However, shortly after EDC addition, the fluorescence of the solution shifts bathochromically to sky-blue, indicating that the Pyr2AcP excimer is dominant at steady state (Figure , Video S3). Dimers or oligomers from adipic acid were not observed (HPLC-MS). Excimer emission reached its maximum after ca. 40-60 min, represented by the broad band at around 485 nm in the fluorescence spectra of Figure and S10. This transient behavior is also reflected in the photoluminescence quantum yield (21-23 % after 60 minutes and 9-11 % before fuel addition, when excited at 340 nm) of the system that is continuously and predictably changing over time upon addition of the fuel (Figure ). Over the four investigated cycles, we found that the ratio of monomer emission (Imax mon ) to excimer emission (Imax exc ) is almost invariant, see inset in the upper right corner of Figure . Leaving aside the aforementioned issues with N-acylurea byproducts, 84 this good reproducibility upon refueling showcases that the acylphosphate reaction cycle is not only robust, but also highly versatile in respect to different types of emergent behavior.