Where is the Best Substitution Position of Amino Groups on Carbon Dots: A Computational Strategy toward Long-Wavelength Red Emission

Carbon dots (CDs) are a promising fluorescent material in biological and optoelectronic applications owing to their high biocompatibility, low cytotoxicity, and high stability. Red emission CDs are not only widely used in biological imaging due to strong tissue penetration, but also improve the white LEDs’ Color Rendering Index. However, the structure–property relationship of CDs is still confusing, and red-emissive CDs are difficult to be fabricated in experiments. In this study, we reveal the relationship between charge transfer and photoluminescence of amino-functionalized CDs through DFT and TDDFT calculation. It is found that the charge transfer has an obvious correlation to the hole contribution of amino groups. Based on the study of amino-substituted position, it is concluded that nucleophilic substitution positions are highly critical for obtaining long-wavelength red emission CDs (LWRE-CDs), which can be prejudged by the calculation of atomic charges. Moreover, the universality of these correlations is demonstrated in different systems composed of polycyclic aromatic hydrocarbons (anthracene, pyrene, tetracene, and phenanthrene) and electron-donating groups (–NH2, –N(CH3)2, –OH, –OCH3, and –CH3). This work provides a theoretical guidance for the preparation of high-performance LWRE-CDs.