Roberto E. Di Paolo

Photodynamics of a PV Trimer in High-Viscosity Solvents and in PMMA Films: A New Insight into Energy Transfer versus Conformational Relaxation in Conjugated Polymers†

Non-Stokes-Einstein relaxation: The rate constant of conformational relaxation of a phenylenevinylene trimer (see picture) in different solvents is proportional to eta(-alpha), with alpha values decreasing from close to unity (low viscosity) to zero at sufficiently high solvent viscosity. This behaviour is attributed to the flexible methylbutyl side chains of the trimer, which partially screen the solvent friction. The p-phenylenevinylene (PV) trimer (MBOPV3) was used to probe the effect of conformational relaxation on fluorescence decays of PV-based polymers in the high solvent viscosity regime, from n-hexadecane (3.45 cP at 293 K) to liquid paraffin (123 cP at 293 K), and in dilute poly(methyl methacrylate) (PMMA) solid films. The effect of intermolecular energy transfer and radiative transport on the fluorescence decays was also analysed by increasing the concentration of MBOPV3 in the PMMA films up to a pure MBOPV3 film. The rate constant of conformational relaxation was found to decrease with increasing solvent viscosity up to about 23 cP, but then becomes viscosity independent. The non-Stokes-Einstein behaviour of k(CR) versus eta over the whole viscosity range apparently results from the presence of the flexible methylbutyl side chains of MBOPV3, which partially screens the solvent friction. Conformational relaxation is not observed in very dilute PMMA solid films, where the fluorescence decay becomes single exponential. The decays become multi-exponential again with an increase of MBOPV3 concentration in the PMMA films, but in this case it is due to intermolecular (interchain) energy transfer from less planar to more planar conformations of MBOPV3. In the pure MBOPV3 film, interchain energy transfer (radiative and non-radiative) is the major process responsible for the observed (tetra-exponential) decays.

Interaction between Poly(9,9-bis(6′-N,N,N-trimethylammonium)hexyl)fluorene phenylene) Bromide and DNA as Seen by Spectroscopy, Viscosity, and Conductivity: Effect of Molecular Weights and DNA Secondary Structure

The interaction between three poly(9,9-bis(6-N,N,N-trimethylammonium)hexyl)fluorene phenylene) bromide (HTMA-PFP) samples of different molecular weights (Mn=14.5, 30.1 and 61.3 kg/mol) and both dsDNA and ssDNA secondary structures has been studied using UV-visible absorption and fluorescence spectroscopies (including steady-state, time-resolved, and anisotropy measurements for the latter), viscosity, and electrical conductivity in 4% (v/v) DMSO-water mixtures. At low nucleic acid concentrations, formation of a 1:1 complex in terms of HTMA-PFP repeat units and DNA bases occurs. This interaction results in quenching of polymer emission. For higher molar ratios of DNA to HTMA-PFP, corresponding to charge neutralization, a second process is observed that is attributed to aggregate formation. From the changes in the absorption spectra, the polymer aggregation constant and the aggregate absorption spectra were calculated by applying an iterative method. Polymer aggregation dramatically quenches HTMA-PFP fluorescence in the region of the electroneutrality point. Under these conditions, the ratio of the emission intensity at 412 nm (maximum) to that at 434 nm (I412/I434) reaches a minimum, the electrical conductivity decreases, and the viscosity of the solution remains constant, showing that the DNA concentration can be determined through various HTMA-PFP physicochemical properties. With respect to the photophysical parameters (emission quantum yield, shape and shift of emission spectra), no significant differences were observed between dsDNA and ssDNA or with conjugated polymer or DNA molecular weight. The two short-lived components in the fluorescence decays are attributed to the presence of aggregates. Aggregates are also suggested to be responsible for the decrease in the fluorescence anisotropy through interchain exciton migration.

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.

Synthesis and optical properties of a new triphenylamine-p-phenylenevinylene-small molecule with applications in high open-circuit voltage organic solar cells

A new oligomer-type para-phenylenevinylene end-capped with triphenylamine groups (TPAPV) was synthesised and its optical properties thoroughly investigated. Fluorescence quenching studies showed that an important fraction of the photogenerated excitons formed in TPAPV is quenched by [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in blend films, therefore indicating efficient exciton dissociation and potential applications in organic photovoltaic (PV) cells. Planar heterojunction organic photovoltaic cells containing TPAPV as the electron-donor were fabricated and devices with structure ITO/TPAPV(6 nm)/C60(50 nm)/BCP(10 nm)/Ag exhibited an open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and power-conversion efficiency (PCE) of 0.92 V, 2.9 mA cm−2, 47.8%, and 1.23%, respectively.

Photophysical properties and biological evaluation of a Zinc(II)-5-methyl-1H-pyrazole Schiff base complex

A new ZnL2 complex containing two molecules of a tridentate Schiff base derived from 5-methyl-1H-pyrazole (HL) is synthesized and characterized. The photophysical properties of HL and ZnL2 are disclosed and supported by CAMB3LYP DFT/TDDFT calculations. It is shown that there is keto-tautomer stabilization upon excitation with an energetically accessible triplet state in HL, not present in ZnL2, this explaining the differences found in the emissions of the compounds. The intrinsic fluorescence of ZnL2 is used as probe for a detailed study of its binding to human serum albumin. The protein-complex association is thermodynamically favourable and it is shown by fluorescence quenching and time-resolved analysis that the fluorescence quenching involves a mixed mechanism with prevalence of static quenching, which corroborates adduct formation at site I, close to the Trp214 residue. The ability of ZnL2 to bind DNA was also evaluated, as well as its cytotoxic activity against MCF7 (breast), PC3 (prostate) cancer cells and hamster V79 fibroblasts. ZnL2 is a moderate DNA intercalator (Kapp = 3.9 × 104 M-1) and depicts a quite low IC50 value at 48 h against MCF7 cells (IC50 = 530 nM), but much higher for PC3 and V79 cells. The relevance of a more careful speciation evaluation of ZnL2 and other potential metal-based drugs in incubation media used in in vitro tests is highlighted.