Radiationless deactivations

Contrary to what happens with the related expression (1.5) for radiative transitions, the Franck-Condon term in (1.7) is made up of a single overlap integral for each vibrational mode, corresponding to the unique isoenergetic transition. In order to discuss the role of the Franck-Condon factor in radiationless transitions, it is worthwhile considering the two archetypal simations shown in Fig. 1.10. 

In intermediate situations (minima nested but with crossing points not too far from the excited-state minimum) it may be more convenient for the molecule to go through the crossing point, because of a more favorable Franck-Condon factor, despite the substantial activation energy required. In these cases, the rates of radiationless transitions may become very sensitive to temperature. 

Before a polyatomic molecule can move over potential energy barriers along a reaction path, the initially introduced vibrational energy is quickly distributed among the various normal vibrations and can be partially or completely dissipated by collisions. This is particularly true for condensed phases where the exchange of vibrational energy with the environment is so fast that thermal equilibrium can be reached in a time as short as I0 s. (Cf. Maier et al., 1977.) 

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Molecular fluorescence and, to a lesser extent, phosphorescence have been used for the direct or indirect quantitative analysis of analytes in a variety of matrices. A direct quantitative analysis is feasible when the analyte s quantum yield for fluorescence or phosphorescence is favorable. When the analyte is not fluorescent or phosphorescent or when the quantum yield for fluorescence or phosphorescence is unfavorable, an indirect analysis may be feasible. One approach to an indirect analysis is to react the analyte with a reagent, forming a product with fluorescent properties. Another approach is to measure a decrease in fluorescence when the analyte is added to a solution containing a fluorescent molecule. A decrease in fluorescence is observed when the reaction between the analyte and the fluorescent species enhances radiationless deactivation, or produces a nonfluorescent product. The application of fluorescence and phosphorescence to inorganic and organic analytes is considered in this section. 

Data on duorescence, phosphorescence, excited-state lifetimes, transient absorption spectra, and dye lasers are tabulated in Ref. 16. The main nonduorescent process in cyanine dyes is the radiationless deactivation Sj — Sg. Maximum singlet-triplet interconversion ( 52 ) methanol for carbocyanines is about 3% (maxLgrp > 0.03), and the sum [Lpj + st] I than 0.10. 

The excited singlet or triplet state returns to the ground state by a radiationless deactivation process (Fig. 15/lV). 

Perun S, Sobolewski AL, Domcke W (2005) Photostability of 9h-adenine mechanisms of the radiationless deactivation of the lowest excited singlet states. Chem Phys 313 107—112... 

From these equations one can approximate kf for benzophenone to be 5 x 10s sec-1. This, however, is the expected rate constant for fluorescence, which should be in competition with radiationless deactivation of the excited state kd. In actuality no fluorescence is observed for benzophenone although the fluorescence techniques are sensitive enough to detect fluorescence occurring with a quantum yield as low as 0/ = 0.001. Therefore kd must be at least 1000 times greater than kf We have... 

The original stabilizer (HBC) was modified as the rapid radiationless deactivation of the stabilizer is (at least partly) due to the intramolecular hydrogen bond, the H-atom was substituted by a methyl group (MBC). This "probe molecule" showed fluorescence and phosphorescence and enabled us to demonstrate the energy transfer to the stabilizer, simply by studying its sensitized luminescence. 

Figure 6. Jablonski diagram for the excited-state proton transfer and energy dissipation in TIN kSo s0> ks,s,-, kT,Tl- rate constants of proton-transfer processes in the ground state, first excited singlet state, and triplet state, respectively, and k,j rate constants of radiationless deactivations and k,- rate constants of intersyslem...

According to Heller and Blattmann ( 2.) the rotation of the hydroxyphenyl group around the central C-N bond may contribute to a rapid radiationless deactivation of the excited states. To understand this the items a) and b) of point 2° with regard to a rotational vibration around the C-N bond can be offered as an explanation. 

Ephardt H, Fromherz P (1989) Fluorescence and photoisomerization of an amphiphilic aminostilbazolium dye as controlled by the sensitivity of radiationless deactivation to polarity and viscosity. J Phys Chem 93(22) 7717-7725... 

