Photochemistry_of_Organic

Metalloporphyrins

In addition to [Ru(bpy)3]2 þ , many other metal complexes can be utilized in photoinduced redox reactions (see also Section 6.8). An important class of such compounds are metalloporphyrins, macrocycles derived from four pyrrole-like subunits with a coordinated metal (haemoglobin or cyanocobalamin (vitamin B12) are the best known derivatives). A free base porphyrin, and also some non transition metal (Al3 þ , Zn2 þ , etc.) complexes, are excellent and widely used photosensitizers1249 to generate singlet oxygen (Section 6.7.1). Many metalloporphyrins can also be involved in photoinduced electron transfer reactions; their redox properties depend on the nature of coordinated metal ion. Complexes of metals possessing a higher positive charge display higher standard potentials of oxidation.1248 Conjugation of the metal d-electrons with the p-electrons of the porphyrin macrocycle causes strong charge transfer and electrostatic interactions upon excitation (the absorption spectrum exhibits significant bands in the visible part of the spectrum). The central heavy metal ion also enhances intersystem crossing due to spin–orbit coupling (Section 4.8).

The interesting dual behaviour of porphyrin derivatives is demonstrated by intramolecular electron transfer between an electron-rich tris(4-methylphenyl)porphyrin zinc complex (ZnTPP) and an electron-deficient tris(4-methylphenyl)octafluoroporphyrin (TPOFP) (Teflon porphyrin) free base connected via an azobenzene moiety (438; Scheme 6.209).1250 Upon photolysis at >440 nm, E ! Z photoisomerization (Section 6.4.1) occurs. (Z)-438 can revert to (E)-438 only thermally, because the excited state of the Z-isomer is efficiently quenched by intramolecular electron transfer from ZnTPP to TPOFP, whereas no such interaction is possible in the E-isomer because the chromophores are too remote. This approach has been suggested for photocontrolled molecular electronics.

Scheme 6.209

6.4.5Problems

1.Explain the following concepts and keywords: photochromism; photoswitching; photoinitiator; photoaffinity labelling; photo-Wolff rearrangement; photoactivatable compound; Barton reaction; photochemical trigger; photolabile linker; photoamination; redox photosensitization.

2.Suggest the mechanisms for the following reactions:

(a)

[ref. 1251]

(b)

[ref. 1252]

(c)

[ref. 1253]

3.Predict the major photoproduct(s):

(a)

N2

hν

O

N

[ref. 1254]

(b)

[ref. 1255]

(c)

C8H17

hν

MeOH

N

OH

[ref. 1256]

6.5 Sulfur Compounds

Sulfur and oxygen (see Section 6.3) compounds are isoelectronic and the discussion of their excited states is analogous. The differences can be understood by recalling two simple facts: sulfur is less electronegative than oxygen and the C¼S p-bond is weaker than the C¼O p-bond. The absorption spectra of thiophenols and thioethers are similar to those of the corresponding phenols (Figure 6.15, top).

Figure 6.15 Absorption spectra of sulfur-containing compounds. Top: thiophenol (in heptane, —), thioxanthone (in hexane, ), thiourea (in water, - - -), bis(trifluoromethyl)thioketene (in

isooctane, . . . . ).280 Bottom: absorption (—) and emission (. . . . ) spectra of adamantanethione in perfluoroalkane solvents.1257 The peak at the red edge of the S0 ! S1 transition coincides with

the 0–0 band of phosphorescence and is attributed to S0 ! T1 absorption

Figure 6.16 Positions of the n,p transitions ( ~v/mm 1) in thiocarbonyl compounds

The n,p transitions of thiocarbonyl compounds lie at much longer wavelengths ( 500 nm) than those of the corresponding carbonyl compounds, because its np lone pair lies at much higher energy. The energy gap between the S1(n,p ) and S2(p,p ) states of thiocarbonyl compounds is therefore fairly large and the Kasha and Vavilov rules (Section 2.1.8) may be violated (Figure 6.15, bottom). Even inefficient reactions proceeding from the S2(p,p ) state can amount to the dominant photoprocess because the S1(n,p ) states are short-lived (energy gap law) and not very reactive due to their low excitation energy. Spin–orbit coupling between the ground state and the lowest triplet state of n,p character is sufficiently large in thiocarbonyl compounds to render singlet–triplet absorption detectable (« 1 M 1 cm 1; Figure 6.15, bottom).

The positions of the n,p transitions in a series of thiocarbonyl compounds are shown in Figure 6.16.1258 The shifts between related compounds follow the trends expected from perturbation theory: inductive acceptors lower the energy of the np orbital on sulfur and mesomeric interactions raise the energy of the p orbital.

Table 6.17 Photoprocesses involving excited sulfur-containing compounds

a[H] ¼ hydrogen atom donor.

hν

Scheme 6.211

Case Study 6.32: Mechanistic photochemistry – reactions from different excited states

Excitation of the alkyl aryl thione 443 with an activated b-hydrogen into either the S1 (n,p ) or S2 (p,p ) state using different irradiation wavelengths was found to produce the cyclopropanethiol 444 via the same 1,3-biradical (Scheme 6.212).1274 In contrast to the reaction from the 1p,p state, the product formation obtained by excitation into S1 was quenched by a triplet quencher, indicating that the lowest (n,p ) triplet must be the reactive state. A very large isotope effect (kH/kD > 17) found for this reaction indicates a substantial tunnelling effect (Special Topic 5.2) in the hydrogen abstraction process.

Scheme 6.212

Experimental details.1274 A benzene solution of 443 (5 10 3 M) was irradiated using a medium-pressure mercury lamp in a Pyrex immersion well through an optical filter (lirr > 420 nm; S1 excitation; Figure 3.9) to complete conversion. Preparative TLC and crystallization from n-pentane afforded the product in 88% chemical yield.

The same benzene solution was irradiated through a Pyrex filter (lirr > 280 nm; S2 excitation) to complete conversion and the product was isolated in 82% chemical yield.

Photocycloaddition to Alkenes

Both aliphatic and aromatic thiocarbonyl compounds can undergo photocycloaddition to unsaturated compounds from either singlet or triplet excited states.1260,1261,1263 The

intersystem crossing quantum yields in aromatic thioketones are usually high and are dependent on the excitation wavelength: F is less than unity ( 0.5) in the case of

excitation to the second excited singlet state, S2 (lexc 300–400 nm), but it approaches F ¼ 1 when the irradiation is done at lexc > 500 nm (S1).1275 Excited thiobenzophenone

(445), for example, reacts with an electron-rich enol ether 446 to produce the thietane 447, analogous to the Paterno`–B€uchi reaction involving carbonyl compounds (Section 6.3.2) or the 1,4-dithiane 448, formed by the reaction of two molecules of thiobenzophenone.1276 Whereas both products are obtained from the n,p excited triplet state (T1) of thiobenzophenone and electron-rich ethoxyethene via a radical mechanism, the thietane 449 is formed exclusively by a stereospecific cycloaddition from the p,p excited singlet state (S2) to an electron-deficient trans-fumaronitrile (450) (Scheme 6.213).

Scheme 6.213