Showing papers by "Luis G. Arnaut published in 2001"

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

Abstract: 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.

Singlet oxygen quantum yields from halogenated chlorins: potential new photodynamic therapy agents

Abstract: Flash photolysis and photoacoustic calorimetry were used to measure the energy-transfer rates and singlet oxygen quantum yields originated by the triplet states of halogenated tetrakisphenylporphyrins and related chlorins in aerated toluene. The chlorins ( λ max ≈660 nm , e≈3×10 4 M −1 cm − 1 ) have long-lived triplet states (>12 μs) in the absence of molecular oxygen, and in its presence the singlet oxygen production quantum yields of the 2-chloro and 2,6-dichlorophenyl derivatives are 0.89±0.05 and 0.98±0.02, respectively. The high absorptivity in the red, the photostability and the efficiency of these chlorins in producing singlet oxygen suggests a ground for the development of better sensitisers for photodynamic therapy.

Intramolecular Charge Transfer of p-(Dimethylamino)benzethyne: A Case of Nonfluorescent ICT State

Abstract: The effect of solvent polarity and external heavy atom on the steady state and time-resolved fluorescence properties of the locally excited (LE) state of p-(dimethylamino)benzethyne (DMABE) has been studied. It has been observed that in less polar environments, intersystem crossing (ISC) is the main nonradiative channel of deactivation for the excited species. However, at higher solvent polarity (ET(30) ≥ 44) the activation barrier dependent intramolecular charge transfer (ICT) reaction is the main nonradiative process dominating over ISC. The results reveal that DMABE undergoes an ICT reaction in polar environments to reach a nonfluorescent intramolecular charge transfer (ICT) state. The efficiency of formation of the ICT state of DMABE is compared with that of p-(dimethylamino)benzonitrile (DMABN). The present work settles the controversy raised by two previous works with conflicting results: a theoretical one, predicting that DMABE will undergo ICT reaction even under isolated conditions (Sobolewski, ...

Tritolylporphyrin dimer as a new potent hydrophobic sensitizer for photodynamic therapy of melanoma.

Abstract: We report the synthesis, photochemical and photophysical properties and preliminary studies on biological effect of a new tritolylporphyrin dimer (T–D). Absorption and emission properties of T–D suggest its possible use in photodynamic therapy. T–D is capable of singlet oxygen production with 0.8 quantum yield. It also has a high photostability. The photodynamic properties of the dimer were examined following the growth of SKMEL 188 (human melanoma) cells irradiated with red light (cut off < 630 nm). The surviving fraction of the cells decreased about 3-fold (vs. non-irradiated cells) for an 81 J/cm 2 dose. Our results suggest that tritolylporphyrine dimer T–D may be an interesting hydrophobic sensitizer for photodynamic therapy.

Photocyclization of triphenylamine: an investigation through time-resolved photoacoustic calorimetry

Abstract: Energetics of formation of the intermediate produced during the photocyclization of triphenylamine (TPA) in alkane and alcoholic solvents have been investigated by employing the time-resolved photoacoustic calorimetric technique. The study, in conjugation with flash photolytic observations, confirms that the long lived intermediate N-phenyldihydrocarbazole (NPDHC) is formed through a cascade of processes. The endothermicity for the process of NPDHC formation from TPA was determined to be ∽53 kcal mol−1 in all the environments studied. The nature and structure of NPDHC have been assigned for the first time. The experimental value of the enthalpy of reaction resembles well that calculated from the semi-empirical (AM1-SCI) method.

Energetics of photocyclization of polyphenylamines and assignment of the intermediate: A time-resolved photoacoustic calorimetric study

Abstract: The photocyclization reactions of diphenylamine (DPA), N-methyldiphenylamine (MDPA) and triphenylamine (TPA) have been studied in methanol solvent through nanosecond/microsecond flash photolysis and time-resolved photoacoustic calorimetry (TRPAC). It is confirmed for all the amines that the long-lived dihydrocarbazole intermediate ( 1 XDHC 0 ) is formed through a cascade of processes. However, DPA behaves differently from the other two members of the series so far as the rates of the individual processes are concerned. The quantum yield of triplet formation and the quantum yield of the photocyclization of the amine triplet have been estimated for the DPA system. In conjugation with the experiments, theoretical calculations, employing AM1-SCI method, have been performed to propose a structure for the intermediate. For all the amines, the intermediate has been proposed to be of zwitterionic nature in the ground state. The enthalpies of formation of the intermediate ( 1 XDHC 0 ) from MDPA and TPA have been determined experimentally to be around 51 kcal mol -1 and the values agree well with the calculated ones.

Theory of electron transfer reactions in blue-copper proteins

Abstract: The rates of electron transfer reactions in azurin and plastocyanin are calculated with the Intersecting-State Model and compared with experimental data. The calculated distance, free-energy and temperature dependencies of the intraprotein rates in Ru-modified azurins are in good agreement with the experiment. These calculations do not require the fitting of any parameters. Significant tunneling contributions to the room temperature rate are found in some systems. In some cases the symmetry or the orientation of the donor and acceptor orbitals are not favorable and the ET rates are reduced by factors exceeding 4 orders of magnitude.

Coexistence of Two Triplets for the Tict Probe Dmabn in Polar Solvents: An Experimental Evidence

Abstract: Room temperature nanosecond/microsecond laser flash photolysis and low temperature phosphorescence studies reveal that two different triplets coexist during the twisted intramolecular charge transfer (TICT) of -N,N-dimethylaminobenzonitrile (DMABN) in polar solvents.