A systematic theoretical study of the electronic structures of porphyrin dimers: DFT and TD-DFT calculations on diporphyrins linked by ethane, ethene, ethyne, imine, and azo bridges

Meso-Formyl, Vinyl, and Ethynyl Porphyrins-Multipotent Synthons for Obtaining a Diverse Array of Functional Derivatives

This review presents a strategy for obtaining various functional derivatives of tetrapyrrole compounds based on transformations of unsaturated carbon-oxygen and carbon-carbon bonds of the substituents at the meso position (meso-formyl, vinyl, and ethynyl porphyrins). First, synthetic approaches to the preparation of these precursors are described. Then diverse pathways for the transformations of the multipotent synthons are discussed, revealing a variety of products of such reactions. The structures, electronic, and optical properties of the compounds obtained by the methods under consideration are analyzed. In addition, there is an overview of the applications of the products obtained. Biomedical use of the compounds is among the most important. Finally, the advantages of using the reviewed synthetic strategy to obtain dyes with targeted properties are highlighted.

Control of spin coupling through a redox-active bridge in a dinickel(II) porphyrin dimer: step-wise oxidations enable isolations of a chlorin-porphyrin heterodimer and a dication diradical with a singlet ground state

A dinickel(II)porphyrin dimer has been used here in which the redox-active pyrrole-moiety, similar to the tryptophan residue in diheme enzymes such as MauG and bCcP, has been placed between two Ni(II)porphyrin centers connected via a flexible, but unconjugated methylene bridge. This arrangement provides a large physical separation between the two metal centers and thus displays almost no communication between them through the bridge. Upon treatment with DDQ as an oxidant, the dinickel(II) porphyrin dimer slowly gets converted into an indolizinium-fused chlorin-porphyrin heterodimer. However, oxidations of the dinickel(II) porphyrin dimer up to two oxidizing equivalents using oxidants such as AgSbF₆ and FeCl₃ resulted in the formation of a dication diradical complex. Interestingly, in order to stabilize such a highly oxidized dication diradical, two non-conjugated methylene spacers undergo facile 2e^-/-2H⁺ oxidation to make the bridge fully π-conjugated for promoting through-bond communication. Through the oxidized and conjugated bridge, two porphyrin π-cation radicals display considerable communications leading to an efficient intramolecular spin coupling to form a singlet state. Interestingly, the redox-active nature of the bridge controls the electronic communication just by simple oxidation or reduction, and thereby, acts as a molecular switch for efficient magnetic relay.

Application of TD-DFT Theory to Studying Porphyrinoid-Based Photosensitizers for Photodynamic Therapy: A Review

An important focus for innovation in photodynamic therapy (PDT) is theoretical investigations. They employ mostly methods based on Time-Dependent Density Functional Theory (TD-DFT) to study the photochemical properties of photosensitizers. In the current article we review the existing state-of-the-art TD-DFT methods (and beyond) which are employed to study the properties of porphyrinoid-based systems. The review is organized in such a way that each paragraph is devoted to a separate aspect of the PDT mechanism, e.g., correct prediction of the absorption spectra, determination of the singlet-triplet intersystem crossing, and interaction with molecular oxygen. Aspects of the calculation schemes are discussed, such as the choice of the most suitable functional and inclusion of a solvent. Finally, quantitative structure-activity relationship (QSAR) methods used to explore the photochemistry of porphyrinoid-based systems are discussed.

Through Bridge Spin Coupling in Homo- and Heterobimetallic Porphyrin Dimers upon Stepwise Oxidations: A Spectroscopic and Theoretical Investigation

