Dithienopyrrole Derivatives with Nitronyl Nitroxide Radicals and Their Oxidation to Cationic High‐Spin Molecules

Abstract Three 1 N‐phenyl nitronyl nitroxide (NN) 4‐substituted dithieno[3,2‐b:2′,3′‐d]pyrrole (DTP) derivatives with R1=4‐phenyl‐, 4H‐, and 4‐methylthiothiophenyl‐ (R1 2 DTP‐Ph‐NN, R1=H, Ph and MeSTh) were designed, synthesized and characterized. The electrochemical properties were studied by cyclic voltammetry (CV). All the molecules exhibited two main oxidation peaks, first for radical cation and next for dication formation. The cation and dication formation were also confirmed by UV/Vis absorption spectroscopy for Ph2DTP‐Ph‐NN and MeSTh2DTP‐Ph‐NN titrated with tris(4‐bromophenyl)aminiumhexachloroantimonate (magic blue). In addition, the cation and dication formation were verified by EPR spectroscopy. Finally, the exchange interactions (J/k B) of NN and radical cation were calculated by DFT studies.


Introduction
Organich igh-spin molecules are attractive due to their flexible and controllable electronic properties, which can be obtained through differents trategies. [1] High-spin molecules have been used for various applicationss uch as spintronic devices,a nd molecular magnets. [1d, 2] The spin-carryingu nits are very important in the high-spinm olecules, because of the kinetic stability issues of materials for furthera pplications.T oe nhance the kinetic stability,n itronylnitroxide (NN) and iminonitroxide (IN) were the most recognized spin units for the high-spin organic molecules. [3] Organic spin-carryingu nits linked with conjugated triarylaminium radical cations are also of interest. [4] Their combination in mixed stable radical and one electron oxidized cation-diradical systemsare more popular,b ecause of their fair kinetic stability and synthetic accessibility.T he parent triphenylaminium radical cation is not stable and quickly reacts via the para positions to form benzidines. Thus, introduction of an electron donating para substituent eliminatest his problem. Stable radicalu nits linked with easilyo xidizable aromatic amines are of particulari nterest for triplet ground state high-spin molecules. [4a,b, 5] Thus, the mixeds table radicalw ith radical-cation molecules were provided for high-spin triplet ground state molecules. [1d, 6] As eries of nitronyl nitroxide (NN) substituted triarylamine (TAA-NN), [7] fused arylamine (FTAA-NN), [8] pyrazine (NNDPP), [9] thianthrene (TA-NN) [10] phenothiazine (PTZ-NN) [11] and pyrrole-based [2b] molecules and their radical-cationic forms were reported (FigureS1, Supporting Information( SI)), most of them afforded ground-state triplet diradicals upon one electron oxidation. In particular, 1-phenyl NN 4substituted-2,5-di(thiophen-2-yl)-1H-pyrrole derivatives (TPT-Ph-NN), [12] and NN-substituted conjugated oligomerso fd ithienyl-N-methylpyrrole with methoxys ubstituents at the inner b-position of thiophene rings type of molecules (DTP-P-NN), [13] were utilized for the synthesis of cationic high-spinm olecules. Therefore our interest focused on the synthesis of stable-radical speciess ubstituted with p-conjugated electron donor systems and their radical cationf ormationf or high-spinf ormation. Dithieno [3,2-b:2,3-d]-pyrrole (DTP)d erivatives are leading to better p-conjugation and lower ionization potential through the electron-donatingn ature of the molecules. They also have two active positions at the thiophene,w hich serve to modify the energy levels to substitute different donor and/ora cceptor units,w hich can assist in the controlo ft he HOMO and LUMO energyl evels of the molecules. In our molecular design, twophenyl (Ph) or -methylthiothiophene (MeSTh) groups were introducedt ot unet he electronic properties of the molecules in the DTP p-unit. [14] We report the design,s ynthesis, and structural characterization of three, NN substituted DTP molecules with R1 = 4phenyl-, 4H-, and 4-di(methylthiothiophenyl dithieno [3,2b:2',3'-d]pyrrole derivatives (R 1 2 DTP-Ph-NN R 1 = H, Ph and MeSTh) together with the formationo ft heir radical cationic (R 1 2 DTP-Ph-NN)C + + forms. Furthermore, intra-moleculare xchange interactions were examined by DFT calculation.

