Introduction:

It has been defined that the bond enthalpy provides an estimate of the average energy needed to break a specific bond. Such bond enthalpies are derived from Thermochemical computations. It is usually seen that the energy needed for the dissociation of a specific bond by light absorption is much greater than its bond enthalpy value. In table shown below, we describe this for some diatomic molecules.

Table: Comparison of Photochemical Dissociation Energies and Bond Enthalpies

Dissociating         l/nm        (Photochemical dissociation         Bond enthalpy

molecules                                energy)   KJ mol^-1                          KJmol^-1                                              

H₂                       84.5                       1420                                  436

C1₂                      478                        250                                   242

Br₂                       511                        235                                   193

HI                        327                        367                                    299

O₂                        176                        682                                    497

Note:

Bond enthalpy of the diatomic molecule points out the energy requirement for its thermal dissociation.

• Value corresponds to the maximum wavelength of light needed for the decompositions via direct irradiation.
• Photochemical dissociation energy for one mole is computed via replacing for l in:

Photochemical reaction = N_Ahv = N_Ahc/l

The main causes for the higher photochemical dissociation energies as compared to the bond enthalpies are represented below:  

• For dissociating a molecule via light absorption, there should be an upper electronic state by suitable energy levels. There is no such limitation for the thermal decomposition.
• Throughout the photochemical dissociation, the product species could be an excited state and/or in ground state. For illustration: decomposition of bromine via absorption of light of wavelength 511 nm can be represented as:

Br₂ + hv → Br^D + Br   l = 511 nm

The atom having asterisk sign points out the excited state. Therefore, photochemical decomposition of bromine requires 235 kJmol^-1. However thermal decomposition of the bromine requires 193 kJ mol^-1 only, as both the bromine atoms are made in ground state. 

The photochemical dissociation of molecules is as well termed as the photolysis. The photolysis can be understood by utilizing potential energy diagrams. 

In the figure illustrated below, the ground state and excited state are represented by employing potential energy diagrams I and II, correspondingly. The quantized vibrational sublevels in each state are described by horizontal lines like AB, EF and so on.

Fig: Electronic Excitation; E (potential energy) vs r (internuclear distance)

Whenever a molecule is excited from zero vibrational level AB of ground electronic state of any of the vibrational levels beneath GH in the upper electronic state, the resulting electronic spectrum exhibits an absorption band with vibration - rotation fine structure. The fine structure is because of the numerous transitions (like III, IV and so on) possible from the zero vibrational level (AB) in the lower electronic state to any of the quantized vibrational levels in the upper electronic state. This can further be comprehended from the fact that each and every vibrational level consists of its own rotational sub-levels.

If a molecule absorbs adequate energy in such a manner that it is transferred from the ground state to or above GH in the upper electronic state, then the molecule experiences photochemical dissociation. The spectrum describes a continuum (that is, lack for discrete lines), once the molecule dissociates. The difference in energy between the levels AB and GH (Ep) is the photochemical dissociation energy. The thermal dissociation energy (Et) is equivalent to the bond enthalpy in case of diatomic molecules and it is the energy difference between the lowest and uppermost vibrational levels (AB and CD) in the ground state. It will be noted that E_p > E_T. 

We are familiar with the photolysis in detail so far as it is the initial reaction in most of the photochemical reactions. The excited atom or radical formed because of photolysis of a molecule often begins a chain reaction.

Some of the photochemical reactions:

Here, we shall talk about the procedure of some photochemical reactions and describe flash photolysis. For the first two reactions represented below, we derive rate equations too. You should go via these derivations cautiously. These two illustrations could provide you an idea as on how the photochemical rate expressions are written.  

The first step in the both illustrations is photolysis. The rate of the photolysis step is deduced as I_a that is the rate of absorption of light (that is, number of quanta absorbed per second). The initial photolysis is obeyed via thermal (or dark) reactions for which kinetic expressions are as identical.

Photochemical Decomposition of Hydrogen Iodide:

Let us now derive expressions helpful in computing the rate of decomposition of HI and the quantum efficiency for this reaction. HI experiences photochemical decomposition beneath 327 um. The method is represented below:

HI + hv → H + I         Rate of Photolysis = I_a

H + HI → H₂ + I

I + I → I₂

These steps are written mainly based on the energy considerations

Note:

As deriving the rate expressions, we try to remove terms having active species by employing steady state principle.

HI is used in two ways as represented in equation (HI + hv → H + I) and equation (H + HI → H₂ + I). The rate of disappearance of HI can be represented as follows:

-d[HI]/dt = I_a + K₂[H][HI]

With respect to the steady state approximation, the concentration of the active species H is constants. In another words, its concentration doesn't differ with time.

