pH-sensitive quantum dots

We have designed organic ligands able to adsorb on the surface of CdSe-ZnS core-shell quantum dots and switch the luminescence of the inorganic nanoparticles in response to hydroxide anions. These compounds incorporate a [1,3]oxazine ring within their molecular skeleton, which reacts with the nucleophilic hydroxide anion to generate a 4-nitrophenylazophenolate chromophore. The chromogenic transformation activates an energy transfer pathway from the quantum dot to the adsorbed chromophores. As a result, the luminescence intensity of the coated nanoparticles decreases significantly in the presence of hydroxide anions. In fact, this mechanism can be exploited to probe the pH of aqueous solutions. Indeed, an increase in pH from 7.1 to 8.5 translates into a 35% decrease in the luminescence intensity of the sensitive quantum dots. Thus, our operating principles for luminescence switching can efficiently transduce a chemical stimulation into a change in the emissive response of semiconductor nanoparticles. In principle, this protocol can be extended from hydroxide anions to other target analytes with appropriate adjustments in the molecular design of the chromogenic ligands. It follows that luminescent chemosensors, based on the unique photophysical properties of semiconductor quantum dots, can eventually evolve from our design logic and choice of materials.     

Colorimetric detection of cyanide with a chromogenic oxazine

[reaction: see text] We have designed a chromogenic oxazine for the colorimetric detection of cyanide. Indeed, the [1,3]oxazine ring of our compound opens to form a 4-nitrophenylazophenolate chromophore in response to cyanide. This quantitative chromogenic transformation permits the detection of micromolar concentrations of cyanide in water. Furthermore, our chromogenic oxazine is insensitive to the presence of large concentrations of fluoride, chloride, bromide, or iodide anions, which are generally the principal interferents in the colorimetric detection of cyanide.     

Photoinduced enhancement in the luminescence of hydrophilic quantum dots coated with photocleavable ligands

In search of strategies to photoactivate the luminescence of semiconductor quantum dots, we devised a synthetic approach to attach photocleavable 2-nitrobenzyl groups to CdSe-ZnS core-shell quantum dots coated with hydrophilic polymeric ligands. The emission intensity of the resulting nanostructured constructs increases by more than 60% with the photolysis of the 2-nitrobenzyl appendages. Indeed, the photoinduced separation of the organic chromophores from the inorganic nanoparticles suppresses an electron-transfer pathway from the latter to the former and is mostly responsible for the luminescence enhancement. However, the thiol groups anchoring the polymeric envelope to the ZnS shell also contribute to the photoinduced emission increase. Presumably, their photooxidation eliminates defects on the nanoparticle surface and promotes the radiative deactivation of the excited quantum dots. This effect is fully reversible but its magnitude is only a fraction of the change caused by the photocleavage of the 2-nitrobenzyl groups. In addition, these particular quantum dots can cross the membrane of model cells and their luminescence increases by ~80% after the intracellular photocleavage of the 2-nitrobenzyl quenchers. Thus, photoswitchable luminescent constructs with biocompatible character can be assembled combining the established photochemistry of the 2-nitrobenzyl photocage with the outstanding photophysical properties of semiconductor quantum dots and the hydrophilic character of appropriate polymeric ligands.     

Bichromophoric photochromes based on the opening and closing of a single oxazine ring

