Luminescent liquid crystalline materials based on palladium(ii) imine derivatives containing the 2-phenylpyridine core

Abstract

In this work we report our studies concerning the synthesis and characterisation of a series of imine derivatives that incorporate the 2-phenylpyridine (2-ppy) core. These derivatives were used in the cyclometalating reactions of platinum(II) or palladium(II) in order to prepare several complexes with liquid crystalline properties. Depending on the starting materials used as well as the solvents employed, different metal complexes were obtained, some of them showing both liquid crystalline behaviour and luminescence properties at room temperature. It was found that, even if there are two competing coordination sites, the cyclometalation process takes place always at the 2-ppy core with (for Pt) or without (for Pd) the imine bond cleavage. We successfully showed that it is possible to prepare emissive room temperature liquid crystalline materials based on double cyclopalladated heteroleptic complexes by varying the volume fraction of the long flexible alkyl tails on the ancillary benzoylthiourea (BTU) ligands.

Structural, Luminescent and Thermal Properties of Heteronuclear PdII–LnIII–PdII Complexes of Hexadentate N2O4 Schiff Base Ligand

“…The absorption spectra of the synthesized Pd II –Ln III –Pd II complexes with H 4 L are characterized by two absorption maxima in the ultraviolet region between 210–290 nm and an additional small band centered at approximately 375 nm, as shown in Figure S9. The higher-energy bands are attributable to the intra-ligand transitions while the small bands could be assigned to metal-to-ligand charge transfer or a mixture of metal-to-ligand charge transfer and ligand-centered transitions [22,23,24]. No absorption bands due to f-f transitions were observed in the absorption spectra of these compounds due to the fact that the f-f transitions in Ln III ions are very weak [25].…”

An overview of phosphorescent metallomesogens based on platinum and iridium

“…Finally, the preparation of some luminescent LC Schiff base complexes of Pd and Pt by Cîrcu and co-workers is noted, although there was no detail concerning the nature of the excited states neither the efficiency of emission. [62][63][64][65][66] Pt complexes with bidentate N^N ligands Bidentate, anionic N^N ligands derived from, for example, 2-(pyrazol-3-yl)pyridines or 2-(1,2,3-triazol-4-yl)pyridine, can lead to highly luminescent cyclometallated platinum complexes, 67 with the azido nitrogen increasing the ligand field by virtue of being a strong -donor. Pyrazolylfunctionalised pyridines turn out to be especially popular, [68][69][70][71][72][73][74][75] so that complex 18a (Figure 13) with tris(alkoxy)phenyl groups, prepared by Hsu et al, 68 exhibited columnar mesophases with very wide temperature ranges.…”

Playing with PtII and ZnII Coordination to Obtain Luminescent Metallomesogens

“…on Ag I[16] and Zn II , Cârcu et al. on Pd II , Cano et al. on Ag I[21–22] and Pucci, Crispini and Ghedini et al.…”

“…Luminescent d-block metallomesogens are promising functional materials for practicala pplications.T hey have mainly been investigated in terms of obtaining polarised emission to be used as backlighting in displays, [1][2][3][4] but also some studies concerning their use as stimuli responsive materials for sensing [5,6] or security inks, [7] or high-hole mobility materials for electroluminescent devices [8] have been reported.D espite recentp rogress with square-planar Pt II and octahedral Ir III complexes, which are considered among the best emitters, there is still considerable work to be done to control the phase type and transition temperatures or to improvet he luminescence in the mesophase. [9] In fact, only af ew studies on the mesophases of luminescent liquid-crystalline transition metal complexes have been carried out, with, just to cite non-exhaustively,t he contributions of Bruce et al on Ir III , [8,[10][11][12] Espinet et al [13] and Tsutsumi et al [14,15] on Au I ,G imenØnez et al on Ag I [16] and Zn II , [17] Cârcu et al on Pd II , [18][19][20] Cano et al on Ag I [21][22] and Pucci, Crispini and Ghedini et al on Ru II , [23] Ag I , [24][25][26] Ir III [27] and Zn II [28,29] complexes. For practical applications,t echnological and economic issueshave to be taken into account,such as thermaland optical stabilities, availability and cost of the precursors and straightforward syntheses.…”