Electronically excited molecules photophysical processes

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

Following photo excitation a solution sample returns to thermal equilibrium by a variety of photochemical and photophysical processes. The faster processes, e.g. vibrational relaxation and solvent relaxation, have only recently begun to be studied by direct kinetic methods (1-5). Picosecond emission spectroscopy has been especially useful in elucidating these ultrafast processes (1,/3, 5). As electronically excited molecules relax, their fluorescence spectrum shows time dependence that reflects the relaxation processes. 

The photochemical and photophysical processes that occur in an electronically excited molecule are well described in many books [11]. Radicals in photoinitiators are produced through several following typical processes [li] ... 

The primary photochemical processes performed by electronically-excited molecules. They can be divided further into photophysical processes and photochemical reaction processes. The former include luminescent processes and nonradiative deactivation. 

The possible fate of excitation energy residing in molecules is also shown in Figure 2. The relaxation of the electron to the initial ground state and accompanying emission of radiation results in the fluorescence spectrum - S0) or phosphorescence spectrum (Tx - S0). In addition to the radiative processes, non-radiative photophysical and photochemical processes can also occur. Internal conversion and intersystem crossing are the non-radiative photophysical processes between electronic states of the same spin multiplicity and different spin multiplicities respectively. 

Electron transfer (Chapter 6), considered as a photophysical process, involves a photoexcited donor molecule interacting with a ground-state acceptor molecule. An ion pair is formed, which may undergo back electron transfer, resulting in quenching of the excited donor. 

This chapter describes the characteristics of the fluorescence emission of an excited molecule in solution. We do not consider here the photophysical processes involving interactions with other molecules (electron transfer, proton transfer, energy transfer, excimer or exciplex formation, etc.). These processes will be examined in Chapter 4. 

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

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