  We propose the heterojunction effect in the analysis of the fluorescence
mechanism of the firefly chromophore. Following this analysis, and with respect
to the HOMO-LUMO gap alignment between the chromophore's functional fragments,
three main heterojunction types (I, II, and I*) are identified. Time-dependent
density-functional theory optical absorption calculations for the firefly
chromophore show that the strongest excitation appears in the deprotonated
anion state of the keto form. This can be explained by its high HOMO-LUMO
overlap due to strong bio-heterojunction confinement. It is also found that the
nitrogen atom in the thiazolyl rings, due to its larger electronegativity,
plays a key role in the emission process, its importance growing when HOMO and
LUMO overlap at its location. This principle is applied to enhance the
chromophore's fluorescence efficiency and to guide the functionalization of
molecular optoelectronic devices.
