Photochemistry_of_Organic

Special Topic 6.15: Photochromism

Photochromism is a reversible chemical transformation, induced in one or both directions by the absorption of light, between two forms having different absorption spectra and other physical properties.624 Originally, the expression photochromic was used only for compounds exhibiting a light-induced reversible change of colour. In the example shown in Figure 6.11, only the thermodynamically stable isomer 2,7-dihydro- 2,2,7,7-tetramethylpyrene absorbs at wavelengths exceeding 330 nm, so that it can be completely converted to the colourless 2,2,7,7-tetramethyldicyclopropa[a,g]pyracene by irradiation at lirr > 330 nm. Irradiation at 313 nm leads to a photostationary state (PSS) (dashed line), because both isomers absorb at this wavelength. However, complete conversion to the more stable dihydropyrene proceeds in the dark at temperatures above 90 C.1092 The reaction medium and the presence of oxygen affect the kinetics and the reversibility of many photochromic systems. Any irreversible (usually photochemical) formation of minor side-products will limit the number of cycles that can be performed and is referred to as fatigue.

Figure 6.11 Photochromism of 2,7-dihydro-2,2,7,7-tetramethylpyrene

There are several families of compounds that display photochromic behaviour involving various mechanisms. The following list of reactions reviews the most common systems (Scheme 6.160). Spiropyrans, spirooxazines, chromenes and fulgides undergo

concerted or non-concerted electrocyclization reactions (Section 6.1.2; see also Case Study 6.4), azobenzenes involve E–Z isomerization (this section),624,1093-1101 quinones exhibit a group transfer,149,1024and polycyclic aromatic hydrocarbons undergo cycloaddi-

tion reactions (Section 6.2.2). Many biological systems are also photochromic; for example, rhodopsin exhibits reversible E–Z photoisomerization (Special Topic 6.1).

Photochromism has many potential and existing applications that take advantage of a change in colour or other physicochemical properties during the process, for example variable-transmission optical materials such as photochromic eyeglass or ophthalmic lenses that darken in sunlight (using spiropyran and spirooxazine systems in addition to

Case Study 6.28: Supramolecular chemistry – photoresponsive crown ethers

Photochemical butterfly-like E ! Z photoisomerization of a bis(crown ether) azobenzene derivative 354 was found to be thermally reversible and the stereoisomers exhibit unique contrasting behaviour in the presence of metal ions.1108 The concentration of the Z-isomer in the photostationary state was noticeably enhanced by the addition of K þ , Rb þ or Cs þ , because the corresponding Z-complex achieved a stable sandwich geometry (Scheme 6.162). As a result, the cations could be selectively extracted by the Z-derivative from an aqueous phase to an organic solvent (o-dichlorobenzene), whereas no complexation (i.e. no transfer) took place in the case of the E-isomer.

Scheme 6.162

Experimental details.1108 An o-dichlorobenzene solution of 354 (2 10 4 M; 100 ml) and an aqueous solution of MOH (M ¼ K, Rb or Cs) (25 ml) placed in a U-tube immersed in a thermostated water-bath were irradiated with a high-pressure mercury lamp (500 W) (Figure 3.9). Liquid–liquid phase transfer of cations between the layers was followed by absorption spectroscopy.

Imines and Oximes

Absorption bands (both n,p and p,p ) of N-alkylimines are generally below 260 nm, whereas those of aryl derivatives are bathochromically shifted. Efficient production of Z-isomers by photoisomerization reaction [e.g. in N-benzalaniline (355); Scheme 6.163] is

usually feasible only at low temperatures because the thermal reversion has a very low activation barrier (Ea 65 kJ mol 1).1061

by steady-state or time-resolved spectroscopy (Scheme 6.167 ).1116,1135–1139 The [3 þ 2] cycloadditions of ylides with dipolarophiles (such as alkenes or carbonyl compounds) provide a convenient method for synthesizing five-membered heterocyclic systems. For example, photolysis of a solution of phenylazirine (361) with an excess of methyl acrylate produces the pyrrolinecarboxylate 362 in 80% chemical yield,1140 and 363 irradiated in the presence of benzaldehyde gives the oxazoline 364 in 20% chemical yield1141 (Scheme 6.168). The ylide intermediates can also be trapped by various nucleophiles.1116

