[4]-Cyclo-2,7-carbazole as host material in high-efficiency
phosphorescent OLEDs: A new perspective for
nanohoops in organic electronics

Clement Brouillac, Fabien Lucas, Denis Tondelier, Joëlle Rault-Berthelot, Christophe Lebreton, Emmanuel Jacques, Cassandre Quinton, Cyril Poriel

<figure><img src="image_1.png" /></figure>

To cite this version:

Clement Brouillac, Fabien Lucas, Denis Tondelier, Joëlle Rault-Berthelot, Christophe Lebreton, et al. [4]-Cyclo-2,7-carbazole as host material in high-efficiency phosphorescent OLEDs: A new perspective for nanohoops in organic electronics. Advanced Optical Materials, inPress, 10.1002/adom.202202191. hal-03923211

HAL Id: hal-03923211

https://hal.science/hal-03923211

Submitted on 17 Jan 2023

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et 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.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# [4]-Cyclo-2,7-carbazole as host material in high-efficiency phosphorescent OLEDs: A new perspective for nanohoops in organic electronics

Clément Brouillard,<sup>a¥</sup> Fabien Lucas,<sup>b,¥</sup> Denis Tondelier,<sup>b</sup> Joëlle Rault-Berthelot,<sup>a</sup> Christophe Lebreton,<sup>c</sup> Emmanuel Jacques,<sup>c</sup> Cassandre Quinton<sup>a</sup> and Cyril Poriel<sup>a*</sup>

email: cyril.poriel@univ-rennes1.fr

¥: equal contribution

Keywords: nanohoops, organic semiconductor, host material, phosphorescent organic light-emitting diode, structure-properties relationship, curved π-conjugated systems.

**Abstract:** In the last ten years, the development of π-conjugated nanohoops has been considerable owing to their remarkable properties. However, to date, their incorporation in organic electronic devices remains very scarce. In this work, we report the first high performance organic electronic device (i.e. Phosphorescent Organic Light-Emitting Diode PhOLED) incorporating a nanohoop ([4]-cyclo-N-butyl-2,7-carbazole [4]C-Bu-Cbz), revealing their potential in electronics. Thus, using the red phosphor Ir(MDQ)₂(acac), the [4]C-Bu-Cbz-based PhOLED displays a high External Quantum Efficiency (EQE) of 17.0 %, a Current Efficiency (CE) of 20.6 cd.A⁻¹ and a Power Efficiency (PE) of 25.8 lm.W⁻¹ demonstrating that the charges injection, transport and recombination are particularly efficient. This performance is significantly higher than that of its linear counterpart, N-butyl-2,7-quartercarbazole [4]L-Bu-Cbz, which presents an EQE of 11.1 %, a CE of 13.0 cd.A⁻¹ and a PE of 15.7 lm.W⁻¹. The significant difference, in term of device performance, between cyclic and acyclic compounds provides a new basis to construct high-performance electronic devices. This study, which includes optical, electrochemical, morphological and charge transport properties, shows that nanohoops can be efficiently used as organic semi-conductors in electronics and opens the way to their practical uses in high-performance optoelectronic devices, which is now the next stage of their evolution.

The research field of hoop-shaped π-conjugated macrocycles, i.e. nanohoops, has significantly grown in recent years.[1] Besides their uncommon electronic properties induced by the radial distribution of the molecular orbitals,[2-7] and their recent applications in many different areas[1], such as imaging tools in biology,[8] complexing agent for single-walled carbon nanotubes[9] or fullerenes,[10-12] carbon-based porous materials,[13, 14] or spin crossover compounds,[15] nanohoops are, over all, appealing π-conjugated materials, which can be potentially applied in electronic devices. However, this field of application remains a new ground to explore for nanohoops. Indeed, despite the fantastic development of organic electronic technologies in the last twenty years,[16, 17] there is only very few experimental data available on the integration of nanohoops in electronic devices, either Organic Light-Emitting Diodes (OLED),[18] Organic Field-Effect Transistors (OFET)[19, 20] or solar cells.[21, 22] This can be assigned to the difficulty to reach sufficient amount of nanohoops for device incorporation. The recent progresses in term of synthesis[20] start to change the deal and will surely allow nanohoops to go one-step further. Considering that linear π-conjugated systems are the molecular pillars, which have allowed the development of organic electronics, investigating their cyclic counterparts appears as an exciting challenge.[21, 23] Recently, the first detailed structure-properties-device performance relationship study of functional materials based on nanohoops i.e. [4]cyclo-N-alkyl-2,7-carbazoles has been reported in literature.[20] This work has shown how the supramolecular arrangement of the nanohoops affected their corresponding OFET characteristics. Field-effect (FE) mobility values of ca 10⁻⁵ cm²·V⁻¹·s⁻¹ were recorded for cyclocarbazoles, whereas their non-bridged analogue [8]cyclo-para-phenylene [8]CPP

