Aggregation-induced emission (AIE) offers a route for the development of luminescent technologies with high quantum efficiencies. Excited-state intramolecular proton transfer (ESIPT) coupled to AIE can produce devices with emission across the visible spectrum. We use a combination of theoretical models to determine the factors that mediate fluorescence in molecular crystals undergoing ESIPT. Using two materials based on 2'-hydroxychalcone as exemplar cases, we analyze how inter- and intramolecular processes determine the emissive properties in the crystal environment. This systematic investigation extends the current interpretation of AIE to polar chromophores with multiple decay pathways. We find that population of nonradiative pathways is dictated by the electronic effects of the substituents and the degree of distortion allowed in the crystal environment. Localization of the electron density is crucial to maximize fluorescence via ESIPT. Our conclusions offer design strategies for the development of luminescent molecular crystals.

Understanding photoinduced processes in molecular crystals is central to the design of highly emissive materials such as organic lasers and organic light-emitting diodes. The modelling of such processes is, however, hindered by the lack of excited state methodologies tailored for these systems. Embedding approaches based on the Ewald sum can be used in conjunction with excited state electronic structure methods to model the localised excitations which characterise these materials. In this article, we describe the implementation of a two-level ONIOM(QM:QM') point charge embedding approach based on the Ewald method, the ONIOM Ewald Embedded Cluster (OEEC) model. An alternative self-consistent method is also considered to simulate the response of the environment to the excitation. Two molecular crystals with opposing photochemical behaviour were used to benchmark the results with single reference and multireference methods. We observed that the inclusion of an explicit ground state cluster surrounding the QM region was imperative for the exploration of the excited state potential energy surfaces. Using OEEC, accurate absorption and emission energies as well as S 1-S 0 conical intersections were obtained for both crystals. We discuss the implications of the use of these embedding schemes considering the degree of localisation of the excitation. The methods discussed herein are implemented in an open source platform (fromage, https://github.com/Crespo-Otero-group/fromage) which acts as an interface between popular electronic structure codes (Gaussian, Turbomole and Molcas).

The study of photoexcitations in molecular aggregates faces the twofold problem of the increased computational cost associated with excited states and the complexity of the interactions among the constituent monomers. A mechanistic investigation of these processes requires the analysis of the intermolecular interactions, the effect of the environment, and 3D arrangements or crystal packing on the excited states. A considerable number of techniques have been tailored to navigate these obstacles; however, they are usually restricted to in‐house codes and thus require a disproportionate effort to adopt by researchers approaching the field. Herein, we present the FRamewOrk for Molecular AGgregate Excitations (fromage), which implements a collection of such techniques in a Python library complemented with ready‐to‐use scripts. The program structure is presented and the principal features available to the user are described: geometrical analysis, exciton characterization, and a variety of ONIOM schemes. Each is illustrated by examples of diverse organic molecules in condensed phase settings. The program is available at https://github.com/Crespo-Otero-group/fromage.

The identification of radiative and nonradiative channels and, how these are affected by aggregation, help rationalise emissive properties and aid in the design of yet more efficient ESIPT fluorophores in the condensed phase.

Organic molecular crystals are attractive materials for luminescent applications due to their promised tunability. However, the link between chemical structure and emissive behaviour is poorly understood due to the numerous interconnected factors which are at play in determining radiative and non-radiative behaviours at the solid state level. In particular, the decay through conical intersection dominates the nonadiabatic regions of the potential energy surface, and thus their accessibility is a telling indicator of the luminosity of the material. In this study, we investigate the radiative mechanism for five organic molecular crystals which display solid state emission, with a focus on the role of conical intersections in their photomechanisms. The objective is to situate the importance of the accessibility of conical intersections with regards to emissive behaviour, taking into account other nonradiative decay channels, namely vibrational decay, and exciton hopping. We begin by giving a brief overview of the structural patterns of the five systems within a larger pool of thirteen crystals for a richer comparison. We observe that due to the prevalence of sheet-like and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited.Additionally, similarly spaced dimers have exciton coupling values of similar order within a 50 meV interval.Next, we focus on three exemplary cases, where we disentangle the role of nonradiative decay mechanisms and show how rotational minimum energy conical intersections in vacuum lead to puckered ones in crystal, increasing their instability upon crystallisation in typical packing motifs. In contrast, molecules with puckered conical intersections in vacuum tend to conserve this trait upon crystallisation, and therefore their quantum yield of fluorescence is determined predominantly by other nonradiative decay mechanisms. File list (2) download file view on ChemRxiv manuscript_v2.pdf (2.23 MiB) download file view on ChemRxiv SI_v2.pdf (10.63 MiB)