New strategies and building blocks for functionalised 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene derivatives, including pyrrolo-annelated derivatives and π-extended systems with intramolecular charge-transfer

Abstract

A range of new functionalised 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (TTFAQ) derivatives have been synthesised from the key di(halomethyl) building blocks, 10-[4,5-bis(bromomethyl)-1,3-dithiol-2-ylidene]anthracene-9(10H)-one 10, 10-[4,5-bis(chloromethyl)-1,3-dithiol-2-ylidene]anthracene-9(10H)-one 11 and 9-[4,5-bis(chloromethyl)-1,3-dithiol-2-ylidene]-10-[4,5-bis(hexylsulfanyl)-1,3-dithiol-2-ylidene]-9,10-dihydroanthracene 18. A Diels–Alder strategy comprising trapping of the transient exocyclic diene 19, which is derived from 18, with 1,4-naphthoquinone leads to the aromatised TTFAQ anthraquinone system 21. Horner–Wadsworth–Emmons olefination of 21 with the anion generated from reagent 22 gave the fused bis(TTFAQ) structure 23. Pyrrolo-annelated derivatives 30–34 have been obtained in a sequence of reactions from compound 10. Mono-formylation of the pyrrole ring of 32 and 33 under Vilsmeier conditions gave 35 and 36 which upon reaction with 2,4,5,7-tetranitrofluorene gave the donor–π-acceptor diads 38 and 39. Cyclic voltammetry (CV) in solution for all the TTFAQ derivatives shows the typical quasi-reversible two-electron oxidation wave of the TTFAQ core at potentials which vary slightly depending on the substituents. For example, the value of E^ox is raised by the electron withdrawing anthraquinone and tetranitrofluorene units of 21 and 38, respectively. The CV of the conjugated TTFAQ dimer 23 showed two, two-electron oxidation waves corresponding to the sequential formation of 23²⁺ and 23⁴⁺ (ΔE^ox = 130 mV) providing evidence for a significant intramolecular electronic interaction, i.e. the dication 23²⁺ acts as a conjugated donor–π-acceptor diad, thereby raising the oxidation potential of its partner TTFAQ unit. Spectroelectrochemical studies on 23 support this explanation. A strong intramolecular charge transfer band at λ_max 538 nm is seen in the UV-Vis spectra of the TTFAQ–π-tetranitrofluorene diads 38 and 39. The X-ray crystal structures are reported for compounds 30, 33 and 34. The pyrrolo–TTFAQ moiety adopts a saddle-shape with the central ring of the dihydroanthracene moiety folded along the C(9)...C(10) vector in each case. Significant intermolecular interactions are observed in the structures.