Rajaa Benchouaia

Nicolas Cissé

Bernard Boitrel

Matthieu Sollogoub

Stéphane Le Gac

Mickaël Ménand

Orchestrating Communications in a Three-Type Chirality Totem: Remote Control of the Chiroptical Response of a Möbius Aromatic System

Among the various types of chirality (central, axial, helical, planar…), that inherent to Möbius topology remains almost unexplored, partly due to the difficulty to access Möbius compounds. Over the last decade, [28]hexaphyrins have revealed among the best candidates to build on Möbius aromaticity. Whereas their flexibility needs to be controlled to get P/M twist enantioselectivity, it could be of great interest to sustain dynamic chirality transfer. In this context, we report herein the first example of a Möbius aromatic ring capped by a cavity, consisting in a Möbius [28]hexaphyrin doubly-linked to an α-cyclodextrin. This unique design affords a "totem" of three different chirality elements arising from the cyclodextrin (fix central chirality), the bridging pattern (dynamic planar chirality) and the hexaphyrin (dynamic topological chirality). Chirality transfers (see TOC graphic) are characterized by a stereospecific planar-to-topological communication (diast. excess > 95 %; the highest asymmetric selectivity reported to date for a Möbius ring) combined to a stereoselective central-to-planar communication (up to 60 % diast. excess). Interestingly, the stereoselectivity is remotely controlled by coordination of an achiral effector to the hexaphyrin, increasing up to five times the chiroptical response of the Möbius aromatic π-system. These results highlight the advantageous use of dynamic chirality transfers to further incorporate Möbius chirality and aromaticity into all kind of stimuli responsive devices.

INTRODUCTION

The conversion of the artistically elegant Möbius strip topology into a molecular architecture has remained elusive until the pioneering report of Walba's bis-crown ether ribbon. 1,[START_REF] For | Aromaticity and Delocalization[END_REF] Two decades later, Herges devised an ingenious multi-step approach to synthesize the first π-conjugated Möbius molecule derived from a benzo-fused annulene scaffold. [START_REF]ppm, long side/long side), vs. distinct chemical shifts for R [26]2 and R [26(δ{ R [26]2} = -150.50 and -154[END_REF] Soon after, the groups of Latos-Grażyński 4 and Osuka [START_REF] Sankar | Unambiguous Identification of Möbius Aromaticity for meso-Aryl-Substituted[END_REF] reported concomitantly the synthesis of heteroannulenic compounds with native Möbius twisted topologies. The conformational flexibility of these expanded porphyrinoids 6 allowed to experimentally evidence the 4n π-conjugated "Möbius aromaticity", unknown in nature but earlier predicted by Heilbronner. [START_REF] Heilbronner | Hückel Molecular Orbitals of Möbius-Type Conformations of Annulenes[END_REF] The straight forward access to regular meso-aromatic expanded porphyrins allowed to further investigate the influence of the Möbius twist on their physico-chemical properties (4n+2 antiaromaticity, 8 baird rules, 9 …). However, despite important findings in this field, it is worth to note that the inherent chirality of Möbius compounds has remained quasi-unexplored. [START_REF] Tanaka | (1.1.1.1.1.1: Enantiomeric Separation, Absolute Stereochemistry, and Asymmetric Synthesis[END_REF][START_REF] Ruffin | Tren-Capped Hexaphyrin Zinc Complexes: Interplaying Molecular Recognition, Möbius Aromaticity, and Chirality[END_REF] Considering the paramount importance of chirality in biological processes, drug design, material sciences and many other fields, scrutinizing Möbius chirality has become a major challenge with promising opportunities. 12 In general, Möbius conformations of regular expanded porphyrins are flexible, making their chiral π-conjugated system dynamic. Therefore, the fluxionality of the macrocycles needs either to be frozen or dynamically controlled to access optically active samples. Scheme 1 summarizes the few successful attempts of stereoselective formation of Möbius compounds, all involving a [28]hexaphyrin scaffold, 13 and emphasizes the chirality transfers at work. Following the "frozen" strategy (Scheme 1a), kinetically trapped Möbius enantiomers were stereoselectively obtained using Pd(II) chiral salts, with however modest enantiomeric excess (23 % e.e.). [START_REF] Tanaka | (1.1.1.1.1.1: Enantiomeric Separation, Absolute Stereochemistry, and Asymmetric Synthesis[END_REF][START_REF] Tanaka | Metalation of Expanded Porphyrins: A Chemical Trigger Used To Produce Molecular Twisting and Möbius Aromaticity[END_REF] On the other hand, the "stereodynamic" strategy allowed to control the Möbius twist winding to the highest diastereomeric excess reported to date (76% d.e.) using labile metal complexes associated with chiral ligands (Scheme 1b). [START_REF] Ruffin | Tren-Capped Hexaphyrin Zinc Complexes: Interplaying Molecular Recognition, Möbius Aromaticity, and Chirality[END_REF] The dynamic stereocontrol [START_REF] Wolf | Dynamic Stereochemistry of Chiral Compounds: Principles and Applications[END_REF] of the Möbius twist can thus turn to be a major advantage for the development of responsive devices and materials with tuneable chiroptical and aromaticity properties. Moreover, switchable stereoisomerism is of particular interest in asymmetric catalysis or for the directional control of motion, which are key features for the design of smart molecular machines. 16 To get any benefit from Möbius stereoisomerism, fine understanding of chirality transfer 17 from or to other stereochemical elements is crucial. Inspired by recent findings A c c e p t e d M a n u s c r i p t in chiral molecular switches and catalysts, featuring hybrid [START_REF] Pizzolato | Central-to-Helical-to-Axial-to-Central Transfer of Chirality with a Photoresponsive Catalyst[END_REF] or compartmentalized 19 transfer of chirality, we devised a new strategy where multiple stereochemical sources would be endogenous to a dynamic Möbius edifice (Scheme 1c). Having a flexible Möbius ring covalently connected to other chiral element(s) would offer a unique opportunity to study chirality transfer phenomena, and ultimately chiral induction of the Möbius twist. We describe herein an unprecedented system composed of three different stereochemical elements arranged in a "totem" fashion referred as central (fix), planar (dynamic) and topological (Möbius, dynamic). Nuclear magnetic resonance (NMR) and electronic circular dichroism (ECD) spectroscopic investigations revealed effective chirality transfers across the three elements according to stereoselective and stereospecific relationships, with a possible remote control, leading ultimately to amplification of the chiroptical response of the Möbius aromatic π-system.

Scheme 1. Simplified depiction of different strategies achieving stereoselective Möbius winding.

(a) "Trapped" coordination approach with chirality transfer occurring in the transition state (23 % e.e.). 10 (b) "Dynamic" coordination approach with an exogeneous chiral ligand offering chirality transfer in the equilibrium of diastereomeric complexes (76 % d.e). 11 (c) Present contribution: "fix" and "dynamic" endogenous chiral sources offering intramolecular chirality transfers in the final equilibrium of diastereomeric coordination complexes.

RESULTS AND DISCUSSION

Design

We recently described hexaphyrin-cyclodextrin hybrid compounds (HCDs) made of an hexaphyrin unit (the six-pyrrole homologue of porphyrin) attached to one or two A,C,Emodified α-cyclodextrin unit(s) by triple or sixfold covalent linkages (Figure 1b). 20 In these hybrids, the hexaphyrin unit adopts up to three different planar conformations (rectangle, dumbbell, and triangle). For each of these, the two oxidation states of the hexaphyrin follow Hückel rules with aromatic character for the oxidized state " [26]" and antiaromatic character for the reduced one " [28]" (4n+2 and 4n conjugated π electrons, respectively). The three planar conformations are interconvertible in both oxidation states using various stimuli. 20 In the particular case of the triangular conformation, independently of the chirality of the cyclodextrin, the triple attachment on one face generates planar chirality (Figure 1b). Indeed, the 5,15,25 and 10,20,30 bridging patterns are mirror images and correspond to opposite R p and S p configurations. 20b Their equal population indicates a lack of stereodiscrimination from the cyclodextrin (static source of central chirality with 30 stereogenic centres, Figure 1a, bottom). [START_REF]See reference 20b and the corresponding SI for a definition of the R p and S p stereodescriptors. 22 See reference 6d for a[END_REF] The planar chirality is dynamic since the diastereomers are in equilibrium through coupled in-out/out-in inversions of the pyrroles (Figure 1a middle, for a related process). Despite the plasticity of these hybrids, the Möbius aromatic conformation ( [28] state) was not detected suggesting that a high strain penalty due to the triple covalent attachment prevents a twisted conformation. We thus turned our attention to less constrained doubly-linked HCDs using an A,Ddifunctionalized cyclodextrin (Figure 1c). The devised 'A,D' pattern was expected to (i) generate planar chirality, (ii) favor Möbius conformation in the [28] state, and ultimately (iii) afford a 3-type chirality totem suitable for chiral transfers. Stereochemical analysis revealed up to 3 possible diastereomers for the most common rectangular conformation and 12 others for the Möbius hexaphyrin one (Figure 1c): (i) the rectangular diastereomers differ by their three possible meso-bridging patterns, two of which generating planar chirality (Figure 1a, middle). Whereas the 5,20-meso bridged hexaphyrin affords an achiral short-side bridging pattern, bridging the 10,25 and 15,30-meso positions affords chiral long-side bridging patterns of opposite R p /S p configurations. The expected dynamic character of these meso-bridging patterns through coupled in-out/out-in inversions of the pyrroles, 20 would offer a possible dynamic control of the planar chirality owing to the cyclodextrin chirality. (ii) introducing half-a-twist in the π-conjugated circuit gives rise to Möbius bands with opposite P/M handedness (Figure 1a top). Considering a simplified rectangular hexaphyrin, the twisting of an inverted pyrrole can process upward (blue arrows) or downward (red arrows) with either left or right orientations, leading to four twisted stereoisomers: two degenerate (C 2 -symmetrically related) M twists and P twists. 22 Bridging opposite meso positions of these stereoisomers lifts their degeneracy affording two different M twists and two different P twists. These four Möbius conformations can occur for each of the three possible meso bridging patterns affording up to 12 possible diastereomers for the targeted 'A,D' doublylinked HCDs (Figure 1c). Similarly to the rectangular conformation, planar chirality arises for the long-side bridging patterns (10,25 and 15,30), affording totems of three types of chirality (Figure 1c). Importantly, the meso-substitution pattern and Möbius isomerization processes are governed by very different energy barriers. The Möbius twisting one amounts approximately for 8 kcal.mol -123 while that of the dynamic planar chirality reaches 23 kcal.mol -1 . 24 ACS Paragon Plus Environment Journal of the American Chemical Society