That is, d[H]/dt = 0 = I_a - K_2[H][HI]

It will be noted that H is formed as given in equation (HI + hv → H + I) and is employed as highlighted in Equation (HI + hv → H + I)

Or K₂ [H] [HI] = I_a

By using equation -d[HI]/dt = Ia + K₂[H][HI] and k₂[H] [HI] = I_a

- (d[HI]/dt) = 2I_a

Experimentally it has been noticed that the quantum efficiency for HI decomposition is 2. It is worth nothing that the rate of decomposition of HI based on the intensity of the absorbed light as represented in equation -d[HI]/dt = 2I_a

Flash Photolysis:

Flash photolysis was introduced by Norrish and Porter in the year 1949. In normal photolysis, the steady state concentrations of the intermediates are very small that these can't be detected through absorption spectrophotometers. In flash photolysis, a high-intensity flash of microsecond duration is employed for the photolysis substance and the products are recognized by using absorption spectrophotometers. The flash duration should match the decay rate of the intermediates. Flash lamps work for a time of around 15 µ s. This limits their use to the study of intermediate of life time around 100 µs. In late years, laser flash sources have been developed. The flash time period is approximately 10^-9 s.

As far as this part is concerned, ensure that you understand the derivations for computing the rate of decomposition of hydrogen iodide and the rate of formation of hydrogen bromide. This could assist you in reaching at the rate expressions for simple photochemical reactions for which reaction series is acknowledged.

Photophysical Processes:

The light absorbed via a molecule is not always employed in producing a chemical reaction. The absorbed energy can be lost via different physical processes. 

The adsorption of ultraviolet or visible light yields in the raise of electronic energy from ground to the excited states. Generally, electronic excitation is as well accompanied via an increase in the rotational and vibrational energy levels. For ease, we shall portray only transitions between the electronic energy levels. In order to comprehend the nature of electronic transitions, it is necessary to know the perception of spin multiplicity. A molecule having electrons pairs and having anti-parallel spins is stated to be in single ground state (S_o). The excited molecule having two of its electrons unpaired and, by anti-parallel spins is stated to be in the excited singlet state like S₁, S₂, S₃... and so on. The excited molecule having two of its electrons unpaired, however with parallel spins is stated to be in the excited triple state like T₁, T₂, T₃... and so on.

Note:

Generally excitation of a ground state molecule leads just to one of the excited singlet states. However in some particular cases, direct excitation from S₀ to a triplet level is possible. We limit our discussion to S₀S₁, S₀S₂ and so on, transitions only. 

Multiplicity of state is represented by the expression 2S + 1, where S (note the italicized type) is the sum of spin values of electrons. This symbol S must not be confused by S (that is, Roman type) for the singlet state.

Assume that a molecule in the singlet state, that consists of two electrons having anti-parallel spins (like º ¹). Then, sum of the electron spins

= - S = 1/2 + 1/2 = 0

Therefore, multiplicity = 2S + 1 = (2 x 0) + 1 = 1

Generally, a molecule in the S₀ state on absorbing a quantum of light obtains two of its paired electron unpaired and gets transferred to S₁ or S₂ or S₃.... and so on, levels, however not to T₁ or T₂ or T₃.... and so on levels. This means that because of excitation, spin multiplicity is not usually modified. This condition is known as the selection rule for electronic transition. In other words, absorption of energy through the molecule in the ground state leads to allowed transition like S₀ → S₁, S₀ → S₂, S₀ → S₃ and so forth. These excitations and the following energy loss while reaching the ground state is illustrated by the Jablonski diagram (figure shown below). The solid arrows pointing upwards refer to the absorption of energy. The solid arrows indicating downwards refer to energy emission as light, termed as radiative transition. The wavy horizontal arrows stand for transition between the excited singlet and triplet state devoid of energy loss, whereas wavy vertical arrows stand for transition between singlet-singlet and triplet-triplet levels having energy loss as heat. Such wavy arrows represent non-radiative transition (that is, transitions without light emission).

Note:

Therefore, the singlet state consists of two electrons having anti-parallel spins and its (2S + 1) value is equivalent to 1. In the presence of a magnetic field, the molecule in the singlet state doesn't split further.

Molecule in the triplet state consists of two electrons with parallel spins (such as º º). The sum of electron spins.

= S = 1/2 + 1/2 = 1

Therefore, multiplicity = (2S + 1) = 3

Thus, a molecule in the triplet state consists of two electrons having parallel spins and its (2S + 1) value is equivalent to 3. In the presence of a magnetic field, a triplet state divides into three energy levels.

