Fundamental Laws of Photochemistry
When molecules interact with light, they must obey specific foundational laws to determine whether a photochemical transformation will occur. The first of these principles is the Grotthus-Draper Law, often referred to as the principle of photochemical activation.
Formulated by Theodor Grotthus and John William Draper, it states that only the light actually absorbed by a chemical system can bring about a photochemical change. Light that is transmitted or reflected is entirely ineffective in causing a reaction.
While the Grotthus-Draper law is qualitative, the Stark-Einstein Law (also known as the Photochemical Equivalence Law) provides a quantitative foundation.
Proposed independently by Johannes Stark and Albert Einstein, this law dictates that each molecule taking part in a photochemical reaction absorbs one and only one photon of the activating radiation.
Mathematically, the energy absorbed per mole of the reactant is given by E = NAhν, where NA is the Avogadro constant, h is Planck’s constant, and ν is the frequency of the absorbed radiation.
It is crucial for competitive exam aspirants to note that the Stark-Einstein law applies strictly to the primary photochemical step (the initial light absorption step).
Secondary thermal reactions following the primary step may involve multiple molecules. This can cause the overall reaction stoichiometry to deviate significantly from the 1:1 photon-to-molecule ratio.
Quantum Yield (Φ)
To measure the efficiency of a photochemical process, chemists use the concept of quantum yield, denoted by the Greek letter Φ (phi).
Quantum yield is defined as the number of reactant molecules undergoing change per photon of light absorbed by the system.
The mathematical expression is given by:
Φ = (Number of molecules reacted or product formed in a given time) / (Number of photons absorbed in the same time)
Quantum yields can vary dramatically across different photochemical systems, ranging from very low fractions to millions:
- Low Quantum Yield (Φ < 1): Occurs when excited molecules deactivate via fluorescence, non-radiative decay, or recombination before reacting. An example is the combination of hydrogen and bromine, where Φ can be a fraction.
- Unity Quantum Yield (Φ = 1): Exactly one molecule reacts per photon absorbed, obeying the strict stoichiometric prediction of the Stark-Einstein law (e.g., the decomposition of hydrogen iodide).
- High Quantum Yield (Φ >> 1): Occurs in chain reactions where a single absorbed photon initiates a cascade of subsequent thermal reactions. A classic example is the formation of hydrogen chloride from hydrogen and chlorine gas, where Φ can reach values between 104 and 106.
Factors that typically decrease quantum yield include the occurrence of reverse reactions, deactivation of the excited state by collision (quenching), and fluorescent or phosphorescent emission of energy.
Photophysical Processes: Fluorescence and Phosphorescence
When a molecule absorbs a photon, an electron is promoted from the ground electronic state (usually a singlet state, S0) to a higher singlet excited state (S1 or S2).
Once in an excited state, the molecule must eventually return to the ground state via various photophysical or photochemical pathways.
Fluorescence is a radiative deactivation process in which an electron falls from an excited singlet state (S1) back down to the ground singlet state (S0) with the concurrent emission of a photon.
Because some vibrational energy is lost non-radiatively prior to emission (a phenomenon known as Kasha’s rule or vibrational relaxation), the emitted fluorescent light always possesses a longer wavelength (lower energy) than the absorbed light.
Fluorescence ceases almost instantaneously (within 10-8 seconds) once the external light source is removed.
Phosphorescence, on the other hand, involves a spin-forbidden transition.
Through a process called intersystem crossing (ISC), the molecule undergoes a spin flip from an excited singlet state to a lower-energy triplet state (T1), where the electron spins are parallel.
Because the transition from the triplet state back to the singlet ground state is quantum mechanically forbidden, the lifetime of the triplet state is significantly longer (ranging from milliseconds to several seconds or even hours).
Consequently, phosphorescence persists long after the exciting light source is turned off.
Photosensitized Reactions
Many molecules do not absorb light directly in the visible or near-UV region, or they decompose destructively when they do. To drive such reactions, chemists employ photosensitization.
In a photosensitized reaction, a foreign molecule known as a photosensitizer (such as mercury vapor, benzophenone, or chlorophyll) is added to the reaction mixture.
The mechanism of photosensitization proceeds in two key steps:
- The photosensitizer molecule absorbs a photon of specific wavelength, transitioning to an electronically excited state.
- The excited photosensitizer collides with the reactant molecule (which cannot absorb light directly), transferring its excess electronic energy to the reactant. The photosensitizer returns to its ground state unchanged, while the reactant is activated and proceeds to form products.
