π-Extended cis- and trans-bis(tetracyanobutadiene) Cu-porphyrins with unusual multiredox behavior

Abstract

This work reports the efficient one-pot synthesis and isolation of two novel β-functionalized TCBD-appended copper porphyrin isomers, trans- and cis-Cu(TCBD)₂, in good yields. Comprehensive characterization by spectroscopy, mass spectrometry, and single-crystal XRD shows that TCBD orients perpendicular to the porphyrin plane to minimize steric strain. Electrochemical studies revealed unprecedented multiredox behaviour, supported by FMO analysis, indicating significant spatial separation consistent with accessible charge-separated states. These unique properties make trans-/cis-Cu(TCBD)₂ a promising candidate for charge-transfer materials and multi-electron catalysis.

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The strategic design of molecular architectures with tailored optoelectronic properties is paramount for advancing technologies ranging from solar energy conversion to molecular electronics. The endeavor to develop economical donor–acceptor (D–A) systems that facilitate long-lived photoinduced charge separation is an underlying theme in artificial photosynthesis, molecular photovoltaics, and related optoelectronic applications.¹ In recent years, 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) has achieved considerable attention as a potent electron acceptor due to its strong electron-withdrawing nature and the facility of its installation through [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction with electron-rich alkynes.² Porphyrins are exceptional electron donors and light harvesters, characterized by their rigid, planar structure, tunable electronic properties, and intense absorption in the visible region. Attaching one or more TCBD units to a porphyrin core generates a substantial donor–acceptor system exhibiting distinct properties, including intramolecular charge transfer (ICT) that extends from the visible to the near-infrared (NIR) region, ground-state dipole moments, enhanced nonlinear optical (NLO) responses, and efficient photoinduced charge separation.³ Within these pursuits, TCBD-appended porphyrins have emerged as a fascinating class of materials, showcasing remarkable electronic interactions and functionalities.

Functionalizing porphyrins at meso- or β-positions with ethynyl-aryl groups provides versatile synthetic handles for post-modification via CA-RE reactions. Demonstrating this potential, Gryko et al. in 2012 showed that meso-phenylethynylporphyrins undergo efficient mono- and di-functionalization with 1,1,4,4-tetracyanoethylene (TCNE), creating push–pull architectures. These TCBD-porphyrins exhibit broadened visible light absorption profiles, indicating strong intramolecular charge transfer from the porphyrin donor to the TCBD acceptor.^4a TCBD conjugation profoundly influences excited-state dynamics beyond these significant ground-state electronic modifications. D’Souza et al. explored multichromophoric systems with a mono-TCBD unit that enable ultrafast photoinduced charge separation. Crucially, electron exchange among TCBD moieties stabilizes energetic radical ion pairs, prolonging charge-separated state lifetimes and enhancing their suitability for charge accumulation and photoconversion processes.^4b,4c Our group has also explored TCBD-appended porphyrins and corroles. In 2022, β-mono-TCBD appended porphyrins were synthesized and investigated for their strong third-order NLO properties and excellent optical limiting behaviour.^4d In 2023, β-bis(TCBD) functionalized copper and silver corroles were reported, demonstrating efficient metal-ion-dependent photoinduced charge transfer and triplet state population.^4b

Although the synthesis and influence of diTCBD substitution at the meso-positions have been studied, its impact continues to offer valuable insights into porphyrin electronic modulation.^4a As part of our ongoing investigation into the synthesis and reactivity of π-extended porphyrins, we herein report the synthesis of β-diTCBD-appended porphyrin isomers, cis- and trans-Cu(TCBD)₂, which show exceptional multi-redox activity and tunable charge-transfer states. These macrocyclic frameworks display rich electrochemical behavior due to their extended conjugation and redox activity. Metalloporphyrins typically undergo two reversible one-electron ring-centred reductions in nonaqueous media, forming a π-anion radical and dianion. Transition metal porphyrins (e.g., Mn(III), Fe(III), Co(II)) can exhibit a third reversible reduction, often metal-centred.^5a–e Substituents influence the reduction potentials: electron-withdrawing groups (–CN, –NO₂) facilitate reduction, while electron-donating groups hinder it.^1f,5f Initial reports documented the deep reduction of porphyrins to tetra- and hexa-anions, yet the corresponding monomeric systems remain largely unexplored, with a few reports describing multi-electron reductions only at low temperature.⁶ To date, such investigations have been confined to more complex structures, namely dimeric porphyrins or those conjugated with BODIPY and fullerene moieties.^7a,b In this work, we delve into our monomeric porphyrins’ rich and versatile electrochemical landscape, uncovering their multi-step redox processes and shedding light on their deeper reduction pathways.

