Synthesis And Photophysical Properties Of D-A Molecules Based On Keto-BODIPY Fluorophore

Pure organic molecules with electron donor-acceptor(D-A)configuration have attracted much attention in the field of materials science due to their outstanding intramolecular charge transfer(ICT)characteristics and unique photoelectric properties.With unique advantages over molecular materials based on heavy metal/heavy atoms,it has been widely used in photoelectric devices(such as organic photovoltaic,field effect transistor,organic light emitting diode),biological imaging,and photocatalysis in recent years.In D-A molecular system,the spatially bonding mode between donor and acceptor,as well as for type of both can have a significant effect on its photophysical properties.So,the construction of D-A molecules with a certain structure,as well as exploration of structure-property relationship combined with the theoretical calculation,has profound and important significance for thorough comprehension of the complex photophysical process,and preparation of D-A molecular materials imparting with excellent optical-electrical properties.In this paper,a series of novel D-A molecular systems based on the keto-BODIPY molecule with strong electron-withdrawing ability and rigid structure were synthesized by introducing different electron-donating groups(diarylethene or carbazole moieties).Regulation of ambient light–induced photocyclization pathway(Norrish-Yang vs.6?)and delayed fluorescence behavior were realized in this D-A system.The main research contents of this paper are as follows:1.A series of D-A molecule based on benzothiophenyl/thienyl-keto BODIPY has been prepared.The widely investigated photochromic diarylethenes undergo 6?photocyclization upon irradiation with ultraviolet light,with the reverse process observed upon visible light irradiation.However,to the best of our knowledge,although the Norrish-Yang cyclization of 1,2-diketones was reported to be triggered by light-emitting diode blue light,most Norrish photoreactions occur under ultraviolet mercury lamp irradiation,which can lead to side reactions and photodamage,especially in biological systems.Furthermore,owing to the low molar extinction coefficient of ketones in the UV-visible region,their Norrish-Yang photocyclizations are always inefficient.To increase the efficiency of visible light–driven photoreactions,we herein incorporate two photoresponsive motifs(diarylethene and ketone)into one chromophore as part of a?-conjugated system,thus pushing n??*/???*transitions to the visible range.Thiophene is an electron donor,whereas ketone is an electron acceptor,the spatial separation of the HOMO and LUMO of these dyes promotes triplet state generation and self-photosensitizes intramolecular photocyclization in the visible light region.The results reveal that simple variation of R substituents on the diaryl moiety allows one to control the intramolecular photocyclization mechanism with high photochemical selectivity,e.g.,under ambient light,methyl-substituted 1d and 2d undergo reversible 6?photocyclization,whereas ethyl-substituted 3d and 6d exclusively undergoes irreversible Norrish-Yang photocyclization,indicating a high photochemical selectivity.Three-dimensional time-resolved and steady-state emission spectra of 3d show that the Norrish-Yang photocyclization takes place within milliseconds with excellent conversion efficiency(96%).And the mechanism of Norrish-Yang photocyclization was first elucidated by theoretical calculations.2.The D-A molecules based on carbazole-keto BODIPY have been synthesized.Due to relatively small?E_ST,thermally activated delayed fluorescence,that excited excitons undergo reverse intersystem crossing(RISC)from T₁ to S₁ generating the final emissive singlet excitons,can improve the exciton efficiency in devices.It is a new generation of molecular material with excellent OLED luminescence performance.As keto-BODIPY is a kind of BF dyes with rigid plane structure and strong electron-withdrawing ability,carbazole substituent was introduced into the isoindolinone unit of this kind of dyes,generating a D-A system with carbazole as the electron donor and keto-BODIPY as the acceptor.Absorption spectra of this molecule,show a LE main absorption peaks with fine vibration structure and an intense CT band at longer wavelength.In the ground state,there are strong ICT effects from carbazole to keto-BODIPY segment,and the LE absorption band are mainly derived from the BF₂ dye core.By meso-cyano modification,the emission color can be regulated from yellow to red.In addition,the luminescent properties are dependent on both oxygen and solvent polarity,that is,both oxygen and large polar solvent have quenching effects on luminescence intensity.The existence of delayed fluorescence was further proved by variable-temperature and time-resolved emission spectra.Furthermore,a long afterglow emission at 77 K with lifetime up to 577 ms has also been observed.The structural factors affecting the properties of delayed fluorescence were investigated by X-ray single crystal diffraction and DFT theoretical calculation.3.Iridium(?)complexes based on 2-difluorophenylpyridine unit generally show blue phosphorescence.The luminescence behavior of this system was regulated by introducing different auxiliary ligands.The electronic effect of auxiliary ligands and the influence of space configuration on the luminescent properties were investigated by X-ray single crystal diffraction,spectroscopic property and theoretical calculation.