# Donor-Acceptor Nanohoops: Impact of the Ratio and Arrangement of the Fluorenone and Carbazole Moieties

Clément Brouillac, Elodie Dureau, Olivier Jeannin, Joëlle Rault-Berthelot, Cyril Poriel, Cassandre Quinton

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Clément Brouillac, Elodie Dureau, Olivier Jeannin, Joëlle Rault-Berthelot, Cyril Poriel, et al.. Donor-Acceptor Nanohoops: Impact of the Ratio and Arrangement of the Fluorenone and Carbazole Moieties. Journal of the American Chemical Society, 2025, 147 (13), pp.11267-11276. 10.1021/jacs.4c18293. hal-05023834

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# Donor-Acceptor Nanohoops: Impact of the Ratio and Arrangement of Fluorenone and Carbazole Moieties.

Clément Brouillac, Elodie Dureau, Olivier Jeannin, Joëlle Rault-Berthelot, Cyril Poriel, Cassandre Quinton*

Univ Rennes, CNRS, ISCR-UMR CNRS 6226, F-35000 Rennes

*email: cassandre.quinton@univ-rennes.fr

Nanohoops, donor-acceptor system, π-conjugated macrocycle, intramolecular charge transfer, solvato-fluorochromism

**ABSTRACT:** We report herein the synthesis and characterization of four donor-acceptor nanohoops incorporating fluorenone and carbazole as electron-poor and electron-rich units. The well-known platinum-mediated cyclization reaction provides here, in high yields, a mixture of four nanohoops possessing different and unexpected molecular arrangements. The four nanohoops have been isolated, characterized and the impact of the number and of the arrangement of the carbazole and fluorenone moieties have been studied by spectroscopic and electrochemical analysis. Thanks to the intramolecular charge transfer resulting from the interaction between the carbazole and fluorenone units, their fluorescence was significantly red-shifted by one hundred nanometers compared with a cyclic 2,7-tetracarbazole. This work highlights the singularity of the platinum-mediated cyclization reaction to construct donor-acceptor nanohoops with molecular arrangements, which are challenging to reach by other synthetic methods.

## INTRODUCTION

Nanohoops, fully conjugated macrocycles with a cylinder shape, are beginning to play a role as functional materials for electronics and are now entering the era of applications.<sup>1-6</sup> However, the synthesis of nanohoops has always been a real challenge since their discovery in 2008,<sup>7</sup> and, despite fantastic progresses, the molecular diversity of nanohoops remains poor. For example, the combination of electron-poor (acceptor) and electron-rich (donor) functional units in the nanohoop backbone is almost absent from literature (see below). This is particularly important because this approach is one of the most studied for linear organic π-conjugated systems, which are the heart of organic electronics. Indeed, incorporation of electron-rich and electron-poor units allows, for example, adjusting the electronic properties such as HOMO/LUMO energy levels for charge injection in devices and make them fit with a chosen device requirements.<sup>8-9</sup> This design approach, which has not yet been significantly developed in nanohoops chemistry, mainly due to synthetic issues, nevertheless appears highly important for their future applications.

The few reported examples of nanohoops incorporating donor and/or acceptor can be divided in four categories (Figure 1).<sup>10</sup> In most reported donor-acceptor nanohoops, phenylenes play the role of a weak donor<sup>11-28</sup> (Figure 1, Category 1) or a weak acceptor<sup>27, 29-33</sup> (Figure 1, Category 2). To our knowledge, only one article, published in 2017, has focused on cyclo-*para*-phenylenes incorporating both donor and acceptor moieties in their core, with phenylenes playing the role of spacer (Figure 1, Category 3).<sup>34</sup> Nanohoops consisting solely of donor and acceptor building blocks without unfunctionalized phenylenes (Figure 1, Category 4) are even more recent, and the only examples reported concern nanohoops in which the electron-rich core is peripherically substituted by acceptors.<sup>35-37</sup> In the present work, we report the first series of nanohoops incorporating only donor and acceptor building units in their core (without phenylene

spacers), enabling us to study precisely the intramolecular charge transfer (ICT) occurring in hoop-shaped molecules (Figure 1, Category 5). These nanohoops, which feature different molecular arrangements, have been synthesized in a single reaction using the well-known platinum-mediated cyclization reaction.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Different categories of donor-acceptor nanohoops with examples from the literature.<sup>27, 32, 34-35</sup> Donor-acceptor nanohoops obtained in this work.</figcaption></figure>

1Cbz-1FO-1Cbz-1FO, which was the initially targeted nano-hoop, displays two fluorenones and two carbazoles alternately arranged. 2Cbz-2FO also incorporates two fluorenones and two carbazoles, but arranged side by side. Nanohoops 3Cbz-1FO and 1Cbz-3FO feature three carbazoles and one fluorenone for the former, and one carbazole and three fluorenones for the latter. The different arrangements of carbazole and fluorenone units lead to unique electronic properties, which have been ra-
tionalized by molecular modelling. This work provides new in-
sights on the donor/acceptor association in nanohoops chemis-
try and on the role played by curvature on resulting properties
such as ICT.

