Self-assembly and phosphorescence of ionic Pd(II) N-heterocyclic allenylidene complexes in nonpolar solvents N-杂环亚丙二烯基钯(II)离子化合物在非极性溶剂中的自组装行为和磷光性质

In sharp contrast with the rich phosphorescent Pt(II) complexes reported in the literatures, phosphorescent Pd(II) complexes in room temperature solutions are scarce, in spite of having the same d electronic configuration and square-planar coordination mode. Due to their low-lying d-d transition state, phosphorescence origin of the Pd(II) complexes are usually ligand-based, rather than much metal participation. Recently, it has been proved to be an effective strategy to construct self-assembled aggregates via metallophilic Pd•••Pd interactions for Pd(II) complexes to achieve metal-metal-to-ligand charge transfer (MMLCT) and corresponding phosphorescent emission in room temperature fluid state. Herein, a cationic N-heterocyclic allenylidene (NHA) Pd(II) complex 3•PF has been synthesized via a facile two-step transmetalation reaction of NHA precursor 2-ethynyl-1,3-dimethyl-1H-imidazol-3-ium hexafluorophosphate (1•PF) and a pincer type [(C^N^N)PdCl] (HC^N^N = 6-phenyl-4-(3,4,5-tris(dodecyloxy)phenyl)-2,2'-bipyridine) 2 with long alkyl chains in the presence of excessive amount of AgO. It is stable towards air, moisture and common organic solvents. The purity of the cationic NHA Pd(II) complex 3•PF was characterized via NMR, HR-MS. And IR spectra revealed its resonance structure between zwitterionic acetylide and N-heterocyclic allenylidene to some extent. Solid state complex 3•PF showed orange emission with microsecond lifetime at both room temperature and 77 K. Temperature-dependent PXRD and TGA/DSC analysis demonstrated the changes of complex 3•PF from an amorphous phase to a stable crystalline one upon raising the temperature. Due to the lipophilic property of the long alkyl chains, ionic complex 3•PF can be soluble in both polar (such as dimethyl sulfoxide (DMSO), CHCN, and dichloromethane) and nonpolar (such as cyclohexane, methylcyclohexane, and decalin) organic solvent, despite its ionic nature. Complex 3•PF in degassed dilute DCM solution showed rather weak photoluminescence with peak maximum at 562 nm (QY < 1%, τ: 17 ns). By increasing the volumetric ratio of cyclohexane which lowered solvent polarity, UV-vis absorption spectra of 3•PF in mixed solvent of DCM/cyclohexane showed a distinct low-energy absorption band with peak maximum at approximately 500 nm, and the corresponding phosphorescence emerged. The NMR spectra recorded in mixed solvent deuterium substituted DCM/cyclohexane also showed shifted H chemical-shift and broaden peaks by increasing the volumetric ratio of cyclohexane, which revealed the deeper degree of aggregation of 3•PF by decreasing the solvent polarity. Meanwhile, enhancement of red-shifted photoluminescence reached its largest absolute QY of 35.4% with emission peak maximum at 612 nm and lifetime of 730 ns in pure cyclohexane. The photophysical data recorded for 3•PF aggregates in other nonpolar solvents (such as toluene and decalin) showed similar broad and featureless phosphorescent emission band with peak maximum at around 620 nm. The origin of the phosphorescence for 3•PF aggregates in nonpolar solvents can be ascribed to MMLCT enabled by the metallophilic Pd•••Pd interactions. To further explore the self-assembly mode of 3•PF in nonpolar solvents, the corresponding temperature-dependent UV-vis absorption spectra have been recorded. The distinct low-energy absorption bands represented the aggregation degrees of the 3•PF monomer in the fluid state. The plot of variable-temperature UV-vis absorbance at the distinct low-energy band (peak maximum at 504 nm) against temperature well-fits the nucleation-elongation model for the self-assembly of 3•PF in nonpolar solvent (such as methylcyclohexane and decalin). In conclusion, we have synthesized an ionic Pd(II) N-heterocyclic allenylidene complex with lipophilic long alkyl chains and manipulated its self-assembly behavior in nonpolar solvents by controlling the polarity and temperature. Corresponding MMLCT phosphorescence was also recorded and correlated with metallophilic Pd•••Pd interactions in the aggregates.