Novel PAMAM light-harvesting antennae based on 1,8-naphthalimide: Synthesis, energy transfer, photophysical and pH sensing properties

This work presents the design, convergent synthesis and spectral characteristics of novel PAMAM dendrons, core and peripherally functionalized with 1,8-naphthalimide fluorophores. The novel compounds were configured as light-harvesting antennae where the system surface is labeled with a different number of blue-emitting 4-allyloxy-1,8-naphthalimide "donor" fluorophores capable of absorbing light and efficiently transferring the energy to a single yellow-green emitting 4-alkylamino-1,8-naphthalimide "acceptor" dye. The overlap between the blue emission of the donor periphery and the absorption of the acceptor (focal dye) was more than 95%. Novel light-harvesting systems showed highly efficient energy transfer (91-94%). Nevertheless fluorescence quenching of the core chromophore was observed, which is related to a possible photoinduced electron transfer (PET) from the 4-alkoxy-1,8-naphthalimide periphery towards the focal 4-alkylamino-1,8- naphthalimide. Novel PAMAM light-harvesting systems were also configured in a "fluorophore-spacer-receptor" format, and as such, displayed sensitive fluorescence signaling over a wide pH scale, which is attributed to a PET process from the tertiary amines in the dendron bone to the fluorophores. This assigns the synthesized light-harvesting antennae to potential use as highly efficient pH chemosensing materials.