Bis[bis-(4-alkoxyphenyl)amino] Derivatives of Dithienylethene,Bithiophene,Dithienothiophene and Dithienopyrrole:Palladium-Catalysed Synthesis and Highly Delocalised Radical Cations

摘要

Five diamines with thio-phene-based bridges-(E)-1,2-bis-{5-[bis(4-butoxyphenyl)amino]-2-thie-nyl}ethylene (1),5,5'-bis[bis(4-methoxy-phenyl)amino]-2,2'-bithiophene (2),2,6-bis[bis(4-butoxyphenyl)amino]di-thieno[3,2-b:2',3'-d]thiophene (3),N-(4-tert-butylphenyl)-2,6-bis[bis(4-methoxy-phenyl)amino]dithieno[3,2-b:2',3'-d]pyr-role (4a) and N-tert-butyl-2,6-bis[bis(4-methoxyphenyl) amino] dithieno [3,2-b:2',3'-d]pyrrole (4b)-have been syn-thesised.The syntheses make use of the palladium(0)-catalysed coupling of brominated thiophene species with dia-rylamines,in some cases accelerated by microwave irradiation.The molecules all undergo facile oxidation,4b being the most readily oxidised at about -0.4 V versus ferrocenium/ferrocene,and solutions of the corresponding radical cations were generated by addition of tris(4-bromophenyl)aminium hexa-chloroantimonate to the neutral species.The near-IR spectra of the radical cations show absorptions characteristic of symmetrical delocalised species (that is,class III mixed-valence species);analysis of these absorptions in the framework of Hush theory indicates strong coupling between the two amine redox centres,stronger than that observed in species with phenylene-based bridging groups of comparable length.The strong coupling can be attributed to high-lying orbitals of the thiophene-based bridging units.ESR spectroscopy indicates that the coupling constant to the amino nitrogen atoms is somewhat reduced relative to that in a stilbene-bridged analogue.The neutral species and the corresponding radical cations have been studied with the aid of density functional theory and time-dependent density functional theory.The DFT-calcu-lated ESR parameters are in good agreement with experiment,while calculated spin densities suggest increased bridge character to the oxidation in these species relative to that in comparable species with phenylene-based bridges.