Marta Liras

Methacrylate‐tethered Analogs of the Laser Dye PM567—Synthesis, Copolymerization with Methyl Methacrylate and Photostability of the Copolymers¶

The synthesis and characterization of new analogs of the laser dye PM567 (4,4‐difluoro‐1,3,5,7,8‐pentamethyl‐2,6‐diethyl‐4‐bora‐3a,4a‐diaza‐s‐indacene) with the 8‐position substituted by a linear chain with n methylenes (n = 1, 3, 5, 10 or 15) tethered with an acetoxy or methacryloyloxy group (PnAc and PnMA, respectively) is described. The monomeric dyes PnMA have been successfully copolymerized with methyl methacrylate (MMA), yielding linear copolymers of high optical quality where the covalently bonded chromophore is separated from the polymeric main chain by a spacer of variable length. The photostability of the solid polymeric materials under UV (310 nm) irradiation (method ASTM G 53‐77) has been compared with those of the model dyes PnAc and PM567 as solid solutions in poly‐MMA (PnAc–PMMA and PM567–PMMA, respectively). In all the cases, the chromophore bound to the polymer is more photostable than that simply dissolved in the same polymer, with photodegradation quantum yields in the range 2.3 × 10−5 to 4.8 × 10−5, which was interpreted as due to additional modes for the dissipation of the absorbed energy along the polymeric chain. In both polymer solutions and copolymers, the length of the polymethylene chain has low or null influence on the photostability. In ethanol solution, PnAc model dyes with polymethylene chains with three or more methylene groups show about the same photostability; this is of an order of magnitude higher than that of the parent dyes P1Ac and PM567 in the same solvent.

New Analogues of the BODIPY Dye PM597: Photophysical and Lasing Properties in Liquid Solutions and in Solid Polymeric Matrices

New tailormade BODIPY dyes have been synthesized by a simple protocol to reach wavelength finely tunable laser action from 540 to 625 nm while maintaining highly efficient and photostable laser emission. The new dyes are analogues of the commercial dye PM597 with the eight position free (PTH8) or substituted by the groups acetoxymethyl (PTAlk) or p-acetoxymethylphenyl (PTAr). The photophysical properties strongly depend on the geometrical distortion from planarity of the indacene core generated by the presence of the bulky 2,6-di-tert-butyl groups and the eight substituent. In both liquid and polymeric solid solutions, lasing efficiencies of up to 63 and 48%, respectively, were observed under transversal pumping at 532 nm with high photostabilities. In the case of PTAlk incorporated into silicon-containing solid organic matrices, the laser emission remained at 92% of its initial intensity value after 100,000 pumping pulses in the same position of the sample at 30 Hz repetition rate. The laser action of the new dyes enhances that of the parent dye PM597 and outperforms the lasing behavior of dyes considered to be benchmarks over the green-yellow to red spectral region.

Structure and Formation of the Fluorescent Compound of Lignum nephriticum

The intense blue fluorescence of the infusion of Lignum nephriticum (Eysenhardtia polystachya), first observed in the sixteenth century, is due to a novel four-ring tetrahydromethanobenzofuro[2,3-d]oxacine which is not present in the plant but is the end product of an unusual, very efficient iterative spontaneous oxidation of at least one of the trees flavonoids.

Photophysical and Lasing Properties of New Analogs of the Boron–dipyrromethene Laser Dye Pyrromethene 567 Incorporated into or Covalently Bounded to Solid Matrices of Poly(methyl methacrylate)¶

Abstract The photophysical, lasing and thermostability properties of newly synthesized analogs of the commercial dye pyrromethene 567 (PM567) have been measured in polymeric matrices of poly(methyl methacrylate) both when used as a dopant and when covalently bounded to the polymeric chain. These analogs have an acetoxy or a polymerizable methacryloyloxy group at the end of a polymethylene chain at Position 8 of the PM567 chromophore core. Clear correlations between photophysical and lasing characteristics are observed. Linking chain lengths with three or more methylene units give the highest fluorescence quantum yields (as high as 0.89) and lasing efficiencies (as high as 41%). The covalent linkage of the chromophore to the polymeric chain via the methacryloyloxy group improves the photostability of the PM567 chromophore.

