António L. Maçanita

Excited-State Dynamics and Self-Organization of Poly(3-hexylthiophene) (P3HT) in Solution and Thin Films

The fluorescence decays of a stereoregular head-to-tail RR-HT poly(3-hexylthiophene), P3HT, in methylcyclohexane (MCH) are described by sums of three or four exponential terms, respectively above and below -10 °C. In the high-temperature region, the polymer lifetime (ca. 500 ps) is accompanied by two shorter decay times (ca. 20 and 120 ps), which are assigned to intrachain energy transfer from high to lower energy excitons on the basis of temperature and wavelength dependence of the fluorescence decays. The absence of conformational (torsional) relaxation is attributed to the small dihedral angle between monomers that is predicted for the stereoregular polymer in the ground state. Below -10 °C, the polymer forms excimer-like aggregates, showing vibrational structured absorption and emission bands similar to those observed in thin films. The vibrational structure is attributed to a deep minimum in the ground-state energy surface of the dimer or aggregate. Below -40 °C, the fluorescence measured at the aggregate emission wavelength (670 nm) basically results from direct excitation of the aggregate and decays with a sum of three exponential terms (decay times of ca. 0.14, 0.6, and 1.5 ns, with similar weights). Because the spectral similarities between film and aggregates indicate similar electronic first singlet excited states (and oscillator strengths), the much shorter decay times (0.05, 0.15, and 0.43 ns) and lower fluorescence quantum yield of P3HT in films are assigned to efficient exciton dissociation and/or phonon-induced internal conversion competing with radiative decay (>1 ns).

Picosecond conformational relaxation of singlet excited polyfluorene in solution

Poly[9,9-di(ethylhexyl)fluorene] was studied by steady-state and time-resolved fluorescence techniques in solution in cyclohexane, methylcyclohexane, tetrahydrofuran, and decalin over the temperature range from 343 to 77 K. A decrease in temperature leads to a decrease in the inhomogeneous broadening of the emission band. Fluorescence decays were biexponential, consistent with a two-state model involving two different polymer conformers. Global analysis of the time profiles of luminescence collected at different emission wavelengths shows a long decay-time of 371.5±1.5 ps, which is temperature and solvent independent. The second shorter time (29±3 ps at 313 K and 100±3 ps at 233 K in methylcyclohexane) appears as a decay-time at the onset of the emission spectrum and as a risetime at longer wavelengths. Whilst the slow process was independent of temperature, the fast process showed Arrhenius type behavior, with an activation energy value of 0.10 eV found in both methylcyclohexane and decalin solutions. Ho...

Three interconverting excited species: experimental study and solution of the general photokinetic triangle by time-resolved fluorescence

Abstract Intermolecular photokinetics with the neutral, the anionic and the tautomeric forms of 7-hydroxy-4-methylcoumarin (7H4MC) is fully resolved using time-resolved fluorescence techniques. From the triple exponential fluorescence decays of these species, and from the separately determined fluorescence lifetimes of the neutral and anionic forms, the six rate constants and the fluorescence lifetime of the tautomeric form are determined. With these rate constants the steady state fluorescence data (fluorescence quantum yields as a function of [H + ]) are accurately reproduced.

Kinetics of ultra-fast excited state proton transfer from 7-hydroxy-4-methylflavylium chloride to water

Abstract Excited state proton transfer from 7-hydroxy-4-methylflavylium chloride to water is reported. From a modified analysis of picosecond time-resolved fluorescence data (not using the lifetime of a parent compound), all rate constants were determined: the deprotonation rate constant of the flavylium cation, k d =1.4×10 11 s −1 , the protonation rate constant of the base form, k p =2.3×10 10 l mol −1 s −1 and the reciprocal fluorescence lifetimes of these species, k AH + =7.8×10 9 s −1 ( τ AH + =128 ps) and k A =7.6×10 9 s −1 ( τ A =132 ps), in water, at 20°C. The value of k d is the largest measured value for an intermolecular proton transfer (to water).

