Organic Photochemistry: The Control of Organic Photochemistry and Photophysics through Intermolecular Interactions

Supramolecular chemistry has evolved from molecular chemistry over the last half century as the science of “chemistry beyond the molecule.” Supramolecular organic chemistry can be described in a number of ways, such as: (1) chemistry of molecular complexes; (2) chemistry of noncovalent bonds; (3) chemistry of the intermolecular bond; and (4) chemistry of molecular recognition. These are appealing, but somewhat fuzzy, descriptions of “chemistry beyond the molecule.”1−3 Nevertheless, they allow the chemist latitude in a wide range of concepts that are useful for deciding what is supramolecular and what is not. Rather than trying to pin down a precise definition of supramolecular organic chemistry, let us try to describe its fundamental intellectual foundation: Supramolecular organic chemistry is based on the concept that whereas organic chemistry is generally interpreted in terms of individual molecular structures and/or dynamics for which covalent bonding is the dominant feature determining molecular structure, there are situations where introduction of more than one interacting molecular structure is required to understand the chemistry under consideration. To a photochemist, the comparative ideas of molecular and supramolecular structures are analogous to the ideas of ground-state and excited-state potential energy surfaces. In this analogy, organic photochemistry is chemistry beyond the groundstate surface: A ground-state potential energy surface determines the ground-state chemistry and properties of a ground-state organic molecule (R), but an electronically excited-state surface determines the photochemical and photophysical properties of the electronically excited state (∗R). Knowledge of the ground-state surface is insufficient to understand the photochemistry of organic molecules. When a single molecular structure is insufficient to describe the ground-state or excited-state chemistry of a molecule under investigation, this is a signature that the transition from molecular organic photochemistry to supramolecular organic photochemistry has occurred. In 925