A Theoretical Analysis of the Potential Role of π–π Stacking Interactions in the Photoprotolytic Cycle of Firefly Luciferin

Firefly oxyluciferin is a photoacid that presents a pH‐sensitive fluorescence, which results from pH‐dependent changes on the conformation of self‐aggregated π–π stacking complexes. Luciferin is a derivative of oxyluciferin with very similar fluorescence and photoacidic properties. This similarity indicates that luciferin is also expected to be able to form π–π stacking complexes, but no pH‐sensitive fluorescence is found for this compound. Here, a theoretical approach is used to rationalize this finding. We have found that luciferin only forms π–π stacking complexes in the ground state at acidic pH. At basic pH and in the excited state, luciferin is present as a dianion. This species is not able to self‐aggregate, owing to repulsive electrostatic interactions. Thus, this emissive species is not subject to π–π stacking interactions; this explains its pH‐insensitive fluorescence.