We have described copper(II)-iron(III) and copper(II)-manganese(III) heterobimetallic porphyrin dimers and compared them with the corresponding homobimetallic analogs. UV-visible spectra are very distinct in the heterometallic species while electrochemical studies demonstrate that these species, as compared to the homobimetallic analog, are much easier to oxidize. Combined Mössbauer, EPR, NMR, magnetic and UV-visible spectroscopic studies show that upon 2e-oxidation of the heterobimetallic complexes only ring-centered oxidation occurs. The energy differences between HOMO and LUMO are linearly dependent with the low-energy NIR band obtained for the 2e-oxidized complexes. Also, strong electronic communication between two porphyrin rings through the bridge facilitates coupling between various unpaired spins present while the coupling model depends on the nature of metal ions used. While unpaired spins of Fe(III) and the porphyrin π-cation radical are strongly antiferromagnetically coupled, such coupling is rather weak between Mn(III) and a porphyrin π-cation radical. Moreover, the coupling between two π-cation radicals are much stronger in the 2e-oxidized complexes of dimanganese(III) and copper(II)-manganese(III) porphyrin dimers as compared to their diiron(III) and copper(II)-iron(III) analogs. Furthermore, coupling between the unpaired spins of a π-cation radical and copper(II) is much stronger in the 2e-oxidized complex of copper(II)-iron(III) porphyrin dimer as compared to its copper(II)-manganese(III) analog. The Mulliken spin density distributions in 2e-oxidized homo- and heterobimetallic complexes show symmetric and asymmetric spread between the two macrocycles, respectively. In both the 2e-oxidized heterobimetallic complexes, the Cu(II) porphyrin center acts as a charge donor while Fe(III)/Mn(III) porphyrin center act as a charge acceptor. The experimental observations are also strongly supported by DFT calculations.

EPR of Photoexcited Triplet-State Acceptor Porphyrins

The photoexcited triplet states of porphyrin architectures are of significant interest in a wide range of fields including molecular wires, nonlinear optics, and molecular spintronics. Electron paramagnetic resonance (EPR) is a key spectroscopic tool in the characterization of these transient paramagnetic states singularly well suited to quantify spin delocalization. Previous work proposed a means of extracting the absolute signs of the zero-field splitting (ZFS) parameters, D and E, and triplet sublevel populations by transient continuous wave, hyperfine measurements, and magnetophotoselection. Here, we present challenges of this methodology for a series of meso-perfluoroalkyl-substituted zinc porphyrin monomers with orthorhombic symmetries, where interpretation of experimental data must proceed with caution and the validity of the assumptions used in the analysis must be scrutinized. The EPR data are discussed alongside quantum chemical calculations, employing both DFT and CASSCF methodologies. Despite some success of the latter in quantifying the magnitude of the ZFS interaction, the results clearly provide motivation to develop improved methods for ZFS calculations of highly delocalized organic triplet states.

Biliazine: a ring open phthalocyanine analog with amesohydrogen bond

The hemiporphyrazine type chelate and phthalocyaninine analog biliazine (H₂BlzH) and its metal complexes can be synthesized in one-step from a monosubstituted diiminisoindoline (DII).

Spin Delocalization in the Radical Cations of Porphyrin Molecular Wires: A New Perspective on EPR Approaches

The spin delocalization in the radical cations of a series of ethyne-linked oligoporphyrins was investigated using EPR spectroscopy. The room-temperature spectral envelope for these oligomers deviates significantly from the benchmark N^-0.5 trend in line width expected for a completely delocalized spin density, in contrast to the butadiyne-linked analogues measured previously. Here, we show, using DFT calculations and complementary low-temperature ENDOR measurements, that this deviation is primarily driven by a more pronounced inequivalence of the ¹⁴N spins within individual subunits for the ethyne-linked oligoporphyrins. Once this ¹⁴N inequivalence is taken into consideration, the room-temperature and ENDOR spectra for both butadiyne-linked and ethyne-linked oligomers, up to N = 5, can be simulated by similar static delocalization patterns. This work highlights the importance of EPR in exploring such spin delocalization phenomena while also demonstrating that the N^-0.5 trend should not be interpreted in isolation but only in combination with careful simulation and theoretical modeling.

Acetylene and trans-Ethylene Bridged BIII -Subporphyrin Dimers

Acetylene and trans-ethylene bridged B^III -subporphyrin dimers were synthesized by cross-coupling reactions of meso-bromo B^III subporphyrin. These dimers display perturbed and red-shifted absorption spectra reaching around 750 nm and fluorescence reaching at around 850 nm with high quantum yields of 0.39 and 0.47, respectively. DFT calculations have revealed that the HOMOs and the LUMOs of both dimers are spread over the two subporphyrin units as an indication of effective conjugation between the two subporphyrin units. The large Stokes shifts and characteristic pico-second time-resolved transient absorption spectra indicated that the S₁ -states of the dimers relax with structural changes, which are larger for the trans-ethylene bridged dimer.

