Interaction of Light with Organic Molecules

If monochromatic light passes through a uniform thickness of an absorbing homogeneous medium with the absorbing centers acting independently of each other, then the energy of light that is absorbed follows the Lambert-Bouguer law. According to this law of physics, the light absorbed is independent of the intensity of the incident light and the intensity of radiation is reduced by the fraction that is proportional to thickness of the absorbing system. In addition, Beer’s law states that absorption is proportional to the number of absorption centers. The two laws are usually combined and expressed as follows:

dI/dl=kcI

where I is the intensity of the radiation, l is the length of the optical path, through the absorbing medium, c is the concentration of the absorbing centers, and k is proportionality constant. While there are no know exceptions to the Lambert-Bouguer law, there are deviations from Beer’s law due to partial ionization, molecular association and complexation, and fluorescence. Portions of organic molecules or whole molecules that have p bonds can absorb light radiation, provided that it is of the right wavelength. Particular groupings or arrangements of atoms in molecules

Fig. 10.2 The orbitals of formaldehyde (from Jaffe and Orchin)

give rise to characteristic absorption bands. Such groups of atoms, usually containing p bonds, are referred to as chromophores. Examples of such molecules with p bonds are compounds that contain carbonyl or nitro groups and aromatic rings. A molecule that serves as an example of carbonyl arrangement, one that is often referred to, is a molecule of the formaldehyde. In this molecule, the carbon atom is linked to two hydrogens and to one oxygen by s bonds. The hybrid sp2 orbitals bond one electron of carbon with one of oxygen in an sp orbital. In addition, there are two unbonded n electrons on oxygen that point away from the carbon atom. The orbitals of formaldehyde, the simplest of the carbonyl compounds, were illustrated by Orchin and Jaffe [88], as shown in Fig. 10.2. As described above, the molecule has s and p bonded skeleton, shown above. The carbon atom is attached to two hydrogenatomsbyasinglebondandtotheoxygenatombyadoublebond.Thisbonding of the carbon to thetwo hydrogensand oneoxygenatomsisbymeansofsp2hybridorbitals.Theorbitals are approximately at 120 angles from each other. In the ground state of the molecules, the pair of electrons that form a bond is paired and has opposite or antiparallel spin. In this state, the formaldehyde molecule is planar. The Pauli exclusion principle [83] states that no two electrons can have identical quantum numbers. That means that if two electrons are in the same orbital and three of their quantum numbers are the same, the fourth quantum number, the spin  quantum number, must be different. The total spin quantum number of a molecule is designated by a letter J and the sum of the spins of the individual electrons by a letter S. The spin quantum number of a molecule J is equal to [2S]+1. This arrangement of electrons in the p orbital can generate p bonding and p* antibonding orbitals. Absorption of light energy by a chromophore molecule results in formation of an excited state and an electronic transition from the ground state to an excited state. Such light may be in the ultraviolet or in the visible region of the electromagnetic spectrum, in the range of 200 mm to approximately 780 mm. Promotion of electrons out of the s bonding orbitals to the excited states requires a large amount of energy and rupture of bonds in the process. On the other hand, the electronic transition to promote one of the n electrons on the oxygen atom in formaldehyde to the antibonding or the nonbonding orbital, n →p*level requires the least amount of energy. The name, antibonding, as one might deduce, is a type of orbital where the electrons make no contribution to the binding energy of the molecule. In formaldehyde, this n → p* transition to the excited state gives rise to an absorption band (at about 270 mm). This is a relatively weak band and it suggests that the transition is a forbidden one (forbidden does not mean that it never occurs, rather than it is highly improbable). It is referred to as a symmetry forbidden transition. The reason for it being forbidden is crudely justified by the fact that the p* is in the xz plane (see Fig. 10.2). The n electrons in the p orbital are in the xz plane and perpendicular to the p* orbital. Because the spaces of the two orbitals overlap so poorly, the likelihood of an electronic transition from one to the other is quite low. As stated above, in the ground (normal) state of the molecules, two electrons are paired. The pairing means that these electrons have opposite or anti-parallel spins. After absorbing the light energy, in the singlet excited