Synthesis and characterization

The synthesis of the doubly-linked HCD hybrids was performed using precursor 5 obtained by alkylation-deprotection of the perbenzylated α-cyclodextrin 6A,6D-diol 4 [START_REF] Pearce | Diisobutylaluminum-Promoted Regioselective De-O-benzylation of Perbenzylated Cyclodextrins: A Powerful New Strategy for the Preparation of Selectively Modified Cyclodextrins[END_REF] in 84% yield over two steps (Scheme 2). This dialdehyde was further engaged in an acid catalyzed macrocyclization with two equivalents of tripyrrane 6, 6h followed by an in-situ oxidation with 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Subsequent purification of the crude material by column chromatography afforded a turquoise blue fraction ( M [28]2 and M [28]3) and a more polar violet fraction ( R [26]1) (Scheme 2). High resolution mass spectrometry (HRMS) analyses were indeed consistent with the formation of doubly-linked HCD hybrids with 28 and 26π electron systems, respectively. The UV-vis-NIR absorption spectra of both blue and violet fractions showed similar absorption profiles, with an intense Soret-like band at respectively 601 and 577 nm and four Q-like bands spanning up to the NIR region, characteristic of an aromatic character (Figure 2). These features are consistent with a Hückel aromatic character for R [26]1 (4n+2 π electrons, planar) and a Möbius aromatic one for M [28]2/ M [28]3 (4n π electrons, singly twisted). The 1 H NMR spectrum of R [26]1 recorded in CDCl 3 displays a well-resolved pattern corresponding to a C 2 -symmetric compound (SI). The chemical shifts of the β-pyrrolic protons (βπ) are splitted into two characteristic regions evidencing a strong diatropic ring current. The shielded part displays a broad singlet (-1.58 ppm, NHin) and two doublets (-2.70 and -2.95 ppm, βπin), while the deshielded one shows four doublets (9. 23, 9.20, 9.12 and 9.02 ppm, βπout). Such a distribution is consistent with a C 2 -symmetric rectangular conformation orienting two NH and four βπ protons in the shielding inner part short sides of the hexaphyrin rectangle (5,20 meso-positions, Scheme 2) as deduced from an unusual 19 F-19 F through space coupling. [START_REF] Hierso | Indirect Nonbonded Nuclear Spin-Spin Coupling: A Guide for the Recognition and Understanding of "Through-Space" NMR J Constants in Small Organic, Organometallic, and Coordination Compounds[END_REF] Indeed, this substitution pattern brings the mesopentafluorophenyl groups on the long sides of the rectangle. Hence, two ortho-fluorine atoms (one of each adjacent mesopentafluorophenyl groups) are in close proximity leading to the coupling/correlation observed in both 19 F and 2D 19 F-19 F COSY NMR spectra (SI). In addition, two O6-benzyl groups belonging to the C 2 -symmetrically related C and F glucose units undergo a marked shielding effect consistent with their location into the confined space between the hexaphyrin and the primary rim of the cyclodextrin. In contrast, the 1 H NMR spectrum (CDCl 3 , 300 K) of the turquoise blue fraction reveals two C 2 -symmetric compounds ( M [28]2 and M [28]3) in a roughly 1:1 ratio, not separated by silica gel chromatography (SI). Whereas sharp signals are observed for the cyclodextrin parts, most of the hexaphyrin protons are strongly broadened over a large temperature range (183 K to 330 K) preventing direct determination of its conformation. Such a behaviour is consistent with an equilibrium occurring on the NMR time scale between Möbius conformations, notably between C 2symmetrically related ones (degenerate conformations). [START_REF] Sankar | Unambiguous Identification of Möbius Aromaticity for meso-Aryl-Substituted[END_REF][START_REF]Considering a transiting C 2 -symmetric rectangular conformation, the same (degenerate) Möbius isomers are obtained from twisting one or the other identical (C 2 -symmetrically related) inverted pyrroles. 28 These long-side patterns are also supported by the 19 F NMR spectra lacking the through-space 19 F-19 F coupling constant, and by the distribution of the para-fluorine atoms resonating at very similar chemical shifts for R[END_REF] Indeed, the energy barrier associated to Möbius conformational exchange is about 8 kcal.mol -1 (Figure 1a, top). [START_REF]Calculated for meso-hexakis(pentafluorophenyl)[END_REF] As determined below, M [28]2 and M [28]3 differ by their bridging patterns which are respectively 10,25 and 15,30 (Scheme 2), affording planar chirality (Figure 1c). 1a, middle and top). It has to be noted that the more rigid triply-linked [28]HCD derivative affords a stable rectangular antiaromatic conformation. 20a,30 In contrast, the doubly-linked [28]

Redox conversion and conformational isomerization

Chirality induction and chiroptical properties with free bases

We next examined the chiroptical properties of M [28]2 and M [28]3 by ECD spectroscopy. 31 Interestingly, these isolated compounds exhibit mirror ECD spectra with intense bis-signate Cotton effect in the Soret band wavelength region (Figure 3). These signatures are in good agreement with those of optically pure Möbius hexaphyrin metal complexes, [START_REF] Tanaka | (1.1.1.1.1.1: Enantiomeric Separation, Absolute Stereochemistry, and Asymmetric Synthesis[END_REF] and give us four main informations: (i) the intense chiroptical response suggests that a high P/M stereoselectivity occurs at the level of the Möbius twist, although quantitative assessment cannot be achieved at this stage due to the dynamic character of the Möbius conformation; (ii) the stereoselectivity of the Möbius twist originates from the 10,25/15,30 bridging patterns (opposite R p /S p planar configurations), without noticeable influence of the chirality of the cyclodextrin. Indeed, the ECD spectrum of the equilibrated M [28]2 and M [28]3 mixture (ca. 1:1 thermodynamic ratio) exhibits a weak signal; (iii) compared to native [28]hexaphyrins, the bridging to the cyclodextrin leads to a significant increase of the M twist ↔ P twist isomerisation barrier. Actually, Möbius isomerization is mainly if not entirely coupled to the planar isomerization process (Figure 1a, top and middle). This gives a unique opportunity to isolate optically active Möbius hexaphyrins in their free base forms; (iv) the negative (/positive) Cotton effect for M [28]2 (/ M [28]3) allows to assign the absolute configurations of the Möbius twists by comparison with isolated enantiopure Möbius hexaphyrins, giving M for M [28]2 and P for M [28]3. [START_REF] Tanaka | (1.1.1.1.1.1: Enantiomeric Separation, Absolute Stereochemistry, and Asymmetric Synthesis[END_REF]32 This further enable to assign the 10,25 (R p C and D). These contrasting geometries are imposed by the relative orientation of the meso-substitutents adjacent to the twisted pyrrole (in red/yellow). Actually, the twisting process projects these meso-substituents (meso-C 6 F 5 and meso-aromatic linker) in opposite directions (up and down compared to the hexaphyrin mean plane). In the case of M [28]2, the meso-C 6 F 5 (in blue, Figure 4a) is oriented upward and away from the cyclodextrin while the meso-aromatic linker is oriented downward pointing the ethylenic spacer towards the cyclodextrin. This situation leads to the observed strain-relieved geometries and is thus favored. In contrast, M [28]3 orients the meso-C 6 F 5 downward leading to steric interactions with the cyclodextrin (Figure 4b). These interactions are amplified by the highly compressed geometries arising from the upward orientation of the meso-aromatic linker which forces the hexaphyrin part to come closer to the cyclodextrin one, thus leading to unfavored geometries. Conversely, the P twist fits better the 15,30 bridging pattern of the M [28]3 isomer than the 10,25 one of M [28]2 (SI). This stereochemical study evidences a strong communication between the planar and Möbius stereogenic elements, but also shows a weak one (if any) with the cyclodextrin's chirality (no stereoselectivity between M [28]2 and M [28]3 of opposite planar configuration). Targeting a full communication across the three levels of this chiral totem, we next investigated the effect of conformational restriction of the Möbius hexaphyrin upon coordination.

Metalation with Zn(II): a three-type chirality totem in action

Dynamic character of the metal complexes

Coordination studies with M [28]2 and M [28]3 focused on the complexation of a labile metal ion, i.e. Zn(II), in order to vs. 0.21 to 0.72 ppm for the three others, SI). In agreement with the literature, this shielding corresponds to a meso-C 6 F 5 adjacent to the twisted pyrrole and projected towards the Möbius ring (Scheme 4, in blue), the three other meso-C 6 F 5 being peripheral. [START_REF] Mori | meso-aromatic substituent cis to the N atom of the twisted pyrrole has been observed with group ten metal complexes of Möbius [28]hexaphyrins, exhibiting a significant localized twist distribution due to a constraining square planar NNNC coordination sphere[END_REF] Moreover, two benzylic groups on the primary rim of the cyclodextrin unit experience a marked shielding (Δδ up to ~4 ppm), indicating that they occupy the confined space between the hexaphyrin and the cyclodextrin units. For each complex, we determined the preferred Möbius stereoisomer among the four possible ones (Figure 1a) following converging evidences: (i) oxidation of the Zn(II) complexes led to a mixture enriched with R [26]2 (SI). Since DDQ oxidation carried out on the corresponding free bases did not affect the 1:1 ratio of planar isomers, this observation indicates that M [28]2•ZnL n is the major species and M [28]3•ZnL n is the minor one; (ii) ECD spectra analysis indicate that the favored M and P Möbius configurations of the free bases are retained upon Zn(II) complexation (see discussion below); (iii) L 1 -type complexes are characterized by a broad signal (-0.93/-1.08 ppm, minor/major species) corresponding to the methyl group of a bound acetato ligand (Figure 5a). [START_REF]groups of the bound L 2 and the linker ethylenic and aromatic protons (< 5 Å) would lead to significant NOE correlations. The only NOE correlations observed with the bound L 2 indicate spatial proximity between its 'in' Me group and outer β-pyrrolic protons, in good[END_REF] Their complexation induced shifts (CIS: Δδ Me ~ -3 ppm, Table 1) indicate that these ligands face the inner part of the Möbius hexaphyrin. In contrast, the two methyl groups of the acac ligand display very different CIS for both the minor and the major L 2 -type complexes (Δδ Me ~ -3 and -0.8 ppm; Figure 5b, Table 1, SI). It suggests that L 2 binds the metal with an offcentred position from the Möbius ring, orienting a methyl group closer to the ring than the other (Scheme 4b). Importantly, both L 1 -and L 2 -type complexes (and even crowded L 3-6 -types, vide infra) did not afford any NOE correlations between bound ligands and either the introverted benzyl groups, the bridging linkages, or the cyclodextrin protons. This is consistent with an exo-coordination of the 'ZnL n ' moiety orienting the L n ligand away from the cyclodextrin unit, 36 the confined space between the hexaphyrin and the cyclodextrin units being occupied by benzyl groups. These observations corroborate the down orientation of the twisted pyrrole pointing the coordinating N atom up, and rules out the opposite orientation (Figure 4 "B" vs. "A" for (M)-M [28]2). Indeed, we recently showed that a capped hexaphyrin in Möbius conformation coordinates 'ZnOAc' at a dipyrrin site involving the adjacent twisted and out pyrroles. [START_REF] Ruffin | Tren-Capped Hexaphyrin Zinc Complexes: Interplaying Molecular Recognition, Möbius Aromaticity, and Chirality[END_REF] In the present case, the NMR and ECD data are in accordance with a similar tetrahedral coordination geometry depicted in Scheme 4 (red and green pyrroles). The metalation process thus corresponds to the stereoselective formation of (M)-M [28]2•ZnL n and (P)-M [28]3•ZnL n out of 12 possible diastereomers (Figure 1c).

