Ln3+-enhanced blue fluorescence from novel excimer of 1,8-naphthalimide-conjugated PAMAM

In this article, the 1,8-naphthalimide-conjugated polyamidoamine (PAMAM) [Dn (n = 3, 4, and 5)] was synthesized by the modification reaction of PAMAM with 1,8-naphthalimide (1,8-NI). The results show that the 1,8-NI groups around the periphery of PAMAM emit strong blue fluorescence at 476 nm and this emission can be enhanced by Ln3+ ions (e.g., La3+, Eu3+, and Tb3+). Compared with the emission of 1,8-NI solution at 386 nm, the enhanced fluorescence at 476 nm by Ln3+ ions in Dn was emitted from the excimer of 1,8-NI groups around the periphery of PAMAM dendrimer. In this system, Ln3+ ions were not emission centers via f–f transitions in organic lanthanide complexes yet, whereas they are the efficient inhibitors of photo-induced electron transfer (PET) between 1,8-NI and nitrogen atoms in PAMAM skeleton. Among used Ln3+ species, La3+ has the strongest enhancement effect. The characteristic emission bands for Eu3+ and Tb3+ were not found in this study. As comparison, all the transition metals, such as Fe3+, Co2+, Cu2+, and Zn2+, showed the quenching effect on 1,8-NI excimer emission. This novel blue fluorescence material will be expected for the potential application for excellent basic color polymeric material.