Novel one-pot π-extended cis-/trans-Cu(TCBD)₂ porphyrins are synthesized in moderate yield and characterized by various spectroscopic techniques. H₂TPP(PE)₄ (TPP = tetraphenylporphyrin, PE = phenylethynyl) was synthesized from H₂TPPBr₄via a Stille coupling reaction, in which H₂TPPBr₄ was charged with tributyl(phenylethynyl)stannane in the presence of a Pd(PPh₃)₄ catalyst, using distilled 1,4-dioxane as a solvent. Copper metalation of H₂TPP(PE)₄ was performed according to the reported method in the literature.^7c–e The synthesized CuTPP(PE)₄ was subsequently reacted with TCNE in 1,2-dichloroethane (DCE), resulting in the formation of cis- and trans-Cu(TCBD)₂ isomers.

A TCBD-functionalized porphyrin is synthesized through [2+2] cycloaddition between the TCNE and the β-tetraphenylethynyl-substituted porphyrin, which is followed by the retro-electrocyclization outlined in Scheme 1. The electron-rich nature of the porphyrin core facilitates the electrophilic cycloaddition of the TCNE, a strong dienophile with a phenylethynyl group. Interestingly, the cycloaddition occurs selectively at one β-phenylethynyl unit of each pyrrole ring in the cis- and trans-patterns (Scheme 1), which may be attributed to steric hindrance and electronic effects introduced by the TCBD group.

Scheme 1 Synthetic scheme of π-extended β-TCBD-appended porphyrin isomers, trans- and cis-Cu(TCBD)₂.

Single-crystal X-ray diffraction analysis unambiguously established the spatial arrangement and connectivity of the trans- and cis-TCBD units at the pyrrolic positions of the porphyrin. Single crystals of trans-Cu(TCBD)₂ were obtained through slow diffusion of methanol into a saturated solution of CH₂Cl₂, whereas the crystals of cis-Cu(TCBD)₂ were obtained from CH₂Cl₂/n-hexane.^1g For both structures, significant disordered solvent-accessible voids were detected that could not be modelled satisfactorily with discrete atoms. Therefore, the PLATON/SQUEEZE procedure was applied to remove the contribution of diffuse electron density from the disordered solvent molecules.^7f The deviation of the 24-core atom of the porphyrin ring from the mean plane is illustrated in Fig. S1 of the SI. Crystallographic parameters and selected average bond lengths and angles for trans-Cu(TCBD)₂ and cis-Cu(TCBD)₂ are compiled in Tables S1 and S2 of the SI.

Fig. 1 shows the ORTEP image (top and side views) of the synthesized Cu porphyrins, and both porphyrins are crystallized in a triclinic crystal system having a P1̄ space group. Analysis of the mean plane deviations of the 24 core atoms (Δ24) and eight β-pyrrolic carbons (ΔC_β) reveals that trans-Cu(TCBD)₂ adopts a saddled distortion, whereas cis-Cu(TCBD)₂ maintains a quasi-planar geometry. The elongation of the C_β–C_β bond is observed due to the extended π-conjugation and electron-withdrawing β-substituents as compared to the unsubstituted C_β′–C_β′ bond (Table S2 of the SI). As expected, both the TCBD substituents are oriented almost perpendicularly (Fig. 1c and d) from the mean porphyrin plane to minimize the repulsive interaction.

Fig. 1 ORTEP diagrams showing top (a) and (b) and side (c) and (d) views of trans-Cu(TCBD)₂ and cis-Cu(TCBD)₂, respectively. H atoms (represented by orange color sphere) are removed for clarity in the side view (c) and (d). Displacement ellipsoids are drawn at 50% probability. Color code: C (slate blue), N (purple), Cu (copper).

The Hirshfeld surface (HS) analysis provides a unique and insightful representation of intermolecular interactions within a crystal structure. The HS is constructed based on the electron density contribution from a molecule relative to its surrounding neighbours and is characterized using parameters such as the normalized contact distance (d_norm), shape index, and curvedness. The HS of trans-Cu(TCBD)₂ and cis-Cu(TCBD)₂, as calculated using crystallographic data, is presented in Fig. S2 and S3, respectively (SI). The d_norm distance is derived from the distance to the nearest internal d_i and external d_e atoms, normalized by their van der Waals radii. A negative d_norm indicates contacts shorter than the van der Waals radii (mainly corresponding to strong interactions such as hydrogen bonding) and is visualized as red regions on the HS. A positive value (blue) reflects longer contacts, while white areas correspond to interactions close to the van der Waals limit (d_norm = 0).^8a Additional surface properties, including curvedness and shape index, offer deeper geometric insights. Curvedness, related to the root mean square curvature of the surface, differentiates flat (green) regions from highly curved (blue) areas. The shape index is sensitive to subtle variations in surface topology, distinguishing between convex bumps (blue) and concave hollows (red), which can reflect complementary molecular packing features.^8b

