Stereoelectronic control of oxidation potentials of 3,7-bis(diarylamino)phenothiazines †

The in ﬂ uence of diarylamino (Ar 2 N – ) substituents on the oxidation potential of 3,7-bis(diarylamino) phenothiazines (Ar 2 N) 2 – PTZ ( 1a – f , a : carbazolyl; b : dihydrodibenzoazepinyl; c : dibenzoazepinyl; d : diphenylamino; e : phenothiazinyl; and f : phenoxazinyl) is investigated, where the Ar 2 N-substituent sequence a / f is aligned in the increasing order of their electron-donating ability. Interestingly, a di ﬀ erent sequence of electron-donating ability for Ar 2 N-substituents was observed for the oxidation potentials of (Ar 2 N) 2 – PTZ : 1a ( E ox 1 ¼ +0.35 V vs. Fc/Fc + ) > 1f (+0.30 V) > 1e (+0.15 V) > 1d ( (cid:2) 0.05 V) > 1c ( (cid:2) 0.19 V) > 1b ( (cid:2) 0.22 V). The observed sequence can be explained by the stereoelectronic e ﬀ ect of the Ar 2 N-substituents to stabilize (Ar 2 N) 2 – PTZ c + . Clear-cut examples are observed in the crystal structure of 1c c + and 1e c + , for which coplanar conformation is observed between the PTZ c + -plane and the planes of the sp 2 -hybridized nitrogen atoms in Ar 2 N-substituents through a large conformational change during the oxidation process of (Ar 2 N) 2 – PTZ .


Introduction
Phenothiazines (PTZs) are typical electron donors and have been used in charge transporting materials, 1 molecular spin materials, 2 dye-sensitized solar cells, 3 p-electron systems for photo-induced electron transfer, 4 organic emitting devices, 5 and charge transfer complexes. 6 Tuning of the oxidation potential of these PTZs is important to widen their applicability. Several 3-or 3,7-substituted PTZs have been synthesized for various purposes. 7 Müller reported the oxidation potentials and optical properties of various 3,7-diamino PTZs. 8 Although PTZ derivatives with considerably low oxidation potentials have been synthesized, systematic substituent-effects based on their conformations conrmed by crystal structure analyses have not been reported.
In this paper, we investigated the diarylamino (Ar 2 N-) substituent effect on the oxidation potentials of 3,7-bis(diarylamino)-10-methyl-10H 0 -phenothiazines (Ar 2 N) 2 -PTZ (1a-f, a: carbazolyl, b: dihydrodibenzoazepinyl, c: dibenzoazepinyl, d: diphenylamino, e: phenothiazinyl, and f: phenoxazinyl in Scheme 1), where the Ar 2 N-substituent sequence a/f is arranged in the ascending order of the electron-donating ability estimated from the oxidation potentials of the corresponding N-phenyldiarylamines Ar 2 N-Ph (2a-f) (Scheme 1). Interestingly, a different sequence was observed for the oxidation potentials of 1a-f: 1a > 1f > 1e > 1d > 1c > 1b. The difference between 2a-f and 1a-f is based on the oxidation moieties in the rst oxidation potential; the oxidation potentials are primarily controlled by Ar 2 N-substituents in Ar 2 N-Ph, whereas the PTZ moieties are oxidized in (Ar 2 N) 2 -PTZ, except for 1f (vide infra). In the latter, a large stabilization is obtained through the conjugation between Ar 2 N-and PTZc + moieties. For instance, in compound 1dc + , the phenyl groups in the Ar 2 N-substituent can rotate around the N-C(sp 2 ) bonds to avoid steric repulsion between the PTZc + and the phenyl moieties, retaining the conjugation between the p-orbital on the nitrogen atom in the Ar 2 Nsubstituent and the singly occupied molecular orbital (SOMO) on the PTZc + moiety; such a conformation is not possible with the carbazolyl group in 1ac + because of the planar conformation. Furthermore, the Ar 2 N-substituents in 1bc + and 1cc + can uniquely adopt a coplanar conformation about the PTZc + plane and the plane of sp 2 nitrogen atom in Ar 2 N-substituents.
Previously, we have reported a unique structure of the PTZ trimer radical cation, the 10-phenyl derivative instead of the 10methyl derivative (1e); the PTZ trimer radical cation has a considerably deformed structure, which is stabilized by the conjugation between the inner PTZc + moiety and the sp 2hybridized nitrogen atoms in the outer PTZs. 9 This paper presents a more general relationship between structures and oxidation potentials using various Ar 2 N-substituents; the proposed stereoelectronic stabilization is directly demonstrated by the crystal structure analyses of a series of (Ar 2 N) 2 -PTZc + .

