Formation of stable biradical triplet state cation versus closed shell singlet state cation by oxidation of adducts of 3,6-dimethoxycarbazole and polychlorotriphenylmethyl radicals.

08028 Barcelona, Spain. E-mail: ljbmoh@cid.csic.es, anglada@iqac.csic.es Abstract We report an experimental and theoretical study of two stable radical adducts of the triphenylmethyl series, 1 and 2 , whose composition and molecular structure are distinguished by the content and position of chlorine atoms in phenyls. The electrochemical study through cyclic shows the existence of two reversible processes, related to a reduction and an oxidation, to stable charged species. The chemical oxidation of both radical adducts gives rise to stable cations, whose fundamental state has a biradical triplet electronic structure or a closed shell singlet character, depending on the electronic conjugation between the donor and acceptor electron moieties. The presence of chlorines adjacent to the nitrogen in 1 breaks the conjugation between both halves, facilitating the formation of a triplet electronic state of the cation, while the absence of the chlorines in these positions in 2 facilitates partial conjugation and stabilizes the closed shell singlet electronic state of the cation. The oxidation of the dimethoxicarbazole adduct of tris(2,3,5,6-tetrachlorophenyl)methyl radical (DTM) involves one electron from the HOMO orbital rather than from the SOMO, to generate a triplet state.

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Introduction
Carbazole derivatives are a kind of organic compounds extensively investigated due to their applications in electronic devices in different technological fields. 1, 2 They have distinguished mainly as hole semiconductors due to the ease with which they lose an electron. 3 The electrochemical oxidation of carbazole derivatives is mainly characterized by the initial loose of an electron from the nonbonding pair on the nitrogen, giving rise to a radical cation. These cationic species are, in general, unstable and suffer dimerization, which can be prevented introducing substituents in the most active positions. 4,5 As an example, polymers bearing pendent 3,6-dimethoxycarbazole units show reversible electrochemical oxidation. 6 In addition, carbazole has been part of organic materials with bipolar semi-conductivity as electron-donor group in donor-acceptor systems, [7][8][9] which are characterized by low ionization potentials and high electronic affinities that make them very favorable as optoelectronic devices.
In the last times, we have been involved in the study of the electronic properties of stable free radical adducts of the TTM [tris(2,4,6-trichlorophenyl)methyl radical] 10 and DTM [tris(2,3,5,6-tetrachlorophenyl)methyl radical] 10 series, using both radical species as electron-acceptor parts in binary systems, preparing radical adducts with bipolar semiconductivity incorporating carbazole derivatives to both organic free radicals. [11][12][13][14][15][16][17][18][19][20] These new radical adducts are characterized by their thermal and chemical stability and are prepared and isolated as pure solids, stable in solution. In turn, they present a) absorption bands in the visible part of the electronic spectrum, as charge transfer bands from the donor to the acceptor part of the molecule, b) electrochemical amphoteric behavior with two redox centers in the molecule that easily undergo oxidation and reduction, respectively, to stable charged species and c) magnetic properties due to the radical character of the molecule.
Very recently, Li and co-workers have produced organic light-emitting diodes (OLEDs) built from adducts between TTM and carbazole derivatives whose high efficiency is due, in great part, to the formation of a biradical in the oxidation step ("hole injection") of the whole process. 21 In connection to this, and taking advantage of the electrochemical stability of the oxidized species of these radical adducts, we now report the chemical oxidation of a new radical adduct of the DTM series, [4-(3,6-dimethoxy-9-carbazolyl)-2,3,5,6-tetrachlorophenyl]bis(2,3,5,6-tetrachlorophenyl)methyl radical adduct (1) (Scheme 1), with a strong electron donating carbazole derivative as substituent, the 3,6dimethoxycarbazole, and whose oxidation leads to the formation of a stable biradical cation. Their electronic structure and those of the oxidized species have been compared with the corresponding ones of the known radical adduct 2 13

