Photophysical deactivation processe

A point which should be stressed is that excited-state reactions must be very fast on the conventional chemical time-scale, since they have to compete with the photophysical deactivation processes. In practice, excited-state reactions must be almost activationless processes. Therefore, the key to understanding excited-state reactivity is the identification of low-energy channels along the excited-state surface leading, perhaps via some surface crossing, to the potential energy minima of the ground-state products. 

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...

A very important bimolecular deactivation process is the electronic energy transfer (ET). In this process, a molecule initially excited by absorption of radiation, transfers its excitation energy by nonradiative mechanism to another molecule which is transparent to this particular wavelength. The second molecule, thus excited, can undergo various photophysical and photochemical processes according to its own characteristics. 

In most cases, the two latter processes have been studied individually by fast techniques (flash photolysis, transient spectra measurements, Raman spectroscopy) in nano-, pico-, and femtosecond time scales as processes accompanying photophysical deactivation steps [64-66]. In Table 3 the data for such individual steps are reported. The data can be summarized as follows ... 

Excited states may be quenched as well via an electron transfer between the excited and quencher molecular entities. The electron can be transferred by two alternative ways, generating a radical anion and cation as a transient species (Figure 4.4). These then react thermally when the reaction leads to reproduction substrate AB and quencher Q in their ground states, the photophysical deactivation occurs when radical ions react with other medium components generating new species, the process belongs to photochemical redox reactions (see Chapter 6). 

The electronic structure of Ni(CO)4 is not as well defined as those of either Cr(CO)6 or Fe(CO)5. This makes the assignment of processes in the early development of the excited-state dynamics somewhat speculative. However there are a number of unique features to the photophysics of CO-loss from Ni(CO)4. Firstly, the CO loss is very slow compared to the other two systems outlined herein taking approximately 600 fs. In addition the Ni(CO)3 fragment is produced in its St state and this state persists because there is no facile deactivation process available based on molecular motions. Deactivation can be achieved only by further CO loss or by radiative processes of either fluorescence or phosphorescence. The overall scheme of potential energy curves and pathways for photoinduced loss of CO from Ni(CO)4 is represented in Fig. 29. 

A few chapters of the current volume describe different state-of-the-art experimental techniques used to unravel photophysical and photochemical properties of complex molecular systems. These chapters are especially tailored for the scholarly description of electronic excited state properties of nucleic acid bases and related species predicting different tautomeric distributions and possible nonra-diative deactivation processes. It is interesting to note that guanine provides particularly challenging case to discuss. Recent theoretical and experimental investigations show the existence of relatively significantly less stable imino tautomers in the... 

In the simplest example, a donor and acceptor pair is activated by electronic excitation of either the donor or the acceptor. In addition to photophysical deactivation or energy transfer, two processes can proceed subsequently—the electronically excited donor donates an electron from its SOMO into the acceptor LUMO or the... 

The preceding deactivation processes are generally considered to be photophysical processes. In addition to these, deactivation can take place via photochemical processes such as photoionization, decarboxylation, dechlorination, and permanent product formation. All these processes can be studied under steady-state or time-resolved domains. 

The lack of fluorescence of TAM dyes in solution is attributed to an extremely rapid, nomadiative deactivation process brought about by intramolecular rotation of the flexible aryl groups. Suppression of molecular rotation, by increasing the viscosity of the medium, by binding of the dye to a polymer or protein, or by selfassociation, diminishes this radiationless process. Coincident with this change in photophysical properties is often a dramatic increase in sensitivity toward photofading due to an increase in the quantum yield of the relatively long-lived and photochemically active triplet state [139-143]. The triplet state of the dye cation... 

As in the case of thermal reactions, the reaction scheme introduced in Section 2.1.1.1 can be used to set up the differential equations. However, the degrees of advancement are primed, since the number of steps can be reduced as will be demonstrated by use of the Bodenstein hypothesis. In the last column of this scheme, the number of moles of light quanta are written for a photochemical step, which are absorbed by the reactant starting this photochemical step. According to this assumption and the different photophysical relaxation processes discussed in Section 1.3 the primary exited molecule A completely deactivates into the lowest level of vibrational energy of the first exited singlet state. Three further steps are possible ... 

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