Photochemistry of the pi-extended 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene system: Generation and characterisation of the radical cation, dication, and derived products

Flash photolysis of bis[4,5-di(methylsulfanyl)1,3-dithiol-2-ylidene]-9,10-dihydroanthracene (1) in chloroform leads to formation of the transient radical cation species 1(.+) which has a diagnostic broad absorption band at lambda (max) approximate to 650 nm. This band decays to half its original intensity over a period of about 80 mus. Species 1(.+) has also been characterised by resonance Raman spectroscopy. In degassed solution 1(.+) disproportionates to give the dication 1(2+) whereas in aerated solutions the photodegradation product is the 10-[4,5-di-(methylsulfanyl)1,3-dithiol-2-ylidene]anthracene-9(10H)one (2). The dication 1(2+) has been characterised by a spectroelectrochemical study [lambda (max) (CH2Cl2) = 377, 392, 419, 479 nm] and by an X-ray crystal structure of the salt 1(2+)(ClO4-)(2), which was obtained by electrocrystallisation. The planar anthracene and 1,3-dithiolium rings in the dication form a dihedral angle of 77.2 degrees; this conformation is strikingly different from the saddle-shaped structure of neutral 1 reported previously.