Three quadrupolar D−π–A−π–D compounds, bearing alkoxy phenyls as mild electron donors and a benzothiadiazole (A), two benzothiadiazoles (B), or a benzothiadiazole linked to two thiophenes (C) as the ...acceptor units, are collectively the object of this study. They proved to be efficient yellow/orange/red fluorophores, respectively, with fluorescence being their preferred deactivation pathway. These systems exhibited positive fluorosolvatochromism and a noticeable decrease of the fluorescence quantum yield in the most polar solvent (with the quenching following the trend B > A > C). These findings point to an intramolecular charge transfer (ICT) nature of the emitting state, whose photoinduced dynamics was investigated by femtosecond-resolved transient absorption and fluorescence upconversion. Remarkable values of hyperpolarizability were estimated by the solvatochromic method. The significant two-photon absorption cross sections measured for A–C (whose trend nicely parallels that of ICT efficiency), coupled with the intense emission, make them promising for applications as environment-sensitive probes in two-photon excited fluorescence imaging.
In this investigation, we report the first hyperpolarizabilities and two-photon absorption cross sections of a large series of 12 push–pull cationic chromophores. All of these dyes show a dipolar ...acceptor+–π–donor structure, where the nature of the donor and acceptor units and π-bridge was synthetically tuned to allow insightful comparisons among the molecules. The hyperpolarizability was obtained through a solvatochromic method, by exploiting the rare negative solvatochromism exhibited by the investigated compounds. The two-photon absorption cross sections were determined through two-photon excited fluorescence measurements by means of a tunable nanosecond laser system for sample excitation. The nonlinear optical properties were discussed relatively to the photoinduced intramolecular charge transfer occurring in these donor–acceptor systems, investigated by femtosecond transient absorption experiments. We found a strong increase in hyperpolarizability upon increasing the molecular conjugation. Unexpectedly, the hyperpolarizability is almost unaffected by an increase in donor/acceptor strength and intramolecular charge transfer degree. Differently, the two-photon absorption cross sections of these dyes are enhanced by an increase in both molecular conjugation and intramolecular charge transfer efficiency. Several recent literature works have reported at the same time scattered information about the hyperpolarizability and two-photon absorption of small organic molecules. Our investigation is, to the best of our knowledge, the first attempt to uncover detailed structure–property relationships for these two nonlinear optical properties. Our results represent a promising route to achieve large hyperpolarizability and two-photon absorption in push–pull dyes and may drive the design of new efficient nonlinear optical materials.
Identifying the composition of the solvated iodoplumbate complexes that are involved in the synthesis of perovskites in different solution environments is of great relevance in order to link the type ...and quantity of precursors to the final optoelectronic properties of the material. In this paper, we clarify the nature of these species and the involved solution equilibria by combining experimental analysis and high-level theoretical calculations, focusing in particular on the DMSO and DMF solvents, largely employed in the perovskites synthesis. The specific molecular interactions between the iodoplumbate complexes and the solvent molecules were analyzed by identifying the most thermodynamically stable structures in various solvent solutions and characterizing their optical properties trough DFT and TD-DFT calculations. A comparison with the experimental UV–vis absorption spectra allows us to define the number of iodide and solvent ligands bonded to the Pb2+ ion and the complex formation constants of the involved species.
A set of styryl- and bis-styryl dyes, varying in length, aromatic surface, net positive charge and steric positioning or bulkiness of substituents, was tested for interactions with various ds-DNA or ...ds-RNA. Most of the compounds showed strong affinity toward ds-DNA/RNA, directly correlated to the synergistic contribution of the aromatic-conjugated surface and net positive charge. The volume or positioning of terminal aromatic substituents directly controlled the binding mode of the core structure, shifting between DNA/RNA groove binding or DNA/RNA intercalation. Consequently, upon binding to DNA/RNA the fluorimetric and induced CD (ICD) response varied for different compounds, for instance one derivative showed specific fluorescence increase with AT-DNA, while another derivative showed specific ICD response with AU-RNA. Preliminary screening on human tumour cell lines revealed an efficient cellular uptake for all dyes. Only mono-styryl-quinoline derivatives showed a strong antiproliferative activity combined with efficient fluorescent localisation, thus showing promising theragnostic potential, while other compounds were negligibly cytotoxic but still efficient fluorescent markers of cytoplasmic organelles.