RESULTS AND DISCUSSION

The four nanohoops were obtained thanks to two consecutive
platinum complexations (Scheme 1), as reported by different
groups.²⁴, ³⁸⁻⁴⁰ First, the bis-boronic precursors **1** and **2** were ob-
tained according to well-known procedures.⁴¹⁻⁴² Then, thanks to
a transmetallation of the bis-boronic-carbazole **2** with two
equivalents of dichloro(1,5-cyclooctadiene)platinum(II), bis-
platinated carbazole **3** was obtained in 83% yield. X-Ray struc-
ture of **3** (CCDC 2391706) shows that the C-Pt-Cl angles are of
91° (inset of Scheme 1), which are of key importance for the
further synthesis of nanohoops. In a second step, equimolar
amounts of bis-platinum-carbazole **3** and bis-boronic-flu-
orenone **1** in the presence of six equivalents of cesium fluoride
in dichloroethane (DCE) were heated at 70 °C for 24 hours.⁴²
The synthesis of this kind of squared intermediates is facilitated
by the C-Pt-Cl angles of 91° found in **3** allowing to perform the
macrocyclization with a low strain. The cyclooctadiene ligands
were then exchanged to triphenylphosphine ligands, favouring
the reductive eliminations and the formation of the targeted na-
nohoop **1Cbz-1FO-1Cbz-1FO** with two fluorenones and two
carbazoles alternately arranged. However, surprisingly, three
other nanohoops were also detected and isolated by silica gel
and size exclusion chromatographies, namely (i) **2Cbz-2FO,**
also built with two fluorenones and two carbazoles but arranged
in a side by side manner, (ii) **3Cbz-1FO** with three carbazoles
and one fluorenone, and (iii) **1Cbz-3FO** with one carbazole and
three connected fluorenones. In the light of literature, this ap-
peared as an unexpected result. Indeed, for similar reactions
Isobe *and coworkers* and Nuckolls *and coworkers* have only re-
ported the alternating –A–B–A–B– pattern for macrocycles
incorporating chrysenylene / chrysenodithiophene and
perylenediimide / bisthiophene, respectively.²⁴, ³⁸, ⁴⁰, ⁴³ Only
Yamago *and coworkers*, in 2011, have described that the unex-

pected [9]CPP and [11]CPP nanohoops can be obtained, in ad-
dition to the targeted [10]CPP, from bis-platinum-biphenyle
and bis-stanylated-terphenyle.³⁹ In our case, the obtention of
**1Cbz-1FO-1Cbz-1FO**, **2Cbz-2FO**, **1Cbz-3FO** and **3Cbz-1FO** implies the formation of the key intermediates tetraplati-
num complexes **4, 5, 6** and **7** (Scheme 1). In addition, Osakada
*and coworkers* have also described some decoordination/reco-
ordination phenomenon also called disproportionation on aryl-
platinum complexes,⁴⁴ especially when the platinum is com-
plexed by a cyclooctadiene (COD) ligand.⁴⁵ Although the de-
tailed mechanism is not fully understood yet, this disproportion-
ation process allows the synthesis of four nanohoops in good
yields, around 10% for each nanohoop with a global yield of the
reaction of 36%. Note that this platinum-mediated reaction can
lead to linear compounds⁴⁶⁻⁴⁷ or nanohoops of different sizes,³¹,
⁴⁸⁻⁴⁹ which may explain why the global yield is lower than that
of the synthesis of cyclocarbazoles with four units (52-64%).⁴²
In the light of the poor diversity of donor-acceptor nanohoops
reported to date and the importance of the synthetic approaches
in this field, we are convinced that this reaction can become a
key tool in the development of nanohoops.

Cyclic voltammetries (CV) were performed in dichloro-
methane (DCM) for oxidation and in DMF for reduction (the
data have also been recorded in DCM, see SI) to experimentally
evaluate the strength of the donor and the acceptor units when
circularly connected (Figures 2A-B). All the CVs show several
successive oxidation processes, with first maxima E^a^ox recorded
at 1.13 V for **1Cbz-1FO-1Cbz-1FO**, 1.04 V for **2Cbz-2FO**,
1.19 V for **1Cbz-3FO** and 0.98 V for **3Cbz-1FO** (Table 1, Fig-
ure 2B). As expected, the nanohoop displaying the highest num-
ber of electron-rich carbazoles, **3Cbz-1FO**, is the easiest to ox-
idize (E^a^ox = 0.98 V), while the one incorporating only one car-
bazole, **1Cbz-3FO**, is the most difficult to oxidize (E^a^ox = 1.19
V). In between, are found the two nanohoops possessing two
carbazoles and two fluorenones which display nevertheless dif-
ferent oxidative properties: E^a^ox = 1.04 V for **2Cbz-2FO** and
E^a^ox = 1.13 V for **1Cbz-1FO-1Cbz-1FO**. In the case of the ad-
jacent arrangement found in **2Cbz-2FO**, the donor unit is con-
stituted by two conjugated carbazoles, which increases the
HOMO level in comparison with the face to face arrangement

