J. Sérgio Seixas de Melo

Heavy-atom effects on metalloporphyrins and polyhalogenated porphyrins

The photophysics of halogenated and metallated tetrakisphenylporphyrins is investigated using single-photon counting, photoacoustic calorimetry and luminescence techniques. The radiationless transition rates in these and related molecules are interpreted with a quantum-mechanical tunnelling model modified to include the effect of spin–orbit coupling in the intersystem crossing rates. It is shown that tetrakisphenylporphyrins with halogens in the ortho positions have long-lived triplet states that are formed in high yields. 2002 Elsevier Science B.V. All rights reserved.

Spectroscopy and photophysics of 4- and 7-hydroxycoumarins and their thione analogs

Acid-base equilibria were studied for 4-hydroxycoumarin, 7-hydroxy-4-methylcoumarin and their thione derivatives in different media for their ground and lowest energy singlet and triplet excited states. The ethoxylated and/or methoxylated derivatives were also investigated. Characterization involves fluorescence spectra, quantum yields, lifetimes, phosphorescence spectra, lifetimes, and triplet‐triplet absorption spectra. From their pKa values it was found that 4 and 7-hydroxycoumarins are more acidic in their lowest excited singlet states than in S0. The origin and character of the lowest singlet and triplet excited states is discussed. q 2001 Elsevier Science B.V. All rights reserved.

Singlet and triplet energies of α-oligothiophenes: A spectroscopic, theoretical, and photoacoustic study: Extrapolation to polythiophene

The triplet state energies, ET(0–0), for the α-oligothiophenes (αn’s) with n=1–7, were determined and vary from 18 000 cm−1 (α2) to 13 000 cm−1 (α7). The data were obtained from spectroscopic and calorimetric measurements. It was found that the triplet energies decrease with the increase ring number (n), leveling off beginning with the α-oligothiophene with n=5. A linear relation was obtained for the triplet energy as a function of the reciprocal of n. The data has allowed the extrapolation to polythiophene (n=∞) and the evaluation of its triplet energy (≅10 000 cm−1). The energies (0–0) of the first singlet excited state of n=1–7 were also evaluated and again a linear relation was found for the E[S1(0–0)] as a function of 1/n. The S1−T1 energy splitting, ΔE(S1−T1), was determined and also found to decrease with the increase in the number ring. Semiempirical molecular orbital theoretical calculations reproduce this same trend of decreasing ΔE(S1−T1) with n. Extended FEMO theory was applied to the singlet ...

S1∼>T1 intersystem crossing in π-conjugated organic polymers

Quantum yields for triplet formation have been determined for seven common π-conjugated polymers in benzene solution using time-resolved photoacoustic calorimetry (PAC) in conjunction with fluorescence quantum yields, singlet and triplet energies. The polymers studied include three poly(thiophenes), poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), a cyano derivative of MEH-PPV, a ladder type poly(p-phenylene) (MeLPPP), and a poly(fluorene). Yields of singlet oxygen formation have also been determined for these polymers in benzene by time-resolved phosphorimetry, and are in reasonable agreement with triplet yields obtained by PAC. Polythiophenes show the highest intersystem crossing yields, which are suggested to result from extensive spin-orbit coupling. Where singlet oxygen yields are less than triplet yields, it is suggested that interaction of molecular oxygen with the ground state of the polymers may be involved.

Photophysics of thiophene based polymers in solution: The role of nonradiative decay processes

An investigation has been undertaken of the photophysics of six thiophene-based polymers. This includes the measurement of fluorescence quantum yields, quantum yields for triplet formation, and determination of fluorescence and triplet lifetimes in benzene solution at room temperature. From the above-mentioned data, the overall set of rates for the deactivation processes (fluorescence, intersystem crossing, and internal conversion), has been evaluated. Mechanisms of nonradiative decay are discussed, and it is shown that both intersystem crossing and internal conversion are important in the decay of the lowest singlet excited state of isolated polythiophenes molecules in solution. Comparison of spectroscopic and photophysical properties of the polymers with analogous oligothiophenes shows that with the polymer, the S1⇝S0 internal conversion deactivation pathway plays a major role, in contrast to the behavior found with the oligomers where this internal conversion deactivation channel is essentially negligible.

