These concepts have been reported in the field of fluorescence spectroscopy, leading to the establishment of a theory known as the red-edge effect (REE) that operates when a subset of the chromophores in a bulk solution can be selectively excited with longer wavelengths of light. The concept of selective excitation can be more simply understood by separating the selection criteria into two categories. The first is the excitation of differing ground state conformations, where heterogeneous distributions in solvation result in a range of (micro)states with varied energy differences. Here, longer wavelengths of light can be used to selectively excite those conformers (chromophore and solvent) with smaller differences in ground and excitedstate energies. The second criteria for site selection considers the process of solvent relaxation surrounding a chromophore immediately after photoexcitation. In a vacuum, there are no external electrostatic forces from a solvent that are acting upon an isolated chromophore, as seen in Figure , with complete homogeneity observed by identical chromophores and wavelength-independent fluorescence profiles observed. When a solvent is present, the interactions between chromophore and solvent minimises the energy of the ground and excited states, shown in Figure . For the situation when this energy is truly minimised, the chromophore is referred to as being in a "relaxed" state, seen in S0 for the case of Microenvironment A (Figure bottom). Being the lowest in energy, this is typically the most abundant state in a bulk solution, with each microenvironment tending towards this state. As many polar chromophores have a difference in ground and excited-state dipole moments (magnitude and/or direction), these microenvironments that are relaxed in S0, are no longer relaxed in S1 directly after photoexcitation (Figure In a solvent, Microenvironment B does not minimise the energy in S0; however, this energy is minimised in S1 after photoexcitation. The solvent relaxation time, 𝜏 R , is the timescale at which solvent molecules take to minimise the energy of the system after transitions between S0 and S1. d) The effect on the absorbance spectrum is seen for a small proportion of the absorption events in the tail due to the cases in Microenvironment B. e) The effect on fluorescence emission is that emissions occurring from Microenvironment B have emissions at longer wavelengths.