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Synthesis of the nanohoops **1Cbz-1FO-1Cbz-1FO**, **2Cbz-2FO**, **1Cbz-3FO** and **3Cbz-1FO**, inset: X-Ray structure of **3** (CCDC 2391706).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Cyclic voltammograms of 1Cbz-1FO-1Cbz-1FO, 2Cbz-2FO, 1Cbz-3FO and 3Cbz-1FO (A) on reduction in DMF (Bu₄NPF₆, 0.1 M) and B) on oxidation in CH₂Cl₂ (Bu₄NPF₆, 0.2 M). Platinum working electrode, sweep rate 100 mV·s⁻¹. C) Energy diagram showing the HOMO and LUMO level obtained by electrochemistry and representation of the HOMO and LUMO (CAMB3LYP/6-311G**, isovalue: 0.04 (e·bohr⁻³)⁻¹/²).</figcaption></figure>

of two conjugated carbazoles, which increases the HOMO level in comparison with the face to face arrangement of by two conjugated carbazoles, which increases the HOMO level in comparison with the face to face arrangement of 1Cbz-1FO-1Cbz-1FO (Figure 2C) incorporating two separated carbazoles linked to two fluorenones. Thus, the donor character of 2Cbz-2FO is stronger than that of 1Cbz-1FO-1Cbz-1FO, the former being easier to oxidize than the latter (E°ox = 1.04 and 1.13 V, respectively).

At negative potentials, the four nanohoops display several successive reduction waves with maxima E°red at -1.30 V for 1Cbz-1FO-1Cbz-1FO, -1.35 V for 2Cbz-2FO, -1.26 and -1.43 V for 1Cbz-3FO and -1.35 and -1.99 V for 3Cbz-1FO (Figures 2A, S24, S27, S30 and S33). In order to reveal the differences between these four nanohoops, we focused on the first inflection-point potentials (known as a reliable method even for irreversible waves).⁵⁰ We note an increase of these potentials as follows: -1.33 V for 3Cbz-1FO, -1.28 V for 1Cbz-1FO-1Cbz-1FO, -1.25 for 2Cbz-2FO and -1.23 V for 1Cbz-3FO (Table 1). Thus, 1Cbz-3FO, possessing the highest number of electron-poor fluorenones, logically presents the highest reduction inflection-point potential, -1.23 eV, while 3Cbz-1FO, possessing only one fluorenone, presents the lowest reduction inflection-point potential, -1.33 V. Moreover, we can note that the side by side arrangement makes 2Cbz-2FO easier to reduce compared to 1Cbz-1FO-1Cbz-1FO possessing an alternated pattern. This can be assigned to the different effects of the surrounding electron-rich carbazole units, which modify the fluorenone reduction potentials. Indeed, the two neighbouring carbazoles slightly decrease the reduction potential of the fluorenone in 1Cbz-1FO-1Cbz-1FO compared to 2Cbz-2FO in which each fluorenone is attached to a carbazole and a fluorenone.

In order to more deeply investigate the impact of the number and arrangement of carbazole and fluorenone units, we determined the number of electrons involved in each redox process using the Randles–Ševčík equation (see SI). Remarkably, while 2Cbz-2FO, 1Cbz-3FO and 3Cbz-1FO transfer only one electron upon both oxidation and reduction processes, two electrons are involved in the case of 1Cbz-1FO-1Cbz-1FO. Thus, each unit, either carbazole in oxidation or fluorenone in reduction, is impacted by the neighbouring units but not by the face-to-face unit. This is why, in the case of 1Cbz-1FO-1Cbz-1FO, the two carbazoles, only impacted by the neighbouring fluorenones and not by the opposed carbazole, are simultaneously oxidized. On

the contrary, in the case of 2Cbz-2FO in which the two carbazoles are adjacent, the oxidation of the first carbazole involves only one electron because it makes the oxidation of the second carbazole more difficult. Similarly, the two fluorenones of 1Cbz-1FO-1Cbz-1FO are reduced at the same potential (a two-electrons transfer is determined), while the first reduction wave of 2Cbz-2FO involves only one electron. Moreover, in the case of 1Cbz-3FO and 3Cbz-1FO, for which two reduction waves are clearly defined, only one electron is involved for each process.

To sum up, the number of fluorenone and carbazole units drive the oxidation and reduction properties: when the number of carbazoles increases and fluorenone decreases, the nano-hoops become easier to oxidize and harder to reduce. However, in the case of the nanohoops with two carbazoles and two fluorenones, the arrangement is also significantly involved in the redox processes. Remarkably, the redox processes of 2Cbz-2FO involve only one electron, while two electrons are transferred in the case of 1Cbz-1FO-1Cbz-1FO. Moreover, the former is easier to both oxidize and reduce than the latter. In other words, the ratio between carbazole and fluorenone influences both frontier orbitals in the same way, while the arrangement between these units can favour both the oxidation and the reduction.

