# Path Selective Photoinduced Energy and Electron Transfer in a Bis(acridinium-Zn(II) Porphyrin)-tetrapyridyl Porphyrin Host-Guest Complex

Federica Ruani, Daniele Veclani, Daniel Sanchez-resa, Amy Edo-osagie, Geordie V Creste, Andrea Barbieri, Valérie Heitz, Henri-pierre Jacquot de Rouville, Nicola Armaroli, Barbara Ventura

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Federica Ruani, Daniele Veclani, Daniel Sanchez-resa, Amy Edo-osagie, Geordie V Creste, et al. Path Selective Photoinduced Energy and Electron Transfer in a Bis(acridinium-Zn(II) Porphyrin)-tetapyridyl Porphyrin Host-Guest Complex. Chemistry - A European Journal, 2025, 202502604, 10.1002/chem.202502604. hal-05343119

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# Path Selective Photoinduced Energy and Electron Transfer in a Bis(acridinium-Zn(II) Porphyrin)-tetpyridyl Porphyrin Host-Guest Complex

Federica Ruani,[a] Daniele Veclani,[a] Daniel Sanchez-Resa,[a] Amy Edo-Osagie,[b]

Geordie Creste,[b] Andrea Barbieri,[a] Valérie Heitz,[*b] Henri-Pierre Jacquot de Rouville,[*b]

Nicola Armaroli,[*a] and Barbara Ventura*[a]

*Dedicated to Jean-Pierre Sauvage on the occasion of his 80th birthday.*

The 1:1 complex of a bis(acridinium-Zn(II)porphyrin) host and a tetpyridyl free-base porphyrin guest shows a remarkable interplay of energy and electron transfer (eT) processes, finely tuned by the solvent polarity or the selection of the photoexcited component. A more polar environment (CH₂Cl₂) favors uncommon parallel intramolecular and intermolecular eT processes, while in an apolar solvent (toluene) the intermolecular eT process

is hampered. Notably, the apolar environment also enables a rare energy transfer process from the porphyrin guest toward the charge-transfer emissive state of the tweezer host. In total, no less than four different excited states were probed, which can be sketched by an inverted pyramid diagram highlighting directional path selection.

## 1. Introduction

Supramolecular architectures featuring photoinduced energy (EnT) and electron transfer (eT) processes are of fundamental importance for the development of artificial photosynthetic systems, because they mimic light harvesting and charge separation events occurring in natural photosynthetic organisms.[1] In these artificial systems, according to the number of donor and acceptor units attached to the photosensitizer, complex photoinduced eT processes can take place.[2] Interestingly, the direction of the eT can be switched by changing the redox states,[2d,3] photo-addressing a specific unit of the system,[4] adding chemicals,[5] stimulating photoconversion[6] or changing the polarity of the solvent.[7] The eT can also be triggered by an earlier EnT process (antenna effect).[8] However, supramolec-

ular systems combining several of these mechanisms are still rare,[9] even though they open up new avenues for controlling directional eT processes. Such systems could prove very useful for optoelectronic and energy harvesting devices, logic gates, switches, and sensors.[10]

Porphyrins have been extensively used in supramolecular chemistry for the construction of highly sophisticated arrays.[11] Metalated porphyrins, in particular, can axially bind selected ligands, enabling the self-assembly of non-covalent light-responsive multicomponent systems.[11a,b,12] Specifically, architectures embedding metalated porphyrins and pyridyl-decorated components have been widely reported,[11c,12b,13] with some of them exhibiting photoinduced EnT or eT processes between the porphyrin moieties.[12b,14]

We recently reported a bis(acridinium-porphyrin) molecular tweezer[15] ($1^{2+}$) made of two Zn(II) porphyrin coordinating sites interconnected via two N-acridinium units.[16] This system responds to electrochemical and photochemical inputs, affording a sophisticated multicomponent switch.[15,16] It was shown that $1^{2+}$ undergoes ultrafast photoinduced eT from the porphyrin donor to the acridinium acceptor,[15] leading to fluorescence quenching of both the porphyrin and the acridinium units. We also studied the ability of $1^{2+}$ to coordinate a non-photoactive 4,4'-bipyridine guest, revealing a twelve-state supramolecular receptor behavior in response to light, redox, and chemical stimuli.[15]

