Design, Synthesis And Electroluminochromism Manipulation Of Methylviologen-Substituted Iridium (Ⅲ) Complexes

Electroluminochromism（ELC）materials are those who exhibit reversible luminescence switching with electrical stimulus,having attracted extensive attention due to its potential application in information display,recording,and encryption.ELC materials are composed of electroactive units and luminophores.The reasonable combination of these two parts is crucial for ELC materials construction.Most electroactive units employed in ELC materials are strong electron donors or receptors while the luminophore employed are usually organic fluorescent dyes.The electron/energy transfer process between the electroactive units and luminophores can be manipulated by electrical stimulus,leading response in luminescence intensity.Methylviologen and its derivatives,which have outstanding structure adjustability and redox reversibility or electroactivity,have been widely used as electroactive units.Additionally,the electron withdrawing and donating ability of methylviologen can be easily adjusted by bridging substituents.During the redox processes under electrical stimulus,viologens undergo reversible structural conversions between neutral and cationic forms,leading to absorption response in the visible region and dramatic color change.Meanwhile,phosphorescent iridium（III）complexes are candidates for ELC luminophores since they have abundant frontier molecular orbitals,environmentally sensitive excited states and stable redox properties.Taking these factors into consideration,methylviologen and iridium（III）complexes have been respectively selected as the electroactive unit and luminophores to construct ELC materials.By attaching viologens to different ligands,a series of iridium（III）complexes were designed and synthesized,and their phosphorescent response to electrical stimulus was systematically studied.The contents are as follows:1.The ELC of the Iridium（III）Complexes with a general structure of[Ir（N^C）₂（N^N）]（PF₆）₃ containing viologen-substituted bipyridyl ligandTwo iridium（III）complexes with a general structure of[Ir（N^C）₂（N^N）]（PF₆）₃ have been designed and synthesized by attaching viologens to bipyridyl ligand.2-Phenylquinoline and 2-phenylbenzo[g]quinoline were coordinated to the complexes as cyclometalating ligands to tune the excited state orbital energy levels and luminescence,respectively.The complexes exhibited no luminescence because of the photoinduced electron transfer（PET）between methylviologen units and iridium（III）complexes cores,which promotes the non-radiative transition of the excited state.The viologen-based reduction couples occurring at-0.6 V versus saturated calomel electrode（SCE）was observed via cyclic voltammetry.Upon applying a voltage of 2 V or adding Na BH₄ as a reducing agent,the methylviologen units were chemically reduced from electron-deficient dication form to electron-rich neutral form,cutting off the PET process and turning on the luminescence.Orange（575nm）and near-infrared（701 nm）luminescence were observed,respectively.Upon applying a reverse voltage,the methylviologen units were oxidized back to dication form and the luminescence turn-off.An“on-off”information recording device was designed and fabricated based on the sensitive and reversible luminescence on-off.ELC in the near-infrared region could not be recognized by the naked eye while recognized by near-infrared luminescence imaging,indicating an application in near-infrared information encryption device.2.The ELC of the Iridium（III）Complexes with a general structure of[Ir（N^C）₂（N^N）]（PF₆）₅ containing viologen-substituted phenylpyridine ligandsSince the excited states of iridium（III）complexes are sensitive to the modification of cyclometalating ligands,three iridium（III）complexes with a general structure of[Ir（N^C）₂（N^N）]（PF₆）₃ have been designed and synthesized by attaching viologens to phenylpyridine ligand.The negatively-charged phenylpyridine ligand increases the charge density of the viologens unit and decreases its oxidation potential(E_red=-0.9 V),cutting off the quenching effect on the excited state of the complexes.The complexes exhibited orange（600 nm）luminescence originating from a dπ（Ir）/π（C^N）→π^（9）（C^N）charge-transfer excited state.Upon applying a voltage of 2 V or adding Na BH₄ as a reducing agent,the methylviologen units were chemically reduced from electron-deficient dication form to electron-rich neutral form,transferring from the electron-withdrawing inductive effect to electron-donating conjugated effect.Since the energy level of phenylpyridine ligands rose,the emissive state of the complexes changed to dπ（Ir）/π（C^N）→π^（9）（N^N）excited state.The three complexes emitted at 550 nm,594 nm and 620 nm according to the energy level of the diimine ligands and were applied in luminescence response information recording.The luminescence response from 600 nm to 594 nm could not be recognized by the naked eye while a 400 ns elongation in luminescence lifetime has been measured.This complex has been used in time-resolved information encryption.3.The ELC of the Iridium（III）Complexes with a general structure of[Ir（N^C）₃]（PF₆）₂containing viologen-substituted phenylpyridine ligandsTwo iridium（III）complexes with a general structure of[Ir（N^C）₃]（PF₆）₂ have been designed and synthesized by attaching viologens to one of the three phenylpyridine ligands.Phenylpyridines and2-Phenylquinolines were served as cyclometalating ligands to tune the excited state orbital energy levels and luminescence,respectively.The blocked PET were restored since a lower oxidation potential of iridium center in the general structure of[Ir（N^C）₃]（PF₆）₂,promoting the non-radiative transition of the excited state and quenches the luminescence.The viologen-based reduction couples occurring at about-1.0 V versus saturated calomel electrode（SCE）were observed via cyclic voltammetry.Upon applying a voltage of 2 V or adding Na BH₄ as a reducing agent,the methylviologen units were chemically reduced from electron-deficient dication form to electron-rich neutral form and cut off the PET,and thus intense luminescence was observed at 541 nm and 599 nm.Upon applying a reverse voltage,the methylviologen units were oxidized back to dication form and the luminescence intensity decreased.This work has achieved breakthroughs in luminescence wavelength or lifetime switching ELC materials.We anticipate that this work opens up a new direction for development of electroluminochromic materials and applications of electroluminochromic devices.