Radiationless deactivation processes

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

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

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. 

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. 

A + RCH2OH - AH. + RCHOH At low concentrations of photoexcited dye and in competition with radiationless deactivation processes the radicals can react with oxygen to form additional radicals and/or hydrogen peroxide. At higher concentrations electron transfer reactions to semi-reduced (A ) and semi-oxidized (A +) anthraquinone compounds can occur which in turn undergo secondary reactions. 

Therefore, the radiationless deactivation process for most emitting species in their excited triplet states is so efficient that phosphorescence spectra can be observed only when the system including the emitting species is frozen at liquid nitrogen temperature. Of course, the environmental aspects of radiationless deactivation do not determine all the whole properties. Pyridine molecules show neither fluorescence nor phosphorescence at room temperature or in the rigid glass state at liquid nitrogen temperature. 

In general, the intensity of emission decreases when the temperature increases, because of the higher probability of other radiationless deactivation processes of excited molecules (Anpo and Che, 1999 Turro, 1978). Furthermore, a much better resolution of the vibrational fine structure of the emission (fluorescence and phosphorescence) can be observed at low temperature, as shown in Section 2.1.2, for the phosphorescence spectrum of highly dispersed tetrahedral vanadium species which exhibits a well resolved vibrational fine structure related to the V=0 double bond. 

H.-D. Ilge, M. Kaschke, and D. Khechinashvili, Photochemistry of phenyl fulgides. XXI ultrafast photoisomerization and radiationless deactivation processes in a,8-di-(4-alkoxyphenyl)-fulgides and a,8-bifluorenyl fulgides, J. Photochem., 33, 349-358 (1986). 

Figure 2 Schematic of the electronic absorption spectrum of a single chromophoric site in a condensed phase host environment at low temperatures. An extremely sharp electronic origin, exhibiting a radiatively limited linewidth is accompanied by a phonon sideband with vibrational sidelines. A second electronic excited state lies at higher energies. Vibrational sidelines and the second electronic excited state are lifetime broadened by rapid radiationless deactivation processes...

If there are leakages from the excited state due to radiationless deactivation processes, Eq. (6) must be used with k = — f, where is the fluorescence quantum yield of A in the absence of reaction. These leakages will increase the last term of the summation in Eq. (14) and hence lower the global conversion efficiency. In the above example, if 0f = 0.5 (k = 0.5), the flux ratio rp is increased from 0.98 to 0.99 and the global conversion efficiency at maximum power is slightly reduced from 0.71 to 0.70. 

Intrinsic lifetime, Tp Lifetime of the triplet in the absence of radiationless deactivation processes (p = 1) 1 Op... 

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