Photochemistry encompasses chemical and physical processes that result from the absorption of light. Two fundamental laws govern the field. The Grotthuss-Draper law states that only light absorbed by a system can produce a photochemical change — light that is transmitted or reflected has no effect. The Stark-Einstein law (the principle of photochemical equivalence) states that each absorbed photon activates exactly one molecule, so the number of molecules activated equals the number of photons absorbed. The energy of a photon E = hν = hc/λ determines whether absorption leads to bond breaking or electronic excitation, with UV photons carrying sufficient energy for photochemical reactions while visible and IR photons typically excite vibrations or promote valence electrons.
The Jablonski Diagram and Photophysical Processes
The Jablonski diagram maps the fate of an electronically excited molecule. Absorption promotes an electron from S₀ (singlet ground state) to higher singlet states (S₁, S₂). From these, the molecule can follow several pathways. Internal conversion (IC) is a non-radiative transition between states of the same multiplicity, rapidly relaxing vibrational energy as heat. Fluorescence is the spin-allowed radiative decay S₁ → S₀, occurring on nanosecond timescales. Intersystem crossing (ISC) involves a spin-forbidden transition to the triplet manifold (T₁), facilitated by spin-orbit coupling, which is especially efficient in molecules containing heavy atoms. Phosphorescence, the radiative T₁ → S₀ transition, is spin-forbidden and therefore slow (microseconds to seconds), making it readily distinguishable from fluorescence by time-resolved measurements.
Quantum Yield and Photosensitization
The quantum yield Φ is the fraction of absorbed photons that lead to a specific process: Φ = number of events / number of photons absorbed. For fluorescence, Φ_F ranges from near unity for highly fluorescent dyes (fluorescein, rhodamine) to essentially zero for molecules where non-radiative decay dominates. The sum of quantum yields for all deactivation pathways equals unity. Photosensitization involves a light-absorbing molecule (the sensitizer) that transfers its excitation energy to a non-absorbing acceptor, initiating chemistry in the acceptor. Perhaps the most biologically significant example is the role of chlorophyll as a photosensitizer in photosynthesis, harvesting sunlight to drive the reduction of CO₂ to carbohydrates. Singlet oxygen photosensitization is exploited in photodynamic therapy, where a photosensitizer generates reactive ¹O₂ to destroy tumor cells.
Stern-Volmer Kinetics
Quenching processes reduce the excited state population and compete with emission. Collisional (dynamic) quenching follows the Stern-Volmer equation: I₀/I = 1 + k_qτ₀[Q], where I₀ and I are fluorescence intensities in the absence and presence of quencher Q, k_q is the bimolecular quenching rate constant, and τ₀ is the unquenched excited-state lifetime. Static quenching (ground-state complex formation) produces a similar linear Stern-Volmer plot but with a different temperature dependence. The Stern-Volmer constant K_SV = k_qτ₀ is obtained from the slope and provides mechanistic insight. Common quenchers include molecular oxygen (a powerful triplet quencher), iodide ions, acrylamide, and heavy atoms that promote ISC.
Photochemical Reactions
Photochemical reactions often follow different pathways than their thermal counterparts because excited states have different electronic distributions and bond orders. Norrish type I reactions involve α-cleavage of carbonyl compounds to form radical pairs, while Norrish type II reactions proceed via a six-membered transition state with γ-hydrogen abstraction, leading to cyclobutanol formation (Yang cyclization) or fragmentation. The Paterno-Büchi reaction is a [2+2] photocycloaddition between a carbonyl and an alkene, yielding oxetanes. The photo-Fries rearrangement of aryl esters produces ortho- and para-hydroxyketones through a radical cage mechanism. Photochemical reactions typically exhibit lower activation barriers than thermal reactions because the excited state already contains the necessary energy, allowing reactions at low temperatures.
Applications of Photochemistry
Photochemistry has transformative applications across science and technology. Photodynamic therapy (PDT) combines a photosensitizer, light, and tissue oxygen to generate cytotoxic species for cancer treatment, with selectivity achieved by targeting light delivery. Organic photovoltaics (OPVs) use photoactive donor-acceptor blends to convert sunlight into electricity, with power conversion efficiencies exceeding 19% in laboratory cells. Photolithography is the enabling technology for semiconductor manufacturing, where photoresists undergo photoinduced solubility changes to create microscopic circuit patterns. Photocatalysis harnesses light to drive chemical transformations using semiconductors (TiO₂, g-C₃N₄) or molecular photocatalysts for applications including water splitting, CO₂ reduction, and environmental pollutant degradation.
