# Acid/Base-Responsive Circularly Polarized Luminescence Emitters with Configurationally Stable Nitrogen Stereogenic Centers

Pablo García-Cerezo, Marcos Codesal, Arthur David, Laura Le Bras, Seifallah Abid, Xuesong Li, Delia Miguel, Masoud Kazem-Rostami, Benoît Champagne, Araceli Campaña, et al.

► To cite this version:

Pablo García-Cerezo, Marcos Codesal, Arthur David, Laura Le Bras, Seifallah Abid, et al. Acid/Base-Responsive Circularly Polarized Luminescence Emitters with Configurationally Stable Nitrogen Stereogenic Centers. Advanced Materials, 2025, 37, pp.2417326. 10.1002/adma.202417326. hal-05142748

HAL Id: hal-05142748

https://hal.science/hal-05142748v1

Submitted on 3 Jul 2025

**HAL** is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published 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_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Acid/Base-Responsive Circularly Polarized Luminescence Emitters with Configurationally Stable Nitrogen Stereogenic Centers

Pablo García-Cerezo, Marcos D. Codesal, Arthur H. G. David, Laura Le Bras,*

Seifallah Abid, Xuesong Li, Delia Miguel, Masoud Kazem-Rostami, Benoît Champagne,

Araceli G. Campaña, J. Fraser Stoddart,* and Victor Blanco*

A way to prevent the fast configurational interconversion of tertiary amines is to invoke Tröger's base analogs, which display methano- or ethano-bridged diazocine cores fused to aromatic rings. These derivatives are configurationally stable, even in acidic media when their structures bear ethylene bridges. Here, a two- to three-step synthesis is presented of methano- and ethano-bridged Tröger's base analogs with two peripheral fluorophores, i.e., anthracene, pyrene, and 9,9-dimethylfluorene units. These compounds, possessing two nitrogen stereogenic centers, exhibit good circularly polarized luminescence (CPL) dissymmetry factors ($|g_{lum}|$ up to $1.2 \times 10^{-3}$) and brightnesses ($B_{CPL}$ up to $26.3 \text{ M}^{-1} \text{ cm}^{-1}$), as well as excellent fluorescence quantum yields, demonstrating the Tröger's base core to be a convenient scaffold to prepare CPL emitters upon functionalization with simple achiral fluorophores. Furthermore, the configurationally stable ethano-bridged Tröger's base analogs are employed to modulate their CPL response, generating a CPL switch through their protonation/deprotonation by consecutive additions of acid and base. The reversibility of the switching process is demonstrated for two cycles without altering the CPL performance of the molecule. It is believed that this straightforward and efficient approach to building CPL emitters employing the Tröger's base core could lead to its incorporation in CPL-based sensors and materials.

stereogenic centers. The vast majority of point chiral stereogenic centers are, however, limited to carbon atoms, although the bond geometry of other atoms, such as nitrogen, can also give rise to chirality. The difficulty in obtaining configurationally stable point chiral nitrogen atoms resides in the fact the lone pair of nitrogen atoms is inverting[1] through quantum tunneling, resulting in the fast configurational inversion of tertiary amines carrying three different substituents. This racemization process can be hampered by the preparation[2] of ammonium cations, which are widely used in medicine[3] and asymmetric catalysis,[4] N-oxides[5] or chloro-aziridines.[6] Other alternatives are Tröger's base analogs,[7] discovered in 1887 by Julius Tröger.[8] Their structures, which were elucidated[9] and confirmed[10] by X-ray crystallography decades later, consist of bicyclic compounds with central methanodiazocine cores and two peripheral aromatic rings. The chiral resolution of these V-shaped compounds, possessing $C_2$ symmetry and two point

chiral nitrogen atoms, was performed in 1944 by Prelog and Wieland.[11] The bicyclic structure of the Tröger's base core locks the configuration at the chiral N centers, precluding its inversion, which is possible only if a covalent bond between the methylene

## 1. Introduction

Chirality is of the utmost importance in nature and life since a plethora of naturally occurring organic compounds include

P. García-Cerezo, M. D. Codesal, A. G. Campaña, V. Blanco
Departamento de Química Orgánica
Facultad de Ciencias

Unidad de Excelencia de Química Aplicada a Biomedicina y
Medioambiente (UEQ)

Universidad de Granada (UGR)

Avda. Fuente Nueva S/N, Granada 18071, Spain

E-mail: victorblancos@ugr.es

<figure><img src="image_3.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202417326

© 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

†Sir Fraser Stoddart passed away on 30 December 2024.

DOI: 10.1002/adma.202417326

A. H. G. David, S. Abid, X. Li, M. Kazem-Rostami, J. F. Stoddart†
Department of Chemistry

Northwestern University
2145 Sheridan Road, Evanston, IL 60208, USA
E-mail: stoddart@hku.hk

A. H. G. David
Laboratoire MOLTECH-Anjou (UMR CNRS 6200)
Université Angers
2 Bd Lavoisier, Angers Cedex 49045, France

L. Le Bras
CNRS

Chrono-environnement (UMR 6249)

Université Marie et Louis Pasteur
Besançon F-25000, France

E-mail: laura.le_bras@univ-fcomte.fr
group and one of the N atoms is broken. Nonetheless, methano-bridged Tröger's base analogues racemize in acidic media.[11,12] The classical proposed mechanism postulates the formation of a methylene-iminium cation, breaking the methano-bridge, and allowing for the configurational inversion of the nitrogen atoms,[12b,13] with a reported enantiomerization barrier of ca. 100 kJ mol⁻¹.[12c] An alternative mechanism based on a retro-hetero Diels-Alder ring opening, followed by a subsequent hetero Diels-Alder closing has also been proposed in the gas phase.[12a] The presence of an ethylene bridge on Tröger's base analogs, however, renders this racemization process under acidic conditions impossible and, hence, affords configurationally stable nitrogen stereogenic centers.[14] Tröger's base analogs, which have found a myriad of applications in organic electronics and photonics,[15] biotechnology[16] and sensing,[17] have also been incorporated as creative building blocks into innovative materials, e.g., polymers[18] and metal-organic frameworks.[19]

Circularly polarized luminescence (CPL) is a property of non-racemic chiral luminescent species describing their preferential emission of left- or right-handed circularly polarized light.[20] The investigation on this chiroptical property is rapidly accelerating[21] as CPL acquisition technology becomes more widely available, leading to the emergence of novel applications ranging from chiroptical security inks,[22] and sensing,[23] to circularly polarized organic light-emitting diodes (CP-OLEDs)[24] and lasers.[25] Among CPL-emissive organic molecules,[26] the

majority of those with covalent point chirality possess asymmetric carbon atoms.[21d,27] The investigation of CPL-emissive compounds with point chiral nitrogen stereogenic centers remains underexplored, with just a few examples reported.[28,29] Thus, Jiang, Wang, and co-workers[28] have built a CPL-emissive methano-bridged Troger's base analog while Yoshigoe and co-workers[29] have recently developed a methano-bridged Troger's base-containing cycloparaphenylene (CPP). The chiral nitrogen atoms in the system prepared by Jiang and Wang,[28] however, are configurationally unstable under acidic conditions on account of the presence of the methylene bridge. Although the CPP-methano-bridged Troger's base hybrid reported by Yoshigoe[29] is configurationally stable on account of the rigidity of the cyclophane prohibiting the chiral inversion process under acidic conditions, its synthesis requires numerous demanding steps. The synthesis of simpler CPL emitters with configurationally stable chiral nitrogen atoms is yet to be explored. In this context, the construction of ethano-bridged Tröger's base analogs with configurationally stable nitrogen stereogenic centers, capable of emitting circularly polarized light, is fundamentally relevant and could be of key importance in the future development of smart photonic materials, sensors, and devices.

Herein, we report the efficient and straightforward three-step syntheses of Tröger's base analogs bearing two peripheral fluorophores namely, anthracene, pyrene or 9,9-dimethylfluorene units while displaying point chirality from configurationally stable nitrogen stereogenic centers (Figure 1). UV-vis absorption and emission spectroscopies have allowed us to visualize the optical properties of the different structures. Moreover, the chiral resolution of the enantiomers by chiral stationary phase high performance liquid chromatography (CSP-HPLC) also led to their study by electronic circular dichroism (ECD) and CPL, demonstrating that the chiroptical properties of the appended fluorophores are induced by the nitrogen-based chiral core. Following this approach, it is possible to confer a CPL response (|g<sub>lum</sub>| up to 1.2 × 10⁻³ and CPL brightnesses (B<sub>CPL</sub>) up to 26.3 M⁻¹ cm⁻¹) to otherwise achiral fluorophores which is in line with other standard organic CPL emitters. Remarkably, the (chir)optical properties of the ethano-bridged Tröger's base analogs can be tuned upon the addition of acid in a reversible manner, leading to the design of a CPL switch.