Three-level chirality amplification

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Journal of the American Chemical Society The chiral induction of the Möbius twist was further evidenced by ECD analysis. The ECD spectra of the NMR tube solutions containing ~4 :1 mixtures of (M)-M [28]2•ZnL 1-2 and (P)-M [28]3•ZnL 1-2 display up to 5 times higher intensity compared to the starting free base mixture (Figure 6). This corresponds formally to a shift from ca. 10 % to 56 and 60 % d.e. according to NMR integrations. These results highlight a rare case of configurationally responsive Möbius aromatic π-system exhibiting an in-situ enhancement of its chiroptical activity. [START_REF] Ruffin | Tren-Capped Hexaphyrin Zinc Complexes: Interplaying Molecular Recognition, Möbius Aromaticity, and Chirality[END_REF] The amplification of the M [28]2/ M [28]3 ratio (1:1 to 4:1) is a consequence of a coordination-driven communication between the static source of chirality (cyclodextrin) and the dynamic ones (the bridging linkages and the hexaphyrin). At first glance, the metal acts as an achiral effector which simply freezes the Möbius ring and strengthens the communication in the totem (Figure 7). As a result, two different stereoselection processes occur: (i) at the lower-half, the central chirality of the cyclodextrin partially discriminates the planar stereoisomers (d.e. ~ 60 %) and (ii) at the upper-half, the planar chirality drastically discriminates the Möbius stereoisomers affording a single twist for each bridging pattern of the Zn(II) complexes. Integrations of the well resolved [START_REF] Tanaka | (1.1.1.1.1.1: Enantiomeric Separation, Absolute Stereochemistry, and Asymmetric Synthesis[END_REF][START_REF] Ruffin | Tren-Capped Hexaphyrin Zinc Complexes: Interplaying Molecular Recognition, Möbius Aromaticity, and Chirality[END_REF] Importantly, it highlights a stereospecific relationship between the planar and Möbius chiralities, that undergo concomitant amplification processes (P → M Möbius twist induction coupled to Sp → Rp planar induction) (Figure 7, blue and red arrows). Tuning chirality amplification using achiral effectors Zn(II)-driven chiroptical responsiveness was further studied with different acac derivatives (Scheme 4a). Symmetrical ones (L 3 -L 5 ) behave similarly to L 1 and L 2 ligands affording a major [(M)-M [28]2•ZnL [START_REF]ppm, long side/long side), vs. distinct chemical shifts for R [26]2 and R [26(δ{ R [26]2} = -150.50 and -154[END_REF][START_REF] Stępień | Expanded Porphyrin with a Split Personality: A Hückel-Möbius Aromaticity Switch[END_REF][START_REF] Sankar | Unambiguous Identification of Möbius Aromaticity for meso-Aryl-Substituted[END_REF] ] and a minor species [(P)-M [28]3•ZnL [START_REF]ppm, long side/long side), vs. distinct chemical shifts for R [26]2 and R [26(δ{ R [26]2} = -150.50 and -154[END_REF][START_REF] Stępień | Expanded Porphyrin with a Split Personality: A Hückel-Möbius Aromaticity Switch[END_REF][START_REF] Sankar | Unambiguous Identification of Möbius Aromaticity for meso-Aryl-Substituted[END_REF] ] (Figure 5c-e, SI). 38 In line with L 2 , half of the bound L 3 -L 5 ligands is strongly shielded (Δδ up to 3.06 ppm, Table 1) in agreement with an off-centred binding geometry (Scheme 4a). Interestingly, the ratio of (M)-M [28]2•ZnL 2-5 and (P)-M [28]3•ZnL 2-5 strongly depends on the ligand size (Figure 5be, Table 1), the highest ratio being obtained for L 2 (R = Me, 78:22) and the lowest one for L 5 (R = tBu, 60:40). This difference in chiral induction is nicely confirmed by ECD analysis of the NMR tube solutions showing the highest chiroptical activity for L 2 and a decreasing chiral induction with bulkier substituents (Figure 6). The linear relation between the d.e. values measured from 1 H NMR integrations and the ECD intensity (Figure 6, right) indicates a chiroptical response influenced only by the P/M ratio. Thus, increasing steric congestion around the bidentate ligand leads to counter-intuitive effects: it weakens chirality transfer with the cyclodextrin (central-to-planar, lower-half communication), but does not affect the stereospecific relationship between planar and Möbius elements (upper-half communication). This ligand is thus acting as an achiral effector affording remote control [START_REF] Mikami | Acyclic Stereocontrol Between Remote Atom Centers via Intramolecular and Intermolecular Stereo-Communication[END_REF][START_REF] Mamiya | Remote Control of the Planar Chirality in Peptide-Bound Metallomacrocycles and Dynamic-to-Static Planar Chirality Control Triggered by Solvent-Induced 3 10 -to-α-Helix Transitions[END_REF] across the whole totem and matching the piled chiral elements (Figure 7, green/purple arrows).

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Journal of the American Chemical Society 5f). Surprisingly, the major complexes (strong pair, 95 %) correspond to those orienting the largest substituent towards the ring (tBuin: L 6 l) while the minor ones (weak pair, 5 %) correspond to the smallest (Mein: L 6 s). Molecular models of L 6 l and L 6 s complexes (Figure 5g) suggest a pinched region due to the "twisted" meso-C 6 F 5 (in blue in Scheme 4), leading to steric hindrance with the tBuout of L 6 s. Interestingly, the stereoselectivity ratios contrast with the stability ones. Indeed, the (M)-M [28]2•ZnL 6 /(P)-M [28]3•ZnL 6 ratios are 82:18 in the L 6 s weak pair and 62:38 in the L 6 l strong pair. These ratios fit those obtained with the corresponding symmetrical ligands L 2 and L 5 , respectively. Combined together, the stability of the complex is governed by the size of the outward substituent (the smallest the size, the highest the stability), while the stereoselectivity is governed by the size of the inward one (the smallest the size, the highest the stereoselectivity). This study shows that subtle steric effects occurring on the naked side of the Möbius ring have important consequences on the bridging pattern that is further discriminated by the cyclodextrin. In other words, coordinationinduced freezing of the Möbius ring is a prerequisite, but is not enough to establish an efficient communication between the cyclodextrin and the planar chiralities. The exogeneous ligand plays a main role as well through an unexpected steric complementarity with the Möbius ring.

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Journal of the American Chemical Society ii) chiral information is conveyed across these three-type chirality totems thanks to a subtle relay involving two dynamic chirality transfers (Figure 7). Spectacularly, the chiral relay occurring between the bridging pattern and the Möbius hexaphyrin is highly efficient affording a single out of four Möbius twists for a given bridging pattern. This illustrates an unprecedented planar-to-topological stereospecific relationship. On the other hand, the chiral relay between the cyclodextrin and the bridging pattern occurs only upon Zn(II) metalation of the Möbius hexaphyrin. The corresponding central-to-planar stereoselectivity is then tuned by an exogenous ligand bound to the metal ion, acting as an achiral effector. The resulting remote control is ensured by the ligand crowding tuning the stereoselectivity up to 60 % d.e. in the case of small size effectors. iii) remarkably, the whole dynamic stereoselective process amplifies up to five times the chiroptical response of the free base Möbius aromatic rings highlighting a fine orchestration of the endogeneous/chiral and exogeneous/achiral components of the totem.

A key point of the system is the dynamic character of the Möbius twist and bridging patterns, which rely on two different hexaphyrin conformational exchanges. The attached cyclodextrin providing a static source of chirality can either be almost silent over this process or active with the help of a remote achiral effector. This dynamic approach appears promising for the further use of Möbius chirality into all kind of stimuli responsive systems including switchable asymmetric catalysts. Work along these lines is in progress in our laboratories.

ASSOCIATED CONTENT Supporting Information

Full experimental details and spectral data. The Supporting Information is available free of charge on the ACS Publications website.

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Figure 1 .

 1 Figure 1. (a) Details of chiral elements constituting the targeted totem edifices: (bottom) representation of the α-cyclodextrin A,D-diol embedding 30 stereogenic centers which are thermodynamically and kinetically stable; (middle) illustration of the dynamic planar chirality arising from bridging the 10,25 and 15,30 meso-positions of a rectangular hexaphyrin. The bridging pattern isomerization occurs through coupled in-out/out-in pyrrole inversions. The labelling of the meso-substitution patterns is defined clockwise relative to a short side substituent (labelled 5) considering the free face of the hexaphyrin; (top) illustration of the equilibria between four possible Möbius stereoisomers (transiting through a rectangular conformation), depending on the up/down and right/left twisting of an inverted pyrrole (highlighted in yellow and converted to blue and red plain arrows). Note that the same degenerate isomerization process is obtained using the opposite inverted pyrrole at the front. The chirality of the Möbius strip is emphasized both with the native writhed view and the stretched writhe-free view of the Möbius strip (grey box). (b) Overview of the triply-linked hexaphyrin-cyclodextrin (HCD) hybrids and their accessible hexaphyrin conformations. 20 Inset: stereochemical analysis of the two diastereomers of the triangular HCD exhibiting planar and central chirality. (c) Stereochemical analysis of the targeted totem design of a three-stereogenic elements assembly from doubly-linked HCD hybrids.

  Scheme 2. Synthesis of HCD hybrids R [26]1, M [28]2 and M [28]3. Labeling for all isomers follows the nomenclature provided on compound R [26]1.

Figure 2 .

 2 Figure 2. Normalized UV-vis-NIR absorption spectra in CH 2 Cl 2 of R [26]1, R [26]2, R [26]3, R [28]1, M [28]2 and M [28]3. (*) contamination with M [28]2 and M [28]3.

  To further study M[28]2 and M [28]3, we decided to explore their oxidized counterparts. Thus, DDQ oxidation of the mixture of M [28]2 and M [28]3 allowed to isolate pure R [26]2 and R [26]3 by chromatography (Scheme 3). Both UV-vis-NIR absorption spectra display absorption bands in accordance with an aromatic character (Figure 2). The 1 H NMR signatures of both C 2 -symmetric compounds were very similar to that of R [26]1 with three shielded signals (2×NHin and 4×βπin), four deshielded ones (8×βπout), and showing two included benzyl groups belonging to the cyclodextrin primary rim (SI). Therefore, the hexaphyrin unit of R [26]2 and R [26]3 also adopts a rectangular conformation. Yet, these hybrids differ from R [26]1 by their bridging pattern, respectively 10,25 and 15,30 (Scheme 3b, vide infra). 28 Besides, R [26]2 and R [26]3 are unstable in solution at room temperature, isomerizing into the thermodynamically more stable R [26]1 isomer (Scheme 3a and Figure 1a middle, SI). Energy barriers of 25.8 and ca. 25.4 kcal.mol -1 were estimated for the conversion of R [26]2 and R [26]3 into R [26]1, respectively (SI). Conversely, reduction of R [26]1 with NaBH 4 in THF-d 8 did not produce the former mixture of M [28]2 and M [28]3 but rather a ACS Paragon Plus Environment Journal of the American Chemical Society

  Scheme 3. (a) Global synthetic network connecting R [26]1, R [26]2, R [26]3, R [28]1, M [28]2 and M [28]3. (b) Structures of R [28]1, R [26]2 and R [26]3.the βπ proton resonances compared to the aromatic R [26]1 isomer. Indeed, the paratropic ring current induces a downfield shift of the four inner βπ protons(14.91 and 14.67 ppm), and an upfield shift for the eight outer ones(3.89, 3.91, 4.71 and 4.82 ppm). R[28]1 isomer is also unstable in solution (THF-d 8 ) at room temperature, isomerizing into the Möbius aromatic mixture of M [28]2 and M [28]3 (Scheme 3a, SI). This process is consistent with a short-side 5,20 meso-bridging pattern in R [28]1 (retained upon reduction of R [26]1), that converts almost equally into the 10,25 and 15,30 long-side ones, coupled to a rectangular-to-Möbius isomerization (Figure1a, middle and top). It has to be noted that the more rigid triply-linked[28]HCD derivative affords a stable rectangular antiaromatic conformation.20a,30 In contrast, the doubly-linked[28]HCD recovers enough flexibility to incorporate a twist into the πconjugated macrocycle. Knowing the redox interconversion and equilibration pathways, we were able to devise a synthetic strategy to access M [28]2 and M [28]3 separately. Therefore, we first oxidized an equilibrated mixture of M [28]2 and M [28]3 with DDQ and the corresponding R [26]2 and R [26]3 were separated by column chromatography. Their reduction with NaBH 4 gave isolated samples of M [28]2 and M [28]3 with respectively 90:10 and 15:85 diastereomeric ratio (d.r.). Their equilibration proceeds slowly at room temperature, thermodynamic ratio (55:45) being reached upon heating overnight at 60 °C (SI). To the best of our knowledge, M [28]2 and M [28]3 are the first examples of cavity-capped Möbius molecular rings.

