Synthesis And Photophysical Properties Of The New Benzimidazol-based Pincer Platinum Complexes

Cyclometallated platinum(II) complexes with tridentate pincer type ligands havearoused a considerable interest in recent years, due to their numerous applications inthe fields such as chemosensing, bioimaging and organic light-emitting diodes(OLEDs). Among various types of platinum complexes bearing NCN pincer ligands,compounds with m-di(2-pyridinyl)benzene(dpyb) ligands have been synthesized andinvestigated extensively. Moreover, Pt(II) pincer complexes with1,3-bis(benzimida-zol-2'-yl)benzene(bzimb) ligands show good optical and electronic propertiesrecently.A series of NCN Pt(II) pincers containing1,3-bis(benzimidazol-2'-yl)benzeneligands were synthesized and characterized. The new pincer complexes weredetermined by elemental analysis, NMR, IR, HRMS and X-ray single-crystalstructure analysis. Simultaneously, the photophysical and spectroscopic behavior ofthese compounds were studied. The main results are as follows?1. The reaction of1,3-bis(benzimidazol-2'-yl)benzene ligands2,12-15,24-26,1,3-bis(benzoxazol-2'-yl)benzene ligand27and1,3-bis(benzothiazol-2'-yl)benzeneligand28with K2PtCl4in HOAc afforded the corresponding neutral pincer Pt(II)chloride complexes3,16,19-21,29-33after reflux. Then the Pt(II) iodide complexes4and17as well as Pt(II) acetylide complexes5-7and18were obtained by treatmentof complexes3and16with KI and aromatic terminal alkynes in methanolic NaOH,respectively. Additionally, chloride abstraction on complex3with AgOTf in thepresence of several neutral N-donor ligands furnished the cationic Pt(II) complexes8-11. X-ray single-crystal structure analyses on complexes3-5,8,16-18,20and29indicate the presence of weak intermolecular interactions including ?…? stackingbetween the interlayer adjacent bzimb rings and/or hydrogen bondings. 2. The photophysical and spectroscopic behavior of bis(benzimidazol) pincerplatinum(II) complexes3-11using chloro, iodo, alkynyl and pyridine as theco-ligands were studied. Complexes3-11show bright luminescence in solution, solidand2-MeTHF glass. The UV-vis and emission spectra of cationic complexes8-11show a visible blue shift in comparison with neutral complexes. Moreover, TD-DFTcalculations have been performed on complexes3-5and8, and their absorptions canbe primarily attributed as ???*(bzimb)1IL and d?(Pt)??*(bzimb)1MLCTtransitions, mixed with minor ?(I)??*(bzimb)1LLCT character for4. For theacetylide complex5, its absorptions are attributed as an admixture of the???*(bzimb)1IL state, d?(Pt)??*(bzimb)1MLCT state and increased?(C?CC6H4OMe-4)??*(bzimb)1LLCT state. In most cases, the luminescencequantum yields in degassed CH2Cl2solution are in the range of0.122to0.150, andthe emission lifetimes are within the microsecond range of2.1-6.8.3. The photophysical and spectroscopic behavior of cycloplatinated pincercomplexes16-21were studied. Complexes16-21show bright luminescence insolution, solid and2-MeTHF glass. In comparison with complex16(509nm) an obvious red-shift is observed in the maximum emission of complex19(532nm),which is attributed to the electron-withdrawing substituent group relative toelectron-donating substituent group. TD-DFT calculations have been performed oncomplexes16and20. The absorptions of16can be primarily assigned as ???*(L)1IL and d?(Pt)??*(L)1MLCT transitions, mixed with ?(Cl)??*(L)1LLCT characterwhereas those of20are composed by ???*(L)1IL and d?(Pt)??*(L)1MLCTtransitions. The luminescence quantum yields in degassed CH2Cl2solution are from0.12to0.15in most cases and the emission lifetimes are in the microsecond range of2.5-6.5.4. The photophysical and spectroscopic behavior of alkyl chain substitutedbis(benzimidazol) pincer platinum(II) complexes29-31and bis(benzoxazol/benzothiazol) pincer platinum(II) complexes32-33were studied. The results showthat the length of the alkyl chain has negligible effect on the transition energies of theabsorption and emission bands, while it has apparent influence on the the quantumyields of the complexes with different alkyl chain length. Complex30has the mostexcellent quantum yield in the solid state(0.28). At the same time, the maximumemission is red-shifted from530nm to553nm and583nm when ligand is changedfrom benzimidazol to benzoxazol and benzothiazol. TD-DFT calculations have beenperformed on complexes29and32-33and their absorptions can be primarilyattributed as ???*(L)1IL and d?(Pt)??*(L)1MLCT transitions.