Antenna Doping: The Key for Achieving Efficient Optical Wavelength Conversion in Crystalline Chromophoric Heterolayers

High‐yield wavelength conversion is one of the key requirements for efficient photon energy harvesting. Attempts to realize efficient conversion by simply stacking layers of chromophores have failed so far, even when using highly crystalline assemblies and employing the recently discovered long‐range (>100 nm) Förster resonance energy transfer (LR‐FRET). Optical conversion efficiency is drastically improved using chromophoric metal–organic framework heterolayers fabricated layer‐by‐layer in connection with an “antenna doping” strategy. Systematic investigations reveal that the LR‐FRET mechanism, reported previously in chromophoric aggregates, is highly anisotropic for neat materials but can be made more isotropic by employing doping strategies. Using optimized fabrication parameters and dopant concentrations, a three‐layer, two‐step cascade with an overall optical conversion efficiency of ≈75% is realized.