A strong impact of fluorophores' charge and length on the binding mode, intracellular distribution and antiproliferative activity; intriguing theragnostic potential.
The identification of novel molecular systems with high fluorescence and significant non-linear optical (NLO) properties is a hot topic in the continuous search for new emissive probes. Here, the ...photobehavior of three two-arm bis(dimethylamino)styrylbenzene derivatives, where the central benzene was replaced by pyridine, furan, or thiophene, was studied by stationary and time-resolved spectroscopic techniques with ns and fs resolution. The three molecules under investigation all showed positive fluorosolvatochromism, due to intramolecular charge-transfer (ICT) dynamics from the electron-donor dimethylamino groups, and significant fluorescence quantum yields, because of the population of a planar and emissive ICT state stabilized by intramolecular hydrogen-bond-like interactions. The NLO properties (hyperpolarizability coefficient and TPA cross-section) were also measured. The obtained results allowed the role of the central heteroaromatic ring to be disclosed. In particular, the introduction of the thiophene ring guarantees high fluorescent quantum yields irrespective of the polarity of the medium, and the largest hyperpolarizability coefficient because of the increased conjugation. An important and structure-dependent involvement of the triplet state was also highlighted, with the intersystem crossing being competitive with fluorescence, especially in the thiophene derivative, where the triplet was found to significantly sensitize molecular oxygen even in polar environment, leading to possible applications in photodynamic therapy.
We report here a joint experimental and theoretical study of a quadrupolar, two-branched pyridinium derivative of interest as a potential non-linear optical material. The spectral and photophysical ...behaviour of this symmetric system is greatly affected by the polarity of the medium. A very efficient photoinduced intramolecular charge transfer, surprisingly more efficient than in the dipolar asymmetric analogue, is found to occur by femtosecond resolved transient absorption spectroscopy. TD-DFT calculations are in excellent agreement with these experimental findings and predict large charge displacements in the molecular orbitals describing the ground state and the lowest excited singlet state. The theoretical study also revealed that in highly polar media the symmetry of the excited state is broken giving a possible explanation to the fluorescence and transient absorption spectra resembling those of the one-branched analogous compound in the same solvents. The present study may give an important insight into the excited state deactivation mechanism of cationic (donor-π-acceptor-π-donor)(+) quadrupolar compounds characterised by negative solvatochromism, which are expected to show significant two-photon absorption (TPA). Moreover, the water solubility of the investigated quadrupolar system may represent an added value in view of the most promising applications of TPA materials in biology and medicine.
In spite of their value as genetically encodable reporters for imaging in living systems, fluorescent proteins have been used sporadically for stimulated emission depletion (STED) super-resolution ...imaging, owing to their moderate photophysical resistance, which does not enable reaching resolutions as high as for synthetic dyes. By a rational approach combining steady-state and ultrafast spectroscopy with gated STED imaging in living and fixed cells, we here demonstrate that F99S/M153T/V163A GFP (c3GFP) represents an efficient genetic reporter for STED, on account of no excited state absorption at depletion wavelengths <600 nm and a long emission lifetime. This makes c3GFP a valuable alternative to more common, but less photostable, EGFP and YFP/Citrine mutants for STED imaging studies targeting the green-yellow region of the optical spectrum.
