Directly linked metalloporphyrins: A quest for the bio-inspired materials

: The directly-linked iron-diporphyrin complexes are appealing candidates and a fundamental precursor for an extended metalloporphyrin array that could potentially mimic the biological design of an energy harvesting material. This impulse us to appraise the layout for the modular fusion of two iron-porphyrin units. Herein, DFT based calculations suggested that the electronic environment of diporphyrin systems could be tuned according to the topological attachment between the porphyrin units. Subsequently, a gradual increase in the electronic interaction between the constituent porphyrin units triggers a decrease in the HOMO−LUMO gap. This is essential to achieve higher electric conductivity. The spin-polarized electronic transmission is another interesting aspect of these iron-diporphyrin systems and promising for spintronic applications. The successive theoretical interpretation of the existence of two dimensional (2D) metalloporphyrin arrays could be the route to design a graphene analog of the covalent metalorganic framework.


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For the first-time, Abel et al. reported the fabrication of a novel two-dimensional (2D) Fe embedded phthalocyanine (poly-FePc) organometallic porous sheet. 25 These newly categorized 2D material exhibit promising conduction and magnetic properties, however, restricted to variable grain boundary conditions. In these 2D systems, the TM (transition metal) atoms are uniformly and separately distributed without clustering and display well-defined geometry and magnetic properties. Subsequently, different TM-Pc monolayers find sophisticated applications in spintronics [26][27][28][29] and gas capture. 30,31 Indeed, the unique and regular structural orientation of the TM atoms in these TM-Pc monolayers makes these 2D architectures an appealing candidate for singleatom-catalysts. 32 In this direction, the DFT based theoretical modeling predicted the application of not-yet-synthesized Co-Pc and Cr-Pc monolayers can be good catalysts for CO oxidation. 33,34 However, to date, the electronic properties of the basic building blocks of the 2D metalloporphyrin sheet i.e., the porphyrin-dimers units have been addressed scarcely.
In this study, we have extended a systematic investigation to develop a better understanding of the electronic behavior of fused metalloporphyrin moiety based on density functional theory (DFT). Taking the analogy from previous reports, there could be significant modulation in the electronic properties of the hybrid system depending on the fusion mode between porphyrin units.
Subsequently, we have also traced the relative changes in the electronic properties of the systems during the transformation from one-dimensional to two-dimensional arrays i.e., sheet-like structure.
The manuscript is organized in the following way. In section 2, we have addressed the computational methodologies. The results and discussions, section 3 (a) consist of an analysis of the DFT results on fused porphyrins at the molecular level along with their photophysical behaviors. The essence of electronic transport through the porphyrin dimers in different fusion modes are assayed in section 3(b). A comprehensive theoretical pursuit of the electronic behavior of the 2D arrays is featured in section 3(c). Finally, we have summarized the key findings of our theoretical simulation along with some futuristic applications of the model systems in the concluding remarks, section 4.

