# Helicene-Bis-Porphyrin Conjugates: Exciton Coupling Chirality and Chiral-Induced Spin Selectivity

Paola Matozzo, Kakali Santra, Anil Kumar, Saurav Parmar, Pierpaolo Morgante, Kais Dhbaibi, Rajatha Rajendran, Nicolas Vanthuyne, Ron Naaman, Jochen Autschbach, et al.

► To cite this version:

Paola Matozzo, Kakali Santra, Anil Kumar, Saurav Parmar, Pierpaolo Morgante, et al. Helicene-Bis-Porphyrin Conjugates: Exciton Coupling Chirality and Chiral-Induced Spin Selectivity. ChemistryEurope, 2026, 4 (1), pp.e202500314. 10.1002/ceur.202500314. hal-05494601

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# Helicene-Bis-Porphyrin Conjugates: Exciton Coupling Chirality and Chiral-Induced Spin Selectivity

Paola Matozzo, Kakali Santra, Anil Kumar, Saurav Parmar, Pierpaolo Morgante, Kais Dhbaibi, Rajatha Rajendran, Nicolas Vanthuyne, Ron Naaman,* Jochen Autschbach,* and Jeanne Crassous*

Following previous work on helicene–porphyrin conjugates in which carbo[6]helicene are connected to zinc-porphyrin via phenyl-bis-ethynyl bridges (Por(Zn)-H[6]¹, series 1), and displaying clear exciton coupling (EC) chirality, novel carbo[6]helicenes derivatives substituted at their 2,15 positions by zinc-porphyrin units are prepared, either through a triple bond (Por(Zn)-H[6]², series 2), or through an alkynyl-phenyl bridge (Por(Zn)-H[6]³, series 3). Series 2 is also synthesized with free porphyrins or different metals [Ni(II) and Pd(II)]. Their photophysical and chiroptical properties (electronic circular dichroism and circularly

polarized luminescence) are characterized, and it is examined how i) the distance between the porphyrin units and ii) the metal type impacted these properties. Experimental and theoretical analyses highlight strong responses originating from EC chirality in combination with the typical helicene-centered optical activity. The Por(Zn)-H[6]² system displaying strong absorption dissymmetry factors is then selected to experimentally examine the chiral-induced spin selectivity effect by magnetic conductive atomic force microscopy; a spin polarization of 50% is measured.

## 1. Introduction

Helicenes are helical π-conjugated molecules formed of ortho-fused aromatic rings. They exhibit strong chiroptical properties (very high

specific optical rotations, intense electronic circular dichroism (ECD), and strong molecular circularly polarized luminescence (CPL)) which are appealing in a variety of research fields, such as optoelectronics, spintronics, or biological imaging.[1–4] Recently, it has been shown that grafting two achiral chromophores at the 2,15 extremities of a carbo[6]helicene scaffold enables efficient tuning and enhancement of the chiroptical activity. Indeed, because the π-conjugation through the whole helical backbone is limited, cova lently grafted achiral chromophores can interact through space, and give access to exciton coupling (EC) chirality.[5–10] It is well-known that when two or more identical chromophores are con nected to vicinal functions such as diols, diamines, or other, within a chiral molecule, they may interact with each other through space and give rise to EC chirality. EC may lead to intense ECD, consisting of bisignate Cotton effects, from the sign of which the absolute con figuration of the molecule can be deduced. Berova, Harada, and Nakanishi have pioneered this domain, developed the nonempirical approach called “CD Exciton Chirality Method”, and exhaustively used it to determine absolute configurations of many classes of chi ral compounds and natural products.[11–13] Typically, the achiral chromophores used in such research exhibit a strong transition electric dipole moment in preferred directions. Porphyrins are archetypes of such chromophores[14] since they display i) highly intense absorption bands, especially the Soret band (epsilon > 10⁵ M⁻¹ cm⁻¹), ii) planar geometry, and iii) an ability to coordinate metal ions.[15–17] In 2021, we described the first example of helicene–porphyrin conjugates to explore and rationalize the role of the helical π-conjugation in the EC phenomenon.[18] We showed that grafting two ethynyl-porphyrin units on the 2,15 positions of a carbo[6]helicene results in a clear, strong EC signature due to the Soret band, with the appearance of an isolated bisignate ECD

P. Matozzo, K. Dhbaibi, R. Rajendran, J. Crassous
Institut des Sciences Chimiques de Rennes

CNRS

UMR 6226

ISCR

University of Rennes

F-35000 Rennes, France

E-mail: jeanne.crassous@univ-rennes.fr

K. Santra, A. Kumar, R. Naaman

Department of Chemical and Biological Physics

Weizmann Institute of Science

Rehovot 76100, Israel

E-mail: ron.naaman@weizmann.ac.il

S. Parmar, P. Morgante, J. Autschbach

Department of Chemistry

University at Buffalo

State University of New York

Buffalo, New York 14260, USA

E-mail: jautschbach@gmail.com

N. Vanthuyne

CNRS

Centrale Med

FSCM

Aix Marseille Univ

13397 Marseille, France

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Supporting information for this article is available on the WWW under https://
doi.org/10.1002/ceur.202500314

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© 2026 The Author(s). ChemistryEurope published by Chemistry Europe and 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.
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couplet at 440 nm. In this prior work, the synergetic effect of helicenes and porphyrins in obtaining enhanced chiroptical responses in an absorption region was highlighted.

