Photochemistry of the p-Extended 9,10-Bis(1,3-dithiol-2-ylidene)- 9,10-dihydroanthracene System: Generation and Characterisation of the Radical Cation, Dication, and Derived Products** Allison E. Jones, [a] Christian A. Christensen, [a] Dmitrii F. Perepichka, [a] Andrei S. Batsanov, [a] Andrew Beeby,* [a] Paul J. Low,* [a] Martin R. Bryce,* [a] and Anthony W. Parker [b] Abstract: Flash photolysis of bis[4,5- di(methylsulfanyl)1,3-dithiol-2-ylidene]- 9,10-dihydroanthracene (1) in chloro- form leads to formation of the transient radical cation species 1 . which has a diagnostic broad absorption band at l max  650 nm. This band decays to half its original intensity over a period of about 80 ms. Species 1 . has also been characterised by resonance Raman spec- troscopy. In degassed solution 1 . dis- proportionates to give the dication 1 2 , whereas in aerated solutions the photo- degradation product is the 10-[4,5-di- (methylsulfanyl)1,3-dithiol-2-ylidene]an- thracene-9(10 H)one (2). The dication 1 2 has been characterised by a spec- troelectrochemical study [l max (CH 2 Cl 2 )  377, 392, 419, 479 nm] and by an X-ray crystal structure of the salt 1 2 (ClO 4  ) 2 , which was obtained by electrocrystallisation. The planar an- thracene and 1,3-dithiolium rings in the dication form a dihedral angle of 77.28 ; this conformation is strikingly different from the saddle-shaped structure of neutral 1 reported previously. Keywords: dithioles ´ electrochem- istry ´ photolysis ´ radical ions ´ structure elucidation Introduction Tetrathiafulvalene (TTF) is a famous p-electron donor molecule which undergoes two reversible one-electron waves to afford the radical cation and dication species, [1] with a gain in heteroaromaticity upon formation of the 1,3-dithiolium cation. [2] Considerable attention has been directed to related bis(1,3-dithiole) systems where p-conjugation is extended between the dithiole rings by the insertion of vinylogous [3] and quinonoid [4] spacer units ; this leads to reduced intramolecular Coulombic repulsion in the oxidised states. [5] Derivatives of the 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene system are particularly interesting in this context as they have a single, quasi-reversible, two-electron oxidation wave that yields a thermodynamically stable dication, for example, for 1, [6] E ox  0.55 V (in MeCN vs. Ag/AgCl) in the cyclic voltammogram. [7] The X-ray crystal structure of 1 [6] and several derivatives [8] have shown that a remarkable conforma- tional change accompanies oxidation to the dication. Whereas the neutral molecules are saddle-shaped, [6, 8] with the central ring of the anthracenediylidene moiety in a boat conforma- tion, the dications have a planar anthracene system with the 1,3-dithiolium cations almost orthogonal to this plane. [7b, 9] Theoretical calculations support the crystallographic data: steric hindrance between the sulfur atoms and the peri hydrogen atoms dictates the conformation of the neutral species whilst, in the dication, there is aromatisation of both the dithiolium rings and the anthracene core. [10] Calculations also suggest that the radical cation largely retains the saddle conformation of the neutral species, thereby restricting any potential gain in aromaticity at this redox stage; this is in accord with the electrochemical data. [10] The strong electron- donor ability of the title ring system has led to its use as a component of intermolecular [7b, 9] and intramolecular [11] charge-transfer systems. The aim of the present work was to explore the photolytic generation of the elusive p-radical cation species 1 . . Herein we report the characterisation of 1 . by UV/Vis and Raman [a] Dr. A. Beeby, Dr. P. J. Low, Professor M. R. Bryce, A. E. Jones, C.A. Christensen, Dr. D. F. Perepichka, Dr. A. S. Batsanov Department of Chemistry, University of Durham Durham, DH1 3LE (UK) Fax: ( 44) 191-384-4737 E-mail : m.r.bryce@durham.ac.uk [b] Dr. A. W. Parker Central Laser Facility, CLRC Rutherford Appleton Laboratory Chilton, Didcot, Oxfordshire, OX11 OQX (UK) [**] Molecular Saddles, Part 6; for Part 5 see ref. [8b]. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/home/chemistry/ or from the author: fluo- rescence spectra obtained by pump (266 nm) ± probe (514 nm) of 1 in degassed chloroform. FULL PAPER Chem. Eur. J. 2001, 7 , No. 5  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0947-6539/01/0705-0973 $ 17.50+.50/0 973