In chemistry, π-effects or π-interactions are a type of non-covalent interaction that involves π systems. Just like in an electrostatic interaction where a region of negative charge interacts with a positive charge, the electron-rich π system can interact with a metal (cationic or neutral), an anion, another molecule and even another π system. Non-covalent interactions involving π systems are pivotal to biological events such as protein-ligand recognition.

The most common types of π-interactions involve:

Graphite consists of stacked sheets of covalently bonded carbon. The individual layers are called graphene. In each layer, each carbon atom is bonded to three other atoms forming a continuous layer of sp² bonded carbon hexagons, like a honeycomb lattice with a bond length of 0.142 nm, and the distance between planes is 0.335 nm.

Cation-π interactions are important for the acetylcholine (Ach) neurotransmitter. The structure of acetylcholine esterase includes 14 highly conserved aromatic residues. The trimethyl ammonium group of Ach binds to the aromatic residue of tryptophan (Trp). The indole site provides a much more intense region of negative electrostatic potential than benzene and phenol residue of Phe and Tyr.

Pi–pi and cation–pi interactions are important in rational drug design. One example is the FDA-approved acetylcholinesterase (AChE) inhibitor tacrine which is used in the treatment of Alzheimer's disease. Tacrine is proposed to have a pi stacking interaction with the indolic ring of Trp84, and this interaction has been exploited in the rational design of novel AChE inhibitors.

${\displaystyle {\ce {\pi - \pi}}}$, ${\displaystyle {\ce {CH-\pi}}}$ and ${\displaystyle {\ce {\pi -cation}}}$ interactions are widely observed motifs in supramolecular assembly and recognition.

${\displaystyle {\ce {\pi-\pi}}}$ concerns the direct interactions between two π-systems; and ${\displaystyle {\ce {cation-\pi}}}$ interaction arises from the electrostatic interaction of a cation with the face of the π-system. Unlike these two interactions, the ${\displaystyle {\ce {CH-\pi}}}$ interaction arises mainly from charge transfer between the C–H orbital and the π-system.

π systems contribute to supramolecular assembly. Some catenanes feature π–π interactions. The major challenge for the synthesis of catenane is to interlock molecules in a controlled fashion. Stoddart and co-workers developed a series of systems utilizing the strong π–π interactions between electron-rich benzene derivatives and electron-poor pyridinium rings. [2]Catanene was synthesized by reacting bis(pyridinium) (A), bisparaphenylene-34-crown-10 (B), and 1, 4-bis(bromomethyl)benzene (C) (Fig. 2). The π–π interaction between A and B directed the formation of an interlocked template intermediate that was further cyclized by substitution reaction with compound C to generate the [2]catenane product.