This paper describes how metal-organic frameworks (MOFs) conformally coated on plasmonic nanoparticle arrays can support exciton-plasmon modes with features resembling strong coupling but that are better understood by a weak coupling model. Thin films of Zn-porphyrin MOFs were assembled by dip coating on arrays of silver nanoparticles (NP@MOF) that sustain surface lattice resonances (SLRs). Coupling of excitons with these lattice plasmons led to an SLR-like mixed mode in both transmission and transient absorption spectra. The spectral position of the mixed mode could be tailored by detuning the SLR in different refractive index environments and by changing the periodicity of the nanoparticle array. Photoluminescence showed mode splitting that can be interpreted as modulation of the exciton line shape by the Fano profile of the surface lattice mode, without requiring Rabi splitting. Compared with pristine Zn-porphyrin, hybrid NP@MOF structures achieved a 16-fold enhancement in emission intensity. Our results establish MOFs as a crystalline molecular emitter material that can couple with plasmonic structures for energy exchange and transfer.
Keywords: conformal coating; metal–organic framework; photoluminescence; plasmonic nanoparticle arrays; surface lattice resonance.