Herein, we investigate the coordination capability of $1^{2+}$ for meso-(5,10,15,20-tetra(4'-pyridyl))porphyrin (TPyP) as a photoactive guest. This porphyrin is particularly interesting since, thanks to its four pyridyl residues, it can act as a multitopic guest and has peculiar electrochemical properties with respect to other free-base porphyrins.[12b,14a] This guest can (i) modify both its and the host's electronic properties through coordination, (ii) change

<footnote>[a] Dr. F. Ruani, Dr. D. Veclani, Dr. D. Sanchez-Resa, Dr. A. Barbieri, Dr. N. Armaroli, Dr. B. Ventura
Istituto per la Sintesi Organica e la Fotoreattività (ISOF), Consiglio Nazionale delle Ricerche (CNR), Via Piero Gobetti 101, Bologna 40129, Italy
E-mail: nicola.armaroli@cnr.it
barbara.ventura@cnr.it</footnote>

<footnote>[b] Dr. A. Edo-Osagie, G. Creste, Prof. V. Heitz, Dr. H.-P. J. de Rouville
Laboratoire de Synthèse des Assemblages Moléculaires Multifonctionnels, Institut de Chimie de Strasbourg, CNRS/UMR 7177, Rue Blaise Pascal, 4, Strasbourg 67000, France
E-mail: v.heitz@unistra.fr
hpjacquot@unistra.fr</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202502604</footnote>

<footnote>© 2025 The Author(s). Chemistry – A European Journal 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.</footnote>
<figure><img src="image_3.png" /></figure>

the direction of eT, and (iii) open the way to additional selec-
tive eT/EnT steps. The complexation has been investigated in two
solvents of different polarity, that is, CH₂Cl₂ and toluene, and a
detailed photophysical investigation, supported by electrochem-
ical and theoretical studies, revealed peculiar photoinduced
dynamics in the 1:1 supramolecular complex. Parallel eT steps
and a rare EnT mechanism that triggers a charge transfer emis-
sion have been found to be operative within the self-assembled
structure. Interestingly, these processes are addressed by playing
on the solvent polarity and on the photoexcited component.

## 2. Results and Discussion

### 2.1. Binding Studies

The association between 1²⁺ and TPyP was characterized in CH₂Cl₂ and toluene (Figures 1 and S11). When followed by absorption spectroscopy, the interaction between the host and the guest was confirmed by i) a red-shift of the Soret band (from 422 to 430 nm in CH₂Cl₂ and from 425 to 437 nm in toluene) and ii) the presence of an isosbestic point at 427 nm in CH₂Cl₂ and at 432 nm in toluene. These spectral changes were observed both when titrating a solution of host with a solution of guest and vice versa (Figures 1a,b and S11a,b). Upon excitation at the isosbestic points, emission titrations revealed that both the already weak fluorescence of the Zn-porphyrin components in 1²⁺ (bands at 600 and 652 nm) and the fluorescence of the TPyP guest (bands at 645 and 712 nm) are substantially quenched upon formation of the complex (Figures 1c and S11c). This unusual outcome indicates that both host and guest undergo a quenching process when involved in the complex. For both titrations, the most meaningful fits were obtained using a 1:1 association model in both solvents (see the Materials and Methods section and Figures S12-S23). This result can be ascribed to an increased steric hindrance between the porphyrins of two hosts around one TPyP guest, thus limiting the formation of the 2:1 complex [(1²⁺)₂·TPyP]. Binding constants (K₃) of 1.2 × 10⁶ L mol⁻¹ in CH₂Cl₂ and 3.2 × 10⁷ L mol⁻¹ in toluene were estimated by averaging the values obtained from the different elaboration methods (see Section S1 and Figures S12-S23 for details).

