Study On Fluorescence And Controlled Crystallization Of Aromatic Aldehyde Azines In Solid State

Aromatic aldehyde azines belongs to the typical Donor-Acceptor-Donor compounds,and its unique photophysical and chemical properties have gradually revealed various application in the field of fluorescent materials.Therefore,in this paper,aromatic aldehyde azines were selected as the model compounds for research.A series of aromatic aldehyde azines with excellent solid-state fluorescence were designed and synthesized by chemical modification.The results showed that the fluorescence properties of these azines were significantly influenced by substitutional groups.In addition,some of the azines were found with polymorphic characters.We construct these azines in two ways,as showing follows:One way is to introduce salicylaldehyde azine as a prototype.The introduction of electron-donating and electron-withdrawing groups into the phenyl ring of salicylaldehyde azine causes red and blue shifts of the fluorescence emission band.The UV/vis spectroscopy,fluorescence spectroscopy and Time-Correlated Single Photon Counting(TCSPC)technique can basically verify that the series fluorescent azines undergo an excited-state intramolecular proton transfer(ESIPT)process after photoexcitation.At the same time,by means of single crystal X-ray analysis,it is found that the greater the structural rigidity and coplanarity of the molecule,the higher the photoluminescence efficiency of the fluorescence system.Thereby realizing the regulation of the solid-state fluorescence behavior of salicylaldehyde azine derivatives(SAAs).Another type of aromatic aldehyde azines is designed by changing the aromatic ring into phenyl/naphthalene rings on the basis of salicylaldehyde azine as the backbone molecule,which include the Pigment Yellow 101(P.Y.101),whose fluorescence mechanism is been controversial over decades.It is found that the P.Y.101 exhibits dual emission with two large Stokes shifts in the spectrum.In view of the dual emission may occur due to the double proton transfer in the ESIPT process,we performed TD-DFT calculation,which demonstrates the feasibility of excited state single intramolecular proton transfer and the thermodynamics and kinetics unfavorable step-wise/asynchronous double proton transfer.Moreover,the stimuli-responsive experiment for P.Y.101 shows the dual fluorescence emission are originated with two polymorphs,and the two crystal forms can realize conversion by grinding/recrystallization.At the same time,we have developed the technique for preparing SAAs crystals by temperature controlling sublimation methods.And this technique has guide the discovery and effective separation of 5-methoxy-salicylaldhyde azine.The rod-like crystals obtained at the sublimation temperature of 170 ~oC showed strong orange-yellow fluorescence,and the lamellar crystals obtained at a sublimation temperature of 220 ~oC showed red weak fluorescence.Single crystal X-ray analysis reveals the two polymorphs were rotational isomers,antiperiplanar(1-anti)and synperiplanar(1-syn)configurations toward the longest chain in the molecule,respectively.Both the experimental analysis and DFT calculation validated the existence of alternative conformations for 5-methoxy-salicylaldhyde azine in the gas phase were kinetic stable form(1-anti)and thermodynamically stable form(1-syn),respectively,and the kinetic stable form can finally converted to the thermodynamically one under thermal treatment.Therefore,the controllable sublimation process can guide the detection and growth of polymorphic materials in regular shapes and good crystallinities.