Luca Scarpantonio

BF2-Azadipyrromethenes: Probing the Excited-State Dynamics of a NIR Fluorophore and Photodynamic Therapy Agent

BF(2)-Azadipyrromethene dyes are a promising class of NIR emitter (nonhalogenated) and photosensitizer (halogenated). Spectroscopic studies on a benchmark example of each type, including absorption (one and two photon), time-resolved transient absorption (ps-ms) and fluorescence, are reported. Fast photodynamics reveal that intense nanosecond NIR fluorescence is quenched in a brominated analog, giving rise to a persistent (21 μs) transient absorption signature. Kinetics for these changes are determined and ascribed to the efficient population of a triplet state (72%), which can efficiently sensitize singlet oxygen formation (ca. 74%), directly observed by (1)Δ(g) luminescence. Photostability measurements reveal extremely high stability, notably for the nonhalogenated variant, which is at least 10(3)-times more stable (Φ(photodeg.) = < 10(-8)) than some representative BODIPY and fluorescein dyes.

Copper Catalyst Activation Driven by Photoinduced Electron Transfer: A Prototype Photolatent Click Catalyst

PET cat. While the copper(II) tren ketoprofenate precatalyst 1 (see picture) is inactive at room temperature in methanol, it is quantitatively and rapidly reduced to its cuprous state upon light irradiation to provide a highly reactive click catalyst. By simply introducing air into the reaction medium the catalysis can be switched off and then switched on again by bubbling argon followed by irradiation.

Incorporation of luminescent CdSe/ZnS core-shell quantum dots and PbS quantum dots into solution-derived chalcogenide glass films

CdSe/ZnS core-shell quantum dots (CSQDs) and PbS quantum dots (QDs) were synthesized using a colloidal method and incorporated into Ge23Sb7S70 glass films via a solution-derived approach to film formation. Photoluminescence (PL) from the QDs inside the glass matrix was observed in the visible (CdSe/ZnS) and near-IR (PbS) regions. Properties of the QDs were found to be environment dependent, with the amine solvent partially quenching the luminescence. The PL lifetime of the CdSe/ZnS CSQDs and PbS QDs in the glass film was decreased to varying degrees from that of the QDs in pure chloroform. Monitoring the steady-state PL intensity and luminescence lifetime of PbS doped films showed that appropriate heat treatment of the deposited film increases the luminescence efficiency by removing residual solvent from the glass matrix.

A Photoreducible Copper(II)‐Tren Complex of Practical Value: Generation of a Highly Reactive Click Catalyst

A detailed study on the photoreduction of the copper(II) precatalyst 1 to generate a highly reactive cuprous species for the copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) click reaction is presented. For the photoactive catalyst described herein, the activation is driven by a photoinduced electron transfer (PET) process harnessing a benzophenone-like ketoprofenate chromophore as a photosensitizer, which is equally the counterion. The solvent is shown to play a major role in the Cu(II) to Cu(I) reduction process as the final electron source, and the influence of the solvent nature on the photoreduction efficiency has been studied. Particular attention was paid to the use of water as a potential solvent, aqueous media being particularly appealing for CuAAC processes. The ability to solubilize the copper-tren complexes in water through the formation of inclusion complexes with β-CDs is demonstrated. Data is also provided on the fate of the copper(I)-tren catalytic species when reacting with O2, O2 being used to switch off the catalysis. These data show that partial oxidation of the secondary benzylamine groups of the ligand to benzylimines occurs. Preliminary results show that when prolonged irradiation times are employed a Cu(I) to Cu(0) over-reduction process takes place, leading to the formation of copper nanoparticles (NPs). Finally, the main objective of this work being the development of photoactivable catalysts of practical value for the CuAAC, the catalytic, photolatent, and recycling properties of 1 in water and organic solvents are reported.

Facile Access to Highly Fluorescent Nanofibers and Microcrystals via Reprecipitation of 2-Phenyl-benzoxazole Derivatives

2-Phenyl-benzoxazole and five derivatives bearing an alkyl or alkoxy substituent on the phenyl ring were used to prepare aqueous suspensions of particles via a solvent-exchange method. In these conditions, the methyl and methoxy derivatives spontaneously gave nanofibers, while the other compounds led to microcrystals. This shows that minor chemical changes are enough to direct the formation of a given type of particle. From a spectroscopic viewpoint, all compounds strongly emit blue light in the solid state, with spectra much broader than those registered in n-heptane and ethanol solutions. The photoluminescence quantum yields reached 38% and were slightly affected in aqueous suspension by the polarity of the environment. The molecular arrangement, deduced from X-ray analysis for the methyl and methoxy derivatives, was used to explain the fluorescence properties in the solid state. This work shows that 2-phenyl-benzoxazole derivatives are interesting candidates for applications as fluorescent nanomaterials, including in aqueous and biological media.