## 2. Results and Discussion

### 2.1. Synthesis

The synthetic route (Scheme 1) starts with the formation of the 2,8-dibromo methano-bridged Tröger's base analog[30] 5 as a result of a condensation reaction between 4-bromoaniline (4) and paraformaldehyde under acidic conditions in 22% yield. Treatment of 5 with 1,2-dibromoethane in the presence of Li₂CO₃ affords 2,8-dibromo ethano-bridged Tröger's base analog[31] 6 in 45% yield. Suzuki–Miyaura coupling[32] between 2,8-dibromo-functionalized intermediates 5 or 6, and the aryl boronic acid or esters of the corresponding fluorophores gave the desired methano- and ethano-bridged Tröger's base analogs **1a,b-3a,b** bearing two peripheral luminescent moieties—namely anthracene, pyrene or 9,9-dimethylfluorene units—in moderate to excellent yields (40–96%). These compounds were characterized

X. Li
Department of Chemistry
University of Wyoming
Laramie, WY 82072, USA

D. Miguel
Nanoscopy-UGR Laboratory, Physical Chemistry Department
UEQ
Faculty of Pharmacy
University of Granada
C. U. Cartuja, Granada 18071, Spain

B. Champagne
Laboratory of Theoretical Chemistry
Namur Institute of Structured Matter (NISM)
University of Namur
rue de Bruxelles, 61, Namur 5000, Belgium

J. F. Stoddart†
Department of Chemistry
The University of Hong Kong
Hong Kong, SAR 999077, China

J. F. Stoddart†
Stoddart Institute of Molecular Science
Department of Chemistry
Zhejiang University
Hangzhou 310027, China

J. F. Stoddart†
ZJU-Hangzhou Global Scientific and Technological Innovation Center
Hangzhou
Hangzhou 311215, China

J. F. Stoddart†
Center for Regenerative Medicine and Department of Medicine
Northwestern University
303 East Superior Street, Chicago, IL 60611, USA

J. F. Stoddart†
School of Chemistry
University of New South Wales
Sydney, NSW 2052, Australia
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Structural formulae of a) (RR) and (SS) methano- and b) ethano-bridged Tröger's base cores displaying the enantiomers and the point chirality of the nitrogen atoms. c) Structural formulae of the photoluminescent point-chiral methano- and ethano-bridged Tröger's base analogs 1a, b-3a, b investigated in this research, bearing two emissive anthracene, pyrene, or 9,9-dimethylfluorene units. The Tröger's base analogs 1b-3b possess configurationally stable stereogenic nitrogen centers in acid media on account of the ethano bridge.</figcaption></figure>

by NMR spectroscopy and high-resolution mass spectrometry (HRMS) (See Supporting Information for details). Noteworthy is the fact that intermediates 5 and 6 offer the opportunity to graft a variety of fluorophores by means of cross-coupling reactions. This synthetic strategy toward the production of chiral compounds with appended fluorophores, and therefore, potential CPL emission, is straightforward and only requires two to three steps, a situation which is rather uncommon for CPL emitters. This approach has a potential for great versatility as, in principle, it could lead to a variety of emissive chiral compounds with CPL activity, by attaching different fluorophores to the Tröger's base core.

## 2.2. Optical Properties

The optical properties of the racemic mixtures of anthracene-, pyrene- and 9,9-dimethylfluorene-functionalized Tröger's base analogs 1a, b-3a, b were investigated by steady-state UV-vis absorption and fluorescence spectroscopies. Both UV-vis absorption spectra (Figure 2a, solid lines) of anthracene-functionalized Tröger's base analogs 1a and 1b are very similar. They exhibit a structured absorption band between 310 and 420 nm with three peaks whose maxima are to be found at 350, 368 (ε = 1.7 × 10⁴ M⁻¹ cm⁻¹ for 1a, and ε = 1.8 × 10⁴ M⁻¹ cm⁻¹ for 1b), and 388 nm. The fluorescence spectra (Figure 2a, dashed lines) of 1a and 1b are also similar and show an emission band between 380 and 510 nm centered at 424 for 1a (Φ<sub>F</sub> = 48%, τ = 4.77 ns) and 427 nm for 1b (Φ<sub>F</sub> = 54%, τ = 5.44 ns). These optical properties can be attributed to the 9-phenylanthracene units,[33] since the methano- or ethano-bridged Tröger's base cores do not show any significant absorbance above 310 nm. Regarding pyrene-functionalized Tröger's base analogs 2a and 2b, their UV-vis absorption spectra (Figure 2b) display similar shapes portrayed by two absorption bands, one between 250 and 300 nm centered on 280 nm, and the other between 300 and 380 nm centered

on 347 nm. Their molar extinction coefficients are, however, different—ε<sub>347</sub> = 6.6 × 10⁴ M⁻¹ cm⁻¹ for 2a, and ε<sub>347</sub> = 5.7 × 10⁴ M⁻¹ cm⁻¹ for 2b. The fluorescence spectra (Figure 2b) of both 2a and 2b display an emission band between 370 and 525 nm. Nonetheless, the fluorescence maximum of 2b (λ<sub>em</sub> = 431 nm, Φ<sub>F</sub> = 61%, τ = 3.05 ns) is red-shifted compared to that (λ<sub>em</sub> = 415 nm, Φ<sub>F</sub> = 66%, τ = 4.27 ns) of 2a. The photoluminescence of both compounds 2a and 2b is characteristic of the peripheral 2-phenylpyrene units.[34] Finally, 9,9-dimethylfluorene-based Tröger's base analogues 3a and 3b show similar UV-vis absorption spectra (Figure 2c) with the main absorption band between 250 and 355 nm, centered on 319 nm. Their molar extinction coefficients, however, are different: ε<sub>319</sub> = 5.8 × 10⁴ M⁻¹ cm⁻¹ for 3a, and ε<sub>319</sub> = 5.0 × 10⁴ M⁻¹ cm⁻¹ for 3b. In common with the pyrene-based Tröger's base analogs, both 3a and 3b emit (Figure 2c) in the same spectral region, between 330 and 480 nm. The emission maximum of 3b (λ<sub>em</sub> = 394 nm, Φ<sub>F</sub> = 76%, τ = 1.91 ns), however, is red-shifted compared to that (λ<sub>em</sub> = 382 nm, Φ<sub>F</sub> = 76%, τ = 1.28 ns) of 3a. The fluorescence of 3a and 3b can be attributed to the peripheral fluorene units.[35] It is worth emphasizing that the quantum yields of all Tröger's base analogs are outstanding: they are higher when a 9,9-dimethylfluorene substituent is incorporated into the structure, reaching a high value of 76%.

## 2.3. Chiroptical Properties

Chiral resolution of Tröger's base analogues 1a, b-3a, b was achieved by using CSP-HPLC.[36] The reader is directed to the Supporting Information for the detailed chiral resolution procedures. It is worth noting that, although in this work we use CSP-HPLC for the enantiomeric resolution, the enantiomers of the Tröger base derivatives could be also accessible by diastereomeric resolution by co-crystallization of the dibromo precursors with (2R,3R)- or (2S,3S)-O,O'-dibenzoyl tartaric acid,[37] followed by the corresponding Suzuki coupling. As proof-of-concept,
<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Synthesis of the fluorescent point chiral methano- and ethano-bridged Tröger's base analogs **1a,b-3a,b**.</figcaption></figure>

we applied this strategy for the preparation of (RR)-2a (see Supporting Information for details).