We have designed bichromophoric photochromes based on the photoinduced opening and thermal closing of a [1,3]oxazine ring. In particular, we have synthesized six compounds incorporating fused 3H-indole and 4-nitrophenoxy fragments and pendant biphenyl, styryl, biphenylvinyl, or stilbenylvinyl groups. The laser excitation of two of these six molecules cleaves a [C-O] bond and opens their [1,3]oxazine ring in less than 6 ns with quantum yields of 0.08 and 0.28, respectively. This photoinduced process generates simultaneously a 4-nitrophenolate anion and a 3H-indolium cation. Both chromophores absorb in the same region of the electromagnetic spectrum. As a result, an intense band appears at ca. 440 nm upon the photoinduced opening of the [1,3]oxazine ring. In both instances, the photogenerated species switches back to the original isomer with first-order kinetics and lifetimes of 38 and 140 ns, respectively. Both compounds have excellent fatigue resistances and retain their photochemical behavior within rigid poly(methyl methacrylate) matrices. However, the thermal reisomerization within the polymer matrix is significantly slower and requires several microseconds to occur. The other four compounds do not undergo ring opening upon excitation under otherwise identical experimental conditions. Indeed, either photoinduced electron transfer or intersystem crossing compete successfully with the ring-opening process.     

Fast and stable photochromic oxazines

We have designed and synthesized two photochromic compounds incorporating fused indoline and benzooxazine fragments. Variable-temperature 1H NMR spectroscopy demonstrates that their central [1,3]oxazine ring opens thermally with free energy barriers ranging from 14 to 19 kcal mol(-1). The ring-opened species reverts rapidly to the original isomer and can only be detected after chemical trapping. Specifically, the nucleophilic attack of a hydroxide anion to the indolium cation of the ring-opened species prevents re-isomerization. Laser excitation of both compounds induces the opening of the [1,3]oxazine ring in less than 6 ns with quantum yields up to 0.1. The photoinduced ring opening generates a 4-nitrophenolate chromophore, which absorbs strongly at 440 nm. The photogenerated species reverts to the original form with a lifetime of 22 ns for both compounds. Thus, these transformations can be exploited to interconvert the two isomers of each species with nanosecond switching speeds. Furthermore, thousands of switching cycles can be repeated consecutively without any sign of degradation, even in the presence of molecular oxygen. These processes can be reproduced efficiently in poly(methyl methacrylate) matrixes. Under these conditions, the thermal re-isomerization occurs with biexponential kinetics in submillisecond time scales. In principle, the fast isomerization kinetics and excellent fatigue resistance of both compounds offer the opportunity to modulate rapidly and efficiently a variety of molecular and macroscopic properties. Thus, our molecular design can evolve into the realization of a new family of photochromic compounds and materials with promising photoresponsive character.     

A fast and stable photochromic switch based on the opening and closing of an oxazine ring

[reaction: see text] We have designed a molecular switch based on the photoinduced opening and thermal closing of an oxazine ring. Ultraviolet excitation of this molecule induces the cleavage of a [C-O] bond to form a p-nitrophenolate chromophore in less than 10 ns with a quantum yield of ca. 0.1. The photogenerated isomer reverts thermally to the original oxazine within 50 ns. Our photochromic switch survives more than 3000 excitation cycles without decomposing, even in air-saturated solutions.     

Luminescent chemosensors based on semiconductor quantum dots

Semiconductor quantum dots are inorganic nanoparticles with unique photophysical properties. In particular, their huge one- and two-photon absorption cross sections, tunable emission bands and excellent photobleaching resistances are stimulating the development of luminescent probes for biomedical imaging and sensing applications. Indeed, electron and energy transfer processes can be designed to switch the luminescence of semiconductor quantum dots in response to molecular recognition events. On the basis of these operating principles, the presence of target analytes can be transduced into detectable luminescence signals. In fact, luminescent chemosensors based on semiconductor quantum dots are starting to be developed to detect small molecules, monitor DNA hybridization, assess protein-ligand complementarities, test enzymatic activity and probe pH distributions. Although fundamental research is still very much needed to understand further the fundamental factors regulating the behavior of these systems and refine their performance, it is becoming apparent that sensitive probes based on semiconductor quantum dots will become invaluable analytical tools for a diversity of applications in biomedical research.     