  HCD recovers enough flexibility to incorporate a twist into the πconjugated macrocycle. Knowing the redox interconversion and equilibration pathways, we were able to devise a synthetic strategy to access M [28]2 and M [28]3 separately. Therefore, we first oxidized an equilibrated mixture of M [28]2 and M [28]3 with DDQ and the corresponding R [26]2 and R [26]3 were separated by column chromatography. Their reduction with NaBH 4 gave isolated samples of M [28]2 and M [28]3 with respectively 90:10 and 15:85 diastereomeric ratio (d.r.). Their equilibration proceeds slowly at room temperature, thermodynamic ratio (55:45) being reached upon heating overnight at 60 °C (SI). To the best of our knowledge, M [28]2 and M [28]3 are the first examples of cavity-capped Möbius molecular rings.

Figure 3 .

 3 Figure 3. Electronic circular dichroism spectra in CHCl 3 of diastereomerically enriched M [28]2 (blue) and M [28]3 (red) (respectively 90:10 and 15:85 d.r.; intensities scaled according to

  s c r i p t their corresponding UV-vis absorption spectra normalized at their maximum absorbance).

  ) and 15,30 (S p ) bridging patterns to M [28]2 and M [28]3 and to their oxidized forms R [26]2 and R [26]3.Indeed, each bridging pattern can formally generate four diastereomeric Möbius conformations, two M twists and two P twists (Figure1a top and 1c). Molecular modelling of the different possible isomers evidenced that only a single one fits a given planar configuration (SI).

  Figure 4 illustrates the case of the M twist for both 10,25 and 15,30 bridging patterns ( M [28]2 and M [28]3 respectively). While M [28]2 isomers adopt a global expanded and strain-relieved geometry, the M [28]3 ones are more compact and constrained (Figure 4, A and B vs.

Figure 4 .

 4 Figure 4. Optimized geometries of the two M Möbius isomers for (a) M [28]2 and (b) M [28]3 (Avogadro software, UFF parameters; benzyl groups were omitted for modelling. Twisted pyrroles are highlighted in red/yellow and meso-C 6 F 5 substituents adjacent to the twisted pyrroles are highlighted in blue). provide conformational restriction to the Möbius ring while keeping the isomerization processes possible (see also approaches"b" and "c" in Scheme 1). Acetate (L 1 ) and a set of acetylacetonate (acac derivatives: L 2 -L 6 ) ligands were used to probe steric factors around the metal (Scheme 4a). Coordination properties were first assessed by 1 H NMR titration experiments with either Zn(OAc) 2 or Zn(OTf) 2 /L 2 in a CDCl 3 /CD 3 OD (9:1) mixture. M [28]2 and M [28]3 were used in various d.r. ratios (90:10, 55:45 and 15:85) to evaluate the dynamics of the metal complexes (SI). 33 All cases led to the same final mixture of mononuclear Möbius complexes with a 4:1 thermodynamic ratio of (M)-M [28]2•ZnL 1-2 and (P)-M [28]3•ZnL 1-2 respectively (Scheme 4a). Interestingly, the intermediate mixtures of Zn(II) complexes equilibrate rapidly (ca. one hour at 298 K), the most striking example being the rapid S p → R p planar isomerization occurring with the starting M [28]3 enriched sample. These observations highlight the dynamic character of both the topological and planar chiralities of the complexes.

  Scheme 4. (a) Zn(II) coordination with M [28]2/ M [28]3 (Ar = C 6 F 5 ) and the ligands (L n ) used in this study. (b) Optimized geometry with different views of (M)-M [28]2•ZnL 2 (Avogadro software, UFF parameters; benzyl groups were omitted for modelling).

Figure 5 . 1 H

 51 Figure 5. 1 H NMR spectra (CDCl 3 /CD 3 OD 9:1, 298 K, 500 MHz, high field region) of complexes (M)-M [28]2•ZnL n /(P)-M [28]3•ZnL n : (a) to(f), respectively L 1 to L 6 . α L and β L stand for the shielded protons of the ligand substituents. π in stands for the βpyrrolic protons of the pyrrole in. (g) Optimized geometries of L 6 l and L 6 s isomers of (M)-M [28]2•ZnL 6 (Avogadro software, UFF parameters, benzyl groups were omitted for modeling; top views with partial mapping of the Connolly solvent-accessible surface areas, L 6 l and L 6 s in space filling).

Figure 6 .

 6 Figure 6. (left) ECD spectra in 9:1 CHCl 3 /CH 3 OH of equilibrated mixture of M [28]2/ M [28]3 (free base) and of complexes (M)-M [28]2•ZnL n /(P)-M [28]3•ZnL n (L 1 -L 6 ) (ECD spectra obtained from NMR tube solutions and scaled according to their corresponding UV-vis absorption spectra normalized at their maximum absorbance). (right) Plot of the maximum ECD intensity vs. the M/P diastereomeric excess obtained from 1 H NMR integrations, showing a linear relationship (L 2 , L 3 , L 4 , L 6 plotted since quasi-quantitative metalation was achieved, i.e. > 90 %; L 1 and L 5 not plotted since metalation was not complete; L 6 corresponds to L 6 l only).

Figure 7 .

 7 Figure 7. Cartoon of the chiral amplification chain in the totem HCD edifices.

  1 H NMR spectra indicate that this upper-half stereoselectivity for a Möbius twist is above 95 % d.e. for both major and minor complexes [(M) for M [28]2•ZnL and (P) for M [28]3•ZnL]. 37 This is the highest chiral induction value reported to date for a Möbius molecular ring (Scheme 1a,b).

Table 1 .

 1 Diastereomeric ratios for (M)-M

[28]2•ZnL n

  R and R' refers to the acac ligands with in and out indicating their relative orientations in the case of dissymmetrical acac. Proportions between brackets indicate the relative proportion of L 6 s vs. L 6 l.

						/(P)-M [28]3•ZnL n
	and the corresponding CIS (Δδ) of L n .
	L n	R/R' a	d.r. b		Δδ (ppm) c
			M/P	 in	 in	 out	 out
	L 1		80:20 M n.a. e	-3.06	n.a.	n.a.
			P	n.a.	-2.91	n.a.	n.a.
	L 2	Me/Me	78:22 M n.a.	-3.15	-0.84	n.a.
			P	n.a.	-3.01	-0.79	n.a.
	L 3	Et/Et	72:28 M -2.80	-3.06 d	-0.89 d	-0.47
			P -2.77	-2.89 d	-0.83 d	-0.42
	L 4	iPr/iPr	67:33 M -2.55 d	-2.80	-0.88	-0.50 d
			P -2.74 d	-2.64	-0.85	-0.47 d
	L 5	tBu/tBu	60:40 M -2.30	n.a.	n.a.	-0.45
			P -2.26	n.a.	n.a.	-0.43
	L 6 s	Me in /tBu out	82:18 M n.a.	-2.84	n.a.	-0.50
		(5%)	P	n.a.	-2.74	n.a.	-0.47
	L 6 l	tBu in /Me out	62:38 M -2.49	n.a.	-0.74	n.a.
		(95%)	P -2.42	n.a.	-0.70	n.a.

a b M/P d.r. refers to (M)-M [28]2•ZnL n /(P)-M [28]3•ZnL n ratio deduced from 1 H NMR integration (experimental error: ± 3%). c Δδ = δ free -δ bound ; α and β refer to the position relative to the carbonyl function. d Average value of diastereotopic protons. e n.a.: not applicable.

AUTHOR INFORMATION

Corresponding Authors * stephane.legac@univ-rennes1.fr * mickael.menand@sorbonne-universite.fr Author Contributions § These authors contributed equally.

ACKNOWLEDGMENT

We are grateful to the Agence Nationale de la Recherche for financial support (ANR research program PRALLOCAT, ANR-16-CE07-0014).

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Lambert Sicard

Cassandre Quinton

Fabien Lucas

Olivier Jeannin

Joelle Rault-Berthelot

Cyril Poriel

Joëlle Rault-Berthelot

-Carbazolyl Spirobifluorene: Synthesis, Structural, Electrochemical and Photophysical Properties

