Förster resonance energy transfer (abbreviated FRET), also known as fluorescence resonance energy transfer, resonance energy transfer (RET) or electronic energy transfer (EET), is a mechanism describing energy transfer between two chromophore

Chromophore

A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...

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A donor chromophore, initially in its electronic excited state, may transfer energy to an acceptor chromophore (in proximity, typically less than 10 nm) through nonradiative dipole–dipole coupling.