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A new series of C-6 unsubstituted tetrahydropyrimidines 6 have been directly synthesized via a convenient urea-catalyzed chemoselective five-component reaction (5CR) under mild conditions. Compounds 6 show typical aggregation-induced emission enhancement (AIEE) characteristics because they are practically no emissive in solution but emit blue or green fluorescence in aggregates with fluorescence yield up to 93%. One of the 5CR products, 6aa, exhibits blue- and green-fluorescence aggregates (bf- and gf-aggregates). The bf- and gf-aggregates are prepared under different conditions and proved to result from different J-aggregations by single-crystal X-ray analysis. In addition, the bf- and gf-aggregates of 6aa show unusual size-independent emission (SIE) characteristics because their maximum emission wavelengths in different sizes (suspension particles, film, powder and crystals) are the same, 434 and 484 nm, respectively. Based on the obtained experimental results, the 5CR mechanism, the origins of AIEE and SIE characteristics are discussed. |
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