Results and Discussion.
The details of the synthesis and spectroscopic characterization of the compounds analyzed in this work are discussed in the Supplementary Information. The UVvis-near infrared spectra of 1 in solvents with different polarity are displayed in Table S1 and Figure S1. Visible and near-infrared spectroscopy and paramagnetic electronic resonance (epr) have been used to characterize and differentiate the electronic structure of the oxidized species of both open-shell compounds. The theoretical work carried out in this investigation include geometry optimization and characterization calculations using the B3LYP 22 density functional approach, calculations on the electronic spectra in the framework of the Time-Dependent density functional theory 23 with the B3LYP functional, and single point energy calculations at DLPNO-CCSD(T) level of theory. 24 Full details of the theoretical methods employed are also discussed in the Supplementary Information.
In Figure 1 we have drawn the optimized structures of radical adducts 1 and 2, computed at B3LYP/6-31+G(2df) level of theory. between the chlorine atoms is minimized. As pointed out in reference 10 this structure prevents the  delocalization between the carbazolyl moiety and the phenyl ring and the unpaired electron is mainly localized over the triarylsubstituted carbon atom which has a computed spin density of 0.73. On the other side, the lack of the two chlorine substituents in 3 and 5 in the dichlorophenyl bridge of 2 allows for a more flexible structure and the angle between the N-carbazolyl moiety and the substituted phenyl bridge is computed to be 65.8, which allows an easier  delocalization between rings. In 2, the radical is still localized over the triarylsubstituted carbon with a computed spin density of 0.67.  shows that for 2, the topology and electronic features of the HOMO, HOMO-1 and SOMO orbitals are quite the same as those in 1. However the SOMO orbital energy in 2 is destabilized by 0.55 eV relative to 1, so that its orbital energy is closer to that of the  part of HOMO.  The electrochemical behavior of radical adduct 1 has been studied through cyclic voltammetry. 1 exhibits two quasi-reversible one-electron processes (Figure 3), reduction by addition of one electron and oxidation by removal of one electron, to yield two stable charged species, anion and cation, respectively. Table 1 shows the redox potentials and the differences between the anodic and cathodic peaks of each process together with values from 2 for comparison. 13  (E p a -E p c (mV)) [b] Eº oxi (V) [c] (E p a -E p c (mV)) [b] EA [ An analysis of the results in Table 1 indicates that the standard potentials for the reduction and oxidation of both radical adducts 1 and 2 are influenced by the electron-acceptor moiety of the molecule, TTM or DTM. Both processes are reversible in 1 and 2, confirming the stability of the reduced and oxidized species.
While the chemical reduction of 1 ( Figure S2 and Scheme S1) does not deserve special mention, we have focused our attention on the oxidation of 1 compared with the oxidation of 2 reported in the literature. 17 For this, we have recorded the UV spectra during the oxidation process, where the bands of the radical adducts 1 and 2 disappear whereas the bands of the corresponding cations appear as the oxidation takes place. Figure 4a   At first glance, Figure 5 shows that there is an overlap of the bands of the radical 2 1 with the bands of cation in its triplet electronic state ( 3 1 + ) in the region between 300 -500 nm. In both cases, these bands correspond to excitations of the unpaired electron from the triarylsubstituted carbon atom to anti-bonding orbitals of phenyl substituents, between 340 -400 nm, and transitions from the  system in the phenyl substituents to the single occupied orbital of the triarylsubstituted carbon atom, close to λ = 500 nm. Figure 5 also shows a band close to λ = 800 nm that is a fingerprint of the cation triplet state that corresponds to a transition from the  system of the carbazole moiety to the single occupied orbital over the nitrogen atom. On the other hand, Figure 5 also shows three bands, one around λ = 600 nm and two between 670 -712 nm that constitute a signature of the closed shell singlet electronic state of the cation ( 1 1 + ), and correspond to excitation from the π system of the phenyl substituents and carbazole moiety to the empty (virtual) orbital of the triarylsubstituted carbon atom (see also as reported in Figure S4 and discussed in the Supplementary Information. Thus, the oxidation in 1 starts with the loss of one electron of the non-bonding pair on the nitrogen, giving a cation-diradical species 3 1  (Scheme 2).   Regarding the single electronic states of cations 1 and 2, ( 1 1 + , and 1 2 + ), it should be pointed out here that we have only considered species having a closed shell structure, although excited electronic states having the same spin structure than the triplet states, namely with two open shells, should also exist. Such kind of singlet electronic states cannot be studied with mono-referential methods but they should lie higher in energy than the corresponding triplet electronic states 26 and, consequently, above the singlet closed shell electronic state in the case of

2.
Page 15 of 45 Physical Chemistry Chemical Physics  Figure S5, and has the appearance of a wide band resulting from the electron-electron interaction. However, the sequence of epr spectra observed as the oxidation of 2 proceeds, leads to a continuous decrease in the intensity of the band until its complete disappearance ( Figure S6). Thus, the initial doublet species is converted into a singlet species at room temperature, as indicated in Scheme 2, being completely transparent in the epr. The structural difference between molecules of 1 and 2, responsible for this different behavior, lies in the presence of the two vicinal chlorine atoms to the carbazole substituent in 1 that are absent in 2. These atoms are responsible for the strong distortion to the planarity between carbazole group and the phenyl bridge in 1, giving rise to a total loss of conjugation between both moieties.
Finally, in order to further support the conclusions raised along this work we have also calculated the ionization energies of the radical adducts 1 and 2 leading to the formation of the corresponding triplet and closed shell singlet electronic states, respectively. The results, collected in Table 2 indicate that the triplet electronic state of the cation 1 + lies lower in energy than the cation in its closed shell singlet state by 0.32 eV whereas for 2, 1 2 + lies lower than 3 2 + by 0.28 eV.
These results support the findings that oxidation of 1 produces the cation in its triplet electronic state whereas oxidation of 2 produces the cation in its closed shell singlet electronic state. (1) Computed at DLPNO-CCSD(T)/cc-pVTZ//B3LYP/6-31+G(2df) level of theory.

Conclusions
The

Conflicts of interest
There are no conflicts to declare. The oxidation of the dimethoxicarbazole adduct of tris(2,3,5,6-tetrachlorophenyl)methyl radical (DTM) involves one electron from the HOMO orbital rather than from the SOMO, to generate a triplet state.    Figure S4. Calculated electronic spectra, in the 300 -1000 nm range, of radical 2 1 (in blue) and the triplet ( 3 1 + in red), closed shell singlet electronic states ( 1 1 + in black), and the cation 2 1H + (in green). A Gaussian function with an arbitrary sigma value of 100 has been used to draw the calculated spectra.