A new contrast agent, LipImage™ 815, has been designed and compared to previously described indocyanine green (ICG)-loaded lipid nanoparticles (ICG-lipidots®). Both contrast agents display similar size (50-nm diameter), zeta potential, high IC50 in cellular studies, near-infrared absorption and emission wavelengths in the "imaging window," long-term shelf colloidal and optical stabilities with high brightness (>106  L mol-1 cm-1) in ready-to-use storage conditions in aqueous buffer (4°C in dark), therefore being promising fluorescence contrast agents for in vivo imaging. However, while ICG-lipidots® display a relatively short plasma lifetime, LipImage™ 815 circulates in blood for longer times, allowing the efficient uptake of fluorescence signal in human prostate cancer cells implanted in mice. Prolonged tumor labeling is observed for more than 21 days.

A series of redox tunable polyoxometalate–bodipy conjugates display variable charge transfer dynamics occuring down to 54 ps.

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 ...

A thorough photophysical study of a photochrome-fluorophore dyad (3), combining a fluorescent laser dye (DCM-type, , Φ(1) = 0.27) and a photochromic diarylethene (2), obtained by click chemistry, is presented. In addition to photochromism, the open form (OF) of 2 exhibits fluorescence (Φ(-OF) = 0.016), whereas the closed form (CF) does not. Fluorescence is switched upon alternate UV/visible irradiation of 2. The emission band of 2-OF matches the absorption band of 1 (400-550 nm), whereas the emission band of 1 overlaps the absorption band of 2-CF (550-700 nm). Therefore, a photoreversible two-way excitation energy transfer (EET), controlled by the state of the photochromic moiety, is obtained in the dyad 3. Their efficiencies are quantified as Φ(EET)(OF→F) = 85% and as Φ(EET)(F→CF) ~ 100% from the comparison of emission and excitation spectra between 1, 2, and 3. These results are fully compatible with the shortening of fluorescence lifetimes (from τ(-OF) = 70 ps and 170 ps essentially to τ(-OF) < 10 ps) and to the values of Förster radii determined for 3 (R(0)(OF → F) = 29 Å and R(0)(F → CF) = 71 Å), evidencing a Förster-type resonance energy transfer mechanism (FRET). An important outcome of this two-way FRET is the possibility to quench 49% of the fluorescence in 3 at PSS upon UV irradiation, corresponding to the conversion extent of the photochromic reaction, which is different from 2 (α(CF) = 91%). This is a clear example of a situation where the presence of FRET between the photochromic unit and the fluorophore affects noticeably the photochromic properties of the dyad molecule 3.

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Bifunctional molecules that combine independent push-pull fluorophores and azo photochromes have been synthesized to create fluorescent structures upon light-induced migration in neat thin films. Their photochromic and emissive properties have been systematically investigated and interpreted in light of those of the corresponding model compounds. Fluorescence lifetimes and photoisomerization and fluorescence quantum yields have been determined in toluene solution. Kinetic analyses of the femtosecond transient absorption spectra reveal that the fluorophores evolve in a few picoseconds into a distorted intramolecular charge-transfer excited state, strongly stabilized in energy. Radiative relaxation to the ground state occurred competitively with the energy-transfer process to the azo moiety. Introduction of a 10 Å-long rigid and nonconjugated bridge between the photoactive units efficiently inhibits the energy transfer while it imparts enhanced free volume, which favors photoactivated molecular migration in the solid state.

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