Fig. 10.3 Illustration of the singlet and the triplet states (from Ravve [91])

Fig. 10.4 Illustration of energy transitions (from Ravve [91])

state the two electrons maintain anti-parallel spins. The n → n * excitation, however, can lead to two excited state, a singlet (S1) and a triplet (T1) one with an absorption band (at about 250 mm). Intersystem crossing to a triplet state from the singlet results in a reversal of the spin of one of the electrons and an accompanying loss of some vibrational energy. This is illustrated in Fig. 10.3. The intersystem crossing from the singlet to triplet states can occur with high efficiency in certain kinds of molecules, particularly in aromatic and carbonyl-containing compounds. Electron–electron repulsion in the triplet state is minimized because the electrons are farther apart in space and the energy is lower in that state than that of the corresponding excited singlet one. Solvents can exert a high influence on the n →n *transitions. While the intersystem crossing is a forbidden transition, it can actually occur with high frequency in aromatic or carbonyl compounds. The chemical mechanism of photo-excitation of organic molecules has been fully described in various books on photochemistry [82, 84, 85, 87]. It will, therefore, be discussed here only briefly. The transitions are illustrated here in a very simplified energy diagram that shows the excited singlet state and the various paths for subsequent return to the ground state in Fig. 10.4. The energy diagram (Fig. 10.4) represents energy states of a molecule that possesses both n and p* electrons. S1 and S2 are the singlet excited states. T1 and T2 are the excited triplet states. Solid lines represent electronic transitions. They are accompanied by absorption or emissions of photons. Radiationless transitions are represented by doted lines. The above diagram shows the lowest singlet state S1, where the electrons are spin-paired, and the lowest triplet state T1, where the electrons are spin-unpaired. The electron is excited by light of a particular wavelength into an upper singlet level, S2. Relaxation follows via an internal conversion process to S level. The excess energy is dissipated by vibrational interactions giving rise to evolution of heat. At the S1 level, there are three possible ways that the excited state becomes deactivated. The return to the ground state from the triplet one requires again an inversion of the spin. In Fig. 10.4, a and a0 represent the energies of light absorbed, b, h, and I the energies of internal conversion, c represents return to the ground state by way of fluorescence, and d return by way of phosphorescence. The Franck and Condon principle states that during an electronic transition the various nuclei in the molecule do not change their position or their moment [90]. What it means is that electronic transitions are much more rapid (l0^─15 s) than nuclear motions (10^─12 s) so that immediately after the transitions the nuclei have nearly the same relative positions and velocities that they had just before the transitions. The energy of various bonding and antibonding orbitals increases for most molecules in the following order,

In molecules with heteroatoms, such as oxygen or nitrogen, however, the highest filled orbitals in the ground state are generally nonbonding, essentially atomic, n orbitals. This, for instance, is a case with ketones and aldehydes. These molecules possess electrons that are associated with oxygen and are not involved in the bonding of the molecules. The n electrons in formaldehyde can be illustrated as follows:

