Computational investigation of structural, electronic, and spectroscopic properties of Ni and Zn metalloporphyrins with varying anchoring groups

Molecular materials design has attracted research attention because of its emerging applications in various scientific fields, such as energy storage and conversion, catalysis, and optoelectronics.¹ Porphyrins have gained significant interest in molecular materials due to their natural abundance, low economic cost, highly conjugated structures, high chemical stability, and ability to form stronger host–guest interactions owing to preorganization.^2–4 Porphyrin is a large ring molecule composed of four pyrroles⁵ linked by a sequence of single and double bonds.^6,7 Metalloporphyrin (MP) compounds, porphyrin coordinating with a metal ion in the center, that widely exist in nature have attracted intensive attention due to their ability to coordinate with almost all the elements and unique π-conjugated systems.⁸ 

Kozlowski and co-workers established a relation between the structural properties of unsubstituted MPs and central metal.^9,10 Liao and Scheiner studied changes in the electronic structure leading to the electronic and optical properties of iron and cobalt porphyrins in the presence of different anchoring groups.¹¹ Introducing different substituents at meso-position modified the physical, chemical, and electrochemical properties of MPs.^12,13 Meso-porphyrins have been reported as the most efficient porphyrins in dye-sensitized solar cell (DSSC) applications.^13,14 Most importantly, porphyrins can undergo electronic structure changes by modifying the central metal and anchoring groups. In recent years, researchers have been focusing on amending the porphyrin molecule to alter the electrochemical behavior of metal-porphyrins, primarily focusing on substitutes, bridge moiety, and anchoring groups.¹⁵ Therefore, enlightening the role of anchoring groups in tailoring electron transport using single molecules is fundamental to molecular device fabrication and accurately measuring and understanding the electrical conductance of molecules, which are the cardinal building blocks of advanced electronics.¹⁶ 

Here, we investigate the metalloporphyrins NiP and ZnP attached with various anchoring groups (–SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, and –PH₂). Phenyl rings were used as internal barriers for charge transfer between the macrocycle porphyrin core and the anchoring groups. Possible correlations between the central metal core and a series of anchoring groups are developed using electronic structure parameters such as highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energies, energy gaps, electronic chemical potentials, global chemical hardness, global chemical softness, and electrophilicity index, investigated using Density Functional Theory (DFT) calculation approach. Furthermore, MP absorption spectra depend on the nature of metal ions and the presence of any ligands, such as anchoring groups, which can alter the electron density distribution in the metalloporphyrin molecule. These factors can cause variations in the absorption spectra, allowing the optical and electronic properties of metalloporphyrins to be fine-tuned for various applications, including catalysis, sensing, and bioimaging.¹⁷ 

We conducted time-dependent density functional theory (TDDFT) based procedures using the Amsterdam density functional (ADF) package to determine the spectroscopic profile in the UV–Vis range.²⁶ It should be noted that PBE, a semi-local functional, is known to underestimate HOMO–LUMO gaps and incorrectly predict excitation energies. This can be corrected using range-separated functionals or by using functionals specifically designed to get the correct asymptotic behavior.^27–29 To aid in obtaining better energies for the HOMO and excitation energies in TDDFT, we carried out single point calculations with unrestricted spin polarization using the LB94 functional, which is known to yield more accurate response properties.³⁰ The TZP basis set was carried out with all electrons without freezing any orbitals.³¹ The excitation spectra of a series of transition porphyrins were calculated to exhibit allowed transitions by employing the Davidson diagonalization algorithm.³² The lowest 30 excitation energies and oscillator strengths of the transition porphyrins were calculated.