Recently, chiral-induced spin selectivity (CISS) has emerged as a highly active area of research and a fascinating topic that connects chirality, charge transport, and magnetism.[19] In this domain, the strong polarizabilities and the long spin relaxation times of porphyrins have demonstrated their benefit in effectively maintaining the spin polarization through a chiral system formed of an L-proline octamer conjugated with a (porphinato)zinc organized into a self-assembled monolayer (SAM) onto a gold-nickel substrate via a thiolate unit.[20,21] Magnetic conducting atomic force microscopy (mc-AFM), spin-dependent electrochemistry, and spin Hall devices were capable of measuring the spin polarizability that accompanies charge polarization. Likewise, owing to their strong chiroptical responses and semiconducting properties, helicenes have revealed efficient spin filtering properties.[22-29]

Only a few helicene-porphyrinoids (porphyrin, phthalocyanines, or porphyrazins) systems have been reported in the literature thus far.[30-38] However, even though the absorption domain could be efficiently tuned down to the near-infrared region and a strong ECD response of the Soret band could be observed in most cases, none of these examples displayed a bisignate EC signature except the helicene-bis-porphyrine conjugates (Por(H₂)-H[6]¹ and Por(Zn)-H[6]¹ in Figure 1) described by our team in 2021.[18] Herein, we explore this novel class of helicene-porphyrin conjugates by

preparing new series of carbo[6]helicenes substituted at their 2,15 positions by porphyrin units either through a triple bond (Por(Zn)-H[6]²), or through an alkynyl-phenyl bridge (Por(Zn)-H[6]³). In this way, we are able to control the distance between the two porphyrin units and investigate the impact on the photo-physical and chiroptical properties of the Zn(II) derivatives and, in particular, the EC response. In series 2, we prepared the free porphyrins, together with diverse metal complexes [Zn(II), Ni(II), and Pd(II)]. These metallic ions were chosen for their different coordination geometries, electronic structures, and optical properties. The resulting experimental photophysical and chiroptical properties were characterized in detail. Furthermore, with the help of theoretical calculations, it was examined how the metal ion impacts, in particular, the EC signature. The Por(Zn)-H[6]² system was further selected to be experimentally studied by mc-AFM in order to investigate its spin filtering ability.

## 2. Results and Discussion

### 2.1. Synthesis of Helicene-Bis-Ethynyl-Porphyrin Systems (Por(M)-H[6]²)

The first derivative was designed starting from the already known enantiopure *P* and *M*–2,15-bis-ethynyl-carbo[6]helicene (H[6]) building blocks.[39] In order to avoid the presence of Glaser

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Different types of carbo[6]helicene-bis-porphyrin conjugates Por(M)-H[6]¹⁻³ were examined by varying the spacer between the helicene and porphyrin units (alkynyl-phenyl-alkynyl in series 1, alkynyl in series 2, and alkynyl-phenyl in series 3.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

by-products, which appeared particularly efficient in the case of a copper-free Sonogashira reaction[40-42] was set up between the ethynyl-helicene and the brominated free porphyrin (Scheme 1), giving the desired compounds Por(H2)-H[6]² with 45% yield (see Electronic Supporting Information). The Zn(II), Ni(II), and Pd(II) complexes, i.e., Por(Zn)-H[6]², Por(Ni)-H[6]², and Por(Pd)-H[6]², were then obtained by metalation of the free-base Por(H2)-H[6]² according to Scheme 1 and following classical procedures (see Electronic Supporting Information). All these derivatives were fully characterized by ¹H and ¹³C NMR spectroscopy and mass spectrometry (see Supporting Information). As an example, the ¹H-NMR spectrum of the free base derivative Por(H2)-H[6]², with the respective assignments for all the protons, is reported on Figure S1.8, Supporting Information In contrast to the previously described Por(H2)-H[6]¹,[18] the helicene unit shows a well-defined set of peaks: the “external” protons resonate as 5 different doublets from 8.31 to 7.98 ppm, while the protons placed at the in *ortho* position with respect to the triple bond, i.e. at the 1,16 inner positions of the helix, resonate as a singlet at 8.57 ppm. Except for the protons on the free porphyrin-helicene derivative, in which the NHs resonate at -2.44 ppm and are not present for the coordinated-porphyrin derivatives, the signal pattern of the chromophore in the Por(M)-H[6]² series remained unaffected by changing the metal. The mass of all these new compounds was ascertained by MALDI experiments (see Supporting Information).

## 2.2. Structural Characterization by Molecular Modeling of (Por(M)-H[6]²) Series

The molecular structures were optimized with density functional theory (DFT) and give a clearer idea about the helicenic environment and of the porphyrin unit geometries (Figure 2 and Supporting Information). In Por(H2)-H[6]², the optimized porphyrins were found to be fully planar and the mesityl substituents fully perpendicular to them. In this structure, the porphyrin displayed coplanarity with the terminal ring of the helicene unit (dihedral angle between both units of around 2.1°), thus ensuring efficient conjugation through the ethynyl bridge. Moreover, the helicity (i.e., dihedral angle between the two terminal rings) displays a 52° value, which is typical for carbo[6]helicenes.[1-4] Interestingly, the theoretical centroid-centroid distance between the two porphyrin units was found to be 15.5 Å. Overall, the two porphyrin units display a mutual orientation (dihedral angle between both porphyrin cycles) of 56°. Very similar features were found for model structures in which mesityl groups were changed to chlorine atoms; a model of such a full (containing mesityl groups) molecule and the substituted molecule is illustrated in Figure 2. Further discussion related to the full versus truncated models can be found in the Supporting Information. The CI-truncated models were used for a subset of the calculations to keep the computational effort manageable.