A classic example in inorganic photochemistry is the dissociation of hydrogen molecules using excited mercury atoms (Hg*).
Mercury vapor absorbs 253.7 nm resonance radiation, and the resulting excited Hg atom collides with H2, causing it to cleave into hydrogen free radicals.
Chemiluminescence
While photochemistry studies chemical reactions driven by light, chemiluminescence is essentially the reverse phenomenon: the production of electromagnetic radiation (light) as a direct result of a chemical reaction.
When a chemical reaction is sufficiently exothermic, the chemical energy released is channeled into populating an electronic excited state of a product molecule.
When this excited product drops back down to its ground state, it emits a photon of visible light, often referred to as “cold light” because it is produced without high thermal temperatures.
Well-known examples include:
- The biochemical oxidation of luciferin in fireflies and marine organisms (bioluminescence).
- The laboratory oxidation of luminol by hydrogen peroxide in the presence of a basic catalyst and a metal ion catalyst, which produces a striking blue glow.
- The glow produced by commercial light sticks via the reaction of aryl oxalates with hydrogen peroxide in the presence of a fluorescent dye.
Key Points to Remember
- Grotthus-Draper Law: Only absorbed light is chemically effective; transmitted light does nothing.
- Stark-Einstein Law: One photon is absorbed per molecule in the primary activation step (E = NAhν).
- Quantum Yield formula: Φ = (Number of molecules reacted) / (Number of photons absorbed).
- Chain Reactions: High quantum yields (Φ > 1) occur when a single photon triggers a self-sustaining cycle of secondary steps (e.g., H2 + Cl2).
- Fluorescence: Spin-allowed radiative transition from excited singlet to ground singlet (S1 → S0), instantaneous lifespan (~10-8 s).
- Phosphorescence: Spin-forbidden radiative transition involving intersystem crossing from triplet state to ground state (T1 → S0), delayed emission.
- Photosensitization: Energy transfer from an absorbing species (sensitizer) to a non-absorbing reactant.
- Chemiluminescence: Light emission directly resulting from an exothermic chemical reaction creating electronically excited products.
Important Facts / Formulas
| Parameter / Concept | Mathematical Expression / Value | Key Significance |
|---|---|---|
| Energy of One Photon | E = hν = hc / λ | Fundamental quantum energy packet for electronic transitions. |
| Einstein Energy (1 Mole of Photons) | E = NAhν = (1.196 × 105) / λ (in nm) kJ/mol | Used to calculate total energy absorbed in stoichiometric calculations. |
| Quantum Yield (Φ) | Φ = Moles of reactant consumed / Moles of photons absorbed | Measures reaction efficiency; can be fractional or up to 106. |
| Fluorescence Lifetime | ~10-8 seconds | Extremely rapid decay; ceases immediately when light source is removed. |
| Phosphorescence Lifetime | 10-4 to several seconds (or hours) | Slow decay due to spin-forbidden triplet-to-singlet transition. |
Previous Year Question Hints
- Question Type 1 (Quantum Yield Calculation): Aspirants are frequently given the total power of a monochromatic light source, its wavelength, and the time of exposure, along with the moles of product formed. You must first calculate the total number of photons emitted using Energy = NA(hc/λ), find the photons absorbed, and then divide the reacted moles by absorbed photons to get Φ.
- Question Type 2 (Identifying Photophysical Pathways): Questions often describe an emission process continuing long after the UV lamp is switched off. Examiners test your conceptual clarity between fluorescence and phosphorescence—remember that delayed emission always points to phosphorescence via triplet states.
Quick Revision Summary
- Photochemistry studies chemical changes induced by electromagnetic radiation absorption, operating independently of thermal activation energies.
- The Grotthus-Draper law mandates light absorption as a prerequisite for photochemical activity.
- The Stark-Einstein law establishes a 1:1 photon-to-molecule absorption ratio in the primary photochemical act.
- Quantum yield (Φ) measures reaction efficiency; values greater than 1 indicate radical chain mechanisms.
- Fluorescence is a fast, spin-allowed singlet-singlet radiative transition.
- Phosphorescence is a delayed, spin-forbidden triplet-singlet radiative transition mediated by intersystem crossing.
- Photosensitizers act as energy-transfer intermediaries for reactants that cannot absorb light directly.
- Chemiluminescence converts chemical reaction energy directly into visible photon emission without thermal excitation.
