The glucose is used by the plant as energy so it can survive. The water is usually absorbed by the plant through its roots while the light energy photons and carbon dioxide are absorbed by the plant through its leaves.
ATP is a substance which is part of the animal and plant cells and helps provide the cell with energy. The above step is usually called Light Reactions. When two molecules of 3GP join together, it forms glucose. Slide 7: Step 1: - the chlorophyll absorbs the photons light. In doing so, the chlorophyll loses one electron.
This process also helps create ATP. The water then turns into oxygen gas. The carbon dioxide is chemically reduced into the precursors of carbohydrates. To obtain the expression for W we will use the Beer-Lambert law Figure 3 , and the fact the absorbances of components in a mixture add up together: Here I0 is the intensity of monochromatic light entering the sample expressed in mol L-1s-1, and are molar absorptivities of the reactant and product M-1 cm-1 , and l is the optical path cm.
The course of a photochemical reaction is often monitored spectrophotometrically at wavelength s different from the irradiation wavelength. By using the Beer-Lambert law we may write for the absorbance measured at the irradiation and observation wavelength at time t: and. By using the absorbance, , measured at the observation wavelength and infinite time and the three equations shown above we obtain Eq.
In the general case, Eq. In the later case we obtain: 4. Theoretical Models of Photochemical Reactions Within the Born-Oppenheimer approximation, potential energy surfaces govern nuclear motion and, therefore, chemical reactivity. However, in studying photochemistry it is also good to keep in mind that this is just an approximation, which is not automatically valid for all possible geometries and experimental conditions.
However, a detailed account of nuclear motion can also be inferred from classical trajectories for a point moving without friction on the potential energy surface.
The moving point may represent a chemically reactive system which consists of one or several molecular species. In the latter case one considers all reactants as a "supermolecule". The forces acting on the nuclei are given by minus the gradient of the potential electronic energy at this point. Recall that the gradient for a function of many variables is a vector formed by the first derivatives with respect to each of the variables.
Points on the surface that are characterized by the gradient vector of zero length are called stationary points. Their location is of primary importance for chemical reactivity. The nature of stationary points is determined by the secondary derivatives, the so-called Hessian matrix.
If all the eigenvalues of this matrix are positive, the point is a minimum, which can be assigned to a reactant, product or intermediate. A first-order saddle point has all positive eigenvalues except for one, which is negative.
It means that it is a maximum with respect to a single coordinate and a minimum in all other directions. Passage from one minimum to another one describes a chemical reaction and a saddle point between the two minima represents the transition state. Because of difficulties in representation of multidimensional hypersurfaces one-dimensional cross-sections through them are frequently used.
The cross-sections may be compared to the potential energy curves of diatomic molecules and may often look similar to such curves.
However, they must be interpreted with caution. For example, a saddle point may appear both as a minimum and maximum on two different cross-sections.
Thermal reactions are generally considered to be adiabatic, i. Another way of putting this is that these reactions occur exclusively in the ground state. Therefore, knowledge of the ground-state potential surface is sufficient for modeling thermal reactivity with reaction rate theories.
In contrast, the theoretical treatment of any photochemical reaction requires information about potential energy surfaces for more than one state. The photoreaction starts from the ground state of the reactant s , necessarily proceeds via electronically excited state s , and ends with the product s in the ground state.
Therefore, photochemical reactions inevitably include diabatic processes, i. This statement should illuminate the complexity of the theoretical analysis of photoreactions, especially because reliable calculations of the potential energy surfaces for electronically excited states of reasonably large molecules still represent a challenge for computational chemistry.
Nevertheless, many fundamental aspects of complex photoinduced reactions still can be understood from qualitative analysis of potential energy surfaces. Figure 5. Franck-Condon Principle.
The vibrational functions of two electronic states are approximately harmonic oscillator-like functions. The most probable position of the nuclei in the ground state corresponds to the maximum of the probability distribution function for the zero level red curve. The energy gap between vibrational levels is usually large enough so that population of excited levels is small.
An electronic transition caused by light absorption is represented by a vertical line block arrow. The highest probability of the transition corresponds to the largest overlap between the ground-state and excited-state vibrational wavefunctions. The overlap is greatest for the S1 vibrational level whose classical turning point is near the equilibrium distance of the ground state.
Upon light absorption, a molecular system may be transferred from the ground state to an electronically excited state. According to the Franck-Condon principle, this transition tends to occur between those vibrational levels of two electronic states that have the same nuclear configurations. In other words, transitions between two potential energy surfaces can be represented by vertical lines connecting them see Figure 5.
