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Química Orgânica Física · Excited states and photoreactions

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O que você vai aprender

  • Read a Jablonski diagram: place absorption, internal conversion, intersystem crossing, fluorescence and phosphorescence, give their typical timescales, and distinguish n,π* from π,π* excited states.
  • Calculate photon energies, quantum yields, rate constants, excited-state lifetimes and excited-state redox potentials from Stark–Einstein, Stern–Volmer and Rehm–Weller relations, and choose a triplet sensitizer or quencher from triplet energies.
  • Draw the radical steps of Norrish type I and type II reactions, Paternò–Büchi oxetane formation and benzophenone photoreduction from the n,π* state, and predict their products and regiochemistry.
  • Predict the products of enone–alkene [2 + 2] photocycloadditions, photochemical electrocyclic and cycloaddition reactions, cis–trans photoisomerization, the di-π-methane rearrangement and singlet-oxygen reactions.

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Absorption and the Jablonski diagram

Absorbing a UV or visible photon takes about 10⁻¹⁵ s and lifts a molecule from its singlet ground state S₀ to a vibrationally hot singlet excited state (S₁, S₂…); electron spin does not change on absorption. Vibrational relaxation and internal conversion (IC) between upper excited states take picoseconds, so almost all emission and photochemistry start from the lowest vibrational level of S₁ or T₁ (Kasha's rule). From S₁ a molecule can fluoresce (S₁ → S₀ with light, lifetimes of about 10⁻⁹–10⁻⁸ s), return to S₀ without light (IC), or undergo intersystem crossing (ISC) to the triplet T₁, which flips one spin. T₁ lies below the S₁ state of the same orbital configuration because two parallel spins avoid each other (Hund's rule). T₁ → S₀ is spin-forbidden, so phosphorescence is slow: milliseconds to seconds for organic molecules in a rigid glass at 77 K. In fluid solution, triplets usually last microseconds because collisions, especially with O₂, deactivate them first. Heavy atoms (Br, I) raise spin–orbit coupling and speed up every singlet–triplet step: 1-bromonaphthalene fluoresces about a hundred times more weakly than naphthalene, and its phosphorescence lifetime at 77 K drops from about 2 s to tens of milliseconds. Benzophenone (shown) crosses to T₁ with a quantum yield of about 1 within about 10 ps, so it hardly fluoresces.

n,π* and π,π* states

A carbonyl group has two low-energy transitions. In n → π* an electron leaves a nonbonding lone-pair orbital in the plane of the C=O group and enters the π* orbital, which is perpendicular to it. The two orbitals barely overlap, so the band is weak (ε ≈ 10–100 M⁻¹ cm⁻¹, acetone 279 nm in hexane) and moves to shorter wavelength in protic solvents, which hydrogen-bond to the lone pair and lower its energy (acetone 265 nm in water). In π → π* both orbitals occupy the same region; the band is strong (ε ≈ 10³–10⁵) and moves to longer wavelength with conjugation. Poor overlap also makes the singlet–triplet gap small for n,π* states (about 25 kJ/mol for benzophenone) but large for π,π* states (about 130 kJ/mol for naphthalene). By El-Sayed's rule, ISC is fast when it also changes orbital type (¹n,π* → ³π,π* or ¹π,π* → ³n,π*), because spin–orbit coupling then has an orbital change to work with; benzophenone's ISC (about 10¹¹ s⁻¹) is therefore far faster than naphthalene's (about 10⁷ s⁻¹), where both states are π,π*. In the n,π* state the oxygen has a half-filled n orbital, so it behaves like an electrophilic alkoxyl radical: it abstracts hydrogen atoms and adds to alkenes. Strong donor substituents change the order of states: in 4-aminobenzophenone, especially in polar solvents, the lowest excited state is a charge-transfer state that puts extra electron density on the oxygen, and it barely abstracts hydrogen. Arrow drawings cannot show excitation, so in this topic the n,π* (or π,π*) excited state is drawn as a 1,2-diradical, for example Ph₂C•–O•, and the radical steps that follow use fishhook arrows.