Theoretical Methods
The quantum mechanical (QM) calculations at the molecular level are undertaken using Gaussian09 software package. 35 All structures were optimized with Becke's three-parameter (a) i. Electronic structure calculation: In an excellent review article Tanaka et al. exclusively discussed the synthetic strategies for different directly linked porphyrin arrays. 51 According to the study, the linked porphyrins are broadly categorized into three types, namely, (a) singly-linked porphyrin oligomers, (b) fused porphyrin oligomers, and (c) triply-linked porphyrin oligomers. These classifications are primarily based on the different modes of chemical attachment between the porphyrin units. In our present study, we have chosen four types of porphyrin oligomers with different bonding patterns. Further, the selected Iron-porphyrin oligomers have well defined synthetic protocols and the promising high-end application prospects are thoroughly explored. Fig. 1: The optimized structures of the systems studied in the present report. According to the standard notations (based on the point of attachments) system 1 is meso-to-meso linked, system 2 is β, meso, β fused and system 3 and 4 are α-β fused oligomers. The geometry optimization is done at B3LYP/cc-pVTZ level of theory. The yellow, blue, gray and white balls represent the Fe, N, C and H atoms, respectively.  The computed Total density-of-states (TDOS) plots along with the location of occupied and virtual orbitals for the four systems are reported in Fig. 2. The green and red lines indicate the occupied and virtual orbitals. The blue line in the plots represents the TDOS of the system.
As we have observed from Fig. 2, the electronic states of the first three systems have close similarities with one-another except small variation near the HOMO level. However, the fourth molecule exhibits significant changes in the electronic states around the HOMO level. A systematic analysis of the plots provides useful insights into the electronic structure. The significant electronic contribution from the p-type atomic orbitals of carbon and nitrogen atoms to the frontier molecular orbitals (FMOs) of the system is evident from the isosurface plots reported in Figure 2 However, π-bonding and π-antibonding nature of the HOMO and LUMO orbitals, respectively, in system 3 associated with an increase in the HOMO-LUMO energy gap. A seemingly different FMO picture is observed for the system 4. Here, the HOMO and LUMO are primarily located at the metal centers associated with an extremely small electronic bandgap refers to the profound metallic nature of the molecule that correlates well to the DOS plot in Fig. 2.
So-called "fused diporphyrin" is an excellent and straightforward synthetic protocol to connect two porphyrins units directly through multiple covalent bonds. Within a coplanar geometrical arrangement, the systems should be quite favorable for the electronic π conjugation.
As we have seen, diporphyrin is the fundamental unit of an extended 1D metalloporphyrin array.
These 1D structures have compelling electronic and magnetic properties and considered as molecular wire or porphyrin-taps by virtue of their rigid shape and extended electronic conjugations. 59 It is now well understood that the extraordinary electronic properties of the metalloporphyrin array are the consequence of a modular sequence of the diporphyrin units. So, it is important to develop a better understanding of the chemical nature as well as the electronic structure of the 'linking-zone' between two single units. anti-aromatic characteristic of the meso-meso bridge system is also realized from the LOL map.

ii. Excited state dynamics: A TDDFT analysis
It is important to understand the photophysical behavior to account the suitable application prospects of a system. The DFT based simulated UV-VIS absorption spectra of the four complexes in water solvent along with the relevant parameters are executed in graphical form and included in  Table 1.  It is observed that in the 1,3-butadiene linked diporphyrine complex (system 1) the low-energy Qband is a sharp peak as compared to that of the other three systems, where the Q-band appeared to be a broad-peak. The Q-band at λmax = 618 nm for system 1 is mainly the result of a The comprehensive discussions on the two sophisticated analytical techniques in terms of computer simulation provide useful insights into the chemical structures of the porphyrin dimers.

(b) Electronic transport behavior of the porphyrine dimers:
In the recent years, density-functional theory (DFT) and nonequilibrium Green's function (NEGF) formalism provide an excellent interface for the theoretical modeling of molecular transport properties in connection to the accomplishments of single-molecule nanodevices. [68][69] It is encouraging to note that the NEGF+DFT formalism, at the GGA level of theory, showing promising results for transport in the strongly coupled regime 69 74 In a molecular bridge, the decrease in conductance due to the appearance of destructive quantum interference (DQI) favors the ferromagnetic coupling between two spin centers while the increased conductance through the bridge can favor antiferromagnetic coupling. 75 It was observed that in the case of a π-conjugated system the destructive interference could block the electronic transmission, contrary to that of a partial cancellation for the saturated system. 76 Similarly, the local transmission through the complexes and their molecular orbitals show substantial dependencies on the various components of the molecular structure that mediate electron transport. 77 It is worth mentioning here that the electronic behavior of a system around the Fermi level determines the versatility of that molecule for nanoscale device fabrication. The present study accounts for a significant modulation of electronic transmission at the two spin states around the Fermi level for all the systems. From Figure 8, it can be assumed that the origin of spindependent electronic transport in the system is due to the spin asymmetry of the electronic structure caused by the presence of paramagnetic metal centers in each porphyrin unit. The  Further, the STM images reported by Nakamura et al. 86 and Grill et al. 97 in their respective studies manifested excellent correlation with our calculated STM surfaces for similar systems. The comprehensive electronic structure calculations in combination with the extensive analytical simulations strongly suggest the experimental feasibility of linked metalloporphyrin two dimensional (2D) arrays and assumed to be the porphyrin analogs of graphene.

Conclusions:
In the present study, we have systematically analyzed the electronic properties of the smallest  Extended metalloporphyrin array could potentially mimic the biological design of an energy harvesting material.  The electronic environment of diporphyrin systems could be tuned according to their topological changes.  The spin-polarized electronic transmission is an interesting aspect of these hybrid system.  2D metaloporphyrin arrays are the graphene analogues of a covalent metalorganic framework.