In this paper we report on a novel fluorescent core skeleton, 9-aryl-1,2-dihydropyrrolo[3,4-b]indolizin-3-one, which we named Seoul-Fluor, having tunable and predictable photophysical properties. Using a concise and practical one-pot synthetic procedure, a 68-member library of new fluorescent compounds was synthesized with diverse substituents. In Seoul-Fluor, the electronic characteristics of the substituents, as well as their positional changes, have a close correlation with their photophysical properties. The systematic perturbation of electronic densities on the specific positions of Seoul-Fluor, guided with the Hammett constant, allows emission wavelength tunability covering the full color range. On the basis of these observations and a computational analysis, we extracted a simple first-order correlation of photophysical properties with the theoretical calculation and accurately predicted the emission wavelength of Seoul-Fluors through the rational design. In this study, we clearly demonstrate that Seoul-Fluor can provide a powerful gateway for the generation of desired fluorescent probes without the need for a tiresome synthesis and trial-and-error process.