Quantum yield, sensitization and quenching

By the Stark–Einstein law, each absorbed photon excites one molecule. A mole of photons (an einstein) carries E = N_A·h·c/λ = 1.196 × 10⁵ kJ nm mol⁻¹ / λ, so 254 nm light carries about 471 kJ/mol and 366 nm light about 327 kJ/mol. The quantum yield Φ is the number of molecules that react (or emit) divided by the number of photons absorbed; the photons absorbed are the lamp's photon flux (measured with a chemical actinometer) × time × the fraction of light the sample absorbs. A primary photoprocess has Φ ≤ 1; secondary dark reactions can raise the overall value (benzophenone photoreduction consumes about two ketones per photon), and radical chains give Φ ≫ 1. For a fluorescent state, Φ_f = k_f·τ. Triplet sensitization transfers triplet energy from a donor to an acceptor by electron exchange (Dexter), which needs contact and conserves spin: ³D* + A → D + ³A*. It is diffusion-controlled when the donor's triplet energy E_T exceeds the acceptor's by roughly 10 kJ/mol or more. Approximate E_T values (kJ/mol): benzene 353, acetophenone 310, benzophenone 289, naphthalene 255, (E)-penta-1,3-diene 247, biacetyl 236, (E)-stilbene 205, anthracene 178, tetracene 123; singlet oxygen lies 94 kJ/mol above ground-state (triplet) O₂. Quenching follows the Stern–Volmer equation, Φ₀/Φ = I₀/I = 1 + k_q·τ₀·[Q], with K_SV = k_q·τ₀, so the slope of a Stern–Volmer plot gives the lifetime of the quenched state once k_q is known (near diffusion control, about 5 × 10⁹–2 × 10¹⁰ M⁻¹ s⁻¹ for exothermic quenching). Because triplets live 10³–10⁶ times longer than singlets, a diene at millimolar to centimolar concentration, or dissolved O₂ (about 2 mM in air-saturated organic solvents), quenches triplet reactions while barely affecting fluorescence from nanosecond singlets, which is how the reactive spin state is identified.

Norrish reactions of ketones

Norrish type I is α-cleavage: the n,π* state breaks the bond between the carbonyl carbon and an α-carbon to give an acyl radical and an alkyl radical. Cleavage occurs on the side that gives the more substituted (more stable) alkyl radical. A cyclic ketone gives an acyl–alkyl diradical that can recombine, transfer a hydrogen to make an unsaturated aldehyde (cyclohexanone gives hex-5-enal) or a ketene, or lose CO. With an open-chain benzyl ketone such as dibenzyl ketone, the arylacetyl radical loses CO and the free benzyl radicals combine. Norrish type II needs a γ-hydrogen. The n,π* oxygen takes it through a cyclic arrangement of six atoms (O, carbonyl C, Cα, Cβ, Cγ, H), giving a 1,4-diradical. This diradical fragments at the Cα–Cβ bond to an enol and an alkene (the enol then tautomerizes to the ketone), closes to a cyclobutanol (Yang cyclization), or gives the hydrogen back. Valerophenone (shown) gives acetophenone and propene. Triplet diradicals must undergo ISC before they can form a bond.

Paternò–Büchi reactions, photoreduction and photoinitiators

The n,π* oxygen of a ketone or aldehyde adds to an alkene to give an oxetane (Paternò–Büchi reaction). With triplet ketones the oxygen bonds first and gives the more stable 1,4-diradical, which closes after ISC. Benzophenone and 2-methylpropene therefore give mainly 3,3-dimethyl-2,2-diphenyloxetane (shown), and furan gives the acetal-type bicyclic oxetane. In propan-2-ol, triplet benzophenone instead abstracts the C–H hydrogen; the 2-hydroxypropan-2-yl radical gives its O–H hydrogen to a second benzophenone, and the two diphenylhydroxymethyl radicals combine to benzopinacol, which is why about two ketones are consumed per photon. Photoinitiators such as 2,2-dimethoxy-2-phenylacetophenone undergo Norrish type I cleavage to benzoyl and dimethoxybenzyl radicals, which start acrylate polymerization.