As explained above, in the triplet state the spin of the excited electron becomes reversed. This results in both electrons having the same spin. From purely theoretical approach, such an electronic configuration is not allowed. Due to the fact that the excited electron cannot take up its original position in the ground state until it assumes the original spin, the triplet state is relative long-lived. For instance, in benzophenone at 77C the lifetime can be 4.7 10─3 s. Orchin and Jaffe wrote [88] that the triplet state has a lifetime of 10─3 s. By comparison, the lifetime of a singlet state is about 10─8 to 10─7 s. Also, in the triplet state the molecule behaves as a free-radical and is very reactive. The carbon atom has a higher electron density in the excited state than in the ground state. This results in a higher localized site for photochemical activity at the orbital of the oxygen. Because the carbonyl oxygen in the excited state is electron-deficient, it reacts similarly to an electrophilic alkoxy radical. It can, for instance, react with another molecule by abstracting hydrogen. At higher frequencies (shorter wavelength) of light, if the light energy is sufficiently high, p ! p* transitions can also take place. All aromatic compounds and all conjugated diene structures possess delocalized p systems. Because there are no n electrons, all transitions in these systems are p ! p*. In general, the excited states of molecule are more polar than the ground states. Polar solvents, therefore, tend to stabilize the excited state more than the ground state. As shown in Fig. 10.4, the triplet state is lower in energy than the corresponding singlet state. This is due to the fact that the electron–electron repulsion is minimized, because they do not share each other’s orbitals according to the Pauli exclusion principle. Thus, less energy is required for the triplet state. The chemical reactivity of organic molecules is determined principally by the electron distribution in that molecule. When the electron distribution changes, due to absorption of light and subsequent transitions, photochemical reactions take place while the molecule is in an electronically excited state. The phenomenon of light absorption, formation of the excited states, and subsequent reactions obey four laws of organic photochemistry, as was outlined by Turro [87]:

1. Photochemical changes take place only as a result of light being absorbed by the molecules.

2. Only one molecule is activated by one photon or by one quantum of light.

3. Each quantum or photon which is absorbed by a molecule has a given probability of populating either the singlet state or the lowest triplet state.

4. In solution, the lowest excited singlet and triplet states are the starting points for the photochemical process.

The relationship between the amount of light or the number of photons absorbed and the number of molecules, that, as a result, undergo a reaction, is defined as the quantum yield. It is defined as the number of molecules involved in a particular reaction divided by the number of quanta absorbed in the process [1, 3]. Another fundamental law of photochemistry was formulated by Grotthus and Draaper [82, 83]. It states that only the light that is absorbed by a molecule can be effective in producing photochemical changes in that molecule. There is also a fundamental law of photochemistry that states that the absorption of light by a molecule is a one-quantum process, so that the sum of the primary processes, the quantum yield, must be unity [82, 83]. Also, the law of conservation of energy requires that the sum of the primary quantum yields of all processes be equal to unity. Mathematically this can be expressed as:

where θ is the quantum yield. The quantum yield of photochemical reactions is important because it sheds light on the mechanisms of the reactions. The number of molecules involved in a particular photoreaction can be established by an analytical kinetic process and the number of quanta absorbed can be measured with the aid of an actinometer. The quantum yield can also be expressed in general kinetic terms [1]:

The above equations signify that a quantum yield of a particular photo process is the product of two or three distinct probabilities. These are: ’ES is the probability that the excited state will undergo the primary photoreaction necessary for the process. The probability that any metastable ground state intermediate will proceed to stable products is Pi and the probability that the excited state will undergo the primary photoreaction necessary of the process is ’R. The concept that matter can only acquire energy in discrete units (quanta) was introduced in 1900 by Max Planck [83]. The corollary of the quantization of energy is that matter itself must be quantized, i.e., constructed of discrete levels having different potential energies. Occupying these particular levels are electrons that obviously possess the energy of the level which they occupy. In a molecule, the intramolecular motions of the electrons and the associated molecular electronic levels must be taken into account. There are, in addition to electronic levels, modes of vibration and rotation that are also quantized. In other words, the absorption of a photon of light by any molecule is a reaction that must promote transitions between quantum states. This requires two conditions. These are: (1) for a molecular state m with energy Em, there must be a state n of higher energy, En, so that hu ¼ En Em; (2) there must be specific interaction between the radiation and the light-absorbing portion of the molecule that results in a change in the dipole moment of the molecule during the transition. If we designate the wave functions of the states m and n as cm and cn respectively, then the transition moment integral that may not equal to zero is:

where P is the electric dipole operator. It has the form of , where e is the electronic charge and ri is the vector that corresponds to the dipole moment operator of an electron i. The increase in the energy of a molecule as a result of absorbing a quantum of radiation can be expressed in the relationship [85]:

where l is the wavelength of the interacting radiation. All reactions that are photochemical in nature involve electronically excited states at one time or other. Each one of these states has a definite energy, lifetime, and structure. The property of each state may differ from one to another and the excited states are different chemical entities from the ground state and behave differently. The return to the ground state from the excited state, shown in Fig. 10.4, can take place by one of three processes [85]:

1. The molecule returns directly to the ground state. This process is accompanied by emission of light of a different wavelength in the form of fluorescence.

2. An inter system conversion process takes place to the T1 state, where the electron reverses its spin. The slower decay of excitation from the triplet state to the ground state is accompanied by emission of phosphorescence.

3. The molecule uses the energy of excitation to undergo a chemical reaction As explained above, in the triplet state the spin of the excited electron becomes reversed. This results in both electrons having the same spin. From purely theoretical approach, such an electronic configuration is not allowed. Due to the fact that the excited electron cannot take up its original position in the ground state until it assumes the original spin, the triplet state is relative long-lived. For instance, in benzophenone at 77C the lifetime can be 4.7 10 3 s. Orchin and Jaffe wrote [88] that the triplet state has a lifetime of 10 3 s. By comparison, the lifetime of a singlet state is about 10 8 to 10 7s. Also, in the triplet state the molecule behaves as a free-radical and is very reactive. The carbon atom has a higher electron density in the excited state than in the ground state. This results in a higher localized site for photochemical activity at the n orbital of the oxygen. Because the carbonyl oxygen in the excited state is electron-deficient, it reacts similarly to an electrophilic alkoxy radical. It can, for instance, react with another molecule by abstracting a hydrogen. At higher frequencies (shorter wavelength) of light, if the light energy is sufficiently high, n → n* transitions can also take place. All aromatic compounds and all conjugated diene structures possess delocalized p systems. Because there are no n electrons, all transitions in these systems are n → n *. In general, the excited states of molecule are more polar than the ground states. Polar solvents, therefore, tend to stabilize the excited state more than the ground state. As shown in Fig. 10.4, the triplet state is lower in energy that the corresponding singlet state. This is due to the fact that the electron–electron repulsion is minimized, because they do not share each other’s orbitals as stated by the Pauli exclusion principle Thus, less energy is required for the triplet state. This dissipation of the excitation energy can also be illustrated as follows:

where A_o represents any organic molecule and A* represents the same molecule in an excited state. In the process of energy dissipation from the singlet and return to the ground states, the light emission by fluorescence is at a different wavelength than that of the light that was absorbed in the excitation. This is because some energy is lost in this process of the electron returning from its lowest excited state to the ground state. The energy, however, may also, depending upon the structure of the molecule, be dissipated in the form of heat, as shown above. And, also, a third form of energy dissipation can occur when the molecule undergoes a chemical reaction. Depending, again on the molecular structure, the chemical reactions can be rearrangement, isomerization, dimerization (or coupling), fragmentation, or attack on another [90–92] molecule. Some examples of such reactions are:

Many other examples can be found in the literature. Most familiar isomerization reaction is that of trans-stilbene to cis-stilbene, shown above. It was observed that the quantum yield of stilbene cis trans isomerization decreased with an increase in viscosity of the medium [89]. In addition, it was also found that in a polymeric matrix, the photo-isomerization is not inhibited, provided that it occurs above the glass transition temperature of the polymer. An example of a fragmentation of a molecule is the decomposition of disulfides upon irradiation with ultraviolet light of the appropriate wavelength:

The same reaction takes place in peroxides. Ketones and aldehydes cleave by the mechanism of the Norrish reaction.

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مواضيع ذات صلة

The Electron Transfer Process

Energy Transfer Process

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