Strong exciton coupling: a practical toolbox for computing interaction energies, wavefunctions, and optical spectra.

The perceived colour and photophysical properties of organic dyes depend not only on their chemical structure, but also how they pack together. When two dyes approach one another, the transition dipole moments associated with their molecular states begin to interact. If this interaction is sufficiently strong, it can lead to hybridization of the excited states, forming new hybrid states with altered photophysical properties. This phenomenon, formally denoted strong exciton coupling, is commonly associated with the terms J- and H-aggregates. Examples are abundant in nature, but less common in technology, though their number may increase as understanding and methods become more developed. Representative examples include light-harvesting complexes in cyanobacteria, which contain strongly coupled bacteriochlorophyll units, and the autumn colours of leaves, which are a function of the packing of carotenoids. In this review, a brief historical outlook is given, followed by a more in-depth discussion of various levels of theories used to model the properties of the hybrid states, ranging from Coulombic exciton models to extensions that incorporate vibronic coupling and charge-transfer interactions. Worked examples are included as tutorials to connect the theoretical frameworks to experimental observables. Finally, a personal reflection on directions where strong exciton coupling can have scientific and technological impact is given.