Fig: Jablonski Diagram

In the above figure, the excitation from the singlet ground state to the excited singlet levels; S₁, S₂ and S₃ are illustrated via vertical arrows marked A. The excited species at S₂ and S₃ have extremely short life span and these species; rapidly lose their energy as heat to the medium in around 10^-11 second and reach S₁ level. Such a singlet-singlet transition is termed as an internal conversion (I_C). The molecule at S₁ state consists of a life time 10^-8 to 10^-10 second. The system at S₁ might experience any of the given transitions.

A = Absorption of quanta leading to excitation to S₁, S₂ and S₃ levels.

It will be noted that the anti-parallel spins at different 'S' levels.

IC = Internal conversion from S₃ to S₂, from S₂ to S₁ and from S₁ to S₀

F = Fluorescence; a transition from S₁ → S₀ by light emission

I_SC = Intersystem crossing; S₁ → T₁ transition. It will be noted that the parallel spins at different T levels

P = Phosphorescence; a transition from T₁ → S₀ by light emission

For clarity, transition to and from T₂ and T₃ are not described.

Note:

The term 'fluorescence' is derived from the name of the mineral, 'fluorite' that emits visible light on exposing to the ultraviolet radiation.

i) Fluorescence:

The excited molecule could experience the transition, S₁ → S₀, by the emission of light. This phenomenon is termed as fluorescence (F). As S₁ → S₀ transition is allowed via selection rule, it is incredibly fast. In another words, substances fluoresce in the presence of the exciting radiation. Once the exciting radiation is stopped, the fluorescence as well stops.

ii) Internal Conversion:

The surplus energy might be lost as heat whereas S₁ → S₀ transition occurs which is again a case of internal conversion and a radiation less transition.

iii) Intersystem Crossing:

The excited molecule could cross over to the first triplet state via S₁ → T₁ transition. Such a transition comprises spin inversion. For such intersystem crossing (ISC) to be proficient, the energy gap between S₁ and T₁ levels should be low.

Ketones encompass very low energy gap between S₁ and T₁ levels and encompass high efficiency for the intersystem crossing. Therefore benzophenone consists of 100% efficiency for intersystem crossing. Compared to ketones, the aromatic hydrocarbons are less proficient in intersystem crossing and olefins are still less efficient. Let us now learn two of the methods through which the molecule in the triplet state could reach the ground state.

Fig: Benzophenone

A) Phosphorescence: The interesting physical method through which an excited species at T₁ level might experience transition to S₀ level is through emitting light; T₁ → S₀ transition with emission of light is known as phosphorescence (P). This is a method having a spin change and is a forbidden transition. Therefore, in contrary to the fluorescence, light emission throughout phosphorescence is slow and it lasts even after the elimination of exciting radiation.

2) Energy Transfer: The other process through which a molecule in the triplet state (termed as a donor molecule) might lose its surplus energy is through energy transfer to an acceptor molecule. This is an illustration of sensitization.

Now consider some of the applications of the study of the physical methods. Study of fluorescent behaviors of substances has led to the growth of the fluorescence spectroscopy. By employing spectrofluorometers, it is possible to recognize some fluorescing substances present in the similar solution; provided they have adequately different fluorescent like 10^-9 g/cm³ could be detected. For illustration, this process is quite helpful in the analysis of pesticides, drugs and atmospheric pollutants that are present in trace amounts. Studies based on the fluorescence and phosphorescence gives significant data on the properties excited states like lifetime, energy and electronic configuration.

On commercial side, the fluorescent lamp is one of the applications of the phenomenon of florescence. The florescent lamp comprises of a glass tube with:

• A small quantity of mercury
• Two electrodes
• A coating of phosphor

A phosphor is a solid substance that emits fluorescent light whenever excited by an ultraviolet radiation. The electrodes initiate an electric arc that assists in vaporizing and exciting mercury atoms. The excited mercury atoms emit the ultraviolet radiation. The phosphor, being excited via ultraviolet radiation, emits the fluorescence.  

A few other commercial applications of fluorescence are represented below:

• The optical brighteners are added to detergents to provide extra-brightness to the clothes. Optical brighteners fluoresce are in sunlight.
• Fluorescent paints are prepared by using appropriate additives.
• TV screens of various colours are generated by utilizing phosphors.

However molecules in the excited singlet and triple states could exhibit interesting chemical behavior, their studies are comprised in Organic Reaction mechanism.

Photosensitization:

Photosensitization is the method of exciting a molecule through energy transfer from an excited molecule. In this method, a donor molecule (D) absorbs a quantum light and forms an excited molecule (D). The excited donor molecule then transfers its excitation energy to the acceptor molecule (A) in the ground state in order to excite it. This can be described by using the given reaction sequence:

• Light absorption: D + hv → D^.
• Sensitization: D + A → A + D

The donor molecule is known as sensitizer. The excited acceptor molecule A could take part either in the chemical reactions or in physical methods.