The effect of radiative transport on fluorescence emission

A model for radiative transport of electronic excitation energy in solution is presented and applied to the time‐resolved and steady‐state fluorescence of DPA (9, 10‐diphenylanthracene) in benzene. The model predicts a nonexponential and wavelength‐dependent decay at high concentrations, in agreement with experimental results. Recovered parameters, along with the time‐resolved emission spectrum, are interpreted on the basis of a progression of the excitation with time farther into the cell, after the excitation pulse.

Viscosity dependence of intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane in alkane solvents as a function of temperature.

Intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane, (1PC(5)1PC) is studied as a function of temperature in a series of alkane solvents and in toluene, covering a wide range of solvent viscosities η, from 0.2 to 125 cP. The rate constant k(a) of the monomer → excimer reaction is determined from the effectively single exponential monomer fluorescence decays. For the viscosity dependence of k(a) in n-alkanes, the Stokes-Einstein relation k(a) ∼ η(-1.0) does not hold. Instead, k(a) is proportional to η(-α), with α increasing upon cooling, from 0.56 at 85 °C to 0.86 at -30 °C. The activation energy E(a) of excimer formation with 1PC(5)1PC, always larger than the activation energy E(T/η) of solvent viscous flow, decreases when the solvent viscosity becomes smaller, from 20.7 kJ/mol in n-hexadecane to 11.8 kJ/mol in n-butane, approaching a value of 11-12 kJ/mol for the low viscosity solvents. As the excimer formation process depends on the restricted diffusion of the 1PC end groups as well as on the C-O and C-C rotations in the -O(CH(2))(5)O- chain, the limiting barrier of 11-12 kJ/mol is attributed to the activation energy E(c) of the multiple bond rotations. This fractional viscosity dependence (α < 1.0) is caused by the multidimensional character of the barrier crossing in the excimer formation process. This multidimensional character should also be taken into account in investigations of polymers and biological media employing excimer formation.

Dynamics of short as compared with long poly(acrylic acid) chains hydrophobically modified with pyrene, as followed by fluorescence techniques.

New low and high molecular weight poly(acrylic acid), PAA, 2000 g mol(-1) and 450,000 g mol(-1), respectively, were tagged with pyrene (low and high contents of probe) and its behaviour in solution was investigated using absorption and fluorescence (steady-state and time-resolved) techniques. Fluorescence data shows that the degree and level of intramolecular association strongly depends on the molecular weight. With the short pyrene-labeled PAA chains in aqueous solution, the excimer-to-monomer fluorescence ratio I(E)/I(M) decreases with the increase of pH, oppositely to the increase in the I(E)/I(M) ratio with the increase in pH previously observed with the long chain PAA. Time-resolved data suggest that excimer formation with the short pyrene-labeled PAA polymers (ca. 28 acrylic acid monomers per chain) in water is largely due to excitation of Ground State Dimers, GSD. The increment of pH, and the consequent gradual ionization of the carboxylic groups in the chain, initially increases the fraction of GSD, possibly due to the occurrence of special micelle-like chain conformations, inside which the pyrene units are accommodated. A further increase of the pH above the pK(a) values, resulting in the full ionization of carboxylic groups, apparently destabilizes such chain conformations, which leads to a pH effect on the photophysical properties identical to that of the long chain polymers. In water, the dynamic data shows the existence of two excimers coexisting with two monomer classes. In methanol and dioxane (good solvents for the pyrene probe) at room temperature, where one excimer and two monomers are present, all rate constants could be obtained, as well as the fractions of ground-state species. It is thus shown that different types of interactions are produced with small- and long-sized PAA polymers, i.e., the size of the polymer matters.

The photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin related to the energy separation of the two lowest-lying singlet excited states

The photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin (ClMMC) was studied as a function of the solvent and temperature. The radiative lifetime τF0=1/kF is essentially solvent independent and its value (≈4.6 ns) is totally commensurate with the fluorescence originating from a 1(π,π*) state as the lowest excited state. From the fluorescence data obtained in 24 solvents plus nine solvent mixtures, and the triplet formation quantum yields for three representative solvents, it was found that the internal conversion rate constant (kIC) dictates the photophysical behavior of ClMMC and changes of two orders of magnitude occur from nonpolar to polar solvents. From the temperature dependence (20 to −100 °C) of the fluorescence lifetimes in five solvents it was found that a change of the internal conversion rate constant of the same order of magnitude occured as above. The rate constants and the activation energies for the radiationless processes were determined. The results show that the reason for the ...

Protein Stabilisation by Compatible Solutes: Effect of Mannosylglycerate on Unfolding Thermodynamics and Activity of Ribonuclease A

Differential scanning calorimetry, optical spectroscopy, and activity measurements were used to investigate the effect of mannosylglycerate, a negatively charged osmolyte widely distributed among thermophilic and hyperthermophilic archaea and bacteria, on the thermal unfolding of ribonuclease A (RNase A). For comparison, assays in the presence of trehalose, a canonical solute in mesophiles, and potassium chloride were also carried out. A thermodynamic analysis was performed by using differential scanning calorimetry data. The changes in the heat capacity for unfolding were similar for the different solutes examined. Mannosylglycerate was an efficient thermostabiliser of RNase A and induced an increase of 6 °C mole‐1 in the melting temperature. Moreover, the performance of mannosylglycerate as a stabiliser depended on the net charge of the molecule, with the maximal effect being observed at pH values above 4.5. Analysis of the enthalpic and entropic contributions to unfolding, derived from calorimetric data, revealed that the stabilisation rendered by mannosylglycerate is primarily achieved through a decrease in the unfolding entropy. Also, the number of protons taken up by RNase A upon denaturation in the presence of mannosylglycerate was considerably higher than with other solutes, a result consistent with a more rigid structure of the native protein. Mannosylglycerate (potassium salt) inhibited the activity of RNase A, albeit to a smaller extent than KCl, and acted as an efficient suppressor of aggregation of the denatured protein, thereby having a remarkable beneficial effect on the inactivation of RNase A upon thermal denaturation. The results are discussed in view of the physiological role of this charged compatible solute.

Photoprotection and the photophysics of acylated anthocyanins.

The proposed role of anthocyanins in protecting plants against excess solar radiation is consistent with the occurrence of ultrafast (5-25 ps) excited-state proton transfer as the major de-excitation pathway of these molecules. However, because natural anthocyanins absorb mainly in the visible region of the spectra, with only a narrow absorption band in the UV-B region, this highly efficient deactivation mechanism would essentially only protect the plant from visible light. On the other hand, ground-state charge-transfer complexes of anthocyanins with naturally occurring electron-donor co-pigments, such as hydroxylated flavones, flavonoids, and hydroxycinnamic or benzoic acids, do exhibit high UV-B absorptivities that complement that of the anthocyanins. In this work, we report a comparative study of the photophysics of the naturally occurring anthocyanin cyanin, intermolecular cyanin-coumaric acid complexes, and an acylated anthocyanin, that is, cyanin with a pendant coumaric ester co-pigment. Both inter- and intramolecular anthocyanin-co-pigment complexes are shown to have ultrafast energy dissipation pathways comparable to those of model flavylium cation-co-pigment complexes. However, from the standpoint of photoprotection, the results indicate that the covalent attachment of co-pigment molecules to the anthocyanin represents a much more efficient strategy by providing the plant with significant UV-B absorption capacity and at the same time coupling this absorption to efficient energy dissipation pathways (ultrafast internal conversion of the complexed form and fast energy transfer from the excited co-pigment to the anthocyanin followed by adiabatic proton transfer) that avoid net photochemical damage.