The HOMO and LUMO are partially separated for all nano-hoops (Figure 2C). This is not a common feature in nanohoops chemistry, which can provide new directions in term of molecular design. The LUMO are localized on the fluorenone unit(s) with no contribution of the carbazole(s). This is not the case for the HOMO, which are delocalized on the entire nanohoop with a main contribution of the carbazole(s). The HOMO and LUMO energies (E°HOMO and E°LUMO) of 1Cbz-1FO-1Cbz-1FO, 2Cbz-2FO, 1Cbz-3FO and 3Cbz-1FO were estimated from the onset anodic oxidation and onset cathodic reduction peak potentials at E°HOMO = -5.41, -5.33, -5.47 and -5.25 eV respectively and E°LUMO = -3.17, -3.21, -3.25 and -3.16 eV respectively.⁵¹ As a result, the HOMO-LUMO gap ΔE°EL decreases from 2.24 eV for 1Cbz-1FO-1Cbz-1FO to 2.22 eV for 1Cbz-3FO to 2.12 eV for 2Cbz-2FO to 2.09 for 3Cbz-1FO. Thus, both HOMO and LUMO energy levels are modified but not in the same extent (0.09 eV for the LUMOs and 0.22 eV for the HOMOs). This can be assigned to the fact that the LUMOs, as significantly localized on the central five membered rings bearing the carbonyl, are less spread than the HOMOs and thus less impacted by
<table><caption>**Table 1.** Selected electronic data of the four nanohoops considered in this work</caption><thead><tr><th></th><th>3Cbz-1FO</th><th>1Cbz-1FO-1Cbz-1FO</th><th>2Cbz-2FO</th><th>1Cbz-3FO</th></tr></thead><tbody><tr><td>λ<sub>abs</sub> [nm]<sup>[a]</sup></td><td>264;290;339;469*</td><td>259;290;336;369*;<br>390*;454</td><td>267; 290; 341;<br>454</td><td>270;288;342; 394*;<br>449</td></tr><tr><td>λ<sub>em</sub> [nm]<sup>[a], [b]</sup></td><td>589</td><td>588</td><td>585</td><td>577</td></tr><tr><td>φ<sub>f</sub><sup>[a], [c]</sup></td><td>0.13</td><td>0.09</td><td>0.08</td><td>0.09</td></tr><tr><td>τ<sub>f</sub> [ns]<sup>[a], [d]</sup></td><td>7.3</td><td>7.3</td><td>6.8</td><td>7.3</td></tr><tr><td>k<sub>r</sub> [ns<sup>-1</sup>]<sup>[a], [d], [e]</sup></td><td>0.018</td><td>0.012</td><td>0.012</td><td>0.012</td></tr><tr><td>k<sub>nr</sub> [ns<sup>-1</sup>]<sup>[a], [d], [e]</sup></td><td>0.119</td><td>0.125</td><td>0.135</td><td>0.125</td></tr><tr><td>E<sup>a</sup><sub>ox</sub> [V vs SCE]<sup>[f]</sup></td><td>0.98; 1.19; 1.48</td><td>1.13; 1.62; 1.92;</td><td>1.04; 1.18;<br>1.42; 1.84</td><td>1.19; 1.52</td></tr><tr><td>E<sup>c</sup><sub>red</sub> [V vs SCE]<sup>[g]</sup></td><td>-1.35; -1.99</td><td>-1.30; -1.83*</td><td>-1.35; -1.83;<br>-2.04</td><td>-1.26; -1.43</td></tr><tr><td>E<sup>(i)</sup><sub>ox</sub> [V vs SCE]</td><td>0.92</td><td>1.00</td><td>1.08</td><td>1.11</td></tr><tr><td>E<sup>(i)</sup><sub>red</sub> [V vs SCE]</td><td>-1.33</td><td>-1.28</td><td>-1.25</td><td>-1.23</td></tr><tr><td>HOMO [eV]<sup>[h]</sup></td><td>-5.25</td><td>-5.41</td><td>-5.33</td><td>-5.47</td></tr><tr><td>LUMO [eV]<sup>[h]</sup></td><td>-3.16</td><td>-3.17</td><td>-3.21</td><td>-3.25</td></tr><tr><td>ΔE<sub>EL</sub> [eV]<sup>[h], [i]</sup></td><td>2.09</td><td>2.24</td><td>2.12</td><td>2.22</td></tr></tbody></table>