Electronic Energy Transfer Modulation in a Dynamic Foldaxane: Proof-of-Principle of a Lifetime-Based Conformation Probe.

Abiotic aromatic oligoamide foldamers are shown to self-assemble in solution to form a double helix, which can accommodate a bichromophoric thread in its central void. While in solution reversible electronic energy transfer is instilled between chromophoric termini of the free, flexible thread as evidenced through delayed luminescence, upon rigidification of the rod the chromophores are mutually distanced and effectively decoupled. Consequently, the chromophores display their individual photophysical characteristics. The observed conformation-dependent changes of dynamic luminescence properties, which are particularly sensitive to distance, offers a new strategy for lifetime-based detection of geometry on the molecular scale as demonstrated through real-time luminescence detection of molecular complexation leading to foldaxane formation.

Hydrosoluble dendritic poly(ethylene oxide)s with zinc tetraphenylporphyrin branching points as photosensitizers

Synthesis of dendrimer-like star-branched poly(ethylene oxide)s (PEOs) with two and three generations having Zn(II) tetraphenylporphyrin (ZnTPP) moieties at the core and at each branching point has been successfully achieved through an iterative convergent approach harnessing “click chemistry”. In the first step, a four-armed poly(ethylene oxide) star-shaped polymer was synthesized by the copper(I)-catalyzed azide–alkyne cycloaddition between α-azido poly(ethylene oxide) branches (N3-PEO-OH) and 5,10,15,20-tetra(propargyloxyphenyl) Zn(II) porphyrin as the core. In the second step, a tetra-armed star PEO bearing 5-(4-phenoxy)-10,15,20-tris(propargyloxyphenyl) Zn(II) porphyrin at each branch end was prepared by divergence of the ω-hydroxyl PEO chain-ends through a Williamson-type etherification reaction. The resulting star PEO with 3 propargyl groups at each arm end was then subjected to a new “click chemistry” reaction with N3-PEO-OH affording the dendrimer star-like PEO of second generation carrying 12 new ω-OH-PEO external branches attached to the four ZnTPP branching points. Repetition of the second step afforded the high molar mass third generation dendrimer-like PEO carrying 36 ω-OH PEO terminal branches with a uniform distribution of 17 zinc tetraphenylporphyrin moieties as internal branching points. The photophysical properties of the resulting polymers in organic or aqueous medium and their efficiency in singlet oxygen generation are reported.

Efficient oxidation and destabilization of zn(cys)4 zinc fingers by singlet oxygen.

Singlet oxygen ((1)O2) plays an important role in oxidative stress in all types of organisms, most of them being able to mount a defense against this oxidant. Recently, zinc finger proteins have been proposed to be involved in its cellular detection but the molecular basis of this process still remains unknown. We have studied the reactivity of a Zn(Cys)4 zinc finger with (1)O2 by combinations of spectroscopic and analytical techniques, focusing on the products formed and the kinetics of the reaction. We report that the cysteines of this zinc finger are oxidized to sulfinates by (1)O2. The reaction of the ZnS4 core with (1)O2 is very fast and efficient with almost no physical quenching of (1)O2. A drastic (ca. five orders of magnitude) decrease of the Zn(2+) binding constant was observed upon oxidation. This suggests that the Zn(Cys)4 zinc finger proteins would release their Zn(2+) ion and unfold upon reaction with (1)O2 under cellular conditions and that zinc finger sites are likely targets for (1)O2.

Photolariats: synthesis, metal ion complexation and photochromism

Photolariat development, as an extension to the family of synthetic photochromic crown ether receptors, or photocrowns, is reported. Incorporated additional chelating groups, namely two anisoles or thioanisoles, contribute in completing the metal ion coordination sphere with different affinities and selectivities for a range of ions. Single-crystal X-ray diffraction analysis suggests that the thermally stable trans-form of the receptor is unsuitable for ion binding, consistent with spectrophotometric and NMR titration results, which is largely improved in the cis-form as the basis for the photocontrolled ion coordination/ejection. In terms of the azobenzene-containing receptor photochemistry, a photostationary state highly enriched in the cis-form (94:6, cis-/trans-) is reached, with slow thermal return (1.1–1.4 × 10− 5 s− 1) in the dark, which can undergo multiple cycles without discernable photodegradation.