4- Tert-butyl-1-(4'-dimethylamino-benzylideneamino)pyridinium perchlorate (BDPP): a novel fluorescent dye

4- Tert-butyl-1-(4'-dimethylaminobenzylideneamino)pyridinium perchlorate (BDPP) is a novel dye showing unusual properties, viz. (i) it gives a yellow-green fluorescence which quenched on heating and in solution, and (ii) by heating at 180° or by simple squeezing, it is transformed into orange, non-fluorescent crystals (i.e. it is piezochromic). Surprisingly, among other 1- (benzylideneamino) pyridinium salts, BDPP was the only dye revealing the above properties.