Next, the chiroptical properties of these enantiopure fluorophore-functionalized Tröger's base analogs (RR)/(SS)-1a,b-3a,b were investigated by a combination of ECD and CPL spectroscopies. **Table 1** summarizes the main optical and chiroptical parameters for the compounds. ECD Spectra (Figure 3a,b, solid lines) of enantiopure anthracene-functionalized Tröger's base analogs (RR)/(SS)-1a,b exhibit several bands within the range 250–415 nm with alternated Cotton effects for 1a and one sign of dichroism value for 1b. The spectra of both compounds are mirror images, as expected for pairs of enantiomers. With respect to the lowest energy transition, (SS)-1a and (SS)-1b show positive Cotton effects at 391 nm (|Δε| ≈ 5 M⁻¹ cm⁻¹, |g<sub>abs</sub>| = |Δε|/ε ≈ 4 × 10⁻⁴), and 393 nm (|Δε| ≈ 8 M⁻¹ cm⁻¹, |g<sub>abs</sub>| ≈ 7 × 10⁻⁴), respectively. For both enantiopure pyrene-functionalized Tröger's base analogs (RR)/(SS)-2a,b, their ECD spectra (Figure 3c,d) show a similar shape composed of several bands in the range 250–395 nm with only one sign of dichroism value. Those spectra are ideal mirror images. With respect to the lowest energy transition, both (SS)-2a and (RR)-2b show very similar behavior since they feature positive Cotton effects at 359 nm (|Δε| ≈ 15 M⁻¹ cm⁻¹, |g<sub>abs</sub>| ≈ 3 × 10⁻⁴), and 357 nm (|Δε| ≈ 16 M⁻¹ cm⁻¹, |g<sub>abs</sub>| ≈ 3 × 10⁻⁴), respectively. Regarding enantiopure 9,9-dimethylfluorene-functionalized Tröger's base analogs (RR)/(SS)-3a,b, their ECD spectra (Figure 3e,f) are mirror images and exhibit again similar shapes consisting of

two bands with alternated Cotton effects. Their magnitude is, however, significantly different since, for the lowest energy transition, (SS)-3a and (SS)-3b display positive Cotton effects at 328 nm (|Δε| ≈ 47 M⁻¹ cm⁻¹, |g<sub>abs</sub>| ≈ 9 × 10⁻⁴), and 328 nm (|Δε| ≈ 26 M⁻¹ cm⁻¹, |g<sub>abs</sub>| ≈ 6 × 10⁻⁴), respectively.

The assignment of the configurations of the Tröger's base analogs was performed by comparison of the experimental and theoretical ECD spectra, which were calculated employing a combination of molecular dynamics (MD) and time-dependent density functional theory (TD-DFT). See the Supporting Information for the computational details. Initially, MD simulations of (RR)-1a-3a and (SS)-1b-3b were performed, highlighting their structural flexibility on account of the rotational motion of the different lateral fluorophores of the Tröger's base analogs. These conformational changes are defined (Figure 4a; Figures S94 and S95, Supporting Information) by a couple of dihedral angles α and β, corresponding to the angle between the benzene ring of the Tröger's base core and the ring of the fluorophore attached to it. Throughout the 20 ns MD simulation (Figure 4b; Figure S95, Supporting Information), anthracene-appended Tröger's base analogs 1a,b exhibit lower flexibilities engendering two main conformations (Table S17, Supporting Information), while pyrene-appended 2a,b and 9,9-dimethylfluorene-appended 3a,b show (Tables S18 and S19, Supporting Information) four and 16 main conformations respectively. These main conformations of Tröger's base analogs (RR)-1a-3a and (SS)-1b-3b were optimized at the CAM-B3LYP/def2tzvp level of theory and their
<figure><img src="image_6.png" /><figcaption>**Figure 2.** UV–vis absorption (solid lines) and normalized emission (dashed lines) spectra of methano- and ethano-bridged Tröger's base analogs a) **1a,b** ($\lambda_{exc}$ = 340 nm), b) **2a,b** ($\lambda_{exc}$ = 340 nm), and c) **3a,b** ($\lambda_{exc}$ = 320 nm) recorded at r.t. in CH$_2$Cl$_2$ at concentrations of ca. $6 \times 10^{-5}$ M (for **1a,b**) and $2 \times 10^{-5}$ M (for **2a,b-3a,b**).</figcaption></figure>

optical and chiroptical properties were computed. The resulting absorption spectra are similar for each conformation and are blue-shifted (Tables S17–S19, Supporting Information) compared to the experimental ones. The absorption maxima $\lambda_{abs}$, found experimentally (Table 1), can be ascribed to HOMO–1/HOMO to LUMO/LUMO+1 transitions with a $n/\pi \to \pi^*$ character (Figures S96–S98, Supporting Information). The ECD spectra computed for the **1a,b-3a,b** conformations (Figure

S99, Supporting Information) exhibit a low (1a,b) or a large (2a,b-3a,b) fluctuation for the same Tröger's base derivative. At this point, the configurational assignment of an enantiomer to a given spectrum is not straightforward since the flexibility of these molecules modifies considerably the resulting ECD properties.

Therefore, in order to increase the accuracy of the computed ECD properties of these compounds,[38] UV–vis spectra (Figure 4c; Figure S95c, Supporting Information) and ECD (Figure 4d; Figures S95d and S100, Supporting Information) spectra have been computed and averaged for Tröger's base analogs (RR)-1a-3a and (SS)-1b-3b over 50 snapshots extracted from MD simulations. The computed UV–vis spectra, for all compounds, are in good agreement with the experimental ones despite a redshift of the theoretical spectra. For (RR)-1a, the global trend of the experimental spectrum is reproduced with alternating negative (310–500 nm), slightly positive (290–310 nm), and negative peaks (under 280 nm) in the ECD theoretical spectrum. For (SS)-1b, the computed ECD spectrum is always positive in the 275–500 nm range (Figure S95, Supporting Information), which is similar to the experimental one. For pyrene-containing Tröger's base analogs, the configurational assignments are not so easy as a clear alternation of the sign of the peaks is found theoretically while experimentally the spectrum is almost fully negative or positive depending on the enantiomers. Nonetheless, for (RR)-2a, the theoretical ECD spectrum reflects two main negative peaks (320–400 and 210–275 nm) which are found experimentally. For (SS)-2b, the computed ECD spectrum, however, is essentially negative and displays a positive peak between 275 and 310 nm. By considering carefully the experimental ECD spectrum of (RR)/(SS)-2b, a brief alternation of the Cotton effect is identified at ca. 273 nm and may be overestimated by the theoretical calculations.[39] Finally, for (RR)-3a and (SS)-3b, the experimental ECD spectrum is reproduced with alternated positive and negative signs for the dichroism value. The combination of MD simulations and TD-DFT calculations appears to be a suitable way to assign configurations to the enantiomers of these flexible fluorophore-appended Tröger's base analogs.

CPL Spectra (Figure 3, dashed lines) of the photoluminescent enantiopure methano- and ethano-bridged Tröger's base analogs (RR)/(SS)-1a,b-3a,b were recorded upon excitation with UV–vis light. The six enantiomeric pairs of the point chiral nitrogen-based compounds (RR)/(SS)-1a,b-3a,b afford mirror image CPL responses between ca. 380 and 500 nm for the anthracene and pyrene-functionalized Tröger's base analogs (RR)/(SS)-1a,b-2a,b, and between 330 and 475 nm for the 9,9-dimethylfluorene-functionalized Tröger's base analogs (RR)/(SS)-3a,b. These compounds exhibit dissymmetry factors, $|g_{\text{lum}}|$, ranging from $2.5 \times 10^{-4}$ to $1.2 \times 10^{-3}$. Furthermore, the signs of the CPL signals are in line with those obtained by ECD spectroscopy for the lowest energy absorption transition for each compound. Although $|g_{\text{lum}}|$ values give valuable information on the CPL response of non-racemic chiral fluorophores, the efficiency of CPL emitters cannot be analyzed exclusively in terms of this parameter, since, for example, the emission intensity is also relevant. As a result, another parameter, the CPL brightness[41,42] ($B_{\text{CPL}}$), has been introduced recently to evaluate and compare the performance of CPL-active compounds. The $B_{\text{CPL}}$ of compounds **1a,b-3a,b** ranges from $1.02 \text{ to } 26.3 \text{ M}^{-1} \text{ cm}^{-1}$. The highest value is obtained for compound **3a**, because of its excellent fluorescence quantum yield
<table><caption>Table 1. Summary of the Photophysical Properties of the Chiral Tröger's Base Analogues 1a,b-3a,b in CH₂Cl₂.</caption><thead><tr><th>Compound</th><th>λ<sub>abs</sub> / nm</th><th>ε / M<sup>-1</sup> cm<sup>-1</sup></th><th>λ<sub>em</sub><sup>a)</sup> / nm</th><th>Φ<sub>F</sub> / %</th><th>τ<sup>b)</sup> / ns</th><th>|g<sub>abs</sub>|<sup>c)</sup></th><th>|g<sub>lum</sub>|</th><th>B<sub>CPL</sub><sup>d)</sup> / M<sup>-1</sup> cm<sup>-1</sup></th></tr></thead><tbody><tr><td>1a</td><td>368</td><td>1.7 × 10<sup>4</sup></td><td>424</td><td>48</td><td>4.77</td><td>4 × 10<sup>-4</sup></td><td>2.5 × 10<sup>-4</sup></td><td>1.02</td></tr><tr><td>1b</td><td>368</td><td>1.8 × 10<sup>4</sup></td><td>427</td><td>54</td><td>5.44</td><td>7 × 10<sup>-4</sup></td><td>4.6 × 10<sup>-4</sup></td><td>2.24</td></tr><tr><td>2a</td><td>347</td><td>6.6 × 10<sup>4</sup></td><td>415</td><td>66</td><td>4.27</td><td>3 × 10<sup>-4</sup></td><td>5.8 × 10<sup>-4</sup></td><td>12.6</td></tr><tr><td>2b</td><td>347</td><td>5.7 × 10<sup>4</sup></td><td>431</td><td>61</td><td>3.05</td><td>3 × 10<sup>-4</sup></td><td>5.3 × 10<sup>-4</sup></td><td>9.21</td></tr><tr><td>3a</td><td>319</td><td>5.8 × 10<sup>4</sup></td><td>382</td><td>76</td><td>1.28</td><td>9 × 10<sup>-4</sup></td><td>1.2 × 10<sup>-3</sup></td><td>26.3</td></tr><tr><td>3b</td><td>319</td><td>5.0 × 10<sup>4</sup></td><td>394</td><td>76</td><td>1.91</td><td>6 × 10<sup>-4</sup></td><td>6.5 × 10<sup>-4</sup></td><td>12.3</td></tr></tbody></table>

and the higher |g<sub>lum</sub>| in comparison with the remaining derivatives. In general, the B<sub>CPL</sub> values, especially those from 2a,b-3a,b, are similar to those exhibited by a variety of organic compounds, including helicenes, usually considered as standard CPL emitters.[42]

