Synthesis and thermoelectric properties of 2- and 2,8-substituted tetrathiotetracenesElectronic supplementary information (ESI) available. CCDC 1811026. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8tc00073e

• Main Authors: Garrett, Mary Robert, Durán-Peña, María Jesús, Lewis, William, Pudzs, Kaspars, U ulis, J nis, Mihailovs, Igors, Tyril, Björk, Shine, Jonathan, Smith, Emily F, Rutkis, Martins, Woodward, Simon
• Format: Article
• Language: English
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/c8tc00073e

Reaction of elemental sulfur with 2-R 1 and 2,8-R 1 ,R 2 -substituted tetracenes ( 2 ) in refluxing DMF affords 5,6,11,12 tetrathiotetracenes ( 1 ) in good yields (74-99%) for a range of substituents where R 1 ,R 2 are: H,H ( a ); Me,H ( b ); MeO,H ( c ); Ph,H ( d ); Me,Me ( e ), iPr,Me ( f , iPr = iso-propyl, CHMe 2 ), Me,MeO ( g ); MeO,MeO ( h ). The reaction rate is limited only by the solubility of the tetracene ( 2 ); 2g-h being both the least soluble and slowest reacting. At partial conversion recovered single crystalline 2g led to its X-ray structure determination. Vacuum deposited (substrate deposition temperature 300 K, pressure 7 × 10 −6 mbar, source temperature 500 K) thin films from 1 (of initial 88-99% purity) show final electrical conductivities, σ (in plane) from 1.40 × 10 −5 S cm −1 ( 1g ) to 3.74 × 10 −4 S cm −1 ( 1b ) for the resultant near pristine films; while 1d proved too involatile to be effectively sublimed under these conditions. In comparison, initially 95% pure TTT ( 1a ) based films show σ (in-plane) = 4.33 × 10 −5 S cm −1 . The purities of 1a-h are highly upgraded during sublimation. Well defined micro-crystallites showing blade, needle or mossy like habits are observed in the films. The Seebeck coefficients ( S b ) of the prepared 1 range from 374 ( 1c ) to 900 ( 1f ) μV K −1 ( vs. 855 μV K −1 for identically prepared 95% pure TTT, 1a ). Doping of films of 1f (R 1 = iPr, R 2 = Me) with iodine produces optimal p-type behaviour: σ (in-plane) = 7.00 × 10 −2 S cm −1 , S b = 175 μV K −1 . The latter's power factor (PF) at 0.33 μW m −1 K −2 is more than 500 times that of the equivalent I 2 -doped TTT films ( 1a , R 1 = R 2 = H), previously regarded as the optimal material for thin film thermoelectric devices using acene radical cation motifs. Thermoelectric properties (conductivity and Seebeck coefficient) are in screening for new tetrathiotetracene lead materials. Iodine doping reveals iPr,Me-TTT as optimal.