Acetyl protected thiol methacrylic polymers as effective ligands to keep quantum dots in luminescent standby mode

Herein, we report evidence for the promising potential of 2-(acetylthio)ethylmethacrytale (AcSEMA), a new monomer, which copolymerizes with methyl methacrylate (MMA) by atom transfer radical polymerization (ATRP) leading to well-defined multidentate protected thiol polymer ligands for CdSe QDs capping by simple ligand exchange. Among the different strategies that can be employed for coating QDs, it is found that thioacetylated polymers, in the presence of hydrophobic QDs, are able to replace trioctylphosphine oxide (TOPO) ligands, leading to hybrid CdSe@PMMA nanoparticles without covalent binding of the polymer to the QD surface. These polymer protected QDs exhibit high stability for long periods of time, in a low luminescence standby mode. Moreover, the fluorescence QD activation of these hybrids is easily achieved at any time from thioacetate hydrolysis to free thiol under mild conditions. This subsequent step, which results in the covalent attachment of PMMA thiol groups to the metallic QD surface, produces compact QDs which exhibit amazingly improved photophysical properties, yielding highly photoluminescent new hybrid materials. For comparison, the same procedure was applied to obtain brush-like QDs coated with PMMA obtained by ATRP with a final thioacetate group. It was then established that the multidentate thiolated polymers provide additional protection against external quenchers, such as 4-amino-TEMPO, to a greater extent than the brush-like PMMA coating and the pristine TOPO capping ligand.

Versatile thiolated thermosensitive polymers synthesized by ATRP of MEO2MA and AcSEMA, a new methacrylic monomer with a protected thiol group

Herein, the synthesis of a new monomer containing a protected thiol group, 2-(acetylthio)ethyl methacrylate (AcSEMA), is introduced. The monomer has been copolymerized via atom transfer radical polymerization (ATRP) with 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) to obtain a series of well-defined hidden-thiol functionalized thermosensitive polymers. The new system exhibits a sharp lower critical solubility temperature (LCST) and after hydrolysis of the acetyl group, the thiolated copolymers exhibit pH responsiveness. Moreover, to show the versatility of AcSEMA, P(MEO2MA-co-AcSEMA) copolymers were in situ hydrolyzed and modified by thiol-ene Michael addition with some acrylate compounds. The click reaction was successfully performed as revealed by NMR and the change in the LCST. We finally demonstrate that the addition of these polymer coatings onto gold nanoparticles (AuNPs) results in the formation of stable, colloidal thermosensitive polymer@AuNP complexes due to bridge formation between the thiol groups of AcSEMA and the metallic NP surfaces. The formation of temperature responsive polymer coated plasmonic nanoparticles shows the promise of P(MEO2MA-co-AcSEMA) copolymers for building multifunctional nanostructures for drug-delivery, diagnosis, tagging, catalysis and organic electronics systems.

Switching to a reversible proton motion in a charge-transferred dye

We report on the steady-state, pico- and femtosecond time-resolved emission studies of 6-amino-2-(2-methoxyphenyl)benzoxazole (6A-MBO) and 6-amino-2-(2-hydroxyphenyl)benzoxazole (6A-HBO) in different solvents. We observed an intramolecular charge transfer (ICT) reaction following by slow (relatively) solvent relaxation, which happened in the same time domain for both molecules. The ultrafast ICT reaction happens in 80-140 fs whereas the solvent relaxation occurs in 0.5-1.1 ps. In 6A-MBO the excited CT species has a lifetime of ∼2.5 ns. However, in 6A-HBO and after the ICT reaction, a reversible excited-state intramolecular proton transfer (ESIPT) reaction takes place in the formed enol charge transfer (ECT*) species producing a keto (K*) type tautomer. Depending on the solvent, the forward ESIPT reaction (ECT* → K*) happens in 40-175 ps while that of the reverse one (ECT* ← K*) occurs in 240-990 ps. Kinetic isotopic effect (OH/OD exchange) study in acetone shows that the reversible ESIPT reaction occurs via tunneling, while we suggest that in acetonitrile solution it evolves along the IHB and solvent coordinates. Our results show a reversible proton motion coupled to charge-transfer reactions opening the way to new explorations of charge- and proton-transfer dynamics and spectroscopy.