Molecular design with DFT calculations
The computations were carriedo ut to understand the electronic structure of the molecules. All the DFT calculations were performed using the Gaussian 09 package. [15] The full geometry optimizations were carried out by UB3LYP/6-31G(d) level for all the neutral radicalm olecules. The optimized structuresa nd spin density distribution of the neutralr adical molecules of DTP-Ph-NN, Ph 2 DTP-Ph-NN,a nd MeSTh 2 DTP-Ph-NN are shown in Figure 1.
In order to get tripletg round state high-spin molecules, one electronc ould be oxidized from HOMO of the neutral molecules. Thep roposed one-electron oxidation mechanism is described in Figure S14 for spin polarized donorr adicals. As imilar mechanism was also reported using molecular orbital theory to analyze one-electron oxidation of the amine-based spin-polarized donor radicala nd pyrrole derivativem olecules. [3b] Therefore, the energy level of the neutral molecules were analyzed using DFT calculations. The energy levels of the HOMO,L UMO, and SOMO were summarized in the Figure 2 and Table S1. The HOMO of the DTP-Ph-NN (À5.18 eV)i s higher energy than HOMO (À5.19 eV) of the Ph 2 DTP-PhNN and MeSTh 2 DTP-Ph-NN (À5.26 eV). The energy level of the SOMO for DTP-Ph-NN (À5.06 eV) is lower than for Ph 2 DTP-PhNN SOMO (À4.85 eV)a nd MeSTh 2 DTP-Ph-NN (À4.22 eV). The energy difference between HOMO and SOMO are gradually increased for DTP-Ph-NN, Ph 2 DTP-PhNN and MeSTh 2 DTP-Ph-NN,r espectively.T he results clearly indicate that the substitutionso nt heir terminal sites in the core DTP p-unit with two-Ph or -MeSTh groups were changed the electronic properties of the molecules.
The bond distance between NN and Ph unit is 1.461 and Ph and DTP-backbone is 1.412, these values are same for all the derivatives. The torsion betweenN Na nd its attached phenyli ss lightly varies for different derivatives such as 7.28, 1.48,a nd 1.78,s imilarly between phenyl and DTP-backbone also varies as 39.9, 48.9, and 41.1 for DTP-Ph-NN, Ph 2 DTP-Ph-NN,and MeSTh 2 DTP-Ph-NN,respectively.

X-ray crystallographic studies
The crystal structure analysisi si mportant to understand magnetic interactions in the solid state. Single crystals suitable for X-ray diffraction analysisw ere obtainedb yslow evaporation of CH 2 Cl 2 solution for MeSTh 2 DTP-Ph-NN and mixtures of CH 2 Cl 2 and PhCN solution for Ph 2 DTP-Ph-NN.
The blue plate-like Ph 2 DTP-PhNN was crystallized with PhCN solventm olecule in orthorhombic, Pbcn space group. The structure of the molecules is given in the Figure 3a.T he tor-  sions betweent he radicalN Na nd phenyla re 14.08.T he torsions between the central phenyl and p-unit is 48.58.F urthermore, the molecular packing is displayed in Figure 3b,p roviding as hort intermolecular distance (3.24 )b etween two oxygen atoms; in addition, short p-p intermoleculard istances were found to be 3.45 for S11···C5 and 3.48 for S11···C4.
The green block MeSTh 2 DTP-Ph-NN was crystallized in dimeric triclinic form with PÀ1s pace group ( Figure S3a). The two structures of the unit cell are given in Figure 3c and d. The two independentm olecules differ mainly in the orientation of the thiophene unit and the torsion angles.I n MeSTh 2 DTP-PhNN-A,t he sulfur atoms are arranged as S5 and S13 in syn-orientations, S8 and S20 in anti-orientations while in molecule MeSTh 2 DTP-Ph-NN-B,b oth S5 and S13, then S8 and S20 are arranged in anti-orientations. The torsions between the radical NN and Ph are also different in both molecules 34.08 and 15.18 for MeSTh 2 DTP-Ph-NN-A,a nd MeSTh 2 DTP-PhNN-B,r espectively.S imilarly slightly different torsion angles were found between the center Ph and DTP core as 30.28 and 27.68 for MeSTh 2 DTP-Ph-NN-A,a nd MeSTh 2 DTP-Ph-NN-B,c orrespondingly.T hese variations are due to intermolecular interaction present between the molecules in the molecular packing.

