(a) (i) What are the phenomena that result in the emission wavelength being longer than the excitation wavelength in…

(a)(i) Why emission is at a longer wavelength than excitation (5 marks)

The emitted light has lower energy than the absorbed light. This shift to longer wavelength is the Stokes shift, and it arises because energy is lost non-radiatively between absorption and emission.

• Vibrational relaxation and internal conversion. Absorption takes S₀ (v = 0) to a higher vibrational level of S₁ (a Franck-Condon transition). The molecule loses the excess vibrational energy to the surroundings in picoseconds and reaches S₁ (v = 0), and emission (Kasha's rule) starts only from there. It ends in a vibrationally excited level of S₀, which then relaxes. Emitted photon energy is smaller than the absorbed photon energy by the sum of these two losses.
• Solvent reorganisation and geometry change. The excited state often has a different dipole moment and geometry, and the solvent shells reorient around it before emission, which lowers its energy further, so polar solvents give a larger shift.
• Intersystem crossing. Where S₁ crosses to the triplet T₁, which lies lower, the emission from T₁ (phosphorescence) is at a still longer wavelength.
• Excimer and exciplex formation gives broad emission at longer wavelength than the monomer.

(a)(ii) Why phosphorescence is slower than fluorescence (5 marks)

Fluorescence is S₁ → S₀, a spin-allowed transition (both singlet, ΔS = 0). It is fast: the radiative rate constant is about 10⁶-10⁹ s⁻¹ and the lifetime is nanoseconds.

Phosphorescence is T₁ → S₀, which changes the spin multiplicity (ΔS ≠ 0) and is forbidden by the spin selection rule. It occurs only because of weak spin-orbit coupling, which mixes a little singlet character into the triplet (strong with heavy atoms such as Br, I or Pb, weak in light-atom molecules). The transition probability, and therefore the radiative rate constant, is small (about 10⁻² to 10³ s⁻¹), so the triplet lives for milliseconds to seconds. In addition, the triplet is usually lost by non-radiative deactivation and collisional quenching in fluid solution, so phosphorescence is seen mainly in rigid glasses at 77 K or in solids. The emitting state is also lower in energy than S₁ (Hund's rule), which further lengthens the wavelength.

(b)(i) Norrish Type I reaction of (C₆H₅)₂CH-CO-CH(C₆H₄CH₃-4)₂ (10 marks)

On irradiation the ketone is excited (n → π*) and undergoes α-cleavage, homolysis of a C-CO bond next to the carbonyl (Norrish Type I). The bond is weak because both radicals formed are stabilised. The reaction goes by a radical pathway:

• (1) α-Cleavage on either side gives an acyl radical and a diarylmethyl radical: Ph₂CH-CO• + •CH(Tol)₂ (Tol = 4-CH₃C₆H₄), or Tol₂CH-CO• + •CHPh₂.
• (2) Each acyl radical loses CO (decarbonylation) very rapidly, because it forms a stable diarylmethyl radical: Ph₂CH-CO• → Ph₂CH• + CO, and Tol₂CH-CO• → Tol₂CH• + CO.
• (3) The result is a pair of radicals, Ph₂CH• and Tol₂CH•, together with carbon monoxide. Recombination of the two radicals gives the products.

Products. Cross-coupling of the pair formed in the same solvent cage gives 1,1-diphenyl-2,2-di(p-tolyl)ethane, Ph₂CH-CH(Tol)₂, as the main product. Radicals that escape the cage combine randomly with each other, giving in addition 1,1,2,2-tetraphenylethane (Ph₂CH-CHPh₂) and 1,1,2,2-tetra(p-tolyl)ethane (Tol₂CH-CHTol₂); a statistical mixture would be about 1:2:1 of homo, cross and homo, and the in-cage cross product raises the proportion of the middle one. Carbon monoxide is also released.

(b)(ii) Norrish Type II: singlet or triplet? (10 marks)

Both, depending on the ketone. The reaction is intramolecular γ-hydrogen abstraction by the excited carbonyl oxygen through a six-membered transition state. This gives a 1,4-biradical, which either fragments to an enol (which tautomerises to a ketone) plus an alkene (Norrish Type II cleavage), or closes to a cyclobutanol (Yang cyclisation), or reverts to the ketone.

The state involved is tested with a triplet quencher such as a conjugated diene (1,3-pentadiene or piperylene), whose triplet energy is lower than that of the ketone, so it removes the ketone triplet at nearly the diffusion-controlled rate. The Stern-Volmer plot, Φ₀/Φ = 1 + kqτ[Q], shows how much of the reaction is from the triplet.