<figure><img src="image_8.png" /><figcaption>**Scheme 1.** Synthetic route to enantiopure helicene-bis-ethynyl-porphyrine derivatives P-Por(M)-H[6]². i) Zn(OAc)₂·2H₂O, CHCl₃/MeOH, 4 h, rt; ii) Ni(OAc)₂·4H₂O, DMF, 12 h, 150 °C; iii) PdCl₂, PhCN/DMF, 4 h, 150 °C, Ar.</figcaption></figure>
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<figure><img src="image_11.png" /><figcaption>**Figure 2.** a) Representation of Por(H2)-H[6]² (containing mesityl groups) free-base, b) representation of a Por(H2)-H[6]² chlorinated free-base. Hydrogens (except inside the porphyrin rings) are not shown for clarity.</figcaption></figure>

The zinc complex Por(Zn)-H[6]² substantially displayed the same structural features as the protonated one, namely full planarity of the zinc-porphyrin units, efficient conjugation between porphyrin and terminal cycles via the ethynyl bond but with a slightly higher dihedral angle between both units of around 8°, a helicity of 50° of the helical core, and an optimized distance between the two Zn atoms of 15 Å. Note that in this case, the two porphyrin units display an overall mutual orientation of 34.3°. Regarding the Por(Ni)-H[6]² derivative, it was necessary to remove the mesityl groups to fully converge to an optimized structure, with the typical porphyrin units displaying a ruffle-type topology.[43] A helicity of 52° was measured for the helicenic core, with again efficient conjugation between each porphyrin and the terminal aromatic cycle of the helix linked through the ethynyl bridge. Interestingly, the Ni–Ni distance is found around 15.3 Å, which is very similar to the other derivatives Por(H2)-H[6]² and Por(Zn)-H[6]². Structurally, the Pd(II) complex Por(Pd)-H[6]² was found to display the same features as the Zn(II) derivatives with similar metrics, for instance a helicity of 52°, a metal–metal distance of 15.5 Å and full planarity of the porphyrin units.

## 2.3. Absorption Properties (UV–vis and ECD Spectroscopies) of Por(M)-H[6]² Series

Due to their 18 π-electrons, porphyrins are macrocycles with extended aromaticity, and their absorption occurs in the visible range. Thanks to their extremely high molar extinction coefficients, porphyrins and their derivatives are among the most intense natural pigments, giving their red color to blood (hemoglobin) or the green color to plants (chlorophylls). The electronic absorption spectra of porphyrins exhibit a characteristic pattern with π–π* transitions grouped into two distinct regions. The “Soret or B band”, located between 380 and 450 nm, corresponds to a transition between the ground state and the second excited state (S₀–S₂). This transition is allowed by the selection rules and shows very high molar extinction coefficients (ε > 10⁵ M⁻¹·cm⁻¹). A shoulder sometimes appears on the high-energy side of the B band, attributed to one vibrational mode of excitation B(1–0). The so-called “Q bands” correspond to the S₀–S₁ transitions and are located between 500 and 750 nm, with molar extinction coefficients on the order of

ε = 10³–10⁴ M⁻¹·cm⁻¹. Generally, there are two or four Q bands, depending on whether it is a free base porphyrin or a metalloporphyrin.[44] The Por(M)-H[6]² series was designed with the aim of changing the extent of the conjugation and the interchromophoric distance in order to modulate the chiroptical response by varying the intensity of the EC couplet and to examine the metal contribution. The UV–vis absorption and ECD spectra were thus recorded in dichloromethane at a concentration of about 10⁻⁵ M (Figure 3a,b). The absorption spectra all display similar features, with contributions from the helicenic part and contributions from the porphyrin units. For instance, in the case of the free porphyrin Por(H2)-H[6]², a set of strong and broad bands is found between 230 and 380 nm with ε values within the range of 35–56 × 10³ M⁻¹·cm⁻¹, which mainly correspond to the helicene-based π–π* transitions (vide infra). At 431 nm, a highly intense Soret band is found with ε values as high around 250–280 × 10³ M⁻¹·cm⁻¹, accompanied by four Q bands of moderate to strong intensities (ε ≈ 7.5–36.1 × 10³ M⁻¹·cm⁻¹) between 525 and 656 nm. Note that some vibronic progression was also observed since a total of 5 bands were found (see Figure 3b and Table 1). Slightly different absorption wavelengths were found between each metallated porphyrins. For instance, the Ni(II) complex had less delocalized π bonds, which decreased the average electron density of the molecule and increased the energy available for electron transitions. Consequently, a blueshift occurred in the Soret bands (427 nm). The Zn(II) porphyrin possessed more delocalized π bonds, which in turn lowered the energy required for electron transitions, leading to a slight redshift in the Soret band (436 nm). Overall, the Pd(II) and free porphyrin systems were found to exhibit stronger absorption spectra, while the Zn(II) and Ni(II) systems showed similar and less intense absorption features. Note however, that the absorptive responses are so intense that such measurements are accompanied by a substantial uncertainty and should be considered with caution. It thus appears safer to consider the dissymmetry factors which are inherent to the chiral molecule properties (see the gₐbs values below).

