Resonant Plasmon-Enhanced Absorption of Charge Transfer Complexes in a Metal–Organic Monolayer

Abstract

Plasmonic enhancement of absorption in charge-transfer (CT) complexes formed under NO2 gas adsorption onto 2D hybrid structure, based on the metal–organic monolayer and gold nanoparticles (AuNPs), is demonstrated. By using Langmuir–Blodgett deposition of low-symmetry zinc phthalocyanine (ZnPc) molecules, the metal–organic monolayer is fabricated with greatly suppressed intermolecular aggregation. Oxidation of the monolayer through coordination of NO2 molecules with axial zinc ions of ZnPc molecules gives rise to the specific absorption band inherited to cation radical ZnPc+. The hybrid AuNPs–ZnPc structure is engineered to maximize exciton–plasmon interaction of CT complexes at the radical form of the metal–organic monolayer. Excellent spectral and spatial overlaps with plasmon resonance boost absorption of CT internal optical transition, so-called “fingerprint” band, by a factor of six from 0.45% to 2.8% in total. The approach paves the way for efficient plasmonic control over photochemical reactions promoted by charge-transfer complexes in metal–organic films. In particular, the plasmonic effect is harnessed to improve NO2 gas sensing properties; the experimental study shows a 15-fold increase of the detection efficiency in the specific band of CT complexes under the gas exposure.