Morphology control for intense solid-state phosphorescence of non-emissive, but potentially emissive crystals of platinum complexes and the mechanistic rationale are described. A series of trans-bis(salicylaldiminato)platinum(II) complexes bearing linear alkyl chains (1a: n=5; 1b: n=8; 1c: n=12; 1d: n=14; 1e: n=16; 1f: n=18) was synthesized and the solid-state emission properties were examined by using crystals/aggregates prepared under various precipitation conditions. Crystals of 1e, prepared using "kinetic" conditions including rapid cooling, high concentrations, and poor solvents, emit intensive yellow phosphorescence (λ(max)=545 nm) under UV irradiation at 298 K with an absolute quantum efficiency of 0.36, whereas all the crystals of 1a-1f prepared using "thermodynamic" conditions including slow cooling, low concentrations, and good solvents were either non- or less emissive with Φ(298K) values of 0.12 (1a), 0.11 (1b), 0.10 (1c), 0.07 (1d), 0.02 (1e), and 0.02 (1f) under the same measurement conditions. The amorphous solid 1e, prepared by rapid cooling and freeze-drying, was also non-emissive (Φ(298K)=0.02, 0.02). Temperature-dependent emission spectra showed that the kinetic crystals of 1e exhibit high heat-resistance towards emission decay with increasing temperature, whereas the amorphous solid 1e is entirely heat-quenchable. This is a rare example of the change from a non-emissive crystal into a highly emissive crystal by morphology control through crystal engineering. Emission spectra and powder X-ray diffraction (XRD) patterns of the emissive, kinetic crystals of 1e are clearly distinct from those of the less emissive, thermodynamic crystals of 1a-1f. Single-crystal XRD unequivocally establishes that the thermodynamic crystals of 1d have a multilayered lamellar structure supported by highly regulated, consecutive π-stacking interactions between imine moieties, whereas the kinetic crystals of 1e have a face-to-edge lamellar structure with less stacking. These results lead to the conclusion that 1) morphology control of long-chained complexes exclusively generates a metastable herringbone-based lamellar packing motif that exhibits intense emission and high heat-resistance, while 2) a thermodynamically stable, highly regulated, consecutive stacking motif is unfavorable for solid-state emission.
The catalytic enantioselective rotation of double-paddled, watermill-shaped palladium complexes is described in this report. The achiral syn-and chiral anti-forms of binuclear transbis(salicylaldiminato)palladium(II) complexes doubly linked with polymethylene bridges (1) were synthesized and subsequently characterized by spectroscopic and single crystal XRD analyses. The heptamethylene-linked complex 1 b undergoes highly controllable rotation between the achiral syn and chiral antiforms at ambient temperature, while the hexamethylene-linked analogue 1 a does not exhibit this rotational motion under the same conditions, due to its shorter bridges, and 1 c (with octamethylene linkers) undergoes uncontrollable rapid rotation even at lower temperatures. The kinetics of the dynamic rotation of syn-1 b to the anti-isomer in solution were examined on the basis of 1 H NMR analyses. The data showed that the rotational rate can be greatly modified by varying the solvent or adding a carboxylic acid. The enantioselective rotation of syn-1 b can be induced using a catalytic amount of (2R,3R)-2,3bis(3,5-dimethylbenzoyl)tartaric acid (2 a) to afford (R)-anti-1 b, with an initial enantiomeric excess of 63 % ee and an equilibrium value of 27 % ee. Curve fitting of data for the parallel reversible transformations of syn-1 b to (R) and (S)-anti-1 b gave the forward and reverse first-order rate constants k 1 , k À 1 (for syn to (R)-anti), k 2 and k À 2 (syn-to (S)-anti), and verified that the enantioselective nature of the present catalytic enantioselective rotation is controlled kinetically during the initial stage (k 1 > k 2 ) and thermodynamically in the equilibrium state (k 1 /k À 1 > k 2 /k À 2 ). The specific changes induced in the transition states in response to the use of various solvents and adducts are discussed herein on the basis of the activation parameters for the rotation of syn-1 b, as determined from plots of the kinetic data as functions of time.
… of trans-bis(salicylaldiminato)PtII complexes bearing long alkyl chains can change typically non-emissive crystals into highly emissive crystals. Crystals prepared using "kinetic" conditions, including rapid cooling, high concentrations and poor solvents, emit intense yellow phosphorescence under UV illumination at an ambient temperature, whereas crystals prepared using ordinary thermodynamic conditions were non-or less emissive. Powder and single-crystal XRD analysis of various crystalline polymorphs revealed that crystal engineering from a typical stacking molecular array to face-to-edge packing motif is a key for the OFF-ON control of solid-state emission. For more details, see the Full Paper by T. Naota et al. on page 9497 ff.
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