The ECD spectra display similar characteristics to the UV-visible, exhibiting the same intensity changes in the signal at the Soret band (Figure 3a). The metalloporphyrin derivatives typically show either one or two ECD-active Q bands, while the protonated chromophore exhibits one, as listed in Table 1 for P enantiomers. It is fundamental to highlight how the intensity of the exciton couplet (EC) increased
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<figure><img src="image_13.png" /><figcaption>**Figure 3.** a) ECD and b) absorption spectra of the *Por(M)-H[6]*² series, measured at room temperature (rt) in CH₂Cl₂ solutions. c) Comparison of *g*<sub>abs</sub> spectra of the *P-Por(M)-H[6]*² derivatives. d) Comparison of *g*<sub>abs</sub> spectra of the *P-Por(Zn)-H[6]*¹⁻³.</figcaption></figure>

<table><caption>**Table 1.** Molar absorption coefficients and Δε values for the soret and Q bands of the *P-Por(M)-H[6]*² derivatives in CH₂Cl₂ solutions at rt.</caption><thead><tr><th rowspan="2">Metal</th><th colspan="2">UV-vis Soret band</th><th colspan="2">ECD Soret band</th></tr><tr><th>λ [nm], ε [10<sup>4</sup> × M<sup>-1</sup> cm<sup>-1</sup>]</th><th>λ [nm], Δε [M<sup>-1</sup> cm<sup>-1</sup>]</th><th>λ [nm], ε [10<sup>3</sup> × M<sup>-1</sup> cm<sup>-1</sup>]</th><th>λ [nm], Δε [M<sup>-1</sup> cm<sup>-1</sup>]</th></tr></thead><tbody><tr><td>P-Por(H2)-H[6]<sup>2</sup></td><td>431, 25.0</td><td>434, -201; 448, +450</td><td>502, 9.2; 526, 12.7;<br>570, 36.1; 601, 7.5; 656, 12.4</td><td>570, +58</td></tr><tr><td>P-Por(Zn)-H[6]<sup>2</sup></td><td>436, 18.1</td><td>437, -249; 450, +460</td><td>556, 15.2; 604, 20.7</td><td>607, +32</td></tr><tr><td>P-Por(Ni)-H[6]<sup>2</sup></td><td>427, 15.0</td><td>427, -84; 446, +300</td><td>535, 16.7; 572, 15.7</td><td>538, +12; 575, +20</td></tr><tr><td>P-Por(Pd)-H[6]<sup>2</sup></td><td>431, 24.2</td><td>430, -233; 444, +602</td><td>495, 7.2; 532, 30.7; 573, 33.7</td><td>530, +12; 575, +61</td></tr></tbody></table>

in system 2, where there is only an ethynyl spacer between the porphyrin and the helicene core, as compared to system 1. For instance, for *P-Por(Zn)-H[6]*², the Δε value for the positive band of the couplet at the Soret band is 460 M⁻¹ cm⁻¹, while comparatively for the positive signal of the helicenic part (330 nm) is only 51 M⁻¹ cm⁻¹. This is also nicely seen in the *g*<sub>abs</sub> spectra plotted in Figure 3c, which show stronger values on the porphyrin-centered bands than on the helicenic ones. It is also worth noting from Figure 3c that all derivatives display very similar *g*<sub>abs</sub> values for the Soret band, highlighting the fact that the distance between the two porphyrin units matters, not the metal. However, it is also interesting to highlight that the ECD bands centered around 350 nm, mainly involving the helicenic transitions (the so-called B-type transition),<sup>[45,46]</sup> show a doubly

structured band whose intensities differ from one metal to another in the order Ni > Pd > Zn > H2 for the lower energy one, strongly suggesting that there is some influence of either electron-vibronic coupling or involvement of the porphyrin units in these transitions (*vide infra*) with an electronic density of the metal having an impact on the rotatory strengths.

### 3. Nonpolarized and Circularly Polarized Emission

The total luminescence of this family of compounds was investigated by measuring the emission of 10⁻⁶ M solutions in CH₂Cl₂, at
<figure><img src="image_14.png" /></figure>

rt, with an excitation at 440 nm. The spectra are reported in **Figure 4a**. Unpolarized luminescence spectra of the **Por(M)-H[6]**² series were measured at rt in CH₂Cl₂ solutions for all the derivatives except for the Pd porphyrin, for which the phosphorescence was investigated under an inert atmosphere. As expected, the Ni derivative showed no emission properties, while the Zn and the free base porphyrins compounds displayed quite similar emission spectra, and in good agreement with literature: two emission bands at 620 and 670 nm for **Por(Zn)-H[6]**², and at 670 and 738 nm for the free base **Por(H2)-H[6]**², i.e. with a 50 nm redshift (see black and red line in Figure 4a). Pleasingly, these latter compounds were also found to be CPL-active (Figure 4b). Overall, the free base and the zinc complex display emission with rather good quantum yields around 30%, substantial *g*<sub>lum</sub> values around 10<sup>-3</sup>, and good CPL brightness (**Table 2**). A different consideration should be made for the Pd derivative. For the palladium porphyrin, the measurements were performed under an inert atmosphere, in order to avoid oxygen quenching, and a predominant peak appeared at 750 nm, as depicted in Figure 4a. Some residual fluorescence signals were observed at 585 and 632 nm. However, no CPL could be detected with our instrument. Note that an interesting optically active cofacial ABCD-Pt(II)-porphyrin dimer was recently reported to display bright circularly polarized phosphorescence with |*g*<sub>phos</sub>| values of 1.0 × 10<sup>-3</sup> and a quantum efficiency Φ of 0.07.[38]

### 3.1. Synthesis of Helicene-Bis-Ethynyl-Phenyl Porphyrin System (Por(Zn)-H[6]³)

2,5-Bis-ethynyl-carbo[6]helicene precursor was reacted with the spacer 1-bromo-4-iodobenzene upon classical Sonogashira conditions to give the dibrominated species **BrH[6]** in a 68% yield (**Scheme 2**). On the other hand, the 5,15-(dimesityl)porphyrin was brominated, metalated, and borylated using classical conditions, thus giving the corresponding boronate ester