A quantitative summary of the various intermolecular interaction contributions derived from Hirshfeld surface analysis is shown in Fig. S4 and S5 of the SI. The 2D fingerprint plots reveal that the central region is dominated by H⋯H interactions, which contribute significantly, ranging from 42% to 48%, to the overall intermolecular interactions. N⋯H interactions are comparable in both structures, but the C⋯H interaction is slightly higher in trans-Cu(TCBD)₂ than in cis-Cu(TCBD)₂. Other intermolecular contact types contribute negligibly to the overall surface.

The optical absorption studies of the trans- and cis-Cu(TCBD)₂ isomers were performed in CH₂Cl₂ at 298 K, as shown in Fig. 2, and the optical data are presented in Table S3. The synthesized porphyrins, i.e., trans-/cis-Cu(TCBD)₂ showed a hypsochromic shift in the Soret band as compared to CuTPP(PE)₄, while the two Q-bands near 574 nm and 625 nm in CuTPP(PE)₄ are replaced by a very broad intramolecular charge-transfer band with end absorptions into the near infrared region.

Fig. 2 Absorption spectra of the synthesized porphyrins in CH₂Cl₂ at 298 K.

Incorporating a TCBD unit at the β-positions of the porphyrin ring resulted in a blue shift of the absorption spectrum of the porphyrin having phenylethynyl substituents. This shift is likely due to the limited conjugation between the porphyrin and the TCBD unit, as the TCBD is almost perpendicular to the porphyrin macrocycle. The presence of the TCBD unit significantly influences the conjugative interaction within the system, thereby modulating the electron density distribution across the porphyrin macrocycle. Both trans- and cis-Cu(TCBD)₂ isomers exhibit bathochromic shifts in their Soret band (Δλ_max ≈ 6 nm) and Q-bands (Δλ ≈ 60–80 nm) relative to the reported β-mono-TCBD appended Cu-porphyrin (Cu-TCBD).^4d This significant red-shift is attributed to the enhanced π-conjugation and more substantial cumulative electron-withdrawing effect imparted by the two TCBD substituents. A blue shift in the Soret band is observed for meso-diTCBD appended porphyrins compared to β-diTCBD appended porphyrins (trans- and cis-Cu(TCBD)₂). This difference stems from the substantially greater steric and electronic perturbation imposed on the porphyrin π-system by β-position substituents relative to meso-aryl substituents.^4a The presence of a low-lying, broad absorption band in the 600–800 nm region, observed for the synthesized porphyrins, indicates the occurrence of charge transfer (CT) processes following the incorporation of the TCBD unit, as reported in the literature.^4c,8c,8d Porphyrins substituted with a phenylethynyl group at the β-position are known to retain their Q bands in the electronic spectra; however, the absence of these bands in the synthesized porphyrins indicates that the presence of the TCBD moiety significantly perturbs the electronic structure. To gain further insight into the absorption behaviour, we recorded the UV-vis spectra in different solvents (Fig. S6) and consistently observed broad absorptions in the Q-band region (tabulated in Table S4). These features can be attributed to charge-transfer interactions between the porphyrin core and the TCBD substituents. The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) and HRMS mass spectrometry of the synthesized porphyrins match well with the molecular ion peaks (m/z) (Fig. S7 and S8).

Trans- and cis-Cu(TCBD)₂ exhibit the EPR spectral profiles shown in Fig. S9 and S10, characterized by distinctive multiline features of copper porphyrin. These features originate from hyperfine coupling interactions between the unpaired electron localised on the Cu^II centre and the magnetic moments of nearby nuclei. The synthesised complexes exhibit g_‖ and g_⊥ values of 2.17 and 2.04, respectively. The corresponding hyperfine coupling constants (A_‖ = 0.67 × 10⁻⁴ cm⁻¹ and A_⊥ = 1.82 × 10⁻⁴ cm⁻¹) further confirm the characteristic features of the Cu^II centre. The observed hyperfine structure arises from interactions involving a single Cu nucleus (I = 3/2), and four equivalent ¹⁴N nuclei (I = 1).^7d