Keywords: Structural, Electrochemical, and Photophysical Properties

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

9,9'-Spirobifluorene (SBF) is one of the most important building block used in the synthesis of Organic Semi-Conductors (OSCs) for Organic Electronics (OE). [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF][START_REF] Poriel | Structure-property relationship of 4-substitutedspirobifluorenes as hosts for phosphorescent organic light emitting diodes: an overview[END_REF][START_REF] Saragi | Spiro compounds for organic optoelectronics[END_REF][START_REF] Ma | Recent progress in fluorescent blue materials[END_REF][START_REF] Poriel | Dihydroindenofluorene positional isomers[END_REF] In recent years, manipulating the substitution pattern of SBF has appeared as a very efficient tool to tune the electronic properties of SBF-based materials. [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF][START_REF] Poriel | Structure-property relationship of 4-substitutedspirobifluorenes as hosts for phosphorescent organic light emitting diodes: an overview[END_REF][START_REF] Quinton | Electron-rich 4-substituted spirobifluorenes: Toward a new Family of high triplet energy host materials for high-efficiency green and sky blue phosphorescent OLEDs[END_REF] In the SBF fragment, there are four positions available for the substitution of each phenyl units numbered 1 to 4 (Chart 1a). This substitution pattern drives the electronic properties of SBF compounds. The position C2 forms a para-substituted biphenyl and has significantly contributed to the development of highly efficient SBF-based materials for OE (chart 1b), mainly as emitters in blue Organic Light-Emitting Diodes (OLEDs) [START_REF] Saragi | Spiro compounds for organic optoelectronics[END_REF][START_REF] Poriel | Dihydroindenofluorene positional isomers[END_REF][START_REF] Wong | Spiro-configured bifluorenes: Highly efficient emitter for UV organic light-emitting device and host material for red electrophosphorescence[END_REF][START_REF] Xing | Spirobifluorene derivative: a pure blue emitter (CIE y ≈ 0.08) with high efficiency and thermal stability[END_REF][START_REF] Lee | Three-dimensional structures based on the fusion of chrysene and spirobifluorene chromophores for the development of blue OLEDs-high performance in non-doped blue oled[END_REF][START_REF] Zhu | Blue fluorescent emitters: design tactics and applications in organic light-emitting diodes[END_REF][START_REF] Tang | A molecular glass for deep-blue organic light-emitting diodes comprising a 9,9′-spirobifluorene core and peripheral carbazole groups[END_REF][START_REF] Wei | Doubly ortho-linked cis-4,4'-bis(diarylamino)stilbene/fluorene hybrids as efficient non-doped sky-blue fluorescent materials for optoelectronic applications[END_REF] but also as electron [START_REF] Bian | Positional isomerism effect of spirobifluorene and terpyridine moieties of "(A)n-D-(A)n" type electron transport materials for long-lived and highly efficient TADF-PhOLEDs[END_REF] or hole [START_REF] Shen | High Tg blue emitting materials for electroluminescent devices[END_REF] transporters in OLEDs and even more recently as very efficient threedimensional non-fullerene acceptors in organic solar cells. [START_REF] Li | Non-fullerene acceptor engineering with three-dimensional thiophene/selenopheneannulated perylene diimides for high performance polymer solar cells[END_REF][START_REF] Li | A spirobifluorene and diketopyrrolopyrrole moieties based non-fullerene acceptor for efficient and thermally stable polymer solar cells with high open-circuit voltage[END_REF][START_REF] Xia | A spiro-bifluorene based 3D electron acceptor with dicyanovinylene substitution for solution-processed non-fullerene organic solar cells[END_REF][START_REF] Yi | A 9,9'-spirobi[9Hfluorene]-cored perylenediimide derivative and its application in organic solar cells as a nonfullerene acceptor[END_REF][START_REF] Wu | Spiro linkage as an alternative strategy for promising non-fullerene acceptors in organic solar cells[END_REF] The three other positions of the SBF fragment have been far less described to date but appear nevertheless highly promising to build new OSCs. [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF][START_REF] Sicard | C1-Linked spirobifluorene dimers: pure hydrocarbon hosts for highperformance blue phosphorescent OLEDs[END_REF][START_REF] Jiang | Novel oligo-9,9′spirobifluorenes through ortho-linkage as full hydrocarbon host for highly efficient phosphorescent OLEDs[END_REF][START_REF] Cui | Meta-Linked spirobifluorene/phosphine oxide hybrids as host materials for deep blue phosphorescent organic light-emitting diodes[END_REF] The first examples of SBF-based materials substituted at C4 (ortho-substituted biphenyl) [START_REF] Jiang | Novel oligo-9,9′spirobifluorenes through ortho-linkage as full hydrocarbon host for highly efficient phosphorescent OLEDs[END_REF] and at C3 (meta-substituted biphenyl) [START_REF] Cui | Meta-Linked spirobifluorene/phosphine oxide hybrids as host materials for deep blue phosphorescent organic light-emitting diodes[END_REF] have only been described in the last ten years, in 2009 and 2013 respectively. The substitution at C1 (meta biphenyl linkage) has only been A c c e p t e d M a n u s c r i p t reported in 2017. [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF] However, and oppositely to the substitution at C2, the particularity of the substitution at C1, C3 or C4 is the restriction of the electronic coupling between the fluorene unit and the attached substituent. This key characteristic has been advantageously used to design high triplet energy materials (E T >2.7 eV), which are particularly attractive for hosting phosphors in Phosphorescent OLED (PhOLEDs). [START_REF] Poriel | Structure-property relationship of 4-substitutedspirobifluorenes as hosts for phosphorescent organic light emitting diodes: an overview[END_REF][START_REF] Sicard | C1-Linked spirobifluorene dimers: pure hydrocarbon hosts for highperformance blue phosphorescent OLEDs[END_REF] Thanks to the combination of a strong steric hindrance and an electronic decoupling, 1-substituted SBF scaffold seems to be the most efficient regioisomer to construct high E T materials (for example, 1-Ph-SBF (chart 1c) possesses an E T of 2.86 eV). [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF] On the other hand, it is widely known when designing OSCs for organic electronics that the incorporation of electron-withdrawing and/or electron donating units is an efficient strategy to modulate electronic properties with the aim of reaching high-performance devices. However, it has been shown that the impact of these electron-rich (-poor) functional units is very different as a function of the SBF isomer involved. [START_REF] Poriel | Structure-property relationship of 4-substitutedspirobifluorenes as hosts for phosphorescent organic light emitting diodes: an overview[END_REF][START_REF] Quinton | Electron-rich 4-substituted spirobifluorenes: Toward a new Family of high triplet energy host materials for high-efficiency green and sky blue phosphorescent OLEDs[END_REF][START_REF] Thiery | 2-Substituted vs 4-substituted-9,9′-spirobifluorene host materials for green and blue phosphorescent OLEDs: a structure-property relationship study[END_REF][START_REF] Thiery | 4-Pyridyl-9,9′-spirobifluorenes as host materials for green and sky-blue phosphorescent OLEDs[END_REF] Indeed, the electronic coupling between the pending substituent and the fluorene unit depends on the SBF substitution pattern and the resulting steric hindrance and/or nature of the linkage (ortho, meta, para). For example, it has been shown that the modulation of HOMO and LUMO energy levels is very different for 2-and 4-substituted SBFs. [START_REF] Yi | A 9,9'-spirobi[9Hfluorene]-cored perylenediimide derivative and its application in organic solar cells as a nonfullerene acceptor[END_REF][START_REF] Wu | Spiro linkage as an alternative strategy for promising non-fullerene acceptors in organic solar cells[END_REF] Thus, in order to increase the molecular diversity of the emerging family of 1-substituted SBFs and to apprehend its electronic properties, we report herein the first example of a 1substituted SBF incorporating an electron-rich fragment, namely the widely known carbazole unit. The present work reports a detailed structure-property relationship study of 1-carbazolylspirobifluorene (1-Cbz-SBF) in regard of its constituting building blocks (Spirobifluorene, SBF, and carbazole, Cbz) and structurally related analogues (1-phenyl-spirobifluorene, 1-Ph-SBF, 1carbazolyl-fluorene, 1-Cbz-F and N-phenyl carbazole N-Ph-Cbz). We show how some electronic properties are driven by the carbazole unit such as the HOMO energy level whereas others such as the triplet state energy level E T are driven by the fluorene. This feature provides an interesting degree of tuning for this family of OSCs.

Experimental section

Experimental

section, synthetic procedures and compounds characterization, structural/electrochemical/photophysical properties, computational details and NMR studies are provided in the Supporting Information.

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Results and discussions

Synthesis. Despite initially developed in 1951, [START_REF] Kharasch | Derivatives of sulfenic acids. V. 1-fluorenone sulfur compounds[END_REF] 1-halogeno-fluorenones have encountered in the last years intense synthetic investigations. [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF][START_REF] George | Synthesis of 1-substituted fluorenones[END_REF][START_REF] Chen | Synthesis of fluorenones from benzaldehydes and aryl iodides: Dual C-H functionalizations using a transient directing group[END_REF][START_REF] Hedidi | Functionalization of pyridyl ketones using deprotolithiation-in situ zincation[END_REF] These key compounds can be advantageously used in the synthesis of 1-substituted-SBFs as recently shown with 1-Ph-SBF. [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF] In this previous work, a synthetic approach towards 1-iodofluorenone was reported, which has nevertheless the disadvantage of being non-regioselective, also providing 3-iodofluorenone. In the present work, the regioselective one-pot approach towards 1-bromofluorenone 1, introduced in 2017 by Sorensen and coworkers, [START_REF] Chen | Synthesis of fluorenones from benzaldehydes and aryl iodides: Dual C-H functionalizations using a transient directing group[END_REF] has been used (Scheme 1). This reaction is a Pd(II)-catalysed C(sp 2 ) functionalization cascade starting from iodobenzene and 2-bromobenzaldehyde. This one pot approach is not only regioselective but also quick and easy to perform, which should allow to deeply develop 1-fluorenones and consequently 1-substituted SBFs in the future. With 1 in hand, the carbazole fragment was then attached via a copper-catalysed Goldberg reaction (CuO/TMEDA/K 3 PO 4 , 180°C) to provide the corresponding 1-carbazolyl-fluorenone 2 with a high yield of 95%. The synthesis of 1-Cbz-SBF was then carried out through a classical two-step procedure: lithium-iodine exchange of 2-iodobiphenyl with n-butyllithium at low temperature, followed by trapping of the lithiated intermediate with fluorenone 2. The resulting fluorenol 3 (not isolated) is finally involved in an intramolecular aromatic electrophilic substitution (AcOH/HCl) to provide the 1-carbazolyl-spirobifluorene 1-Cbz-SBF with a yield of 68% over the two steps. We should mention that, despite a sterically hindered environment, the C1 position allows to incorporate substituents and even a spirolinked fluorene with good yields. Note that 1-carbazolylfluorenone 2 can be a useful synthetic intermediate to further introduce other spirolinked fragments of interest for OE such as phenylacridine, [START_REF] Romain | D Properties modulation of organic semi-conductors based on a donor-spiro-acceptor (D-spiro-A) molecular design: New host materials for efficient sky-blue PhOLEDs[END_REF] xanthene, [START_REF] Poriel | Incorporation of spiroxanthene units in blue-emitting oligophenylene frameworks: A new molecular design for OLED applications[END_REF] or quinolinophenothiazine. An important and characteristic in 1-substituted SBFs is the particular face to face arrangement of the pending substituent (colored in blue in chart 1c) and the non-substituted cofacial fluorene (colored in green in chart 1c). Such an arrangement appears unique in the SBF family and the possible interactions between these two fragments have therefore never been studied to date. In order to determine the impact of this cofacial arrangement on the hydrogen atoms resonances, complete [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF] H NMR assignations were performed by 2D NMR spectroscopy for 1-Cbz-SBF and compared to relevant model compounds SBF, 1-Ph-SBF and Cbz (Figure 1). All the chemical shifts are gathered in Table S1.

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The Journal of Physical Chemistry A c c e p t e d M a n u s c r i p t In the model compound SBF, the chemical shifts of H1-H4 are respectively detected at 6.68, 7.13, 7.39 and 7.89 ppm (Figure 1). [START_REF] Thirion | Violetto-blue tunable emission of aryl-substituted dispirofluorene-indenofluorene isomers by conformationally-controllable intramolecular excimer formation[END_REF] In 1-Ph-SBF, H1 is detected at 6.70 ppm, at an almost identical chemical shift than that of SBF (= 6.68 ppm, Table S1), this proton being therefore not influenced by the cofacial phenyl. On the other hand, H4 is significantly shielded by ca 0.5 ppm in 1-Ph-SBF compared to its homologue in SBF (7.40 vs 7.89 ppm, Figure 1). One can note that the chemical shift difference between the fluorene hydrogen atoms of SBF and those of 1-Ph-SBF gradually increases from H1 to H4. This shielding effect may be assigned to the increased overlap of the two face-to-face aromatic rings of 1-Ph-SBF. Indeed, transannular  interactions are usually accompanied by high field shifts in 1 H NMR spectra and have been observed in many different systems such as fluorene, [START_REF] Rathore | Synthesis, structure, and evaluation of the effect of multiple stacking on the electron-donor properties of π-stacked polyfluorenes[END_REF][START_REF] Nakano | Synthesis, structure, and photophysical and electrochemical properties of a π-stacked polymer[END_REF][START_REF] Thirion | Tuning the optical properties of aryl-substituted dispirofluorene-indenofluorene isomers through intramolecular excimer formation[END_REF] paracyclophane, [START_REF] Wang | Effect of transannular π-π interaction on emission spectral shift and fluorescence quenching in dithia[3.3]paracyclophane-fluorene copolymers[END_REF] thiophene, [START_REF] Cornelis | Terchromophoric interactions in chiral X-type π-conjugated oligomers: A linear and nonlinear optical study[END_REF] pyrenes, [START_REF] Nandy | Intramolecular π-stacking interaction in a rigid molecular hinge substituted with 1-(pyrenylethynyl) units[END_REF] and dihydroindeno[2,1a]fluorene. [START_REF] Poriel | Dihydroindenofluorene positional isomers[END_REF][START_REF] Thirion | Violetto-blue tunable emission of aryl-substituted dispirofluorene-indenofluorene isomers by conformationally-controllable intramolecular excimer formation[END_REF][START_REF] Thirion | Tuning the optical properties of aryl-substituted dispirofluorene-indenofluorene isomers through intramolecular excimer formation[END_REF][START_REF] Poriel | Blue emitting 3 π-2 Spiro terfluorene-indenofluorene isomers: A structure-properties relationship study[END_REF] In the present 1-Cbz-SBF, the shielding effect is even accentuated compared to 1-Ph-SBF. Indeed, H1-H4 are respectively detected at 6.65, 6.79, 6.90 and 6.80 ppm, the latter being impressively shielded by more than 1 ppm compared to SBF (= 7.89 ppm). Thus, due to the inclination of the carbazole unit (see X-ray structure in Figure 2), H4 is the most influenced by the anisotropy cone and hence the most shielded. As a function of the substituent grafted at C1, the chemical shifts of the cofacial fluorene (and hence the strength of the  interactions) can be therefore modulated. This is an interesting characteristic of 1-substituted SBFs. The hydrogen atoms of the carbazole unit are also strongly affected by the cofacial arrangement. Indeed, the comparison of the chemical shifts of the hydrogen atoms of the carbazole unit in 1-Cbz-SBF and in Cbz clearly leads to the same conclusions. Thus, hydrogen atoms H5-H8 in Cbz are respectively

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The Journal of Physical Chemistry A c c e p t e d M a n u s c r i p t detected at 7.42, 7.42, 7.21 and 8.06 ppm and at 6.20, 6.83, 6.94 and 7.75 ppm in 1-Cbz-SBF. In this case, the most influenced hydrogen is H5 in accordance with the inclination of the carbazole unit.