Figure 3 provides the lowest energy molecular conformations for NiDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂). The planarity of the porphyrin ring remains unchanged after introducing the Ni central metal in the MP systems (shown in Fig. 3). However, meso-positioned phenyl rings and functional groups (such as –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) display torsion angles deviating from the plane. Planarity is the most desirable and essential characteristic that should be preserved in molecules used for electronic charge transfer.^33–35 Average bond lengths of Ni-MP macrocycle NiDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems are listed in Table I. The Ni–N bond length range of 1.96–1.99 Å is close to the experimental value (∼1.957 Å) (see Table S4).³⁶ Although the theoretical key parameters are close to experimental values, MPs derived from different anchoring groups show slightly distinctive geometric structures, and the Ni–N bond lengths are slightly longer than their experimental analogs. High conjugation throughout the molecular structure of the MP backbone results in the similarity of the N–C1_pp, C1_pp–C_β, and C_β–C_β^. bonds in all systems. The twist, due to steric constraints, is characterized by a dihedral angle, as defined elsewhere.³⁷ It is the angle between the direction along the bond between the C atom (C_pp) in the macrocycle and the C atom (C_ph) in the phenyl ring. The dihedral angle data show that the phenyl rings are rotated in all NiDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems concerning the pyrrole rings. For example, TeH, F, Cl, Br, and PH₂ induce more tilting by dihedral angles 70.51°, 71.02°, 70.79°, 70.63°, and 70.44° than SH, SeH, CN, and NH₂ by angles 69.62°, 69.41°, 69.20°, and 67.18° in NiDPPX molecules. Controlling the phenyl ring orientation is desired in developing nanotechnologies such as memory storage, nanotransistors, and molecular machines.^33,34,38

The HOMO and LUMO levels were investigated to get insights into the electronic properties, distribution patterns, and densities of the frontier molecular orbitals (FMOs). The electronic structure of NiDPPX molecules is shown in Figs. 4 and S1 in the supplementary material. The electron densities of HOMOs for all Ni-based MPs (NiDPPX; X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) are delocalized predominantly over Ni central metal. The LUMO orbitals are delocalized at the center of the MP plane, indicating a good intramolecular charge transfer (ICT) between donor (Ni) and acceptor (porphyrin core) subunits.³⁹ Therefore, the major orbital distributions of HOMO and LUMO of NiDPPX (X = SH, F, and CN) systems are located on the MP plane, which agrees with the previous theoretical reports.^39,40 A noteworthy aspect is that the meso-phenyl-X anchoring groups have negligible HOMO and LUMO electron densities in all of the NiDPPX molecules studied here. A comparison of the frontier molecular orbitals of the selected NiDPPX systems revealed similarities in the shapes of the HOMOs and LUMOs of all the systems. There are commonalities in the distribution of electron density on the porphyrin core structures. As one changes the anchoring group, there should be discernible differences in the HOMO, LUMO, and HOMO–LUMO gap energies.⁴¹ 

The energy of HOMO and LUMO is an essential factor in determining the electronic properties of a molecule. It is a critical aspect that affects molecular chemical stability and electron conductivity, which in turn play a vital role in molecular electrical transport.^42,43 Figure 5 shows the HOMO and LUMO energies of the studied NiDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems. The HOMO and LUMO energy values for the NiDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems range from −4.57 to −5.35 and −3.12 to −3.63 eV, respectively. NiDPPCN has the lowest HOMO energy, whereas NiDPPNH₂ has the highest HOMO energy. Conversely, NiDPPCN has the lowest LUMO energy level, while NiDPPNH₂ has the highest LUMO energy. These results indicate that CN decreases while NH₂ increases both HOMO and LUMO energy levels compared to other anchoring groups.

The data of the HOMO–LUMO gaps of NiDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems are listed in Table S2. Interestingly, the smallest HOMO–LUMO gap is 1.46 eV for NiDPPNH₂. Meanwhile, both NiDPPSeH and NiDPPTeH have the largest gaps (1.73 eV). The HOMO–LUMO gaps of NiDPPX systems were reduced mainly due to decreasing LUMO energies. These changes in HOMO and LUMO energies can be deduced by analyzing the push–pull effect resulting from an effective charge separation mechanism that localizes HOMO on the donor while anchoring groups and acceptors are populated by LUMO. Figures 4 and S1 in the supplementary material demonstrate that altering the anchoring groups leads to significant changes in the spatial distribution of molecular orbitals, which can alter the position and energy of the HOMO and LUMO levels. The total electron density plots in Fig. S2 in the supplementary material for NiDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems show the symmetric charge distribution throughout the entire molecule. According to the Mulliken charge analyses (Table II), varying charge distributions depend on the anchoring group. The charge distribution of different deviations of NiDPPX (X = SH, SeH, TeH, F, Cl, Br, CN, NH₂, and PH₂) mainly accumulated on the Ni metal in the center and the N atoms in the porphyrin ring. However, a noticeable differential charge distribution is observed at C2_pp and the anchoring groups, particularly with the substitution of CN and NH₂ groups. For instance, in the case of CN, C2_pp has the least charge accumulation, while more charge accumulates on CN due to its higher electronegativity. On the other hand, in the case of NH₂, the C2_pp has more charge accumulation, while NH₂ has the least charge accumulation. These changes in Mulliken charge in each system depend on the anchoring groups, which affect the geometric parameters and delocalization of HOMO–LUMO, leading to changes in the charge distribution throughout the system.