Photoswitchable fluorescent dyads incorporating BODIPY and [1,3]oxazine components

We designed and synthesized three compounds incorporating a BODIPY fluorophore and an oxazine photochrome within the same molecular skeleton and differing in the nature of the linker bridging the two functional components. The [1,3]oxazine ring of the photochrome opens in less than 6 ns upon laser excitation in two of the three fluorophore-photochrome dyads. This process generates a 3H-indolium cation with a quantum yield of 0.02-0.05. The photogenerated isomer has a lifetime of 1-3 μs and reverts to the original species with first-order kinetics. Both photochromic systems tolerate hundreds of switching cycles with no sign of degradation. The visible excitation of the dyads is accompanied by the characteristic fluorescence of the BODIPY component. However, the cationic fragment of their photogenerated isomers can accept an electron or energy from the excited fluorophore. As a result, the photoinduced transformation of the photochromic component within each dyad results in the effective quenching of the BODIPY emission. Indeed, the fluorescence of these photoswitchable compounds can be modulated on a microsecond time scale with excellent fatigue resistance under optical control. Thus, our operating principles and choice of functional components can ultimately lead to the development of valuable photoswitchable fluorescent probes for the super-resolution imaging of biological samples.     

Photophysical properties of near-infrared-emitting Ln(III) complexes with 1-(9-Anthryl)-4,4,4-trifluoro-1,3-butandione (Ln = Nd and Er)

We report the synthesis and photophysical properties of Nd(III) and Er(III) complexes with 1-(9-anthryl)-4,4,4-trifluoro-1,3-butandione (9-ATFB). The complexes of [Nd(9-ATFB)4]- and [Er(9-ATFB)4]- produced sensitized near-infrared (NIR) luminescence via the excitation of anthracene. This suggests that the intramolecular energy transfer occurred from the singlet excited state of anthracene to the resonance levels of the metal ions, since the phosphorescence of anthracene is forbidden under normal conditions. The observed quantum yield of the visible luminescence showed that the energy transfer is more efficient for [Nd(9-ATFB)4]- than for [Er(9-ATFB)4]-. The lifetimes of the NIR luminescence of the complexes were in the microsecond range. The quantum yields of the sensitized NIR of the complexes were estimated using the lifetime and the energy-transfer quantum yield.     

Designed Long‐Lived Emission from CdSe Quantum Dots through Reversible Electronic Energy Transfer with a Surface‐Bound Chromophore

The size‐tunable emission of luminescent quantum dots (QDs) makes them highly interesting for applications that range from bioimaging to optoelectronics. For the same applications, engineering their luminescence lifetime, in particular, making it longer, would be as important; however, no rational approach to reach this goal is available to date. We describe a strategy to prolong the emission lifetime of QDs through electronic energy shuttling to the triplet excited state of a surface‐bound molecular chromophore. To implement this idea, we made CdSe QDs of different sizes and carried out self‐assembly with a pyrene derivative. We observed that the conjugates exhibit delayed luminescence, with emission decays that are prolonged by more than 3 orders of magnitude (lifetimes up to 330 μs) compared to the parent CdSe QDs. The mechanism invokes unprecedented reversible quantum dot to organic chromophore electronic energy transfer.     

Photophysical properties and energy transfer pathway of Er(III) complexes with Pt-porphyrin and terpyridine ligands

The photophysical properties of Er(III) complexes coordinated with platinum[5,10,15-triphenyl-20-(4-carboxyphenyl)-porphyrin] (PtP) and terpyridine (tpy) ligands in organic solution were investigated. The Er(III) complex emitted sensitized near-IR (NIR) luminescence when the PtP ligands were excited under deoxygenated conditions. The quantum yield (PhiLn) of the sensitized luminescence was 0.015%, as evaluated from luminescence lifetime. The photophysical studies and theoretical calculations suggest that the F?rster resonance mechanism is very suitable for the energy transfer from PtP to the Er(III) ion and occurred through the first triplet excited state of PtP. The 12.3% energy transfer from the triplet state to the 4F9/2 and 4I9/2 states of Er(III) occurred with a rate distribution of 3.36x10(5) and 6.67x10(4) s(-1), respectively. In addition, the observed triplet quantum yield of the PtP ligand in [Ln(PtP)3(tpy)] proved that the energy transfer from the singlet excited state of the PtP ligand to the Er(III) ion did not take place.     

