João Seixas de Melo

Picosecond Structural Relaxation of Abietic Acid Based Amine End Capped Para-Phenylenevinylene Trimers in Solution

The synthesis and photophysical properties of six new abietic acid based amine end-capped p-phenylenevinylene trimers (AECPV3) in their lowest excited singlet states are presented. The AECPV3 compounds show a large red-shift of both the absorption (25-30 nm) and emission (37-42 nm) maxima with respect to those of the corresponding trimers. Picosecond time-resolved fluorescence data reveal the presence of a fast conformational relaxation process (40-62 ps) of the initially excited compounds, leading to more planar conformers. The conformational relaxation time is proportional to the volume of both the side chain and the amine groups.

Photochromic and photophysical properties of new benzo- and naphtho[1,3]oxazine switches

A new set of photochromic hindered benzo- and naphtho[1,3]oxazines with fast switching speed between the closed and open form were synthesised. Laser excitation of these uncoloured molecules (with ∼10 ns FWHM pulses) leads to the cleavage of the C-O bond and opening of the oxazine ring in less than 20 ns, generating a thermally unstable coloured zwitterionic species that reverts in few nanoseconds to the initial oxazine closed form. For these derivatives a detailed spectroscopic and photophysical study was undertaken involving measurements of absorption and phosphorescence spectra, phosphorescence quantum yields (ϕ(Ph)), phosphorescence lifetimes (τ(Ph)), triplet energies (E(T)), singlet oxygen (ϕ(Δ)) and ring opening photoreaction (ϕ) quantum yields, reisomerization kinetics and the zwitterionic form lifetime. A remarkable solvent effect on the lifetime of the photogenerated species was observed.

A Spectroscopic Study of the Interaction of the Fluorescent β-Carboline-3-carboxylic Acid N-methylamide with DNA Constituents: Nucleobases, Nucleosides and Nucleotides

Interaction between β-carboline-3-carboxylic acid N-methylamide, βCMAM, and nucleobases, nucleosides and nucleotides is studied in the ground state with UV-visible, 1H NMR and 31P NMR spectroscopies and in the first excited state, with steady-state and time-resolved fluorescence spectroscopy. Job plots show a predominant 1:1 interaction in both electronic states. Association constants are estimated from changes in the absorption spectra, and show that the strongest interaction is produced with the nucleosides: 2′-deoxyadenosine (dAdo) and thymidine (Thd), and with the mononucleotides: 2′-deoxycytidine 5′- monophosphate (5′-dCMP) and uridine 5′- monophosphate (5′-UMP). These results are corroborated by the upfield shifts of two 1H NMR resonances of the βCMAM indole group. The 31P NMR resonance of nucleotides is shifted downfield, suggesting the presence of electrostatic or hydrogen bond interaction with βCMAM. In the first electronic singlet excited state, static and dynamic quenching of βCMAM emission is achieved upon addition of nucleobases, nucleosides and nucleotides. This has been analysed using Stern–Volmer kinetics.

Synthesis, spectroscopy, photophysics and thermal behaviour of stilbene-based triarylamines with dehydroabietic acid methyl ester moieties

Novel stilbene-based triarylamines with dehydroabietic acid methyl ester moieties have been synthesised by palladium catalysed C–N coupling reactions. The presence of the bulky dehydroabietic acid group increases solubility, hinders crystallisation and permits their sublimation while preserving their electronic and optical characteristics. This makes them good candidates for various molecular electronic applications. We report a detailed characterisation of their electrochemical, spectroscopic and photophysical properties. In addition, the thermal stability and behaviour is discussed with reference to the potential for preparation of stable thin films.

Photochemistry for Cultural Heritage

Why do certain ancient natural dyes, such as indigo, preserve their colour so well while others, like brazilein, seem to degrade much faster? And how did mauveine change the world of colour? Will modern binding media, as vinyl paints, perform as well as a medieval tempera? Will it be possible to predict their durability? Photochemistry can answer many important questions about materials’ stability, providing new tools for the conservation of treasured artworks.

Effect of the Phospholipid Chain Length and Head Group on Beta‐Phase Formation of Poly(9,9‐dioctylfluorene) Enclosed in Liposomes

We have studied the effect of head group and alkyl chain length on β‐phase formation in poly(9,9‐dioctylfluorene) (PFO) solubilized in phospholipid liposomes. Systems studied have three different alkyl chain lengths (1,2‐dimyristoyl‐sn‐glycero‐3‐phosphatidylcholine [DMPC], 1,2‐didodecanoyl‐sn‐glycero‐3‐phosphatidylcholine [DLPC], 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphatidylcholine [DPPC]) and head groups (1,2‐dimyristoyl‐sn‐glycero‐3‐phosphate monosodium salt [DMPA], 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphoethanolamine [DMPE] and 1,2‐dimyristoyl‐sn‐glycero‐3‐phospho‐l‐serine sodium salt [DMPS]). Changes in liposome size upon addition of PFO are followed by dynamic light scattering. All the phospholipids induce the formation of PFO β‐phase, which is followed by the emission intensity and deconvolution of the absorption spectra. Both the head group and alkyl chain length affect the yield of β‐phase. The photophysics of PFO incorporated in liposomes is characterized by stationary and time‐resolved fluorescence, whereas the polymer‐phospholipid interactions have been studied by the effect of the PFO concentration on the phospholipid phase transitions (differential scanning calorimetry [DSC]).