Victor N. Nemykin

In this report, we present the formation and the X-ray crystal structure of the [SnIVNCP(3-)Py2](SnCl3) complex upon direct metalation of the H2NCP(2-) ligand (NCP = 5,10,15,20-tetraphenyl-N-confused porphyrin) with a large excess of SnCl2 in pyridine in dry anaerobic conditions. Reaction of this complex with dry and wet (NBu4)Cl results in the formation of the anionic [SnIVNCP(3-)Cl2]− and neutral SnIVNCP(2-)Cl2 complexes, respectively, indicating the ability to manipulate the electronic structure of the NCP ligand by stabilizing the external NH proton in part via the formation of a hydrogen bond. Similarly, reaction of the [SnIVNCP(3-)Py2]+ cation with (NBu4)OH or water results in the formation of [SnIVNCP(3-)(OH)2]− and SnIVNCP(2-)(OH)2 complexes, respectively. UV–vis-NIR and magnetic circular dichroism (MCD) spectroscopies are indicative of a significantly larger Qx-Qy energy gap for the NCP(2-) core compared to the NCP(3-) ionic form. Density Functional Theory (DFT) calculations confirm that a much smaller difference between ΔHOMO and ΔLUMO values for the NCP(2-) ligand (ΔHOMO is the energy gap between Gouterman’s “a1u” and “a2u” MOs, and ΔLUMO is the energy gap between Gouterman’s “eg(x)” and “eg(y)” MOs) is responsible for quenching of the magnetic orbital moment of the Qx-band in all NCP(2-)-containing complexes, reflected in MCD signal magnitudes. Time-Dependent DFT (TDDFT) calculations on all complexes correlate well with the experimental data and reveal specific spectroscopic signatures of the NCP(2-) and NCP(3-) systems as a function of the axial ligands and intermolecular hydrogen bond.

BOPHYs are excellent fluorophores exhibiting high fluorescence quantum yield, strong visible light absorption, tunable emission colors, controllable electron-accepting properties, and high photostability. These properties make BOPHYs attractive as potential nonfullerene electron acceptors (NFAs). Recent studies on Y6, a benchmark NFA, have collectively suggested that charge-transfer character in the excited state promotes efficient charge generation. Here, the charge-transfer character in the excited state of a series of BOPHYs is investigated using electroabsorption and Stark fluorescence spectroscopies. Methyl-substituted, ester- and phenyl-substituted, and pyridone-fused BOPHYs are comparatively studied. Fluorescence quantum yield was reduced in the presence of electric fields, which was attributed primarily to an enhancement of intramolecular nonradiative deactivation processes. Time-dependent density functional theory (TD-DFT) calculations and analyses of electron density redistribution induced by photoexcitation indicate that molecular symmetry determines the patterns of local charge redistribution and consequently, the magnitude of the net dipole moment change (|μ|). Among the three derivatives, pyridone-fused BOPHY exhibits the largest |μ| upon photoexcitation. These results indicate that the lowest excited state of pyridone-fused BOPHY has substantial intramolecular charge-transfer character. This study provides a basis for the functionalization of BOPHY for future photovoltaic and organic optoelectronic applications in addition to the conventional π-conjugation extension approach.

Here, we summarize the general use of the symmetry-adapted perturbation theory (SAPT) toward empirical prediction of the energy (and splitting when present) of the Qx- and Qy-bands in a large variety of substituted phthalocyanines (Pcs) and their analogues (i.e., tetraazaporphyrins (TAPs), naphthalocyanines (Ncs), and anthracocyanines (Acs)). It was shown that SAPT can be used as a reliable and quick empirical tool for the accurate (within 0.05 eV) estimation of both Qx- and Qy-band energies and splittings. Simple empirical rules for a 24 nm redshift per added benzene ring at the β,β′-positions (96 nm rule going from TAP to Pc to Nc to Ac) and 30, 15, 8, 5, and 4 nm rules for redshift per added amino, thiol, alkoxy, aryl, and alkyl group at the α-position of the phthalocyanine core, respectively, were established. The SAPT accuracy was tested against the data from more time-consuming and resource-intensive time-dependent density functional theory (TDDFT) calculations for several series of compounds.

We report the synthesis of a BODIPY 8, which bears ester substituents at the 2,6-positions and is coupled to a benzoquinone unit through an ethanediylidene linker. The hydroxyphenyl precursor, 7, was obtained via Knoevenagel condensation and oxidized chemically or electrochemically to yield the benzoquinone-BODIPY 8 conjugate. The magnetic properties of 8 were probed by EPR spectroscopy, VT 1H NMR spectroscopy, and DFT/TDDFT calculations. The benzoquinone-BODIPY 8 exhibited a predominantly closed-shell singlet ground state, with the open-shell singlet and triplet states being higher in energy, consistent with the absence of a room-temperature EPR signal in solution. Electrochemistry and spectroelectrochemistry revealed stepwise oxidation from the dianionic precursor [7]2– to a mixed-valence anion radical [7]–•, featuring a broad IVCT band at 1980 nm (TDDFT-predicted at 1790 nm), before full conversion to BODIPY 8. These results demonstrate fine-tuning of diradical character and charge-transfer behavior in BODIPY chromophores through conjugation and redox modulation. We also discovered the formation of an unexpected blue-colored BODIPY dimer 9 upon treatment of compound 8 with methanol. In this dimer, two BODIPY units are connected by a single sp3–sp3 carbon bond located at the carbon atoms directly attached to the α-position of the BODIPY core.