[a] In cyclohexane. [b] $λ_{exc} = 400$ nm. [c] Fluorescence quantum yield ($φ_f$) determined with sulfate quinine in 1 M $H_2SO_4$ as the reference, $λ_{exc} = 370$ nm. [d] $λ_{exc} = 310$ nm. [e] $k_r = φ_f / τ_f$ and $k_{nr} = (1/τ_f) × (1-φ_f)$. [f] in dichloromethane. [g] in DMF. [h] From electrochemical data. [i] |HOMO-LUMO|. *shoulder

neighbouring fragments. Thus, the HOMO seems to have more fluorenones and the carbazoles impact on the gap evolution than the LUMO and increasing the number of electron-poor fluorenones increases the HOMO-LUMO gap, while increasing the number of electron-rich carbazoles decreases it. Moreover, the arrangement between the brings some important subtleties. From an adjacent arrangement in 2Cbz-2FO to a face to face arrangement in 1Cbz-1FO-1Cbz-1FO, the HOMO-LUMO gap increases by more than 0.1 eV, leading to a value even higher than that of 1Cbz-3FO enriched in fluorenones (2.24 eV for 1Cbz-1FO-1Cbz-1FO versus 2.22 eV for 1Cbz-3FO). This is an important consideration in the understanding of the electronic properties, which should be considered for future design of donor-acceptor nanohoops.

The absorption and emission properties of the four nanohoops in cyclohexane were also investigated (Figure 3, Table 1). In the absorption spectra, the nanohoops display a main band at around 340 nm as classically observed²⁷, ³¹, ⁴², ⁵² and a low-energy band at around 450 nm. According to TD-DFT calculations, this low energy band is due to two or three transitions only localized on the fluorenones in the case of the nanohoops incorporating two or three fluorenones respectively, while it displays a charge transfer character in 3Cbz-1FO (see NTO in Figures S35-38). In this last case, the low energy band is due transitions involving occupied molecular orbitals localized on

the three carbazoles and the LUMO localized on the fluorenone. For this reason, its oscillator strength and therefore the intensity of the band are lower for 3Cbz-1FO compared to the three other nanohoops. The position of this band is red-shifted with the increase of the number of carbazole: 449, 454, 454 and 469 nm for 1Cbz-3FO, 2Cbz-2FO, 1Cbz-1FO-1Cbz-1FO and 3Cbz-1FO. This tendency is well modelized with theoretical calculations, which predict maxima at 390, 394, 395 and 405 nm for 1Cbz-3FO, 2Cbz-2FO, 1Cbz-1FO-1Cbz-1FO and 3Cbz-1FO. The red-shift observed for 3Cbz-1FO is linked to its charge transfer character while the blue-shift observed for 1Cbz-3FO is due to its low HOMO level, as shown by electrochemistry (see above). Indeed, the HOMO is the main orbital involved in this transition in the cases of 1Cbz-3FO, 2Cbz-2FO and 1Cbz-1FO-1Cbz-1FO (see Tables S7-10). As shown by electrochemistry and confirmed by theoretical calculations, due to the incorporation of three electron-poor fluorenones, 1Cbz-3FO displays the lowest HOMO, which increases the energy transition and decreases the absorption wavelength. Thus, electrochemical, spectroscopic and theoretical studies are in perfect agreement showing that the donor behaviour increases with the number of carbazoles.

The emission spectra of the four nanohoops are large and centered at 577, 585, 588 and 589 nm for 1Cbz-3FO, 2Cbz-2FO, 1Cbz-1FO-1Cbz-1FO and 3Cbz-1FO, following the same
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Absorption (top) and emission (bottom) spectra of 1Cbz-1FO-1Cbz-1FO, 2Cbz-2FO, 1Cbz-3FO and 3Cbz-1FO in cyclohexane</figcaption></figure>

trend than that observed in absorption for the low-energy band. From these comparisons, it is clear that increasing the number of carbazoles shifts emission to higher wavelengths, whereas the face to face or adjacent arrangement (in 1Cbz-1FO-1Cbz-1FO and 2Cbz-2FO respectively) has a less significant effect.

Moreover, the fluorescence bands of all these nanohoops are also highly red-shifted compared to that of 4Cbz ($\lambda_{em}=483$ nm),⁴²,⁴⁹ which is constituted by four carbazole units (see structure in Figure S1), showing the high impact of fluorenone(s) incorporation. Indeed, the fluorenone(s) reduce the LUMO level, from -2.4 eV for 4Cbz to -3.2 eV for the four nanohoops, which in turn red-shift the fluorescence. The highest red-shift is obtained for 3Cbz-1FO. Hence, it is more efficient to change only one carbazole of 4Cbz for a fluorenone as in 3Cbz-1FO (red-shift of 106 nm) than modifying three units as in 1Cbz-3FO (red-shift of 94 nm). This is due to the charge transfer character of the transition in 3Cbz-1FO (see above). This highlights the role played by the ratio between carbazole and fluorenone moieties on the donor-acceptor interaction.

