Relaxation: radiationless

A form of radiationless relaxation in which an analyte moves from a higher vibrational energy level to a lower vibrational energy level in the same electronic level. 

Another form of radiationless relaxation is internal conversion, in which a molecule in the ground vibrational level of an excited electronic state passes directly into a high vibrational energy level of a lower energy electronic state of the same spin state. By a combination of internal conversions and vibrational relaxations, a molecule in an excited electronic state may return to the ground electronic state without emitting a photon. A related form of radiationless relaxation is external conversion in which excess energy is transferred to the solvent or another component in the sample matrix. 

Fig. 8.10 Schematic illustration of LIES ST and reverse LIESST oidicf complex. Spin allowed d-d transitions are denoted by arrows and the radiationless relaxation processes by wavy lines (from [21])...

Hudock et al. [126] used the ab initio molecular dynamics multiple spawning method to go beyond the static picture based on PES and include the time dependent dynamical behavior and predict time-resolved photoelectron spectroscopy results. According to these results the first ultrafast component of the photoelectron spectra of uracil corresponds to relaxation on the S2 minimum rather than nonadiabatic transitions to the Si state. The authors suggest that the radiationless relaxation from... 

Several base pairs of adenine-thymine, including the WC pair, have also been studied [238], It is found that a charge transfer state exists about 1.5 eV higher than the local nn states. Proton transfer between the bases stabilizes a charge transfer state which then crosses with the ground state facilitating radiationless relaxation. This mechanism is not energetically favorable for non WC pairs. 

One of the most popular applications of molecular rotors is the quantitative determination of solvent viscosity (for some examples, see references [18, 23-27] and Sect. 5). Viscosity refers to a bulk property, but molecular rotors change their behavior under the influence of the solvent on the molecular scale. Most commonly, the diffusivity of a fluorophore is related to bulk viscosity through the Debye-Stokes-Einstein relationship where the diffusion constant D is inversely proportional to bulk viscosity rj. Established techniques such as fluorescent recovery after photobleaching (FRAP) and fluorescence anisotropy build on the diffusivity of a fluorophore. However, the relationship between diffusivity on a molecular scale and bulk viscosity is always an approximation, because it does not consider molecular-scale effects such as size differences between fluorophore and solvent, electrostatic interactions, hydrogen bond formation, or a possible anisotropy of the environment. Nonetheless, approaches exist to resolve this conflict between bulk viscosity and apparent microviscosity at the molecular scale. Forster and Hoffmann examined some triphenylamine dyes with TICT characteristics. These dyes are characterized by radiationless relaxation from the TICT state. Forster and Hoffmann found a power-law relationship between quantum yield and solvent viscosity both analytically and experimentally [28]. For a quantitative derivation of the power-law relationship, Forster and Hoffmann define the solvent s microfriction k by applying the Debye-Stokes-Einstein diffusion model (2)... 

Webber NM, Litvinenko KL, Meech SR (2001) Radiationless relaxation in a synthetic analogue of the green fluorescent protein chromophore. J Phys Chem B 105 8036-8039... 

Ultrafast charge separation and radiationless relaxation processes from S2 excited electronic states of directly linked Zinc-porphyrin-acceptor dyads... 

Alternatively, the molecule could cross from S, into an excited vibrational level of T,. Such an event is known as intersystem crossing (ISC). After the radiationless vibrational relaxation R3, the molecule finds itself at the lowest vibrational level of T,. From here, the molecule might undergo a second intersystem crossing to S0, followed by the radiationless relaxation R4. All processes mentioned so far simply convert light into heat. 

Radiationless Relaxation and Optical Dephasing of Molecules Excited by Wide- and Narrow-Band Lasers. II. Pentacene in Low-Temperature Mixed Crystals, T. E. Orlowski and A. H. Zewail, J. Chem. Phys. 70, 1390 (1979). 

In the early 1970s, Demas and Crosby [73, 74] postulated that in transition metal complexes with unfilled d shells luminescence should only be observed from the lowest excited or thermally populated higher excited states. The condition for this thermalization is a fast radiationless relaxation between the excited states. It was then suggested by Watts et al. [75] that the relaxation between the states may be hindered if the excited states have a different orbital parentage and the energy difference between the states is small ( < 300 cm-1). 

The energy stored in an excited state is dissipated by the unimolecular radiative and radiationless relaxations. For strongly allowed electronic transitions, Strickler and Berg have obtained an expression for the rate constant of the excited-state radiative decay (Equation 6.67).31... 

The rate constant for the radiationless relaxation of the excited states of inorganic compounds has been derived on the basis of the Fermi s golden rule (Equation 6.69). 

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