<table><caption>**Table 2.** Emission properties of *P-Por(M)-H[6]*² (M = H2, Zn) in CH₂Cl₂ and *P-Por(Zn)-H[6]*³.</caption><thead><tr><th>Compound</th><th>PLQY<sup>a)</sup></th><th>g<sub>lum</sub> [λ]<sup>b)</sup></th><th>ε<sub>440 nm</sub></th><th>B<sub>CPL</sub><sup>c)</sup></th></tr></thead><tbody><tr><td>P-Por(Zn)-H[6]<sup>2</sup></td><td>0.28</td><td>1.1 × 10<sup>-3</sup><br>(620 nm)</td><td>172 × 10<sup>3</sup></td><td>26</td></tr><tr><td>P-Por(H2)-H[6]<sup>2</sup></td><td>0.32</td><td>1.3 × 10<sup>-3</sup><br>(680 nm)</td><td>215 × 10<sup>3</sup></td><td>45</td></tr><tr><td>P-Por(Zn)-H[6]<sup>3</sup></td><td>0.08</td><td>±1 × 10<sup>-3</sup></td><td>200 × 10<sup>3</sup></td><td>0.8</td></tr></tbody></table>

**Por(Zn)Bester**[47] (see Supporting Information). The so-obtained compounds were then involved in the Suzuki cross-coupling reaction to get the desired molecule **Por(Zn)-H[6]**³ in a 40% yield.

As depicted in Figure S1.9, Supporting Information, the ¹H NMR spectrum of **Por(Zn)-H[6]**³ displays the typical signals for the helicene moiety and the two doublets of the phenyl protons of the spacer at 8.26 and 7.78 ppm. The porphyrin core shows the same set of peaks as the previous compound **Por(Zn)-H[6]**², with the proton of the free *meso* position resonating at 10.21 ppm and the β-pirrolic ones represented by the four doublets between 8.88 and 9.38 ppm. Since the porphyrin substitution has been modified, we observed a signal at 7.34 ppm corresponding to the aromatic protons of the mesityl unit, as well as two singlets at 1.88 and 2.69 ppm for the –CH₃ in ortho- and para-position of the mesityl unit, respectively.

### 3.2. UV-vis and ECD Spectra of Por(Zn)-H[6]³

The absorption spectrum of **Por(Zn)-H[6]**³ is depicted in **Figure 5b**. In the high-energy region, three absorption bands are observed at 300 nm (ε = 31,260 M⁻¹ cm⁻¹), 360 nm (ε = 16,780 M⁻¹ cm⁻¹), and 387 nm (ε = 20,000 M⁻¹ cm⁻¹), which

<figure><img src="image_15.png" /><figcaption>**Figure 4.** a) Emission and b) CPL spectra of the **Por(M)-H[6]**² series, measured at rt in CH₂Cl₂ solutions (see Supporting Information for further information).</figcaption></figure>

<footnote><sup>a)</sup>Relative quantum yield (TPP in dichloromethane as reference, see Electronic Supporting Information); <sup>b)</sup>dissymmetry factor, *g*<sub>lum</sub>, of each compound; <sup>c)</sup>CPL brightness.[68] Excitation at 440 nm for *P-Por(M)-H[6]*² (M = H2, Zn) and 410 nm for *P-Por(Zn)-H[6]*³.</footnote>
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Scheme 2.** a) Synthetic route for the helicene derivative BrH[6]; b) Suzuki coupling between the helicene BrH[6] and the porphyrin Por(Zn)BEster to give Por(Zn)-H[6]<sup>3</sup>.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Figure 5.** a) ECD and b) UV-vis spectra of Por(Zn)-H[6]<sup>3</sup> in CH<sub>2</sub>Cl<sub>2</sub> solutions at 298 K. c) Total luminescence and d) CPL spectra of Por(Zn)-H[6]<sup>3</sup> in CH<sub>2</sub>Cl<sub>2</sub> solution at 298 K.</figcaption></figure>

can be assigned to π-π* transitions of the helicene unit; the Soret band appears at 414 nm (ε = 210,755 M⁻¹ cm⁻¹) and moving toward the low-energy region of the spectrum one signal for the Q bands of Zn porphyrins is observed at 541 nm (ε = 12,860 M⁻¹ cm⁻¹). Overall, it shows very similar features to Por(Zn)-H[6]<sup>1</sup>,<sup>[18]</sup> whose bridges differ only by an additional ethy-nyl spacer. Due to the variation in this conjugation extent, the signals undergo a blueshift of about 30 nm, and the molar extinc-tion coefficients are lower. It is also worth noting that the por-phyrins bear a phenyl in their meso position, and it is known that small shifts may occur in the absorption maxima upon sub-stitution in the para-positions of tetraphenylporphyrins.<sup>[48]</sup> Regarding the ECD (Figure 5a), the helicenic part is characterized by the typical negative (Δε = −73 M⁻¹ cm⁻¹ at 300 nm) and posi-tive signals (Δε = +12 and +75 at 267 and 369 nm, respectively) for the *P* enantiomer (opposite sign values are depicted for *M*-Por(Zn)-H[6]<sup>3</sup>), as well as the bisignature of the EC for the Soret band (Δε = −38 at 407 nm and +112 at 420 nm). The over-all ECD bands intensities are however, more than twice lower compared to *P* and *M*-Por(Zn)-H[6]<sup>1</sup> enantiomers, revealing the

beneficial implication of the triple bonds in the π-conjugated pathway and in the resultant optical activity.