The electronic and chemical properties, namely, the frontier energies (i.e., E_HOMO, E_LUMO, and E_gap), the electronic chemical potential (μ), the global hardness (η), global softness (σ), and electrophilicity index (ω) of NiDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems obtained from the optimized structure are given in Table III. The motivation was to see how the reactivity descriptors change with different anchoring groups in the NiDPP system. The value of μ defines the electron escaping tendency of a system in equilibrium. Among the studied systems, the μ value is the lowest for NiDPPCN (−4.49 eV) and the highest for NiDPPNH₂ (−3.84 eV), calculated from Eq. (1). The value of η, which is the resistance to charge transfer, was calculated from Eq. (2). NiDPPSeH and NiDPPTeH have the highest η (1.73 eV), while NiDPPNH₂ (1.46 eV) shows the lowest value. Therefore, NiDPPSeH, NiDPPTeH, and NiDPPCN are more stable, requiring more energy to reach an excited state since large energy gaps characterize η. Whereas σ is opposite to global hardness and measures the polarizability and ease of charge transfer, it was calculated from Eq. (3). NiDPPNH₂ has the highest σ value, indicating that NiDPPNH₂ is softer and more polarizable than other systems. The value of ω defines the electrophilic power and tendency of a system to accept electrons from an electron reservoir.⁴⁴ The ω is highest for NiDPPBr (5.95 eV), indicating that NiDPPBr is more electrophilic. Among all NiDPPX systems, the NiDPPSeH system with the higher bandgap, μ, and η values and the lower value of ω is highly stable and less reactive.

The UV–vis spectrum of metalloporphyrins will typically exhibit two main groups of absorption bands in the 300–800 nm region of the spectrum. A strong band is found in the 330–500 nm region of the spectrum, and a group of weak bands is found in the 500–700 nm region.⁴⁵ In our study, the UV–vis spectrum of NiDPPX exhibits absorption bands in the 470–1000 nm region of the spectrum (Fig. 6). Interestingly, there is only one primary peak observed for each of the NiDPPX systems. The origin of the absorption peaks is somewhat dependent on the anchoring group. For example, for NiDPPX, where X = PH₂, CN, F, Cl, Br, the absorption peak corresponds to transitions originating from the HOMO and transitioning to the LUMO+1. The molecular orbitals participating in the electronic transitions include the methine bridges, the p orbital of N atoms, and the β carbons of porphyrin to the macrocycle ring, respectively. In addition, we observed a small electronic density on the Ni atom. In NiDPPSH as well as NiDPPSeH, strong absorption spectra were observed at 618.75 and 660.6 nm, respectively. These absorption characteristics come from the contribution of thiol and selenol fragments in the porphyrin molecule to the macrocycle ring. The insertion of selenol into nickel-containing porphyrins suggests a cause for the spectra to undergo redshift relative to the thiol anchoring group. In NiDPPTeH, two strong peaks were observed at 553.1 and 911.9 nm. The peaks consist of transitions from HOMO-2 and HOMO to LUMO+2, with electron density primarily concentrated on the pyrrole ring and two mesocarbons that are connected to phenyl moieties, with very little residing on the Ni atom. The second peak arises from the electronic transition of the Te orbital (Fig. S3). In NiDPPNH₂, the absorption at 584.43 nm shows the contributions of electronic transitions come from HOMO, which is the methine bridge and p orbitals of N atoms, and the molecular orbitals of phenylamine engaged in these electronic transitions to LUMO+2, whose corresponding orbitals are the macrocycle ring and the small electronic density on Ni atom. It is obvious that the nickel atom contributing to these absorptions indicates a stronger metal–ligand interaction, resulting in a more distorted coordination environment.