did not display any FE mobility. In addition, this work has also shown that cyclocarbazoles-based devices were stable at the year time scale under inert atmosphere, and under electrical stress showing the real potential of these nanohoops in electronics. In the light of the importance of charge transport in all electronic devices,[24] these first data have shown that functional nanohoops can be used as organic semi-conductors (OSC). However, in this field, there is a long way to go in terms of fundamental knowledge and practical applications and to date, all the organic electronic devices incorporating a naonohoop display low performance. In the present work, we aim to go one-step further by reporting the first high-performance organic electronic device using a nanohoop. Thus, [4]-cyclo-N-butyl-2,7-carbazole, [4]C-Bu-Cbz cyclic assembly of four carbazole units substituted at C2 and C7, has been efficiently incorporated as a host material, in a red emitting phosphorescent OLED (PhOLED) using Ir(MDQ)₂(acac) as emitter. This device displays a very high external quantum efficiency (EQE) of 17.0 %, a current efficiency (CE) of 20.6 Cd.A⁻¹, a power efficiency (PE) of 25.8 lm.W⁻¹ and a low threshold voltage (Von) of 2.3 V. These performances translate excellent charges injection, transport and recombination. More importantly, in identical experimental conditions, its linear analogue, N-butyl-2,7-quartercarbazole [4]L-Bu-CBz (with the same C2/C7 linkages) presents significantly lower performance: EQE of 11.1 %, CE of 13.0 cd.A⁻¹ and PE of 15.7 lm.W⁻¹. Thus, this work shows not only that [4]C-Bu-Cbz can efficiently host a phosphorescent emitter within a PhOLED but also reveals, by comparison with its acyclic counterpart [4]L-Bu-Cbz, the strong advantages of cyclic vs linear structures. As the curvature of non-planar aromatic molecules is known to play a key role in achieving a desired function,[25] the present

<footnote><sup>a</sup> Univ Rennes, CNRS, ISCR-UMR 6226, F-35000 Rennes, France, <sup>b</sup> LPICM, CNRS, Ecole Polytechnique, IPParis, 91128 Palaiseau, France
<sup>c</sup> Univ Rennes, CNRS, IETR-UMR 6164, F-35000 Rennes, France</footnote>
finding can provide a new basis to construct high-performance electronic based on nanohoops.

<figure><img src="image_3.png" /><figcaption>Figure 1. Synthesis of cyclic ([4]C-Bu-Cbz) and linear ([4]L-Bu-Cbz) quatercarbazoles.</figcaption></figure>

One of the main weakness of nanohoops research field in materials science remains their synthesis, which has strongly hindered their development and especially electronic applications. Herein, [4]C-Bu-Cbz has been synthesized at the gram scale following optimized experimental conditions previously reported in literature (i.Pt(cod)Cl₂/CsF in DCE at 70°C for 24 h, ii. PPh₃ in o-DCB at RT for 1 h and at 180°C for 48 h), Figure 1. Compared to previous works,²⁰ this reaction has been scaled up by a factor 4 (1 g of starting material 2) while maintaining an identical yield of 58%. As the scale-up is one of the inherent difficulty in nanohoop chemistry in general, this result appears beneficial for further application in materials science. The approach developed for linear N-butyl-2,7-quatercarbazole [4]L-Bu-Cbz is short, i.e. 2 steps, involves the same starting material (1) than that used for its cyclic analogue and displays an overall yield of 42% from 1.

To insure efficient T₁/T₁ energy transfer from the host to the guest in a PhOLED, the energy level of the lowest triplet excited state (E<sub>T1</sub>) of the host material should be higher than that of the guest. However, the data on the phosphorescence properties of nanohoops remain very scarce.²⁶⁻²⁹ These data are nevertheless crucial to define design rules of nanohoop-based host materials. At 77 K, in 2Me-THF, the emission spectrum of [4]C-Bu-Cbz displays two main contributions: the fluorescence contribution with a main band centred at 470 nm and the phosphorescence contribution with two bands at ca 548 and 584 nm (Figure 2A), providing a white light emission. The origin of these bands is ascertained by the gated emission as the fluorescence contribution is completely vanished. The decay curve for the band at 548 nm (λ<sub>exc</sub> = 312 nm, Figure 2B) provides a very long lifetime of 0.65 s in accordance with an emission arising from the T<sub>1</sub> state. The E<sub>T1</sub> is measured at 2.26 eV from the maximum of the first phosphorescent band. The linear analogue [4]L-Bu-Cbz presents a blue shifted phosphorescent contribution spectrum with a main band detected at 520 nm, leading to an E<sub>T1</sub> value of 2.38 eV, higher to that of [4]C-Bu-Cbz. The lifetime of [4]L-Bu-Cbz is also significantly longer, 2.85 s, than that of [4]C-Bu-Cbz. Thus, the high strain of [4]C-Bu-Cbz (and nanohoops in general) leads to a decrease of both E<sub>T</sub> and lifetime. One can hence conclude that the strain and the

