Novel Two Dimentional Oligopyrrole Conjugated Fused Oligomers. Design,Synthesis,and Properties

Conjugated two-dimensional carbon materials like graphenes constructed with benzene moiety as the basic building block have aroused great interest in nanoelectronics and energy conversion/storage devices due to their remarkable electronic properties.Modulation over these 2D carbon materials by nitrogen-doping results in novel catalysts with great potential applications even in photo/electro-catalytic fields.In consideration of the intrinsic nitrogen-containing conjugated nature with tuneable central metal ions,the conjugated two dimensional molecular materials composed of oligopyrrole building block are of significant scientific and technological importance in the fields of nanoelectronics,energy conversion/storage devices,and photo/electro-catalysts.Tetrapyrrole pigments of life,porphyrins(Pors)and phthalocyanines(Pcs)with 18 ?-electron conjugated system and highly planar molecular structure have attracted robust investigations ranging from photosynthetic antenna models to photo dynamic therapy(PDT)due to their unique optical and favorable redox features.Tripyrrole pigments,ring-contracted Por and Pc analogues,containing three pyrrole units with 14 ?-electron aromatic system and cone-shaped molecular conformation have also received intense attention as new functional pigments in recent years.Particularly,subphthalocyanines(SubPcs)show unique properties such as non-aggregated behavior,electron-deficient nature,and tunable optical properties.Fusing these different oligopyrrole chromophores together would lead to new two-dimensional functional molecular systems with a wide range of application potentials in photosynthetic antenna models,molecular sensors,NIR dyes,and PDT materials associated with the effective electronic communication between/among the fused chromophores.Herein,a series of oligopyrrole-fused oligomers were designed,synthesized,and structurally characterized.Meanwhile,their optical and electrochemical properties have also been studied.The specific contents are as following:(1)Phthalocyanine and porphyrin chromophores have been fused through the quinoxaline moiety,resulting in the heteroleptic tetrapyrrole-fused dimers and trimers.The heteroleptic-tetrapyrrole nature has been clearly revealed based on the single crystal X-ray diffraction analysis of the zinc dimer.Electrochemical analysis,theoretical calculations and time-resolved spectroscopic results disclose that the two/three-tetrapyrrole-fused skeletons behave totally as one ?-conjugated system due to the strong conjugative interaction between/among the tetrapyrrole chromophores.In particular,the effectively extended ?-electron system through the fused-bridge induced strong electronic communication between Pc and Por moieties and large transition dipole moments in the Pc-Por-fused systems,providing high fluorescence quantum yields(>0.13)and relatively long excited state lifetimes(>1.3 ns)in comparison with their homo-tetrapyrrole-fused analogues.(2)Porphyrin and subphthalocyanine chromophores have been fused through the quinoxaline moiety,resulting in the first hybridized porphyrin-subphthalocyanine-fused dimer with a unique plane-bowl molecular structure revealed on the basis of single crystal X-ray diffraction analysis.The spectroscopic and electrochemical results in combination with theoretical calculations disclose the delocalization of its HOMO over the entire molecular skeleton,indicating the strong electronic interaction between SubPc and Por moieties at ground state.On the other hand,in LUMO,its electron density is condensed on the quinoxaline moiety rather than the SubPc and Por moieties due to the electron deficient nature of the quinoxaline ring.This feature indicates the intramolecular charge transfer from both tetrapyrrole and tripyrrole chromophores to the fused quinoxaline moiety,which is in accordance with the solvatochromic fluorescence,red-shifted and diminished emission upon the increase of solvent polarity.(3)Optically active binaphthyl units have been introduced onto the periphery of the phthalocyanine chormophores in the phthalocyanine-porphyrin-fused oligomers,resulting in the first(R)-and(S)-enantiomers of Pc-Por-fused dimer and trimer with induced chirality nature.This represents the largest chiral tetrapyrrole-based conjugated system,to the best of our knowledge.Electronic absorption,MCD,and CD spectroscopic results in combination with theoretical calculations disclose the effective transfer of the chiral information from the binaphthyl groups to both the central tetrapyrrole-fused chromophores,indicating the effective delocalization of the ?-electron density over the fused systems due to the effective electronic communication between/among the tetrapyrrole chromophores.(4)The largest phthalocyanine-porphyrin-fused pentameric molecular arrays have been synthesized and spectroscopically characterized.The saddled molecular conformation revealed for the pentamer by DFT-D3 calculation in combination with the bulky peripheral substituents precludes effective face-to-face ?-?intermolecular interaction.As a consequence,intermolecular C-H ? interactions together with the ubiquitous dispersion force arrays help to self-assemble the representative metal-free pentameric molecules into the three-dimensional supramolecular structures with nanaorod morphology in CHC13 and n-butanol.Powder X-ray diffraction(XRD)analysis and selected area electron diffraction(SAED)disclose the gradually increased long range of molecular ordering in the nanorods along with the increase in the substrate temperature from 30,40,50,to 60?.This in turn results in an increase in the semiconductivity of the single nanorod in the same order from 9.4×0-9 to 3.8×0-8,7.6×10-7,and 6.3×10-5 S/m.(5)Linear oligoisoindole derivatives including diisoindole,triisoindole,tetraisoindole,and hexaisoindole species,representing the new family rather than the traditional cyclic oligoisoindole ones as exemplified by the well known subphthalocyanines(SubPcs),phthalocyanines(Pcs),and extended phthalocyanines,have been synthesized and spectroscopically characterized for the first time.Variable temperature NMR experiments and DFT calculation results reveal the tautomerization of the N-H protons between the outer and inner isoindole units in triisoindole,tetraisoindole,and hexaisoindole species at room temperature.The conjugated electronic structure nature of these oligoisoindole compounds has been clearly clarified on the basis of their single crystal molecular structure,electronic absorption,fluorescence,and electrochemical properties together with theoretical calculations.Moreover,? Mayer bond order calculation suggests these oligoisoindole compounds have two types of strong conjugated regions including the delocalized conjugated line across the whole skeleton and the localized conjugated circles at the benzene moieties,which are united together into a whole conjugated system by weak ? linking bonds.In particular,the ?bonds that pervade over all the linear conjugated oligoisoindole skeletons are comparable to those in the traditional cyclic oligoisoindole skeletons.This,in combination with the existence of the well-defined coordination moiety/moieties for these linear oligoisoindoles,suggests their promising application potential in diverse fields including sensors and electronic devices as their cyclic counterparts.