Features of the influence of stabilizing ligands on luminescence properties of cadmium selenide colloidal quantum dots

It has been shown that immediately after the synthesis, the luminescence efficiency of CdSe quantum dots stabilized by n-octadecylphosphonic acid together with one of the auxiliary ligands decreases in the order: oleylamine, hexadecylamine, trioctylphosphine oxide, 1-octadecene, and stearic acid, which is due to combination of the energy of their binding with surface atoms of the nanoparticles and the packing density of the ligands in the shell. In the course of post-synthetic ripening of the quantum dots in a solution, changes in their luminescence quantum yield occur, depending on the solvent polarity and due to rearrangement of the ligands in the shells. The effect of dark recovery of trap luminescence from UV-irradiated quantum dots stabilized by octadecylphosphonic acid and a long-chain amine has been found.     

Synthesis of 1,3-diketo derivatives of calix[4]arene with nonyl substituents at the lower rim as novel efficient sensibilizers of Tb<Superscript>3+</Superscript> luminescence

Novel bis- and tetra-1,3-diketo derivatives of calix[4]arene with nonyl substituents at the lower rim have been synthesized. Their conformational and tautomeric composition have been determined; spectral parameters and complex formation properties with Tb³⁺ have been studied. Substitution of two 1,3-diketo groups with bromine under going from tetra- to bis 1,3-diketone derivatives was resulted in twofold increase luminescence intensity in their terbium complexes. Lifetime of the excited state of the terbium luminescence of the corresponding complexes as well as lifetime of excited triplet level of the 1,3-diketonate ligands have been determined.     

Luminescence quenching in self-assembled adducts of [Ru(dpp)3]2+ complexes and CdTe nanocrystals

We have investigated chloroform solutions containing tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) and CdTe nanocrystal quantum dots (5.6 nm diameter). The electronic levels of these two components are such that the Ru complex can act as an energy donor towards the quantum dot, which can thus behave as an energy acceptor. Steady-state and time-resolved spectroscopic experiments indicate that the Ru complexes and the CdTe nanocrystals self-assemble to give stable 1?:?1 adducts, in which the luminescence of the former units is strongly quenched. Such a quenching can be ascribed to either energy transfer to the CdTe quantum dot, or to electron transfer from the CdTe valence band to the excited Ru complex. However, no supporting evidence for the occurrence of photoinduced energy transfer in the adduct could be found. The CdTe luminescence is also slightly quenched in the presence of the ruthenium complex. The strong association of the metal complexes with the nanocrystals suggests that self-assembly strategies may be effectively employed to achieve surface functionalization of semiconductor quantum dots with molecular units.     

Synthesis of the new ring system 2-oxo-[1,4]oxazino[3,2-e]indole, heteroanalogue of Angelicin

A convenient synthesis of the 2-oxo-[1,4]oxazino[3,2-e]indole ring system, an heteroanalogue of Angelicin, is reported. Our synthetic approach consisted of the annelation of the oxazine ring on the indole moiety using 4-amino-5-hydroxy indoles as building blocks. The antiproliferative activity of the new compounds either in the dark or under UVA irradiation was investigated.     

Reversible cyclization of N-(3-hydroxy-1-alkenyl)pyridinium salts to pyridooxazines

Partially hydrogenated pyrido[2,1-b][1,3]oxazine, cyclopenta[d]pyrido[2,1-b][1,3]oxazine and pyrido[1,2-a]-[3,1]benzoxazine ring systems were easily formed in the reaction of 3-hydroxy-2-(2-hydroxy-1,3-dioxo-2-indanyl)-2-alken-1-one derivatives with tosyl chloride and pyridine bases. A facile interconversion between pyridooxazines and the corresponding pyridinium salts was also realized.     