Push-pull quinoidal porphyrins

A family of push-pull quinoidal porphyrin monomers has been prepared from a meso-formyl porphyrin by bromination, thioacetal formation, palladium-catalyzed coupling with malononitrile and oxidation with DDQ. Attempts at extending this synthesis to a push-pull quinoidal/cumulenic porphyrin dimer were not successful. The crystal structures of the quinoidal porphyrins indicate that there is no significant contribution from singlet biradical or zwitterionic resonance forms. The crystal structure of an ethyne-linked porphyrin dimer shows that the torsion angle between the porphyrin units is only about 3°, in keeping with crystallographic results on related compounds, but contrasting with the torsion angle of about 35° predicted by computational studies. The free-base quinoidal porphyrin monomers form tightly π-stacked layer structures, despite their curved geometries and bulky aryl substituents.

Stepwise oxidations of a nickel(ii)–iron(iii) heterobimetallic porphyrin dimer: structure, spectroscopic and theoretical investigation

A novel nickel(ii)–iron(iii) heterobimetallic ethene-bridged porphyrin dimer has been synthesized which upon two-electron oxidation produces a nickel(ii)–iron(iii) dication diradical complex where radicals undergo extensive conjugation through the bridge for all possible interactions.

Single-Acetylene Linked Porphyrin Nanorings

The synthesis of ethyne-linked porphyrin nanorings has been achieved by template-directed Sonogashira coupling. The cyclic hexamer and octamer are predicted by density functional theory to adopt low symmetry conformations, due to dihedral twists between neighboring porphyrin units, but their symmetries are effectively D_6h and D_8h, respectively, in solution by ¹H NMR. The fluorescence spectra indicate that the singlet excited states of these nanorings are highly delocalized.

On the Influence of the Bridge on Triplet State Delocalization in Linear Porphyrin Oligomers

The extent of triplet state delocalization is investigated in rigid linear zinc porphyrin oligomers as a function of interporphyrin bonding characteristics, specifically in meso-meso singly linked and β,meso,β fused structures, using electron paramagnetic resonance techniques. The results are compared with those of earlier measurements on porphyrin oligomers with alkyne linkers exhibiting different preferred conformations. It is shown that dihedral angles near 90° between the porphyrin planes in directly meso-to-meso linked porphyrin oligomers lead to localization of the photoexcited triplet state on a single porphyrin unit, whereas previous work demonstrated even delocalization over two units in meso-to-meso ethyne or butadiyne-bridged oligomers, where the preferred dihedral angles amount to roughly 30° and 0°, respectively. The triplet states of fused porphyrin oligomers (i.e., porphyrin tapes) exhibit extended conjugation and even delocalization over more than two porphyrin macrocycles, in contrast to meso-to-meso ethyne or butadiyne-bridged oligomers, where the spin density distribution in molecules composed of more than two porphyrin units is not evenly spread across the oligomer chain.

Computational Studies on Optoelectronic and Nonlinear Properties of Octaphyrin Derivatives

The electronic and nonlinear optical (NLO) properties of octaphyrin derivatives were studied by employing the DFT/TDFT at CAM-B3LYP/6-311++G (2d, 2p) level of the theory. Thiophene, phenyl, methyl and cyano moieties were substituted on the molecular framework of octaphyrin core, in order to observe the change in optoelectronic and nonlinear response of these systems. The frontier molecular orbital studies and values of electron affinity reveals that the studied compounds are stable against the oxygen and moisture present in air. The calculated ionization energies, adiabatic electron affinity and reorganization energy values indicate that octaphyrin derivatives can be employed as effective n-type material for Organic Light Emitting Diodes (OLEDs). This character shows an enhancement with the introduction of an electron withdrawing group in the octaphyrin framework. The polarizability and hyperpolarizability values of octaphyrin derivatives demonstrate that they are good candidates for NLO devices. The nonlinear response of these systems shows enhancement on the introduction of electron donating groups on octaphyrin moiety. However, these claims needs further experimental verification.

meso-Nitro- and meso-Aminosubporphyrinatoboron(III)s and meso-to-meso Azosubporphyrinatoboron(III)s

meso-Nitrosubporphyrinatoboron(III) was synthesized by nitration of meso-free subporphyrin with AgNO₂ /I₂ . The subsequent reduction with a combination of NaBH₄ and Pd/C gave meso-aminosubporphyrinatoboron(III). meso-Nitro- and meso-amino-groups significantly influenced the electronic properties of subporphyrin, which has been confirmed by NMR and UV/Vis spectra, electrochemical analysis, and DFT calculations. Oxidation of meso-aminosubporphyrinatoboron(III)s with PbO₂ cleanly gave meso-to-meso azosubporphyrinatoboron(III)s that exhibited almost coplanar conformations and large electronic interaction through the azo-bridge.