Formation of the 1:1 complex was confirmed by ¹H NMR spec-
troscopy in CD₂Cl₂ (Figure 2). The presence of the free base was
first evidenced by the signal of the tautomeric protons (δₙₐ =
−3.75 ppm). In addition, the three different aromatic protons of
the TPyP guest appear as three broad signals (δ(Hα) = 6.07,
δ(Hβ) = 7.11 and δ(HPy) ≈ 7.96 ppm) all integrating for eight pro-
tons. This result suggests a dynamic binding since the expected
symmetry breaking in a kinetically inert complex should lead
to at least two sets of signals for each proton.[17] Notewor-
thy, the unusual chemical shifts observed for Hα and Hβ reflect
the influence of the cone of anisotropy of the ZnTPP cores on
the pyridine rings. However, the protons of 1²⁺ are clearly less
affected than the guest (−0.16 < Δδ < +0.04, Table S3). This
observation supports a metal-ligand interaction and excludes
π-π stacking interactions.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Top: schematic representation of guest complexation in which both *cis* and *trans* forms are represented (for related discussion, see the theoretical section). a) Absorption titration of 1²⁺ (2 × 10⁻⁷ mol L⁻¹) with TPyP in CH₂Cl₂ from 0 (red) to 14 equivalents (blue) after subtraction of the absorption spectra of corresponding reference solutions of TPyP. Inset: absolute absorbance variation as a function of the added equivalents of TPyP with the relative fitting curves obtained with the Wilcox function.</figcaption></figure>
<figure><img src="image_5.png" /></figure>

b) Absorption variation upon titration of TPyP (5 × 10⁻⁷ M) with ¹²⁺ in CH₂Cl₂ from 0 (red) to 5 equivalents (blue), after subtraction of the absorption spectra of corresponding reference solutions of ¹²⁺ and TPyP. Inset: absolute absorbance variation as a function of the added equivalents of ¹²⁺ with the relative fitting curves obtained with the Wilcox function. c) Emission spectra of the solutions in a) ($\lambda_{exc}$ = 427 nm, solid lines) with the spectra of the corresponding reference solutions of TPyP (dashed lines) for the sake of comparison. Inset: spectra of the mixtures subtracted by both the spectra of ¹²⁺ and TPyP.

<figure><img src="image_6.png" /><figcaption>**Figure 2.** ¹H NMR spectra (500 MHz, CD₂Cl₂, 298 K) of ¹²⁺ and ¹²⁺·TPyP complex. (For clarity reasons, only the cis-¹²⁺·TPyP was drawn; see below).</figcaption></figure>

In order to further investigate the nature of the ¹²⁺·TPyP complex, molecular modeling simulations were performed in CH₂Cl₂. Two different geometries for the complex were considered, namely i) cis-¹²⁺·TPyP, where two adjacent TPyP pyridines are coordinated with both ZnTPP cores of ¹²⁺ and ii) trans-¹²⁺·TPyP, where two opposite TPyP pyridines are coordinated with both ZnTPP cores of ¹²⁺ (Figure 3). The root mean square deviation (RMSD) plot (Figure S48a, red and blue curves) shows that both ¹²⁺·TPyP structures exhibit reduced fluctuation (less than 10 Å) compared to isolated ¹²⁺ (Figure S48a, yellow curve), resulting from an increased structural rigidity gained upon complexation. In contrast, the isolated TPyP displays minimal RMSD fluctuation (less than 2 Å), due to the low flexibility that characterizes its structure (Figure S48a, dark green curve). TPyP remains coordinated to ¹²⁺ throughout the simulation, as evidenced by the distance plots between the Zn atom of ¹²⁺ and the N atom of TPyP (d̄ Zn-μ¹-Nₘₚₘ = 2.25 Å) over time (Figure S49a). A similar trend is observed for the distance between the Zn atom of ¹²⁺ and the center of mass (com) of TPyP (d̄ Zn-TPyPcom ~ 9.0 Å) as a function of time (Figure S49b). In both investigated geometries, the combined distribution function (CDF) plot reveals that the region of highest probability is located at distances between Zn and Nₘₚₘ close to 2.3 Å, and at N-Zn-Nₘₚₘ angles around 80°, thus indicating the formation of square-pyramidal coordination complexes for each ZnTPP moiety (Figure S49c-f). These results suggest that it might be experimentally challenging to distinguish between the cis and trans forms of ¹²⁺·TPyP.