resulting deformations of the phenyl rings decrease the E<sub>T1</sub>. This is an interesting finding for further molecular designs. Theoretical calculations confirm this feature with an E<sub>T1</sub> evaluated at 2.20 eV for [4]C-Bu-Cbz and at 2.30 eV for [4]L-Bu-Cbz. The triplet spin density distribution (SDD, Figure 2C) is mainly located, for both molecules, on two connected carbazole units, but the contribution of the two other carbazoles is slightly more intense in the case of [4]C-Bu-Cbz than in [4]L-Bu-Cbz, in accordance with the higher value of the former. In 2020, Ruiz Delgado, Huang and their coworkers have reported the phosphorescence of a meta-linked [4]cyclo-3,6-carbazole.³⁰ Despite being a positional isomer of [4]C-Bu-Cbz, this macrocycle is flat and therefore structurally different. Its phosphorescent contribution, due to the meta linkages is blue shifted to 472 nm, leading to a strong increase of the E<sub>T1</sub>, measured at ca 2.63 eV. In addition, its linear analogue (3,6-quatercarbazole) has also been studied and presents an identical E<sub>T1</sub> of 2.63 eV (470 nm). So, there is no difference between the two meta-substituted quatercarbazoles and the cyclic vs linear nature does not significantly influence the T<sub>1</sub> state. In our case, [4]C-Bu-Cbz and [4]L-Bu-Cbz display a different behaviour as there is a difference of ca 0.12 eV between the two compounds in term of E<sub>T1</sub>. This highlights the influence of the strain on the T<sub>1</sub> state in nanohoops, which induces a significant difference compared to other macrocycles.

With an E<sub>T1</sub> of 2.26 eV, [4]C-Bu-Cbz can be used to host the red emitter Ir(MDQ)<sub>2</sub>acac, which presents E<sub>T1</sub> values of 2.02/2.08 eV at room temperature/77 K.[31] Prior to device fabrication, a thin film of Ir(MDQ)<sub>2</sub>acac, dispersed into either [4]C-Bu-Cbz or [4]L-Bu-Cbz (exactly corresponding to the EML of the PhOLED studied below) has been deposited on quartz by thermal evaporation. The resulting spectrum shows no trace of the emission of [4]C-Bu-Cbz / [4]L-Bu-Cbz, indicating an efficient energy transfer from the guest to the host (Figure 3A). In these conditions, the E<sub>T1</sub> of Ir(MDQ)<sub>2</sub>acac, is measured at 2.01/2.02 eV from the peak maximum (Figure 3A), in full accordance with previous reports.[31]
<figure><img src="image_4.png" /><figcaption>Figure 2. (A) Normalized emission and delayed emission at 77 K, in 2-Me-THF, of [4]C-Bu-Cbz (blue lines, λ<sub>exc</sub>= 340 and 400 respectively) and of [4]L-Bu-Cbz (black lines, λ<sub>exc</sub>= 360 nm for both); (B) Decay of [4]C-Bu-Cbz (λ<sub>exc</sub>= 312 nm, λ<sub>em</sub>= 543 nm) and [4]L-Bu-Cbz (λ<sub>exc</sub>= 312 nm, λ<sub>em</sub>= 520 nm); (C) Triplet spin density distribution: TD-DFT b3lyp 6-311+g(d,p).</figcaption></figure>

The evaluation of defect density in the EML is a key
point to obtain optimal recombination between hole and
electron. This defect density is directly linked to the film
organization and could be estimated by analysing charge
transport. OFET electrical characterizations and I-V
measurements in two-terminal devices have been performed
to evaluate the charge transport properties of [4]C-Bu-Cbz[20]
and [4]L-Bu-Cbz (See figures S17-S19). Note that charge
transport studies of nanorings are almost absent from
literature[19, 20, 32, 33] and comparison with linear analogues
never reported to date. They nevertheless constitute the
fundations of high efficiency devices First, the OFETs
possessing a bottom-gate bottom-contact architecture (Figure
S17[34]), in which the active layer is evaporated in the last step
(in order to prevent structural defects resulting from the
deposition of other layers[35, 36]) were used to extract the FE
mobility. Used as active layer in such a type of device, [4]C-
Bu-Cbz / [4]L-Bu-Cbz display mobility μ<sub>FE-sat</sub> of 1.04 × 10<sup>-5</sup> / 1.10 × 10<sup>-4</sup> cm<sup>2</sup>·V<sup>-1</sup>·s<sup>-1</sup> in saturated regime (Figure S20, Table
S6). In linear regime, the mobility μ<sub>FE-lin</sub> of [4]C-Bu-Cbz
remains identical, 1.03 × 10<sup>-5</sup> cm<sup>2</sup>·V<sup>-1</sup>·s<sup>-1</sup>, indicating that the
linear mobility μ<sub>FE-lin</sub> is almost unaffected by traps into the
OSC. Oppositely, μ<sub>FE-lin</sub> of [4]L-Bu-Cbz is significantly
decreased compared to μ<sub>FE-sat</sub> (3.48 × 10<sup>-5</sup> vs 1.10 × 10<sup>-4</sup>
cm<sup>2</sup>·V<sup>-1</sup>·s<sup>-1</sup>), showing the importance of traps in the layer
organization. The ability to accumulate charges at the
OSC/insulator interface is reflected through two key electrical
parameters: the threshold voltage (V<sub>TH</sub>, gate-source voltage
needed for the channel to be populated) and subthreshold
slope (SS, voltage required to increase the current at the
OSC/insulator interface by one order of magnitude). The SS
of [4]C-Bu-Cbz is significantly lower than that of [4]L-Bu-
Cbz, 0.89 and 4.24 V/dec respectively. The V<sub>TH</sub> values
measured are also in accordance with this trend, -12.8 v<sub>S</sub> -
20.6 V for [4]C-Bu-Cbz and [4]L-Bu-Cbz respectively
(Table S6). These data fully corroborate those obtained with
μ<sub>FE-lin</sub>/μ<sub>FE-sat</sub> and suggest that the nanohoop promotes a better
organization of the molecules in thin films whereas, in the
linear oligomer, the density of structural defects is increased,
which traps the charge carriers.

