Study On The Synthesis,Self-assembly Controlling And Application Of Solid-state Fluorescent Squaranine Dyes

As is well known,squaraine dyes with versatile structures have been widely applied in a number of advanced applications including solar cell,optical probes,photodynamic therapy,biological cell imaging and nonlinear optics due to their unique photoelectrical properties.Nevertheless,those derivatives are greatly limited by the typical aggregation-caused quenching(ACQ)in the practical applications.As such,in this study we first summarized the synthesis methods of those versatile derivatives with different substituents at diverse positions,and then focused on the recent progress in the design,synthesis and application of the reported squaraine dyes with emission in the solid-state.Upon those results,a series of novel squaraine dyes with efficient emission in the solid-state was designed and synthesized,and their self-assembly emission control and practical applications in optical detection as well as cell imaging was studied in detail.Combining with in-depth theoretical calculations,the relationship between their molecular configuration,self-assembly and emission behavior observed in the experiments was fully deciphered.The main results were as follows:Firstly,two benzimidazole-based 1,2-SQs(SQM and SQB)were designed and synthesized,which exhibited strong emission from bright-yellow to red colors in well-ordered assemblies.Their self-assembling behaviors showed that the SQM assemblies were 1D nanorods,whereas SQB assemblies can be tuned as 1D nanorods(Z-SQB·CH2Cl2)and 2D nanoplates(E-SQB·2CH3OH)via a simple solution-based self-assembly method.Combining experimental and theoretical analysis,the varied self-assembly behaviors came from their variable molecular conformations and packing modes in different assembly solutions.Accordingly,distinct optical behaviors in those SQ-based microstructures were demonstrated.Moreover,the multiple reversible vapochromic behaviors were first observed in the SQB·sovlent assemblies,which makes it a potential "naked-eye" fluorescent sensor for organic solvent vapors.Secondly,a new 1,3-SQ was designed and synthesized,and based on its self-assembly microcrystals a reversible and specific TFA detection was developed through hydrogen-bonding docking strategy.The results showed that the reversible luminescent alternation between green-emitting R-SQ crystals and cyan-emitting SQ-2TFA crystals can be achieved by the annealing and TFA fuming treatment,which was ascribed to the crystal rearrangements with formation and collapse of organic frameworks between SQ and TFA molecules triggered by hydrogen-bonding docking behavior,thus leaded to a detection of TFA vapor with excellent specificity and reusability.Moreover,various emission behaviors such as AIE,CIEE and mechanochromism of the crystals were observed,which permits it a promising skeleton for stimuli-responsive emissive organic crystals.Finally,guided by in-depth TDDFT calculations,a bidirectional TICT(twisted intramolecular charge transfer)/ESIPT(exited-state intramolecular proton transfer)controls switching was firstly decoded for intense red/NIR emission both in solution and crystalline state,based on twisting conjugated squaraine dyes(SQDP,SQHTPE and SQTPE).From detailed structural and DFT analysis,the enhanced ?-conjugation and subsequent rigidity prevented intramolecular phenyl rotations and TICT process vibration,thus allowed their strong red emission in dilute solution(?PL of SQHTPE up to 57.5%at 586/622 nm).Meanwhile,a synergistic hydrogen-bonding dominated packing mode and ESIPT mechanism responded to the intense red/NIR emission in their crystalline state(?PL of SQTPE up to 73.1%at 656 nm).Moreover,efficient red bioimaging in HUVECs has been achieved with naked SQHTPE,which provided a new vision for directly applying squaraine dyes in biological applications.