The ground state molecular conformations of ZnDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) are in Fig. 7. Similar to NiDPPX, the structure maintains the macrocycle in one plane, with the phenyl-anchoring group twisted out of the plane. Table IV shows that the calculated Zn–N bond lengths agree well with previously reported values for all anchoring groups (see Table S4).^36,46 From the table, there is no significant variation in the C–C bond lengths by changing the anchoring group. Tilting of the phenyl groups of all ZnDPPX molecules changes with anchoring groups (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂). The TeH, F, Cl, Br, and PH₂ induce more tilting by dihedral angles 65.01°, 65.86°, 65.66°, 65.56°, and 65.83° than SH, SeH, CN, and NH₂ by angles 64.60°, 64.66°, 64.61°, and 62.19° in ZnDPPX molecules.

Figures 8 and S3 in the supplementary material show the orbital density for ZnDPPX molecules. In the case of HOMO, orbital density is observed on the anchoring groups for ZnDPPX (X = SH, F, and CN) (Fig. 8). In addition, there is no localization of orbital density on the Zn-center, which is different from Ni. However, delocalization across the macrocycle extends through the phenyl substituent. The LUMOs exhibit orbital density across the macrocycle and phenyl-group; however, no orbital density is located on the Zn or anchoring group. This is also observed for other ZnDPPX systems (Fig. S4 in the supplementary material). The closed-shell nature of the Zn metal ion is one of the reasons for the lack of localized d-orbitals in the system. Although the meso-phenyl moieties contributed to both the HOMOs and the LUMOs in ZnDPPSH, ZnDPPF, and ZnDPPCN molecules, the SH, F, and CN anchoring groups contributed more significantly to HOMOs than LUMOs. Functionalization of meso-phenyl of MP with different anchoring groups (–SH, –F, and –CN) produces a push–pull effect, enhancing the polarizability of the π-conjugation of phenyl groups. In addition, the anchoring groups (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) in the case of ZnDPPX influenced the electron density of porphyrin core structures in HOMO orbitals, as shown in Figs. 8 and S4 in the supplementary material.

Figure 9 provides the HOMO and LUMO energies for all of the ZnDPPX systems studied. The HOMO and LUMO energy values for the ZnDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems were calculated and found to be in the range of −4.45 to −5.36 and −2.72 to −3.49 eV, respectively. ZnDPPCN is determined to have the lowest HOMO energy level, whereas ZnDPPNH₂ has the highest HOMO energy level. Conversely, ZnDPPCN has the lowest LUMO energy level, while ZnDPPNH₂ has the highest LUMO energy level. These results indicate that CN can decrease both HOMO and LUMO energy levels, while NH₂ can increase both HOMO and LUMO energy levels compared to other anchoring groups. Current theoretical calculations suggest that the CN and NH₂ groups have a similar impact on the electronic properties of ZnDPPX systems as observed in NiDPPX systems.

Table S3 in the supplementary material provides the HOMO–LUMO gaps of ZnDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems. The HOMO–LUMO gaps of studied ZnDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) range from 1.73 to 1.89 eV. Figures 8 and S4 in the supplementary material demonstrate that altering the anchoring groups leads to significant changes in the spatial distribution of molecular orbitals, which can alter the position and energy of the HOMO and LUMO levels. The total electron density plots of ZnDPPX (X = –SH, –SeH, –TeH, –F, –Cl, –Br, –CN, –NH₂, –PH₂) systems in Fig. S5 in the supplementary material show the symmetric charge distribution over the entire molecule, which accounts for their chemical stability. Regardless of the anchoring group, the charge distribution of ZnDPPX remains concentrated on the Zn and N atoms, as evident from the Mulliken charge analysis (Table V). However, in the case of CN, the C2_pp has the least charge accumulation, while more charge accumulates on CN due to its higher electronegativity. On the other hand, in the case of NH₂, C2_pp has more charge accumulation, while NH₂ has the least charge accumulation.

