Blue-Shifting Intramolecular Charge Transfer Emission by Nonlocal Effect of Hyperbolic Metamaterials

Metallic nanostructures permit controlling various photophysical processes by coupling photons with plasmonic oscillation of electrons confined in the tailored nanostructures. One example is hyperbolic metamaterial (HMM) leading to an enhanced spontaneous emission rate of emitters located nearby. Noting that emission in organic molecules is from either π-π* or intramolecular charge-transfer (ICT) states, we address here how HMM modifies ICT emission spectral features by comparing them with a spectral shift dependent on the local polarity of the medium. The 7.0 nm blue shift is observed in ICT emission from 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dispersed into a polymer matrix prepared on HMM multilayered structure, while no spectral shift is observed in π-π* emission from perylene diimide. In the frame of the Lippert-Mataga formalism, the blue shift is explained by the HMM nonlocal effects resulting from 8% decrease in refractive index and 18% reduction in dielectric permittivity. This phenomenon was also shown in a hemicurcuminoid borondifluoride dye yielding 15.0 nm blue shift. Such a capability of spectral shift control in films by HMM structure opens new prospects for engineering organic light-emitting devices.