Calculation of the binding Gibbs free energies (ΔG) between ¹²⁺ and TPyP (Figure 3) was performed on a selection of the ten most representative conformations (see Materials and Methods

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Box charts related to ΔG<sub>bin</sub> for trans-1²⁺·TPyP and cis-1²⁺·TPyP calculated by averaging the Gibbs free energy of ¹²⁺ in both the open (trans-1²⁺·TPyP<sub>open</sub> and cis-1²⁺·TPyP<sub>open</sub>) and closed conformations (trans-1²⁺·TPyP<sub>closed</sub> and cis-1²⁺·TPyP<sub>closed</sub>). "Open" stands for extended chain, where ZnTPP and Acr are separated (see Figure S47 in Supporting Information); "closed" stands for folded ¹²⁺, with π-π interaction between ZnTPP and Acr (see Figure S47 in Supporting Information). Box heights indicate the 25th-75th percentile range, the line inside the box is the median, the small box is the mean value, the line outside the box represents the 5th-95th percentile range. The normal distribution of the calculated ΔG<sub>bin</sub> is also reported.</figcaption></figure>

in Supporting Information). To achieve a better distribution of ΔG, the latter has been calculated by considering the free energies of both conformations of ¹²⁺, open (ΔG<sub>O</sub>) and closed (ΔG<sub>C</sub>, see Supporting Information). Indeed, RMSD plot versus time of ¹²⁺ showed the evolution of the initial open conformation, by folding of the flexible alkene spacer, to a closed conformation stabilized by π-π interactions between one ZnP moiety and an acridinium unit of the tweezer (Figure S47b). The ΔG<sub>C</sub> values were calculated to be -9.2 kcal mol⁻¹ (K<sub>a</sub> = 5.2 × 10⁶ L mol⁻¹) for the trans-1²⁺·TPyP complex and -11.8 kcal mol⁻¹ (K<sub>a</sub> = 4.8 × 10⁸ L mol⁻¹) for the cis-1²⁺·TPyP complex. These calculated K<sub>a</sub> values are in good agreement with the experimental one of 1.2 × 10⁶ L mol⁻¹. In comparison, the ΔG<sub>O</sub> values obtained using the open conformation of the host are largely overestimated (-30.4 kcal mol⁻¹ for trans-1²⁺·TPyP, K<sub>a</sub> = 1.8 × 10²² L mol⁻¹ and -33.0 kcal mol⁻¹ for cis-1²⁺·TPyP, K<sub>a</sub> = 1.6 × 10²⁴ L mol⁻¹). These results suggest that complexation occurs between TPyP and the predominant closed conformation of ¹²⁺, representing approximately 85% of the simulation time (Figure S47a). It is worthwhile to note that the ΔG<sub>C</sub> values (2.6 kcal mol⁻¹) for the cis and trans
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Cyclic voltammograms (C₂H₄Cl₂, WE: Pt, CE: Pt, RE: SCE, 100 mV s⁻¹, TBAPF₆ 0.1 mol L⁻¹) of a solution of 1²⁺·TPyP (c = 1 × 10⁻³ mol L⁻¹). Potentials are referenced against the Fc⁺/Fc couple.</figcaption></figure>

complexes are very similar, thus indicating the possibility of an
equilibrium between both geometries in solution.