The fluorescence quantum yields $\phi_f$ and the singlet state lifetimes $\tau_s$ were measured in cyclohexane at $\phi_f = 0.09$ and $\tau_s = 7.3$ ns for 1Cbz-1FO-1Cbz-1FO, $\phi_f = 0.08$ and $\tau_s = 6.8$ ns for 2Cbz-2FO, $\phi_f = 0.09$ and $\tau_s = 7.3$ ns for 1Cbz-3FO and $\phi_f = 0.13$ and $\tau_s = 7.3$ ns for 3Cbz-1FO, leading to radiative rates around $0.015 \text{ ns}^{-1}$ and non-radiative rates of $0.125 \text{ ns}^{-1}$ for the

four nanohoops (Table 1). The series of nanohoops display similar radiative rates and similar non-radiatives rates, which indicates that the desexcitation pathway is similar for the four nanohoops, whatever the number and arrangement of carbazole and fluorenone units. We can note that the fluorescence quantum yield of the four donor-acceptor nanohoops is maintained at around 0.1, which is only half of $\phi_f$ usually observed for nanohoops based on eight phenylene units such as 4Cbz ($\phi_f = 0.2$), while donor-acceptor interaction often drastically decreases the quantum yield.⁵³⁻⁵⁶ According to a theoretical study previously performed on CPPs incorporating donor and acceptor units,⁵⁷ the quantum yield conservation for the four donor-acceptor nanohoops compared to 4Cbz is assigned to similar frontier molecular orbitals in the emissive state and similar fluorescence mechanism. This is corroborated by their similar radiative and non-radiative rates. Red-shifting the emission by one hundred nanometers ($\lambda_{em}= 577-589$ nm for the four nanohoops vs 483 nm for 4Cbz) while maintaining the quantum yield is a very interesting characteristic, which can be advantageously used for bio-imaging for example.

Absorption spectra of the four nanohoops in a spin-coated thin film from a THF solution were recorded (Figure S10). The shapes and maxima are similar to those in solution. They are, however, slightly larger and shifted according to the different environments (solid vs solution). However, the fluorescence spectra could not be recorded because fluorescence quantum yields in thin film are surely very low ($< 0.01$ for the four nanohoops), surely due to the intermolecular donor-acceptor interaction. This is easily understandable as the quantum yields in solution are already very low (around 0.1).

As discussed all along this work, the interaction between the donor and the acceptor is a central notion. Solvatochromism, that is modification of photophysical properties induced by the polarity of the media, is a common phenomenon for π-conjugated systems incorporating donor and acceptor fragments. When the chromophore absorbs light, solvatochromism occurs due to differential solvation of the ground and first excited states. By recording absorption and emission spectra of the four nanohoops, in five different solvents of varying polarity, changes in the absorption and emission bands were investigated (Figure 4 and Figures S2-5). We first note that the position of the absorption bands of the four nanohoops are weakly sensitive to the environment translating that the ground-state dipole moment is weakly affected by the polarity of the medium. As often observed, the emission spectra are more influenced by the nature of the solvent due to important solvations effects on the excited states. The emission bands of the four nanohoops are thus red-shifted by 82 nm for 1Cbz-1FO-1Cbz-1FO (from 588 nm in non-polar cyclohexane to 670 nm in polar acetonitrile), by 101 nm for 2Cbz-2FO (from 585 to 686 nm), by 84 nm for 1Cbz-3FO (from 577 to 661 nm) and by 58 nm for 3Cbz-1FO (from 589 to 647 nm). By increasing solvent polarity, a positive solvatochromism is observed for all nanohoops. The bathochromic shift is weak in absorption and more important in emission, meaning that the excited state of the nanohoops is more stabilised by polar solvents than the ground state. Particularly, 2Cbz-2FO well stabilizes the emissive state compared to the ground state, inducing a remarkable positive solvatofluorochromism of 101 nm. Thus, in order to increase the charge transfer character, it seems to be more efficient to incorporate the same number of electron-poor and -rich units and to arrange them in an adjacent way. Note that the emission spectra of
**3Cbz-1FO** are less impacted by the nature of the solvents compared to the other nanohoops, although it displays the strongest donor character (the highest HOMO) and the lowest overlap between the HOMO and LUMO. Moreover, for **3Cbz-1FO**, the polarity of the solvent is not the main parameter driving the red

shift. Chloroform and dichloromethane are polar solvents with a hydrogen bond donor ability (Kamlet-Taft parameters $π*=0.58$ and $0.82$, $β=0.00$ and $0.00$, and $α= 0.44$ and $0.30$)$^{58}$ which can thus interact with the oxygen of the carbonyl

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Emission spectra in various solvents of **1Cbz-1FO-1Cbz-1FO** (top left), **2Cbz-2FO** (top right), **1Cbz-3FO** (bottom left) and **3Cbz-1FO** (bottom right).</figcaption></figure>

group, inducing a red-shift of the fluorescence. In the other nanohoops, the multiple interactions between the two or three carbonyl groups and the solvent can compensate each other, which could explain why they are less impacted than **3Cbz-1FO** incorporating only one fluorenone.

