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Radical cations and dications of bis[1]benzothieno[1,4]thiazine isomers

Radical cations and dications of three isomeric bis[1]benzothieno[1,4]thiazines are (electro)chemically generated, spectroscopically characterized and assigned by (TD)DFT calculations.


Introduction
Molecular electronics based upon organic molecules has become increasingly important over the past decade and the quest for tailor-made functional small molecules is an ongoing task for chemistry. 1,2 Since the efficiency of electronic devices correlates with electronic properties of the constituting molecules, a rational design of novel molecule based semiconductors commences with high lying HOMO levels, which in turn correspond to low oxidation potentials. [3][4][5][6][7][8] Electron-rich heterocycles, such as phenothiazine, are prominent structural motifs used in electroactive materials for batteries, [9][10][11] OLEDs, [12][13][14] and DSSCs. [15][16][17][18][19] They possess low oxidation potentials with fully chemically reversible oneelectron oxidations, low band gaps, and luminescent properties. [20][21][22] In addition to their convenient accessibility, their intrinsic properties are easily fine tunable by synthetic functionalization. 22,23 Furthermore, the remarkable stability of their radical cations is crucial for providing efficient charge transfer as hole conductors. 24,25 Recently, we successfully proposed the two-fold thieno expansion of phenothiazine to bis [1]benzothieno [1,4]thiazines (BBTT) that can be considered to be novel electron-enriched phenothiazine congeners (Scheme 1). 26 The resulting compounds likewise possess discretely adjustable properties that depend on the mode of anellation, as for dithienothiazines, 27 as well as on the substitution pattern. Different steric hindrance unequivocally causes the absence of measurable luminescence in syn-syn BBTTs (bis [1]benzothieno [2,3- [1,4]thiazines) with fluorescence quantum yields drastically increased to 45%. [26][27][28][29][30] Furthermore, anti-anti BBTTs represent the first known fused thiazines with antiaromatic character in the crystalline solid state. Likewise, being identified as Weitztype-redox systems, 31 BBTTs are outscoring their progenitors, i.e. phenothiazines, by significantly lower oxidation potentials with reversible two step oxidations, forming radical cations and dications. 26,28 Herein, we report the first synthesis of BBTTs' radical cations and dications by chemical and electrochemical
Spectra with hyperfine coupling patterns of three equidistant lines reflect the coupling of the unpaired electron with the nitrogen nucleus in each case. Furthermore, the range of the g-factors (5 + : 2.0106, 6 + : 2.0050, 7 + : 2.0078) indicates the predominant localization of the radicals on the nitrogen atoms. 41,50 The Mulliken atomic spin density on the nitrogen atoms decreases from syn-syn over syn-anti to anti-anti derivative, which is in accordance with the descending order of hyperfine coupling constants. Hence, an increasing spin delocalization across the BBTT units for anti-anellated benzo[b]thiophene wings can be deduced. The stronger delocalization matches with an increased stability of the corresponding radical cation 7 + , which is consistent with the experimentally determined semiquinone formation constants K SEM , which also gives largest values for anti-anti radical cation 7 + . The delocalization of spin density of the radical cations is unequivocally supported by the calculated Wiberg bond orders ( Figure  The bond orders of the systems should gradually converge upon oxidation due to increasing orbital overlap by planarization resulting in an increased resonance stabilization. All three regioisomers show the most significant changes in bond orders in the 1,4-thiazine core, where CN and CS bond orders increase while CC bond orders decrease. This effect is most pronounced for the syn-syn regioisomer 5/5 + while the anti-anti isomer 7/7 + is most marginally affected. However, anti-fused benzo[b]-thiophene wings result in higher bond order changes on the fused thiophene in turn. This also confirms the previously observed increased spin delocalization upon introduction of anti-anellated wings. Nevertheless, even with anti-anellation, the spin density on the benzo ring is not high enough for causing observable coupling with the protons' nuclear spins. Although the calculated spin density distributions ( Figure 1) suggest this conclusion, no hyperfine coupling patterns of the protons are experimentally found. For hexachloroantimonate salts of dications 5 2+ , 6 2+ , and 7 2+ , DFT calculations claim formation of singlet dications being thermodynamically favored over triplet dications by energy differences of 92 kJ mol -1 (5 2+ ), 89 kJ mol -1 (6 2+ ), and 90 kJ mol -1 (7 2+ ). Consequently, EPR-silent but NMR-active target compounds should be present. NMR spectra of the bis(hexachloroantimonate) salts of 5 2+ , 6 2+ , and 7 2+ were recorded and compared to the spectra of the native compounds 5, 6, and 7, thus, confirming the formation of diamagnetic dications, i.e. in their singlet ground states ( Figure 4). As expected, the majority of the multiplets shift towards lower field, underlining the electron-deficient dicationic nature of the compounds. Due to structural differences of the regioisomers, parts of the signals appear shielded in high field. DFT-calculated geometry optimizations show planar BBTT units with orthogonally twisted N-p-fluorophenyl substituents, in analogy to the radical cations. Syn-anellated wings place benzo protons into the anisotropy cone of the N-aryl substituent's diamagnetic ring current. Thereby, their signals are shifted to high field.

