Department of Chemistry, Graduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-8531, Japan.
Chemistry. 2013 Jul 15;19(29):9497-505. doi: 10.1002/chem.201301087. Epub 2013 Jun 18.
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.
描述了一种通过形态控制来实现强固态磷光的方法，这种方法适用于原本不发光但具有潜在发光性的铂配合物非晶态晶体。我们合成了一系列带有线性烷基链的反式-双（水杨醛亚胺）铂（II）配合物（1a：n=5；1b：n=8；1c：n=12；1d：n=14；1e：n=16；1f：n=18），并通过在各种沉淀条件下制备晶体/聚集体来研究其固态发光性质。晶体 1e 的晶体，使用包括快速冷却、高浓度和不良溶剂在内的“动力学”条件制备，在 298 K 下用 UV 辐射照射时发出强烈的黄色磷光（λ（最大值）=545nm），绝对量子效率为 0.36，而所有使用包括缓慢冷却、低浓度和良好溶剂在内的“热力学”条件制备的晶体 1a-1f 均为非发光或发光较弱，在相同的测量条件下，其 298 K 时的Φ值分别为 0.12（1a）、0.11（1b）、0.10（1c）、0.07（1d）、0.02（1e）和 0.02（1f）。通过快速冷却和冷冻干燥制备的无定形固体 1e 也不发光（Φ（298 K）=0.02，0.02）。温度依赖的发射光谱表明，动力学晶体 1e 在温度升高时对发射衰减表现出高耐热性，而无定形固体 1e 则完全可以被热猝灭。这是通过晶体工程的形态控制，将非发光晶体转变为高度发光晶体的罕见实例。发光的动力学晶体 1e 的发射光谱和粉末 X 射线衍射（XRD）图谱明显不同于那些发光较弱的热力学晶体 1a-1f。单晶 XRD 明确表明，热力学晶体 1d 具有由高度有序的连续π堆积相互作用支撑的多层层状结构，而动力学晶体 1e 具有面对面的层状结构，堆积较少。这些结果得出以下结论：1）长链配合物的形态控制专门产生一种亚稳的人字形层状堆积模式，表现出强烈的发光和高热稳定性，而 2）热力学稳定、高度有序的连续堆积模式不利于固态发光。