Interestingly, the comparison of the CPL responses of the compounds studied shows a better performance of the 9,9-dimetyfluorenyl derivatives, in special the methano-bridged Tröger's base analog 3a. In general, the chiroptical response in terms of g<sub>lum</sub> value depends on the magnetic and electric dipole transition moments, |m| and |μ|, respectively, and the angle between them (g<sub>lum</sub> = 4(|μ|·|m|·cos θ)/(|μ|² + |m|²) ≈ 4|m|·cos θ/|μ|). Thus, the |g<sub>lum</sub>| value increases if the magnetic dipole transition moment increases, the electric dipole transition moment decreases or the angle between them gets closer to 0° or 180°. The rotational freedom around the bond between the Tröger's base core and the fluorophore unit hampers a more precise study of the electronic transitions of the compounds by TD-DFT because of the high variability of the calculated chiroptical properties upon changing the dihedral angle. From the structure of the compounds, however, we can get some insights that can qualitatively explain to some extent the observed results. It was recently demonstrated[43] that in helical structures the magnetic dipole transition moment increases with the inner area, and |m| also benefits from the electron density being delocalized over the whole structure. Although Tröger's base derivatives are not helical, we can approximate them to being half a turn. In this context, the substitution pattern of the fluorophores, which influence the structures, could also influence the chiroptical response. The substitution of the 9,9-dimethylfluorene in position 2 results in the whole fluorophore extending the "half-turn" and increasing the area and, therefore, |m|. On the contrary, with the anthracene moiety, the extension of the structure is lower on account of its functionalization in the 9-position. Inspection of the dihedral angles between the fluorophore and the Tröger core shows that compounds 3a,b display angles closer to 0° and 180° than derivatives 1a,b-2a,b (Figure 4b; Figure S95b, Supporting Information), resulting in more planar structures. This flatter geometry implies a higher electronic delocalization, as can be seen in the calculated molecular orbitals (Figures S97 and S98, Supporting Information). In contrast, for the anthracene derivatives, the dihedral angles closer to 90° result in no significant delocalization being observed, unlike 3a,b, in which the frontier orbitals are more delocalized over the Tröger core.

The CPL results demonstrate that the functionalization with fluorophores of methano- and configurationally stable ethano-bridged Tröger's base scaffolds is a remarkable and versatile strategy to prepare organic CPL emitters in a straightforward way, with a reduced number of synthetic steps. Thus, the attachment of different fluorophores to this core confers upon them a chiral environment that induces chiroptical responses, in particular, CPL. This strategy could lead to the transformation of a variety of achiral fluorophores into chiral emitters in a synthetically accessible manner.

## 2.4. Acid/Base-Driven Fluorescence and CPL Switching

The presence of aniline rings in the structure of compounds 1a,b-3a,b could enable the tuning of the optical properties, mainly fluorescence, triggered by protonation or deprotonation of the nitrogen atoms. This methodology is especially appealing in the case of configurationally stable derivatives 1b-3b, as it would produce acid/base-controlled CPL switches.[27d,e,44] Hence, we focused on investigating the switching of emission properties of the ethano-bridged Tröger's base analogs. The addition of increasing amounts of trifluoroacetic acid to 1b-3b resulted in a hypochromic shift of their emission bands, which can be attributed to the loss of the electron-donor character of the aniline N atom (Figure 5a; Figures S75-S80, Supporting Information).[45] The changes in the fluorescence, however, are more pronounced in the case of compounds 2b (Figure 5a) and 3b (Figure S79, Supporting Information), with a shift in the λ<sub>em</sub> from 431 to 397 nm (Δλ<sub>em</sub> = -34 nm) and from 394 to 365 nm (Δλ<sub>em</sub> = -29 nm), respectively. This hypochromic shift of the emission band upon protonation is accompanied by a decrease of the fluorescence intensity, as a result of a decrease of the quantum yield (Φ<sub>F</sub> = 45% for 2b and Φ<sub>F</sub> = 47% for 3b) after CF₃CO₂H addition. In contrast, the Tröger's base analog 1b undergoes a less marked shift of only 6 nm (from 427 to 421 nm) with a significant increase (Figure S75, Supporting Information) in the fluorescence intensity and fluorescence quantum yield (Φ<sub>F</sub> = 67%). Clear variations in fluorescence intensity are advisable for the development of CPL switches. In this context, an analysis of the emission profile of 2b revealed (Figure 5b) certain wavelengths at which a drastic change in the emission intensity is observed. In particular, at 430 nm there is a ca. 50% decrease in the fluorescence intensity upon addition of acid, while at 395 nm the intensity

<footnote><sup>a)</sup> Obtained from the fluorescence spectrum of the compounds using the following excitation wavelength: λ<sub>exc</sub> = 340 nm (for 1a,b, and 2a,b); 320 nm (for 3a,b); <sup>b)</sup> Corresponds to τ<sub>1</sub> or τ<sub>average</sub>, see Table S14 (Supporting Information) for more details; <sup>c)</sup> The |g<sub>abs</sub>| values given in this Table correspond to the dimensionless dissymmetry factor values at the maximum of the lowest energy transitions; <sup>d)</sup> Calculated at λ<sub>exc</sub> = 368 nm (for 1a,b); 347 nm (for 2a,b); 319 nm (for 3a,b).</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Chiroptical properties of the photoluminescent methano- and ethano-bridged Tröger's base analogs (RR)/(SS)-1a,b-3a,b. ECD (solid lines) and CPL (dashed lines) spectra of: a) (RR)/(SS)-1a ($\lambda_{exc}$ = 403 nm), b) (RR)/(SS)-1b ($\lambda_{exc}$ = 403 nm), c) (RR)/(SS)-2a ($\lambda_{exc}$ = 372 nm), d) (RR)/(SS)-2b ($\lambda_{exc}$ = 372 nm), e) (RR)/(SS)-3a ($\lambda_{exc}$ = 340 nm), and f) (RR)/(SS)-3b ($\lambda_{exc}$ = 342 nm) recorded at r.t. in CH₂Cl₂ at concentrations of ca. 6 × 10⁻⁵ M (for 1a,b) and 2 × 10⁻⁵ M (for 2a,b-3a,b).</figcaption></figure>

doubles. These changes can be attributed to a decrease in the emission intensity of the signal corresponding to the neutral species ($\lambda_{em}$ = 431 nm) and the appearance of a new band corresponding to the protonated form ($\lambda_{em}$ = 395 nm), with a clear isosbestic point at 410 nm. Fluorescence lifetime analyses ex-

hibited (Figure S78, Supporting Information) a different behavior between neutral and cationic species. In contrast to the single fluorescence lifetime observed for 2b in neutral conditions ($\tau$ = 3.05 ns), addition of trifluoroacetic acid results in the presence of three emissive species with $\tau_1$ = 0.7 ns; $\tau_2$ = 3.0 ns and
<figure><img src="image_8.png" /><figcaption>**Figure 4.** a) Structural formulae of methano-bridged Tröger's base analogs (RR)-1a-3a showing the two dihedral angles $\alpha$ and $\beta$. b) Representation of the dihedral couple ($\alpha$; $\beta$) of (RR)-1a (left), (RR)-2a (middle), and (RR)-3a (right) along the 20 ns MD simulation, where each point represents a snapshot of the simulation. For 1a,b-2a,b, because of steric hindrance, only the half-rotation is possible and the dihedral angles extend over a 180° range (either [0;180] or [-180;0]). Note that for 3a,b, since the complete rotation of the fluorene moieties is possible, the dihedral angles range from -180 to +180°. Averaged calculated c) UV-vis absorption and d) ECD spectra of (RR)-1a (green lines, left), (RR)-2a (red lines, middle), and (RR)-3a (blue lines, right) for 50 extracted snapshots of MD simulation (full width at half maximum (FWHM) = 0.3 eV). The results for (SS)-1b-3b are presented in the Supporting Information.</figcaption></figure>

