Chemistry driven by electronic excitation: absorption of photons, excited-state relaxation pathways, photo-induced reactions. Bridges L1 optics / quantum-mechanical transitions to L2 reactivity. Covers singlet/triplet manifolds, internal…
photochemistry
Jablonski diagram
Schematic energy-level diagram showing ground singlet S₀, excited singlets S₁, S₂, …, and triplet manifold T₁, T₂, …, with radiative…
Intersystem crossing (ISC)
Non-radiative transition between electronic states of different spin multiplicity (typically S₁ → T₁). Formally spin-forbidden; rate…
Photochemical quantum yield
Φ = (number of events of a given type) / (number of photons absorbed). Defined per process: Φ_f for fluorescence, Φ_r for product…
Photoinduced electron transfer
Transfer of an electron from a donor to an acceptor mediated by excitation of one of the two species by a photon. Rate is governed by…
Förster resonance energy transfer (FRET)
Non-radiative dipole–dipole energy transfer from an excited donor fluorophore to a nearby acceptor, with efficiency E = 1 / (1 + (r/R₀)⁶),…
Franck-Condon principle
Electronic transitions occur vertically (nuclei frozen); overlap of vibrational wavefunctions sets band intensity. Explains Stokes shift,…
Kasha's rule
Emission typically from lowest excited state (S1→S0 or T1→S0) regardless of excitation energy. Rapid IC/IVR relaxation within excited-state…
Conical intersection
Point where two BO potential energy surfaces touch; nuclear dynamics bypass slow radiative decay. Sub-ps photochemistry: vision (retinal),…
Semiconductor photocatalysis
TiO₂/CdS/g-C₃N₄ absorb hν > Eg → e⁻/h⁺ pair → surface redox. Water splitting (Honda-Fujishima 1972), pollutant degradation, solar fuels.
Ruthenium/iridium photoredox catalysis
[Ru(bpy)₃]²⁺* and Ir(ppy)₃* strong single-electron oxidants/reductants; MacMillan + Yoon + Stephenson revolution in organic synthesis…
Jablonski diagram (detail)
S₀ absorption → S_n; IC (ps), ISC (ns), fluorescence (ns), phosphorescence (μs-s); ΦF + ΦIC + ΦISC = 1 rates.
El-Sayed rule (ISC)
ISC rate large when Δ(orbital character): π-π* → n-π* allowed; enables spin-orbit coupling via orbital angular momentum change.
Stern–Volmer quenching analysis
F₀/F = 1 + k_q τ₀ [Q]; distinguishes dynamic from static quenching; FRET, collisional, heavy-atom mechanisms.
Photosensitized ¹O₂
³Sens* + ³O₂ → Sens + ¹O₂ (1Δ_g, 95 kJ/mol); in PDT, photo-oxidation; porphyrins, phenothiazines.
Norrish I/II photochemistry
Carbonyl n-π* excited: I α-cleavage, II γ-H abstraction followed by β-scission or cyclization (Yang).
Triplet-triplet annihilation upconversion
2 ³Sens* → ¹Sens* + Sens; low-power UC of NIR to visible; solar concentrators, bioimaging.
Retinal photoisomerization
11-cis → all-trans in rhodopsin in 200 fs with Φ=0.65; first step in vision; bacterial proton-pumping rhodopsins analog.
Photopolymerization (radical/cationic)
UV-cure; radical (acrylate) vs cationic (epoxy) initiators; 3D printing, dental resins, coatings.
Solar photocatalytic H₂ production
TiO₂ rutile/anatase, BiVO₄, g-C₃N₄; Z-scheme water splitting; quantum yield challenges at red wavelengths.
Einstein A_21 / B_21 = 8 pi h nu^3 / c^3 with Planck-consistent B_12 = g_2 / g_1 B_21 (photochemistry)
Chemistry application of L1 einstein-coefficients and rabi-oscillation. Einstein's 1917 derivation introduces three rate coefficients…
Electric-dipole selection rules: Delta S = 0, Delta L = +-1, Delta m_L in {-1,0,+1}, parity flip (photochemistry)
Chemistry application of L1 selection-rules and fermi-golden-rule. Electric-dipole (E1) transitions have non-zero matrix element <f | e r…