Photosensitizers

As explained in Sect. 10.3, photosensitizers are molecules that absorb the energy of light and act as donors by transferring this energy to acceptor molecules. The molecules that receive the energy may in turn undergo various reactions, such as polymerizations, isomerizations, couplings, and others. Many different molecules can act as photosensitizers, but the most useful ones are various aromatic compounds. In Table10.1arelistedsome common photosensitizes that appeared in various publications  

in the literature. The process of photosensitization and energy transfer involves formation of charge transfer complexes. A good photosensitizer, therefore, is not only a molecule that readily absorbs light energy, but also one that readily transfers it to another molecule. Some compounds are capable of forming such transfer complexes in the ground state, but many more form exiplexes in the excited state. Others can form complexes between a compound in the ground state and another one in the excited state and are called excimers or excited dimers. The difference between the excited state of a dimer and an exiplex is that the dimers possess binding energy in the ground state, while exiplexes lack any binding energy in the ground state. This is described in Sect. 10.3.1. The emission spectra from two molecules that are capable of forming exciplexes depend upon the distances between the two molecules. An equation for the excited state wavefunction of a one-to-one exciplex that forms from a donor molecule D and an acceptor molecule A was written by Guellet as follows [94]:

The first two terms on the right side of the above equation correspond to charge resonance states and the last two to the excitation resonance states. Thus, a photosensitizer, as explained in Sect.10.3, can act in two ways, by energy transfer and by electron transfer. To be exact, one may feel that a true photosensitizer is one that acts by energy transfer alone. This, however, is not always the case. Also, in the event of electron transfer, the process can lead to photo-induced decomposition via electron transfer [102]. The rate of absorption of light by a sensitizer that corresponds to excitation from the ground state to the excited singlet can be expressed as [94]:

The measurement of fluorescence and phosphorescence spectra of photosensitizers is very important in providing information about the energy of the excited states. It also allows identification of the phenomena.

The process of energy transfer requires that the excited donor diffuse to the proximity of an acceptor within the time period of its excited lifetime. This is subject to the viscosity of the medium and the efficiently of the collision process and the range r in which the collisions can occur. The observed rate constant for energy transfer kET is governed by the molecular rate constant kdiff for diffusion-controlled reaction. This is defined by the Debye equation:

where α is the probability of energy transfer. R is the universal gas constant, T is the temperature in Kelvin, is the viscosity of the medium in poise. The Schmoluchowski [94] equation defines the diffusion constant in terms of the diffusion coefficient of the sensitizer and the acceptor:

where Ds and Da are the diffusion coefficients of the sensitizer and the acceptor Rs and Ra are the molecular radii of the sensitizer and the acceptor, Na is the Avogadro number, and t0 is the lifetime of the excited state of the sensitizer.

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مواضيع ذات صلة

Polymers with Functional Chalcone Groups

Polymers with Pendant Cinnamoyl Functional Groups

Polymers That Photocross-link by Formation of Cyclobutane Rings

Photocross-linkable Polymers

The Charge Transfer Processes in Polymers

The Electron Transfer Process

Energy Transfer Process

Interaction of Light with Organic Molecules

The Nature of Light

Electricity-Conducting Polymers

Immobilized Reagents

Immobilized Nonenzymatic Catalysts