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Molecular-based Fluorescent Organic Nanoparticles (FONs) are versatile light-emitting nano-tools whose properties can be rationally addressed by bottom-up molecular engineering. A challenging property to gain control over is the interaction of the FONs' surface with biological systems. Indeed, most types of nanoparticles tend to interact with biological membranes. To address this limitation, we recently reported on two-photon (2P) absorbing, red to near infrared (NIR) emitting quadrupolar extended dyes built from a benzothiadiazole core and diphenylamino endgroups that yield spontaneously stealth FONs. In this paper, we expand our understanding of the structure-property relationship between the dye structure and the FONs 2P absorption response, fluorescence and stealthiness by characterizing a dye-related series of FONs. We observe that increasing the strength of the donor end-groups or of the core acceptor in the quadrupolar (D-π-A-π-D) dye structure allows for the tuning of optical properties, notably red-shifting both the emission (from red to NIR) and 2P absorption spectra while inducing a decrease in their fluorescence quantum yield. Thanks to their strong 1P and 2P absorption, all FONs whose median size varies between 11 and 28 nm exhibit giant 1P (106 M−1.cm−1) and 2P (104 GM) brightness values. Interestingly, all FONs were found to be non-toxic, exhibit stealth behaviour, and show vanishing non-specific interactions with cell membranes. We postulate that the strong hydrophobic character and the rigidity of the FONs building blocks are crucial to controlling the stealth nano-bio interface. [ABSTRACT FROM AUTHOR]

[Display omitted] • Uncaging efficiency is retained in dimeric DEAC conjugated structures. • Two-photon absorption properties (2PA) are enhanced by up to two order of magnitude. • The aromatic cores influence both the uncaging efficiency and the 2PA properties. • Both electron-releasing and electron-withdrawing cores lead to unprecedented 2P uncaging sensitivities (550 GM). A series of symmetrical conjugated dimeric coumarinyl cages has been synthesized by grafting 7-diethlyaminocoumarin (DEAC) cages via π-linkers onto different aromatic cores displaying various electronic character (electron-withdrawing: benzobisthiazole BBTz, electron-donating: carbazole or ambivalent: fluorenyl). Due to their symmetrical design, all conjugated dimeric derivatives showed enhanced two-photon absorption at 730 nm, the electron-withdrawing core leading to the highest response (over 3000 GM). All dimeric derivatives retained the uncaging ability of the DEAC cage, with a strong influence of the nature of the core on the uncaging quantum yield. While the fluorenyl core led to an uncaging quantum yield similar to DEAC, the BBTz core led to a twice larger uncaging quantum yield and the carbazole core to even larger uncaging quantum yield (60 %). As a result, both electron-donating and electron-withdrawing cores lead to symmetrical dimeric DEAC derivatives showing unprecedented two-photon uncaging photo-sensitivity (up to 550 GM). [ABSTRACT FROM AUTHOR]

This contribution employs quantum chemistry methods to describe the variations of the second nonlinear optical responses of molecular switches based on benzazolo-oxazolidine (BOX) units, connected by π-linkers, along their successive opening/closing. Under the fully closed forms, all of them display negligible first hyperpolarizability (β) values. When one BOX is opened, which is sketched as C→O, a push–pull π-conjugated segment is formed, having the potential to enhance β and to set the depolarization ratio (DR) to its one-dimensional-like value (DR = 5). This is observed when only one BOX is open, either for the monoBOX species (C→O) or for the diBOX (CC→CO) and triBOX (CCC→CCO) compounds, i.e., when the remaining BOXs stay closed. The next BOX openings have much different effects. For the diBOXs, the second opening (CO→OO) is associated with a decrease of β, and this decrease is tuned by controlling the conformation of the π-linker, i.e., the centrosymmetry of the whole compound because β vanishes in centrosymmetric compounds. For the triBOXs, the second opening gives rise to a Λ-shape compound, with a negligible change of β, but a decrease of the DR whereas, along the third opening, β remains similar and the DR decreases to the typical value of octupolar systems (DR = 1.5). [ABSTRACT FROM AUTHOR]

[Display omitted] • Synthesis of molecular photoswitches based on 2-substituted N H and N CH 3 imidazole. • Tuning of switching properties by preventing imidazole tautomerism and benzene substitution. • Pentafluorosulfanyl derivative showed the highest stability of the (Z)–isomer. • Insertion of SF 5 group proved to be useful strategy towards a bistable photoswitch. Azo–imidazole photoswitches based on N H– and N –methylimidazole were designed and prepared via a convenient synthetic route. These azo-compounds were investigated by means of XRD analysis, cyclic voltammetry, thermal analysis, UV–Vis absorption spectroscopy and DFT calculations. The photoswitching was mostly affected by imidazole N -substitution with the half-life of (Z)–isomers found within the range of tens of seconds (N H-imidazoles) vs. tens of hours (N -methylimidazoles). N -Methylimidazole switches can be further tuned by altering electronic effects of the appended substituents (acyl, bromine, trifluoromethyl and pentafluorosulfanyl). The latter group proved to be particularly interesting in stabilizing (Z)–isomer. Its large half-life (∼24 h), high photoconversion (89 %) and diminished nonlinear optical response in contrast to the (E)-isomer (β E / Z = 3.82) point to a very useful strategy in introducing SF 5 group in the structure of molecular switches. In summary, this detailed study demonstrates that photoswitching properties of azo-imidazoles can be significantly tuned in a broad extent by (preventing) imidazole tautomerism as well as by altering electronic effects of appended substituents. [ABSTRACT FROM AUTHOR]