Table VI shows that ZnDPPCN has the lowest μ (−4.43 eV), while ZnDPPNH₂ has the highest (−3.58 eV) μ value. The η value is the maximum for SeH, TeH, and F (1.89 eV) and the minimum for ZnDPPNH₂ (1.73 eV). The values of σ for all systems are very close to each other. ZnDPPCN (5.26 eV) is the most electrophilic, while ZnDPPNH₂ (3.71 eV) is the least electrophilic, based on the values of ω. Despite variations in the anchoring groups, the shifts in HOMO and LUMO energies are quite consistent. Overall, there are no significant differences in the reactivity descriptors for ZnDPPX systems. Nevertheless, the ZnDPPX materials exhibit higher hardness than softness values, indicating stability. This feature allows designers and manufacturers to select and utilize these materials in various electronic applications.

The UV–vis spectrum of ZnDPPX exhibits absorption bands in the 300–800 nm region of the spectrum (Fig. 10).⁴⁷ The most noticeable peak of ZnDPPSH appears at 420.3 nm, with the corresponding transition from the HOMO-1 to the LUMO+1. These transitions arise from density localized on pyrrole carbons to the antibonding π orbital of the macrocycle ring, respectively. In ZnDPPSeH and ZnDPPTeH, the peak consists of transitions from the HOMO-3 to LUMO+1 at 409.9 nm and the HOMO-2 to LUMO at 578.6 nm, respectively. The absorption in ZnDPPSeH can be attributed to the transition from pyrrole carbons to pyrrole rings and two methine bridges (=CH–). In ZnDPPTeH, the transition comes from all methine bridges and p orbitals of N atoms to the macrocycle ring [Fig. 10(a)].

In the UV–vis spectrum of ZnDPPF, the absorption at 408.41 nm shows the contributions of electronic transition from HOMO-1 to LUMO+1. The transition from π to π* comes from the electrons in the payroll carbons in the macrocycle ring. ZnDPPCl has a peak at 430.09 nm with a shoulder at around 402.2 nm. The strong absorption confirmed at 430.09 nm corresponds to transitions from the HOMO-4 to the LUMO. The molecular orbitals participating in the electronic transitions are phenyl substituents and anchoring groups (Cl) to the macrocycle ring. In ZnDPPBr, the peak consists of transitions from HOMO-5 to LUMO+1 at 402.44 nm. The orbital coefficients of HOMO-5 are localized at the orbitals of the β carbons and N atoms of the porphyrin ring and the LUMO+1 antibonding π orbital of the macrocycle ring of the porphyrin molecule. We also observed a peak at 453.55 nm from HOMO-4 localized at the phenyl substituents and anchoring groups (Br) orbitals to the LUMO that exhibits orbital contributions located on the macrocycle ring. Figure 10(b) shows that the presence of bromine atoms as anchoring groups in zinc-based porphyrins can cause a blue shift in the peak compared to fluorine and chlorine, which is attributed to a combination of the inductive and resonance effects of the halogens on the macrocycle.

In the absorption spectra of ZnDPPPH₂, the major absorption at 420.3 nm can be attributed to the electronic transition from HOMO-1 to LUMO+1. The orbital densities participating in the transitions are pyrrole carbons to pyrrole rings and two methine bridges of porphyrin. The peak of ZnDPPCN shows a strong absorption at 404.97 nm that is attributed to electronic transitions of the p orbital of N atoms and π electrons of the β carbons to the antibonding orbital of the macrocycle ring. In addition, weak absorption at 466.7 nm comes from HOMO, which is attributed to all methine carbons and N atoms in LUMO+3, which are phenyl substituents and CN anchoring groups. In ZnDPPNH₂, the peak at 402.6 nm corresponds to transitions from the HOMO-5 to the LUMO. The molecular orbitals participating in the electronic transitions are the β carbons and N atoms in the macrocycle ring of porphyrins [Fig. 10(c)].

