Excited state nonradiative deactivation

The photoluminescence of dipyridophenazine complexes of ruthenium ) in the presence and absence of DNA has been well-characterized (38-40, 46-52). Excitation of the dppz complexes with visible light (440 nm) leads to localized charge transfer from the metal center (39, 40). In aqueous solution, the emission resulting from the metal-to-ligand charge-transfer excited state is deactivated via nonradiative energy transfer... 

The lifetime of an analyte in the excited state. A, is short typically 10 -10 s for electronic excited states and 10 s for vibrational excited states. Relaxation occurs through collisions between A and other species in the sample, by photochemical reactions, and by the emission of photons. In the first process, which is called vibrational deactivation, or nonradiative relaxation, the excess energy is released as heat thus... 

After excitation has occurred, there are several processes which are important in the deactivation of the excited states. Those discussed in this section will be nonradiative, that is, they do not involve the emission of light. 

Fig. 10 Energy level diagram showing the excited states involved in the main photophysical processes (excitation solid lines radiative deactivation dashed lines, nonradiative deactivation processes wavy lines) of the 2 Nd3+ [Ru(bpy)2(CN)2] three-component system. For the sake of clarity, naphthyl excimer energy level has been omitted...

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

The effective lifetimes of all these excited states are determined by radiative as well as collisional deactivation, and which contribution is the more significant depends on pressure and transition probability. The simultaneous recording of the absorption and fluorescence spectra yields information about the ratio of radiative to collisioninduced nonradiative decays. This ratio is proportional to the quotient of total fluorescence from the excited level to total absorbed laser light. Such experiments have been started by Ronn oif... 

Photophysical processes, that is, ones not involving any change in composition of an A, have become of much interest to the inorganic photochemist, particularly in terms of excited state kinetic schemes. A brief discussion of the phenomenology and theory of radiative and nonradiative deactivations follows. 

One solution to the problem of semiconductor photodecomposition is to modify the spectral response of a stable wide-band-gap semiconductor so that solar energy can be efficiently utilized. This can be accomplished by adding to the electrolyte a dye that has absorption features that overlap the solar spectrum. The short excited-state lifetimes of molecular systems limit the distance an excited state can be expected to diffuse prior to nonradiative deactivation. Thus,... 

Interrelationships of Excited-State Decay Routes. The iLV curves conveniently display the competitive nature of photocurrent and luminescence intensity as excited-state deactivation pathways. Our analysis is limited in the sense that we have obtained absolute numbers for X but have had to content ourselves with relative < > r measurements. We lack measures of nonradiative recombination efficiency (4>nr) although they now appear to be... 

The temperature dependent term describes the transfer of CT energy to d-d excited states and constitutes an additional nonradiative deactivation pathway. (See Fig. 2.) Caspar and Meyer185 calculated Ea for [Ru(bpy)3]2+ to be 3560 cm 1 supporting Van Houten and Watts original estimate of ca. 3600 cm 1174. ... 

The photophysical properties of compounds 31—34 are presented in Table 7. Compared to the Pd-based materials, the Pt-based ones exhibit longer emission lifetimes. This is rationalized by the more stable Pt-Pt and Pt-L bonds compared to the more photolabile Pd-Pd and Pd-L ones. Therefore, the energy-wasting photo-induced cleavage (here bond cleavage process) does not occur or at least in a much less efficiency for the Pt-materials, thus reducing efficient nonradiative excited state deactivation. 

Although the existence of the M.I.R. may have appeared counter-intuitive to many chemists, photophysicists had a different point of view, since an inverted" relationship of the rate constant of nonradiative transitions and the energy difference between the states is well established [91]. This energy gap law results from the decreasing vibrational overlap of electronic states, the so-called Franck-Condon factor. It predicts an exponential relationship of the rate constant of nonradiative deactivation of excited states with the energy gap, of the form ... 

In addition to mastering the various processes leading to electronic excitation of the lanthanide ions, one has to prevent excited states to de-excite via nonradiative processes. The overall deactivation rate constant, which is inversely proportional to the observed lifetime r0bs, is given by ... 

Nonradiative deactivation of the Yb(2F5/2) excited state occurs through vibrational states of surrounding molecular groups. Since the contributions of these molecular groups to the overall nonradiative deactivation rate constant are known to be additive, the following equation can be written ... 

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