Chapter 2 - Photochemical and Photophysical Behavior of Thiophene

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Abstract

The photophysical properties and the photochemical behavior of thiophene and thienyl derivatives has been described. Photobiochemical properties of naturally occurrinf thiophenes are reported. The reactivity of thiophene in common reactions such as isomerization, Norrish Type I, dimerization, cycloaddition, arylation, cyclizations, photooxidation, and photopolymerization has been examined. The role of thienyl ring in the synthesis of photochromic substances and in the preparation of optically active material has been discussed. The photochemistry of both thiophene S-oxide or sulphone and dihydrothiophene was examined. The photochemistry of inorganic complexes in the presence of thiophene has been rewieved.

Introduction

The photochemistry of heterocyclic compounds was the object of a book published in 1976 and edited by Buchardt (76MI1). Here, the photochemistry of thiophene covers eight pages written by Lablache-Combier (76MI123). Some reviews have appeared since on the photochemical behavior of pyrrole and furan derivatives (89G419, 96H(43)1305, 96H(43)1529) but apparently none on thiophene photochemistry.

Section snippets

Thienyl derivatives and oligomers

UV properties of some thienyl derivatives appear in Table 1 (59G540) along with some 3-substituted derivatives (58SA350). UV and IR spectra of 2,2′-bithienyl derivatives appear in Table 2 (67T4419, 91JCP(95)4783). Optical spectra of oligothiophene derivatives (nT) with n = 2–11 have been studied in solution (93SM(60)23a, 94JCP(101)1787, 95JPC16991, 95PAC9, 96JPC18683), in thin films (93JCP(97)7427, 94JCP(101)1787, 98CP(227)33), and in crystals (96CPL(260)125). Spectra of nT in the gas phase have

Photochemical and photobiochemical properties of naturally occurring thiophenes and related structures

Bithiophenes and terthiophenes were isolated in several members of the Compositae. α-Terthiophene was found in Tagetes minuta (47JA270, 47JA273). After this first identification, several different compounds were found in all the family of the Compositae (58ACSA765, 58ACSA771, 62CB2934, 65CB155, 65JCS7109, 66TL4227, 70CB834, 76P1309, 78P2097, 80P2760, 81P743, 81P825, 82P1795, 85P615). From a biogenetic point of view, there derived from the fat acids metabolism, such as polyacetylenes (65CB876,

Photochemical isomerization

A photochemical isomerization of thienyl alkenes has been described (Scheme 2) (92CB2583). In this case, the cyclization reaction plays only a marginal role.

The photochemical isomerization of pentaatomic heterocyclic compounds has been the object of several reviews, starting with that of Lablache–Combier in 1971 (71BSF679). After this work, reviews of the whole field (76MI123, 77CRV473, 80MI501) or part of it (85MI745, 95MI803, 95MI1063) appeared. In spite of the amount of work done in this

Norrish-type i reaction

UV irradiation of 65 in benzene (about 4 mM) with a medium-pressure Hg Hanovia lamp using Pyrex glassware as a filter (λ > 290 nm) revealed products that are consistent with the expected α-cleavage and decarbonylation followed by radical–radical reactions (07OL4351). 2,3-Dithiophenylbutane 66 is obtained by a radical–radical combination reaction, while 3-isopropylthiophene 67 and 3-(propen-2-yl)thiophene 68 form by a radical–radical disproportionation process. The solid-state reaction was shown

Dimerization

The irradiation of 2-thiophenemethanol (69) gave dimeric products 70 and 71 (91JCR(S)166). The same behavior was observed when a bithienyl derivative 73 was used as substrate. A photosubstitution reaction to give 77 was observed in methanol.

Cycloaddition reactions

Thophene gave a 2+2 cycloaddition reaction when irradiated in the presence of maleic anhydride (Scheme 8) (63CB498; 70 JCS(CC)1474; 70MI28, 71MI9, 73JCS(P1)2322, 86JP(32)363). But performed in the presence of benzophenone as triplet sensitizer, the main product was the Paternò-Büchi adduct.

Thiophene reacts with phthalimide (78TL125). With halogenated phthalimide, the main reaction is the formation of the dimer 78 of the starting material, while a substitution product 79 was obtained in very low

Photochemical coupling of halogeno-substituted derivatives with thiophene

Allyl iodide reacts with thiophene giving a mixture of 2-allylthiophene (63.8%) and 3-allylthiophene (36.2%) (77JOC1570). Some other halogeno-substituted derivatives react with thiophene giving the corresponding thienyl derivatives (Scheme 14) (82JOC4520, 86H(24)799, 86JOC3453, 87BCJ1847, 91JHC1481).

Photochemical arylation

2-Iodothiophene reacts with PhS to give the coupling product through an SRN1 reaction (87JOC5382). 2-Iodothiophene (100) can be arylated in good yields when irradiated in aromatic solvents (

Stylbene-like cyclization

In an approach intermediate between photochemical arylation and stylbene cyclization, the reaction of chlorothienylanilides has been described (Scheme 20) (75JOC3001, 89SC1325, 91H(32)2323, 91JHC1997, 95H(41)1659, 95H(41)2691). It is noteworthy that this reaction can be selectively performed.