Observation of enhanced energy transfer in individual quantum dot-oligophenylene vinylene nanostructures

The temporal and spectral properties of luminescence from individual CdSe quantum dot-oligophenylene vinylene nanostructures (single quantum dots with conjugated organic ligands coordinated to the surface) are profoundly modified relative to blended films of the same components. These kinds of composite quantum dot-conjugated organic systems have attracted significant interest as a way to improve efficiency in photovoltaic device applications. By direct functionalization of the dot surface with the conjugated organic ligands, we realize a significant enhancement in energy transfer and luminescence stability.     

Fluorescent II-VI semiconductor quantum dots in living cells: nonlinear microspectroscopy in an optical tweezers system

In this work we used a setup consisting of an optical tweezers combined with a nonlinear microspectroscopy system to perform scanning microscopy and obtain emission spectra using two photon excited (TPE) luminescence of captured single living cells labeled with core-shell fluorescent semiconductor quantum dots (QDs). The QDs were obtained via colloidal synthesis in aqueous medium with an adequate physiological resulting pH. Sodium polyphosphate was used as the stabilizing agent. The results obtained show the potential presented by this system as well as by these II-VI fluorescent semiconductor quantum dots to perform spectroscopy in living trapped cells in any neighborhood and dynamically observe the cell chemical reactions in real time.     

Wire-type ruthenium(II) complexes with terpyridine-containing [2]rotaxanes as ligands: Synthesis, characterization, and photophysical properties

[2]Rotaxanes based on the 1,2-bis(pyridinium)ethane subset[24]crown-8 ether motif were prepared that contain a terminal terpyridine group for coordination to a transition-metal ion. These rotaxane ligands were utilized in the preparation of a series of heteroleptic [Ru(terpy)(terpy-rotaxane)]2+ complexes. The compounds were characterized by 1D and 2D 1H NMR spectroscopy, X-ray crystallography, and high-resolution electrospray ionization mass spectrometry. The effect of using a rotaxane as a ligand was probed by UV/Vis/NIR absorption and emission spectroscopy of the Ru(II) complexes. In contrast with the parent [Ru(terpy)(2)]2+ complex, at room temperature the examined complexes exhibit a luminescence band in the near infrared region and a relatively long lived triplet metal-to-ligand charge-transfer (3MLCT) excited state, owing to the presence of strong-electron-acceptor pyridinium substituents on one of the two terpy ligands. Visible-light excitation of the Ru-based chromophore in acetonitrile at room temperature causes an electron transfer to the covalently linked 4,4'-bipyridinium unit and the quenching of the MLCT luminescence. The 3MLCT excited state, however, is not quenched at all in rigid matrix at 77 K. The rotaxane structure was found to affect the absorption and luminescence properties of the complexes. In particular, when a crown ether surrounds the cationic axle, the photoinduced electron-transfer process is slowed down by a factor from 2 to 3. Such features, together with the synthetic and structural advantages offered by [Ru(terpy)2]2+-type complexes compared to, for example, [Ru(bpy)3]2+-type compounds, render these rotaxane-metal complexes promising candidates for the construction of photochemical molecular devices with a wire-type structure.     

Photoluminescence of cadmium selenide quantum dots in polymer solutions

The effect of solvent on the photoluminescence of cadmium selenide quantum dots stabilized by oleic acid is examined with the example of two organic solvents: toluene and THF. It is found that THF favors desorption of ligands and formation of surface defects to a greater extent than toluene; as a result, the maximum of the photoluminescence band shifts to the red spectral region and its intensity decreases. The addition of polymers to the solution of quantum dots causes changes in the efficiency of photoluminescence and in the kinetics of its quenching. In the range of low concentrations (≤2 wt %) of quantum dots in polymer solutions, the intensity of luminescence first grows and then passes through a maximum and decreases. This effect may be explained both by the increasing number of surface defects and by quenching via energy transfer to polymers, especially in the case of polymers containing aromatic groups.