Photochemistry is the branch of chemistry concerned with the chemical effects of light. Specifically, it deals with chemical reactions that are initiated by the absorption of ultraviolet, visible, or infrared radiation.
Thermal vs. Photochemical Processes
Chemical reactions can be broadly classified into thermal (dark) reactions and photochemical reactions. Understanding their differences is fundamental to the study of photochemistry.
| Feature | Thermal Processes (Dark Reactions) | Photochemical Processes |
|---|---|---|
| Driving Force | Activated by thermal energy (heat and molecular collisions). | Activated by absorption of light (photons). |
| Temperature Dependence | Highly dependent on temperature. The rate increases significantly as temperature rises. | Negligible dependence on temperature. Temperature only influences secondary thermal steps. |
| Selectivity | Non-selective; thermal energy is distributed among all vibrational degrees of freedom. | Highly selective; only molecules that absorb specific wavelengths of light are excited. |
| Free Energy Change (ΔG) | Must always be negative (ΔG < 0) for the reaction to occur spontaneously. | Can occur even if ΔG is positive (ΔG > 0) because light energy compensates for the endergonic nature. |
| Activation Energy | Obtained through intermolecular collisions in the ground electronic state. | Obtained directly through absorption of light quanta, elevating molecules to excited electronic states. |
Laws of Photochemistry
Grotthuss-Draper Law (The First Law of Photochemistry)
“Only those radiations which are absorbed by a reacting system can bring about a chemical change.”
When light falls on a substance, a portion of it is reflected, a portion is transmitted, and the remaining portion is absorbed. According to this law, the reflected and transmitted light have no chemical effect. Only the absorbed light is capable of producing a photochemical change. However, the absorption of light does not guarantee a chemical reaction; the absorbed energy may also be dissipated as heat, fluorescence, or phosphorescence.
Stark-Einstein Law of Photochemical Equivalence (The Second Law of Photochemistry)
“Each molecule taking part in a photochemical reaction absorbs exactly one quantum (photon) of radiation that causes the reaction.”
This law establishes a one-to-one relationship between the number of absorbing molecules and the number of photons absorbed in the primary step of a photochemical reaction. The energy absorbed by one mole of reacting substance is called one Einstein (E) of energy.
E = NAhν = NAhc / λ
Where:
- NA is Avogadro's number (6.023 × 1023 molecules/mol)
- h is Planck's constant (6.626 × 10-34 J·s)
- ν is the frequency of light
- c is the velocity of light (3 × 108 m/s)
- λ is the wavelength of light
Jablonski Diagram and Excited State Processes
The Jablonski diagram is a classic visual representation of the electronic states of a molecule and the radiative and non-radiative transitions that can occur when a molecule absorbs a photon and becomes excited.
When a molecule is in its ground electronic state, the electron spins are typically paired, resulting in a spin multiplicity of 1 (Singlet Ground State, S0). Upon absorbing light, an electron is promoted to a higher electronic singlet state (S1, S2, etc.). If the spin of the excited electron flips, the molecule enters a triplet state (T1, T2, etc.) with a spin multiplicity of 3 (parallel spins). Triplet states are always lower in energy than their corresponding singlet states due to Hund's rule of maximum multiplicity.
Radiative Processes
Radiative transitions involve the emission of an electromagnetic photon as the molecule transitions to a lower energy state.
- Fluorescence: This is a radiative transition between states of the same spin multiplicity, typically from the first excited singlet state to the ground singlet state (S1 → S0). Because this transition is spin-allowed, it occurs very rapidly (lifetime of 10-9 to 10-6 seconds) and ceases immediately when the exciting light source is removed.
- Phosphorescence: This is a radiative transition between states of different spin multiplicity, typically from the lowest excited triplet state to the ground singlet state (T1 → S0). Since this transition is spin-forbidden, the probability of occurrence is low, resulting in a much longer lifetime (10-4 to several seconds or even hours). Consequently, the emission continues (glow-in-the-dark) even after the exciting light source is removed.
Non-Radiative Processes
Non-radiative transitions involve the loss of electronic excitation energy without the emission of a photon, typically dissipated as vibrational heat to the surroundings.
- Internal Conversion (IC): A non-radiative transition between two electronic states of the same spin multiplicity (e.g., S2 → S1 or T2 → T1). It is highly rapid (10-12 seconds).
- Intersystem Crossing (ISC): A non-radiative transition between two electronic states of different spin multiplicity (e.g., S1 → T1). Although theoretically forbidden, it occurs due to spin-orbit coupling and vibrational overlap. It is slower than internal conversion (10-8 seconds).
Quantum Yield (Quantum Efficiency)
The quantum yield (represented by Φ) is a measure of the efficiency of a photochemical reaction. It is defined as the ratio of the number of reacting molecules to the number of photons absorbed in the same time interval.
Φ = (Number of molecules reacting in a given time) / (Number of photons absorbed in the same time)
Alternatively, Φ can be defined in terms of reaction rates:
Φ = (Rate of chemical reaction) / (Intensity of light absorbed, Ia)
Classification of Quantum Yields
| Quantum Yield Class | Value of Φ | Underlying Cause | Example Reactions |
|---|---|---|---|
| High Quantum Yield | Φ > 1 (often up to 106) | The primary photochemical step generates highly reactive intermediates (like free radicals) that initiate a secondary thermal chain reaction. | H2 + Cl2 → 2HCl (Φ ≈ 104 to 106) |
| Low Quantum Yield | Φ < 1 | Deactivation of excited molecules before they react, recombination of primary dissociation products (cage effect), or endothermic secondary steps. | H2 + Br2 → 2HBr (Φ ≈ 0.01 at room temperature) |
| Moderate/Normal Quantum Yield | Φ ≈ 1 to 2 | The primary step is followed by simple, non-chain secondary reactions. | Photolysis of Hydrogen Iodide (2HI → H2 + I2, Φ = 2) |
Photosensitized Reactions and Quenching
Photosensitized Reactions
A photosensitized reaction is a process where a chemical reaction is initiated not by the reactant itself, but by a foreign substance called a photosensitizer. The photosensitizer absorbs the light energy, becomes excited, and subsequently transfers this energy to the reactants through molecular collisions, returning to its ground state.