The carrier mobilities were also extracted in a two-
terminal device by applying the Mott-Gurney model to the I-
V measurements (space-charge-limited current SCLC
transport, Figure S18). The carrier mobility, μ<sub>carrier</sub>, was
measured at 2.78 × 10<sup>-4</sup> cm<sup>2</sup>·V<sup>-1</sup>·s<sup>-1</sup> for [4]C-Bu-Cbz, nearly
two times higher than that of [4]L-Bu-Cbz (1.49 × 10<sup>-4</sup>
cm<sup>2</sup>·V<sup>-1</sup>·s<sup>-1</sup>), Table S6. Note that the hole mobility of [4]C-
Bu-Cbz is also significantly higher by one decade to that very
recently reported for a nanohoop incorporating graphenic
hexabenzocoronene unit.[33] More interestingly, the applied
voltage needed to fill the traps (trap free region in SCLC
model) in the case of [4]L-Bu-Cbz is higher, 28 V, than that
needed, 19 V, in the case of [4]C-Bu-Cbz, confirming a
higher trap density in the film made of [4]L-Bu-Cbz. From
these studies, we can conclude that [4]C-Bu-Cbz displays
better transport characteristics with less defects density than
its linear analog [4]L-Bu-Cbz. This feature can be beneficial
for the PhOLEDs investigated below.

Finally, red PhOLEDs (Figure 3) with the following architectures: ITO/Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/ 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (45 nm)/ 4,4',4"-tris(carbazol-9-yl)-triphenylamine (TcTa) (8 nm)/ [4]C-Bu-Cbz or [4]L-Bu-Cbz:Ir(MDQ)<sub>2</sub>(acac) (20 wt%, 20 nm)/1,3,5-tris(6-(3-(pyridin-3-yl)phenyl)pyridin-2-yl)benzene (TmPyPB) (55 nm)/Lithium fluoride (LiF) (1.2 nm)/Al (100 nm) were fabricated. PEDOT:PSS and LiF were selected as the hole- and electron-injection layers, and, TAPC and TmPyPB employed as the hole- and electron-transporting layers, respectively and TcTa as exciton-blocking layer. HOMO and LUMO energy levels of [4]C-Bu-Cbz (-5.18 / -2.40 eV[20]) and [4]L-Bu-Cbz (-5.30 / -2.17 eV) were obtained by cyclic voltammetry (Figure S14), those of TAPC, TcTa and TmPyPB were taken from literature (see Figure S16 for molecular structures).[37]

First, regarding the optical properties, no contribution of
the host is detected in the electroluminescent (EL) spectrum,
the devices exhibiting a red emission arising exclusively from
Ir(MDQ)₂acac (Figure 3C). This confirms good excitons
transfer from the matrix to the dopant with both [4]C-Bu-Cbz
and [4]L-Bu-Cbz. The EL spectrum of the device is identical
to the thin-film one presented above (Figure 3A), showing the
similitude of the radiative deactivation processes involved.
<figure><img src="image_5.png" /><figcaption>Figure 3. Normalized steady state photoluminescence, $\lambda_{exc} = 400$ nm (A), and time-resolved photoluminescence, $\lambda_{exc} = 310$ nm, $\lambda_{em} = 650$ nm (B), of 20 % doped Ir(MDQ)₂acac into [4]L-Bu-Cbz or into [4]C-Bu-Cbz. Normalized electroluminescence spectra (C) and performances: Current density and luminance as a function of the bias voltage (D); power efficiency and current efficiency as a function of the current density (E); Roll-off EQE as a function of the luminance (F) of red PhOLEDs using as host matrix either [4]C-Bu-Cbz (blue lines), [4]L-Bu-Cbz (black lines) or CBP (red lines). Schematic representations of the PhOLED stack used in this work (G and H).</figcaption></figure>

