João Avó

Convenient Synthesis of 3-Vinyl and 3-Styryl Coumarins

A variety of 2-hydroxy aldehydes on reaction with 3-butenoic acid afford in a one-pot reaction the corresponding 3-vinylcoumarins. As expected, extension of the delocalized π-electron system accomplished by Heck coupling reactions with aryl halides results in an increased fluorescence of the compounds whose applicability is yet to be established.

The contributions of molecular vibrations and higher triplet levels to the intersystem crossing mechanism in metal-free organic emitters

Intense, simultaneous, room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) is observed in a series of donor-acceptor-donor (D–A–D) molecules. This dual-luminescence is stronger in the “angular” isomers, compared to their “linear” regioisomers, which is consistent with an enhanced intersystem crossing (ISC) in the former. Herein, we demonstrate that the small energy gap between the triplet levels, T1-Tn, below the lowest singlet state, S1, in the “angular” regioisomers, enhances the coupling between S1 and T1 states and favors ISC and reverse ISC (rISC). This is consistent with a spin-vibronic mechanism. In the absence of this “triplet ladder”, due to the larger energy difference between T1 and Tn in the “linear” regioisomers, the ISC and rISC are not efficient. Remarkably the enhancement on the ISC rate in the “angular” regioisomers is accompanied by an increase on the rate of internal conversion (IC). These results highlight the contributions of higher triplet excited states and molecular vibronic coupling to harvest triplet states in organic compounds, and casts the TADF and RTP mechanisms into a common conceptual framework.

Design and Synthesis of Photoactive Ionic Liquids

Two ionic liquids with photoisomerizable p-hydroxycinnamic acid moieties were synthesized and characterized by X-ray crystallography and DSC, and their photochemistry was studied in solution and neat conditions. Irradiation at absorption maxima led to trans-cis photoisomerization and resulted in significant reduction of melting temperatures of the ionic liquids. X-ray structures of both compounds show an intricate network of supramolecular interactions before irradiation. Physical and chemical transformations are completely reversible upon irradiation at lower wavelengths of ionic liquid solutions in acetonitrile.

Phase transition thermochromism based on C–H acidity of 4-alkylflavylium compounds in Pluronic F-127

In moderately acidic water/ethanol solutions, 7-(naphthalen-2-ylmethyl)-5,6-dihydrobenzo[c]xanthen-12-ium perchlorate shows C–H proton acidity giving rise to an ethylenic base species, instead of the hydration products of the typical flavylium network of chemical reactions, hemiketal, cis-chalcone and trans-chalcone. The kinetics of this acid–base process is much slower, kobs(s−1) = 3.5 × 10−3 + 0.16[H+], than those commonly observed for O–H deprotonation. In the presence of the triblock copolymer Pluronic F-127, the ethylenic base is dramatically stabilized shifting the pKa from 1.35 (in ethanol : water (1 : 1) to −0.35. Taking profit from the well-described temperature dependence of this polymer, a phase transition thermochromic system was achieved. The system was cycled between 5 and 35 °C with negligible changes in absorbance after 20 cycles.

Photorheological Ionic Liquids

Two room temperature ionic liquids (ILs) bearing coumarin and p-hydroxycinnamic acid moieties are synthesized, and their photochemistry is studied in solution and neat conditions. Irradiation at absorption maxima leads to photochemical transformations and results in changes of their rheological properties which are evaluated by rotational rheometry. Samples of ionic liquids are also studied by Hyper-Rayleigh scattering, and the effect of their photochemistry on ionic nanoaggregation is discussed.

Synthesis and multistate characterization of bis-flavylium dications – symmetric resorcinol- and phloroglucinol-type derivatives as stochastic systems

Two symmetric bis-flavylium dications were readily synthesized and further evaluated for their multistate profile. Both systems were shown to be fully stochastic and behave like a single compound suggesting that the two flavylium moieties do not communicate via the bridge linking them. Global pKa values of ca. 4 regarding the acid–base reaction between flavylium cation and quinoidal base were calculated using a stopped flow technique. It was further demonstrated that the equilibrium between AH+–AH+ and indistinguishable flavylium-quinoidal base isomers AH+–A (A–AH+) can be calculated by subtracting 0.3 pH units from the observed acid–base constant. On the other hand, the equilibrium between both flavylium-quinoidal base isomers and the bis-quinoidal base A–A is obtained by adding 0.3 pH units. Moreover, both systems do not have a cis–trans isomerization barrier and the rate constants of interconversion between flavylium cation and quinoidal base as a function of pH are fitted with a mono-exponential and follow a bell-shaped curve. The systems proved also to be photochromic and from the fitting of the bell-shaped curve, flash photolysis measurements and quantum yields, it is possible to calculate all rate and equilibrium constants and construct a global energy-level diagram. It was also proved that the rate constant to form both isomers AH+–Ct from the bis-flavylium upon a pH jump from 1 to less acidic solutions is twice that of the observed value and the formation of the bis-trans-chalcone from both isomers AH+–Ct is equal to the observed rate constant. An energy-level diagram of all the multistate species was constructed from the equilibrium constants.

Revealing the Hierarchical Mechanical Strength of Single Cellulose Acetate Electrospun Filaments through Ultrasonic Breakage.

The tensile strength of single cellulose acetate electrospun fibers is determined through sonication-induced fragmentation in water using a model previously developed by Terentjev and co-workers. The fragmentation of the electrospun fibers results in a gradual shortening of their length until a constant modal length is achieved. A single electrospun CA fiber tensile strength of ≈ 150 MPa (55-280 MPa) is determined based on fracture statistics. It is also observed that the fragmented fibers show bunches of nanofilaments at their ends with similar diameters to those of round structures observed in the cross-section of the initial electrospun fibers (≈ 38 nm). The sonication of these nanofilaments gives rise to spherical particles with similar diameter dimensions, which allows the estimation of a value of the tensile strength of the order of 2 MPa for these nanostructures. The aggregation and the alignment of the nano filaments inside the electrospun fiber should be the source of its higher strength value.

Exploring the diethylaminoflavylium derivatives multistate system of chemical reactions in the presence of CTAB micelles: thermodynamic reversibility achieved through different kinetic pathways

Aminoflavylium compounds like anthocyanins are involved in multiple reversible, pH-dependent chemical reactions. In very acidic media, the thermodynamically stable species is the flavylium cation, while it is trans-chalcone in near-neutral media. Different from anthocyanins, aminoflavylium compounds achieve the thermodynamic reversibility through different kinetic pathways depending on if the flavylium is transformed into the trans-chalcone upon a direct pH jump or the protonated trans-chalcone if converted into flavylium upon a reverse pH-jump. Three aminoflavylium compounds were prepared and investigated in the presence of CTAB micelles. Two of the compounds bear bridges linking rings C and B of the flavylium core, and the third lacks this bridge. The bridge has the effect of destabilizing the chalcones and the hemiketal. Only in the presence of CTAB micelles does the system respond to light stimuli, a powerful tool that provides a small cis–trans isomerization barrier for the three compounds. The relatively slow response of the system to direct and reverse pH jumps is due to the small mole fractions of Cc available for isomerization (giving Ct) and B available for dehydration (giving AH+). The mathematical expression that accounts for the pH-dependent kinetic processes was revisited to account for the consequences of amine protonation in very acidic media. The crystal structures of the compounds possessing the bridge were obtained, and for one of the compounds, two polymorphic forms were isolated from different crystallization solutions.