Results and discussion
Syntheses of 3,7-bis(diarylamino)-10-methyl-10H 0phenothiazines and their radical cationic species Compounds (Ar 2 N) 2 -PTZ were synthesized from 3,7-dibromo-PTZ 3 (Scheme 2). Compound 3 was prepared according to the reported methods. 10 Compound 3 was converted to the desired PTZ derivatives 1a-f by using Buchwald-Hartwig cross-coupling reactions with the corresponding diarylamines 4a-f. 7 Single crystals of (Ar 2 N) 2 -PTZ were obtained by recrystallization under suitable conditions (see Experimental section). However, we failed to obtain single crystals of 1b. We could obtain the crystal structure of the model compound 1b 0 (Scheme 3) using N-p-tolyl group instead of the N-methyl group in 1b. The reference compounds 2a-f were also synthesized from the corresponding diarylamines and bromobenzene using Buchwald-Hartwig reactions.
Oxidation potentials of 3,7-bis(diarylamino)-10-methyl-10H 0phenothiazines The cyclic voltammograms are shown in Fig. 1 for 1a-e and Fig. S1 † for 1f, and their oxidation potentials are listed in Table  1. In order to gain insight into the electron donating ability of the Ar 2 N-substitutents, oxidation potentials of Ar 2 N-Ph (2a-f) and N-methyl-10H-phenothiazine (5) were also measured (Table  1), which reects the electron donating ability of the Ar 2 Nsubstituent: Table 1). The oxidation potentials of (Ar 2 N) 2 -PTZ were considerably lower than those of Ar 2 N-Ph, except for 1f (Table 1). Compound 1a has a lower oxidation potential (+0.35 V vs. Fc/Fc + ) than that (+1.08 V) of 2a, because the oxidation in 1a occurs in the central PTZ moiety rather than the outer carbazolyl-moieties (cf. E ox 1 for 5 and 2a). The oxidation potentials of 1b and 1c with sevenmembered ring systems (E ox 1 z À0.2 V) are considerably lower than those of 2b and 2c (E ox 1 z +1.0 V) and even lower than that of 5 (+0.31 V). The observed sequence of the oxidation potentials is totally different from those of the Ar 2 N-substituents. 13 X-ray diffraction studies revealed that the compounds with low oxidation potential (1b, 1c, and 1d) can adopt a coplanar conformation about the PTZc + plane and the plane of sp 2hybridized nitrogen atom in the radical cation states by a structural change from the neutral states (vide infra), as observed in the PTZ trimer radical cation. 9 However, 1ac + and 1fc + cannot undergo such conformational changes due to their rigid planar structures of Ar 2 N-substituents, 14 which causes steric repulsion between H a (H b ) and H c (H d ) in the structure of (Ar 2 N) 2 -PTZ, as shown in Scheme 1.
It is to be noted that the oxidation wave for 1f (+0.3 V) is assigned to sequential two-electron oxidations of outer phenoxazines (Table 1, Fig. S1 †). This assignment is compatible with the following three observations; (1) slightly lower oxidation potential of 2f (+0.24 V) than those of 2e (+0.26 V) and 5 (+0.31 V), (2) crystal structure of 1fc + $GaBr 4 À exhibiting neutral PTZ with a buttery structure (vide infra, Fig. 3(l) and S2 †), and (3) UV-vis-NIR spectrum of 1fc + $GaBr 4 À showing a strong absorption at $500 nm assigned to the terminal phenoxazine radical cation (Fig. S3 †). These results indicate that the oxidation potential of the central PTZ moiety in 1f should be more positive than +0.30 V.
X-ray crystal structure analysis and substituent effect in the oxidation of (Ar 2 N) 2 -PTZ To demonstrate the conformation-dependent substituent effect, we analyzed the X-ray crystal structure of both neutral and radical cationic species ( Fig. 2 and 3). 11 The molecular structures of neutral species 1a, 1b 0 (as a model compound of 1b), 1c-f, and radical cationic species 1a-fc + are shown in Fig. 3. The central PTZ moiety of (Ar 2 N) 2 -PTZ takes a buttery structure (dihedral angles between planes A/B: 35-50 ) ( Table 2, Fig. 2 and 3), 4a,9 whereas that of (Ar 2 N) 2 -PTZc + except for 1fc + has an almost planar structure (A/B < 17.7 ), indicating that the central PTZ moiety of 1a-ec + is in the radical cationic state. The structure of radical cation 1fc + is different from those of the other radical cations 1a-ec + . The central PTZ moiety of 1fc + has a buttery structure (A/B: 45.4 ) similar to that of 1f (A/B: 45.4 ), showing that the central PTZ moiety of 1fc + is in the neutral state. It is also interesting that one of the two outer phenoxazine moieties has a planer structure with shorter C-O bond lengths ( Fig. S2 †). These results indicate that the planar phenoxazine moiety is in the radical cationic state, whereas the other phenoxazine moiety is in the neutral state. Table 2 lists the bond lengths R(N1-C6 (7)) and R(S-C1 (12)) of the central PTZ moiety in the neutral and the radical cationic states. These N-C(sp 2 ) and S-C(sp 2 ) bond lengths in the radical cationic state are slightly shorter than those in the neutral state by reection of the antibonding character of these bonds in SOMOs of 1a-ec + (Fig. S4 †).