Photosensitized Chemical Reactions:  

Whenever the excited molecule A has acquired adequate energy, it will get dissociated and begin a chemical reaction. The benefit in photosensitized dissociation of a molecule is that it is sufficient to transfer energy equivalent to its bond enthalpy to dissociate it. This is thus as the photosensitized molecule gets dissociated in the ground state. This is in dissimilarity to direct photochemical decomposition for which much higher energy is needed because of the necessity of exciting a molecule to upper electronic state. Therefore, the energy needed for photosensitized dissociation of a molecule is less as compare to that of the photochemical dissociation.

Let us take an illustration. Whenever irradiated by 253.7 nm light, hydrogen and oxygen react in presence of mercury vapor however not in its absence. Mercury vapor acts as the sensitizer. The reaction series is illustrated below:

Hg + hv → Hg^D

Hg^D + H₂ → H + H + Hg

H + O₂ → OH + O

The chain reaction carries on further. The energy transferred to hydrogen molecule via excited mercury atom is equivalent to 472 kJ mol^-1 (as represented in the equation, Photochemical reaction = N_Ahv = N_Ahc/l where l = 253.7nm). This energy is adequate for the thermal dissociation of hydrogen as its bond enthalpy is around 436 kJ mol^-1.

It will be noted that excited mercury atom can't directly dissociate oxygen molecule as its bond enthalpy is higher (approx 497 kJ mol^-1). Moreover in the absence of mercury vapor, light of 253.7 nm (or 472 kJ mol^-1 energy) can't photolyse hydrogen or oxygen directly, as the energies required for their photochemical dissociation are much higher (that is, 1420 kJ mol^-1 and 682 kJ mol^-1 correspondingly). Therefore, mercury vapor is necessary as a sensitizer for H₂ - O₂ photochemical reaction as l = 253.7 nm.

A recognized photosensitized reaction is photosynthesis. Chlorophyll ('chl') and other plant pigments act as the photosensitizer in the synthesis of starch from carbon-dioxide and water. A simplified reaction series is as shown below:

chl + hv → chl·     Starch

chl CO₂ + H₂O → (1/n) (CH₂O) + O₂ + chl

Note:

Chlorophyll is the name provided for a group of compounds having minor variation in structure. Chlorophyll absorbs efficiently in the red region of sunlight; the red light is in plenty in sunlight.

The reaction procedure is very complex. Energy computations illustrate that apart from the chlorophyll; there should be other colored light-absorbing materials (that is, pigments) which as well provide energy needed for the synthesis of starch.

Photosensitization is often employed via chemists for preparing compounds which can't be manufactured by thermal or direct irradiation methods.  

Sensitized Fluorescence:

Consider this juncture; discuss a physical method that carries on via sensations. Thallium vapor doesn't give rise to fluorescence whenever directly irradiated by light of wavelength 253.7 nm. However if mercury vapor is as well comprised in the reaction vessel, thallium shows fluorescence. Mercury atoms get excited first and transfer energy to the thallium atoms to excite them. The excited thallium atoms release fluorescent as they go down to the ground level.

Applications of Photochemistry:

Synthetic organic chemists encompass increasingly started utilizing photochemical processes for the synthesis because of greater efficiency and selectively as compared to the dark reaction. Photochemistry offers a procedure of conducting reactions that are not thermodynamically possible.

In the analysis of pollutants, photochemistry plays a significant role. For illustration, photochemical studies have pointed how ozone layer is influenced via chlorofluorocarbons (Freon) employed as refrigerants, solvents and spray-propellants. The simplest Freon is CF₂Cl₂. Freon is chemically inert and remains as such for years. However whenever it stratosphere (10 to 50 km above the surface of earth), Freon decomposes and provides free chlorine atoms. Such chlorine atoms can react by ozone decomposing it. It can cause depletion on the ozone layer. This is a matter of serious concern as ozone layer protects our planet from low wavelength portion of sun's rays (that is, 290nm to 320nm wavelength). Irradiation by such high energy radiation could product skin cancer. As an outcome of photochemical studies, alternatives are tried for Freon.

The present energy calamity has compelled the scientists to look for options. Solar energy, if correctly utilized via appropriate photochemical reactions, could offer a solution for this energy problem.

Notes:

Three common kinds of cells employed for transforming light into electricity are represented below:

a) Photoelectric cells or photo cells transform light to electricity by employing photosensitive cathode.

b) Photovoltaic cells encompass two dissimilar silicon (or germanium) crystals in close contact. The Irradiation of light causes flow of electrons from one crystal to other. Solar cells employed in calculators are photovoltaic cells.

c) Photo galvanic cells transform light to electricity via chemical reactions.

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