In order to fully characterize the ICT properties of the four nanohoops, changes in absorption and emission maxima as a function of the Reichardt solvent polarity parameter $E_T$ (30)

have been studied (Figure 5). First, the effect of solvatochromism on the absorption of the four nanohoops is weak but the trend is significant. The Reichardt slope becomes important when the number of fluorenones increases (from $-23×10^3$ mol.cm$^{-1}$.kcal$^{-1}$ for **3Cbz-1FO** to $-35×10^3$ mol.cm$^{-1}$.kcal$^{-1}$ for **2Cbz-2FO** to $-40×10^3$ mol.cm$^{-1}$.kcal$^{-1}$ for **1Cbz-3FO**), showing that the dipole moment in the ground state $μ_g$ increases with the number of fluorenones. What is of key importance is the
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Dependence of the ICT absorption maximum (top) and emission maximum (bottom) of 1Cbz-1FO-1Cbz-1FO, 2Cbz-2FO, 1Cbz-3FO and 3Cbz-1FO on Reichardt solvent polarity parameters E<sub>T</sub>(30).</figcaption></figure>

<figure><img src="image_8.png" /></figure>

arrangement of the carbazole and fluorenone units, as shown by the higher slope observed for 1Cbz-1FO-1Cbz-1FO compared to 2Cbz-2FO (-46×10³ mol.cm⁻¹.kcal⁻¹ versus -35×10³ mol.cm⁻¹.kcal⁻¹). The alternating arrangement increases so much the dipole moment at the ground state that it becomes even higher than that of 1Cbz-3FO yet enriched in fluorenones (slope of -40×10³ mol.cm⁻¹.kcal⁻¹). These results indicate that the arrangement of the carbazole and fluorenone units in nanohoops drives the dipole moment in the ground state µ<sub>g</sub> while the impact of their ratio is weaker. This is the opposite of what has been observed for electronic properties, which are more influenced by the ratio than by the arrangement (see above). The nanohoops also present remarkable solvatofluorochromism as evident by the slopes: -87×10³ mol.cm⁻¹.kcal⁻¹ for 3Cbz-1FO, -171×10³ mol.cm⁻¹.kcal⁻¹ for 1Cbz-1FO-1Cbz-1FO and -182×10³ mol.cm⁻¹.kcal⁻¹ for 1Cbz-3FO. Such values, reported herein for the first time in the field of nanohoops, allow us to define the first design rules to obtain nanohoops displaying an ICT. Thus, the increase of the number of fluorenones increases the solvatochromism and the strength of the ICT. What is even more important is the arrangement of the fluorenone and carbazole units since the highest ICT is observed for 2Cbz-2FO incorporating the same number of electron-rich and -poor units arranged in an adjacent manner (slope of -202×10³ mol.cm⁻¹.kcal⁻¹). The ICT of the four nanohoops can also be explained in terms of the Stokes shift (Δν), which is the difference in wavenumber between the shortest wavenumber absorption (ν<sub>a</sub>) and the fluorescence emission (ν<sub>f</sub>) maxima. The Stokes shifts (Δν) versus solvent orientation polarizability [Δf (ε,n)] were plotted in Figure 6. Correct linear Lippert-Mataga relationships are found for all nanohoops (regression coefficient R² =0.75-0.83) with slopes of 4662, 6256, 5331 and 3081 cm⁻¹ for 1Cbz-1FO-1Cbz-1FO, 2Cbz-2FO, 1Cbz-3FO and 3Cbz-1FO, respectively. As seen in the Lippert-Mataga Equation (1) below,⁵⁹ a larger slope means a larger dipole moment variation Δμ, which is the difference between the excited state dipole moment μ<sub>e</sub> and the ground state dipole moment μ<sub>g</sub>.

$$ \Delta\nu = \frac{2(\Delta\mu)^2}{r^3 h c} \Delta f + C \quad \text{with} \quad \Delta f = \left(\frac{\varepsilon-1}{2\varepsilon+1} - \frac{n^2-1}{n^2+1}\right) \quad \text{Equation (1)} $$

The dipole moment variation Δμ is calculated at 7.9, 9.6 and 10.4 D for 3Cbz-1FO, 1Cbz-1FO-1Cbz-1FO and 1Cbz-3FO respectively, showing that Δμ increases with number of fluorenones. Nevertheless, as exposed above, the arrangement of the carbazole and fluorenone units drives the dipole moment

variation as 2Cbz-2FO presents a higher dipole moment variation (Δμ = 11.2 D) than that of 1Cbz-1FO-1Cbz-1FO (Δμ = 9.6 D) and even than that of 1Cbz-3FO (Δμ = 10.4 D) incorporating three fluorenones. The value obtained for 2Cbz-2FO is close to the highest value reported for a nanohoop (Δμ =11.6 D), a [10]CPP incorporating an acenaphthylene (Figure 1).⁶⁰ Using the adjacent arrangement of the electron-poor and -rich units seems hence to be an efficient strategy to increase the dipole moment variation between the ground and excited states.