The total luminescence spectrum showed two bands, a lower intensity band at 600 nm and a major one at 638 nm, with oppo-site intensities as for the previously described Por(Zn)-H[6]<sup>1</sup> compound. In addition, Por(Zn)-H[6]<sup>3</sup> was found to be CPL active, with a glum value of ±1 × 10<sup>-3</sup> but with a noisy signal (Figure 5d). The photoluminescence quantum yield of 0.08 was obtained in dichloromethane, using TPP again as reference. This value was found to be smaller compared to Por(Zn)-H[6]<sup>1</sup>, highlighting again the importance of the triple bond present in the spacer to obtain strong emission through a stronger π-conjugation path-way. Note also that the brightness is much lower in Por(Zn)-H[6]<sup>3</sup> than in the Por(M)-H[6]<sup>2</sup> series (see Table 2).

### 3.3. Theoretical Analysis and Influence of EC in the Por(M)-H[6]<sup>2,3</sup> Series

Full computational details can be found in Section S2 in the Supporting Information. In a nutshell, time-dependent density
<figure><img src="image_19.png" /></figure>

functional theory (TD-DFT) linear response theory was used to calculate absorption and ECD spectra with the ωB97X-D[49] range-separated hybrid functional and a split-valence polarized basis set SV(P). The ωB97X[50] functional was used to optimize the excited state geometries and to calculate the excited state harmonic frequencies and emission spectra because of unphysical geometries caused by the dispersion corrections in ωB97X-D. Regarding general strategies for calculating natural optical activity parameters, in particular with TD-DFT, see, for example, Ref. [51] and [52].

Calculated versus experimental absorption and ECD spectra of the *M* isomers of compounds **Por(H<sub>2</sub>)-H[6]<sup>2</sup>**, **Por(Ni)-H[6]<sup>2</sup>**, **Por(Zn)-H[6]<sup>2</sup>**, and **Por(Pd)-H[6]<sup>2</sup>** are shown in **Figure 6**. The excited-state optimizations and exciton-coupling calculations used the mesityl → Cl replacement model systems mentioned earlier (**Por(H<sub>2</sub>)Cl-H[6]<sup>2</sup>**, **Por(Ni)Cl-H[6]<sup>2</sup>**, etc.), similar to a previous study of a related helicene-porphyrin system.[18] Section S2 in the Supporting Information shows extensive test calculations demonstrating that these substitutions do not affect the calculated absorption and ECD in the visible and near-UV spectral ranges because the mesityl groups absorb at shorter wavelengths. Consequently, except where necessary, the following discussion does not specifically distinguish between the full and the Cl-substituted systems. However, **Por(Zn)-H[6]<sup>3</sup>** was investigated without the substitution. The calculated UV-vis and ECD spectra for the *M*-isomer of the compound are shown in Figure S2.8, Supporting Information. A global energy shift of -0.25 eV was applied to all excitation energies prior to their conversion to wavelengths, to better align the calculated and experimental spectra. Energy shifts like this are typically necessary in calculations of helicene systems when range-separated hybrid functionals are employed.[18,53,54] We note in passing that range-separated hybrid functionals are recommended for porphyrin EC systems to avoid spurious charge-transfer transitions.[18,55] The energy-shifted calculated absorption and ECD spectra match the most intense experimental peaks both in terms of the absolute intensity as well as relative peak positions.

As experimentally observed, the absorption spectra of the systems are characterized by three groups of bands, namely helicene-centered, porphyrin-based Soret (B), and porphyrin Q bands. The calculated spectra are in satisfactory agreement with the experiments, with some deficiencies in the Q-band regions that were previously noted also for tetraphenyl-porphyrin (TPP): a selection of different functionals, including nonhybrid, global hybrid, and range-separated hybrids, were not able to reproduce the experimentally observed splitting between the Q band peaks,[53,54] which is also seen in the present calculations. Accordingly, in the low-energy region (>550 nm), ωB97X-D predicts multiple transitions, as evident in the "stick spectra" shown in Figure S2.13-S2.17 in the Supporting Information, but only one peak with the chosen broadening. The calculations describe the spectral region of the Soret band well, with four excitations underlying the high-intensity band. The high-energy part of the spectrum (<300 nm) is also satisfactorily reproduced. For **Por(Zn)-H[6]**³, the four dominant excitations in the Soret bands are well accounted for. However, the Q-bands are conspicuous by

their absence both in the experimental and in the calculated spectrum (Figure S2.17, Supporting Information).

The calculated ECD spectrum of *M-Por(H<sub>2</sub>)-H[6]*² is dominated by what appears to be an EC peak structure around 450 nm associated with the porphyrin Soret bands. Additional calculations were performed on model systems in which the helicene moiety and the alkynyl bridges were removed, to understand how the presence of the helicene influences this dominant spectral feature. To summarize a large set of calculated data, as detailed in Section S2.4 in the Supporting Information, although the porphyrin rings in their relative orientations in **Por(H<sub>2</sub>)-H[6]**² are responsible for some of the ECD intensity observed in the 450 nm region, the intensity of the EC is considerably stronger when the helicene is present. Therefore, the ECD intensity in this spectral region cannot be explained simply by EC of the porphyrin Soret band transitions, as we hypothesized initially. This finding lets us conclude that there must be substantial interactions between the helicene core and the porphyrin as far as the electronic transitions are concerned. On a related note, the negative sign of the ECD Q-band is correctly reproduced by the calculations on the full systems. The Q-band ECD intensity becomes significantly reduced when one of the porphyrin rings is removed, and the ECD becomes negligible on the scale of the plots when the helicene moiety and alkenyl bonds are removed. The influence of the helicene on the porphyrin Q-band EC ECD therefore, likewise appears to be rather strong.

