Energy Transfer Process

Photosensitizers are used in many photonic applications. To understand how they function, it is necessary to understand the energy transfer process. The term energy transfer [92, 93] refers specifically to one-step radiationless transfer of electronic excitation from a donor molecule to another, qualified, acceptor molecule, from one chromophore to another one. This excludes what is referred to as trivial energy transfers that result from the donor emitting light that is subsequently absorbed by an acceptor. Based on the energy and spin conservation laws, there are two a priori requirements for efficient energy transfer: (1) the process must be thermoneutral or exothermic to occur with highest efficiency, because the activation energies have to be low due to short lifetimes of electronically excited state, and (2) no net spin changes should occur. If a donor molecule was in the triplet state at the time of the energy transfer process, the acceptor molecule is then also promoted to the triplet state. Transfer of singlet to singlet energy should be possible, but it occurs less frequently, because of the shorter lifetimes of the singlet states [92, 93]. Energy transfer is thus a process by which excitation energy passes from one photo-excited molecule, often referred to as a sensitizer and in this case designated as S*, to another adjacent molecule in its ground state, often referred to as a quencher, in this case designated as Q. The quencher must have a thermodynamically accessible excited state, one whose energy is lower than that of S*. A donor molecule must possess sufficiently long lifetime to be an efficient sensitizer. The reaction of energy transfer can be illustrated as follows:

where * designates an excited state. In the process of energy transfer, S* returns (or relaxes) to the ground state S. Energy transfer is further categorized as involving singlet (paired electron spins) or triplet (unpaired electron spins) states. Symmetry rules, as explained above, require a singlet S* to produce a singlet Q* and a triplet S³⁺ to produce a triplet Q³⁺. The quenching reaction of the excited state was expressed in a equation by Stern–Volmer. The reaction shown below is based on a quenching reaction that is accompanied by a release of heat:

The equation is written as follows:

In experimental studies of energy transfer, it is convenient to express the experimental results in another form of the Stern–Volmer equation, as follows,

Where

θ₀ is the quantum yield for a particular process in the absence of a quenching molecule

θ_Q is the quantum yield of the quenched process

 kq is the bimolecular rate constant for the quenching process

 t is the lifetime of the state in the absence of a quenching molecules. It is equal to 1/(k1 + k3), and

[Q] is the concentration of the quenching molecules

Twoprocesses were proposed to explain the mechanism of energy transfer. In the first one, energy transfers result from the interactions of the dipole fields of the excited donors and ground state acceptor molecules (long-range: Forster (dipole–dipole)) [86, 90]. This is referred to as the resonance transfer mechanism. Such transfer is rapid when the extinction coefficients for absorption to the donor and acceptor-excited states involved in the process are large (104–105 at the maximum). When the dipolar interactions are large, resonance transfers are possible over distances of 50–100 A ˚ . Close proximities of donors and acceptors, however, are required for weakly absorbing molecules. In the second mechanism [90] (short-range: Dexter (exchange)), the excited donor and acceptor are in very close proximity to each other, (up to 15 A ˚) such that their electronic clouds overlap slightly. In the region of the overlap, the location of the excited electron is indistinguishable. It may be at any one instant on either the donor or on the acceptor molecule. Should the pair separate when the excited electron is on the acceptor molecule, energy transfer has been achieved by the mechanism of electron transfer, discussed in the next section. Both absorption and emission processes may be intramolecular, localized in a single molecule. On the other hand, they can also involve whole crystals that may act as absorbers and emitters. Such energy transfers can manifest themselves in different ways that include sensitized fluorescence or phosphorescence, concentration depolarization of fluorescence, photo-conduction, and formation of triplet acceptor molecules. Intermolecular energy transfer can be electronic and vibrational and can take place in solid, liquid, and gaseous phases. In addition, the sensitized excitation of Q by S* has to take place within the time that the molecule S remains in the excited state. In summary, theoretical and empirical considerations suggest two modes of transfer, described above:

1. Only when the two molecules are in very close proximity to each other and their centers are separated by the sum of their molecular radii will transfer take place.