$\tau_3$ = 14.5 ns. The contribution of the species with the shortest lifetime ($\tau_1$ = 0.7 ns) is almost negligible. The intensity profile of the species with the lifetime of 3.0 ns is in accordance with the emission of the neutral species. The emission with the longer lifetime, however, is shifted to shorter wavelengths, matching the new emission band attributed (Figure S78, Supporting Information) to the protonated species. Remarkably, this fluorescence shifting is reversible. Addition of Et₃N restores the position of the emission band and its intensity to almost its original value (only a 3% intensity decrease was observed) (Figure S87, Supporting Information). A second cycle of CF₃CO₂H and Et₃N additions was performed (Figure S87, Supporting Information), demonstrating

the switching reproducibility. For 1b, an increase in fluorescence intensity at ca. 422 nm is observed, although the variation is less intense (ca. 16%), together with a very small increase of the fluorescence lifetime ($\tau_1$ = 6.07 ns). The ethano-bridged Tröger's base analog 3b also shows a ca. 70% quenching of the fluorescence at 398 nm upon addition of trifluoroacetic acid despite the fact that no significant change of the lifetime was observed ($\tau_{av}$ = 1.26 ns).

From the fluorescence switching studies, the configurationally stable Tröger's base analog 2b was identified as the most promising candidate for CPL switching studies [46] on account of its clear changes in the fluorescence profile. Hence, addition of an excess
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Switching of the fluorescence and CPL signals of ethano-bridged Tröger's base analog (RR)/(SS)-2b upon the addition of acid and base. a) Fluorescence emission spectra (λ<sub>exc</sub> = 340 nm) of 2b in CH<sub>2</sub>Cl<sub>2</sub> after the addition of increasing quantities of a CF<sub>3</sub>CO<sub>2</sub>H solution (0 equiv. – 50 equiv.) and b) evolution of the fluorescence intensity at different wavelengths. c) CPL (λ<sub>exc</sub> = 372 nm) spectra of (RR)/(SS)-2b in CH<sub>2</sub>Cl<sub>2</sub> in the presence (orange lines) and the absence (pink lines) of 100 equiv. of CF<sub>3</sub>CO<sub>2</sub>H. d) In situ switching of the CPL signal (λ = 431 nm) of (RR)-2b after consecutive addition of CF<sub>3</sub>CO<sub>2</sub>H (cycles 0.5 and 1.5, orange squares) and Et<sub>3</sub>N (cycles 0, 1 and 2, pink squares).</figcaption></figure>

of CF<sub>3</sub>CO<sub>2</sub>H promotes a hypsochromic shift of the CPL emis-
sion band (Figure 5c), which appears between 360 and 480 nm.
This CPL band matches the emission profile of the species with
the longest fluorescence lifetime (τ<sub>3</sub> = 14.5 ns) (Figure S78, Sup-
porting Information), which is, therefore, responsible of the CPL
emission. This wavelength shift is accompanied by a decrease
of the |g<sub>lum</sub>| value to ca. 3 × 10<sup>-4</sup>. These changes are reversed
upon the addition of Et<sub>3</sub>N (see Figure 5d; Figures S89–S90, Sup-
porting Information), demonstrating that the CPL modulation is
triggered by the addition of acid/base as stimuli. Two consecu-
tive acid-base cycles were performed with no significant differ-
ences between cycles. The changes in the CPL emission can be
monitored at 431 nm, where the normalized ΔI varies from ca.
5.3 × 10<sup>-4</sup> and ca. 2 × 10<sup>-4</sup> with the addition of acid, while the
addition of base restores the original value. Therefore, ethano-
bridged Tröger's base analogs, not only makes it possible to con-
fer CPL emission to simple polycyclic aromatic hydrocarbon-
based achiral fluorophores by attaching them to the chiral core,
but also enable the modulation of the CPL by acid/base addition
on account of the configurationally stable nature of the N atoms
in acid media.

**3. Conclusion**

In summary, we have presented a straightforward and effi-
cient syntheses of CPL emitters based on simple achiral fluo-
rophores attached to methano- or ethano-bridged Tröger's base
cores with configurationally stable nitrogen stereogenic centers.

As a proof-of-concept, we have developed six analogs function-
alized with pyrene, anthracene, or 9,9'-dimethylfluorene moi-
eties. On account of the high fluorescence quantum yields and
the good emission dissymmetry factors (up to 1.2 × 10⁻³) of
these fluorophore-appended Tröger's base analogs, the values
obtained for their CPL brightnesses (up to 26.3 M⁻¹ cm⁻¹) are
similar to those exhibited by common small organic molecules,
especially helicenes,[⁴²] frequently viewed as promising organic
CPL emitters. The strength of this approach lies in the fact that
it yields CPL emitters in only three synthetic steps, while its
modular nature opens up the way to a large number of simple
achiral luminophores that could potentially be attached to the
Tröger's base to yield CPL-active compounds. For instance, a po-
tential possibility could be the functionalization of the Tröger's
base core with multi-resonance thermally activated delayed flu-
orescence emitters for the development of suitable materials to
build CP-OLEDs. In addition, the configurationally stable nitro-
gen stereogenic centers in the ethano-bridged derivatives offer
the possibility to modulate the chiroptical responses in the pres-
ence of acid or base. The switching of the CPL signal of the
pyrene-appended ethano-bridged Tröger's base analog is demon-
strated for two consecutive acid-base addition cycles without sig-
nificantly altering the chiroptical response. The straightforward
syntheses of these fluorophore-appended configurationally sta-
ble Tröger's base analogs provides us with the incentive to ap-
ply their remarkable CPL properties in sensing and organic elec-
tronic applications. We also believe that compounds based on the
Tröger's base core might be of considerable interest in the field
of spintronics.[⁴⁷]
**Supporting Information**

Supporting Information is available from the Wiley Online Library or from the author.

**Acknowledgements**

P.G.-C. and M.D.C. contributed equally to this work. This work was supported by grants PID2020-112906GA-I00 and PID2020-113059GB-C21 funded by MICIU/AEI/10.13039/501100011033 and grants PID2021-127521NB-I00, PID2023-146433NB-I00 and PID2023-146801NB-C31 funded by MICIU/AEI/10.13039/501100011033 and by ERDF/EU. The authors would like to thank Northwestern University (NU) for its contin-
ued support of this research. The authors acknowledge the Integrated Molecular Structure Education and Research Center (IMSERC) at NU for providing access to equipment for the experiments. The authors thank Saman Shafaie for HRMS measurements. L. L. B thanks the supercomputer facilities of the Mésocentre de calcul de Franche-Comté.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

**Keywords**

chiroptical switch, circularly polarized luminescence, configurationally
stable nitrogens, fluorophores, Tröger's Base

Received: November 9, 2024

Revised: April 8, 2025

Published online:

[1] J.-M. Lehn, in *Dynamic Stereochemistry*, Vol. 15, (Eds.: J. E. Baldwin, R. H. Fleming, J. M. Lehn, W. Tochtermann), Springer-Verlag, Berlin **1970**, pp. 311–377.

[2] a) E. Fröhlich, E. Wedekind, *Chem. Ber.* **1907**, *40*, 1009; b) M. P. Walsh, J. M. Phelps, M. E. Lennon, D. S. Yufit, M. O. Kitching, *Nature* **2021**, *597*, 70.

[3] a) N. J. Gross, *N. Engl. J. Med.* **1988**, *319*, 486; b) C.-S. Yuan, J. F. Foss, *Drug Dev. Res.* **2000**, *50*, 133; c) G. N. Tytgat, *Drugs* **2007**, *67*, 1343.

[4] a) U.-H. Dolling, P. Davis, E. J. J. Grabowski, *J. Am. Chem. Soc.* **1984**, *106*, 446; b) M. J. O'Donnell, W. D. Bennett, S. Wu, *J. Am. Chem. Soc.* **1989**, *111*, 2353; c) E. J. Corey, F. Xu, M. C. Noe, *J. Am. Chem. Soc.* **1997**, *119*, 12414; d) E. J. Corey, F.-Y. Zhang, *Org. Lett.* **1999**, *1*, 1287; e) M. Cortigiani, A. Mereu, M. G. Healy, M. F. A. Adamo, *J. Org. Chem.* **2019**, *84*, 4112.