The calculated ECD spectra of the metalloporphyrin systems (nickel, zinc, and palladium) display qualitatively the same features as the **Por(H<sub>2</sub>)-H[6]**² spectrum. Some differences between the calculated and experimental spectra are observed in the low-energy region, where only one negative band (around 600 nm) is observed instead of two for the *M* enantiomer. The position of the calculated peak is slightly shifted toward higher wavelengths, but the intensity and the sign correctly match the experiments. In the Soret band region, the calculated and experimental bands have similar intensities for **Por(Zn)-H[6]**² and **Por(Pd)-H[6]**², but they appear somewhat overestimated for **Por(Ni)-H[6]**².

Notably, the calculated ECD spectrum for **Por(Zn)-H[6]**³, which is shown in Figure S2.17, Supporting Information with absolute intensities, does not indicate a dramatic change in the intensity of the ECD Soret bands as a function of the (relatively minor) differences in the distance between the two porphyrin moieties when compared to the Series 2 analog. A factor that may be of relevance here is the interplay between the Davydov splitting and the rotatory strengths of the coupled transitions at the intermediate porphyrin distances encountered in the present systems. Namely, the resulting broadened intensity of the EC couplets is sensitive to these parameters. Certainly, however, given the aforementioned important and very strong helicene contributions to these bands, a simple long-range EC-ECD intensity decrease with increasing porphyrin distance is perhaps not to be expected. Instead, the computations indicate that the situation is more complex.

Further analysis of the excitations reveals that the orbitals in the transitions of the Soret band region are mainly centered on the porphyrin systems. However, contributions from the helicene
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 6.** Experimental and calculated absorption (left) and ECD (right, *M* isomer) spectra along with frontier molecular orbitals (isosurfaces, ±0.03) of the S₀ structure of a) *Por(H₂)-H[6]²*, b) *Por(Ni)-H[6]²*, c) *Por(Zn)-H[6]²*, and d) *Por(Pd)-H[6]²* in CH₂Cl₂.</figcaption></figure>
<figure><img src="image_22.png" /></figure>

rings are clearly visible, which bolsters the notion that the helicene directly takes a strong influence on the porphyrin EC couplets. The helicene contributions are evident in Figure 6, which shows the HOMO-1, HOMO, LUMO, and LUMO + 1 for Por(H₂)-H[6]², Por(Ni)-H[6]², Por(Zn)-H[6]², and Por(Pd)-H[6]². The excitations in the high-energy region of the spectra mix contributions from the porphyrin rings and the helicenes more strongly.

Experimental emission data are available for Por(H₂)-H[6]², Por(Zn)-H[6]², and Por(Pd)-H[6]². The S₁ equilibrium structures of the Cl-substituted molecules (M isomers) were optimized using TD-DFT. The fluorescence and CPL spectra, corresponding to the S₁ to S₀ transition, were calculated for the molecules and are shown in Figure S2.9-2.12, Supporting Information. The two bands (one between 600-700 nm and another between 700-800 nm) in the emission spectra of Por(H₂)-H[6]² and Por(Zn)-H[6]² are well reproduced by the calculations. In the case of Por(Pd)-H[6]², the calculation only produces the intense lowest-energy band located between 700-800 nm. It is at present unclear what might be the origin of the CPL observed experimentally at shorter wavelengths. We note in passing that the calculated CPL spectra of the compounds Por(H₂)-H[6]², Por(Zn)-H[6]², and Por(Zn)-H[6]³ have the same sign (negative for the M isomer) as the intense ECD band at lower energies.

### 3.4. CISS Effect Measurements: mc-AFM

Since mc-AFM measurements involve the deposition of chiral molecules on a gold-coated nickel surface (Au/Ni), the arrangement of these molecules on the surface significantly impacts the efficiency of spin filtering. However, achieving precise molecular orientation is challenging. To address this, two main approaches were pursued: generating SAMs, using chiral molecules with anchoring groups such as -SH or -OH,[56] and harnessing flat, π-conjugated molecules that can autonomously organize into chiral, columnar arrangements.[57,58] In the first approach, the selectivity of spin is measured along the primary axis of the molecule, which stands perpendicular to the plane of the substrate. In contrast, the second approach involves polymers arranged on the Au/Ni surface, and the measurement of the CISS effect is taken perpendicular to the molecule's primary axis. However, both methodologies exhibit certain limitations. Approach (a) necessitates intricate modifications either before or after synthesizing the chiral molecule, while

approach (b) requires meticulous design and extensive assessment of the supramolecular polymer's properties. Considering these challenges, finding new spin filters that are suitable compromises between easy processability and high performance is crucial to enable the practical application of the CISS effect in devices.

Thanks to the compromise between a high *g*<sub>abs</sub> value and the significant Δε for the EC at the Soret band, Por(Zn)-H[6]² (see comparison on Figure 3d) was selected as a suitable candidate for the CISS effect measurements through mc-AFM. The substrates for the mc-AFM studies were prepared using the e-beam evaporation deposition technique on a Si (100) wafer. A 100 nm layer of Ni and a 7 nm layer of Au were deposited on a Si wafer: the gold layer has the function to protect Ni from oxidizing. The deposited multilayer surfaces were cleaned by immersing them first in acetone and then in ethanol for 10 min. The role of the Ni/Au surfaces is to allow spin polarization of electrons injected from the surface into the chiral molecules induced by a magnetic field.

The chiral molecules were deposited on the Ni/Au surfaces by spin coating a 1 mM solution in dichloromethane at 3000 rpm for 40 sec. The mc-AFM images showed an amorphous nanoparticle-like assembly of the molecules onto the substrate (Figure 7). The calculation of the roughness of the surfaces was implemented using a higher resolution AFM.

