Abstract:The photophysical properties of a Keggin-type polyoxometalate (POM) covalently bounded to a benzospiropyran (BSPR) unit have been investigated. These studies reveal that (i) both closed and open forms are emissive with distinct spectral features (λem, (closed form) = 530 nm, λem, (open form) = 670 nm); (ii) the fluorescence of the BSPR unit of the POM-based hybrid is considerably enhanced compared to BSPR reference compounds. While the fluorescence excitation energy of the BSPR reference compounds (370 nm) is close to the intense absorption responsible of the photochromic character (350 nm), the fluorescence excitation of the hybrid is shifted to lower energy (400 nm), facilitating the population of the emissive state. Combined NOESY NMR and theoretical calculations of the closed form of hybrid give an intimate understanding of the structural conformation adopted by the hybrid and show that the nitro-aryl moieties of the BSPR is folded toward the POM, which should affect the electronic properties of the BSPR.Photo-responsive molecules and materials are currently receiving considerable attention regarding their potentiality for the development of advanced photonic devices in various applications such as information storage, optical switches, protection, smart labelling and super-resolution microscopy. [1] Particularly, the elaboration of molecular systems combining photo-switching and fluorescence properties constitute a promising research field because of the high sensitivity, resolution, contrast and fast response times of the fluorescence. Furthermore, the emergence of single-molecule fluorescence spectroscopy opens the route to high memory density nanodevices, in which a single molecule would work as one bit of memory, [2] and thus strongly supports the molecular approach. Until now, the development of molecular systems displaying photo-modulated emissive properties has mostly relied on the association of luminescent and photochromic components in a molecular assembly. [3] In these systems, the transformation of the photochromic component is exploited to modulate the emission intensity of the luminescent component on the basis of electron and/or energy transfer. [4] However, molecular systems displaying tuneable fluorescence features, i.e. having different emission energies, according to the form of the photochromic unit, have been scarcely reported in the literature. [5] The coexistence of photochromic and fluorescence properties, in a single photoactive unit, is rarely achieved since in case of fast internal conversion only one photophysical property (photochromic vs fluorescence) is observed. Furthermore the ns timescale of fluorescence [6] is considerably slower than the fs to ps timescale of the ring opening/closure dynamics occurring in the photochromic events. [7] POMs are nanosized molecular oxo cluster that are currently receiving considerable attention because of their wide range of properties and their potential applications in various fields such as analytical chemistry, medicine and ...

The first systems associating in a single molecule polyoxotungstates (POTs) and photochromic organic groups have been elaborated. Using the (TBA)4[PW11O39{Sn(C6H4I)}] precursor, two hybrid organic-inorganic species where a spiropyran derivative (SP) has been covalently grafted onto a {PW11Sn} fragment via a Sonogashira coupling have been successfully obtained. Alternatively, a complex containing a silicotungstate {PW11Si2} unit connected to two spiropyran entities has been characterized. The purity of these species has been assessed using several techniques, including (1)H and (31)P NMR spectroscopy, mass spectrometry, and electrochemical measurements. The optical properties of the hybrid materials have been investigated both in solution and in the solid state. These studies reveal that the grafting of SPs onto POTs does not significantly alter the photochromic behavior of the organic chromophore in solution. In contrast, these novel hybrid SP-POT materials display highly effective solid-state photochromism from neutral SP molecules initially nonphotochromic in the crystalline state. The photoresponses of the SP-POT systems in the solid state strongly depend on the nature and the number of grafted SP groups.