<table><caption>Table 1. PhOLEDs characteristics using as host matrix either [4]C-Bu-Cbz, [4]L-Bu-Cbz or CBP.</caption><thead><tr><th>Host</th><th>V<sub>ON</sub><br>(V)</th><th>EQE<br>(%)</th><th>CE<br>(cd.A<sup>-1</sup>)</th><th>PE<br>(lm.W<sup>-1</sup>)</th><th>EQE<br>(%)</th><th>CE<br>(cd.A<sup>-1</sup>)</th><th>PE<br>(lm.W<sup>-1</sup>)</th><th>CIE coordinates<br>(x ; y)</th></tr></thead><tbody><tr><td></td><td colspan="4">At 10 mA.cm<sup>-2</sup></td><td colspan="3">Max (at J (mA.cm<sup>-2</sup>))</td><td>At 10 mA.cm<sup>-2</sup></td></tr><tr><td>[4]C-Bu-Cbz</td><td>2.3</td><td>12.6</td><td>15.1</td><td>6.8</td><td>17.0<br>(0.01)</td><td>20.6<br>(0.01)</td><td>25.8<br>(0.01)</td><td>0.65 ; 0.35</td></tr><tr><td>[4]L-Bu-Cbz</td><td>2.2</td><td>8.9</td><td>10.4</td><td>6.8</td><td>11.1<br>(0.07)</td><td>13.0<br>(0.07)</td><td>15.7<br>(0.07)</td><td>0.65 ; 0.35</td></tr><tr><td>CBP</td><td>2.5</td><td>17.2</td><td>19.9</td><td>10.1</td><td>18.4<br>(0.08)</td><td>21.4<br>(0.08)</td><td>22.4<br>(0.08)</td><td>0.65 ; 0.35</td></tr></tbody></table>
[4]C-Bu-Cbz as host in Ir(MDQ)₂acac-based device (Figure 3D-F, blue lines) displays a very high EQE of 17 % (at 0.01 mA.cm⁻²) with a low threshold voltage (VON) of 2.3 V. The corresponding current (CEmax) and power (PEmax) efficiencies are measured at 20.6 cd.A⁻¹/25.8 lm.W⁻¹ at 0.01 mA.cm⁻². These data translate the efficiency of this device in terms of charges injection, transportation, and recombination, showing the efficiency of [4]C-Bu-Cbz as a host. Interestingly, [4]L-Bu-Cbz was also incorporated as a host with the same device architecture, displaying significantly lower performances. Indeed, the EQEmax is measured at 11.1 %, CEmax at 13.0 cd.A⁻¹ and PEmax at 15.7 lm.W⁻¹ (at 0.07 mA.cm⁻²), Figure 3D-F, black lines. At higher current densities, 10 mA.cm⁻², the EQE of [4]C-Bu-Cbz remains significantly higher than that of [4]L-Bu-Cbz: 12.6 % vs 8.9 %. These data demonstrate that [4]C-Bu-Cbz allows a better charge injection from adjacent layers to the EML and excitons transfer within the EML than [4]L-Bu-Cbz. Moreover, as shown with the charge transport studies, the intrinsic lower trap density of [4]C-Bu-Cbz vs [4]L-Bu-Cbz might help to increase the ratio of charge recombination and extraction of photons.

To interpret the different performances, phosphorescence lifetimes of the EMLs were first investigated (Figure 3B). The EMLs were exactly those used in the above mentioned PhOLEDs (20 wt% Ir(MDQ)₂acac). [4]C-Bu-Cbz and [4]L-Bu-Cbz based EMLs display radiative deactivation lifetimes of 0.47 and 0.57 µs at 650 nm, respectively. Thus, one can note that the lifetime is shorter for [4]C-Bu-Cbz than for [4]L-Bu-Cbz, which might help to reduce the triplet states density and the possibility of triplet-triplet annihilation (TTA),[38-42] resulting in higher device performances. The link between short deactivation of the EML and the efficiency of the device has been recently shown and is a key parameter in PhOLED efficiency.[37]

To gain insight into the surface morphology, AFM studies have been performed on the PhOLED active layer with the same stack (PEDOT-PSS, 40 nm)/TAPC, 45 nm/TcTa, 8 nm/ [4]C-Bu-Cbz or [4]L-Bu-Cbz: Ir(MDQ)₂acac 20 wt%, 20 nm, Figure 4) in order to well visualize the interface effects. This study reveals equivalent film roughness for both [4]L-Bu-Cbz and [4]C-Bu-Cbz with root mean surface roughness Rq of 0.57 and 0.59 nm respectively (Table S7). Maximum vertical distance between the highest and lowest peak is also equivalent with Rmax values of 4.8 and 4.3 nm respectively. The interface between the TcTa and the emissive layer is then not responsible in the different PhOLEDs performances. Focusing on the general aspect of AFM images, [4]C-Bu-Cbz film is nevertheless smoother than that of [4]L-Bu-Cbz with spreading peaks. This feature could be linked to larger grain sizes and thus less defects density into [4]C-Bu-Cbz film than into [4]L-Bu-Cbz, in accordance with the higher performance of the former.

Another important feature arises from molecular orbital alignment in the device. Indeed, [4]C-Bu-Cbz displays a lower LUMO level than [4]L-Bu-Cbz (-2.40 vs -2.17 eV) leading to a better alignment with the LUMO of the electron transporting layer, TmPyPB (-2.70 eV). This characteristic is also surely involved in the higher performance obtained for the nanohoop. It is known for CPPs that their LUMO energy levels are lower than those of their linear analogues with the same number of phenyl units.[43] Herein, [4]C-Bu-Cbz and [4]L-Bu-Cbz display an identical trend. In PhOLEDs, this

characteristic of nanohoops can be advantageously used to adjust the alignment of LUMOs, which in turn increases the performance.

