Migration Of Triplet Excitations Research Articles

Migration of Triplet Excitations in Chrysene Glass Films

ABSTRACT Research results of delayed fluorescence decay kinetics in solid chrysene films deposited on quartz substrates in vacuum are presented. It is found that at the temperatures which are close to the temperature of liquid nitrogen, phosphorescence and annihilation delayed fluorescence decay kinetics are described by non-exponential function. The data obtained were discussed from the point of formal kinetics and percolation T–T annihilation models.

Migration of triplet excitations of complex molecules in disordered media and in systems with a confined geometry

The paper presents the results of analysis of migration of a triplet excitation in inhomogeneous media such as mixed molecular crystals, solid solutions of organic compounds in low-molecular solvents and polymers, as well as porous matrices activated by complex molecules. The experimental data obtained under conditions of steady-state excitation and time resolution are analyzed within different approaches used for describing the energy transport in disordered systems.

Percolation of triplet excitation in molecularly doped polymer, copolymer and rigid solution matrices

The triplet energy migration in benzophenone and 1,4 dibromonaphthalene doped polystyrene, methylmethacrylate–styrene copolymer and ethanol–methanol rigid glassy solution has been studied under steady state excitation. The probability of donor excitation energy capture by the traps shows a critical concentration dependence on benzophenone molecules. The percolation model has been successfully applied to evaluate the critical exponents. It is observed that in rigid glassy solution and in polystyrene matrices a three-dimensional triplet excitation migration topology is observed, whereas in copolymer matrix it is two-dimensional.

Migration of triplet excitations generated by ionizing radiation in organic materials

The problem of triplet excitations migration for the case both of the diffusion - controlled triplet-triplet annihilation and of the concentration - controlled one has been considered.

Percolation of triplet excitation in restricted geometries

Migration of benzophenone triplet excitations in polymethylmethacrylate and methylmethacrylate-styrene copolymer has been studied under steady state excitation by using 1-chloronapthalene as a trap. The excitation energy capture efficiency by a trap has been obtained as a function of the donor concentration at various trap concentrations. Percolation model has been successfully applied to evaluate the critical exponents. The evaluated critical exponents are in very good agreement with three dimensional triplet excitation migration topology in polymethylmethacrylate and two dimensional one in the copolymer.

Triplet excitation migration for compound molecules in solid solutions

Migration of benzaldehyde triplet excitations in solid solution has been studied by the technique of capture by traps using steady-state excitation. The probabilities of excitation capture by a trap have been obtained as a function of the concentration of the indicated compound at various temperatures and trap concentrations. Analysis of the experimental dependencies for scaling in the system “benzaldehyde-1-bromonaphthalene in ethanol” results in values of the critical exponents β=0.41 and γ=1.7 coinciding with the theoretical ones, obtained in the scope of percolation theory for three-dimensional space. The values of the critical exponents and the character of the dependencies are different when toluene or PMMA are used as matrices. We analyze the obtained experimental data within the cluster model of energy migration and the Blumen-Silby model.

Percolation of triplet excitation of compound molecules in polymeric matrices

The electronic excitation energy migration over the triplet levels of complex molecules in polymeric matrices has been investigated by the method of capture by traps and the annihilation delayed fluorescence method in the 77–320 K temperature range. From experiments with steady-state excitation for benzophenone in PMMA values of the critical exponents β=0.18 and γ=1.8 have been obtained. It is shown that the deviation of the value of the critical exponent β from the theoretical value for the three-dimensional space is due to the influence of the microscopic organization of the amorphous polymer. For comparison, we have investigated the migration of triplet excitations of benzophenone in ethanol. The thus obtained critical exponents β=0.41 and γ=1.8 are in good agreement with theory for the case of three-dimensional space. The application of the notion of connectivity to the description of the energy migration process in the systems under investigation is considered. The dependence of the connectivity radius on the system's properties and experimental conditions for the case of the exchange interaction between molecules is shown.