## 2.2. Electrochemistry

Cyclic voltammetry experiments were made on a preformed 1:1 mixture of TPyP and 1²⁺ at a concentration of 1 × 10⁻³ mol L⁻¹ (91% complex formation in solution). Three quasi-reversible redox processes (75 < ΔE < 100 mV, Figure 4 and Table S4), namely one in the anodic and two in the cathodic regime, were monitored for 1²⁺·TPyP.[18] The first oxidation (E₁/₂⁽¹⁾ = +0.275 V vs. Fc⁺/Fc) and reduction waves (E₁/₂⁽²⁾ = -0.952 V vs. Fc⁺/Fc) were respectively assigned to the oxidation of both Zn(II) porphyrin cores and the reduction of both acridinium cores of the receptor 1²⁺ in the complex. The third event (E₁/₂⁽³⁾ = -1.473 V vs. Fc⁺/Fc) was associated with the reduction of the complexed TPyP ligand.

When compared to the isolated tweezer 1²⁺, the first oxidation of the Zn(II) porphyrin cores is cathodically shifted (ΔE₁/₂ = -12 mV) upon coordination of TPyP. This shift allows the estimation of a binding constant (K<sub>Ox</sub>) of 3.1 × 10<sup>6</sup> L mol<sup>-1</sup> in

<figure><img src="image_10.png" /><figcaption>**Figure 5.** Square scheme analysis leading to the estimation of a binding constant for 1⁴⁺·TPyP and 1⁰·TPyP complexes, K<sub>Ox</sub> and K<sub>Red</sub>, respectively. Potentials are referenced against Fc⁺/Fc. (For clarity reasons, only the cis-1²⁺·TPyP was drawn).</figcaption></figure>

the 1⁴⁺·TPyP complex according to a square scheme analysis (Figure 5).[19] This good binding constant reflects the stability of the assembly upon oxidation (98% complex present in solution) on account of i) the ditopic coordination of the TPyP guest to the Zn(II)-porphyrin moieties and ii) the easy possible rearrangement of the complex provided by the four pyridyl groups of the TPyP ligand. Interestingly, an enhanced binding constant (K<sub>Red</sub>) of 3.1 × 10<sup>7</sup> L mol<sup>-1</sup> and an anodic shift (ΔE<sub>1/2</sub> = +41 mV) were assessed for the 1⁰·TPyP complex by a second square scheme analysis. These values indicate that the reduction/oxidation of the receptor does not affect much the 1:1 complex speciation at the studied concentrations.

## 2.3. Photophysics of Complex 1²⁺·TPyP

In order to get a deeper insight into the mechanisms of quenching of both host and guest in complex 1²⁺·TPyP, Vis-NIR transient absorption measurements with femtosecond resolution were performed in the two solvents and with two excitation wavelengths: 565, where the Zn porphyrins of 1²⁺ predominantly
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 6.** Top: TA spectra of complex 1<sup>2+</sup>·TPyP in CH<sub>2</sub>Cl<sub>2</sub> (*E* = 8 µJ/pulse) upon excitation at a) 565 nm and b) 510 nm. Middle: TA kinetics at selected wavelengths with the relative fittings (lines) of 1<sup>2+</sup>·TPyP upon excitation at c) 565 nm and d) 510 nm. An amplification factor of ca. 1.5 has been applied to some of the kinetic traces for clarity. Bottom: energy level diagram and photoinduced processes of complex 1<sup>2+</sup>·TPyP in CH<sub>2</sub>Cl<sub>2</sub>.</figcaption></figure>

absorb (> 70%), and 510 nm, where almost selective excitation of TPyP is achieved (Figure S24). Comparison with relevant models (1<sup>2+</sup>, ZnTPP, and TPyP) was also made (see the Supporting Information for details).