Thus, the LUMO energy level, the organization of the active layer and the deactivation lifetime appear as first arguments to explain the higher performances of [4]C-Bu-Cbz vs [4]L-Bu-Cbz.

<figure><img src="image_6.png" /><figcaption>Figure 4. 2D (500 × 500nm²) AFM images of (A) [4]L-Bu-Cbz and (B) [4]C-Bu-Cbz.</figcaption></figure>

Finally, in order to evaluate the efficiency of [4]C-Bu-Cbz with state of the art materials, similar devices with the same architecture but using the well-known benchmark and efficient host, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl, CBP, were investigated, Figure 3D-F and Table 1. CBP represents one of the most used commercially available host material for PhOLEDs incorporating Ir(MDQ)₂acac as red phosphorescent guest.[44-46] Thus, an emissive layer constituted of CBP doped with Ir(MDQ)₂acac leads to a device with a VON of 2.5 V, and EQEmax of 18.4 %, CEmax of 21.4 cd.A⁻¹ and PEmax of 22.4 lm.W⁻¹ at 0.08 mA.cm⁻². These performances are higher than those of [4]C-Bu-Cbz but nevertheless highlights the potential of nanohoops as host materials for PhOLEDs.

To summarize, we report herein the first high-performance electronic device using a nanohoop as active layer. Compared to its linear analogue [4]L-Bu-Cbz, [4]C-Bu-Cbz displays a lower ET₁, 2.26 vs 2.38 eV, showing the impact of the nanohoop strain on the ET₁. This unexpected finding reveals an unknown aspect of nanohoop that is the impact of strain on the phosphorescence properties. If the electronic properties of nanohoops start to be known especially for the flagship family of nanohoops CPPs, their applications, especially in organic electronics, remain almost unexplored. The charge transport studies also reveal better electrical properties for [4]C-Bu-Cbz vs [4]L-Bu-Cbz, due to a better organization of the film, highlighting a new appealing characteristic for nanohoops. Incorporated as host material in the EML of a PhOLED, [4]C-Bu-Cbz efficiently hosts the red phosphor Ir(MDQ)₂acac with a high EQE of 17.0 %, a CE of 20.6 cd.A⁻¹ and a PE of 25.8 lm.W⁻¹. The PhOLED also displays a low VON of 2.3 V, translating a good charge injection. These performances are significantly higher than those of its linear analogue [4]L-Bu-Cbz (EQE = 11.1 %, a CE of 13 cd.A⁻¹ and a PE of 15.7 lm.W⁻¹), revealing the
influence of the cyclic vs linear geometry. These performances are already in the same range than those of benchmark materials such as **CBP** investigated herein. These findings show the potential of nanohoop structures in the emerging technologies of organic electronics. As the synthesis of nanohoops becomes now as efficient as linear ones (Figure 1), their use in electronics will continue to grow. With more accurate molecular designs, there is no doubt that the performance of nanohoops will also rapidly increase. By increasing the $E_{T1}$, green or even blue phosphors could be, for example, used. Recent works have shown that the nanohoop size has a significant impact on several electronic properties, e.g. HOMO and LUMO energies and fluorescence.[3, 7, 27, 47-53]

[54] However, the evolution of $E_{T1}$ as a function of the nanohoop size (and its resulting strain) is still in its infancy.[27-29] This is a key step for further exploring materials designs, in order to tune $E_{T1}$ of nanohoops, allowing in turn to fit the energy transfers within the PhOLEDs. This will deeply contribute to the development of nanohoops as organic materials for electronics. We are currently working in this direction.

## Experimental Section

Experimental Details can be found in Supplementary Information.

## Acknowledgements

This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 699648 (FRODO). We thank GENCI (Project N°AD0100805032R1), the ANR (n°19-CE05-0024), the Agence de l'Environnement et de la Maîtrise de l'énergie (ADEME) for PhD grant (CB), Dr Bruno Lafitte (ADEME), the CRMPO (Rennes). We also thank Dr B. Le Guennic (Rennes) for his help in theoretical calculations, Dr J.F Bergamini (Rennes) for the graphical abstract design and Dr Ludovic Favereau and Dr Julien Boixel (Rennes) for assistance in spectroscopy and Inorganic Theoretical Chemistry Team at ISCR (Rennes) for allocation of computational time.

## References

[1]E. J. Leonhardt, R. Jasti, *Nat. Rev. Chem.* **2019**, *3*, 672-686.

[2]H. Omachi, Y. Segawa, K. Itami, *Acc. Chem. Res.* **2012**, *45*, 1378-1389.

[3]E. R. Darzi, R. Jasti, *Chem. Soc. Rev.* **2015**, *44*, 6401-6410.

[4]M. R. Golder, R. Jasti, *Acc. Chem. Res.* **2015**, *48*, 557-566.