Emission spectra of radical cations are obtained by vacuum UV ionization and subsequent laser excitation in noble-gas matrices (see below), or by electron-impact ionization of a beam of neutral parent molecules at energies above the first ionic excited state. After internal conversion to the first excited state, emission may compete more or less successfully with radiationless deactivation. If the experiment is carried out on a supersonic molecular beam one obtains highly resolved emission spectra which, in the case of small molecules, may contain sufficient information to allow a determination of the molecular structure. 

The intramolecular processes responsible for radiative and radiationless deactivation of excited states we have considered so far have been uni-molecular processes that is, the processes involve only one molecule and hence follow first-order kinetics. 

The various mechanisms which affect the Ti-<- Sq intensity and thus the radiative lifetime have been discussed earlier. For the class of aromatic hydrocarbons spin-orbit couphng is small and a t5q3ical value of about 30 sec for Tr seems appropriate However, the observed phosphorescence lifetimes vary greatly, demonstrating in most cases a dominating influence of the radiationless contribution. Siebrand and Williams > have noticed a very interesting correlation (Fig. 27) between the radiationless deactivation rate = 1 /rri and the triplet... 

Nitroanilines and nitronaphthylamines (group III) show remarkable stability towards both photoreduction and photosubstitution. Photoreduction (including intramolecular hydrogen abstraction) occurs, however, after acylation of the amino group in nitroanilines 46-48). Xhe stability of liitroanilines towards photoreduction is evidently due to the chcirge transfer character of the lowest excited state. Another possibility could be the increased probability of radiationless deactivation due to smaller separation of the ground state and the lowest excited states. 

Notwithstanding the excellent analytical features inherent in molecular phosphorimetric measurements, their use has been impeded by the need for cumbersome cryogenic temperature techniques. The ability to stabilize the "triplet state" at room temperature by immobilization of the phosphor on a solid support [69,70] or in a liquid solution using an "ordered medium" [71] has opened new avenues for phosphorescence studies and analytical phosphorimetry. Room-temperature phosphorescence (RTF) has so far been used for the determination of trace amounts of many organic compounds of biochemical interest [69,72]. Retention of the phosphorescent species on a solid support housed in a flow-cell is an excellent way of "anchoring" it in order to avoid radiationless deactivation. A configuration such as that shown in Fig. 2.13.4 was used to implement a sensor based on this principle in order to determine aluminium in clinical samples (dialysis fluids and concen-... 

Radiationless deactivation processes, of excited state to ground state, 32 47 Radicals... 

In summary the qualitative data on fluorescence and phosphorescence cannot explain satisfactorily the behavior of excited azines. More quantitative work is needed. In many cases, the sum of fluorescence and phosphorescence yields is not unity and processes of radiationless deactivation must be considered. 

Pyrimidine (1,3-diazine) and pyrazine (1,4-diazine) exhibit weak fluorescences73,74 in solutions or as vapors at room temperature, and strong phosphorescences 76-79 in dilute solid solutions at low temperatures (77 or 90°K). The phosphorescent quantum yields have never been accurately measured in these solid solutions. In the vapor phase or in ordinary solutions, at room temperature, these two compounds do not phosphoresce. Radiationless deactivation processes must be considered again and a deactivation through an isomer cannot be excluded. 

Spin equilibria are thermal intersystem crossing processes. The ground state and the excited state lie within a few hundred wavenumbers of each other and both are thermally populated. There are two photophysical processes in excited states related to the dynamics of thermal spin equilibria. One is the radiationless deactivation of an excited state to a ground state of different spin multiplicity. The other is intersystem crossing between excited states. 

The results obtained from thermal spin equilibria indicate that AS = 1 transitions are adiabatic. The rates, therefore, depend on the coordination sphere reorganization energy, or the Franck-Condon factors. Radiationless deactivation processes are exothermic. Consequently, they can proceed more rapidly than thermally activated spin-equilibria reactions, that is, in less than nanoseconds in solution at room temperature. Evidence for this includes the observation that few transition metal complexes luminesce under these conditions. Other evidence is the very success of the photoperturbation method for studying thermal spin equilibria intersystem crossing to the ground state of the other spin isomer must be more rapid than the spin equilibrium relaxation in order for the spin equilibrium to be perturbed. 

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