To investigate the electronic effects induced by the TCBD units, cyclic voltammetry measurements were performed in CH₂Cl₂ containing 0.1 M TBAPF₆ as the supporting electrolyte at 298 K and shown in Fig. 3. The corresponding DPV diagrams are presented in Fig. S11 and S12 of the SI. The electrochemical data of the synthesized porphyrins are tabulated in Table S5 of the SI. The trans- and cis-Cu(TCBD)₂ isomers revealed distinct electrochemical redox profiles. trans-Cu(TCBD)₂ displayed five well-resolved, reversible reductions and two reversible oxidations. The first two reduction waves were assigned to the stepwise reductions of the two equivalent TCBD units (TCBD^0/−1 and TCBD^−1/−2).^3b,4c The enhanced current observed for both waves arises because each wave represents a concerted two-electron transfer: the first wave corresponds to simultaneous one-electron reduction (TCBD⁰ → TCBD⁻¹) of both TCBD units, yielding a net two-electron process at a single potential. Similarly, the second wave also results in a net two-electron process. Subsequent peaks represent three discrete one-electron ring-centered reductions.

Fig. 3 Cyclic voltammograms (CVs) of the trans- and cis-Cu(TCBD)₂ porphyrins in CH₂Cl₂ at 298 K.

In contrast, cis-Cu(TCBD)₂ displayed six reversible reductions and three reversible oxidations. Likely, the first two reductions were assigned to the TCBD units (TCBD⁰/⁻¹ and TCBD⁻¹/⁻²), followed by four ring-centered one-electron reductions. The greater number of redox processes observed for the cis isomer is attributed to its lower molecular symmetry than the highly symmetric trans isomer. Remarkably, both isomers undergo multi-redox processes involving ≥10 electrons overall. To the best of our knowledge, these monomeric porphyrins having diTCBD units at the β-positions represent the first examples exhibiting such a versatile multiredox electrochemical behaviour at room temperature, particularly in their extensive reduction sequences.

To examine the impact of substituents at the β-pyrrolic position of the porphyrin macrocycle and to validate the experimental results, we performed the optimization of the ground state geometries for the π-extended trans- and cis-Cu(TCBD)₂ porphyrins in the gas phase using Gaussian 16 software. The calculations were performed with the B3LYP functional set and LANL2DZ basis set. The synthesized porphyrins were fully optimized, and the corresponding average bond lengths and angles were determined and summarized in Table S6 of the SI. The degree of nonplanarity was assessed by calculating the Δ24 and ΔC_β values. Both the trans- and cis-Cu(TCBD)₂ isomers exhibited a slightly saddle-shaped geometry. The ground state dipole moments of the synthesized porphyrins were calculated and displayed in Fig. S13 of the SI. The frontier molecular orbitals and energy levels of the trans- and cis-Cu(TCBD)₂ porphyrins are shown in Fig. S14 and S15 of the SI and illustrate that the electron density in the HOMO is mainly contributed from the porphyrin core, which acts as the electron donor. In contrast, the LUMO is contributed from the TCBD moiety, which behaves as the electron acceptor group. Based on the spatial distribution of these orbitals, the formation of a charge-transfer state (Por^δ+–TCBD^δ−) or a charge-separated state (Por˙⁺–TCBD˙⁻) can be anticipated.

In conclusion, we have successfully synthesized and isolated cis- and trans-Cu(TCBD)₂ porphyrins in good yields. Comprehensive characterization by spectroscopic methods, single-crystal XRD, and mass spectrometry confirmed their structures. SCXRD analysis revealed that both isomers adopt a triclinic lattice, with each TCBD unit oriented perpendicularly to the porphyrin mean plane to minimize steric repulsion. Electrochemically, these systems exhibit unprecedented multiredox behaviour, representing the first monomeric porphyrins demonstrating such extensive electron-transfer processes at room temperature. Frontier molecular orbital analysis further reveals significant spatial separation, suggesting that efficient photoinduced charge-transfer/charge-separated states can be achieved in these architectures. Given their exceptional multi-redox activity and tuneable charge-transfer states, these novel porphyrin systems present compelling opportunities for future exploration in multi-electron catalysis, advanced energy storage materials, and next-generation molecular electronic devices.

M. S. thanks the Science and Engineering Research Board (SERB/CRG/2020/005958), New Delhi, India, for the financial support. R. J. and A. S. B. thank the Ministry of Education, India, for their fellowship.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary Information: synthesis and characterization data. See DOI: https://doi.org/10.1039/d5cc04942c.

CCDC 2470071 (trans-Cu(TCBD)₂) and 2470072 (cis-Cu(TCBD)₂) contain the supplementary crystallographic data for this paper.^9a,b

Notes and references

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