Fluorescence imaging is constantly searching for new far-red emitting probes whose turn-on response is selective upon the interaction with specific biological targets. Cationic push-pull dyes could ...indeed respond to these requirements due to their intramolecular charge transfer (ICT) character, by which their optical properties can be tuned, and their ability to interact strongly with nucleic acids. Starting from the intriguing results recently achieved with some push-pull dimethylamino-phenyl dyes, two isomers obtained by switching the cationic electron acceptor head (either a methylpyridinium or a methylquinolinium) from the ortho to the para position have been scrutinized for their ICT dynamics, their affinity towards DNA and RNA, and in vitro behavior. By exploiting the marked fluorescence enhancement observed upon complexation with polynucleotides, fluorimetric titrations were employed to evaluate the dyes' ability as efficient DNA/RNA binders. The studied compounds exhibited in vitro RNA-selectivity by localizing in the RNA-rich nucleoli and within the mitochondria, as demonstrated by fluorescence microscopy. The para-quinolinium derivative showed some modest antiproliferative effect on two tumor cell lines as well as improved properties as an RNA-selective far-red probe in terms of both turn-on response (100-fold fluorescence enhancement) and localized staining ability, attracting interest as a potential theranostic agent.
Small organic molecules arouse lively interest for their plethora of possible biological applications, such as anticancer therapy, for their ability to interact with nucleic acids, or bioimaging, ...thanks to their fluorescence emission. Here, a panchromatic series of styryl-azinium bicationic dyes, which have already proved to exhibit high water-solubility and significant red fluorescence in water, were investigated through spectrofluorimetric titrations to assess the extent of their association constants with DNA and RNA. Femtosecond-resolved transient absorption spectroscopy was also employed to characterize the changes in the photophysical properties of these fluorophores upon interaction with their biological targets. Finally, in vitro experiments conducted on tumor cell lines revealed that some of the bicationic fluorophores had a peculiar localization within cell nuclei exerting important antiproliferative effects, others were instead found to localize in the cytoplasm without leading to cell death, being useful to mark specific organelles in light of live cell bioimaging. Interestingly, this molecule-dependent behavior matched the different amphiphilicity featured by these bioactive compounds, which are thus expected to be caught in a tug-of-war between lipophilicity, ensured by the presence of aromatic rings and needed to pass cell membranes, and hydrophilicity, granted by charged groups and necessary for stability in aqueous media.
Display omitted
•The photocatalytic activity of a series of iridium complexes toward water oxidation is explored.•Differential manometry allows simplified detection of O2 evolution.•KIE and 1H NMR ...studies disclose important aspects concerning the reaction mechanism and catalyst oxidative transformation.
The results of a systematic investigation on the photocatalytic water oxidation activity of eight Ir complexes are reported. In particular, Cp*IrLnXm 1, Ln=(H2O)3, X−m=(NO3−)2; 2, Ln=N-dimethylimidazolin-2-ylidene and (Cl−)2; 4, Ln=1,1′-dimethyl-3,3′-ethylene-diimidazol-2,2′- diylidene and Cl− X−=PF6−; 5, Ln=2-phenylpyridine (ppy) and NO3−; 6, Ln=2-benzoylpyridine (bzpy) and NO3−; 8, Ln=2,2′-bipyridine (bpy) and Cl−, Ir(HEDTA)ClNa (3) and Ir(ppy)2(OH2)2OTf (7), whose water oxidation activity driven by chemical oxidants was already proved, were tested as mediators (10μM–100μM) in photocatalytic experiments exploiting Ru(bpy)3Cl2 (1mM) as photosensitizer and Na2S2O8 (2.5mM–30mM) as electron acceptor, at pH 5.2 and pH 8 by Na2SiF6/NaHCO3 and Na2B4O7 buffers, respectively. All complexes showed to be competent catalysts for the photolytic water oxidation. Best performances were obtained with 2 at pH 5.2, where TOF>2.5min−1, TON>80 and quantum yield>0.1. H/D kinetic isotopic effect was evaluated for 1, 2 and 8 and related to their tendency to undergo oxidative transformation as deduced by 1H NMR studies, using CH3COOH as molecular probe.
PDF