UV-vis and magnetic circular dichroism (MCD) spectroscopies were used to characterize electronic transitions in [Pc(2−)Fe(CN)2]−and Pc(1−)Fe(CN)2 (Pc(2-) and Pc(1-) are phthalocyanine dianion and cation-radical ligands, respectively). The expected splittings for the 2Eg and 3Eu ground-state Jahn–Teller distortions were investigated by DFT and TDDFT calculations. It was demonstrated that several Jahn–Teller distortions lead to energetically similar D2h and C2h point group structures, as well as structures significantly higher in energy. TDDFT calculations for both compounds suggest that metal-to-ligand charge-transfer (MLCT) transitions mostly contribute to the UV absorption region, while the visible absorption region is dominated by intraligand π–π* and ligand-to-metal charge-transfer (LMCT) transitions. Triplet-multiplet and charge-transfer (LMCT) excited states are also predicted in the NIR region. This work represents the first attempt to explore the influence of the Jahn–Teller distortions on geometry, electronic structure, energetics, and spectroscopy of the low-spin iron(III) phthalocyanines with degenerate ground states.

We report the synthesis and characterization of homoleptic Co(II), Ni(II), Cu(II), and Zn(II) complexes of a phenol-substituted aza-dipyrromethene ligand. The complexes adopt distorted tetrahedral geometries, display ligand-centered redox activity, and exhibit panchromatic absorption. Chemical and electrochemical oxidation of the ML₂ complexes generate quinone/bis(diradicaloid) species that exhibit strong near-infrared absorption and antiferromagnetically coupled ligand-centered spins in the Zn(II) complex. Charge-transport measurements reveal modest electron mobilities and unusually high hole mobilities (≈10^-3 cm² V^-1 s^-1) for the quinone/bis(diradicaloid) complexes, making them potential hole transport materials. DFT and TD-DFT calculations confirm ligand-dominated frontier orbitals with metal-dependent electronic modulation. These results highlight aza-dipyrromethene metal complexes as promising platforms for redox-active and charge-transport materials.

Synthetic porphyrins have been crucial as models for heme units and other biological porphyrins, as well as components of advanced materials and catalysts. Much of the work on synthetic porphyrins has focused on the meso-substituted 5,10,15,20-tetraphenylporphyrin (TPP) or the pyrrole-substituted 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP). In this report, we present a comprehensive spectroscopic, electrochemical, and computational study of free-base, zinc(II), and copper(II) 5,10,15,20-tetrapentylporphyrin (TPeP). TPeP can be prepared via a two-step, one-flask reaction of pyrrole and hexanal mediated by Montmorillonite K10, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in yields of around 40%. Similar to TPP, the TPeP systems have the highest occupied molecular orbital (HOMO) with a_2u symmetry and are, in general, easier to oxidize than their corresponding TPP or OEP analogues.

Biliazines, tetradentate chelates similar to the bilin class of heme metabolites, are ring open phthalocyanine analogs where the chelate ring is closed by a hydrogen bond. Linked dimers of biliazines, incorporating both pyrazole and indazole units, can be produced via a self-assembly reaction between the diiminoisoindoline-pyrazole and indazole chelate precursors and zinc ion in a 4 : 3 ratio. The dimers are comprised of zinc metalated biliazines linked by a third tetrahedral zinc metal ion. The closure of the ring with zinc ion does not induce the formation of an aromatic ring current in either the pyrazole or indazole-based dimers.

Optical spectroscopy of functional dyes is the most used tool for organic, organometallic, and inorganic chromophores. This technical note discusses the coupling of magnetic circular dichroism (MCD) spectroscopy with a conventional spectroelectrochemical (SE) cell setup. The discussed approach allows for the study of in situ ligand and metal centered reduction and oxidation processes. Advantages and limitations of the proposed method will be discussed on the basis of transition-metal phthalocyanines and chlorophyll A chromophores. Reported methods can be used not only for SE-MCD experiments with organic chromophores but also to study redox processes in inorganic and organometallic complexes.

UV-vis-NIR spectroscopy and magnetic circular dichroism (MCD) were used to characterize electronic transitions in the dicyano-substituted phthalocyanines [Pc ^tBu (2[ ]Cr(III)(CN) ₂ ] ⁻ , [Pc(2[ ]Fe(II)(CN) ₂ ] ²⁻ , and [Pc(2[ ]Co(III)(CN) ₂ ] ⁻ . Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were used to validate transition assignments, corresponding energies, and orbital contributions. Specifically, calculations were employed to provide insight into the multiple overlapping transitions present in the 250–500 nm region and to propose assignments for all transitions. In addition, energies corresponding to the charge-transfer (CT) transitions were analyzed to study the impact of closed-shell (Fe(II) and Co(III)) and open-shell (Cr(III)) configurations on ligand-centered, charge-transfer, and triplet–multiplet transitions. For dicyano-substituted Cr(III) phthalocyanine, we provide convincing evidence for reassignment of the lowest-energy transitions in the NIR to triplet–quartet transitions and suggest that the lowest-energy ligand-to-metal charge transfer transition is located near 500 nm. This work presents a comprehensive experimental and theoretical investigation of the electronic behavior of three dicyano-substituted metal phthalocyanines (MPcs) and provides a broader context for general trends in ligand-centered and CT transitions in metalated macrocycles.