Pyrene Derivatives Functionalized By Pyridine:Influence On ’Site-effect’ And Its Bioimaging Application

The structure-property relationship of organic materials has always been a concern topic for material chemistry researchers.For the pyrene derivatives,even if the same substituent is linked to different sites of pyrene,their opto-electronic properties will be significantly different,which is termed as "site-effect" of pyrene in this paper.Pyrene is a prototypical polycyclic aromatic hydrocarbon(PAH)and a well-studied fundamental fluorophore.The specific properties of pyrene,such as its long-lived excited state,high fluorescence quantum yield,and easy to form excimer,have attracted many scientists to develop and extend the applications of pyrene in wide variety of modern science.Up to now,lots of pyrene derivatives have been designed and prepared for use as organic semiconductors or fluorescent probes.Although pyrene itself seems simple and highly symmetrical,its ten peripheral reactive sites(C-H bonds)can easily be divided into two types and three classes depending on the symmetry and chemical environment,each possess unique electronic distribution and sterical environment.1-,3-,6-,and 8-position of pyrene are the "common sites",which possess the maximum contributions of the HOMO and are favorable for electrophilic aromatic substitution.Actually,most pyrene derivatives result from reactions at these sites.In contrast,the 2-and 7-positions of pyrene lie on a nodal plane of both the HOMO and LUMO but are the least hindered sites and are thus favorable for sterically controlled reactions.The 4-,5-,9-and 10-position of pyrene,also known as K-region,lack either steric or electronic advantages.The C4-C5 and C9-C10 bonds at K-region normally present the character "localized" double bond,thus are favorable for some oxidation or hydrogenation with moderated yields.To discover the dependence of opto-electronic properties of pyrene derivatives on the site of substituents connecting to pyrene(termed as "site-effect"),6 series of regioisomeric pyridyl-pyrene conjugates which focused on mono-,di-,and tetra-substituted at 1-,2-,and 4-position of pyrene were designed and synthesized.The pyridyl group was chosen as the substituent because of its versatile potential as a precursor of cationic fluorescent dyes and as an effective ligand to potential applications in organometallic complexes or ploymers.Accurately construct regioisomers and carefully compare the differences of each group of isomers in photophysical properties and bioimaging applications.A complete and systematic study of "structure-property"relationship has been formed from the structures,properties and applications of the compounds.In this work,totally 26 pyridyl-pyrene conjugates were synthesized and 25 crystal structures of them were determined as the solid foundation of "strutures".Correspondingly,their general "properties",such as chemical synthesis and melt points,HNMR shifts,photon absorption and emission properties,theoretical ionization energy and energy levels,were extensively studied.Especially,these compounds could be classified into 6 series of exact regioisomer,which enable us clearly find the "site-effect"behind these pyrene derivatives.1.The site-effect in chemical synthesis.Although the 1-site of pyrene could be directly and coupled further,the 2-site of pyrene can only be highly selective borylated then brominated and finial coupled.However,the 4-substituted pyrene compounds have to be synthesized through multi-steps route(reduced-substituted-oxdized)because the direct activation of 4-site of pyrene is still unavailable.Thus the order of difficulty for synthesis is:4->2->1-.2.The site-effect in the crystal structures:The dihedral angle formed by pyridine ring and pyrene ring is related to the sites of pyrene,namely,the order of dihedral angle in the compounds substituted at three sites is:1-≈4->2-.On the other hand,it is related to the N atom in the substituent group,the order of dihedral angles in pyridine pyrene compounds is p->o-,while the order of dihedral angles in vinylpyridine pyrene compounds is the opposite.3.The site-effect in photophysical properties:due to the unique symmetry of molecular orbitals,2-substituted pyrene derivatives show the characteristics of large macroscopic stokes shifts because of their weak S0→S1 transistion.However,1-and 4-substituted pyrene derivatives break the molecular orbital symmetry,and the absorption and emission spectra show significant intramolecular charge transfer(ICT)characteristics.For 1-,2-,4-substituted pyrene derivatives,from mono-substituted,bis-substituted to tetra-substituted,the compounds show "addition effect" in the spectra,that is,more substitutes result in smaller energy gaps and larger spectral red shifts,especially 1-substituted pyrene derivatives are the most significant.4.The site-effect in two-photon absorptions:The vinylpyridine-substituted compounds show large Stokes shifts and excellent two-photon properties.The relationship between the two-photon properties of this series compounds and pyrene sites are 1->4->>2-.The two-photon absorption/emisison cross-section values of 1368-doPP and 16-dpPP-8 are up to 652/417 GM and 5014/752 GM respectively,which is much higher than that of pyrene derivatives in the literatures.5.Pyridine-pyrene compounds show excellent solid-state luminescence properties,while vinylpyridine-pyrene compounds have poor solid-state luminescence properties and excellent solution-state luminescence properties.The quantum yield in DMSO solution of 16-pPP and 1-doPP is as high as 98%,which is comparable to that of fluorescein.6.The application of vinylpyridyl-pyrene compounds for bioimaging:It was found that the N-alkylated cationic pyridyl-pyrene specifically target mitochondria,thus could achieve imaging of mitochondria in mice liver tissues and visualization of mitochondria with a high signal-to-noise ratio at a depth of up to about 50 μm.