Recently, molecular fluorophores were shown to be formed in the bottom-up chemical synthesis, contributing to the emission of carbon dots (CDs), derived from a citric acid precursor. We applied an ammonothermal synthesis toward CDs, employing two reactants citric acid and supercritical ammonia functioning as both solvent and precursor. The resulting nanoparticles are identified as amorphous aggregates of molecular fluorophores based on citrazinic acid derivatives, which resemble many of the emission features typically reported to be characteristic for CDs. The aggregates absorb and emit at short and long wavelengths of the spectrum, a feature prior ascribed to intrinsic CD core and surface states, respectively. We identify three emission states: a high energy and a low energy aggregate state as well as an energy transfer state between both. Energy transfer is triggered only upon excitation within a narrow high energy spectral range, resulting in a characteristic blue-green double emission. The high energy aggregate state exhibits a trapping mechanism elongating emission lifetime. To further analyze aggregated molecular fluorophores, we studied aqueous solutions and films of citrazinic acid and polyvinylpyrrolidone and demonstrated their concentration dependent optical behavior. Since fluorophore aggregates reproduce the emissive features of CDs, the contribution of sp²/sp³ carbonized products and graphitic domains to the emission features of CDs must be carefully evaluated in future studies.

中文翻译：

---

碳点氨热合成中的聚集分子荧光团

最近，分子荧光团被证明是在自下而上的化学合成过程中形成的，有助于从柠檬酸前体衍生的碳点（CDs）的发射。我们对CD进行了氨热合成，采用柠檬酸和超临界氨这两种反应物作为溶剂和前体。所得的纳米颗粒被鉴定为基于瓜氨酸衍生物的分子荧光团的无定形聚集体，其类似于通常据报道具有CD特有的许多发射特征。聚集体在光谱的短波长和长波长处吸收和发射，这是先前分别归因于固有CD核和表面态的特征。我们确定了三种发射状态：高能量和低能量聚集状态以及两者之间的能量转移状态。仅当在狭窄的高能谱范围内激发时才触发能量转移，从而导致特征性的蓝绿色双发射。高能量聚集态表现出俘获机制，延长了发射寿命。为了进一步分析聚集的分子荧光团，我们研究了瓜氨酸和聚乙烯吡咯烷酮的水溶液和薄膜，并证明了其浓度依赖性的光学行为。由于荧光团聚集体可复制CD的发射特征，因此sp的贡献 我们研究了瓜氨酸和聚乙烯吡咯烷酮的水溶液和薄膜，并证明了其浓度依赖性的光学行为。由于荧光团聚集体可复制CD的发射特征，因此sp的贡献 我们研究了瓜氨酸和聚乙烯吡咯烷酮的水溶液和薄膜，并证明了其浓度依赖性的光学行为。由于荧光团聚集体可复制CD的发射特征，因此sp的贡献CD的² / sp ³碳化产物和石墨域的排放特征必须在以后的研究中仔细评估。