Photocycloadditions and selection rules

Light reverses the thermal pericyclic rules. Suprafacial [2 + 2] and [4 + 4] cycloadditions (4n electrons) are photochemically allowed while the suprafacial [4 + 2] becomes forbidden, and electrocyclic reactions swap modes: 4n electrons disrotatory, 4n + 2 conrotatory. Cyclohepta-1,3-diene therefore closes to cis-fused bicyclo[3.2.0]hept-6-ene, and norbornadiene, whose two C=C bonds face each other, closes with a triplet sensitizer to the strained quadricyclane, which stores about 90 kJ/mol. Cyclic enones such as cyclohex-2-enone (shown) add to alkenes from their π,π* triplet. The reaction goes stepwise through 1,4-diradicals, so alkene geometry is lost and trans-fused bicyclo[4.2.0]octan-2-ones can form. Electron-rich alkenes such as 1,1-dimethoxyethylene give the head-to-tail adduct, 7,7-dimethoxybicyclo[4.2.0]octan-2-one, in which the substituted alkene carbon is bonded to the enone β-carbon. In the solid state the crystal packing chooses the dimer (topochemical control): α-cinnamic acid, packed head-to-tail, gives α-truxillic acid, and β-cinnamic acid, packed head-to-head, gives β-truxinic acid.

Photoisomerization, di-π-methane and singlet oxygen

Excited alkenes twist about the C=C bond toward a perpendicular geometry that decays to either isomer, so irradiation interconverts (E)- and (Z)-stilbene. At the photostationary state the forward and reverse rates are equal, which gives [Z]/[E] = (ε_E·Φ_E→Z)/(ε_Z·Φ_Z→E): light that the E isomer absorbs better enriches the Z isomer, even though Z is less stable. Triplet sensitizers set their own ratio. A sensitizer whose E_T is above both stilbene triplet energies ((E) ≈ 205, (Z) ≈ 240 kJ/mol) excites both isomers at similar rates, but one whose E_T lies between them excites the E isomer much faster than the Z isomer, so Z accumulates. 1,4-Dienes with an sp³ central carbon undergo the di-π-methane rearrangement to vinylcyclopropanes. In an unsymmetrical diene the three-membered ring of the intermediate opens so as to keep the better-stabilized radical, so a diphenyl-substituted alkene carbon ends up in the cyclopropane ring. Singlet oxygen, made by energy transfer from the triplet of a dye sensitizer such as rose bengal or methylene blue to ground-state triplet O₂, reacts as a closed-shell O=O: it undergoes ene reactions with alkenes that carry allylic hydrogens (allylic hydroperoxides, with the C=C shifted) and [4 + 2] cycloadditions with dienes (endoperoxides such as ascaridole).

Photoinduced electron transfer and photoredox catalysis

An excited state is both a better oxidant and a better reductant than its ground state: excitation leaves a hole in a low-lying orbital, which can accept an electron, and puts an electron in a high-lying orbital, from which it is easily removed. Its potentials follow from the ground-state couples and the excitation energy E₀,₀ (for one electron, an energy in eV shifts a potential by the same number of volts): E(A*/A•⁻) = E(A/A•⁻) + E₀,₀ and E(A•⁺/A*) = E(A•⁺/A) − E₀,₀. 9,10-Dicyanoanthracene (DCA, shown; E(A/A•⁻) = −0.89 V vs SCE, E₀,₀ = 2.89 eV) is a mild oxidant in the ground state but a strong one, +2.00 V, when excited. The free energy of electron transfer from a donor D to the excited acceptor is ΔG_ET ≈ F·[E_ox(D) − E(A*/A•⁻)] (the Rehm–Weller estimate, with the small Coulomb term neglected in polar solvents); 1 eV corresponds to 96.5 kJ/mol, and transfer is fast when ΔG_ET is clearly negative. Photoredox catalysts such as Ru(bpy)₃²⁺, fac-Ir(ppy)₃ and the organic dye 4CzIPN obey the same arithmetic: compare the excited catalyst's potential with the substrate's to decide whether reductive quenching (the excited catalyst oxidizes a donor) or oxidative quenching (it reduces an acceptor) is downhill. These electron transfers create radical ions, which this topic teaches with questions rather than arrows.