In the course of a photochemical reaction there is a considerable time interval when the molecular system is out of the thermal equilibrium a few ps in condensed phase, up to ms in low pressure gas phase reactions. It means that the population of vibrational energy levels may differ strongly from that predicted by the Boltzmann distribution see Basic Photophysics. As a consequence of "vertical" electronic transitions and different equilibrium geometries of the ground and first excited state Figure 5 , immediately after excitation the molecular system will likely be in an excited vibrational state "hot" molecule.
The amount of extra energy available for nuclear motion is a function of the excitation energy wavelength. Vibrational excitation may also result from internal conversion or intersystem crossing, when electronic energy is converted into kinetic energy of the nuclei. It is known that internal conversion from S1 to S0 can be so fast in some systems that the thermal equilibration is first achieved only in the ground state.
In solution, "hot" molecules in the first excited or ground state are quickly cooled down via interactions with the surroundings. Thermal equilibrium is normally established within a few picoseconds. Nevertheless this time is long enough to comprise several vibrational periods. The excess of kinetic energy may help the reactant s to overcome the barrier and relax into a new minimum. Chemical reactions of this type are called "hot". They preferentially occur in the gas phase at lower pressure where molecular collision frequency is much smaller than in the condensed phase.
We have already discussed that theoretical analysis of thermal reactions can be accomplished when minima and saddle points on the ground-state surface are allocated. The situation is much more complex for photochemical reactions. Difficulties emerge when one needs to explore several potential energy surfaces in detail. Luckily, only a few excited-state surfaces are of importance for the majority of photoreactions. Even so, topology of the three surfaces, S0, S1 and T1, which are almost without exception needed to understand the photoreaction mechanism, may be extremely complex.
Minima on S1 and T1 surfaces may be anticipated in the regions near the ground-state equilibrium geometries and near geometries, corresponding to intermolecular complexes. The latter minima reflect much larger polarizability of excited species and therefore higher affinity to other molecules.
Excited complexes can be formed from two molecules of the same type excimer , or from two different molecules exciplex. Formaldehyde provides an example for large geometrical distortions in the excited state, the molecule is planar in S0, and pyramidal in S1 and T1.
Figure 6. Schematic representation of the energy profiles corresponding to the ground state and the first excited state a for a system that undergoes an excited-state reaction but achieves no chemical conversion upon returning to the ground state and b for a system with partial conversion upon jumping to the ground state.
Light absorption is represented by red block arrows, light emission by white block arrows. In addition to localizing minima on the potential surfaces, finding the regions where the surfaces may cross or come very close to each other is of primary importance.
The Born-Oppenheimer approximation is generally invalid in the vicinity of surface crossings and additional effects must be taken into account to describe the time evolution of the molecular species. The non-crossing rule states that potential energy curves can cross only if the electronic states have different symmetry spatial or spin. Therefore the wavefunctions in the crossing region predicted by the simplest approximation has to be modified to avoid crossing of the potential energy curves Figure 7.
The non-crossing rule is strictly valid only for diatomic molecules. Intersection or touching of potential energy surfaces in polyatomic systems is generally allowed even if they belong to the states of the same symmetry.
Recent studies showed that such crossing, also called conical intersection because of the topology of the surfaces at the crossing point, is quite common. The question whether a true conical intersection or avoided crossing is observed for a particular system of interest can be answered only with quantum mechanical calculations of high accuracy. These calculations recently became feasible for relatively large organic molecules, but reliable data are available just for a few systems.
Figure 7. Adiabatic solid and non-adiabatic energy curves dashed for the S0 and S1 states. The light absorption is a vertical transition block red arrow. Nuclear motion after excitation is governed by the S1 curve. Blue arrows show the motion in the case of avoided crossing and the black broken arrow corresponds to the allowed crossing. Two hypothetical surfaces for the ground- and an excited state are depicted in Figure 8.
The fact that multidimensional potential-energy surfaces may have numerous regions where they come very close to each other is of great importance for understanding photochemical mechanisms. First, non-radiative transitions such as internal conversion and intersystem crossing have much higher probability in these regions.
Second, conical intersections or weakly avoided crossings serve as bottlenecks through which the photoreaction passes on the way from excited-state species to the ground-state products. In this sense crossing points are analogous to the transition states on the adiabatic surfaces. An essential distinct feature of the conical intersection is the presence of two independent pathways for the reaction path f as compared to the single path through the saddle point.
