Triplet-doublet Interactions Research Articles

Time-Resolved Continuous-Wave and Pulse EPR Investigation of Photoinduced States of Zinc Porphyrin Linked with an Ethylenediamine Copper Complex

Electron spin dynamics of the photoexcited state of a new porphyrin system built of a zinc porphyrin molecule with a linked complex of copper ion (ZnPCu) was studied using electron paramagnetic resonance (EPR) methods. The time-resolved continuous-wave and echo-detected EPR data show that electron spins of the ZnPCu system are polarized. The shape of the EPR spectrum and the echo-detected nutation spectroscopy data indicate that there is a relatively small spin–spin interaction between two subunits, namely, less than the zero-field splitting parameter of the excited triplet ZnP molecule. Results have been interpreted assuming that the S = 1 Zn porphyrin subunit electron spins are polarized via the triplet mechanism and this polarization is transferred to the S = 1/2 copper subunit via a flip-flop process induced by the exchange interaction between these subunits. The formation of the integral spin polarization in the triplet–doublet system as a function of the value and type of the triplet–doublet interaction was analyzed.

Application of the short and long consecutive pairs model to the triplet-doublet interaction in molecular crystals

We have adapted the model of two consecutive pairs to the study of the triplet-doublet (T-D) interaction in molecular crystals. We have applied this model to the modulation of the photoconductivity in crystalline anthracene by a static magnetic field (MFE) and a microwave field (PDMR). We were able to reproduce, for the first time, quite perfectly two types of experiments with the same set of kinetic constants.

Optimal magnetization in liquids, generated by triplet—doublet interaction

This study is an extension of a recent theoretical treatment for calculating the magnetization generated by the encounters of radicals and photoexcited triplets in solution. Whereas the previous study employed a restricted analytical approach to the problem, the present study takes into account a general numerical formulation for the solution of the stochastic Liouville equation to calculate the electron spin polarization generated in the radical, following its encounter with the triplet. This method considers the efficiency of triplet quenching by the radical, which is an important factor in determining the radical polarization and the triplet lifetime in the solution. In addition, numerical calculation of the diffusion process is used to obtain the overall magnetization of the radical and its time dependence. The theory presented here complies with the experimental results and allows for efficient optimization of the magnetization in terms of magnitude and overall lifetime. Such an optimization is accomplished by the proper choice of the chemical system, which is exposed to light excitation, solvent properties and temperature. The ultimate goal of this study is to achieve photo-controlled high magnetization, which can be used in a variety of novel microwave applications.

Optoelectronic properties of sexithiophene single crystals

UV—Vis absorption, photoluminescence (PL) and photoconductivity (PC) measurements have been carried out on sexithiophene (α6T) single crystals grown from the vapor phase. UV—Vis absorption data were used to determine the absorption coefficient and refraction index under polarized light. The spectrum at polarization perpendicular to the unique axis b is similar to that measured on polycrystalline films, whereas that at light polarized parallel to b presents a weak structured absorption. The refractive index is 1.656, independent of the wavelength, under parallel polarization, and follows the Sellmeier equation, with n ∞ = 1.867, under perpendicular polarization. The PL excitation spectrum with parallel polarization follows exactly the corresponding weak absorption, whereas, at perpendicular polarization, it saturates when the absorption length compares the crystal thickness, and then decreases down to a minimum at 3.5 eV. This lowering mirrors a decrease of the PL yield at higher energies. The PC action spectrum shows an opposite behavior: it presents first a rise at 2.4 eV, parallel to the absorption onset, then a second rise at 3.0 eV up to a maximum at 3.3 eV. The first rise corresponds to the generation of singlet excitons, which then give birth to charges, through either exciton breaking, or more likely charge detrapping. The second rise could correspond to a direct ionization process. The magnetic field effect on PC has been studied. A relative decrease of up to 6% at a field of 4000 G has been detected. The decrease is attributed to the lowering of the triplet—doublet interaction constant R.

In VivoExperimental Studies on the Role of Free Radicals in Photodynamic Therapy

The effect of excited Photofrin II molecules on the zymosan stimulated and luminol dependent chemiluminescence (CL) of macrophages, known as respiratory burst, has been studied. Excitation was carried out by light irradiation in the vicinity of the maximal CL by varying light doses. In separate experiments potassium salt of 3,5-di-tert-butyl-4-hydroxyphenyl propionic acid radical inhibitor was added to the system and its consumption measured spectroscopically. Based on the experimental results the decrease in the steady state radical concentrations under the influence of radical inhibitor and of triplet sensitizer generatedin situhas been calculated. It has been established that the corresponding photodynamic effect is an inhibition-like process proceeding by triplet–doublet interactions.

Absolute magnitude of spin polarization in the radical-triplet pair mechanism: CIDEP generation by level crossings in a triplet-doublet interaction

Chemically induced dynamic electron polarization (CIDEP) generated through the interaction of the excited triplet state of benzophenone with the 2,2,6,6,-tetramethyl-1-piperidinyloxyl (TEMPO) radical was investigated by time-resolved ESR spectroscopy in benzene solution. We carefully examined what factors control the CIDEP intensities. By comparing the CIDEP intensity of TEMPO obtained in the triplet benzophenone-TEMPO system with the intensity in the C 60-TEMPO system, the absolute magnitude of net emissive polarization was determined to be −6.9 in the unit of Boltzmann polarization. The emissive polarization is attributed to state mixing between a quartet and a doublet in the radical-triplet pair induced by the zero-field splitting interaction of the counter triplet molecule. Our result is quantitatively explained by the theory that the net CIDEP is generated predominantly in regions where the quartet and doublet levels cross. This indicates that the quenching of the excited triplet benzophenone by TEMPO in benzene can proceed via an electron-exchange interaction.

Physicochemical Modeling of the Role of Free Radicals in Photodynamic Therapy. III. Interactions of Stable Free Radicals with Excited Photosensitizers Studied by Kinetic ESR Spectroscopy

The loss of paramagnetism of the stable free radical 4,4′-di-tert.-butyl-diphenyl-1-nitroxy during illumination of the photosensitizer meso-tetrahydroxyphenyl chlorin by light above 620 nm in the absence of oxygen has been followed by kinetic ESR spectroscopy. Addition of the spin trap α-(4-pyridyl-1-oxide)-N-tert.-butyl-nitrone reduces the initial rate of the disappearance of the free radical enabling to separate triplet-doublet interactions from processes between radicals stemming from the triplet sensitizer and the stable free radical. Kinetic treatment of the mechanism suggested yielded the rate constant of the triplet-doublet interaction proceeding via electron transfer being about 6% of the rate constant of the overall interaction including both energy transfer and electron transfer.

Perturbation par une puissance micro-onde de l'interaction triplet-doublet dans les cristaux moléculaires. Application à la photoconductivité de l'anthracène.

We study via photoconductivity the modulation of the triplet-doublet interaction by a microwave field in molecular crystals and we apply it to crystalline anthracene. The kinetic model indicates the presence of microwave transitions between the Triplet-Doublet (T-D) pair spin states in the form of a spectrum of three rays which merge into a single ray for a magnetic field direction corresponding to the crossing of the pair levels. The experiment agrees with the existence of these rays and allows us to evaluate a pair life time of the order of 2×10 -9 s

Spin polarization generated in the triplet-doublet interaction: hyperfine-dependent chemically induced dynamic electron polarization

Spin polarization generated in the triplet-doublet interaction: hyperfine-dependent chemically induced dynamic electron polarization