Cyclic voltammetry studies
Ap rerequisite for generating ar adical cationic molecule is that the arylamine of DPT moiety has al ower oxidation potential than those of the NN radical. Hence,t he electrochemical properties of all the moleculesw erei nvestigated by the cyclic voltammetry in acetonitrile for R 1 2 DTP-Ph-NN,( R 1 = H, and Th), and benzonitrile for Ph 2 DTP-PhNN at room temperature. The cyclic voltammograms are given in Figure 4f or Ph 2 DTP-Ph-NN,a nd for R 1 2 DTP-Ph-NN, (R 1 = H, andT hi nF igure S4). The oxidative process with half-wave potentials are summarized versusf errocene/ferrocenium (Fc/Fc + )i nT able 1.
The first oxidation wave occurs at + 0.23, and + 0.33, (AE 0.02) Vv ersus Fc/Fc + ,f or Th 2 DTP-Ph-NN and Ph 2 DTP-Ph-NN respectively.T he first oxidation potentials were apparently lower than those of the NN unit, [2b, 12] and similar compounds of DTP-Ph, Ph 2 DTP-Ph,a nd MeSTh 2 DTP-Ph reported in the literature. [14d-f] All the molecules showed two reversible and one irreversible oxidationw aves. The second and third oxidation   Figure S4b). The extension of the p-bridgei sb eneficial for better donor ability to the p-core. These two molecules exhibited both the first and second oxidation potentials lower than those of the NN radical. It means that the first and second steps were radical cation and dicationf ormations in the oligomer moieties (Scheme 2).

UV/Vis absorption studies
The opticalp roperties were studied for R 1 2 DTP-Ph-NN (R 1 = H, Ph and MeSTh)b yU V/Vis spectroscopy.T he absorption spectra are displayed in FigureS5. Twom ain absorption bands appeared, one around 280-450 nm for p-p*t ransitionso ft he donor p-unit and another about 500-750nmf or n-p*t ransitions of NN radicalu nits which is similar to typically reported nitronylnitroxide molecules. [20] The chemical oxidation reactions were conducted by tris(4bromophenyl)aminiumhexachloroantimonate (magic blue, (BrC 6 H 4 ) 3 NC + SbCl 6 À )a st he oxidant at room temperaturei na ir, and theser eactions were monitored by UV/Vis absorption spectroscopy.D uring first oxidation two new peaks were formed at 521 and 805 nm for Ph 2 DTP-Ph-NN ( Figure 5) and at 539 and 945 nm for MeSTh 2 DTP-Ph-NN ( Figure S6) duet op roductiono fr adical cation, at the mean time the absorption band at 380 for Ph 2 DTP-Ph-NN and 425 for MeSTh 2 DTP-Ph-NN for the neutral compounds weres uperseded. The absorption in the longerw avelength regionsw as assigned as the HOMO-SOMO and shorter for SOMO-LUMO transition, respectively.F or second oxidation, another newp eak appeared at 730 nm for Ph 2 DTP-Ph-NN and 887 nm for MeSTh 2 DTP-Ph-NN and former peaks decreased.