The use of microspectrofluorimetry for the characterization of lake pigments

In this paper, the potential of confocal microfluorescence spectroscopy is explored for the characterization of selected red lake pigments and paints based on alizarin, purpurin and eosin (weak, medium and strong emitters). The anthraquinone pigments have been used since ancient times by artists, and eosin lakes were used by impressionist painters. Reconstructions of artists paints based on 19th century recipes are examined. The paints were made using the lake pigments bound in a range of binding media including gum arabic, collagen, a vinyl emulsion and linseed oil. The acquisition of the spectra is rapid, with high spatial resolution and the data reliable and reproducible. Together with full emission spectra, it was possible to acquire well-resolved excitation spectra for purpurin, alizarin and eosin based colors. The present investigation suggests that micro-emission fluorescence can also be used as a semi-quantitative method for madder lake pigments, enabling the determination of purpurin lake ratio in a mixture of purpurin and alizarin, which is important for provenance studies. The data obtained with microfluorescence emission with those acquired with fiber-optic fluorimetry are compared. The spatial resolution used, 8microm, is appropriate for the analysis of individual pigments particles or aggregates in a paint film. Micro-emission molecular fluorescence proved to be a promising analytical tool to identify the presence of selected red lake pigments combined with a range of binding media.

Triplet state dynamics on isolated conjugated polymer chains

Triplet state behaviour has been studied with several conjugated polymers in dilute benzene solutions by flash photolysis, photoacoustic calorimetry (PAC) and pulse radiolysis/energy transfer. With polythiophenes and the ladder poly(p-phenylene) MeLPPP, singlet–triplet intersystem crossing (ISC) is relatively efficient. In contrast, it is inefficient with poly(p-phenylenevinylene)s (PPVs) and polyfluorene, while with cyano-substituted PPV, there is no evidence for any long-lived triplet state. Energy transfer from triplet biphenyl to MEH-PPV is diffusion controlled and triplet state lifetimes are typically tens or hundreds of μs. All the triplet states are quenched by molecular oxygen, leading to formation of singlet oxygen with yields which are generally close to those for triplet formation. With pulse radiolysis at high doses, it is possible to have more than one triplet state per polymer chain. This can lead to delayed fluorescence via intrachain triplet–triplet annihilation. Kinetic analysis of this shows slow movement of triplets by hopping along the chain.

Preparation and photophysical characterisation of Zn–Al layered double hydroxides intercalated by anionic pyrene derivatives

Zn–Al hydrotalcite-like compounds intercalated by 1,3,6,8-pyrenetetrasulfonate (PTS), 1-pyrenesulfonate (PS) and 1-pyrenecarboxylate (PC) anions were synthesised by an ion-exchange procedure. The materials were characterised by powder X-ray diffraction at different temperatures, thermogravimetric analysis, FTIR, 13C{1H} CP/MAS NMR and photoluminescence techniques. In the fully hydrated states, the interlayer distances are 13.7 A for Zn–Al–PTS, 18.9 A for Zn–Al–PS, and 24.8 A for Zn–Al–PC. These can be ascribed to a monolayer arrangement for intercalated PTS anions and bilayer arrangements for the 1-pyrenyl derivatives. The samples exhibit different thermal decomposition pathways, and in the case of Zn–Al–PTS the removal of physisorbed and interlayer water leads to a change in the orientation of the organic anion with respect to the hydroxide layers. The structural transformation is fully reversible upon hydration. The photophysical characterisation of the bulk materials was based on the determination of their emission and fluorescence excitation spectra, and the fluorescence lifetimes. From the steady-state (monomer and “excimer-like” bands) and time-resolved (triple exponential decays) data, evidence for the presence of a structure with similar characteristics to pyrene dimer together with monomer (by comparison with the emission of dilute solutions of PS and PC), pre-associated and (possibly) dynamic excimer species could be presented.

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...