## 2.4. Studies in CH<sub>2</sub>Cl<sub>2</sub>

Upon excitation at 565 nm, formation of the charged species Acr\* and ZnP\*\+ is observed from the typical features of Acr\* (bands at 480 and 520 nm)<sup>[15,16b,20]</sup> and ZnP\*\+ (bands between 600 and 700 nm and at ca. 900 nm)<sup>[15,20b,c,21]</sup> that appear and decay in 0.6 and 3 ps, respectively (Figure 6a,c). The charge-separated (CS)

state Acr\*-ZnP\*\+ is thus forming and recombining in the complex with fast kinetics, identical to those observed in the uncomplexed 1<sup>2+</sup> (Figure S52).<sup>[15]</sup> Interestingly, two other bands at around 450 and 930 nm are formed within the same time range, 0.6 ps, and decay with a lifetime of about 250 ps (Figures 6a,c, and S53). They are attributed to the guest radical anion (TPyP\*\-), based on spectroelectrochemical data (Figure S50), theoretical simulations (see Supporting Information) and literature reports.<sup>[22]</sup> The observation of the radical anion TPyP\*\- supports the coexistence of another (intercomponent) CS state formed by eT from the host to the guest, that is, ZnP\*\+-TPyP\*\-, in addition to the intramolecular one Acr\*-ZnP\*\+. Notably, this second CS state shows a much slower recombination rate (250 ps).<sup>[23,24]</sup>
<figure><img src="image_13.png" /></figure>

Following excitation of the complex at 510 nm (almost selec-
tive on TPyP), the formation and decay of the band of TPyP<sup>-</sup>
at 930 nm is clearly evidenced (Figure 6b), even if partially over-
lapped with the ¹TPyP signal at 1050 nm (Figure S54a). This
band is formed within 6 ps (matching the quenched lifetime
of ¹TPyP inside the complex, Figure S55) and decays in about
250 ps (Figure 6d). The features of the zinc porphyrin radical
cation ZnP<sup>+</sup> are also clearly observed, showing a rise of 5–6 ps
followed by a decay of ca. 250 ps (Figure 6b, d), that is, the
same kinetics of the guest anion TPyP<sup>-</sup>. This indicates that the
intercomponent ZnP<sup>+</sup>-TPyP<sup>-</sup> CS state forms with a longer time
constant (6 ps) upon excitation of the TPyP guest, while it decays
with the same lifetime.[25].

A global analysis of the TA maps of 1<sup>2+</sup>·TPyP for both exci-
tation wavelengths (Figure S56) helps in extracting the profiles
of the various species. It is evident that, upon excitation of the
Zn-porphyrin of 1<sup>2+</sup> (Figure S56a), two CS states are formed, that
is, Acr<sup>-</sup>-ZnP<sup>+</sup> and ZnP<sup>+</sup>-TPyP<sup>-</sup>, living 3 and 250 ps, respec-
tively. On the other hand, upon excitation of TPyP at 510 nm
(Figure S56b) only the formation of one CS species is observed,
that is, ZnP<sup>+</sup>-TPyP<sup>-</sup>, which lives about 250 ps. The spectrum of
the species with a long lifetime is the singlet of the unbound
TPyP.

Based on the above results, we can rationalize the light-
induced processes of 1<sup>2+</sup>·TPyP in CH<sub>2</sub>Cl<sub>2</sub> by means of the energy
diagram reported in Figure 6 (bottom). Upon excitation of the
porphyrin moieties of 1<sup>2+</sup>, two parallel eT processes occur from
the Zn porphyrin singlet, which generate Acr<sup>-</sup>-ZnP<sup>+</sup> and ZnP<sup>+</sup>-
TPyP<sup>-</sup>. The lifetime of the 1<sup>ZnP</sup> is quenched to 0.6 ps and both
CS states form with the same kinetics. Eventually, they recom-
bine to the ground state with different lifetimes, the longer-lived
one being ZnP<sup>+</sup>-TPyP<sup>-</sup>, likely due to the supramolecular versus
covalent binding. Notably, parallel eT processes are only rarely
found in the literature and are always related to systems where
porphyrins are coupled to strong electron acceptors of different
nature.[2e,4,26] On the other hand, when excitation is addressed
to TPyP, only the ZnP<sup>+</sup>-TPyP<sup>-</sup> species is formed in about 6 ps,
then recombining with the same lifetime of ca. 250 ps. It can be
noticed that we found no experimental evidence of the genera-
tion of Acr<sup>-</sup>-ZnP<sup>+</sup> from ZnP<sup>+</sup>-TPyP<sup>-</sup>, via TPyP<sup>-</sup> → Acr<sup>+</sup> charge
shift, although the process appears to be thermodynamically
allowed.