As a conclusion, 3Cbz-1FO displays in this series the lowest dipolar moment at the ground state μ<sub>g</sub> and the lowest dipole moment variation and thus the lowest dipole moment at the excited state μ<sub>e</sub>, showing that several fluorenones are essential to increase the ICT properties. Moreover, it should be stressed that, unlike electronic properties, solvato(fluo)chromism properties are more impacted by the arrangement than by the ratio of fluorenone and carbazole units. Thus, the alternating arrangement increases the dipole moment at the ground state of

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Stokes shifts as a function of the solvent orientation polarizability of 1Cbz-1FO-1Cbz-1FO, 2Cbz-2FO, 1Cbz-3FO and 3Cbz-1FO</figcaption></figure>

1Cbz-1FO-1Cbz-1FO while 2Cbz-2FO, displaying the adjacent arrangement, presents the highest ICT, experimentally determined by the highest values of dipole moment variation Δμ (11.2 D), slope of emission wavelength vs Reichardt parameter E<sub>T</sub>(30) (-202×10³ mol.cm⁻¹.kcal⁻¹) and slope of Lippert-Mataga fit (6256 cm⁻¹). Thus, ratio and more importantly arrangement of donor and acceptor moieties must be considered in order to
design some bipolar nanohoops displaying high ICT. This is the first step toward nanohoops-based materials.

## CONCLUSIONS

In summary, we report herein a series of four donor-acceptor nanohoops built on the different arrangement of electron-rich carbazole and electron-poor fluorenone moieties. While only one nanohoop was initially targeted, **1Cbz-1FO-1Cbz-1FO**, thanks to disproportionation reactions, three other molecular arrangements have been isolated. Electrochemical and photo-physical properties were investigated to shed light on the impact of the arrangement of the donor and acceptor units on the electronic properties and particularly on the ICT. We have shown that **3Cbz-1FO**, incorporating three electron-rich units, exhibits the highest electron-rich character with the highest HOMO and that **1Cbz-3FO**, incorporating three electron-poor units, exhibits the highest electron-poor character with the lowest LUMO. Thus, the ratio between electron-rich and electron-poor units is the main parameter driving the energy of molecular frontier orbitals although their arrangement may also play a role, as underlined by the differences between **1Cbz-1FO-1Cbz-1FO** and **2Cbz-2FO**. In addition, **3Cbz-1FO** has the narrowest HOMO-LUMO gap (2.09 eV) and a red-shifted emission at 589 nm. Despite their red-shifted fluorescence, all four nanohoops display a fluorescence quantum yield of 0.1, which is only half that recorded for **4Cbz** consisting of four carbazole units. Due to its balanced ratio of carbazole to fluorenone units, **2Cbz-2FO** exhibits intermediate electronic characteristics compared to **3Cbz-1FO** and **1Cbz-3FO** ($\lambda_{abs} = 469 > 454 > 449$ nm, $\lambda_{em} = 589 > 585 > 577$ nm, HOMO = -5.25 > -5.33 > -5.47$ eV and LUMO = -3.16 > -3.21 > -3.25$ eV for **3Cbz-1FO**, **2Cbz-2FO** and **1Cbz-3FO**, respectively). Nevertheless, thanks detailed solvatochromic studies, we have shown that the balanced number of electron-poor and electron-rich units together with their adjacent arrangement confers to **2Cbz-2FO** the highest ICT of the four nanohoops with values of dipole moment variation $\Delta\mu$ (11.2 D), slope of emission wavelength $\nu$ Reichardt parameter $E_T(30)$ (-202×10³ mol.cm⁻¹.kcal⁻¹) and slope of Lippert-Mataga fit (6256 cm⁻¹) among the highest reported in the literature for nanohoops. This work lays the first foundations for the synthesis of bright red-emitting nanohoops, which may be useful for bio-imaging applications.

## ASSOCIATED CONTENT

The Supporting Information with the experimental section including synthetic procedures, NMR and mass spectra, electrochemical properties, photophysical properties and molecular modelling is available free of charge at https://pubs.acs.org.

## AUTHOR INFORMATION

### Corresponding Author

* Cassandre Quinton – Univ Rennes, CNRS, ISCR-UMR 6226, F-35000 Rennes, France

## ACKNOWLEDGMENT

CB thanks the ANR (N°19-CE05-0024- SpiroQuest Project) and the ADEME (Ecoelec Project, Dr Bruno Lafitte) for PhD grant. CB thanks the ANR (N°22-CE07-0024-Evolution Project) for post-

doctoral position. CQ thanks the Université de Rennes for the allocation of a Défi Scientifique 2024-Politique de pôles. We also thank the CRMPO (Rennes) for mass analyses, Christine Deponge (Rennes) for fruitfull discussions on NMR and Dr Corinne Lagrost and Dr Yann Leroux (Rennes) for their help in electrochemistry. This work was granted access to the HPC resources of TGCC/CEA/CINES/IDRIS under the allocation 2024 AD010814136R1 awarded by GENCI.

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