Influence of plasmons on the luminescence properties of solvatochromic merocyanine dyes with different solvatochromism

Abstract

The effect of localized surface plasmon resonance (LSPR) of a system consisting of a highly dipolar merocyanine dye and a silver nanoparticle (NP) was studied experimentally and theoretically. A theoretical model for estimating the fluorescence quantum yield (φ_fl) using quantum chemical calculations of intramolecular and intermolecular electronic transition rate constants was developed. Calculations show that the main deactivation channels of the lowest excited singlet state of the studied merocyanines are internal conversion (k_IC(S₁ → S₀)) and fluorescence (k_r(S₁ → S₀)). The intersystem-crossing transition has a low probability due to the large energy difference between the singlet and triplet levels. In the presence of plasmonic NPs, the fluorescence quantum yield is increased by a factor of two according to both experiment and computations. The calculated values of φ_fl, when considering changes in k_r(S₁ → S₀) and the energy-transfer rate constant (k_transfer) from the dye to the NP was also twice as large at distances of 6–8 nm between the NP and the dye molecule. We also found that the LSPR effect can be increased or decreased depending on the value of the dielectric constant (ε_m) of the environment.