5,11-Bis(phenylethynyl)benzo[1,2-f:4,5-f′]diisoindole-1,3,7,9(2H,8H)-tetraone 1H was designed as an application of the theoretical design principle for fluorescent molecules which is derived from the vibronic coupling density analysis. For solubility reasons, tertiary-butylated 1H, 2,8-di-tert-butyl-5,11-bis(phenylethynyl)benzo[1,2-f:4,5-f′]diisoindole-1,3,7,9(2H,8H)-tetraone 1 was synthesized and its fluorescence properties were measured. It is found that the photoluminescence quantum yield of 1 was 96%. We discuss the rationale for designing 1H as a highly efficient fluorescent molecule, and compare the theoretical calculations for 1 with the observed absorption and photoluminescence spectra.