Confining Nitrogen Inversion to Yield Enantiopure Quinolino[3,2,1-k]Phenothiazine Derivatives Cassandre Quinton, Lambert Sicard, Nicolas Vanthuyne, Olivier Jeannin, Cyril Poriel To cite this version: Cassandre Quinton, Lambert Sicard, Nicolas Vanthuyne, Olivier Jeannin, Cyril Poriel. Confining Ni- trogen Inversion to Yield Enantiopure Quinolino[3,2,1-k]Phenothiazine Derivatives. Advanced Func- tional Materials, Wiley, 2018, 28 (39), pp.1803140. 10.1002/adfm.201803140. hal-01902029 HAL Id: hal-01902029 https://hal-univ-rennes1.archives-ouvertes.fr/hal-01902029 Submitted on 7 Apr 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Confining Nitrogen Inversion to Yield Enantiopure Quinolino[3,2,1-k]Phenothiazine Derivatives Cassandre Quinton, Lambert Sicard, Nicolas Vanthuyne, Olivier Jeannin, and Cyril Poriel* molecular orbital level and of the spin– The first examples of optically pure 9H-quinolino[3,2,1-k]phenothiazine orbit coupling) has been demonstrated.[5] (QPTZ)-based molecules are reported. The inversion of the nitrogen atom, The QPTZ fragment incorporated in locked in the QPTZ fragment, is confined with a high-energy barrier that has spiroconfigured materials has thus led allowed the isolation at room temperature of each stereoisomer. Considering to highly efficient blue phosphorescent OLEDs, displaying better performance the growing necessity in molecular electronics to introduce chiral charac- than those obtained with the widely used teristics within highly efficient molecular fragments, a molecular strategy to phenylacridine.[5] In the present work, we generate enantiopure derivatives constructed on the very promising electron wish to focus on the dynamic behavior of rich core QPTZ is provided. As this work aims to report the foundations of the nitrogen atom in QPTZ-based mole- the QPTZ chirality, the present findings may open avenues towards the use of cules. Indeed, N-pyramidal amines with this fragment in its optically pure form. three different substituents attached to the nitrogen atom can be optically active if there is a thermodynamically predomi- nant conformer in the conformational 1. Introduction equilibrium, but the resulting conformers often cannot be iso- lated as the nitrogen inversion barrier is too low.[7] In fact, for The incorporation of chirality in highly efficient molecular configurational stability to be observed, the nitrogen inversion fragments is one of the chief future directions for all three energy barrier should be higher than 100 kJ mol−1 at room tem- types of organic electronics devices,[1] namely organic light- perature.[8] This value can be reached by inserting the nitrogen emitting diodes (OLEDs),[2] field effect transistors,[3] and solar atom into a ring, adding strain in the planar transition state.[9] cells.[4] The incorporation of a chiral compound in device can Interestingly, in the rigid QPTZ fragment investigated herein, enable the detection and emission of chiral light or a enhance the confinement of the nitrogen atom within a dissymmetric charge transport.[1] However and despite recent promising bridged structure increases the energy of this inversion bar- studies, only a few examples have been reported to date. This rier which therefore enables the isolation of the resulting iso- is partly due to the limited number of chiral fragments dis- mers. Moreover, the sulfur atom seems to contribute to the playing a high efficiency when incorporated in devices. Two structural and chiral characteristics described herein. Indeed, years ago, a new electron rich core, 9H-quinolino[3,2,1-k]phe- the sulfur/carbon linkage rigidifies the system compared to the nothiazine (QPTZ), has appeared in literature displaying its phenylacridine fragment but retains enough flexibility, unlike potential in the field of phosphorescent OLEDs.[5] This frag- indoloacridine,[10] to allow the inversion of the nitrogen atom ment consists of a phenylacridine core[6] to which a sulfur when heated. Note that a structurally related analogue pos- bridge was added between the top and side phenyl rings. In sessing an oxygen atom instead of the sulfur atom has been this previous work, the crucial role played by this sulfur atom reported with nevertheless no mention of nitrogen inversion.