What to carry forward

First decide which excited state reacts: n,π* or π,π*, singlet or triplet, using lifetimes, quenching and sensitization data. Then draw the excited state as a diradical and continue with ordinary radical steps (α-cleavage, 1,5-hydrogen abstraction, addition to an alkene, ring closure after ISC), or apply the reversed pericyclic rules for concerted singlet reactions. For electron transfer, add or subtract the excitation energy to get the excited-state potentials. Quantum yields, Stern–Volmer plots and redox potentials turn these mechanisms into numbers you can test.

Problemas de prática (40)

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Desenhe o mecanismo 9

Carbonyl photochemistry

  • Photoreduction of benzophenone to benzopinacol in propan-2-olDraw hydrogen abstraction from propan-2-ol by triplet benzophenone, hydrogen transfer from the 2-hydroxypropan-2-yl radical to a second benzophenone, and coupling of the two ketyl radicals to benzopinacol.
  • Norrish type I cleavage of the photoinitiator DMPA and initiation of methyl acrylateDraw the α-cleavage of excited DMPA into benzoyl and dimethoxybenzyl radicals, the addition of benzoyl to methyl acrylate, and the β-scission of the dimethoxybenzyl radical to methyl benzoate and a methyl radical.
  • Norrish type I photolysis of cyclohexanone to hex-5-enalDraw the α-cleavage of excited cyclohexanone to an acyl–alkyl diradical and the intramolecular hydrogen transfer that gives hex-5-enal.
  • Norrish type II cleavage of valerophenoneDraw the 1,5-hydrogen abstraction by triplet valerophenone, the fragmentation of the 1,4-diradical to an enol and propene, and the acid-catalysed ketonization to acetophenone.
  • Paternò–Büchi reaction of benzophenone with 2-methylpropeneDraw the stepwise Paternò–Büchi reaction of triplet benzophenone with 2-methylpropene through the more stable 1,4-diradical, and explain why 3,3-dimethyl-2,2-diphenyloxetane is the main regioisomer.
  • Norrish–Yang cyclization of butyrophenone to a cyclobutanolDraw γ-hydrogen abstraction by triplet butyrophenone and the Yang ring closure of the 1,4-diradical to 1-phenylcyclobutan-1-ol, and explain what decides cyclization against fragmentation.

Photocycloaddition, isomerization and rearrangement

  • Photochemical ring closure of cyclohepta-1,3-dieneDraw the photochemical 4-electron electrocyclic closure of cyclohepta-1,3-diene to bicyclo[3.2.0]hept-6-ene and explain why the disrotatory, cis-fused outcome needs light.
  • Di-π-methane rearrangement of a 1,4-diene to a vinylcyclopropaneDraw the di-π-methane rearrangement of 3,3-dimethyl-1,1,5,5-tetraphenylpenta-1,4-diene as bridging to a cyclopropyldicarbinyl diradical, ring opening to a 1,3-diradical and ring closure to the vinylcyclopropane.
  • Photocycloaddition of cyclohex-2-enone and ethyleneDraw the stepwise triplet [2 + 2] photocycloaddition of cyclohex-2-enone and ethylene through a 1,4-diradical to bicyclo[4.2.0]octan-2-one, and use loss of alkene geometry as evidence for the diradical.

Preveja o produto 8

Quantum yield, quenching and electron transfer

  • Which electron transfer can 4CzIPN do?Decide which photoinduced electron transfer is thermodynamically downhill by comparing excited-state and substrate potentials.

Excited states and the Jablonski diagram

  • Add a bromine to naphthalenePredict how an internal heavy atom changes fluorescence and phosphorescence.

Photocycloaddition, isomerization and rearrangement

  • Photodimerization in a crystalPredict the constitution of a solid-state [2 + 2] photodimer from the packing of the monomers in the crystal.
  • Which end ends up in the ring?Predict the regiochemistry of an unsymmetrical di-π-methane rearrangement from radical stability in the intermediate diradicals.
  • Head-to-head or head-to-tail?Predict the regiochemistry of an enone [2 + 2] photocycloaddition with an electron-rich alkene.
  • Norbornadiene under a sensitizerPredict the product of an intramolecular [2 + 2] photocycloaddition of norbornadiene.