Example 1, aliphatic ketones (2-pentanone or 2-hexanone). Intersystem crossing is slow (about 10⁸ s⁻¹) and is comparable with the rate of γ-H abstraction from S₁, so reaction occurs from both S₁ and T₁. Diene quenching removes only the triplet part: the quantum yield falls but levels off at a non-zero value, and a plot against the diene concentration curves, showing that a fraction of the product comes from S₁ and is not quenched.

Example 2, aryl alkyl ketones (valerophenone, butyrophenone). Intersystem crossing in these is extremely fast (about 10¹⁰-10¹¹ s⁻¹), much faster than any singlet reaction, so S₁ is depleted before it can react, and the n,π* triplet is the reacting state. Diene quenches the product formation completely (linear Stern-Volmer plot to zero), and the triplet lifetime deduced is about 10⁻⁸-10⁻⁷ s.

So the multiplicity is not fixed by the reaction type but by the competition between intersystem crossing and hydrogen transfer in the particular ketone.

(c) Products and mechanisms (20 marks)

(i) NaBH₄ in EtOH on methyl 3-formylbenzoate (4 marks). Sodium borohydride is a mild hydride donor. It reduces the aldehyde much faster than the ester (the ester carbonyl is stabilised by the OMe lone pair), so the product is methyl 3-(hydroxymethyl)benzoate. Mechanism: hydride from BH₄⁻ adds to the carbonyl carbon of CHO through a four-centre transition state, giving an alkoxide-borane; this is protonated by the ethanol solvent (or during work-up) to give the primary alcohol, and each BH₄⁻ can deliver up to four hydrides. The ester is unchanged.

(ii) LiAlH₄ then H₂O on 4-tert-butylcyclohexanone (4 marks). The tert-butyl group is locked equatorial. A small hydride such as AlH₄⁻ attacks from the axial direction. This path avoids the torsional strain (eclipsing with the adjacent C-H bonds) that equatorial attack would incur, and a small nucleophile is not troubled by the axial hydrogens at C3 and C5. Hydride delivered axially pushes the oxygen to the equatorial position, so the major product (about 90%) is trans-4-tert-butylcyclohexanol, with the OH equatorial, and about 10% is the cis (axial OH) isomer. Water then protonates the aluminium alkoxide.

(iii) Na, NH₃(l), ether, ethanol on anisole (4 marks). This is the Birch reduction. The product is 1-methoxycyclohexa-1,4-diene, in which the methoxy group stays on a double-bond carbon. Mechanism: (1) an electron from sodium (solvated in liquid ammonia) adds to the ring to give a radical anion; (2) ethanol protonates it, at a carbon ortho to OMe, to give a cyclohexadienyl radical; (3) a second electron gives the cyclohexadienyl anion, in which the electron-donating OMe destabilises the negative charge on the carbon that carries it; (4) ethanol protonates the anion at the central carbon of the pentadienyl system (para to OMe), which gives the non-conjugated 1,4-diene rather than the 1,3-diene. The substituent stays on an sp² carbon because a donor group disfavours negative charge at its own carbon.

(iv) SeO₂ on RCH=C(CH₃)₂ (4 marks). This is the Riley allylic oxidation. The oxidation occurs at the more substituted end of the double bond, at an allylic methyl group, and it prefers the methyl group trans to R (the E-methyl), giving the (E)-allylic alcohol RCH=C(CH₃)CH₂OH. Mechanism: (1) an ene reaction of the alkene with SeO₂: an allylic hydrogen from the CH₃ is transferred to selenium oxygen while Se bonds to the carbon RCH, in a cyclic six-membered transition state, shifting the double bond to give an allylic seleninic acid, R-CH(SeO₂H)-C(CH₃)=CH₂; (2) a [2,3]-sigmatropic rearrangement moves oxygen to the terminal carbon, giving the selenite ester RCH=C(CH₃)-CH₂-O-Se-OH; (3) hydrolysis releases the allylic alcohol and Se(II). With excess reagent the alcohol can be oxidised further to the enal.

(v) Pb(OAc)₄ on 2-cyanohydroquinone (4 marks). Lead tetra-acetate oxidises the hydroquinone to the quinone, 2-cyano-1,4-benzoquinone. Mechanism: (1) one OH exchanges with an acetate ligand, giving the aryloxy-lead(IV) species ArO-Pb(OAc)₃ and AcOH; (2) the para OH loses its proton and its electrons flow through the ring, forming the second C=O, while the O-Pb bond and a second ligand leave, so that Pb(IV) is reduced to Pb(II); (3) the products are the quinone, Pb(OAc)₂ and two molecules of acetic acid. HQ + Pb(OAc)₄ → Q + Pb(OAc)₂ + 2AcOH. The electron-withdrawing CN makes the quinone a strong oxidising agent, so it is best isolated quickly.