**2.5. Studies in Toluene**

Upon prevalent excitation of 1<sup>2+</sup> in 1<sup>2+</sup>·TPyP at 565 nm, only
the features of Acr<sup>-</sup> and ZnP<sup>+</sup> are detected, while the bands of
the TPyP<sup>-</sup> radical anion at 450 and 930 nm were not observed
(Figure 7a). Accordingly, no eT between the host and the guest
was found to take place. The Acr<sup>-</sup> / ZnP<sup>+</sup> bands decay with
a lifetime of 200 ps and no rise was detected (Figures 7c and
S59), indicating that the ZnP → Acr<sup>+</sup> eT within 1<sup>2+</sup> in the adduct
becomes ultrafast, while charge recombination remains in the
order of 200 ps as in free 1<sup>2+</sup> (Figure S58b).[15] This is compatible
with the weak CT emission detected in the NIR region (Figure
S63a). It can be noticed that a small fraction (8%) of free compo-

nents 1²⁺ and TPyP accounts for the bands observed at 430 and
1300 nm (¹ZnP) and 1070 nm (¹TPyP).

Under selective excitation of TPyP at 510 nm in 1<sup>2+</sup>·TPyP (Figure 7b), the ¹TPyP band at 1070 nm has a double exponential decay, with lifetimes of ca. 60 ps and more than 7 ns (Figures 7d and S62). The shorter component, which is also coherent with the rise (ca. 50 ps) of the ZnP<sup>+</sup> radical cation bands (Figure 7d), suggests an EnT from ¹TPyP to the intramolecular charge transfer (CT) state of 1<sup>2+</sup>, typical of this solvent (8% of free TPyP occurring in solution is responsible for the longer component). This (Acr<sup>-</sup>-ZnP<sup>+</sup>)<sub>CT</sub> state exhibits the same transient spectral features of the CS state Acr<sup>-</sup>-ZnP<sup>+</sup> (lifetime ca. 200 ps), despite the likely lower extent of charge separation with respect to more polar media, including CH<sub>2</sub>Cl<sub>2</sub> (see above). Further evidence of the occurrence of this EnT process lies in the excitation spectrum of the complex recorded at 1000 nm, where only the CT emission is present (Figure S63b). In this spectrum, both the absorption features of 1<sup>2+</sup> (acridinium and Zn-porphyrin) and the first Q band of TPyP guest at 510 nm are detected. Remarkably, an EnT process from a singlet to a CT state is a very rarely observed phenomenon.[8c].

An energy level diagram (Figure 7, bottom) summarizes the
processes occurring in toluene upon light excitation. Addressing
light to the individual components of 1<sup>2+</sup> brings to the pop-
ulation of the emissive intramolecular CT state (Acr<sup>-</sup>-ZnP<sup>+</sup>)<sub>CT</sub>.
Likewise, also the excitation of the singlet state of the TPyP
guest populates this level via EnT.[27] Since the value taken
for the radius of the porphyrins strongly affects the calculated
energy of ZnP<sup>+</sup>-TPyP<sup>-</sup> state with the Weller equation, the
experimental uncertainty is large. It is thus difficult to conclude
whether this state lies above or below the TPyP singlet. Since
no evidence of ZnP → TPyP eT is found when exciting ¹TPyP,
the relatively high energy gap between ¹TPyP and (Acr<sup>-</sup>-ZnP<sup>+</sup>)<sub>CT</sub>
might favor the EnT between those two states.

