Phosphorescent Sensor Based on Iridium Complex/Poly(vinylcarbazole) Orderly Assembled with Layered Double Hydroxide Nanosheets: Two-Dimensional Föster Resonance Energy Transfer and Reversible Luminescence Response for VOCs

Abstract

The application study of phosphorescence complexes is important for further investigation and exploration of novel optofunctional materials. In this work, neutral poly(vinylcarbazole) (PVK) and tris[2-(4,6-difluorophenyl)pyridinato-C2,N]iridium(III) (Ir(F2ppy)3) were assembled with LDH nanosheets to form ordered ultrathin films (UTFs). These inorganic/organic composite UTFs exhibited cyan luminescence from Ir(F2ppy)3, peaking at 471 and 491 nm, due to triplet metal-to-ligand charge transfer and ligand-centered states, respectively. Under PVK maximal excitation at 294 nm, the photoluminescence spectra of the UTFs showed emission from Ir(F2ppy)3 rather than from PVK, demonstrating PVK transfer of resonance energy to Ir(F2ppy)3. Temporal luminescence spectroscopy revealed that the phosphorescence lifetime of Ir(F2ppy)3 molecules increased to 885 ns and fluorescence lifetime of PVK fell to 1.39 ns in the UTFs, which was typical character of the FRET process. This FRET process occurred within the interlayers of LDH nanosheets and can be described as a two-dimensional (2D) process with high efficiency (0.892). Moreover, it was found that the presence of volatile organic compound (VOC) vapors can interrupt this 2D process because of the unique hydrophobic character of the organic interlayers within the UTFs. This function was utilized for a phosphorescent sensor that enables reversible two-state photoemission switching (ON for cyan light of Ir(F2ppy)3 vs OFF for blue light of PVK). This phosphorescence sensor demonstrated a fast, highly sensitive, and reversible response toward common VOCs, and this 2D FRET with energy transfer involving the D(singlet) → A(triplet) process was more efficient compared with that of singlet–singlet processes. That is, to get the same luminescence intensity, these UTFs can be realized by less frequent excitation of PVK or lower concentrations of Ir(F2ppy)3 compared with intrinsic excitation of Ir(F2ppy)3.