[11] in the electronic properties (i.e., raising of the highest occupied Thus, this work presents our investigations on the nitrogen inversion of the QPTZ fragment and the first examples of opti- cally pure QPTZ-based molecules (1–3, Figure 1) described in Dr. C. Quinton, L. Sicard, Dr. O. Jeannin, Dr. C. Poriel literature. Constraining the nitrogen atom in a pyramidal con- Univ Rennes figuration at room temperature appears hence as an efficient CNRS ISCR-6226, F-35000 Rennes, France strategy to generate enantiopure compounds. Furthermore, the E-mail: [email protected] spiro carbon of spirofluorene-based compounds also allows the [12] Dr. N. Vanthuyne introduction of chirality in such systems. Aix Marseille Univ Three molecular models based on the fluorene-spiro-QPTZ CNRS (F-s-QPTZ) architecture have been chosen for this study: the Centrale Marseille unsubstituted F-s-QPTZ (2) and two substituted F-s-QPTZ pos- iSm2, Marseille F-13397, France sessing different substitution patterns (1 and 3) in order to The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201803140. investigate the impact of the steric hindrance on the nitrogen inversion. Considering electronic device applications for the DOI: 10.1002/adfm.201803140 QPTZ core, it is important to know whether the inversion of Figure 1. Molecular structures of the investigated molecules. a) 1, b) 2, and c) 3 and their relationships obtained by molecular modelling (optimized structures with b3lyp/6-31g(d)). The view angle chosen to highlight the phenothiazine flip shows the two phenyl rings of the acridine moiety and the two sigma bonds of the spiro bridge as superimposed (different views of 2NR can be found Figure S48 in the Supporting Information). Sulfur atom in yellow and nitrogen atom in blue. the nitrogen atom can occur or not and, if it can, how quickly 1-phenyl-9-fluorenone for 1, 9-fluorenone for 2, 3-phenyl- and at which temperature. 9-fluorenone for 3 (Scheme 1). The corresponding fluorenols were not isolated and were involved in an intramolecular elec- trophilic cyclisation reaction to give the three targets 1–3 in 2. Results and Discussion good yields. After synthesis, the 1H and 13C NMR spectra of the three Compounds 1–3 were readily obtained through a short and compounds 1–3 provide the first piece of interesting data (see efficient synthetic approach by coupling the halogeno-aryl Figures S49–S68 in the Supporting Information). Indeed, 10-(2-bromophenyl)-10H-phenothiazine (obtained from the 3 displays a complex spectrum, characteristic of a mixture of copper catalysed Goldberg coupling of phenothiazine and diastereoisomers, very different to that of the unsubstituted 2 2-bromoiodobenzene), with the corresponding 9-fluorenone: and that of the 1-substituted 1. Scheme 1. Syntheses of compounds 1–3. The presence of diastereoisomers in 3 suggests at least phenothiazine core. Indeed, the QPTZ moiety is not flat due two stereogenic centers, identified in our case as the nitrogen to the intense deformations induced by the phenothiazine and atom and the spiro carbon (the fluorene being dissymmet- acridine cores (Figure 2). In fact, the nitrogen atom is pyramidal rical). These stereochemical features have been confirmed by with values of nitrogen torsion angle found between 157° for ° chiral high pressure liquid chromatography (HPLC) studies 3NRCS and 3NSCR and 162 for 3NSCS (see Figure S17 and Table (see Figures S2, S4, and S6 in the Supporting Information), S3 in the Supporting Information), as expected for nitrogen with which have allowed to separate and isolate each diastereoi- one or more aromatic substituents.[13] This shows that the triva- somer of 3: 3NRCR, 3NSCS, 3NRCS, and 3NSCR (Figure 1c). lent nitrogen in these compounds is in an intermediate hybridi- 2 3 X-Ray crystallography of the two diastereoisomers 3NSCS and zation state between sp and sp . Thus, the phenothiazine can 3NRCS (Figure 2c) clearly confirms the inversion of the nitrogen move from one side of the fluorene to the other through a atom in the QPTZ fragment, which is caused by a flip of the structural reorganization of the acridine, much like a wing flap- ping. The same kind of stereoisomers would have been expected for 1 which displays the same two stereogenic centres. However, only two stereoisomers (enantiomers) have been isolated by chiral HPLC: 1NRCS and 1NSCR (Figures 1a and 2a). Indeed, the steric hindrance induced by the pendant phenyl ring in position 1 of the fluorene prevents the flipping of the phenothiazine above dis- cussed and hence the formation of the other pair of enantiomers. This structural feature provides an efficient diastereocontrol and can be easily visualized on the X-ray struc- tures of the two enantiomers 1NRCS and 1NSCR (Figure 2a). The phenyl ring in C1 displays two important characteristics. First, it is almost orthogonal to the fluorene with a dihedral angle between the mean plane of the phenyl ring in C1 and that of the fluorene ° ° of 83.3 for 1NRCS and of 84.1 for 1NSCR suggesting a strongly sterically hindered environment (see Figures S13 and S14 in the Supporting Information). This structural characteristic is at the origin of the emer- gence of one-substituted fluorenes recently reported in literature.[14] In addition, the pen- dant phenyl ring is very close to the QPTZ fragment (three C/C intramolecular distances are indeed shorter than the sum of the Van der Walls radii, see Figures S15 and S16 in the Supporting Information).