Aggregation and ESIPT together inspired naked-eye fluorescence color switching of new π-conjugated dyes carrying double proton transfer segments

New conjugated dye C1 (2, 2′-((5-(2-(4-methoxyphenyl)ethenyl)-benzene-1, 3-diyl)-bis-(nitrilomethylylidene))-diphenol) carrying double proton transfer segments and its reference C2 (N, N′-(5-(2-(4-methoxyphenyl)-ethenyl)-benzene-1, 3-diyl)-bis-(1-phenylmethanimine)) were synthesized. The SEM and TEM images showed that cubic-shaped aggregates of C1 were formed with sizes of 300-500 nm in THF/H2O mixed solution at 12 h. The spectral properties of aggregation states of the molecules in different ratios of THF/H2O were further investigated. The fluorescence emission spectra under different aggregation time intervals indicated that C1 exhibited remarkable aggregation induced emission enhancement. The keto emission of C1 showed an increase firstly and then a decrease with increasing aggregation time. Hence, the enhanced fluorescence color switching from bright yellow to pure blue under UV lamp was observed by our naked eyes. In contrast, C2 could not show obvious AIEE properties under identical experimental conditions. This is the first successful attempt to achieve naked-eye fluorescence chromism of new organic chromophores inspired by aggregation and ESIPT together.