[5] a) J. F. Traverse, Y. Zhao, A. H. Hoveyda, M. L. Snapper, *Org. Lett.* **2005**, *7*, 3151; b) S. Bhadra, H. Yamamoto, *Angew. Chem., Int. Ed.* **2016**, *55*, 13043; c) R.-M. Liu, Y.-H. Wang, Z.-Y. Chen, L. Zhang, Q.-H. Shi, Y. Zhou, Y.-P. Tian, X.-L. Liu, *Org. Chem. Front.* **2022**, *9*, 6881; d) Z. Luo, M. Liao, W. Li, S. Zhao, K. Tang, P. Zheng, Y. R. Chi, X. Zhang, X. Wu, *Angew. Chem., Int. Ed.* **2024**, *63*, 202404979.

[6] a) A. Forni, I. Moretti, A. V. Prosyanik, G. Torre, *J. Chem. Soc., Chem. Commun.* **1981**, 588; b) S. Brückner, A. Forni, I. Moretti, G. Torre, J.

Chem. Soc., Chem. Commun. **1982**, 1218; c) M. Bucciarelli, A. Forni, I. Moretti, G. Torre, *J. Org. Chem.* **1983**, *48*, 2640; d) G. V. Shustov, G. K. Kadorkina, R. G. Kostyanovsky, A. Rauk, *J. Am. Chem. Soc.* **1988**, *110*, 1719; e) G. V. Shustov, A. V. Kachanov, V. A. Korneev, R. G. Kostyanovsky, A. Rauk, *J. Am. Chem. Soc.* **1993**, *115*, 10267.

[7] a) B. Dolenský, J. Elguero, V. Král, C. Pardo, M. Valík, *Adv. Heterocycl. Chem.* **2007**, *93*, 1; b) Ö. V. Rúnarsson, J. Artacho, K. Wärnmark, *Eur. J. Org. Chem.* **2012**, 7015.

[8] J. Tröger, *J. Prakt. Chem.* **1887**, *36*, 225.

[9] M. A. Spielman, *J. Am. Chem. Soc.* **1935**, *57*, 583.

[10] S. B. Larson, C. S. Wilcox, *Acta Crystallogr., Sect. C: Cryst. Struct. Commun.* **1986**, *42*, 224.

[11] V. Prelog, P. Wieland, *Helv. Chim. Acta* **1944**, *27*, 1127.

[12] a) O. Trapp, V. Schurig, *J. Am. Chem. Soc.* **2000**, *122*, 1424; b) O. Trapp, G. Trapp, J. Kong, U. Hahn, F. Vögtle, V. Schurig, *Chem.-Eur. J.* **2002**, *8*, 3629; c) D. A. Lenev, K. A. Lyssenko, D. G. Golovanov, V. Buss, R. G. Kostyanovsky, *Chem.-Eur. J.* **2006**, *12*, 6412.

[13] A. Greenberg, N. Molinaro, M. Lang, *J. Org. Chem.* **1984**, *49*, 1127.

[14] a) Y. Hamada, S. Mukai, *Tetrahedron Asymmetry* **1996**, *7*, 2671; b) D. A. Lenev, D. G. Golovanov, K. A. Lyssenko, R. G. Kostyanovsky, *Tetrahedron Asymmetry* **2006**, *17*, 2191; c) C. Michon, A. Sharma, G. Bernardinelli, E. Francotte, J. Lacour, *Chem. Commun.* **2010**, *46*, 2206; d) A. Sharma, L. Guénée, J.-V. Naubron, J. Lacour, *Angew. Chem., Int. Ed.* **2011**, *50*, 3677; e) A. Sharma, C. Besnard, L. Guénée, J. Lacour, *Org. Biomol. Chem.* **2012**, *10*, 966.

[15] a) I. Neogi, S. Jhulki, A. Ghosh, T. J. Chow, J. N. Moorthy, ACS *Appl. Mater. Interfaces.* **2015**, *7*, 3298; b) T. Braukyla, N. Sakai, M. Daskeviciene, V. Jankauskas, E. Kamarauskas, T. Malinauskas, H. J. Snaith, V. Getautis, *Chem. Asian J.* **2016**, *11*, 2049; c) S. Urnikaitė, T. Braukyla, A. Magomedov, E. Kamarauskas, T. Malinauskas, V. Getautis, *Dyes Pigment.* **2017**, *143*, 48; d) T. Braukyla, R. Xia, M. Daskeviciene, T. Malinauskas, A. Gruodis, V. Jankauskas, Z. Fei, C. Momblona, C. Roldán-Carmona, P. J. Dyson, V. Getautis, M. K. Nazeeruddin, *Angew. Chem., Int. Ed.* **2019**, *58*, 11266; e) I. Neogi, V. Darshan, A. Linet, P. K. Anjalikrishna, A. Sebastian, G. Mohanty, A. Morimoto, C. H. Suresh, S. Yagi, Y. D. Posner, F. Grynszpan, N. Unni, *Synth. Met.* **2022**, *291*, 117185; f) S. M. Rivero, M. J. Alonso-Navarro, C. Tonnelé, J. M. Marín-Beloqui, F. Suárez-Blas, T. M. Clarke, S. Kang, J. Oh, M. M. Ramos, D. Kim, D. Casanova, J. L. Segura, J. Casado, *J. Am. Chem. Soc.* **2023**, *145*, 27295; g) H. Yu, X. Guo, Y.-H. Li, J.-H. Yuan, X.-Z. Shang, J.-Y. Luan, Q. Zhang, S. Chen, K.-P. Wang, Z.-Q. Hu, *Dyes Pigment.* **2024**, *224*, 112037.

[16] a) E. B. Veale, D. O. Frimannsson, M. Lawler, T. Gunnlaugsson, *Org. Lett.* **2009**, *11*, 4040; b) R. B. P. Elmes, M. Erby, S. A. Bright, D. C. Williams, T. Gunnlaugsson, *Chem. Commun.* **2012**, *48*, 2588; c) S. Banerjee, S. A. Bright, J. A. Smith, J. Burgeat, M. Martinez-Calvo, D. C. Williams, J. M. Kelly, T. Gunnlaugsson, *J. Org. Chem.* **2014**, *79*, 9272; d) B. R. Manda, M. Alla, R. J. Ganji, A. Addlagatta, *Eur. J. Med. Chem.* **2014**, *86*, 39; e) S. Shanmugaraju, B. la Cour Poulsen, T. Arisa, D. Umadevi, H. L. Dalton, C. S. Hawes, S. Estalayo-Adrián, A. J. Savyasachi, G. W. Watson, D. C. Williams, T. Gunnlaugsson, *Chem. Commun.* **2018**, *54*, 4120; f) E. Calatrava-Pérez, J. M. Delente, S. Shanmugaraju, C. S. Hawes, C. D. Williams, T. Gunnlaugsson, E. M. Scanlan, *Org. Biomol. Chem.* **2019**, *17*, 2116; g) Y. Zhao, K. Chen, E. A. Yildiz, S. Li, Y. Hou, X. Zhang, Z. Wang, J. Zhao, A. Barbon, H. G. Yaglioglu, H. Wu, *Chem.-Eur. J.* **2020**, *26*, 3591.

[17] a) E. M. Boyle, S. Comby, J. K. Molloy, T. Gunnlaugsson, *J. Org. Chem.* **2013**, *78*, 8312; b) C. Yuan, Y. Zhang, H. Xi, X. Tao, *RSC Adv.* **2017**, *7*, 55577; c) S. Shanmugaraju, D. Umadevi, L. M. González-Barcia, J. M. Delente, K. Byrne, W. Schmitt, G. W. Watson, T. Gunnlaugsson, *Chem. Commun.* **2019**, *55*, 12140.

[18] a) M. Carta, R. Malpass-Evans, M. Croad, Y. Rogan, J. C. Jansen, P. Bernardo, F. Bazzarelli, N. B. McKeown, *Science* **2013**, *339*, 303; b) Z.-Z. Yang, H. Zhang, B. Yu, Y. Zhao, G. Ji, Z. Liu, *Chem. Commun.*
[2015, 51, 1271; c) Z. Yang, R. Guo, R. Malpass-Evans, M. Carta, N. B. McKeown, M. D. Guiver, L. Wu, T. Xu, Angew. Chem., Int. Ed. **2016**, *55*, 11499; d) H. A. Patel, J. Selberg, D. Salah, H. Chen, Y. Liao, S. K. M. Nalluri, O. K. Farha, R. Q. Snurr, M. Rolandi, J. F. Stoddart, ACS Appl. Mater. Interfaces **2018**, *10*, 25303; e) S. M. Meckler, J. E. Bachman, B. P. Robertson, C. Zhu, J. R. Long, B. A. Helms, Angew. Chem., Int. Ed. **2018**, *57*, 4912; f) X. Du, Y. Yuan, T. Dong, X. Chi, Z. Wang, ACS Appl. Energy Mater. **2021**, *4*, 13327; g) X. Chen, Y. Fan, L. Wu, L. Zhang, D. Guan, C. Ma, N. Li, Nat. Commun. **2021**, *12*, 6140; h) Y. Zhang, J. Dai, H. Lai, Y. Wan, Y. M. Lee, Y. Zhuang, Macromolecules **2023**, *56*, 4681; i) Z. Tang, S. Xu, N. Yin, Y. Yang, Q. Deng, J. Shen, X. Zhang, T. Wang, H. He, X. Lin, Y. Zhou, Z. Zou, Adv. Mater. **2023**, *35*, 2210693.

