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A family of propeller‐shaped donor‐acceptor hexapyrrolylbenzenes (HPBs) were designed and synthesized by sequential nucleophilic substitution of hexafluorobenzene with π‐extended pyrroles. In particular, four hybrids were obtained, containing various combinations of electron‐rich and electron‐poor acenaphthylene‐fused pyrroles. Additionally, to probe the efficiency of ortho transfer interactions, a system was designed containing unique donor and acceptor subunits spatially separated with four unfunctionalized pyrroles. DFT calculations showed propeller‐shaped geometries of all HPB molecules and separation of frontier molecular orbitals between donor and acceptor subunits. Steady‐state and time‐resolved photophysical measurements revealed charge‐transfer (CT) character of the emission with strong positive dependence on solvent polarity. The principal CT pathway involves ortho‐positioned pairs of donors and acceptors and requires bending of the acceptor in the excited state. [ABSTRACT FROM AUTHOR]

Symmetry-breaking charge transfer is one of the key process in photosynthetic reaction centers and specific artificial optoelectronic devices such as dye-sensitized solar cells. Here we report the synthesis of aromatic-bridged BF2-smaragdyrin dimers, meso-free BF2-smaragdyrin, and its meso-meso-linked BF2-smaragdyrin dimer. The decays of S1-states of these dimers are accelerated with an increase in solvent polarity and a decrease in the distance between the two BF2-smaragdyrin units, suggesting symmetry-breaking charge transfer. The fluorescence lifetimes of the dimers become shortened in polar solvents. However, ultrafast transient absorption spectroscopy do not detect charge-separated ion pairs. On the basis of these results, we conclude that the decays of the excited states of the BF2-smaragdyrin dimers are accelerated by solvation-induced symmetry-breaking charge transfer, depending on the degree of the electronic interaction between the smaragdryin units as a rare case for porphyrinoids. The degree of charge transfer is larger for dimers with larger electronic interactions. Symmetry breaking charge separations have been found to be important for charge transfer in bacteriochlorophyll pairs in photosynthetic reaction centers, but engineering symmetry breaking charge separations in artificial dimers of related chromophores is challenging. Here, the authors report the design and synthesis of aromatic-bridged as well as meso-meso-linked BF2-smaragdyrin dimers that exhibit symmetry-breaking charge transfer properties. [ABSTRACT FROM AUTHOR]

Self‐assembly of porphyrins is a fascinating topic, not only for mimicking chlorophyll assemblies in photosynthetic organisms, but also for the potential of creating molecular‐level devices. Herein, zinc porphyrin derivatives bearing a meta‐pyridyl group at the meso position were prepared and their assemblies studied in chloroform. Among the porphyrins studied, one with a carbamoylpyridyl moiety gave a distinct 1H NMR spectrum in CDCl3, which allowed the supramolecular structure in solution to be probed in detail. Ring‐current‐induced chemical‐shift changes in the 1H NMR spectrum, together with vapor‐pressure osmometry and diffusion‐ordered NMR spectroscopy, among other evidence, suggested that the porphyrin molecules form a trimer with a triangular cone structure. Incorporation of a directly linked porphyrin–ferrocene dyad with the same assembling properties in the assemblies led to a rare example of a light‐harvesting/charge‐separation system in which an energy gradient is incorporated and reductive quenching occurs. [ABSTRACT FROM AUTHOR]

Replacement of the meso methine carbon atoms of porphyrins with heteroatoms is a powerful strategy for tuning their optical and electronic properties. In particular, 5‐oxaporphyrin is an attractive target due to its importance as an intermediate in heme catabolism. In this work, we describe the synthesis and properties of a free‐base 5‐oxaporphyrinium cation, which was prepared by the ring‐closure of a bilindione with trifluoromethanesulfonic anhydride. This free‐base 5‐oxaporphyrinium cation exhibits dual fluorescence originating from its unique NH tautomerism. In contrast to normal porphyrins, the cis form of the 5‐oxaporphyrinium cation is more stable than the trans form due to the effective delocalization of its positive charge. We thus demonstrate here that meso‐modified heteroporphyrins enable the manipulation of NH tautomerism in porphyrinic macrocycles. [ABSTRACT FROM AUTHOR]

