,respectively.The results showed increased fluorescence quantum yields of 0.11 and 0.19 for B1 and B2 respectively,while keep excellent photostability,illustrating that the D-π-A structure with larger torsion angles has higher fluorescence quantum yield.