EPR studies
The EPR spectra of neutral radicals of DTP-Ph-NN, Ph 2 DTP-Ph-NN,a nd MeSTh 2 DTP-Ph-NN in toluene (~10 À4 m)i na rgon-saturated solutions were measured at room temperature and in frozen solution at 130 K. The experimentala nd simulated spectra are displayed in Figure S7a-c. All the molecules showed five equallys eparated lines at room temperature assigned to hyperfine coupling of two equivalent nitrogen nuclei of the NN unit. The g values (g = 2.0070, AE 0.02) are nearly the same for all three neutral radicals. The variablet emperature EPR spectra of the Ph 2 DTP-Ph-NN and Th 2 DTP-Ph-NN are displayedi nF igure S9. Moreover,t he frozen solution EPR spectra are asymmetric with anisotropic components, providing different numbersofs houlders on the low and high field range . Initially,w et ried to oxidizeu sing magic blue as an oxidant in the oxidation process, but the reaction could not be monitored clearly by EPR spectroscopy.I tm ay be that the unreactedm agic blue affectedt he resolution of the spectra.F urthermore, the chemical oxidation reactionw as carried out for all the neutralr adicals by the silver hexafluoroantimonate (AgSbF 6 )a nd the reactionw as monitored with EPR spectroscopy. DTP-Ph-NN in CH 2 Cl 2 (blue solution) was titrated with AgSbF 6 as an oxidanta nd the spectraa re given in Figure S8a. Duringa ddition of the oxidant, the blue solution of DTP-Ph-NN turned to green and the intensity of the five line spectra decreased. Finally,a ll the EPR lines disappeared, which is due to decomposition of the NN, and polymerization of the molecules or some other side reactionoccurred.
The blue solution of Ph 2 DTP-Ph-NN in CH 2 Cl 2 become brown during addition of the AgSbF 6 ,i nw hicht he EPR spectrum was recorded, there is ac learc hange in the spectrum where the center is shifted upfield and the total width narrowed asg iven in Figure S8ba nd Figure6.A fter addition of one equivalent oxidant, the 30 line EPR spectrumw as obtained Oxidative process with half-wavep otentials versus Fc/Fc + in Vw ith (AE 0.02 V).
Scheme2.Oxidation and reductionoft he R 2 DTP-Ph-NN and oxidationo f (R 2 DTP-Ph-NN) + + C molecules.  Figure S10b). The one very weak signal was found at 160 K, this is due to dimerizationo ccurring at lower temperatures. Therefore, no zero field splitting could be observed. After heating, the variable temperature EPR spectrum for the oxidized species of the (Ph 2 DTP-Ph-NN)C + + was measured ( Figure S10b). While decreasing the temperature, the intensity of the EPR line also decreased until at 200 K, It is just above the melting point( 176.5 K) of CH 2 Cl 2 .F or MeSTh 2 DTP-Ph-NN in CH 2 Cl 2 the light blue/green solution became dark green during addition of the AgSbF 6 ,a nd formed ad ark-green precipitate. The poorly soluble materials might be dimerized radicalc ations. The new EPR line appeared during addition of AgSbF 6 along with MeSTh 2 DTP-Ph-NN (5 line) but the spectra are not resolved, as demonstrated in Figure S8d. The p-dimerization of dithienylpyrrole radical cation molecules is known from the literature. [13,22] Further addition of one more equivalent of the AgSbF 6 ,k ept at À10 8Cf or 6h,f ive-line EPR spectra were retained for Ph 2 DTP-Ph-NN and MeSTh 2 DTP-Ph-NN,t he spectra for which are given in FigureS8e,f. These results indicate the dicationf ormation on the DTP core without decompositionoft he NN radical.