[5]S. E. Lewis, *Chem. Soc. Rev.* **2015**, *44*, 2221-2304.

[6]S. Yamago, E. Kayahara, T. Iwamoto, *Chem. Rec.* **2014**, *14*, 84-100.

[7]Y. Segawa, A. Kukazawa, S. Matsuura, H. Omachi, S. Yamaguchi, S. Irle, K. Itami, *Org. Biomol. Chem.* **2012**, *10*, 5979-5984.

[8]B. M. White, Y. Zhao, T. E. Kawashima, B. P. Branchaud, M. D. Pluth, R. Jasti, *ACS Cent. Sci.* **2018**, *4*, 1173-1178.

[9]K. Miki, K. Saiki, T. Umeyama, J. Baek, T. Noda, H. Imahori, Y. Sato, K. Suenaga, K. Ohe, *Small* **2018**, *14*, 1800720.

[10]T. Iwamoto, Z. Slanina, N. Mizorogi, J. Guo, T. Akasaka, S. Nagase, H. Takaya, N. Yasuda, T. Kato, S. Yamago, *Chem. Eur. J.* **2014**, *20*, 14403-14409.

[11]Y. Nakanishi, H. Omachi, S. Matsuura, Y. Miyata, R. Kitaura, Y. Segawa, K. Itami, H. Shinohara, *Angew. Chem. Int. Ed.* **2014**, *53*, 3102-3106.

[12]Y. Xu, R. Kaur, B. Wang, M. B. Minameyer, S. Gsänger, B. Meyer, T. Drewello, D. M. Guldi, M. von Delius, *J. Am. Chem. Soc.* **2018**, *140*, 13413-13420.

[13]H. Sakamoto, T. Fujimori, X. Li, K. Kaneko, K. Kan, N. Ozaki, Y. Hijikata, S. Irle, K. Itami, *Chem. Sci.* **2016**, *7*, 4204-4210.

[14]T. A. Schaub, E. A. Prantl, J. Kohn, M. Bursch, C. R. Marshall, E. J. Leonhardt, T. C. Lovell, L. N. Zakharov, C. K. Brozek, S. R. Waldvogel, S. Grimme, R. Jasti, *J. Am. Chem. Soc.* **2020**, *142*, 8763-8775.

[15]M. J. Heras Ojea, J. M. Van Raden, S. Louie, R. Collins, D. Pividori, J. Cirera, K. Meyer, R. Jasti, R. A. Layfield, *Angew. Chem. Int. Ed.* **2021**, *60*, 3515-3518.

[16]J. L. Bredas, S. R. Marder, *The WSPC References on Organic Electronics: Organic Semi-Conductors*, World Scientific

[17]Z.-Q. Jiang, C. Poriel, N. Leclerc, *Mater. Chem. Front.* **2020**, *4*, 2497-2498.

[18]Y.-Y. Liu, J.-Y. Lin, Y.-F. Bo, L.-H. Xie, M.-D. Yi, X.-W. Zhang, H.-M. Zhang, T.-P. Loh, W. Huang, *Org. Lett.* **2016**, *18*, 172-175.

[19]F. Lucas, L. Sicard, O. Jeannin, J. Rault-Berthelot, E. Jacques, C. Quinton, C. Poriel, *Chem. Eur. J.* **2019**, *25*, 7740-7748.

[20]F. Lucas, N. McIntosh, E. Jacques, C. Lebreton, B. Heinrich, B. Donnio, O. Jeannin, J. Rault-Berthelot, C. Quinton, J. Cornil, C. Poriel, *J. Am. Chem. Soc.* **2021**, *143*, 8804-8820.

[21]M. Ball, Y. Zhong, B. Fowler, B. Zhang, P. Li, G. Etkin, D. W. Paley, J. Decatur, A. K. Dalsania, H. Li, S. Xiao, F. Ng, M. L. Steigerwald, C. Nuckolls, *J. Am. Chem. Soc.* **2016**, *138*, 12861-12867.

[22]O. Koçak, I. P. Duru, I. Yavuz, *Advanced Theory and Simulations* **2019**, *2*, 1800194.

[23]M. Ball, C. Nuckolls, *ACS Cent. Sci.* **2015**, *1*, 416-417.

[24]V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R. Silbey, J. L. Brédas, *Chem. Rev.* **2007**, *107*, 926-952.

[25]W.-S. Wong, M. Stepień, *Trends in Chemistry* **2022**, *4*, 573-576.

[26]K. Itami, H. Shudo, M. Kuwayama, M. Shimasaki, T. Nishihara, Y. Takeda, T. Kuwabara, A. Yagi, Y. Segawa, *ChemRxiv* **2021**, *1-6*.
[27]M. Fujitsuka, C. Lu, B. Zhuang, E. Kayahara, S. Yamago, T. Majima, *J. Phys. Chem. A* **2019**, *123*, 4737-4742.

[28]D. A. Hines, E. R. Darzi, R. Jasti, P. V. Kamat, *J. Phys. Chem. A* **2014**, *118*, 1595-1600.

