Subtle structural changes in octupolar merocyanine dyes influence the photosensitized production of singlet oxygen

Abstract

The photophysical properties of two indoline-based octupolar merocyanine dyes and of the corresponding quinoline-based dyes were examined. This seemingly subtle structural change in the chromophore of these molecules has an appreciable effect on the yields with which these respective compounds sensitize the production of singlet molecular oxygen, O₂(a¹Δ_g). The indoline-based dyes are reasonably efficient O₂(a¹Δ_g) sensitizers (ϕ_Δ ∼ 0.35), whereas the quinoline-based dyes are poor O₂(a¹Δ_g) sensitizers (ϕ_Δ ∼ 0.005). A series of experiments, including Laser-Induced Optoacoustic Calorimetric (LIOAC) measurements, reveal that this difference principally reflects the fact that the excited singlet state of the quinoline-based dyes rapidly and efficiently decays via nonradiative channels to regenerate the ground state molecule. It is likely that a charge-transfer state mediates this efficient coupling between the excited and ground states. Such subtle, structure-dependent effects are important in elucidating and ultimately understanding phenomena that influence the efficiency of photosensitized O₂(a¹Δ_g) production. In turn, the knowledge gained facilitates the rational design and preparation of O₂(a¹Δ_g) sensitizers with explicitly controlled properties.