Photophysical Properties Of Highly Twisted π-Conjugated Molecules

Due to its excellent optical properties,low cost and low processing energy consumption,pure organic luminous materials have a wide range of applications in the fields of photoelectric conversion,sensors,biomedical engineering and anticounterfeiting encryption.One kind of special molecules,aromatic molecules with highly twisted structure,not only promote phosphorescent emission by inducing orbital angular momentum change during electron intersystem crossing,but also generate special aggregation-induced emission by avoiding aggregation-caused quenching generated by π-π stacking between aromatic rings,which has attracted wide attention.These molecules have been studied to some extent,but the theoretical basis of their promotion of phosphorescent production and the stability of these materials under physiological conditions still need further investigation.During my postgraduate study,I devoted myself to the theoretical research of phosphorescence of this kind of molecule and the physiological stability of aggregationinduced emission and discussed the photophysical properties and application prospects of this kind of molecule from theory to practice.The details are as follows.1.Study on intersystem crossing of biphenyls and characterization of their room temperature phosphorescence properties.Through the clever design of molecular structure,the restriction of electron transition by the traditional El-Sayed’s rule can be broken.We have designed and synthesized biphenyl molecules with electron donor and electron acceptor,so that the energy difference between singlet and triplet states can be reduced by charge transfer state during the excitation process,and the electrons can jump in different π orbitals during the intersystem crossing.Then,we adjust the steric hindrance by modifying different numbers of methyl groups in the ortho-positions of the two phenyl groups,and gradually increase the twisted structure of the molecule(the dihedral angle between the two benzene rings),so that the change of orbital angular momentum of the electron in the transition process to compensate for the change of spin angular momentum,which is conducive to the occurrence of intersystem transiting,thus promoting the generation of phosphorescence.We can induce room temperature phosphorescence at different wavelengths,from blue to green,by doping molecules into different polymer substrates.In addition,by calculating the two-dimensional spinorbit coupling matrix elements,we study the intersystem crossing coefficients in different excited states in detail and find that the molecules designed by EI-Sayed’s rule analogy are more likely to occur intersystem crossing at higher excited energy levels.The conclusion is proved by experiments and calculations.Our study contributes to a deeper understanding of the relationship between molecular structure and emission,and provides ideas for the design of new room temperature phosphorescent molecules.2.Stable aggregation-induced emission materials under physiological conditions and simple applications in cell imaging.In addition to phosphorescent properties,highly twisted aromatic molecules are often characterized by aggregation-induced emission.These materials has been extensively studied,but their stability and photophysical properties under physiological conditions are still the hotspot of recent research.We designed and synthesized two kinds of aggregation-induced emission molecules using tetraphenylethylene as the core and phosphocholine group to provide hydrophilicity.The difference between them is that one has a carbon chain as a linker,while the other one does not.We found that both molecules exhibit good aggregationinduced emission properties,and the system containing linker has stronger fluorescence emission and better self-assembly properties.We were intrigued by the huge difference in molecular properties caused by the short linker with only two carbons.We hypothesized that the twisted tetraphenylethylene molecules are very sensitive to water molecules,and that the distance between the two would result in strong photophysical properties changes.In addition,we performed preliminary cell imaging experiments using these two molecules and found that the cells were successfully stained and fluoresced green.Our research provides a new design idea for aggregation-induced emission materials.