Study On Synthesis And Properties Of 9-Phenylfluorene Mechanochromic Luminescent Materials

Mechanochromic luminescence(MCL)is a type of stimuli-responsive functional material that can perceive external stimuli(such as mechanical force,heat and solvent evaporation)and switch the emitted light color in the solid state.It has great application prospects in the fields of organic optoelectronics,soft robotics,information security,and intelligent displays.Among the numerous organic luminescent materials,9-phenylfluorene-based materials have advantages such as easily modifiable structure,excellent photoelectric efficiency and low cost,making them promising candidate "star materials" for constructing mechanically responsive luminescent semiconductors.Therefore,exploring the molecular design,performance regulation and scenario application of fluorene-based mechanochromic luminescent materials is of significant research value.Based on the above discussions,we selected 9-phenylfluorene as the molecular skeleton,and prepared a series of D-A type 9-phenyl derivatives by structural modification.We carried out systematic research on their pressure-induced coloration properties through molecular structure characterization,photophysical characterization,crystallographic parameters,thermodynamic characterization and theoretical calculations,and discovered the regularities of 9-phenylfluorene-based MCL effects.The relevant work is presented in the form of three chapters in this thesis,as follows:1.Based on previous work within our group,we began to explore molecular-design strategies to investigate key factors in initiating MCL behavior.Using the "organic semiconductor quaternary design theory" as guidance,we introduced different functional molecular fragments onto the 9-phenylfluorene skeleton through electronic structure modulation,ultimately discovering the critical elements required for 9-phenylfluorene-based molecules to exhibit MCL effects and undergo the transformation from "OFF" to "ON" through mechanical coloration.By modifying the 9-phenylfluorene backbone with N-ethylcarbazole(D)as the electron donor and 4,6-diphenyl-1,3,5-triazine(A)as the electron acceptor,we designed and synthesized three molecules based on 9-phenylfluorene molecular skeleton,named PFCz,TRZDPh FO and TRZPFCz.Through photophysical characterization after applying pressure via grinding to the three materials,we discovered that only when both N-ethylcarbazole and 4,6-diphenyl-1,3,5-triazine are introduced onto the 9-phenylfluorene skeleton can MCL effects be initiated.Subsequently,we analyzed photophysical data,crystallographic dataand thermodynamic data to infer that the reason behind the MCL effect is due to a transition from a crystalline state to an amorphous state,which is accompanied by molecular skeleton twisting and subsequent excitation state changes.Therefore,our work suggests that the D-A type molecular design strategy induces a strongly distorted molecular skeleton and separation of the molecular frontier orbitals,making it a critical molecular design strategy for initiating MCL effects.Specifically,using 9-phenylfluorene as the molecular backbone,molecules modified with just D or A group cannot induce MCL effect,and only simultaneous modification with both D and A groups can induce MCL effect.This work represents the first step towards pressure-induced coloration of 9-phenylfluorene derivatives and elucidates the necessary conditions for achieving MCL effect in fluorene-based systems.Furthermore,it has provided molecular design experience for the development of new 9-phenylfluorene-based mechanically responsive luminescent materials.2.We previously achieved MCL in 9-phenylfluorene-based materials by introducing D-A fragments onto the molecular skeleton.However,achieving practical applications that demand high contrast,sensitivity and cycling stability is of paramount importance for MCL.To address this,we utilized steric hindrance control strategies from the "organic semiconductor quaternary design theory" to design two new D-A-type 9-phenylfluorene-based molecules,named molecule 1 and 2.Compared with 1,molecule 2 possesses a rigid hindrance group,namely incorporating a 9-phenylfluorene fragment onto the molecular skeleton.This modification significantly improves the extent of emission-color change induced by MCL and increases the stability of "write-erase" cycles.Through systematic measurments and characterizations,we concluded that the introduction of a hindrance weakened the intermolecular forces between the molecules and made the crystal structure more susceptible to damage,ultimately resulting in enhanced MCL behavior.3.Guided by the quaternary design mechanism of organic semiconductors,our previous two work-sections successfully initiated the fluorene-based mechanochromic luminescence behavior from the two dimensions of regulating electronic structure and introducing steric hindrance,and improved its color-changing wavelength and write-erase cycle stability.However,the MCL behavior is caused by multi-effect synergy.Single factors such as excited state and weak force are important factors affecting MCL behavior,but not decisive reasons.Therefore,based on the D-A structure and steric hindrance,we deeply explore the multi-effect synergy between fluorene-based MCL behavior and energy level distribution,force form and stacking mode.Based on the second work,we prepared two novel MCL molecules,namely DTRZ-MOM and DTRZ-Ph,by the strategy of electron-rich group methoxy and planar phenyl substituted N-ethyl,and characterized the photophysical and crystallographic properties of the two molecules.By comparing the above two molecules with the previous work,it is found that the distribution of energy levels is crucial to the behavior of MCL.At the same time,the rich intermolecular weak interaction and stacking mode induced by C-H bond are also the factors that must be considered in the design of efficient MCL materials.This chapter demonstrates that the influence of multi-effect synergy such as energy and stacking mode on MCL behavior,hoping to open up the way for subsequent work from the perspective of molecular design and lay the foundation for the practical application of fluorene-based MCL.