[19] W. Gong, M. Kazem-Rostami, F. A. Son, S. Su, K. M. Fahy, H. Xie, T. Islamoglu, Y. Liu, J. F. Stoddart, Y. Cui, O. K. Farha, J. Am. Chem. Soc. **2022**, *144*, 22574.

[20] a) J. P. Riehl, F. S. Richardson, Chem. Rev. **1986**, *86*, 1; b) G. Longhi, E. Castiglioni, J. Koshoubu, G. Mazzeo, S. Abbate, Chirality **2016**, *28*, 696.

[21] For selected examples of recent CPL-emissive systems, see: a) S. Sato, A. Yoshii, S. Takahashi, S. Furumi, M. Takeuchi, H. Isobe, Proc. Natl. Acad. USA **2017**, *114*, 13097; b) C. M. Cruz, S. Castro-Fernández, E. Maçóas, J. M. Cuerva, A. G. Campaña, Angew. Chem., Int. Ed. **2018**, *57*, 14782; c) T. Zhao, J. Han, X. Jin, Y. Liu, M. Liu, P. Duan, Angew. Chem., Int. Ed. **2019**, *58*, 4978; d) Y. B. Tan, Y. Okayasu, S. Katao, Y. Nishikawa, F. Asanoma, M. Yamada, J. Yuasa, T. Kawai, J. Am. Chem. Soc. **2020**, *142*, 17653; e) J. Wade, J. R. Brandt, D. Reger, F. Zinna, K. Y. Amsharov, N. Jux, D. L. Andrews, M. J. Fuchter, Angew. Chem., Int. Ed. **2021**, *60*, 222; f) M. A. Medel, R. Tapia, V. Blanco, D. Miguel, S. P. Morcillo, A. G. Campaña, Angew. Chem., Int. Ed. **2021**, *60*, 6094; g) O. G. Willis, F. Petri, G. Pescitelli, A. Pucci, E. Cavalli, A. Mandoli, F. Zinna, L. Di Bari, Angew. Chem., Int. Ed. **2022**, *61*, 202208326; h) A. Garci, S. Abid, A. H. G. David, M. D. Codesal, L. Đorđević, R. M. Young, H. Sai, L. Le Bras, A. Perrier, M. Ovalle, P. J. Brown, C. L. Stern, A. G. Campaña, S. I. Stupp, M. R. Wasielewski, V. Blanco, J. F. Stoddart, Angew. Chem., Int. Ed. **2022**, *61*, 202208679; i) S. Parzyszek, J. Tessarolo, A. Pedrazo-Tardajos, A. M. Ortuño, M. Bagiński, S. Bals, G. H. Clever, W. Lewandowski, ACS Nano. **2022**, *16*, 18472; j) N. F. M. Mukthar, N. D. Schley, G. Ung, J. Am. Chem. Soc. **2022**, *144*, 6148; k) M. Kazem-Rostami, A. Orte, A. M. Ortuño, A. H. G. David, I. Roy, D. Miguel, A. Garci, C. M. Cruz, C. L. Stern, J. M. Cuerva, J. F. Stoddart, J. Am. Chem. Soc. **2022**, *144*, 9380; l) Y. Wang, J. Gong, X. Wang, W.-J. Li, X.-Q. Wang, X. He, W. Wang, H.-B. Yang, Angew. Chem., Int. Ed. **2022**, *61*, 202210542; m) S. Míguez-Lago, I. F. A. Mariz, M. A. Medel, J. M. Cruz, E. Maçóas, C. M. Cruz, A. G. Campaña, Chem. Sci. **2022**, *13*, 10267; n) K. Dhbaibi, M. Grasser, H. Douib, V. Dorcet, O. Cador, N. Vanthuyne, F. Riobé, O. Maury, S. Guy, A. Bensalah-Ledoux, B. Baguenard, G. L. J. A. Rikken, C. Train, B. Le Guennic, M. Atzori, F. Pointillart, J. Guassous, Angew. Chem., Int. Ed. **2023**, *62*, 202215558; o) X. Tang, X. Liao, X. Cai, J. Wu, X. Wu, Q. Zhang, Y. Yan, S. Zheng, H. Jiang, J. Fan, S. Cai, W. Zhang, Y. Liu, Angew. Chem., Int. Ed. **2023**, *62*, 202216310; p) S.-M. Guo, S. Huh, M. Coehlo, L. Shen, G. Pieters, O. Baudoin, Nat. Chem. **2023**, *15*, 872; q) X. Tian, K. Shoyama, B. Mahlmeister, F. Brust, M. Stolte, F. Würthner, J. Am. Chem. Soc. **2023**, *145*, 9886; r) M. Toya, T. Omine, F. Ishiwari, A. Saeki, H. Ito, K. Itami, J. Am. Chem. Soc. **2023**, *145*, 11553; s) Y. Appiarius, S. Míguez-Lago, P. Puylaert, N. Wolf, S. Kumar, M. Molkenthin, D. Miguel, T. Neudecker, M. Juríček, A. G. Campaña, A. Staubitz, Chem. Sci. **2024**, *15*, 466.

[22] a) L. E. MacKenzie, R. Pal, Nat. Rev. Chem. **2021**, *5*, 109; b) I. López-Sicilia, A. M. Ortuño, P. Reiné, D. Otero, M. T. Martin-Romero, L. Camacho, L. Alvarez de Cienfuegos, A. Orte, J. J. Giner-Casares, D. Miguel, J. M. Cuerva, J. Mater. Chem. **C** **2023**, *11*, 2591.

[23] a) D. Parker, J. D. Fradgley, K.-L. Wong, Chem. Soc. Rev. **2021**, *50*, 8193; b) B. H. Jhun, S. Y. Park, Y. You, Dyes Pigment **2023**, *208*, 110786.

[24] a) D.-W. Zhang, M. Li, C.-F. Chen, Chem. Soc. Rev. **2020**, *49*, 1331; b) L. Frédéric, A. Desmarchelier, L. Favereau, G. Pieters, Adv. Funct. Mater. **2021**, *31*, 2010281; c) Y. Yang, N. Li, J. Miao, X. Cao, A. Ying, K. Pan, X. Lv, F. Ni, Z. Huang, S. Gong, C. Yang, Angew. Chem., Int. Ed. **2022**, *61*, 202202227; d) X. Wu, X. Yan, Y. Chen, W. Zhu, P.-T. Chou, Trends Chem. Sci. **2023**, *5*, 734.

[25] a) W. Dai, Y. Wang, R. Li, Y. Fan, G. Qu, Y. Wu, Q. Song, J. Han, S. Xiao, ACS Nano. **2020**, *14*, 17063; b) Z.-F. Liu, J. Ren, P. Li, L.-Y. Niu, Q. Liao, S. Zhang, Q.-Z. Yang, Angew. Chem., Int. Ed. **2023**, *62*, 202214211.

[26] T. Mori, Circularly Polarized Luminescence of Isolated Small Organic Molecules, 1st ed., Springer Nature, Singapore **2020**.

[27] For selected examples of CPL-emissive molecules with point chiral asymmetric carbon atoms, see: a) H. P. J. M. Dekkers, L. E. Closs, J. Am. Chem. Soc. **1976**, *98*, 2210; b) P. H. Schippers, H. P. J. M. Dekkers, J. Am. Chem. Soc. **1983**, *105*, 145; c) G. Longhi, E. Castiglioni, S. Abbate, F. Lebon, D. A. Lightner, Chirality **2013**, *25*, 589; d) S. Ito, K. Ikeda, S. Nakanishi, Y. Imai, M. Asami, Chem. Commun. **2017**, *53*, 6323; e) A. Homberg, E. Brun, F. Zinna, S. Pascal, M. Górecki, L. Monnier, C. Besnard, G. Pescitelli, L. Di Bari, J. Lacour, Chem. Sci. **2018**, *9*, 7043; f) H. Shigemitsu, K. Kawakami, Y. Nagata, R. Kajiwara, S. Yamada, T. Mori, T. Kida, Angew. Chem., Int. Ed. **2022**, *61*, 202114700; g) K.-F. Zhang, N. Saleh, M. Swierczewski, A. Rosspeintner, F. Zinna, G. Pescitelli, C. Besnard, L. Guénée, T. Bürgi, J. Lacour, Angew. Chem., Int. Ed. **2023**, *62*, 202304075.