In these radical cations, the small dihedral angle between planes A and C (A/C) (or B and D (B/D)) permits the ease of conjugation between the PTZ p-orbitals and the adjacent nitrogen p-orbital in the Ar 2 N-substituents. Interestingly, compounds 1cc + and 1ec + experienced large conformational changes to diminish A/C and B/D. The compounds having low oxidation potentials, i.e. 1b, 1c, and 1d, have small average dihedral angle in their radical cationic state: 4.8 (1bc + ), 6.4 (1cc + ), and 7.8 (1dc + ). Compound 1a had the largest dihedralangle in the radical cation state except for 1fc + and exhibited the most positive oxidation potential. Furthermore, effective conjugations about sp 2 nitrogen atom and the central PTZc + for the low-oxidation-potential compounds (1b, 1c, and 1d) are re-ected by shorter bond lengths R(N2(3)-C3 (10)) (1.37-1.38Å) in the radical cation structures. These results clearly indicate the importance of radical cation structures for the oxidation potentials.
It is interesting to consider the mechanism of oxidation of the adsorbed neutral state (Ar 2 N) 2 -PTZ for cyclic voltammogram measurements; oxidation is accompanied by the large conformational change to stabilize the (Ar 2 N) 2 -PTZc + . The oxidation of neutral (Ar 2 N) 2 -PTZ on the electrode surface starts Table 1 Oxidation potentials of 5, (Ar 2 N) 2 -PTZ (1a-f), and Ar 2 N-Ph Conditions of cyclic voltammetry: nBu 4 N + $ClO 4 À as an electrolyte; glassy carbon and Pt wire as a working and counter electrode, respectively; in dichloromethane; Fc/Fc + ¼ +0.49 V vs. SCE. b Half wave potential of a reversible oxidation wave. c Two-electron oxidation processes of phenoxazine moieties. d Peak potential, respectively. E ox 1 , E ox 2 : rst and second oxidation potentials. from the oxidation of the adsorbed molecular edge of the Ar 2 N moiety rather than the central PTZ moiety covered by the bulky groups to produce Ar 2 Nc + -PTZ; it then quickly changes to more stable hole-shied Ar 2 N-PTZc + with large conformational change. These processes occur very rapidly, so that the cyclic voltammograms exhibit usual reversible waves (Fig. 1). In order to obtain insight into such hole-shi processes, we previously investigated photo-induced electron transfer of a (PTZ trimer)anthraquinone (PTZ3-PTZ2-PTZ1-B-AQ, B: bridge) dyad 4a in which electron transfer via through-bond interaction from PTZ trimer to the excited anthraquinone (AQ) to give PTZ3-PTZ2-PTZ1c + -B-AQc À . The hole shi process (PTZ3-PTZ2-PTZ1c + -B-AQc À / PTZ3-PTZ2c + -PTZ1-B-AQc À ) with large conformational change was directly monitored by the transient absorption spectroscopy using a time constant of 6 ns. Thus, the oxidation of (Ar 2 N) 2 -PTZ is associated with both neutral and radical cation geometries. We also carried out theoretical calculation on the stereoelectronic effects by theoretical calculation using Gaussian 09 on the basis of (U)B3LYP/6-31G** level of theory in order to obtain a general relationship between structures and oxidation potentials. 15,16 The calculated HOMO energy levels of 1a-e are not correlated to the oxidation potentials (HOMO energy of 1a (À5.18 eV) > 1e (À5.00 eV) > 1d (À4.58 eV) > 1b (À4.24 eV) > 1c (À4.19 eV)) ( Fig. S6 †). The calculated SOMO energy levels of 1aec + are in the order of 1ac + (À7.92 eV) > 1ec + (À7.64 eV) > 1cc + (À7.54 eV) > 1b (À7.53 eV) > 1dc + (À7.46 eV) (Fig. S4 †) are also not correlated to oxidation potentials; the sequence agrees with the experimentally observed oxidation potentials except for the position of 1d (Fig. 4, inset). This consideration also indicates the importance of radical cationic geometry in the oxidation potentials of 1a-e. Finally, the stabilization-energy gain from the neutral to the radical cation state, i.e. the calculated heat of formation differences between optimized structures in neutral À hydrogen atoms, counter anions, and crystal solvents were omitted for clarity.  and radical cation states (DHF) are considered: the plots of DHF (À139.23 kcal mol À1 (1a), À119.70 (1b), À118.97 (1c), À124.98 (1d), À128.85 (1e)) vs. E ox 1 provides a good correlation (correlation coefficient: R 2 ¼ 0.983; Fig. 4). As apparent from the crystal structures of 1a-ec + , the central PTZc + can be stabilized by the conjugation with the nitrogen p-orbitals in Ar 2 N-substituents. These p-conjugation gives rise to broad and intense intramolecular charge transfer (CT) bands in near infrared region: 1ac + (l max ¼ 1200 nm in dichloromethane), 1bc + (950 nm), 1cc + (918 nm), 1dc + (1118 nm), and 1ec + (963 nm) (Fig. 5). These differences are almost consistent with the results of theoretical calculations by TDDFT method (see, ESI †).