Structural properties. Single crystals of 1-Cbz-SBF were grown from CHCl 3 /MeOH. The first and evident structural particularity of 1-Cbz-SBF is the cofacial arrangement between the carbazole and the cofacial fluorene. Thanks to the substitution at C1, one can note a very high dihedral angle of 84.1° between the mean plane of the substituted fluorene and that of the carbazole. This large dihedral angle is at the origin of the electronic decoupling noticed below in the physico-chemical properties. In addition, one can note that this angle is higher than that reported for 1-Ph-SBF (75.4°) [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF] and highlights the importance of the substituent itself and especially its size on the structural characteristics. As previously shown for 4-substituted SBFs, [START_REF] Poriel | Structure-property relationship of 4-substitutedspirobifluorenes as hosts for phosphorescent organic light emitting diodes: an overview[END_REF][START_REF] Jiang | Novel oligo-9,9′spirobifluorenes through ortho-linkage as full hydrocarbon host for highly efficient phosphorescent OLEDs[END_REF] this dihedral angle can be therefore used to modulate the electronic coupling between the fluorene and the pending substituent. The molecular structure of 1-Cbz-SBF displays many short intramolecular distances between the two cofacial fragments (Figure 2a andFigures S3-S7). Some C/C distances are shorter than the sum of their van der Waals radii (3.4 Å) [START_REF] Bondi | van der Waals volumes and radii[END_REF] and translate a sterically hindered environment. The shortest distance is measured between the two carbon atoms C1 of the cofacial fluorene and C5 of the carbazole (3.27 Å). In order to precisely evaluate the strength of these interactions between the carbazole and the cofacial fluorene, three structural parameters have been evaluated for the four cofacial phenyl rings, namely ring-centroid/ringcentroid distance d C-C , vertical displacements d 1 and d 2 and slippage angles  1 and  2 (Figure 2d and Figure S1 for definitions and calculation methods). As discussed by Janiak and coworkers, these three parameters can reflect the strength of the interactions between two phenyl rings. [START_REF] Yang | From model compounds to protein binding: syntheses, characterizations and fluorescence studies of [RuII(bipy)(terpy)L]2+ complexes (bipy = 2,2′-bipyridine; terpy = 2,2′:6′,2″-terpyridine; L = imidazole, pyrazole and derivatives, cytochrome c) Dalton Trans[END_REF][START_REF] Craven | Dimethyl-2, 2′-bipyridine)(IDA)copper(II) and structural comparison with aqua(IDA)(1, 10-phenanthroline)copper(II) (IDA = iminodiacetato) Anorg[END_REF][START_REF] Janiak | A critical account on π-π stacking in metal complexes with aromatic nitrogencontaining ligandsJ[END_REF][START_REF] Banerjee | Polymethylene spacer regulated structural divergence in cadmium complexes: Unusual trigonal prismatic and severely distorted octahedral coordination[END_REF] In 1-Cbz-SBF, the ring-centroid/ring-centroid distances between two cofacial phenyl rings of the carbazole and fluorene units are estimated at ca. 3.78 Å and 3.84 Å (Figure S2 and Table S2). The smallest vertical displacements between two phenyl rings are very short (0.65 Å and 0.39 Å, Table S2) and their corresponding ring slippage angles are very low (9.9° and 5.9°, Table S2). In the light of Janiak's works such values of ring-centroid/ring-centroid distance (<3.8 Å), vertical displacements (<1.5 Å), and ring slippage angles (<25°) indicate strong interactions between carbazole and fluorene.

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The Journal of Physical Chemistry In oxidation, 1-Cbz-SBF presents two distinct waves with maxima at 1.17 and 1.87 V (Figure 3a, red line) followed by an additional oxidation at a potential higher than 2.25 V (potential limit reached). Thus, the first oxidation of 1-Cbz-SBF occurs at almost identical potential value than that of Cbz (1.16 V, Figure 3c, black line) translating an electron transfer centered on the carbazole unit. This is in accordance with the HOMO distribution of 1-Cbz-SBF (Figure 7) and of the spin density of its cation-radical (Figure 3e) both localized on the carbazole unit. From the onset potential of the first oxidation wave, 1.07 V, the HOMO energy level of 1-Cbz-SBF is evaluated at -5.47 eV. As HOMO level of 1-Cbz-SBF is distributed on the carbazole unit, its energy is higher than that of SBF (-5.95 eV, HOMO localized on fluorene) and almost identical to that of Cbz (-5.50 eV).

At this stage, it is interesting to compare the oxidation of 1-Cbz-SBF to that of another model compound, N-phenyl-Carbazole (N-Ph-Cbz), [START_REF] Quinton | Electron-rich 4-substituted spirobifluorenes: Toward a new Family of high triplet energy host materials for high-efficiency green and sky blue phosphorescent OLEDs[END_REF] the carbazole having a phenyl group substituting the nitrogen atom (See structure in chart 1a). Thus, as presented Figure 3d (green line), the first oxidation of N-Ph-Cbz is shifted to more anodic values (1.30 V) compared to that of both 1-Cbz-SBF and Cbz (1.17 V), all CVs being recorded in identical conditions (CH 2 Cl 2 /[Bu 4 N][PF 6 ] 0.2 M). The potential difference between Cbz and N-Ph-Cbz can be explained by the negative inductive effect of the phenyl ring which leads to a carbazole unit more difficult to oxidize in N-Ph-Cbz than in Cbz. It is indeed known that the substituent borne by the nitrogen atom has a consequent impact on the first oxidation potential of the carbazole core. [START_REF] Ambrose | Anodic oxidation pathways of carbazoles I . carbazole and N-substituted derivatives[END_REF] For example with a

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The Journal of Physical Chemistry A c c e p t e d M a n u s c r i p t methyl or an ethyl group borne by the nitrogen atom, the oxidation of the carbazole unit is shifted to lower potentials. [START_REF] Ambrose | Anodic oxidation pathways of carbazoles I . carbazole and N-substituted derivatives[END_REF] It should be specified that, with high dihedral angles of ca 50° reported for different molecules possessing the N-phenyl-carbazole fragment, [START_REF] Ding | Novel spiro-based host materials for application in blue and white phosphorescent organic light-emitting diodes[END_REF][START_REF] Tang | Tunable photoluminescent materials based on two phenylcarbazole-based dimers through the substituent groups[END_REF] there is no or only a very weak electronic coupling between the carbazole and the phenyl (no conjugation extension). As 1-Cbz-SBF is also built with a N-phenyl-carbazole fragment, the anodic shift between 1-Cbz-SBF and N-Ph-Cbz cannot be explained by simple inductive effects and another effect should be invoked. We have shown above that the position of the carbazole at C1 of the SBF core induces a strong interaction with the cofacial fluorene. It is known in literature that the oxidation of -stacked systems is more facile than their non-stacked analogues. [START_REF] Rathore | Synthesis, structure, and evaluation of the effect of multiple stacking on the electron-donor properties of π-stacked polyfluorenes[END_REF][START_REF] Chebny | Molecular Actuator: Redox-Controlled Clam-Like Motion in a Bichromophoric Electron Donor Org[END_REF] In the case of fluorenes based systems, several examples have been provided by Rathore and coworkers [START_REF] Rathore | Synthesis, structure, and evaluation of the effect of multiple stacking on the electron-donor properties of π-stacked polyfluorenes[END_REF][START_REF] Qi | Electrochemistry and electrogenerated chemiluminescence of π-stacked poly(fluorenemethylene) oligomers. Multiple, interacting electron transfers[END_REF] or by us. [START_REF] Thirion | Violetto-blue tunable emission of aryl-substituted dispirofluorene-indenofluorene isomers by conformationally-controllable intramolecular excimer formation[END_REF][START_REF] Thirion | Tuning the optical properties of aryl-substituted dispirofluorene-indenofluorene isomers through intramolecular excimer formation[END_REF][START_REF] Poriel | Modulation of the electronic properties of 3π-2spiro compounds derived from bridged oligophenylenes: A structure-property relationship[END_REF] In the present case, the strong interaction between the cofacial fluorene and the carbazole fragment can be responsible of the shift observed between 1-Cbz-SBF and N-Ph-Cbz. To confirm this hypothesis, another model compound, 1-carbazolylfluorene 1-Cbz-F, has been synthesized and studied (See structure in chart 1a). In this molecule, there is the same N-carbazole-1-fluorene fragment but without any cofacial fluorene. Studying this compound should highlight the through-space interaction in 1-Cbz-SBF. The first oxidation wave of 1-Cbz-F displays a maximum at 1.3 V (Figure 3d, sky blue line) shifted by 0.13 V compared to that of 1-Cbz-SBF (1.17 V). This shift clearly confirms that the  intramolecular interactions between the carbazole and its facing fluorene are at the origin of the shift to less positive values of the first oxidation potential in 1-Cbz-SBF (1.17 V). This strategy could be advantageously used to finely tune the first oxidation potential (and hence the HOMO energy level) of 1-substituted SBF based materials. Note that the first oxidation waves of 1-Cbz-F and N-Ph-SBF are both detected at a very similar potential, showing the similar impact of a fluorene or a phenyl ring on the oxidation of these compounds.

For both 1-Cbz-SBF and Cbz, the first oxidation is irreversible showing the high reactivity of the charged species (Figure 3c). The weak reduction waves recorded at the reverse scan for both compounds may be assigned to the reduction of a small amount of oligomers precipitated on the electrode surface during the former oxidation. However, as no new oxidation wave appears on the CV upon successive cycles, one may conclude that these species are soluble in solution in these experimental conditions.

The second oxidation of 1-Cbz-SBF, with its maximum at 1.87 V, appears more difficult to assign with confidence. As shown by the differential pulse voltammetry (Figure 3b), three electrons are involved in this second oxidation process. As this oxidation occurs in the potential range of the two successive oxidation waves of SBF (1.67 & 1.86 V, Figure 3a) two of these electron may be therefore ascribed to the oxidation of the SBF unit. This is in agreement with molecular modelling which shows that the electron density of 1-Cbz-SBF bis radical cation is spread out on both cofacial fluorene and carbazole (Figure 3f), indicating that the second oxidation involves the cofacial fluorene leading to 1-Cbz .+ -SBF .+ . The following oxidation processes are nevertheless difficult to modelize because of polymerization.

