Förster (FRET) R⁻⁶ dipole-dipole vs. Dexter e^(−2R/L) exchange
Förster Resonance Energy Transfer (FRET) proceeds via long-range dipole-dipole coupling: k_F = (1/τ_D)(R₀/R)⁶, where R₀ (2–8 nm) is set by spectral overlap J and quantum yield. Dexter transfer requires orbital overlap and falls off exponentially: k_D ∝ J·exp(−2R/L), L ≈ 1 Å. FRET dominates at nm distances and underpins biological light harvesting (LH1/LH2 complexes in photosynthesis, FRET-based biosensors). The 50% efficiency point at R=R₀ is used as a "molecular ruler."