Figure 8. Potential energy surfaces of the ground and an excited state with various pathways dashed lines following the light absorption red arrow. Thermal equilibrium may be established during the lifetime of the excited state, meaning that vibrational relaxation takes place and the photoreaction starting from a minimum on the excited-state surface is said to have an excited-state intermediate path a. Return from the first or even the second minimum reached on the excited-state surface often does not produce a new species right part of path c and the whole sequence may be considered as a photophysical process.
A typical example is the protolytic dissociation of 1-naphthol in the singlet excited state Scheme 3. It has to be noted that Scheme 3 does not account for all photoprocesses occurring in 1-naphthol solutions. Scheme 3 The primary excited-state intermediate in Figure 8 may produce a new molecule in the excited state, which undergoes further modifications path b , or returns to a new minimum on the ground-state surface left part of path c.
A jump from the excited-state surface can be accomplished via non-radiative transition path c or light emission path d. An illustrative example of an excited-state intermediate in the photochemical reaction is the interaction of 9-cyanophenathrene with tetramethylethene in benzene that forms a cycloadduct via a singlet exciplex Scheme 4. Scheme 4 The reaction sequences represented by motion on the excited-state adiabatic surface are usually called adiabatic reactions.
If the loss of excitation occurs anywhere on the reaction path between the points corresponding to reactants and products, then such photoreaction may be referred to as diabatic also called non-adiabatic.
It is also possible that the vibrational relaxation first occurs in the ground state path e in Figure 8. Such a photoreaction is called "direct". A direct reaction proceeds through a funnel path f , which is a region of the potential energy surface where the probability for a jump from one energy surface to another one is very high.
Funnels usually correspond to conical intersections or weakly avoided crossings. To characterize a molecule in a funnel one needs not only the positions of the nuclei but also their velocity vectors. In some systems passage through a conical intersection may also be separated from the excited-state minimum initially populated by a small barrier paths a and c assuming that surfaces now cross at the point corresponding to path c.
The presence of a S1-S0 conical intersection separated from the "vertical" geometry by a small barrier has been predicted for benzene. This funnel is responsible for the opening of efficient deactivation channel leading to disappearance of fluorescence and isomerization Scheme 5 when the benzene molecule has enough vibrational energy to overcome the barrier. Scheme 5 5. Factors Determining Outcome of a Photochemical Reaction The wide variety of molecular mechanisms of photochemical reactions makes a general discussion of such factors very difficult.
The chemical nature of the reactant s is definitely among the most important factors determining chemical reactivity initiated by light. However, a better understanding of this aspect may be gained from a closer examination of the individual groups of chemical compounds. The nature of excited states involved in a photoreaction is directly related to the electronic structure of the reactant s. Environmental variables, i.
It is useful to distinguish between variables that are common for thermal and photochemical reactions, and those that are specific for the reactions of excited species.
The first group includes reaction medium, reaction mixture composition, temperature, isotope effects to name the most important. The distinctive feature of photochemical reactions is that these parameters almost always operate under conditions when one or more photophysical processes compete with a photoreaction. The result of a photoinduced transformation can only be understood as the interplay of several processes corresponding to passages on and between at least two potential energy surfaces.
We saw that even the simplest system, shown in Scheme 2, corresponds to parallel reactions in terms of reaction kinetics. Reaction medium may directly modify the potential energy surfaces of the ground and excited states and hence affect the photoreactivity. The outcome of the two reactions presented in Schemes 3 and 4 changes dramatically when solvent polarity and hydrogen bonding capacity are changed.
The protolytic photodissociation of 1-naphthol is completely suppressed in aprotic solvents because of unfavorable solvation energies both for the anion and proton. Under such conditions, proton transfer reaction cannot compete with the deactivation.
The formation of two new products Scheme 6 in the reaction of 9-cyanophenathrene with tetramethylethene is observed in methanol, because the exciplex dissociated into radical ions.
It means that the potential energy minimum corresponding to the ion-radical pair shifts below that of the exciplex in polar solvents. The ion-radical formation is often followed by proton transfer reactions. Scheme 6 Solvent viscosity will strongly affect photoreactions where the encounter of two reactants or a substantial structural change are required. In highly viscous or solid solutions the loss of excitation via light emission or unimolecular non-radiative deactivation is more probable than a chemical modification of the excited species.
