Primary photophysical processes

This absorption in combination with the excitation of a molecule A from the electronic and vibrational ground state at thermal equilibrium A to an energy level of high vibrational and electronic excitation symbolised by A is given in Fig. 1.1 [6, 7], This figure represents a schematic energy level 

Energies of electromagnetic radiation relevant to photochemistry in different units 

After absorption of photons a variety of processes can take place. These processes are either isoenergetic (no change in energy) or combined with an energy transfer to other molecules or between different types of energy levels of the molecule itself  

It can change its state of rotation. The levels of the rotational energies are closely spaced. The difference amounts to approximately 0.4 kJ. That means a molecule can be excited in a higher rotational state by absorption of radiation in the far infrared. The excitation of rotational states by use of conventional intensities of light cannot induce a photoreaction. 

The state of electronic energy can be changed by absorption of photons in the visible and ultraviolet. In combination with this energy uptake the 

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The primary photophysical processes occuring in a conjugated molecule can be represented most easily in the Jablonski diagram (Fig. 1). Absorption of a photon by the singlet state So produces an excited singlet state S . In condensed media a very fast relaxation occurs and within several picoseconds the first excited singlet state Si is reached, having a thermal population of its vibrational levels. The radiative lifetime of Si is in the order of nanoseconds. Three main routes are open for deactivation ... 

There is evidence that the primary photophysical process produces a long-lived intermediate. Parker and Hatchcard detected a long-lived but reversible optical absorption on flash photolysis of a U(VI)-oxalate solution. Photolysis at 77 K of a concentrated U02 /oxalic acid system yielded an ESR signal displaying g-factor anisotropy with an average... 

Therefore, as in the case of parent phenyl azide 47 and its simple derivatives, the photochemistry of polynuclear aromatic azide, especially that of naphthyl azides 79 and 80, is now well understood. Specifically, the dynamics of the primary photophysical processes as well as the subsequent photochemical steps have been directly investigated using a variety of modem and conventional experimental techniques and compntational chemistry. It is clear now, that the difference between the photochemistry of phenyl azide (and its simple derivative) and polynuclear aromatic azide is caused mainly by the difference in the thermodynamics of the singlet nitrene rearrangement to azinine type species. 

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. 

By definition, the sum of the primary quantum yields for all photochemical and photophysical processes taken together must add up to unity, i.e.,... 

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

In conclusion, photogeneration is one of the several photophysical processes in competition with one another in which the excited state may be involved. In this view, the primary photogeneration quantum yield (), defined as the number of charge pairs that are formed per absorbed light... 

Primary (photo)process See primary photochemical process. The term primary (photo)process for photophysical processes is apt to lead to inconsistencies, and its use is therefore discouraged. 

Primary Photochemical Process a term originally used (12) to describe the entire set of photochemical and photophysical primary processes. 

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