A series of excited-state intramolecular proton transfer (ESIPT) poly(amido amine) (PAMAM) dendrimers with 4-(benzo[d]thiazol-2-yl)-3-hydroxyphenyl modified at the periphery were synthesized (G0—G4). Functionalization of PAMAM dendrimers was performed through the condensation reaction of dendrimer peripheral amine units and carboxylic groups in DMSO catalyzed by 1-ethyl-3-(3-dimethyllaminopropyl) carbodiimide hydrochloride (EDC). The structures of synthetic compounds G0—G4 were characterized by IR, ¹H NMR, ¹³C NMR, and MS. The average functionalization extents of the peripheral amine groups were 99%, 88%, 80%, 83%, and 78% for G0—G4, respectively. The solubility of functionalized dendrimers is dramatically affected by peripheral groups and the dendritic backbone. Model compound dissolves in THF homogeneously, exhibiting enol emission only due to the freely intramolecular rotation and consequently few ESIPT process occurrence. On the contrary, dendrimers form aggregates in THF and present keto emission of ESIPT chromophores, which increases with the dendrimer generation increasing from 0 to 2 and decreases in generation 3 and 4, indicating that the emission of ESIPT chromophores is affected by the PAMAM backbone and peripheral units. From G0 to G2, the increasing congestion of aggregates restrains the intramolecular rotation of ESIPT chromophores, which facilitates the ESIPT process and results in more keto emission. However, a considerable enol emission presents in G3 and G4 because larger dendritic backbone provides more free space and higher polarity microenvironment. The protonated dendrimers, G1-H—G4-H, are soluble in water and form aggregates with a diameter of ca. 20 nm demonstrated by dynamic light scattering experiments. TEM image of aggregates was also obtained revealing vesicle like assemblies. The absorption of ESIPT groups is not affected by protonation of dendritic backbones giving typical absorption of the ESIPT chromophore. The ESIPT chromophore exhibits keto emission exclusively due to the restricted conformation by confined aggregation microenvironments in water, and the emission intensity is slightly affected by dendritic backbones. The lower emission intensity of G4-H can be attributed to the loose microenvironment in the G4-H assembly caused by the larger and flexible dendrimer backbone.

Key words: PAMAM dendrimer, excited-state intramolecular proton transfer (ESIPT), photophysics