Several recent studies of carbon dots (CDs) synthesized by bottom-up methods under mild conditions have reported the presence of organic molecular fluorophores in CD dispersions. These fluorophores have a tendency to aggregate, and their properties strongly depend on whether they are present in the form of discrete molecules or aggregates. The aggregation becomes more prominent in the solid state, which motivates the study of the properties of the fluorophores associated with CDs in the solid state. Here, we report the solid-state characterization of N⁴,N¹¹-dimethyldibenzo[a,h]phenazine-4,11-diamine (BPD) – a molecular fluorophore that forms CDs. Discrete BPD molecules show excitation-wavelength-independent photoluminescence (PL) emission in the green wavelength region at ∼520 nm. However, additional blue PL is also observed due to aggregation, making the PL emission significantly broad. For detailed studies, BPD is mixed in different solid matrices, and it is observed that the PL quantum yield (PLQY) of BPD films strongly depends on the concentration of BPD in the solid matrices. Increasing the concentration of BPD results in a considerable decrease in the PLQY. The PLQY of the films with an optimum concentration of BPD is 75.9% and 40.2% in polymethyl methacrylate and polystyrene, respectively. At higher concentrations, these PLQY values decrease to ∼11%. The significant decrease in the PLQY is ascribed to reabsorption and nonradiative exciton decay that is facilitated by BPD aggregation at higher concentrations. Finally, light-emitting devices (LEDs) were fabricated with almost pure white emission color, having CIE (International Commission on Illumination) coordinates of (0.35, 0.37) using BPD in the color-converting layer of blue-pumped LEDs. The device shows a luminous efficiency 3.8 lm W⁻¹ and luminance of 43 331 cd m⁻².

中文翻译：

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用于发光器件的碳点衍生分子荧光团的高固态光致发光量子产率

最近对在温和条件下通过自下而上方法合成的碳点（CD）进行的几项研究报告了 CD 分散体中存在有机分子荧光团。这些荧光团有聚集的倾向，它们的特性很大程度上取决于它们是否以离散分子或聚集体的形式存在。聚集在固态中变得更加突出，这激发了对与固态 CD 相关的荧光团特性的研究。在这里，我们报告了N ⁴ , N ¹¹ -二甲基二苯并[ a , h ]吩嗪-4,11-二胺 (BPD)（一种形成 CD 的分子荧光团）的固态表征。离散 BPD 分子在~520 nm 的绿色波长区域显示出与激发波长无关的光致发光 (PL) 发射。然而，由于聚集，还观察到额外的蓝色 PL，使得 PL 发射范围显着扩大。详细研究中，BPD 混合在不同的固体基质中，观察到 BPD 薄膜的 PL 量子产率 (PLQY) 强烈依赖于固体基质中 BPD 的浓度。增加 BPD 浓度会导致 PLQY 显着降低。具有最佳BPD浓度的薄膜在聚甲基丙烯酸甲酯和聚苯乙烯中的PLQY分别为75.9%和40.2%。在较高浓度下，这些 PLQY 值降低至 ∼11%。 PLQY 的显着降低归因于重吸收和非辐射激子衰减，这是由较高浓度的 BPD 聚集促进的。最后，在蓝色泵浦 LED 的颜色转换层中使用 BPD，制造出具有几乎纯白色发射颜色的发光器件 (LED)，其 CIE（国际照明委员会）坐标为 (0.35, 0.37)。该器件显示出3.8lm W ^-1的发光效率和43 331 cd m ^-2的亮度。