Ni and Zn-based MPs with different anchoring groups exhibit distinct electronic structures due to their atomic properties and electronegativity differences. The large atomic size of Ni and its lower electronegativity difference with nitrogen (N) generate a more symmetrical electron density distribution in its metalloporphyrin, leading to symmetric HOMO–LUMO shapes. This symmetry is further highlighted by the delocalization of HOMO electron density predominantly over the Ni central metal, while LUMO orbitals are delocalized at the center of the metalloporphyrin plane. In contrast, the smaller atomic radius of Zn and higher electronegativity difference with nitrogen led to an asymmetric HOMO–LUMO distribution, with orbital density observed on the anchoring groups of the HOMO. Unlike Ni, there is no localization of orbital density on the Zn center, and delocalization extends across the macrocycle and the phenyl substituent. Additionally, the anchoring groups in Zn-MPs influence the electron density of porphyrin core structures in HOMO orbitals, which is unlike Ni-MPs. These differences in electronic structure highlight the impact of atomic properties on the orbital characteristics and electron density distribution, with implications for the bandgap and optical absorption spectra of the material. For example, there is a general redshift in each UV–vis spectra for NiDPPX compared to ZnDPPX. The observance of the redshift is due to the change in the distribution of electron density at the metal center. Additionally, while the NiDPPX species exhibits a single peak for each anchoring group except TeH, this exception is similar to the Zn case; the case is different for ZnDPPX. In the case of ZnDPPNH₂, CN, and Br, the absorption spectrum has two peaks. While for ZnDPPCl, there is a distinct shoulder, and in the remaining Zn-porphyrins, there is one peak. These differences indicate the importance of the central metal atom and the influence of the anchoring group on optical absorbance spectra.

We investigated Ni-diphenyl porphyrin and Zn-diphenyl porphyrin molecules functionalized with various anchoring groups (S, Se, Te, F, Cl, Br, CN, PH₂, and NH₂). The observed variations in the geometric structures can be attributed to several factors, including the molecular configurations, the distribution of electrons in the central metal d orbitals, and the interactions between the metal center and meso-phenyl-X anchoring groups. The structural analysis of NiDPPX and ZnDPPX (X = SH, SeH, TeH, F, Cl, Br, CN, NH₂, and PH₂) systems proved to be a useful approach for studying charge transfer and conjugative interactions in molecular systems. The HOMOs and LUMOs of the NiDPPX and ZnDPPX (X = SH, SeH, TeH, F, Cl, Br, CN, NH₂, and PH₂) are different, possibly caused by the different electron delocalization arising from the interaction between porphyrin ring ligands and central metals. Global reactivity parameters were associated with a large bandgap with higher hardness values, lower softness values, and lower electrophilicity index values. The demonstration of a generic trend of the effect of metal with different anchoring groups described in this study may have a broader influence on functional porphyrin design, molecular devices, and electron transfer studies. In conclusion, our study shows that NiDPPX has a noticeable redshift in the UV–Vis spectrum when compared to zinc-containing porphyrins. This redshift can be attributed to increased electron density at the metal center, which results in the stabilization of energy levels involved in electronic transitions. We found that, in contrast to zinc orbitals, nickel orbitals take an active part in these transitions. These results provide insights into the modulation of optical properties in this system and its potential applications. All collected spectra show that the LUMO involved in the electronic transitions includes the antibonding π orbital of the macrocycle porphyrin ring. This observation highlights the highly conjugated nature of the system.

See the supplementary material for electron density plots, spatial distribution for the highest occupied and lowest unoccupied molecular orbitals, spin multiplicities, energy gaps, and optical spectra.

Financial support from the National Science Foundation (Award No. 2055668) is gratefully acknowledged. This project was supported by resources provided by the Office of Research Computing at George Mason University (URL: https://orc.gmu.edu) and funded in part by grants from the National Science Foundation (Award Nos. 1625039 and 2018631).

The authors have no conflicts to disclose.

Beenish Bashir: Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (lead). Maha M. Alotaibi: Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Andre Z. Clayborne: Conceptualization (lead); Funding acquisition (lead); Project administration (lead); Writing – original draft (equal); Writing – review & editing (lead).

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