Another interesting feature related to the oxidation of 1-Cbz-SBF is the electrodeposition process observed during recurrent cycles when cycling up to or over the second oxidation potential. It is indeed widely known that SBF derivatives can be polymerized by anodic oxidation leading to 3D materials. [START_REF] Rault-Berthelot | Anodic oxidation of 9,9′-spirobifluorene in CH 2 Cl 2 +0.2 M Bu 4 NBF 4 . Electrochemical behaviour of the derived oxidation product[END_REF][START_REF] Poriel | Organic crosslinked electropolymers as supported oxidation catalysts: Poly((tetrakis(9,9'spirobifluorenyl)porphyrin)manganese) films[END_REF][START_REF] Ferrand | Asymmetric heterogeneous carbene transfer catalyzed by optically active ruthenium spirobifluorenylporphyrin polymers[END_REF][START_REF] Poriel | Anodic oxidation and physicochemical properties of various porphyrin-fluorenes orspirobifluorenes: Synthesis of new polymers for heterogeneous catalytic reactions[END_REF][START_REF] Poriel | Synthesis and stereochemical studies of di and tetra 9,9′-spirobifluorene porphyrins: New building blocks for catalytic material[END_REF] However, anodic polymerization of 1-substituted SBFs has never been reported to date. As presented Figure 4-left, one observes the appearance and the regular growth of three new redox waves centered at 0.90, 1.20 and 1.60 V when cycling up to 1.80 V or at 1.00, 1.30 and 1.80 V when cycling up to 2.25 V.

Depending on the anodic limit, the intensity of the new waves increases in a different manner. When cycling to 1.80 V, the currents measured after ten cycles are ca 2 to 3 times higher than those measured at the first cycle. However, when cycling to 2.25 V, the three waves increase more intensively (ca 7 times). This feature can be assigned to the number of positions involved in the electrodeposition process (mainly carbazole units in the first case and both carbazole and fluorene units in the second). 0.60 and 2.13 V. The corresponding HOMO of the deposit is evaluated at -5.20 eV, 0.27 eV higher than that of its corresponding monomer 1-Cbz-SBF. When the deposition is performed at high potential (up to 2.25 V), the CV is less defined (higher currents are nevertheless reached) displaying only three distinct redox waves weakly shifted compared to those of the previously described deposit. One can note that the HOMO of the polymer is slightly lower, ie -5.27 eV, characteristic of a shorter conjugation length. For all the deposits, the range of stability is wide showing a high electrochemical stability between 0.50 and 2.40 V.

The electrochemical behavior of deposits prepared during anodic oxidation is presented Figure 5 and compared to that of Poly(Cbz) and Poly(SBF). The deposit obtained from the oxidation of Cbz possess the highest HOMO (E onset ox : 0.57 V, HOMO: -4.97 eV) whereas that of SBF the lowest (E onset ox : 1.10 V, HOMO: -5.50 eV), translating very different conjugation pathways between the polymers (Figure 5). In between, begins the p-doping process of the deposit derived from 1-Cbz-SBF (E onset ox : 0.87 V, HOMO: -5.27 eV). Such a behavior confirms that carboncarbon couplings leading to the electrodeposition process of 1-Cbz-SBF involve both the carbazolyl and the fluorenyl units. The nature of the deposit is however difficult to precisely determinate because at least six carbon atoms may be involved in bonds formation (2 carbon atoms in para positions of the nitrogen atom for carbazole and 2 carbon atoms in para positions of the biphenyl linkage for both fluorenes).

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 ] 0.2 M) free of any electroactive species, platinum disk working electrode modified by: poly(1-Cbz-SBF) (red line), anodic limit 2.4 V, poly(Cbz) (black line), anodic limit: 2.13 V or poly(SBF) (blue line), anodic limit 1.76 V.

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The Journal of Physical Chemistry a: from electrochemical data, b: vs SCE, c: in cyclohexane, d: referenced to quinine sulfate, e:  exc =310 nm, f: 2-MeTHF at 77 K Optical properties. The UV-vis absorption spectra of 1-Cbz-SBF and of the two building units SBF and Cbz recorded in cyclohexane are presented Figure 6 (left). The absorption spectrum of 1-Cbz-SBF is well structured and displays three main bands at 338, 323 and 310 nm. The band at 338 nm (modelized at th = 287 nm, see Figure 7) is red shifted by 6 nm compared to that of Cbz (exp = 332 nm, modelized at th = 285 nm) and possesses a higher molar absorption coefficient (4.3 ×10 3 L.mol -1 .cm -1 for 1-Cbz-SBF vs 3×10 3 L.mol -1 .cm -1 for Cbz, Figure 6). In 1-Cbz-SBF, this band is due to a transition involving two contributions: HOMO→LUMO+3 (56%) and HOMO→LUMO+1 (28%). For the first contribution (HOMO→LUMO+3), molecular orbitals are both localized on the carbazole and for the second contribution (HOMO→LUMO+1) the HOMO is spread out on the carbazole whereas the LUMO+1 is located on the cofacial fluorene (Figure 7). The HOMO→LUMO transition of Cbz (th=285 nm) involves orbitals with the same shape than the first contribution of the first transition in 1-Cbz-SBF. Therefore, this first contribution can be fully assigned to the carbazole fragment. The second contribution (HOMO→LUMO+1) involving a through space transfer from carbazole to fluorene can explain both the red shift and the higher molar absorption coefficient of 1-Cbz-SBF compared to Cbz. The second band of 1-Cbz-SBF is

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The Journal of Physical Chemistry A c c e p t e d M a n u s c r i p t experimentally found at 324 nm (th=264 nm) and possesses a molar absorption coefficient of 3.5 ×10 3 L.mol -1 .cm -1 . This band is similar to that found in Cbz at 319 nm (=3.5 ×10 3 L.mol -1 .cm -1 , th=264 nm) and is mainly due to a  transition involving orbitals localized on the carbazole fragment (major contributions: HOMO-1→LUMO+3 for 1-Cbz-SBF and HOMO-1→LUMO for Cbz). The third band is experimentally found at 311 nm (th=271 nm) and possesses a molar absorption coefficient of 1.2 ×10 4 L.mol -1 .cm -1 . This band, which is also found in SBF at 308 nm (=1.4 ×10 3 L.mol -1 .cm -1 , th=268 nm) [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF] has been assigned to a  transition having two contributions: HOMO-2→LUMO (46%) and HOMO-3→LUMO (30%), the three molecular orbitals involved being localized on the SBF fragment. One can indeed note that there is a small red shift of 3 nm between 1-Cbz-SBF and SBF (which is also found in theoretical calculations: =271 and 268 nm respectively) which can be assigned to an alteration of the spiroconjugation (conjugation between the two spiro connected fluorenes) [START_REF] Geuenich | Anisotropy of the induced current density (ACID), a general method to quantify and visualize electronic delocalization[END_REF][62][START_REF] Hintschich | The complex excited-state behavior of a polyspirobifluorene derivative: The role of spiroconjugation and mixed charge transfer character on excited-state stabilization and radiative lifetime[END_REF][START_REF] Lukes | Theoretical investigation of the structure and the electron-vibrational dynamics of 9,9′-spirobifluorene[END_REF][START_REF] Kowada | Synthesis, structures, and optical properties of heteroarene-fused dispiro compounds[END_REF] probably due to the particular cofacial arrangement.

In the light of TD-DFT analyses, it is finally important to mention that the HOMO→LUMO contribution is involved in two transitions (th=272 and 271 nm) having small oscillator strengths (0.0009 and 0.008). Indeed, as exposed in the electrochemical part, the position 1 induces a conjugation breaking between the fluorene and the carbazole leading to the complete spatial separation of HOMO (spread out on the carbazole) and LUMO (spread out on the substituted fluorene). This HOMO→LUMO transition is hence strongly disfavored in accordance with a through space electron transfer. [START_REF] Romain | D Properties modulation of organic semi-conductors based on a donor-spiro-acceptor (D-spiro-A) molecular design: New host materials for efficient sky-blue PhOLEDs[END_REF][START_REF] Pan | Through-space charge-transfer emitting biphenyls containing a boryl and an amino group at the o,o′-positions[END_REF][START_REF] Romain | Donor/Acceptor dihydroindeno[1,2-a]fluorene and dihydroindeno[2,1-b]fluorene: Towards new families of organic semiconductors[END_REF] To conclude, the absorption spectrum of 1-Cbz-SBF displays the characteristics of each building block, ie SBF and Cbz but the red shift of the low energy band translates the interactions between the two cofacial units. The fluorescence spectrum of 1-Cbz-SBF presents two maxima at 344 and 357 nm (Figure 6 right). This violet emission is significantly red shifted compared to both SBF and Cbz showing the impact of the cofacial arrangement on the emission properties. Thus, despite a -conjugation disruption between the carbazole and SBF fragments, the intramolecular interaction of 1-Cbz-SBF induces a red shift of the fluorescence spectrum (as commonly observed when there is an electronic coupling between two -systems) compared to its two constituted fragments. The superposition of the optimized geometries of ground state S 0 and first singlet excited state S 1 (obtained by TD-DFT b3lyp using 6-31g(d) basis set, see SI) shows that there is only a slight difference between these two states. This allows to conclude that the cofacial arrangement in 1-Cbz-SBF has a key role in its fluorescence, by avoiding strong molecular rearrangements between S 0 and S 1 (Figure 7-Right).

We also suggest that the red shift in fluorescence arises from the cofacial arrangement (and not from a more intense electronic coupling in S 1 ) since the angles between the carbazole and the substituted fluorene planes are the same in S 0 and S 1 (90°). The structured fluorescence spectrum and the small Stokes shift (7 nm) of 1-Cbz-SBF can also be explained by this very weak reorganization between S 0 and S 1 . This is a different behavior to those previously reported for 2and 4-substituted SBFs but similar to 1-Ph-SBF and therefore a characteristic of 1-substituted SBFs family. [START_REF] Quinton | Electron-rich 4-substituted spirobifluorenes: Toward a new Family of high triplet energy host materials for high-efficiency green and sky blue phosphorescent OLEDs[END_REF][START_REF] Thiery | 2-Substituted vs 4-substituted-9,9′-spirobifluorene host materials for green and blue phosphorescent OLEDs: a structure-property relationship study[END_REF] The fluorescence decays were successfully fitted by single exponentials (See SI), indicating a unique radiative pathway from S 1 to S 0 . The lifetime of 1-Cbz-SBF ( exc = 310 nm, Figure S16) is recorded at 8.1 ns, longer than the lifetimes of SBF (4.6 ns) [START_REF] Sicard | Spirobifluorene regioisomerism[END_REF] and Cbz (7.7 ns, Figure S17), showing once again that the fluorescence of 1-Cbz-SBF is not the superposition of the emission of Cbz and SBF but comes from a single fluorophore. Finally, the quantum yield  of 1-Cbz-SBF was calculated in solution at 0.39 and is identical to that of SBF (0.40) 1 and higher than the one of Cbz (0.25). As the non-radiative constant of 1-Cbz-SBF (k nr = 7.5×10 7 s -1 ) is smaller than those of the fragments SBF and Cbz (resp. k nr = 13.0×10 7 s -1 and 9.7×10 7 s -1 ), one can conclude that the internal conversion processes are limited for 1-Cbz-SBF compared to both SBF and Cbz.

At this stage, it seems relevant to compare the photophysical data of 1-Cbz-SBF to those of 1-Ph-SBF in order to analyze the impact of the C1-subsituted SBF scaffold on the photophysical data.