Carbonyl photochemistry

  • Which side breaks in Norrish type I?Predict which α C–C bond of an unsymmetrical cyclic ketone breaks in Norrish type I cleavage.
  • Benzophenone and furanPredict the regiochemistry of a Paternò–Büchi reaction with furan from the stability of the intermediate diradical.

Classificar 4

Excited states and the Jablonski diagram

  • Assign the acetone bandClassify a weak carbonyl absorption band as n → π* from its intensity and solvent shift.

Photocycloaddition, isomerization and rearrangement

  • Spot the di-π-methane substrateRecognize the 1,4-diene motif with an sp³ central carbon that the di-π-methane rearrangement requires.
  • What does light allow?Classify pericyclic processes as photochemically allowed or forbidden with the Woodward–Hoffmann rules.

Carbonyl photochemistry

  • Dibenzyl ketone loses COClassify the photodecarbonylation of dibenzyl ketone as a Norrish type I reaction.

Comparar 1

  • How fast is each Jablonski process?Order absorption, vibrational relaxation, fluorescence and phosphorescence by their typical timescales.

Calcular 8

Excited states and the Jablonski diagram

  • Singlet–triplet gap of benzophenoneCalculate a singlet–triplet energy gap in kJ/mol from the 0–0 band wavelengths of the S₀–S₁ transition and of phosphorescence.

Quantum yield, quenching and electron transfer

  • How oxidizing is excited DCA?Calculate the excited-state reduction potential of an organic photosensitizer from its ground-state potential and singlet energy.
  • Energy of 313 nm lightCalculate the energy of one einstein of UV photons in kJ/mol from the wavelength.
  • Quantum yield from actinometryCalculate a reaction quantum yield from product formed, photon flux, irradiation time and the fraction of light absorbed.
  • Is electron transfer to excited DCA downhill?Estimate the free energy of photoinduced electron transfer from donor and acceptor potentials and the excitation energy.
  • Quenching rate constant from a Stern–Volmer pointCalculate a bimolecular quenching rate constant from fluorescence intensities and the unquenched lifetime.
  • How long does the reactive triplet live?Calculate the lifetime of a reactive triplet state from quantum yields with and without a diene quencher.

Photocycloaddition, isomerization and rearrangement

  • Photostationary state of stilbeneCalculate the composition of a photostationary state from the absorption coefficients and isomerization quantum yields of the two isomers.

Encontre o erro 1

  • Does the reaction come from S₁?Diagnose a wrong spin-state assignment from triplet-quencher and fluorescence data.

Interprete as evidências 2

Excited states and the Jablonski diagram

  • Two emission bands from naphthaleneAssign fluorescence and phosphorescence bands from their wavelengths and lifetimes.

Quantum yield, quenching and electron transfer

  • A quantum yield of 2000Interpret a quantum yield far above 1 as evidence for a radical chain reaction.

Trabalhe de trás para frente 1

  • Which ketone gives acetone and 2-methylpropene?Work backwards from Norrish type II fragmentation products to the ketone that was irradiated.

Planeje uma síntese 1

  • Pick a triplet quencher for benzophenoneChoose a triplet quencher that accepts energy from a given sensitizer and does not absorb the irradiation wavelength.

Explique por quê 5

Carbonyl photochemistry

  • Why is 4-aminobenzophenone so hard to photoreduce?Explain how a donor substituent that changes the lowest excited state from n,π* to charge transfer shuts down hydrogen abstraction.
  • Norrish type II, one event at a timePut the photophysical and chemical events of a triplet Norrish type II reaction in order.

Excited states and the Jablonski diagram

  • Why benzophenone crosses to the triplet so fastUse El-Sayed's rule to explain why intersystem crossing is fast from an n,π* singlet and slow between π,π* states.

Photocycloaddition, isomerization and rearrangement

  • Sensitizer energy and the stilbene ratioExplain how a sensitizer's triplet energy controls the photostationary state of a sensitized E/Z isomerization.
  • How a dye makes singlet oxygenExplain how a triplet dye sensitizer converts ground-state triplet oxygen into singlet oxygen.

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