Scheme 6.168

3H-Diazirines

3H-Diazirines have been recognized as important photochemical and thermal precursors to carbenes (Section 5.4.1).1115,1142–1145 The N¼N bond in diazirines, constrained to a

three-membered ring, generally displays strong absorption between 310 and 350 nm. Various short-lived intermediates, including singlet excited diazirine, singlet carbene (365) and biradical (366), may be involved in photolysis of 367 (Scheme 6.169). These reactive species then undergo various rearrangement or bimolecular reactions. Diazirines can also photoisomerize to diazo compounds.1145 Mechanistic studies of diazirine photochemistry often utilize state-of-the-art methods, such as low-temperature matrix photochemistry (Section 3.10), to trap and detect reactive intermediates.

Diazo Compounds

Diazoalkanes display a weak absorption band between 300 and 500 nm (Figure 6.9).1115 An excited singlet state, formed upon irradiation in this region, eliminates nitrogen

Scheme 6.169

[DC¼N2(diazomethane) 200 kJ mol 1]1146 to form a singlet carbene intermediate (Scheme 6.170). Subsequent reactions of the carbene moiety are generally the same as those discussed in the previous paragraph.

Scheme 6.170

a-Diazocarbonyl compounds represent an important class of photolabile compounds because of their applications in lithographic (see also Special Topic 6.27) production of

integrated circuits used by the computer industry, photoaffinity labelling (Special Topic 6.16) and DNA cleavage experiments,1147 or organic synthesis.1110,1148 A typical

mechanism of the photodegradation of the a-diazocarbonyl compound 368 is the photoWolff rearrangement,1149–1151 which has been suggested to proceed via either

simultaneous elimination of nitrogen and rearrangement to a ketene 369 or via a carbene 370 intermediacy (Scheme 6.171).1152,1153

For example, steady-state photolysis of 2-diazoindan-1,3-dione (371) in alcohol gives a diester 372 in two photochemical steps (Scheme 6.172).1154,1155

O

OR

372

Scheme 6.172

Case Study 6.29: Mechanistic photochemistry – singlet–triplet interconversion of carbenes

Diphenylcarbene (diphenylmethylene) can be generated from diphenyldiazomethane (373) by direct irradiation or by triplet sensitization.1156 The intermediate multiplicity then controls the subsequent reactions: the singlet carbene inserts into the O H bond of methanol, whereas the triplet carbene adds to an alkene (Scheme 6.173). It has been

found that singlet and triplet diphenylcarbenes are in rapid equilibrium relative to the rates of reactions.1157,1158 Competitive quenching experiments (to obtain k1 and kTS)

and laser flash spectroscopy (Section 3.7; to obtain k2 and kST) allowed the determination of the free energy difference between the singlet and triplet states of carbene ( 20 kJ mol 1).

Experimental details.1157 A solution of 373 in acetonitrile (2.5 10 3 M) containing methanol (0.05 M) and isoprene (0.10–10 M) was purged with nitrogen and irradiated

with a high-pressure xenon lamp (150 W) through optical filters (lirr 366 nm; Figure 3.9). The products shown in Scheme 6.173 were analysed by GC.

Scheme 6.173

Diazonium Salts

Unsubstituted benzenediazonium ion in solution shows strong absorption maxima at lmax 300 and 261 nm.1111 As with thermal decomposition, its principal photoreaction is

release of the nitrogen molecule [e.g. DC N(benzenediazonium) 154 kJ mol 1]1159 to give an aryl cation,1160,1161 which can be readily attacked by a nucleophile such as water

(Scheme 6.174). In contrast, an aryl radical is formed in the presence of an electron donor (such as methanol) by electron transfer, followed by radical reactions, such as hydrogen abstraction from an H-atom donor ([H]).

- N2, - MeOH

Scheme 6.174

Azides

Unsubstituted organic azides absorb appreciably in the near-UV region (<380 nm). Their

direct irradiation leads to the extrusion of molecular nitrogen to form singlet nitrene (Section 5.4.2), which can intersystem cross to triplet nitrene (Scheme 6.175).1112,1114,1162

Triplet nitrene can also be obtained by photosensitization. In general, nitrenes are reactive intermediates that can undergo various reactions.