Molecular and Crystalline Requirements for Solid State Fluorescence Exploiting Excited State Intramolecular Proton Transfer

Aggregation induced emission offers a route to the development of emissive technologies based on solely organic systems. However, maximising fluorescence quantum efficiencies (QE) is a formidable challenge in attaining first-principles materials design, due to the interplay between the electronic structure of the chromophore and the molecular crystal. The identification of radiative and nonradiative channels, and how these are affected by aggregation, can rationalise the emissive properties of materials and aid in the design of yet more efficient fluorophores. In the current work, we examine the mechanism behind aggregation induced emission in two related families of compounds with lasing properties, which undergo excited state intramolecular proton transfer (ESIPT). We systematically investigate competing excited state decay channels in a total of eleven crystals to evaluate the factors needed for efficient ESIPT fluorophores, aided by a full evaluation of the crystal structures, exciton coupling, and exciton hopping rates. We show that in addition to the restriction of nonradiative pathways, an efficient ESIPT is essential to maximise the QE in the solid state. This extensive study of structure-property relationships for fluorophores based on the ESIPT mechanism bridges the understanding of molecular photophysics with crystal structure, accelerating the development of highly efficient solid state emitters.