Magnetic interactionc alculations using DFT calculations
The intra molecular interaction (J intra,calc )o fN Na nd radicalcation was calculated for (R 1 2 DTP-Ph-NN)C + + for all the molecules with density functional theory (DFT)a nd hybridf unction BLYP,a nd basis set 6-31G(d) in the gas phase using the Gaussian 09 package. [15] The X-ray structure geometry of Ph 2 DTP-Ph-NN, MeSTh 2 DTP-Ph-NN and optimized geometry for DTP-Ph-NN with positive charge were used for the calculations.T he J intra,calc valuesa re À3580 K, + 5000 K, + 965 Kf or DTP-Ph-NN, Ph 2 DTP-Ph-NN,a nd MeSTh 2 DTP-Ph-NN,r espectively.T he exchange interaction between the NN and the radical-cation is positive and ferromagnetic for Ph2DTP-Ph-NN and MeSTh 2 DTP-Ph-NN,w hereas for DTP-Ph-NN it is negative indicating antiferromagnetic interaction. The spin densities were also calculated for both the neutral R 1 2 DTP-Ph-NN and charged( R 2 DTP-Ph-NN)C + + molecules. The spin density is mostly localized on Ph-NN for the neutral molecules while delocalized over the extended p-unit for charged molecules in Figure7 for Ph 2 DTP-Ph-NN (Ph 2 DTP-Ph-NN)C + + and see in Figure S11a nd S12 for R 2 DTP-PhNN, R 1 2 DTP-Ph-NN and (R 1 2 DTP-Ph-NN)C + + (R = H, and Th). The J intra,calc is nearly five times higher for (Ph 2 DTP-Ph-NN)C + + than for (MeSTh 2 DTP-Ph-NN)C + + because the positive chargesa re distributed equally over the entire molecule for (Ph 2 DTP-Ph-NN)C + + ,w hereas in MeSTh 2 DTP-Ph-NN the positive charge and the spin are better delocalized over the extended p-unit and there is less spin on the central phenyl.
The J intra,calc values indicate antiferromagnetici nteraction for (DTP-PhNN)C + + butf erromagnetic interaction for (Ph 2 DTP-PhNN)C + + and (MeSTh 2 DTP-PhNN)C + + .T oi nvestigate this variation, vertical ionization potential( IP ver = E cation ÀE neutral )c alculation wasu sed. These (IP ver )c alculations were performed by the DFT with BLYP,6 -31G(d) basis set in the gas phase using the Gaussian09p ackage (Figures S15 and S16, SI). The NN was deleted from the X-ray structures for Ph 2 -DTP-Ph and MeSTh 2 -DTP-Ph,w hereas for DTP-Ph and Ph-NN,o ptimizeds tructures were used for calculation. The calculated IP ver were 5.54 eV and 4.98 eV for Ph 2 -DTP and MeSTh 2 -DTP-Ph,r espectively.T hese are much lower than 6.24 eV of NN-Ph ( Figure S15). Therefore, neutralm olecules of Ph 2 DTP-Ph-NN, MeSTh 2 DTP-Ph-NN are easily oxidized andf orm high spin ground state molecules. In case of DTP-Ph the vertical ionization potential is IP ver = 6.37 eV,w hich is highert han the IP ver = 6.24 eV for phenyl nitronyl nitroxide radical( Ph-NN,F igure S13). Consequently,i n DTP-Ph-NNC + + the nitroxide is charged with al ow-spin ground state and the triplet is much higher in energy.
As mentioned before, to generate ah igh-spin biradical the arylamine of the DPT moiety must possess al ower oxidation potentialt han the phenyl-NN moiety.T he important factor now is that the energy difference between the HOMO and SOMO must be high enough. In the case of DTP-Ph-NN,t he energy level of the SOMO is À5.18 eV and HOMO is À5.06 eV. Then, the energyd ifferenceb etween HOMO and SOMO is only (0.12 eV),w hereast he HOMO of the phenyl nitronyl nitroxide (Ph-NN) is À5.17 eV.I tis very close to the SOMO of the DTP- Figure 6. EPR spectra of the (Ph 2 DTP-Ph-NN)C + + biradicalcation monomer obtained by chemicaloxidation titrated with AgSbF 6 as an oxidant in CH 2 Cl 2 at room temperature. Ph-NN (À5.18 eV). Therefore, we cannot determinev ery clearly that the removalo ft he first electron is either from the SOMO or HOMO for DTP-Ph-NN during oxidation. In other cases, the energy differenceb etween the HOMO and SOMO were À0.39 and À1.04 for Ph 2 DTP-PhNN and MeSTh 2 DTP-Ph-NN,r espectively,w hicha re sufficient to generate the cationic molecules.

Conclusions
Three Ph-NN substituted donor p-core of (DTP) derivatives (R 2 DTP-Ph-NN)m olecules were prepared and characterized. The moleculars tructures and packing of Ph 2 DTP-Ph-NN and Th 2 DTP-Ph-NN were examined by single-crystal X-ray structure analysis. The Th 2 DTP-Ph-NN is crystalized in the dimeric form with shorteri ntermolecular distance of NÀO···CÀPh (3.31 ), whereas Ph 2 DTP-Ph-NN crystallized as am onomeric structure with smalleri ntermolecular distance found betweent wo oxygen atoms for NÀO···NÀO( 3.24 )i nm olecular packing. Upon one-electron oxidation, the (R 1 2 DTP-Ph-NN,R 1 = Ph and MeSTh)C + + offered triplet ground state radical-cationic high spin molecules. Although for (DTP-Ph-NN)C + + the calculated magnetic interaction is antiferromagnetic between the NN and radicalcation. Further charged molecules will be isolated and analyzed for magnetic properties by magnetic susceptibility.S yntheses of similarr adicalc ationic molecules are under way.T he Ph 2 DTP-Ph-NN and MeSTh 2 DTP-Ph-NN type molecules are suitable for spintronic and molecular magnetic materials applications.

Experimental Section
Full experimental details can be found in the Supporting Information.