On the other hand, the radiative relaxation of 3.1 is faster than any backward conformational changes, thus ruling out the possibility of reversible processes that would eventually achieve the equilibrium distribution among different populations. Therefore, the fate of pyrene excimer appears to be kinetically controlled, and the initial variety of clusters 1-8 is eventually funneled into a single fluorescence emission channel (Figure ). More precisely, the typical fluorescence response is determined by the most stable conformation of the excited state dimer. We also confirmed that 3.1 fulfills the requirement to provide an optimized geometry only in the excited state. A somewhat different structure is obtained if it is relaxed on the ground state, as would occur after the emission of a photon. This difference between the two PESs involved in the transition is responsible for the broadening and the absence of any vibronic patterns in the fluorescence band of the excimer 10 (see Figure ), as the transition dipole auto-correlation function 〈𝜇 𝑆 1 𝑆 0 (𝑅(𝑡)) 𝜇 𝑆 1 𝑆 0 (𝑅(𝑡 0 ))〉 exhibits a very rapid decay when the PES governing nuclear dynamics, 𝑅(𝑡), is switched to the ground state one. As expected from the localized nature of their S0 -S1 transition, orthogonal conformers, represented in cluster 9, give rise to an emission profile in the monomer's region of experimental spectra (Figure ). Its vibronic substructure could be resolved following a path integral formalism implemented in the ORCA software that exploits the harmonic oscillator approximation for vibrational normal modes. The close resemblance between the predicted and experimental spectra suggests that these weakly bound structures may contribute to the final spectrum, hidden under the typical emission of the pyrene monomer.