The Impacts Of Through-Space Conjugation And Heteroatoms On The Optical Property Of AIEgens

The molecules that glow a weak light or even go dark in dilute solutions but show robust emission in aggregated states are called aggregation-induced emission（AIE）luminogens.AIE materials show great potential applications in organic optoelectronic devices,chemical sensors,biological probes and fluorescence imaging.Studies on AIE mechanism and structure-property correlations will help to design highly efficient solid-state luminescent materials.In this thesis,the impacts of through-space conjugation and heteroatoms on AIE properties are systematically studied experimentally and theoretically.In chapter 3,we focused on the through-space conjugation in a new class of foldamers with a folded tetraphenylethene（TPE）core.We have studied its impact on the photophysical properties through experimental measurements and theoretical calculations.It is found that S₀→S₁ transition is mainly attributed to the TPE core,and through-space conjugation makes significant contributions to the high energy（S₀→S_n）absorption band in these foldamers.Moreover,these foldamers exhibit aggregation-enhanced emission（AEE）and apparent blue shifts in their emission spectra from solution to aggregates.The structural changes are smaller in the aggregated state than those in the isolated state during the excited-state relaxation process,which results in lower reorganization energies,thus blue-shifted and enhanced emissions in the aggregates.In chapter 4,we have studied the photophysical properties of tetraphenylethene foldamers with heteroatoms.The results show that there are strong through-space conjugations between the face-to-face aligned chains in the foldamers.The interactions of benzene-thiophene and benzene-furan are greater than that of benzene-benzene.In addition,we can see that the S₁→S₀ transition in the benzene-benzene foldamers pertains to the TPE fragment,while the S₁→S₀ transitions in benzene-thiophene and benzene-furan foldamers are dominated by the double bond in TPE and the overlapped fragments in foldamers.It is obviously that through-sapce conjugation plays an important role in the emission of benzene-thiophene and benzene-furan foldamers.For most organic molecules,the spin-orbit coupling is small and the intersystem crossing is negligible.In chapter 5,we find that intersystem crossing matters a lot in benzothiophene and dithiophene derivatives,while after oxidizing on thiophene,the intersystem crossing is weakened apparently.In isolated state,internal conversion rate is predominant,leading to weak emission,while in aggregates,internal conversion is greatly hindered,resulting in higher fluorescence quantum yields.Due to smaller spin-orbit coupling and lower S₁ energy levels after oxidizing on thiophene,the S₁→T_n channels are blocked,and the intersystem crossing rates in benzothiophene and dithiophene oxides are smaller.Meanwhile theπ?πinteraction becomes wearker among the molecules with oxided thiophene ring.As a result,the emission intensity of the molecules is increased greatly.