[28] C. Qian, Y. Chen, Q. Zhao, M. Cheng, C. Lin, J. Jiang, L. Wang, Beilstein J. Org. Chem. **2021**, *17*, 52.

[29] Y. Yoshigoe, H. Shimada, T. Takaki, Y. Imai, S. Saito, Chem.-Eur. J. **2024**, *30*, 202304059.

[30] J. Jensen, K. Wärnmark, Synthesis **2001**, *12*, 1873.

[31] Y. Ishida, H. Ito, D. Mori, K. Saigo, Tetrahedron Lett. **2005**, *46*, 109.

[32] N. Miyaura, A. Suzuki, Chem. Rev. **1995**, *95*, 2457.

[33] N. Kitamura, E. Sakuda, Y. Iwahashi, K. Tsuge, Y. Sasaki, S. Ishizaka, J. Photochem. Photobiol. A. **2009**, *207*, 102.

[34] A. Stockmann, J. Kurzawa, N. Fritz, N. Acar, S. Schneider, J. Daub, R. Engl, T. Clark, J. Phys. Chem. A. **2002**, *106*, 7958.

[35] M. Belletète, S. Beaupré, J. Bouchard, P. Blondin, M. Leclerc, G. Durocher, J. Phys. Chem. B **2000**, *104*, 9118.

[36] CSP-HPLC has been frequently employed for the resolution of Tröger's Base derivatives, see the following selected examples: a) S. Sergeyev, F. Diederich, Chirality **2006**, *18*, 707; b) C. Benkhäuser-Schunk, B. Wezisla, K. Urbahn, U. Kiehne, J. Daniels, G. Schnakenburg, F. Neese, A. Lützen, ChemPlusChem **2012**, *77*, 396; c) C. A. Weatherly, Y.-C. Na, Y. S. Nanayakkara, R. M. Woods, A. Sharma, J. Lacour, D. W. Armstrong, J. Chromatogr. B. **2014**, 955-956, 72.

[37] For the racemic resolution of **5** by co-crystallization, see: a) D. L. Jameson, T. Field, M. R. Schmidt, A. K. DeStefano, C. J. Stiteler, V. J. Venditto, B. Krovic, C. M. Hoffman, M. T. Ondisco, M. E. Belowich, J. Org. Chem. **2013**, *78*, 11590; b) M. Kazem-Rostami, J. Med. Nanomater. Chem. **2020**, *2*, 138.

[38] a) P. Pisani, P. Piro, S. Decherchi, G. Bottegoni, D. Sona, V. Murino, W. Rocchia, A. Cavalli, J. Chem. Theory Comput. **2014**, *10*, 2557; b) J. Juárez-Jiménez, X. Barril, M. Orozco, R. Pouplana, F. J. Luque, J. Phys. Chem. B. **2015**, *119*, 1164.

[39] a) M. Monti, M. Stener, M. Aschi, J. Comput. Chem. **2022**, *43*, 2023; b) A. C. Evans, A. S. Petit, S. G. Guillen, A. J. Neurkich, S. V. Hoffmann, N. C. Jones, RSC Adv. **2021**, *11*, 1635.

[40] Note that the glum values are calculated as $g_{\text{lum}} = 2(I^L - I^R)/(I^L + I^R)$, where $I^L$ and $I^R$ correspond to the intensities of left- and right-handed circularly polarized light.

[41] $B_{CPL}$ is defined as $B_{CPL} = \varepsilon_{\lambda} \times \Phi_{F} \times |g_{\text{lum}}|/2$, being $\varepsilon_{\lambda}$, the molar extinction coefficient at the excitation wavelength $\lambda$; $\Phi_{F}$, the fluorescence quantum yield; and $|g_{\text{lum}}|$, the emission dissymmetry factor. See ref. 42.

[42] L. Arrico, L. Di Bari, F. Zinna, Chem.-Eur. J. **2020**, *27*, 2920.
[43] a) R. G. Uceda, C. M. Cruz, S. Míguez-Lago, L. Álvarez de Cienfuegos, G. Longhi, D. A. Pelta, P. Novoa, A. J. Mota, J. M. Cuerva, D. Miguel, *Angew. Chem., Int. Ed.* **2024**, *63*, 202316696; b) R. G. Uceda, A. Gijón, S. Míguez-Lago, C. Moreno-Cruz, V. Blanco, F. Fernández-Álvarez, L. Álvarez de Cienfuegos, M. Molina-Solana, J. Gómez-Romero, D. Miguel, A. J. Mota, J. M. Cuerva, *Angew. Chem., Int. Ed.* **2024**, *63*, 202409998.

[44] For selected examples of CPL switching with discrete molecules, see: a) H. Maeda, Y. Bando, K. Shimomura, I. Yamada, M. Naito, K. Nobusawa, H. Tsumatori, T. Kawai, *J. Am. Chem. Soc.* **2011**, *133*, 9266; b) Y. Hashimoto, T. Nakashima, D. Shimizu, T. Kawai, *Chem. Commun.* **2016**, *52*, 5171; c) H. Isla, M. Srebro-Hooper, M. Jean, N. Vanthuyne, T. Roisnel, J. L. Lunkley, G. Muller, J. A. G. Williams, J. Autschbach, J. Crassous, *Chem. Commun.* **2016**, *52*, 5932; d) P. Reiné, A. M. Ortuño, S. Resa, L. Álvarez de Cienfuegos, V. Blanco, M. J. Ruedas-Rama, G. Mazzeo, S. Abbate, A. Lucotti, M. Tommasini, S. Guisán-Ceinos, M. Ribagorda, A. G. Campaña, A. Mota, G. Longhi, D. Miguel, J. M. Cuerva, *Chem. Commun.* **2018**, *54*, 13985; e) K. Takaishi, M. Yasui, T. Ema, *J. Am. Chem. Soc.* **2018**, *140*, 5334; f) Y. Imai, J. Yuasa, *Chem. Commun.* **2019**, *55*, 4095; g) A. H. G. David, R. Casares, J. M. Cuerva, A. G. Campaña, V. Blanco, *J. Am. Chem. Soc.* **2019**, *141*, 18064; h) J.-L. Ma, Q. Peng, C.-H. Zhao, *Chem.-Eur. J.* **2019**, *25*, 15441;

i) K. Takaishi, K. Iwachido, T. Ema, *J. Am. Chem. Soc.* **2020**, *142*, 1774; j) M. Ikeshita, M. Mizugaki, T. Ishikawa, K. Matsudaira, M. Kitahara, Y. Imai, T. Tsuno, *Chem. Commun.* **2022**, *58*, 7503; k) L. Qu, X. Zhou, J. Song, B. Zhang, Q. Yang, X. Zhou, J. Liu, H. Xiang, *Adv. Opt. Mater.* **2023**, *11*, 2300779; l) W.-T. Xu, X. Li, P. Wu, W.-J. Li, Y. Wang, X.-Q. Xu, X.-Q. Wang, J. Chen, H.-B. Yang, W. Wang, *Angew. Chem., Int. Ed.* **2024**, *63*, 202319502.

[45] Although protonation results in many cases in a red-shift of the photoluminescence band respect to that of the non-protonated species on account of an increased electron-withdrawing character, there are reported examples of fluorophores functionalized with aniline groups that undergo hypsochromic shifts upon protonation. See for example: a) J. Chen, S. Ma, J. Zhang, L. Wang, L. Ye, B. Li, B. Xu, W. Tian, *J. Phys. Chem. Lett.* **2014**, *5*, 2781; b) T. Antón-Cánovas, S. Achelle, M. P. Fernández-Lienpres, A. Navarro, F. Alonso, J. Rodríguez-López, *J. Mol. Liq.* **2023**, *380*, 121758.

[46] Note that photodegradation was observed for compound 3b in acidic medium after exhaustive irradiation during the CPL measurements.

[47] a) H. J. Eckvahl, N. A. Teyrulnikov, A. Chiesa, J. M. Bradley, R. M. Youn, S. Carretta, M. D. Krzyaniak, M. R. Wasielewski, *Science* **2023**, *382*, 197; b) H. J. Eckvahl, G. Copley, R. M. Young, M. D. Krzyaniak, M. R. Wasielewski, *J. Am. Chem. Soc.* **2024**, *146*, 24125.