Photophysical properties and spectroelectrochemistry
For further insight into electronic transitions of oxidized species, i.e. radical cations and dications, spectroelectrochemical measurements were performed in an OTTLE (Optically Transparent Thin Layer Electrochemical) cell, monitoring the in situ formation of radical cations and dications from the neutral precursors 5, 6, and 7 (Table 1). Hence, spectroelectrochemical UV/Vis absorption spectra were recorded by steadily increasing the applied potential in steps of 0.1 V. Initially, UV/Vis spectra of all three parent regioisomers 5, 6, and 7 were recorded showing multiple absorptions bands in the UV region with the longest wavelength maxima at 309 nm (5) and 311 nm (6). For compound 7 the longest wavelength maximum is found in the visible at 424 nm ( Figure 5).  All oxidation processes proceed without formation of intermediates, as supported by the occurrence of isosbestic points. Moreover, the oxidations from the neutral BBTTs to the radical cations reflect the largest changes in the UV/Vis spectra and consequently in the electronic structure (vide infra). This originates from major geometry changes from butterfly structures to the planar alignments of the BBTT units (cf. phenothiazines and DTT). 38,52,53 Consequently, oxidized compounds yielded by chemical as well as electrochemical oxidation are deeply colored ( Figure 6).
By gradual increase of the potential the formation of radical cations is accomplished, revealing significantly bathochromically shifted absorption bands (Figure 7). Therefore, compound 5 + shows an intense maximum at 680 nm. A strongly broadened absorption band ranging from 450 to 850 nm with shoulders at 566 and 617 nm in conjunction with a maximum at 694 nm is found for radical cation 6 + . The two prominent absorption maxima for oxidation product 7 + appear at 534 and 717 nm.  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58 59 60  Further increase of the potential allows monitoring of the formation of dications (Figure 8). For dication 5 2+ , the longest wavelength absorption maximum at 680 nm shifts slightly hypsochromically, resulting in a band at 672 nm while simultaneously a second band of lower intensity appears at 526 nm. In addition, the most intense band appears at 346 nm. A comparable absorption band is found at 324 nm for dication 6 2+ . The shoulders observed for radical cation 6 + merge into a new maximum at 595 nm, whereas the maximum at 694 nm decreases to a shoulder at 709 nm. Upon oxidation of radical cation 7 + to dication 7 2+ , the two intense bathochromically shifted maxima of the radical cation result in the formation of two new maxima at 561 and 657 nm for the dication. Likewise, an intense band appears in the UV at 337 nm. Finally, the spectra from spectroelectrochemical measurements were compared with the spectra of the synthesized hexachloroantimonate radical cation salts (Figure 9).
The superposition shows that the salts unambiguously correspond to the spectroelectrochemically generated specimen and a good overall fit is obtained. Differences in the intensity of the absorption bands in the UV region of the spectra (230-380 nm) can be accounted to counter ion effects. 25 Likewise, the longest wavelength absorption maxima for the hexachloroantimonate salts of the radical cations are only slightly bathochromically shifted (5 + SbCl 6  :  = 171 cm 1 , 6 + SbCl 6  :  = 305 cm 1 , 7 + SbCl 6  :  = 173 cm 1 ). As known for Weitz-type redox systems, the radical ions are remarkably stable due to their high semiquinone formation constants K SEM (vide supra). 31 However, the dications could not be successfully measured due lacking stability under prolonged measuring conditions.