Conclusions
We designed and prepared a series of 3,7-bis(diarylamino) phenothiazines (Ar 2 N) 2 -PTZ (1a-f) and investigated relationship between their oxidation potentials and their molecular structures. X-ray crystal structural analyses and theoretical calculations indicated a unique stereoelectronic effect on PTZs, which strongly depends on the conformation of Ar 2 Nsubstituent.
The oxidations of 1c and 1e are accompanied by the large conformational changes. In compound 1a, the freedom of Ar 2 Nsubstituent is restricted only by rotation of the carbazolyl group.
Although the manner of stabilization of (Ar 2 N) 2 -PTZc + (1a-ec + ) is highly substituent-dependent, these substituent-dependent oxidation potentials were shown to be proportional to the energy gain from the neutral state to the radical cation state.
The radical cationic species (Ar 2 N) 2 -PTZc + were quite stable under aerobic conditions in both solution and the solid states. In addition, the radical cationic species 1a-ec + showed intense intermolecular CT bands in near infrared region.
Although this study was particularly investigated for the PTZ derivatives, the observed stereoelectronic substituent-effect must be uniformly applicable to electron rich aromatic or benzo-condensed p-conjugated systems.

Experimental
General information 1 H and 13 C NMR spectra were recorded on a Bruker NanoBay 300 spectrometer. MALDI-TOF-MS was measured on a Shimadzu-Kratos AXIMA-CFR Plus spectrometer using dithranol as a matrix reagent. Absorption spectra were measured with a JASCO V-750 UV-vis spectrometer and Hitachi U-3500L spectrometer. X-ray data were collected by a Rigaku Saturn CCD system with graphite monochromated Mo-Ka radiation. Melting points were measured with a hot-stage apparatus and the values are uncorrected. Redox potentials were measured using ALS electrochemical analyzer MODEL 610A in a conventional three-electrode cell equipped with a glassy carbon as a working electrode and a platinum wire as a counter electrode with a SCE reference electrode. The measurements were carried out with a sweep rate of 100 mV s À1 in suitable solvent containing 0.1 M tetra-n-butylammonium perchlorate (nBu 4 -N + $ClO 4 À ) as an electrolyte. The redox potentials were nally corrected by the ferrocene/ferrocenium couple (Fc/Fc + ).
Silica gel 60 (100-200 mesh) was used for column chromatography. All commercially available compounds were reagent grade and used without further purication. Dichloromethane (CH 2 Cl 2 ), and acetonitrile (CH 3 CN) were dried and distilled over calcium hydride. Toluene, benzene, tetrahydrofuran (THF), and diethyl ether (Et 2 O) were dried and distilled over sodium. Ethanol (EtOH) and methanol (MeOH) were dried and distilled over magnesium.

Synthetic procedures
Thianthrenium tetrabromogallate (THc + $GaBr 4 À ). Oxidant THc + $GaBr 4 À was prepared according to the same procedure of thianthrenium radical cations reported by our research group. 12 The oxidation of thianthrene was carried out in an electrochemical cell, which has two compartments (each 25 mL) separated by glass lter and equipped with platinum electrodes (15 Â 15 mm 2 ). An electrolyte solution of tetra-n-butylammonium tetrabromogallate (nBu 4 N + $GaBr 4 À ) (1.35 g, 2.13 mmol) in CH 2 Cl 2 (50 mL) was prepared. Half of the solution was placed to the cathodic compartment in the cell. Thianthrene (381 mg, 1.76 mmol) was dissolved in the remaining electrolyte solution, and this solution was placed to the anodic compartment. Both compartments in the cell were purged argon for a few minutes.