Single-Crystal X-ray Structures of conductive π-Stacking Dimers of Tetrakis(alkylthio)benzene Radical Cations

Salts containing radical cations of 1,2,4,5-tetrakis(isopropylthio)benzene (TPB) and 1,2,4,5-tetrakis(ethylthio) benzene (TEB) have been successfully synthesized with . These newly synthesized salts have been characterized by UV-Vis absorption, EPR spectroscopy, conductivity measurement, single crystal X-ray diffraction analysis as well as DFT calculation. This study raises the first crystal structure of conductive π-stacking radical cation with single phenyl ring and reveals their conductivity has relationship with the stack structure which affected by the substituent.

Scientific RepoRts | 6:29314 | DOI: 10.1038/srep29314 indicating that the radical cations are stable under these conditions. Upon oxidation with NOSbF 6 , TPB and TEB successfully converted to soluble radical cation salts (Eq. 1). TPB SbF 6 at − 20 °C, and the details of the crystallographic data are shown in support information (Tables S1, S2, SI). The single crystal X-ray diffraction reveals that TPB ·+ has π -dimer structure   Scientific RepoRts | 6:29314 | DOI: 10.1038/srep29314 (Fig. 3). Dimeric pairs are separated by counter anions. Cations and anions are linked by considerable H… F and S… F contacts (Fig. 4). One stack is neighboring to six others by aromatic-aromatic interactions (3.6898(7) Å; Fig. 4). Close packings of the TPB ·+ in dimeric pairs lead to staggered arrangements of the iPr groups to avoid steric crowding (Fig. 5). In a π -dimer, two independent radical cations have essentially the same structures (Fig. 3). The closest interplanar C-C distance (3.3183(3) Å) and S-S (3.5678(1) Å) between the two radical cations in the dimeric pairs are less than the sum of the van der Waals radii of these atoms (C-C 3.40 Å; S-S 3.70 Å) respectively, indicating electronic couplings between them. However, the closest interdimeric S-S distance (5.6861(1) Å) is too long, thus no interaction is observed between the pairs (Fig. 6).
The blue needle-like crystals cooling from the solution obtained from equivalent TEB and NOSbF 6 have totally different structure compared with ⋅+ − TPB SbF 6 ( Fig. 7).

⋅+ −
TEB SbF 6 stacks along a axis as dimers and crystallizes in the space group P-1 (Tables S1, S2; Figures S1, S2, SI). In a stack, the closest interplanar S-S distance between the two radical cations is 3.4518(3), indicating the electron can migrate well between them, while the closest interdimeric S-S distance (3.8356(4) Å) of two neighboring dimers is a little longer than 3.70 Å, implying a weak electronic couplings.
Crystals of ⋅+ − 2TEB (TEB)2SbF 6 were obtained by using excess amount of TEB. The single crystal X-ray diffraction demonstrated neutral TEBs inserted in the stacks (Fig. 8, Tables S1, S2; Figures S3, S4, SI). As shown, TEB ·+ stacks as dimers and between two dimers there is a neutral TEB (Table S2, SI) 14,26 , the dimers and the neutral molecules are arranged at regular intervals. The closest interplanar S-S distance (3.5903(8) Å) between the two radical cations in the dimeric pairs is shorter than the sum of the van der Waals radii of a sp 2 S-S interaction (3.70 Å), indicating strong electronic couplings between them. However, the closest S-S distance (3.8163(3) Å)   between one radical cation and one neutral molecule is longer than 3.70 Å. The insulation of the neutral TEB make the electron could not migrate well across the neutral molecule. Therefore, the π -reaction could not convey thoroughly via the stacks.   The EPR spectrum of solution samples (Fig. 11) gave a powerful support of the formation of the tetrakis(akylthio)benzene radical cations. The hyperfine coupling of hydrogen atoms in the solution EPR spectra are in good agreement with the patterns resulting from interactions with H ring (2H) and H akyl (SEt or SiPr) atoms 1 . This can be rationalized as the single charge is stabilized by the whole molecule seen from the experimental and simulated results as well as the calculated spin density maps (Fig. 12)  TEB SbF 6 at room temperature gave σ = 3.9 × 10 −7 S/cm, 2.04 × 10 −6 S/cm, and 1.09 × 10 −5 S/cm along the axises on which stacks lie, respectively (Fig. 13, Table 1). Spin density maps showed the electron cloud on S atoms contribute a lot to the delocalization of charge. It is known that the conductivity of π -stacked radical cations originates from the migration of π -interactions between chains while anions have no contribution to conductivity 11 . It can be expected that conductivity would be higher along the stacking direction (i.e., through π -stacks) because the axis of the stacking directions are not overlap with the axises of the unit cells (This can be seen from the stereoviews of the crystal structures in Fig. 4, Figures S2 and S4, SI) 10,27 . Comparing

Conclusions
In conclusion, radical cations of 1,2,4,5-tetrakis(isopropylthio)benzene (TPB) and 1,2,4,5-tetrakis(ethylthio)benzene (TEB) have been successfully isolated as stable crystals, and their structures were determined by X-ray crystallography. Upon oxidation with NOSbF 6 , TPB and TEB form stacks of radical cation π -dimers. Conductivity measurement reveals the interdimeric distance between two dimer pairs determines the ability of charge transformation. Thus our work not only raise the first example of π -stacking radical cation with single phenyl ring of weak conductivity, but also lead to a steric control synthesis and provide a systematic study of conductive π -stacking radical cations. Isolation of such radical species together with their structural determination will open up a new avenue for electrical conductors 21 .

Methods
General Procedures. All experiments were carried out under a nitrogen atmosphere using standard Schlenk techniques and a glove box. 1,2,4,5-tetrakis(isopropyl-thio)benzene (TPB) and NOSbF 6 (Alfa Aesar) were purchased and used upon arrival. 1,2,4,5-tetrakis(ethylthio)benzene (TEB) were prepared according to literatures 25 . Solvents were dried prior to use. EPR spectra were obtained using Bruker EMX-10/12 at room temperature. UV-Vis spectra were recorded on Lambda 35 spectrometers. Element analyses were performed on Elementar Vario EL III at Shanghai Institute of Organic Chemistry, the Chinese Academy of Sciences. X-ray crystal structures were obtained by Bruker APEX-II CCD and PHOTON100 CMOS detectors. Single crystals were coated with Paratone-N oil and mounted using a glass fiber. Crystal data and structure refinement details are listed in the supporting information (Table 1, SI). For conductivity measurements, single-crystal samples were affixed on glass carriers and silver paste was used to connect samples and electrodes along the crystallogarphic axises. I-V curves were measured by using a computer-controlled Keithley 2400 source meter.