A deprotection strategy of a BODIPY conjugated porous polymer to obtain a heterogeneous (dipyrrin)(bipyridine)ruthenium(II) visible light photocatalyst

This work shows that a deprotection strategy of BODIPY conjugated porous polymers (CMPBDPs) can be successfully applied to synthesize a new (dipyrrin)(bipyridine)Ru(II) (CMPBDP-Ru) efficient heterogeneous photocatalyst for iminium ion generation under visible light. CMPBDP-Ru shows high thermal and photochemical stability under irradiation, and it could be reused several times.

Versatile Approach for the Fabrication of Functional Wrinkled Polymer Surfaces

A simple and versatile approach to obtaining patterned surfaces via wrinkle formation with variable dimensions and functionality is described. The method consists of the simultaneous heating and irradiation with UV light of a photosensitive monomer solution confined between two substrates with variable spacer thicknesses. Under these conditions, the system is photo-cross-linked, producing a rapid volume contraction while capillary forces attempt to maintain the contact between the monomer mixture and the cover. As a result of these two interacting forces, surface wrinkles were formed. Several parameters play a key role in the formation and final characteristics (amplitude and period) of the wrinkles generated, including the formulation of the photosensitive solution (e.g., the composition of the monomer mixture) and preparation conditions (e.g., temperature employed, irradiation time, and film thickness). Finally, in addition, the possibility of modifying the surface chemical composition of these wrinkled surfaces was investigated. For this purpose, either hydrophilic or hydrophobic comonomers were included in the photosensitive mixture. The resulting surface chemical composition could be finely tuned as was demonstrated by significant variations in the wettability of the structured surfaces, between 56° and 104°, when hydrophilic and hydrophobic monomers were incorporated, respectively.

Synthesis and photophysics of novel biocompatible fluorescent oxocines and azocines in aqueous solution.

The spectroscopic properties in water solution of the different prototropic forms of the strongly fluorescent hemiacetal 4,9-dihydroxy-1,2-dihydro-4,11a-methanooxocino[4,5-b]benzofuran-5(4H)-one (1a, monardine), the aza analogue 4,9-dihydroxy-3,4-dihydro-1H-4,11a-methanobenzofuro[2,3-d]azocin-5(2H)-one (2a, azamonardine) and the respective 2-carboxyl derivatives (1b, 2b) have been studied by experimental and quantum-chemical methods. Monardine and carboxymonardine are the major products of new fluorogenic, room-temperature reactions of hydroxytyrosol or salvianic acid in aqueous solution, respectively, and present unique photophysical properties. Near neutral pH (pKa = 7.2) monardine switches from a weakly emitting, UV-absorbing (382 nm) neutral species to a VIS-absorbing (426 nm), blue emitting (464 nm) anion form, with a fluorescence quantum yield ϕF = 1 and single-exponential decay τF = 2.74 ns. This binary-like spectroscopic change from the neutral to the anionic form was interpreted based on time-dependent density functional theory (TDDFT) calculations as due to (i) the reversal of (n,π*) and (π,π*) lowest-lying singlet excited states, and (ii) a change in the triplet-state distribution accompanying monardine ionization which may abolish de-excitation via intersystem crossing. A similar fluorogenic reaction takes place with catecholamines such as dopamine and DOPA, to yield fluorescent azocines 2a and 2b which, depending on pH, may be present as cationic, neutral or anionic species. TDDFT computations of these forms were also carried out to assign the corresponding excitation transitions and emission properties. Besides the analytical interest of the fluorogenic reactions, the photochemical stability and biocompatibility of the bright-dark pH-controlled molecular switches 1a and 1b may facilitate novel labels and probes to be developed for superresolution fluorescence microscopy.