The Molecular Origin of Perylene Fluorescence in Gas, Solution and Crystal Phases: A Comparative Theoretical Study

Abstract Perylene is a widespread molecular motif in materials science thanks to its phase-dependent optical response and its good excitation transport properties, making it ideal for many optoelectronic applications. However, the molecular-level understanding of how changing the aggregation phase determines the optical modulation remains elusive. This work addresses this gap using density functional calculations at ground and excited state. Using these data we present the systematic assignment of experimental spectra in dilute solutions, dimeric aggregates and the α-polymorph to the corresponding molecular structures through a comparison to simulated spectra obtained with the inclusion of two elements with pronounced interplay: (i) explicit vibrational effects, arising from strong coupling between electronic structure and molecular vibrations, and (ii) stacking of monomers in aggregates, to give dimers as fundamental optical units. Incorporating these elements yields accurate absorption and emission spectra and reveals the central role of the dimer and of its different stacking configurations in the modulation of the spectra observed in the crystal. The detailed structural data generated allow us to hypothesize a coherent mechanism able to explain the dual emission of α-perylene as based on the competition of two minima on S1, ultimately identified as responsible for the well-known “Y” and “E”-fluorescence.