Design,Synthesis,Photophysical And Photochemical Properties Of Donor-Acceptor Imidazole Derivatives

Luminescent materials are closely related to people's production and life.Imidazole is a classical five-element aromatic heterocyclic ring containing nitrogen.Its luminescent derivatives exhibit some excellent photophysical properties,such as deep blue emission,narrow spectral characteristics,high quantum efficiency,and efficient proton transfer luminescence.So it has been widely used in organic optoelectronic fields such as electroluminescence,optopump laser,and chemical sensing,et al.In addition,imidazole structure also has unique photochemical properties,which can produce photooxidation reaction under certain conditions,so it has potential application prospects in photodynamic therapy,oxygen monitoring and other biomedical fields.The donor-acceptor(D-A)structure molecule would bring the change of photophysical and photochemical properties,such as delayed fluorescence emission,accelerated intersystem channeling,facilitated electron transfer and energy transfer,and sensitizated ground state oxygen,has recently become a research hotspot.Therefore,the design and synthesis of novel donor-acceptor structure imidazoles and studied their photophysical and photochemical properties are conducive to the discovery of luminescent materials with excellent photoelectric properties,which is of great research value.(1)First,a series of imidazole derivatives were designed and synthesized by using phenylimidazole(PIM)as the receptor and aromatic amine groups with different electron donating capacities as the donor.PL tests showed that such materials displayed blue or deep blue emission,and the full width at half maxima was narrow.Among them,the compound DPA-PIM with diphenylamine as the donor had photochromic phenomenon.The aggregation packing structure indicated that due to the large steric hindrance of the receptor and the distortion of the conformation of the molecule,the intermolecular distance of this kind of compounds was relatively far and the intermolecular force was small,so the luminescence properties of these compounds mainly depended on the single molecule.Solvatochromic testing and the front orbital theory calculation showed that the introduction of donor with stronger donating ability,the more rigid,the charge transfer(CT)excited states composition of obtained molecular was higher,the CZ-PIM exhibited pure local excited(LE)state,DPA-PIM and TPA-PIM were hybridized local and charge transfer(HLCT)state,DMAC-PIM and POZ-PIM were pure CT state,PTZ-PIM with different conformation exhibited LE and CT states characteristics respectively.The quantum efficiency tests showed that the solid state quantum efficiency of HLCT state compound DPA-PIM was the highest.These results showed that through structural design,such kind of molecules were expected to become high-efficiency,narrow-emission,dark blue photoluminescent materials.(2)We studied the photochromic phenomenon of DPA-PIM in depth and found that this photochromic phenomenon derived from its own photo-oxidation reaction.The single-crystal structure proved that in the photooxidation process,the carbon-carbon double bond on the ring of imidazole was broken under the attack of oxygen,then benzoamide and benzoimide groups were formed to produce the photo-oxidation product DPA-PYZ.In-situ NMR and UV-vis spectra showed that there was no byproducts in the photooxidation process,and DPA-PYZ was the only reaction product.Through the ultraviolet spectrum test under the control variables,we determined that oxygen and light are the two conditions for photo-oxidation reaction of DPA-PIM.The electron spin resonance spectroscopy(ESR)indicated that the reactive oxygen species directly involved in the oxidation reaction was singlet oxygen.From the characteristics of this reaction,since the photooxidation reaction of DPA-PIM did not need any sensitizers and the hydrogen on the 1-nitrogen on the imidazole ring was substituted by the benzene ring,which was different from the photo-oxidation reaction of imidazole derivatives reported in the literature,we believed that this was a newly discovered photo-oxidation reaction of imidazole.In addition,because this reaction occured rapidly(< 30 s),the fluorescent color difference between the raw material and the product was large,and the luminous efficiency was high,so this photo-oxidation reaction could be applied in photoetching,oxygen sensing and trace oxygen detected in aqueous solution and its sensitivity could reach up to 2‰(volume ratio).(3)In order to explore the mechanism of this newly imidazole photo-oxidation reaction.We designed and synthesized a series of imidazole derivatives by introducing receptor groups at various substitution sites of PIM and studied their photochemical properties.By in-situ UV test,it was found that 3-carbon substitution position of imidazole ring has a decisive influence on the photo-oxidation activity and only when the aromatic amine connected to the receptor in p-C type,this kind of imidazole derivatives could be oxidized under UV irradiation,while imidazole derivatives with N type connection had photostability.The ESR spectra showed that more superoxide anion radicals could be detected in p-C type imidazole derivatives solution under irradiation,while more singlet oxygen could be detected in N type imidazole derivatives solution.This may be due to the fact that p-C type imidazole derivatives could happen charge transfer with oxygen under irradiation and gave an electron to oxygen formed its own oxidation state and generated superoxide anion radical.This oxidation state structure would undergo structural rearrangement and was vulnerable to be attacked by the singlet oxygen generated by self-sensitization,resulting in photooxidation reaction.However,N type imidazole derivatives were difficult to form this planar oxidation state structure due to the large steric hindrance between the donor and receptor,so they cannot consume the singlet oxygen generated by sensitization and thus have photostability.The UV-vis spectra at high voltage showed that the p-C imidazole derivatives had newly absorption peaks which were not found in the N type imidazole derivatives after loss of electrons,which also proved the existence of this planar oxidation state structure.Therefore,we tried to propose the mechanism of this kind of novel imidazole photo-oxidation reaction: there were energy transfer and charge transfer process between derivatives and oxygen and generated singlet oxygen and superoxide anion free radicals,dervatives losed an electron to form planar oxidation state structure after rearrangement,singlet oxygen attack 5-carbon of imidazole ring form superoxide free radicals,and then continue to attack the neighbouring carbon generated the four-member ring.This four-member ring was not stable and would decompose into end-product containing two carbanyl groups.(4)We designed and synthesized a series of ionized imidazole luminescent small molecules by introducing a side chain to the 3-nitrogen on the imidazole ring.We found that these compounds had good photostability.In addition,the introduction of heavy atomic anions(I-)helped to enhance the intersystem crossing rate of such materials,making them exhibit room temperature phosphorescent properties.The introduction of different donors could also adjust the phosphorescent emission peak location and phosphorescent lifetime of such materials,but if the rigidity of the donor was weak,the introduction of charge transfer state would quench the phosphorescence.Among this kind of ionized imidazole materials,DPA-PIM-ICH3 showed green room temperature phosphorescence in crystal state and blue fluorescence in amorphous phosphorescence quenching,so there was a luminescence blue-shift phenomenon in the grinding process.PTZ-PIM-ICH3 exhibited obvious wavelength dependence phenomenon due to the existence of intersystem crossing channel in higher excited state.Therefore,this kind of material had a broad application prospect in the fields of sensing,anti-counterfeiting and imaging fields.