Example 1: Mercury-sensitized dissociation of Hydrogen:
Hg + hν → Hg*
Hg* + H2 → Hg + 2H
Example 2: Photosynthesis in plants, where Chlorophyll acts as a photosensitizer by absorbing visible light and transferring the energy to convert CO2 and H2O into carbohydrates.
Quenching
Quenching is the process by which the intensity of emission (such as fluorescence or phosphorescence) from an excited state molecule is decreased by physical or chemical interaction with another species known as a quencher. The quencher accepts the excitation energy and deactivates the excited state molecule without undergoing a permanent chemical reaction.
D* + Q → D + Q*
Where D* is the excited donor molecule and Q is the quencher.
Chemiluminescence
Chemiluminescence is the emission of light as a result of a chemical reaction at room temperature without the generation of significant heat (hence termed “cold light”). It is the exact reverse of a photochemical reaction; instead of using light to drive a chemical reaction, a chemical reaction produces an electronically excited product which then emits light upon returning to its ground state.
A + B → C* + D
C* → C + hν (emission of light)
Example: The oxidation of 3-aminophthalhydrazide (Luminol) in basic solution with hydrogen peroxide in the presence of a catalyst yields an excited aminophthalate ion, which glows with a brilliant blue light. Bioluminescence (seen in fireflies) is a form of natural chemiluminescence catalyzed by enzymes.
Kinetics of Photochemical Reactions
1. Photochemical Reaction between Hydrogen and Bromine (H2 + Br2 → 2HBr)
The photochemical combination of hydrogen and bromine takes place through a chain mechanism initiated by light absorption by Br2 molecules.
Mechanism:
- Initiation: Br2 + hν → 2Br (Rate = Ia)
- Propagation: Br + H2 → HBr + H (Rate constant k2)
- Propagation: H + Br2 → HBr + Br (Rate constant k3)
- Inhibition: H + HBr → H2 + Br (Rate constant k4)
- Termination: 2Br → Br2 (Rate constant k5)
Kinetics and Rate Law Derivation:
Applying the Steady State Approximation (SSA) for the highly reactive intermediates, [H] and [Br]:
For the H atom:
d[H]/dt = k2[Br][H2] - k3[H][Br2] - k4[H][HBr] = 0
For the Br atom:
d[Br]/dt = Ia - k2[Br][H2] + k3[H][Br2] + k4[H][HBr] - 2k5[Br]2 = 0
Adding these two steady-state equations simplifies the system directly:
Ia - 2k5[Br]2 = 0
[Br] = (Ia / 2k5)1/2
Now, solve for [H] from the first steady-state equation:
[H] = k2[Br][H2] / (k3[Br2] + k4[HBr])
The rate of formation of HBr is given by:
d[HBr]/dt = k2[Br][H2] + k3[H][Br2] - k4[H][HBr]
By rearranging the steady-state equation of [H], we see that: k3[H][Br2] - k4[H][HBr] = k2[Br][H2]. Substituting this into the rate equation:
d[HBr]/dt = 2 k2 [Br] [H2]
Substituting the steady-state concentration of [Br] into this expression:
d[HBr]/dt = 2 k2 [H2] (Ia / 2k5)1/2
To express this in terms of all parameters including inhibition, we substitute [H] fully, yielding the classic experimental rate law:
d[HBr]/dt = (2 k2 (Ia/2k5)1/2 [H2]) / (1 + (k4[HBr] / k3[Br2]))
Important Observations:
- Low Quantum Yield: At room temperature, the activation energy of the propagation step
Br + H2 → HBr + His extremely high (approximately 75 kJ/mol), making this step highly endothermic and slow. Hence, the overall quantum yield is very low (Φ ≈ 0.01) at room temperature. - Inhibition: The presence of HBr in the denominator shows that as the reaction proceeds and HBr accumulates, it inhibits its own rate of formation by competing with Br2 for the reactive H atoms.
2. Photochemical Decomposition of Hydrogen Iodide (2HI → H2 + I2)
The photochemical decomposition of HI is a non-chain process with a quantum yield of exactly 2. The mechanism consists of a photochemical primary step followed by a rapid thermal secondary step.
Mechanism:
- Primary Photochemical Step (Initiation): HI + hν → H + I (Rate = Ia)
- Secondary Thermal Step: H + HI → H2 + I (Rate constant k2)
- Secondary Thermal Step (Recombination): I + I → I2 (Rate constant k3)
Kinetics and Rate Law Derivation:
Let us write the rate of decomposition of HI:
-d[HI]/dt = Ia + k2[H][HI]
Applying the Steady State Approximation (SSA) on the intermediate H atom:
d[H]/dt = Ia - k2[H][HI] = 0
k2[H][HI] = Ia
Substituting this back into the rate of decomposition equation:
-d[HI]/dt = Ia + Ia = 2Ia
Conclusion & Quantum Yield Analysis:
The rate of decomposition of HI is directly proportional to the intensity of light absorbed (Ia). Since the rate of decomposition of HI is 2Ia while the rate of absorption of photons is Ia, the quantum yield is precisely 2:
Φ = (-d[HI]/dt) / Ia = 2Ia / Ia = 2
This shows that for every single photon absorbed, exactly two molecules of HI are decomposed.