Radiative and Nonradiative Decay Processes

2 Radiative and Nonradiative Decay Processes - Due to the potential application of these compounds as photosensitizers for photodynamic therapy the photophysical properties of porphyrins and phthalocyanines, and their corresponding metal complexes, have been investigated extensively over the past decade. The photophysical properties of water-soluble metalloporphyrins, and especially the tetraphenylsulfonates, have been re-examined but nothing new has been found. The disulfonated metallophthalocyanines (MPcS2, where M = Al , Ga , or Zn ) form complexes with fluoride ions for which the fluorescence yields and lifetimes are decreased with respect to the parent dyes while there are 

The fluorescence properties of substituted polyphenyls have been recorded in 

The first reported fluorescence lifetime of a diazirine has appeared and gives a lifetime for adamantyldiazirine of 240 ps at ambient temperature. A report has 

Phosphorescence lifetimes of 2-amino-l-methyl-6-phenylimidazo[4,5-b]pyri-dine and benzo[f]quinoline increase with decreasing temperature in glucose glasses. A series of articles has addressed the properties of the excited triplet state of 4H-l-benzopyrane-4-thione in perfluoroalkane solvents.Delayed S2-S0 fluorescence arises via triplet-triplet annihilation. 

A common problem encountered in photophysical investigations concerns 

2 Radiative and Nonradiative Decay Processes - The use of cyclodextrins or surfactant dispersions to promote room-temperature phosphore ence from organic molecules is a particularly simple but elegant way in which to characterise triplet excited states. This field has been greatly extended by the synthesis of modified cyclodextrins bearing chromophoric groups that can be included into the cavity.Additional studies have reported binding constants for the 

The photophysical properties of polyacene molecules depend markedly on the number of rings and the fluorescence behaviour of hexacene has now been compared with that of earlier members of the series. A similar comparison has been made of the photophysical properties of catacondensed aromatic poly-cycles. Fluorescence from an upper-excited singlet state has been described for benz[a]azulene derivatives while the fluorescence properties of some antiaromatic molecules have been described in detail. Several thiopyrylium and pyrylium salts have been studied and the effects of various substituents attached to the heterocycle have been examined in terms of the triplet yield. A full evaluation of the photophysical properties of 4-aminonaphthalimide, and its 

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In the treatment of radiationless transitions presented above, we have mainly considered the case of a closed channel decaying into a single open channel, which latter consists of the dense vibronic manifold of some one electronic state (statistical limit). That description is obviously incomplete, since both radiative and nonradiative decay processes occur simultaneously. Clearly, a complete theoretical description of the radiationless transition... 

By taking into account the radiative and nonradiative decay processes described above, Eq. 8 gives the overall decay rate constant of the emitting state of the metal. In this equation, kt is the radiative rate constant and knr and knr(T) are the nonradiative temperature independent and temperature dependent decay rate constants, respectively. 

The first optical laser, the ruby laser, was built in 1960 by Theodore Maiman. Since that time lasers have had a profound impact on many areas of science and indeed on our everyday lives. The monochromaticity, coherence, high-intensity, and widely variable pulse-duration properties of lasers have led to dramatic improvements in optical measurements of all kinds and have proven especially valuable in spectroscopic studies in chemistry and physics. Because of their robustness and high power outputs, solid-state lasers are the workhorse devices in most of these applications, either as primary sources or, via nonlinear crystals or dye media, as frequency-shifted sources. In this experiment the 1064-mn near-infrared output from a solid-state Nd YAG laser will be frequency doubled to 532 nm to serve as a fast optical pump of a raby crystal. Ruby consists of a dilute solution of chromium 3 ions in a sapphire (AI2O3) lattice and is representative of many metal ion-doped solids that are useful as solid-state lasers, phosphors, and other luminescing materials. The radiative and nonradiative relaxation processes in such systems are important in determining their emission efficiencies, and these decay paths for the electronically excited Cr ion will be examined in this experiment. 

Quantum yield of the fluorescence processes is defined by the ratio of emitted photons to absorbed photons which can be further expressed in the form of radiative and nonradiative decay rate ... 

The radiative decay of singlet excitons is clearly an important process in the operation of polymer LEDs. This rate is denoted by kr, where for PPV, (k, rl 1200 ns.19 Radiative decay competes with various nonradiative decay processes, such as quenching of excitons by defects, exciton dissociation, and intersystem crossing to form triplet states. Assuming that both radiative and nonradiative decays are monoexponential, the photoluminescence quantum efficiency, PLeff, defined as the number of photons emitted per photon absorbed, is given by... 

The second reason to introduce the derivation (6 -9) is to note that all that is required to evaluate the absorption and emission probability F A (t, r) of (9) are matrix elements of the evolution operator exp(-i//r/h). (These matrix elements are the conventional probability amplitudes When considering a situation in which many different kinds of decay processes are involved, e.g. radiative and nonradiative decay, it is not always convenient to deal directly with the matrix elements of exp(-itfr/h), the af(t). Rather, it is simpler to introduce (imaginary) Laplace transforms 16) in the same manner that electrical engineers use them to solve ac circuit equations 33L Thus, if E is the transform variable conjugate to t, the transforms of af(t) are gf(E). The quantities gf (E) can also be labeled by the initial state k and are denoded by Gjk(E). It is customary in quantum mechanics to collect all these Gjk(E) into a matrix G(E). Since matrix methods in quantum mechanics imply some choice of basis set and all physical observables are independent of the chosen basis set, it is convenient to employ operator formulations. If G (E) is the operator whose matrix elements are Gjk(E), then it is well known that G(E) is the Green s function i6.3o.34) or resolvent operator... 

The model (9.73)—(9.75) was presented as an initial value problem We were interested in the rate at which a system in state 0) decays into the continua L and R and have used the steady-state analysis as a trick. The same approach can be more directly applied to genuine steady state processes such as energy resolved (also referred to as continuous wave ) absorption and scattering. Consider, for example, the absorption lineshape problem defined by Fig. 9.4. We may identify state 0) as the photon-dressed ground state, state 1) as a zero-photon excited state and the continua R and L with the radiative and nonradiative decay channels, respectively. The interactions Fyo and correspond to radiative (e.g. dipole) coupling elements between the zero photon excited state 11 and the ground state (or other lower molecular states) dressed by one photon. The radiative quantum yield is given by the flux ratio Yr = Jq r/(Jq r Jq l) = Tis/(Fijj -F F1/,). 

In the absence of triplet quenchers two processes compete for triplet deactivation radiative (phosphorescence) and nonradiative decay of the triplet to the ground state ... 

The third process sensitive to heavy-atom perturbation is the radiative decay from the triplet to the ground state (phosphorescence). Since phosphorescence is commonly not observed in fluid solution at room temperature, the rate of phosphorescence in the presence of heavy-atom perturbation relative to the rate of intersystem crossing and nonradiative decay need not be considered. At low temperatures in a rigid glass, however, phosphorescence... 

To test the above ideas, Weitz etal.(i2) performed experiments on the fluorescence decay from a thin layer of europium(III) thenoyltrifluoracetonate (ETA) deposited on a glass slide covered with Ag particles approximately 200 A in diameter. The fluorescence decay rate was found to increase by three orders of magnitude in comparison with that of ETA in solid form. In addition to the large increase in decay rate, there was also evidence for an increase in overall fluorescence quantum efficiency. It is not possible from Eq. (8.11) to say anything about the manner in which is partitioned between radiative and nonradiative processes, y should be written in terms of a radiative part yr and a nonradiative part ynr y = yr + y r. Since the radiative rate for dipole emission is given by... 

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