Electronic structure of the radical cations and dications
For a deeper insight into the electronic structure of the absorption behavior of neutral, radical cation and dication BBTTs TDDFT calculations 54,55 ((u)B3LYP 43,44,56 /6-311++G**, IEFPCM CH 2 Cl 2 ) were performed. First, the optical transitions of the experimentally observed absorption bands of the parent compounds 5, 6, and 7 were assigned (Table 2, Figure 10). The longest wavelength absorption bands are reasonably well reproduced by transitions at 339 nm (5) and 422 nm (7). They predominantly consist of HOMOLUMO transitions representing a charge transfer (CT) from the thiazine core to the benzo[b]thiophene wings. The calculated longest wavelength absorption band of 6 appears at 331 nm and can be predominantly assigned to a HOMOLUMO+1 transition. Again, this transition is accompanied by a CT from the central thiazine to the benzo[b]thiophene units, with a dominant shift of coefficient density due to anti-anellation.  Likewise, for assigning the observed UV/Vis absorption of the radical cations, TDDFT calculations with the uB3LYP/6-311++G** functional were performed to prevent spin pairing and to cause separate orbitals occupation (Table 3, Figure 11).  This leads to a singly occupied molecular orbital (SOMO). The longest wavelength absorption bands are almost accurately reproduced by HOMO-1SOMO transitions at wavelengths of 665 nm (5 + ), 714 nm (6 + ) and 701 nm (7 + ). For cation 7 + , the measured second absorption band at 534 nm is best represented by a HOMO-3SOMO transition at 538 nm, while the shoulders of structure 6 + are best reproduced by calculated bands at 617 nm (HOMO-2SOMO) and 577 nm (HOMO-3SOMO). All these transitions are predominantly characterized by CT contributions from the anellated benzo[b]thiophene wings to the central thiazine core, while coefficient density on the N-aryl substituents is absent. Finally, TDDFT calculations (uB3LYP/6-311++G**) were also carried out to assign the absorption bands of the dications 5 2+ , 6 2+ , and 7 2+ (Table 4, Figure 12). The longest wavelength absorption bands are best represented by calculated HOMOLUMO transitions at wavelengths of 736 (5 2+ ), 709 (6 2+ ), and 747 nm (7 2+ ) that can be interpreted as CT transitions from the outer benzo[b]thiophene part to the central thiazine unit. The following experimental bands in the visible are assigned to transitions arising from HOMO-4LUMO at 517 nm (5 2+ ), HOMO-1LUMO at 617 nm (6 2+ ), and HOMO-2LUMO at 579 nm (7 2+ ). These transitions are characterized by CT contributions from the N-aryl moiety to the thiazine core for structure 5 2+ , whereas for structure 6 2+ by a shift of coefficient density from the benzo[b]thiophene parts to the thiazine core. For structure 7 2+ , the transition can be accounted as a combination of both preceding coefficient density shifts. The preceding calculations reveal that BBTT are strongly polarizable systems with large CT contributions. Accordingly, neutral BBTT are potent donors due to their electron richness, whereas the oxidized compounds are accompanied by strong acceptor properties.

General procedure -synthesis of bis[1]benzothieno[1,4]thiazines (BBTTs)
In a dry Schlenk tube with a magnetic stir bar under Ar atmosphere were placed dibrominated sulfane 1, 2, or 3 (2.3 g, 5.0 mmol), Pd(dba) 2 (0.14 g, 5.0 mol%) and ligand (10 mol%). By glovebox technique sodium tert-butoxide (1.4 g, 15 mmol) was added into the reaction tube. Subsequently, p-fluoroaniline (0.47 mL, 5.0 mmol) und dry toluene (30 mL) were added by syringe before stirring the reaction mixture at 110 °C (oil bath) for 65 h. After cooling to room temp the reaction was terminated by addition of saturated aqueous sodium sulfite solution (400 mL). The aqueous phase was extracted with dichloromethane (3 x 300 mL), the combined organic phases were dried with anhydrous magnesium sulfate and the solvents were removed under vacuo. After adsorption to Celite ® the residue was purified by chromatography on silica gel. After recrystallization from n-hexane the products 5, 6, or 7 were isolated as crystals. Deviations from this general procedure are individually highlighted.

Conclusions
The electronic properties of oxidized species of all three anellative bis [1]benzothieno [1,4]thiazine (BBTT) isomers were extensively characterized by experimental and computational methods. All three regioisomeric N-p-fluorophenyl-BBTTs (synsyn, syn-anti and anti-anti) were successfully transformed into the corresponding radical cation and dication salts by using antimony pentachloride as an oxidant on a preparative scale. By EPR spectroscopy the radical character was unambiguously proven and the predominant localization of the unpaired spin on the central 1,4-thiazine moiety was supported by the resulting hyperfine coupling with the nitrogen nucleus. Moreover, experimentally determined hyperfine coupling constants and computationally determined spin density distributions and Wiberg bond orders underline that a stronger delocalization arises from anti-alignment of the benzo[b]thiophene units. The diamagnetic singlet character as well as geometric structure of the dications was assigned by NMR spectroscopy, especially supported by high-field shifted signals in diamagnetic anisotropy cones of syn-aligned wings due to planarization of the BBTT core. Spectroelectrochemical measurements by in situ generation of the oxidized species allowed characterization of the electronic absorption bands. Finally, the experimentally observed absorption bands could be reproduced TDDFT calculations and assigned to the underlying molecular orbital transitions. The calculated electronic structure of the radical cations and dications underlines for all transitions a high polarizability of BBTT upon photonic excitation, as revealed by pronounced charge transfer from the benzo[b]thiophene moieties or the Naryl unit to the central 1,4-thiazine core, stating a strong acceptor character of oxidized BBTT. Further studies on the synthesis of functionalized BBTTs focusing on enhancing their neutral antiaromatic character as well as addressing their electroactive and electro-optical applications are currently underway.

Conflicts of interest
There are no conflicts to declare.