One-dimensional migration of triplet excitations in polymers at low temperatures: polyquanylic acid—lTb 3+ complex

The energy migration in the frozen polymer Poly(G) doped with Tb 3+ ions-energy traps is studied at 77 K. It is shown that the decay of the Tb 3+ luminescence is in good agreement with the theory of migration in low-dimensional systems. The approximation gives the upper limit of the jump rate to be 1.2 × 10 5 s −1. The influence of the energy disordering upon the excitation migration along the polymer chain is discussed.

Time resolved studies of naphthalene mixed crystals. Fractal and Euclidian behaviors of the migration kinetics

Time resolved experiments have been used in order to study the migration of triplet excitations at low temperature (T<4.2 K) in napthalene D8 crystals doped with 5% to 20% of naphthalene H8 containing a small amount of betamethylnaphthalene. The analysis of the time dependence of the fusion of two triplet excitations leads to clearly deomonstrate the fractal behavior of the exploration at short time and the Euclidian behavior at longer times. The spectral dimension has been measured for the smallest studied concentrations (c<12%); it ranges between 1.33 and 1 which is in very good agreement with a 3D percolation model. The time T0 at which the crossover between the fractal and Euclidian exploration occurs, has been approximately determined for the concentrations. It turns out that the 20% sample does not exhibit this fractal behavior on the studied time scale; this is also in agreement with the percolation model. We have also demonstrated that the time dependence of the fusion of two excitations gives a better insight in the energy migration processes than the time dependence of the supertrapping processes.

BEHAVIORS OF THE TRIPLET MOLECULE IN NEAT LIQUID OF NAPHTHALENE DERIVATIVES AT ROOM TEMPERATURE

Abstract Triplet excited species in neat liquids of 1-methylnaphthalene and 1-chloronaphthalene were investigated by means of flash photolysis. The triplet excited dimers are assumed on the bases of analysis of T-T absorption spectra. A rapid T-T energy transfer from 1-chloronaphthalene to trans-stilbene indicates a triplet excitation migration.

Triplet—Triplet Annihilation and Delayed Fluorescence in Molecular Aggregates

This paper deals with triplet—triplet annihilation in pure and mixed organic crystals. In crystals containing a small concentration of impurity traps, triplet excitation migration may proceed from trap to trap on a time scale which is short compared with the long triplet state lifetime but which is long compared with the normal fluorescence lifetime. Nearest-neighbor and long-range mutual annihilation of two triplets may then take place giving rise to delayed fluorescence. The rates of long-range triplet excitation migration and annihilation show a concentration dependence, a temperature dependence, and a solvent dependence. Providing the triplet—triplet annihilation rate is not too fast, the intensity of the delayed fluorescence can be shown to depend upon the square of the intensity of the exciting light. This expectation is borne out by experiments, briefly reported here, on delayed fluorescence in dilute isotopic mixed crystals. In crystals containing high concentrations of such impurity traps, or in pure crystals, the annihilation rate becomes extremely rapid and this mechanism effectively quenches phosphorescence in many, but not all, classes of pure organic crystals. The kinetics of the over-all process are discussed in both the limits of fast and slow annihilation rates. A theoretical investigation of the origin of the annihilation matrix element is carried out, and it is shown that exchange interactions play the largest role in determining annihilation rates. Past work [H. Sponer, Y. Kanda, and L. A. Blackwell, J. Chem. Phys. 29, 721 (1958); N. W. Blake and D. S. McClure, ibid., p. 722] on delayed fluorescence of assumed pure naphthalene crystals containing very small amounts of β-methyl naphthalene (traps) can be understood within the framework of this paper. The need in organic crystals for the major delayed fluorescence mechanism to be based upon ionization and electron trapping seems now to be considerably lessened.

Retardation of singlet and triplet excitation migration in organic crystals by isotopic dilution

The mixed crystal C/sub 10/H/sub 8/in C/sub 10/D/sub 8/ provides a 100- cm/sup -1/ trap. A crystal of high chemical purity containing 0.5% C/sub 10/H/ sub 8/ was studied in emission between 4.7 and 77 deg K on a 2-m grating spectrograph. Results and conclusions are listed. (L.N.N.)