The first difference is linked to the quantum yield, which is around 2/3 lower for 1-Cbz-SBF (0.39) than for 1-Ph-SBF (0.60). As the non-radiative constants are almost identical for both 1-Cbz-SBF and 1-Ph-SBF 1 (k nr= 7.5×10 7 s -1 and 7.2×10 7 s -1 respectively), one can conclude that the efficiency of the internal conversion processes is similar in the two molecules. Therefore, the lower quantum of 1-Cbz-SBF finds its origin in the radiative constant k r which is much lower for 1-Cbz-SBF (4.8×10 7 s -1 ) compared to that of 1-Ph-SBF (1.22×10 8 s -1 ). The loss of quantum yield of 1-Cbz-SBF is therefore due to a lower electronic transition moment. From a molecular point of view, this feature can be assigned to the carbazole itself and not to the 1-substituted-SBF scaffold (in other words the substitution pattern) as both SBF and 1-Ph-SBF possesses almost identical k r /k nr . Finally, at 77 K, the emission spectrum of 1-Cbz-SBF presents a phosphorescence contribution, with a first band centered at 438 nm followed by two other at 467 and 501 nm leading to a very high E T of 2.84 eV (Figure 8, left). An important feature needs to be stressed out. Indeed, the E T of 1-Cbz-SBF is almost identical to that of SBF evaluated at 2.88 eV 1 (and smaller than that of Cbz, 3.05 eV, measured and determined in identical conditions, Figure S18) highlighting that the pendant substituent has a very weak influence on the T 1 state being hence different to that of S 1 (red shift for 1-Cbz-SBF compared to SBF observed both in absorption and in emission, see above). The high E T of 1-Cbz-SBF can be explained by the localization of the triplet exciton, which is exclusively spread out on the substituted fluorene with no contribution of the pendant substituent nor its cofacial fluorene (Figure 8, middle). Therefore, the position C1 which combines an electronic decoupling thanks to the meta linkage and a strong steric hindrance seems to be an ideal position to keep a very high E T . As the potential applications of C1-linked SBFs are in the field of host materials for PhOLEDs, this feature appears promising. In addition, superposition of optimized geometries of the ground state S 0 and the first triplet excited state T 1 shows that the high rigidity of the 1-SBF scaffold strongly prevents reorganization at the first triplet excited state. As for S 0 , the carbazole-fluorene dihedral angle is very high (90°) at T 1 maintaining the conjugation breaking between the two fragments. The E T decrease of 1-Cbz-SBF compared to SBF can then be assigned to the cofacial arrangement of the 1-substituted SBF scaffold. Radiative deactivation of the triplet state of 1-Cbz-SBF appears to be very slow under these experimental conditions: 5.0 s. This phosphorescence lifetime is slightly lower compare to that of SBF previously reported, 5.3 s 1 (and smaller than that of Cbz, 7.4 s) in accordance with a triplet exciton localized on the fluorene. This highlights that the substitution at C1 only slightly modifies the phosphorescent lifetime of the T 1 state.

Conclusions

The present work reports the synthesis and the study of an example of a new emerging family of spirobifluorene based OSCs, namely 1-substituted spirobifluorenes. Thanks to a structure-property relationship study of 1-carbazolyl-spirobifluorene 1-Cbz-SBF with its building blocks (SBF and Cbz) and structurally related compounds (N-Ph-Cbz and 1-Ph-SBF) we show that i) there is a complete conjugation breaking between the SBF fragment and the pending substituent at C1, and ii) there is an intramolecular interaction between the pending substituent at C1 and the cofacial fluorene (both in solution and in the solid state). The conjugation disruption allows to tune the HOMO energy level of 1-Cbz-SBF. Indeed, the electronic density is mainly spread out on the pending carbazole leading to a strong increase (by ca 0.5 eV) of the HOMO energy of 1-Cbz-SBF, -5.47 eV, compared to SBF, -5.96 eV. Interestingly, the E T of 1-Cbz-SBF is nevertheless maintained at a very high value (2.84 eV) almost identical to that of SBF, 2.88 eV. Thus, some electronic properties are driven by the pending carbazole whereas others by the SBF fragment. Furthermore, the strong carbazole/fluorene through space interaction modifies some electronic properties (red shift of the absorption and fluorescence spectra), keeping other almost unaltered (phosphorescence). This - interaction between the C1-linked substituent and the cofacial fluorene is therefore an interesting tool to modulate the electronic properties of SBF based materials. In the light of the development of new generations of SBF regioisomers in the last ten years, we believe that 1-substituted SBFs constitute an interesting platform for future organic materials in OE, notably as hosts for PhOLEDs. We are currently working in this direction.

There are no conflicts to declare.

A c c e

  p t e d M a n u s c r i p t Chart 1. a) Model compounds used in this study and numbering of the substitution position on one phenyl of the SBF unit, b) Examples of SBF positional isomers incorporating electron rich fragments: 2-substituted SBFs, 25-28 3-substituted SBFs 29-30 and 4-substituted SBFs. 6, 26-27 c) 1 substituted SBFs: 1-Cbz-SBF and 1

Figure 1 . 1 H

 11 Figure 1. 1 H NMR spectra in CD 2 Cl 2 of 1-Ph-SBF, Cbz, SBF and 1-Cbz-SBF (From top to bottom respectively).

  A c c e p t e d M a n u s c r i p t

Figure 2 .

 2 Figure 2. Top) Molecular structure of 1-Cbz-SBF from X-ray diffraction (two different views)Bottom) Representation of the crystallographic parameters to evaluate the strength of  interactions as described by Janiak and coworkers.[START_REF] Yang | From model compounds to protein binding: syntheses, characterizations and fluorescence studies of [RuII(bipy)(terpy)L]2+ complexes (bipy = 2,2′-bipyridine; terpy = 2,2′:6′,2″-terpyridine; L = imidazole, pyrazole and derivatives, cytochrome c) Dalton Trans[END_REF][START_REF] Craven | Dimethyl-2, 2′-bipyridine)(IDA)copper(II) and structural comparison with aqua(IDA)(1, 10-phenanthroline)copper(II) (IDA = iminodiacetato) Anorg[END_REF][START_REF] Janiak | A critical account on π-π stacking in metal complexes with aromatic nitrogencontaining ligandsJ[END_REF][START_REF] Banerjee | Polymethylene spacer regulated structural divergence in cadmium complexes: Unusual trigonal prismatic and severely distorted octahedral coordination[END_REF]

Figure 3 .

 3 Figure 3. a) CVs recorded at 100 mV s -1 (potential range of 0.2/2.2 V) in CH 2 Cl 2 /[Bu 4 N][PF 6 ] 0.2 M in the presence of 1-Cbz-SBF (red line), SBF (blue line), and Cbz (black line). b) Differential pulse voltammetry of 1-Cbz-SBF. c) CVs recorded at 100 mV s -1 (potential range of -0.2/1.45 V) in CH 2 Cl 2 /[ Bu 4 N][PF 6 ] 0.2 M in the presence of 1-Cbz-SBF (red line) and Cbz (black line). d) CVs recorded at 100 mV s -1 (potential range of 0.2/2.25 V) in CH 2 Cl 2 /[Bu 4 N][PF 6 ] 0.2 M in the presence of 1-Cbz-SBF (red line), N-Ph-Cbz (green line) and 1-Cbz-F (sky blue line). Intensity is normalized at E ox 1 for all the compounds. Spin densities of (e) the cation radical and the bis radical cation (f) of 1-Cbz-SBF (isovalue = 0.004, cam-b3lyp/6-31g(d)).

Figure 4 .

 4 Figure 4. Left: CV (100 mV s -1 , CH 2 Cl 2 /[Bu 4 N][PF 6 ] 0.2 M) in the presence of 1-Cbz-SBF, 2.7 10 -3 M, potential range of the ten successive cycles : 0.2-1.8 V (top) or 0.2-2.25 V (bottom), platinum disk working electrode. Right: CV (100 mV s -1 , CH 2 Cl 2 /[Bu 4 N][PF 6 ] 0.2 M) free of any electroactive species, potential limit of the three successive cycles: 0.2-2.13 V (top) or 0.2-2.40 V (bottom), platinum disk working electrode modified by the deposits obtained along the ten cycles in presence of 1-Cbz-SBF.

Figure 5 .

 5 Figure 5. CV (100 mV s -1 , CH 2 Cl 2 /[Bu 4 N][PF 6] 0.2 M) free of any electroactive species, platinum disk working electrode modified by: poly(1-Cbz-SBF) (red line), anodic limit 2.4 V, poly(Cbz) (black line), anodic limit: 2.13 V or poly(SBF) (blue line), anodic limit 1.76 V.

Figure 6 .

 6 Figure 6. Absorption spectra (left) and normalized emission spectra (right) of 1-Cbz-SBF (red line), SBF (blue line) and Cbz (black line) in cyclohexane at room temperature.

Figure 7 .

 7 Figure 7. Left: Representation of the energy levels and the main molecular orbitals involved in the electronic transitions of 1-Cbz-SBF obtained by TD-DFT CAM-B3LYP/6-311+G(d,p), shown with an isovalue of 0.04 [e bohr -3 ] 1/2 . For clarity purposes, only the major contributions (>10%) of each transition is shown (see SI for details). Right: Superposition of optimized geometries of S 0 (ground state, light blue) and S 1 (first singlet excited state, red).

Figure 8 .

 8 Figure 8. Left: emission spectrum of 1-Cbz-SBF at 77 K (2-Me-THF, λexc = 300 nm). Middle: Spin density of triplet of 1-Cbz-SBF (isovalue= 0.004). Right: Superposition of optimized geometries: S 0 (ground state, light blue) and T 1 (first triplet excited state, blue)

  [START_REF] Thiery | Modulation of the physicochemical properties of donor-spiro-acceptor derivatives through donor unit planarisation: Phenylacridine versus indoloacridine-new hosts for green and blue phosphorescent organic light-emitting diodes (PhOLEDs)[END_REF]

Table 1 .

 1 Selected electronic data of 1

-Cbz-SBF, SBF and Cbz

			1-Cbz-SBF	SBF 1	Cbz
	HOMO (eV) a		-5.47	-5.95	-5.50
	Potential	of	1.17, 1.87, 2.25 1.67, 1.86	1.18
	peak oxidation
	(V) b
	 abs (nm) c		310, 323, 338	308	319, 332
	 em (nm) c		345, 357	310, 323,	332, 346,
				340 (sh)	361 (sh)
	 c,d		0.39	0.40	0.25
	 (ns) c,e		8.1	4.6	7.7
	k nr (s -1 ) c		7.5×10 7	13.0×10 7	9.7×10 7
	k r (s -1 ) c		4.8×10 7	8.7×10 7	3.3×10 7
	 T (nm) f		438, 467, 501	431, 463, 490,	406
				500
	E T (eV) f		2.84	2.88	3.05
	 (s) f		5.0	5.3	7.4

Acknowledgments

The authors would like to thank the CINES (Montpellier N° 2018-A0040805032) for computing time, S. Thiery for preliminary works on 1-carbazolyl-fluorenone and the ANR (n°14-CE05-0024) for a PhD grant (LJS) and a post-doctoral position (CQ) and for financial support.

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Supporting Information Available

General Experimental Methods, synthesis and characterization, details on NMR, electrochemical and photophysical studies, X-Ray and theoretical modelling. Copy of NMR spectra.

Abbreviations

PhOLED, phosphorescent organic light-semitting diode; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; SBF, 9,9'-spirobifluorene; OSC, organic semi-conductor; OE, Organic Electronics; OLED, organic light emitting diode; E T , triplet state energy; TMEDA, N,N,N′,N′-tetramethylethylenediamine; NMR, nuclear magnetic resonance; CV, cyclic voltammetry; DPV, differential pulse voltammetry; TD-DFT, time-dependent density functional theory; THF, tetrahydrofurane; HFlP, Hexafluoro-2-propanol; TFA, trifluoroacetic acid; SCE, saturated calomel electrode.

Conflicts of interest

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