On the other hand, slow diffusion in viscous solutions may prevent self-deactivation of the triplet state via a bimolecular process called triplet-triplet annihilation and enhance the efficiency of a photoreaction from this state. Triplet-triplet annihilation belongs to electronic-energy transfer processes, which may be classified as quenching of excited states. Quenching rate is a very important factor in discussing effects of medium and reaction mixture composition on photoreactivity.
Quenching of excited states is a general phenomenon that is realized via different mechanisms. Any process that leads to the disappearance of the excited state of interest may be considered as quenching.
In general it can be represented as: Scheme 7 Notice that the quencher molecule Q may belong to the same kind of chemical species as the excited molecule, and be either in the ground or in an excited state. S' corresponds to the ground state or to an excited state of lower energy. For the purpose of our discussion we separated quenching described by Scheme 7 from all other processes, including the photoreaction of interest introduced in Scheme 2.
Obviously, this separation is just a matter of convention. Generally, any chemical reaction of the excited species can be considered as a quenching process for fluorescence. Scheme 7 can easily be incorporated into the reaction scheme see Scheme 8 and into our kinetic analysis as an additional pseudo-unimolecular rate constant kq[Q].
Scheme 8. Kinetic scheme for a simple system with a photoreactive singlet state in the presence of a quencher. In the presence of a quencher, Q, the observed lifetime of the excited molecule and therefore the quantum yield of the photoreaction may be significantly reduced.
If we would consider the fluorescence quantum yield instead of the photoreaction yield, we would obtain a similar equation, which is known as the Stern-Volmer equation.A photochemical event involves the absorption of light to create an excited species that may subsequently undergo a number of different reactions. Several mechanisms involving collisional quenching, ground-state aggregation and energy transfer to the aggregates has been proposed to account for this phenomenon. Energy transfer In some cases the excited species may simply transfer its excess energy to a second species. Any process that leads to the disappearance of the excited state of interest may be considered as quenching. The results suggest that B is formed in a triplet-state reaction. A photochemical reaction differs notably from a thermally, or heat, induced reaction in that the rate of a photochemical reaction is frequently greatly accelerated, and the products of the photochemical reaction may be impossible to produce otherwise.
We have already discussed that theoretical analysis of thermal reactions can be accomplished when minima and saddle points on the ground-state surface are allocated. The nature of stationary points is determined by the secondary derivatives, the so-called Hessian matrix. Turro, N. Triplet-triplet annihilation belongs to electronic-energy transfer processes, which may be classified as quenching of excited states.
This law relates photochemical activity to the fact that each chemical substance absorbs only certain wavelengths of light, the set of which is unique to that substance. Meanwhile, photons are also being absorbed by pigment molecules in the antenna complex of Photosystem I and excited electrons from the reaction center are picked up by the primary electron acceptor of the Photosystem I electron transport chain. Exciplexes are typically more reactive, and provide examples for combined physical and chemical quenching see Schemes 4 and 6. In the later case we obtain: 4.
In contrast, the efficiency of photoconversion from the triplet state is usually increased because the triplet lifetime remains sufficiently long in the presence of a quencher, and the overall effects is largely determined by an increase in the yield of the triplets. The chloroplast is enclosed by a membrane.
The nature of stationary points is determined by the secondary derivatives, the so-called Hessian matrix. This energy corresponds to near-IR radiation with a wavenumber of Figure 8. Here the electron may be accepted by an electron acceptor molecule of an electron transport chain see Fig.
State the reactants and the products for the light-independent reactions. The light-dependent reactions are of two forms: cyclic and non-cyclic. Two general mechanisms of the energy transfer are distinguished: radiative and nonradiative. Therefore, the presence of CFCs interrupts the natural ozone cycle by consuming the oxygen atoms that should combine with molecular oxygen to regenerate ozone.
CAM plants store the CO 2 mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. The net result is that ozone is removed from the stratosphere while the chlorine atoms are regenerated in a catalytic process to continue the destructive cycle. Thermal equilibrium may be established during the lifetime of the excited state, meaning that vibrational relaxation takes place and the photoreaction starting from a minimum on the excited-state surface is said to have an excited-state intermediate path a. Plants, algae, and bacteria known as cyanobacteria are known as oxygenic photoautotrophs because they synthesize organic molecules from inorganic materials, convert light energy into chemical energy, use water as an electron source, and generate oxygen as an end product of photosynthesis.