**3. Conclusion**

It has been demonstrated that a bis(acridinium-Zn(II)porphyrin)
host efficiently encapsulates a photoactive guest, namely
tetapyridylporphyrin, leading to a peculiar photophysical
behavior of the formed adduct. The photophysics of the system
is strongly affected by the polarity of the solvent and the moiety
selectively addressed, making possible a fine tuning of the
photoinduced processes. A polar environment, such as CH₂Cl₂,
favors the occurrence of two eT within the supramolecular
assembly: an intercomponent eT between the porphyrin moiety
and the TPyP guest in parallel with an intramolecular one
between ZnP and Acr⁺. On the other hand, a less polar solvent,
such as toluene, hinders the intercomponent eT process while
providing a route for EnT from the singlet state of the TPyP
guest to the charge transfer ¹(Αcr⁺-ZnP⁺)ₚt state of the host.

Unusual photoinduced processes that are rarely observed
separately, such as parallel eTs and a singlet-to-CT-state energy
transfer, are recorded here in a single system by the com-
bination of multi-responsive building blocks, that is, Zn(II)
porphyrins, acridinium moieties, and a free-base porphyrin. The
presence of many orthogonal stimuli that can trigger different
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 7.** Top: TA spectra of complex 1<sup>2+</sup>·TPyP in toluene (E = 8 µJ/pulse) upon excitation at a) 565 nm and b) 510 nm. Middle: TA kinetics at selected wavelengths with the relative fittings (lines) of 1<sup>2+</sup>·TPyP upon excitation at c) 565 nm and d) 510 nm. An amplification of ca. 2.5 has been applied to some of the kinetic traces for clarity. Bottom: energy level diagram and photoinduced processes of complex 1<sup>2+</sup>·TPyP in toluene.</figcaption></figure>

photophysical behaviors makes such complex supramolecular assemblies of potential interest in the domains of information processing and storage networks,[15] molecular wires and switches, photoactive components for transistors, chemical sensors, and photodiodes.[20c]

**Supporting Information**

Additional experimental details, materials and methods, absorption, emission and transient absorption spectra, electrochemical and photophysical data, DFT calculations. The authors have cited additional references within the **Supporting Information**.[15,20,21,28]

**Acknowledgments**

We thank the European Union for financial support for the H2020-MSCA-ITN-2017-765297 project "NOAH" and the H2020-LC-SC3-2020-RES-RIA-101006839 project "CONDOR".

Open access publishing facilitated by Consiglio Nazionale delle Ricerche, as part of the Wiley - CRUI-CARE agreement.

**Conflict of Interest**

The authors declare no conflict of interest.
<figure><img src="image_16.png" /></figure>

Data availability statement

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

Keywords: acridinium · charge transfer state · photoinduced electron transfer · tetpyridylid porphyrin · Zn porphyrin

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[23] The band at 450 nm shows a decay and a rise with the same time (0.6 ps), followed by a longer-lived component (Figure 6c and S53). The decay of 0.6 ps can be attributed to the decay of ¹ZnP, while the rise with the same lifetime is the formation of ¹TPyP⁻. Finally, the last decay (250 ps) is the lifetime of the ¹ZnP⁺⁻¹TPyP⁻ state.

[24] These kinetics are somehow faster than those observed in a (ZnP)₈P(¹TPyP)₂ complex in CH₂Cl₂ ($τ_{CS} = 2$ ps and $τ_{CR} = 650$ ps[¹⁴c]).

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[27] Notably, in our previous work on a bis-Zn porphyrin tweezer complexing ¹TPyP, an eT from the Zn-porphyrin tweezer to the ¹TPyP guest was observed in toluene. In that case, the energy gap between ¹TPyP and the CS state was higher compared to the present case, due to the different Zn-porphyrin chemical environment related to the presence of the acridinium. Furthermore, the presence of a ¹(Adr⁻·ZnP⁺)₃⁻ state at low energy can drive, in the present case, the deactivation of the ¹TPyP singlet by EnT.
<figure><img src="image_17.png" /></figure>

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Manuscript received: September 3, 2025

Revised manuscript received: September 10, 2025

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