The gold-coated nickel surface (Au/Ni), with the chiral molecules, was magnetized perpendicularly to the surface, with the north pole pointing either up (toward the surface) or down. Depending on the direction of the external magnetic field (0.50 T), the spin of the ejected electrons from the nickel substrate to the deposited chiral molecular layer varies. The current displayed for both *M*- and *P*-Por(Zn)-H[6]² strongly dependent on the direction of the magnetization of the substrate (Figure 8). Specifically, the *M*-enantiomer exhibits higher currents when the substrate is magnetized with the magnetic field up. In contrast, in the case of *P*-Por(Zn)-H[6]², the current is lower with the same magnetization but higher for magnetization down. These results are perfectly in line with former studies.[24,25]

Magnetic field-dependent current-voltage (I-V) characteristics of the prepared samples were determined using a multimodal scanning magnetic probe microscopy system equipped with a Beetle Ambient AFM and an electromagnet with an R9 electronic controller (RHK Technology). Voltage spectroscopy for the I-V measurements was performed by applying voltage ramps with a nonmagnetic Pt

<figure><img src="image_23.png" /><figcaption>**Figure 7.** AFM images of the Ni/Au coated with *P*-Por(Zn)-H[6]².</figcaption></figure>
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /><figcaption>**Figure 8.** Current versus voltage (I-V) plots recorded for spin-coated a) M-, and b) P-Por(Zn)-H[6]² on Ni-Au substrates. In both cases, the Ni substrates were magnetized with the north pole pointing up (the black line) and down (the red line) orientations; c) Spin Polarization plot for both compounds.</figcaption></figure>

tip (DPE-XSC11, µmasch with spring constant 3–5 Nm⁻¹) in contact mode. At least 50 I-V curves were scanned for each sample.

The spin-selective electron transport was quantified (SP, spin polarization) as the relative ratio of currents with the two different configurations of the magnet (UP and DOWN),

$$SP = \frac{(I_{UP} - I_{DOWN})}{(I_{UP} \mp I_{DOWN})} \times 100 = 50 \pm 5\% \quad (1)$$

The SP value is about constant for the range of ±0.1 to ±1.5 V. Overall, the value is quite high considering that there are no anchoring groups or supramolecular polymerization or organization.

the nature of the EC chirality in addition to π-conjugation with the helicene extremities. Interestingly, when adding a phenyl spacer, such as in Por(Zn)-H[6]³, smaller EC responses were observed, in agreement with a longer interchromophoric distance. Derivative Por(Zn)-H[6]² was selected as a favorable candidate for CISS measurements through mc-AFM, and spin polarization as high as 50% was obtained. Overall, these results nicely illustrate how molecular engineering using functionalization with strongly polarizable units in close proximity enables the fine-tuning of the chiroptical signatures. This strategy also allows access to CPL active porphyrin derivatives, which still remain quite rare, and opens up new opportunities for the design of suitable candidates for spintronics applications.

## 4. Conclusions

The chemical design of helicenes with porphyrin substituents allowed us to fine-tune this platform by changing the linker between the helicene and the porphyrin to achieve higher g<sub>abs</sub> and g<sub>lum</sub> values. A noteworthy enhancement was achieved with the family of molecules Por(Zn)-H[6]², where the 2,5-bis-ethynyl carbo[6]helicene is linked to the porphyrins, avoiding the spacer, so that the interchromophoric distance is reduced. In the ECD spectrum, the EC bis signature at the Soret band appeared to be the prominent signal compared to the helicene bands, and the g<sub>abs</sub>/g<sub>lum</sub> values were remarkably improved. Therefore, the investigation of the properties was extended to the influence of the metal, and several derivatives were prepared. Within the same series “2”, no significant changes were observed with the different metals, suggesting the exclusive role of the interchromophoric distance on the chiroptical response of the Soret band and further emphasizing

## 5. Computational Details

All quantum chemical calculations employed Kohn-Sham (KS) DFT and TD-DFT linear response methods. Geometry optimizations (M isomers of all molecules) and harmonic vibrational frequency calculations for the ground states were performed with the B3LYP[59–61] hybrid generalized gradient approximation functional and the Gaussian-type basis set def2-SV(P).[62] ωB97X was used to optimize the lowest excited state geometries. A scalar relativistic effective core potential matching the def2-SV(P) basis was employed for the palladium atom. All structures were characterized as minima on their potential energy surfaces. The calculations were performed using the Conductor-like Polarizable Continuum Model[63,64] with parameters for dichloromethane. All calculations were performed with the Gaussian 16 program.[65] See Section S2 in the Supporting Information for additional
<figure><img src="image_26.png" /></figure>

comments regarding the geometry optimizations performed herein. Absorption and ECD spectra were calculated with TD-DFT. The transitions were Gaussian-broadened with a wavelength-dependent $\sigma$ according to Ref. [66].

## Acknowledgements

The authors acknowledge the Ministère de l'Education Nationale, de la Recherche et de la Technologie, the Centre National de la Recherche Scientifique (CNRS), Rennes Métropole, and the French National Agency (ANR projects LumoMat-E, 18-EURE-0012 and CHIFTS, ANR-23-CE09-0022-03). The European Commission Research Executive Agency (Grant Agreement number: 859752 – HEL4CHIROLED – H2020-MSCA-ITN-2019) is thanked for financial support. J.A. thanks the National Science Foundation for financial support of the theoretical component of this research from grant CHE-2503332. We acknowledge the Center for Computational Research (CCR) at the University at Buffalo[67] for providing computational resources. Part of this work has been performed using the PRISM core facility (Biogenouest, Univ Rennes, Univ Angers, INRAE, CNRS, FRANCE). J.C. warmly thanks Dr. Ludovic Favereau for fruitful discussions and technical help.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** chirality · chiral-induced spin selectivity · circularly polarized luminescence · exciton coupling · helicene

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