The Research On [5] Helicene Luminescent Materials Modified By Boron

Circularly polarized luminescence(CPL)materials have broad application prospects in the fields of 3D display,optical information storage and processing systems,optical quantum communication,molecular photoswitches,biological probes,CPL lasers and CPL sensors and they are attracting more and more attention.The photoluminescence quantum yields and the luminescence dissymmetry factor are two important parameters for measuring the luminescence properties of CPL materials.Small organic molecules are the most promising CPL luminescent materials for their simple structure,finely tunable emission wavelength and easiness of manufacturing processes.However,it is difficult for current CPL small organic molecules to have both high quantum yields and high luminescence dissymmetry factor,and most of them have severe fluorescence quenching in the solid state,which greatly limit their practical applications.Helicenes are ortho-fused polycyclic aromatic compounds with a helically twisted structure,which makes these compounds have good chiral optical properties.Therefore,these compounds have great potential in the development of new CPL materials.However,the unmodified helicenes have very low quantum yields due to the symmetry-forbidden of the frontier orbitals,which cannot meet the requirements of CPL materials.So it is a good strategy to break the symmetry of the frontier orbitals by introducing an electron donor and an electron acceptor onto the skeleton of helicene to form a "push-pull"electronic structure.So we propose a molecular design idea for a CPL luminescent small organic molecules obtained by modifying helicene skeleton with a bulk BMes2.Due to the effect of boron's empty p-orbital,BMes2 is not only a good electron acceptor,but also has strong Lewis acidity,which can effectively complex with Lewis basic fluoride ion,thereby providing a possible route to regulate CPL properties.At the same time,the steric hindrance of BMes2 can effectively suppress the fluorescence quenching in the solid state.Based on the above molecular design ideas,this paper has carried out the following two aspects of work.1.Firstly,we used[5]helicene as a model compound to design and synthesize the[5]helicene derivatives which are modified with either BMes2(7B-HC),or both BMes2 and NMe2(8B5NMe2-HC and 7B5NMe2-HC),to study the effect of the introduction of BMes2 on the optical properties of[5]helicene.In the synthesis process,we utilized a newly reported synthesis method of[5]helicene,and built up the efficient synthetic route to prepare[5]helicene derivatives modified by boron and nitrogen with asymmetric structure.Moreover,it was first proved that BMes2 has the similar directing effect as Br on the electrophilic substitution reaction.A series of characterizations on the target products have been carried out,and we found that the introduction of BMes2 into[5]helicene is effective to enhance the transition probability of S1 and shift the emission to longer wavelength.With the introduction of NMe2,the emission wavelengths of 8B5NMe2-HC and 7B5NMe2-HC are further red-shifted.All of these compounds have high quantum yields in cyclohexane,and their solid quantum yields are also high due to the large sternc hindrance of BMes2.Moreover,the addition of fluoride to 8B5NMe2-HC and 7B5NMe2-HC induces significant blue shift of fluorescence,which provides the possibility to achieve fluoride-responsive CPL.Unfortunately,these compounds do not have a stable chirality and we need to further modify its structure to obtain products with stable chirality.2.The work of the first part shows that the introduction of BMes2 and NMe2 is veryeffective to improve the quantum yields of the parent helicene,but the obtained compounds are liable to undergo configuration flipping and do not have stable chirality.Therefore,based on the first part of the work,we further modified the[5]helicene skeleton by introducing a phenyl group at the 2nd and 13th positions,to increase the steric hindrance of both ends and inhibit the configuration flip.Using a synthetic route similar to the first part,we first synthesized and characterized the[5]helicene derivative 7B-PhHC containing only one BMes2.This compound has a photoelectric property very similar to that of 7B-HC,but it has a stable chirality,and we have obtained its optically pure isomer by chiral resolution.It's luminescence dissymmetry factor in cyclohexane is 2.67 × 10-3,quantum yields is 0.51,which are relatively high for small organic molecules.We anticipate that the further introduction of NMe2 into the molecule can make its emission wavelength red shift greatly and obtain[5]helicene derivatives with fluoride-responsive CPL property.