 2. When the two molecules are at distances that exceed their collision diameters, resonance transfer or long-range electronic excitation takes place though Coulombic interactions. The transfers that take place by mechanism 1 are limited by diffusion of molecules in solution and should be affected by the viscosity of the medium. Transfers by mechanism 2, on the other hand, should be much less sensitive to the viscosity of the medium. It was shown by Foster [86] that the rate constant of resonance-energy transfer (mechanism 1), as a function of distance, is

where tS is the actual mean lifetime of S*, R is the separation between the centers of S* and Q, and R is the critical separation of donor molecules and the acceptor molecule. The efficiency of energy transfer was expressed by Turro et al. [94] as follows:

The transfer by long-range excitation or mechanism 2 can be in the form a singlet–singlet transfer, a triplet–singlet transfer, and a triplet–triplet transfer. Due to the fact that the lifetime of triplet state of molecule is longer than the singlet one, it is more probable to be the one to participate in energy transfer. Molecules that undergo intersystem crossing with high efficiency, like benzophenone, are efficient triplet sensitizers. Such molecules must possess high energy in the triplet state and a lifetime of at least 10 ^-4 s. The two types of intermolecular energy transfers can be expressed as follows:

The nomenclature that was developed in connection with energy and charge transfer processes is as follows, an eximer is a transient dimer formed by the combination of an excited (usually aromatic) molecule and a second similar (usually unexcited) molecule. Such a dimer bonds only in the excited state and promptly dissociates in losing its excitation energy. The term exiplex, explained by Birks [92], describes a complex between two molecules, one a donor and the other one, an acceptor, which subsequently dissociate in a deactivation process. One of the components of the exiplex, either the donor or the acceptor, is in excited state while the counterpart, acceptor or donor, is in the ground state. An eximer is then just a special case in which the two constituent molecules are identical. While numerous charge-transfer complexes can form between certain molecules in the ground state, a number of compounds can form only charge–transfer complexes when either the donor or the acceptor is in an excited state. Formation of eximers was observed in a number of aromatic polymers, such as polystyrene, poly(vinyl naphthalenes), poly(vinyl toluene), and others [93]. An exterplex is composed of three molecules and often takes an important role in photophysical and photochemical processes. Polymers with pendant aromatic chromophores and dimeric compounds often show efficient exterplex formation due to high local chromophore concentration in their structure. It was observed that exiplex emission spectra from a chromophore is usually broad, structureless, and red-shifted to the corresponding monomer fluorescence. The extend of such a shift is a function of the distance between the two components of the complex. It is also strongly affected by the polarity of the media. Martic et al. [95] obtained emission spectra of the exiplexes of anthracene and N,N, dimethyl-p-toluidine in toluene and in polystyrene. While the maximum band of the emission spectra in toluene at 30C is at 616 nm, in polystyrene it is shifted to 400 nm. The exiplex emission spectra in a copolymer of styrene with 4-N,N-diaminostyrene is at 480 nm. The maxima of the emission spectra are temperature-dependent. The maxima shifts in toluene solution to shorter wavelength and in polystyrene it is the opposite, and it shifts to longer wavelength with an increase in temperature. The maxima approaches common value at the glass transition temperature of polystyrene. Similar results were reported by Farid et al. [96] who studied formation of exiplexes of 4-(1-pyrenyl) butyrate in different solvents and in polymers. Chemical and physical changes take place in molecules when they absorb energy and reach an excited state. This is particularly true of carbonyl compounds. There is a change, as already stated,in the dipole moments of the molecules. This is due to the fact that dipole moments depend upon the distribution of the electrons. In carbonyl compounds, this change is particularly large. Also the geometry of the molecule changes from the ground to the excited states. In addition, the chemical properties of the molecules change. Thus, phenol, for instance, is a weak acid, but in the excited state it is a strong acid. This can be attributed to the n → n*transition where one of the pair of pelectrons is promoted to an antibonding orbital. By the same reason, the acid strength of benzoic acid is less than in the excited state because the charge in this case is transferred to carbonyl group. The excited states of both phenol and benzoic acid can be illustrated follows [93]:

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مواضيع ذات صلة

The Electron Transfer Process

Interaction of Light with Organic Molecules

The Nature of Light

Electricity-Conducting Polymers

Immobilized Reagents

Immobilized Nonenzymatic Catalysts

Immobilized Catalysts

Special Gels for Drug Release

Support Materials Other Than Polystyrene

Materials Based on Polystyrene

Polymer Supports for Reagents, Catalysts, and Drug Release

Degradation of Polymeric Materials by Ionizing Radiation