[29]D. A. Hines, E. R. Darzi, E. S. Hirst, R. Jasti, P. V. Kamat, *J. Phys. Chem. A* **2015**, *119*, 8083-8089.

[30]H. Zhu, I. Badía-Domínguez, B. Shi, Q. Li, P. Wei, H. Xing, M. C. Ruiz Delgado, F. Huang, *J. Am. Chem. Soc.* **2021**, *143*, 2164-2169.

[31]F. Lucas, C. Quinton, S. Fall, T. Heiser, D. Tondelier, B. Geffroy, N. Leclerc, J. Rault-Berthelot, C. Poriel, *J. Mater. Chem. C* **2020**, *8*, 16354-16367.

[32]E. Kayahara, L. Sun, H. Onishi, K. Suzuki, T. Fukushima, A. Sawada, H. Kaji, S. Yamago, *J. Am. Chem. Soc.* **2017**, *139*, 18480-18483.

[33]S. Wang, X. Li, K. Wei, X. Zhang, S. Yang, G. Zhuang, P. Du, *Eur. J. Org. Chem.*, e202101493.

[34]J.-D. Peltier, B. Heinrich, B. Donnio, O. Jeannin, J. Rault-Berthelot, E. Jacques, C. Poriel, *J. Mater. Chem. C* **2018**, *6*, 13197-13210.

[35]J.-D. Peltier, B. Heinrich, B. Donnio, E. Jacques, J. Rault-Berthelot, C. Poriel, *ACS Appl. Mater. Interfaces* **2017**, *9*, 8219-8232.

[36]S. Bebiche, P. Cisneros-Perez, T. Mohammed-Brahim, M. Harnois, J. Rault-Berthelot, C. Poriel, E. Jacques, *Mater. Chem. Front.* **2018**, *2*, 1631-1641.

[37]Q. Wang, F. Lucas, C. Quinton, Y.-K. Qu, J. Rault-Berthelot, O. Jeannin, S.-Y. Yang, F.-C. Kong, S. Kumar, L.-S. Liao, C. Poriel, Z.-Q. Jiang, *Chem. Sci.* **2020**, *11*, 4887-4894.

[38]A. Köhler, H. Bässler, *Mat. Sci. Eng. C-Reports* **2009**, *66*, 71-109.

[39]F. Steiner, J. Vogelsang, J. M. Lupton, *Phys. Rev. Lett.* **2014**, *112*, 137402.

[40]M. A. Baldo, C. Adachi, S. R. Forrest, *Phys. Rev. B* **2000**, *62*, 10967-10977.

[41]Y.-K. Wang, Q. Sun, S.-F. Wu, Y. Yuan, Q. Li, Z.-Q. Jiang, M.-K. Fung, L.-S. Liao, *Adv. Funct. Mat.* **2016**, *26*, 7929-7936.

[42]Q. Wang, Q.-S. Tian, Y.-L. Zhang, X. Tang, L.-S. Liao, *J. Mater. Chem. C* **2019**, *7*, 11329-11360.

[43]T. Iwamoto, Y. Watanabe, Y. Sakamoto, T. Suzuki, S. Yamago, *J. Am. Chem. Soc.* **2011**, *133*, 8354-8361.

[44]Y. L. Chang, Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, Z. H. Lu, *Org. Electron.* **2012**, *13*, 925-931.

[45]J. P. Duan, P. P. Sun, C. H. Cheng, *Adv. Mater.* **2003**, *15*, 224-228.

[46]C. Zang, X. Peng, H. Wang, Z. Yu, L. Zhang, W. Xie, H. Zhao, *Org. Electron.* **2017**, *50*, 106-114.

[47]M. Fujitsuka, D.-W. Cho, T. Iwamoto, S. Yamago, T. Majima, *Phys. Chem. Chem. Phys.* **2012**, *14*, 14585-14588.

[48]R. Jasti, J. Bhattacharjee, J. Neaton, C. R. Bertozzi, *J. Am. Chem. Soc.* **2008**, *130*, 17646-17647.

[49]G. Povie, Y. Segawa, T. Nishihara, Y. Miyauchi, K. Itami, *J. Am. Chem. Soc.* **2018**, *140*, 10054-10059.

[50]L. Wang, S.-H. Chen, D. He, Q.-J. Li, Y.-L. Liu, M.-S. Wang, *J. Phys. Chem. C* **2020**, *124*, 11081-11091.

[51]H. Jia, G. Zhuang, Q. Huang, J. Wang, Y. Wu, S. Cui, S. Yang, P. Du, *Chem. Eur. J.* **2020**, *26*, 2159-2163.

[52]Y. Nakagawa, R. Sekiguchi, J. Kawakami, S. Ito, *Org. Biomol. Chem.* **2019**, *17*, 6843-6853.

[53]L. Sicard, F. Lucas, O. Jeannin, P. A. Bouit, J. Rault-Berthelot, C. Quinton, C. Poriel, *Angew. Chem. Int. Ed.* **2020**, *59*, 11066-11072.

[54]F. Lucas, J. Rault-Berthelot, C. Quinton, C. Poriel, *J. Mater. Chem. C* **2022**, *10*, 14000-14009.