Directing the solid-state photochromic and luminescent behaviors of spiromolecules with Dawson and Anderson polyoxometalate units

Abstract

A series of three new polyoxometalate/spiromolecule compounds – including the (TBA)₃[AlMo₆-SN] (SN = spironaphthoxazine) and (TBA)₃[AlMo₆-SP] (SP = spiropyran) Anderson-type polyoxometalates (POMs) and the unprecedented (TBA)₅[P₂W₁₅V₃-SP] Dawson-type species – has been synthesized and characterized using a full panel of techniques. These hybrids have been thoroughly investigated by (spectro)electrochemical techniques, allowing not only to extract the electrochemical data characterizing these POMs but also to deeply investigate the redox mechanisms involved, including the dimerization process occuring during the oxidation step. The solid-state photochromic properties of these species have been fully studied and the photochromic kinetic parameters determined. It revealed that while in (TBA)₅[P₂W₁₅V₃-SP] the merocyanin form is highly stabilized, (TBA)₃[AlMo₆-SN] represents highly robust optical switch. Importantly, the room temperature solid-state luminescent properties of such assemblies have been investigated for the first time. It has been evidenced that (TBA)₃[AlMo₆-SP] exhibits a strong red emission under UV irradiation, while the luminescence of the related (TBA)₃[MnMo₆-SP] is weaker and that (TBA)₅[P₂W₁₅V₃-SP] does not exhibit luminescent properties. These emission intensity differences have been interpreted in terms of differences in photoinduced electron transfers from the excited state of the merocyanine isomer to the inorganic units. A correlation between its efficiency and the electron acceptor ability of the POMs determined from electronic absorption spectroscopic measurement and electrochemical data has been proposed.