Structure-Dependent Electronic Interactions in Ethyne-Bridged Porphyrin Arrays Investigated by Single-Molecule Fluorescence Spectroscopy

By using single-molecule fluorescence spectroscopy, we have investigated the electronic interaction of ethyne-bridged porphyrin arrays (ZNE) depending on their structure. The fluorescence dynamics of ZNE show a large amount of one-step photobleaching behaviors, indicating the high degree of π-conjugation. The ratio of one-step photobleaching behavior decreased as the number of porphyrin units increased. This behavior indicates that the linear and shortest Z2E shows a strong electronic coupling between constituent porphyrin moieties. Structural properties and orientation of ZNE were also measured by wide-field excitation fluorescence spectroscopy (ExPFS) and defocused wide-field imaging (DWFI). The ExPFS and DWFI show that the structure of absorbing and emitting units of Z2E and Z3E are linear. On the other hand, star-shaped pentamer with five porphyrins acts as an absorbing unit, but unidirectional trimer moiety acts as an emitting unit in the Z5E molecule. Collectively, these studies provide further information on the electronic interaction depending on their structure and length.

Time dependent DFT investigation of the optical properties of artificial light harvesting special pairs

Simulated absorption spectra of (ZnTriPP)₂DPB dimer in which Q band is enhanced 50 times for visibility.

Oxidation triggers extensive conjugation and unusual stabilization of two di-heme dication diradical intermediates: role of bridging group for electronic communication

Synthetic analogs of diheme enzyme MauG have been reported. Unlike the bis-Fe(iv) state in MauG, the 2e-oxidation stabilizes two ferric hemes, each coupled with a porphyrin π-cation radical.

MauG is a diheme enzyme that utilizes two covalently bound c-type hemes to catalyse the biosynthesis of the protein-derived cofactor tryptophan tryptophylquinone. The two hemes are physically separated by 14.5 Å and a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes undergoes reversible oxidation and reduction to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis-Fe(iv) MauG. The present work describes the structure and spectroscopic investigation of 2e-oxidations of the synthetic diheme analogs in which two heme centers are covalently connected through a conjugated ethylene bridge that leads to the stabilization of two unusual trans conformations (U and P′ forms) with different and distinct spectroscopic and geometric features. Unlike in MauG, where the two oxidizing equivalents are distributed within the diheme system giving rise to the bis-Fe(iv) redox state, the synthetic analog stabilizes two ferric hemes, each coupled with a porphyrin cation radical, a scenario resembling the binuclear dication diradical complex. Interestingly, charge resonance-transition phenomena are observed here both in 1e and 2e-oxidised species from the same system, which are also clearly distinguishable by their relative position and intensity. Detailed UV-vis-NIR, X-ray, Mössbauer, EPR and ¹H NMR spectroscopic investigations as well as variable temperature magnetic studies have unraveled strong electronic communications between two porphyrin π-cation radicals through the bridging ethylene group. The extensive π-conjugation also allows antiferromagnetic coupling between iron(iii) centers and porphyrin radical spins of both rings. DFT calculations revealed extended π-conjugation and H-bonding interaction as the major factors in controlling the stability of the conformers.

Dielectric properties and electronic absorption: a comparison of novel azo- and oxo-bridged phthalocyanines

Measurements showed that the dielectric permittivity of azo-bridged phthalocyanines is significantly higher than that of the oxo-bridged homologs. This result can be attributed to the limited electron delocalization ability of oxo-bridged homologs.

TD-DFT calculations of the excited states of metalloporphyrins relevant to organic solar photovoltaic cells

The molecular orbital energies and symmetries, electronic state energies and symmetries, and orbital compositions and oscillator strengths for one-photon radiative transitions up to an energy of 4 eV have been calculated by DFT and TD-DFT methods for 15 d0 and d10 metalloporphyrins. Data for both singlet and triplet excited states are reported and used to identify potential candidates for use as photon upconverters by homomolecular triplet–triplet annihilation.