This type of reaction has been used in the synthesis of biological active compounds (03JMC4516). Compounds 185f, 185h, and 186 had the strongest inhibition, considering the antitumor activity. Compound 185h

Photochromism

Several reviews consider the photochromic properties of thiophene derivatives (98BCJ985, 02MI167, 04CSR85, 04JPP(C)(5)169, 05JPC(A)7343, 06CL1204, 07JCS(CC)781, 08MI1617).

A photochromic system between the colorless trienes 241a–c and their corresponding closed-ring species 242 of yellow color was devised (02JOC5208, 02OL1099).

The various photochromic parameters which were taken into account in the case of 243 were absorption wavelengths of the closed and open forms, rate constants of thermal

Photolysis of azido derivatives

Photolysis of 4-azidobenzo[b]thiophene (320) in diethylamine gave 4-aminobenzo[b]thiophene (321) (35%) and 4-(4-benzo[b]thienyl)azobenzo[b]thiophene (322) (20%) (72JCS(CC)879). However, photolysis of 5-azidobenzo[b]thiophene (323) gave 324 (24%) (72JCS(CC)879).

The proposed mechanism involved the ring opening reaction of an azirine (Scheme 26).

Photopolymerization

2-iodothiophene has been mixed with an aqueous solution of hydriodic acid, HI, by a ratio of 1:1. The entire mixture has been spin-coated on a silicon substrate, after all the fluid solution has been irradiated by UV-laser light with a wavelength of 248 nm (02MI34).

After irradiation, a silver-colored thin polymer layer totally covers the liquid phase. During the irradiation process, the color of the liquid phase has changed from yellow to a deep red one. The red color is due to a charge transfer

Optically active thiophene materials

Organic thin-film transistors have received intense interest in recent years for their potential as a low-cost alternative to amorphous hydrogenated silicon thin-film transistors for various electronic applications. OTFT-based array/circuits are particularly suited for large-area devices where high transistor density and switching speeds are not essential; they may also be attractive for applications in low-end microelectronics where the high cost of packaging silicon circuits becomes a

Photooxidation

Despite the fact that optical applications require thin films of poly(3-alkylthiophene)s, the photochemistry of these materials has been characterized in solution but only scarcely in the solid state. The UV/Vis spectra of these films of poly(3-butylthiophene) show an absorption band in the visible range corresponding to a π−π* transition whose energy depends on π-electron delocalization.

Photoirradiation experiments of P3-BT were carried out under nitrogen, air, or oxygen atmosphere using lamps

Photochemical cyclization

An intramolecular photocyclization of tris(2-benzo[b]thienyl)methyl alcohol (664a) to afford five- and seven-membered products has been reported (02TL8669). The irradiation of 664a gave cyclopentanone 665a (43%) and benzo[b]thiophene-condensed tropone derivative 666a (23%) along with recovered 664a (ca. 30%).

A similar photoreaction was induced also in methyl-substituted derivative 664b, giving rise to 665b (33%) and 666b (30%).

The interconversion between 665a and 666a was not induced under the

Photochemistry of thiophene s-oxide and sulphone

The photodimerization of 2-methylthianaphthene 1,1-dioxide (MeTND) as a function of solvent, concentration of MeTND, sensitizers, and quenchers (49JCS381, 50NAT(166)108, 56JA6174, 77AJC173, 79JA2157) showed the photodimerization gave a single product in good yield. We have found that irradiation at 313 nm of rigorously degassed solutions of MeTND produces two dimers in about a 9:1 ratio. When oxygen is present, the quantum yield decreases and the two dimers are produced in similar proportions.

Dihydrothiophenes

The authors reported that methyl 4-mercapto-2-alkenoates 710, obtained by irradiation of 2(5H)-thiophenones 709 in MeOH, undergo light-induced S–H bond homolysis to give thio radicals 710 (84HCA2198, 85HCA2350, 87HCA125). Intermediate 710 can be trapped by alkenes to afford 3-thiahex-5-enyl radical 711, which cyclize selectively to radicals 712, precursors of thiolane-3-acetates 714.

Reaction of 711 with terminal alkynes occurs regioselectively living 3-thiahexa-1,5-dienyl radicals 715 which

Photochemistry of inorganic complexes

The complex (C5Me5)Rh(PMe3)H2 was irradiated in the presence of thiophene in C7D14 solution at −40̊C in order to look for intermediates in the C–S cleavage reaction (92JA151). The major product observed was 738 (75%), although a second (C5Me5)Rh(PMe3)-containing product was observed by 1H NMR spectroscopy (25%). The ratio of products 738 and 739 remained 3:1 throughout the photolysis, suggesting that they are formed in parallel reactions rather than sequential reactions.

When a Rhodium complex 7

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