Acetylene and trans‐ethylene bridged BIII‐subporphyrin dimers were synthesized by cross‐coupling reactions of meso‐bromo BIII 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. [ABSTRACT FROM AUTHOR]

A family of propeller-shaped donor-acceptor hexapyrrolylbenzenes (HPBs) were designed and synthesized by sequential nucleophilic substitution of hexafluorobenzene with π-extended pyrroles. In particular, four hybrids were obtained, containing various combinations of electron-rich and electron-poor acenaphthylene-fused pyrroles. Additionally, to probe the efficiency of ortho transfer interactions, a system was designed containing unique donor and acceptor subunits spatially separated with four unfunctionalized pyrroles. DFT calculations showed propeller-shaped geometries of all HPB molecules and separation of frontier molecular orbitals between donor and acceptor subunits. Steady-state and time-resolved photophysical measurements revealed charge-transfer (CT) character of the emission with strong positive dependence on solvent polarity. The principal CT pathway involves ortho-positioned pairs of donors and acceptors and requires bending of the acceptor in the excited state., (© 2023 Wiley-VCH GmbH.)

A bright near-infrared (NIR) fluorescent molecule was developed based on the donor-acceptor-donor (D-A-D) approach using an aza-BODIPY analog called pyrrolopyrrole aza-BODIPY (PPAB) as an electron-accepting chromophore. Directly introducing electron-donating triphenylamine (TPA) to develop a D-A-D structure caused redshifts of absorption and emission of PPAB into the NIR region with an enhanced fluorescence brightness of up to 5.2×10 4  m -1  cm -1 , whereas inserting a phenylene linker between the TPA donor and the PPAB acceptor induced solvatochromic behavior in emission. Transient absorption spectra and theoretical calculations revealed the presence of a highly emissive hybridized locally excited and charge-transfer state in the former case and the contribution of the dark charge-separated state to the excited state in the latter case. The bright D-A-D PPAB as a novel emitter resulted in a NIR electroluminescence with a high external quantum efficiency of 3.7 % and a low amplified spontaneous emission threshold of ca. 80 μJ cm -2 , indicating the high potential for NIR optoelectronic applications., (© 2021 Wiley-VCH GmbH.)

Self-assembly of porphyrins is a fascinating topic, not only for mimicking chlorophyll assemblies in photosynthetic organisms, but also for the potential of creating molecular-level devices. Herein, zinc porphyrin derivatives bearing a meta-pyridyl group at the meso position were prepared and their assemblies studied in chloroform. Among the porphyrins studied, one with a carbamoylpyridyl moiety gave a distinct 1 H NMR spectrum in CDCl 3 , which allowed the supramolecular structure in solution to be probed in detail. Ring-current-induced chemical-shift changes in the 1 H NMR spectrum, together with vapor-pressure osmometry and diffusion-ordered NMR spectroscopy, among other evidence, suggested that the porphyrin molecules form a trimer with a triangular cone structure. Incorporation of a directly linked porphyrin-ferrocene dyad with the same assembling properties in the assemblies led to a rare example of a light-harvesting/charge-separation system in which an energy gradient is incorporated and reductive quenching occurs., (© 2020 Wiley-VCH GmbH.)

N-Alkylation significantly changes the electronic and optical properties, as well as the reactivity of nitrogen-containing π-conjugated molecules. In this study, it is found that treating 5,15-diazaporphyrins with methyl triflate selectively affords the corresponding N-methyl-5,15-diazaporphyrinium cations in good yield. N-Methylation substantially alters the electronic properties and reactivity of diazaporphyrins. The electron-accepting properties of the N-methyl-5,15-diazaporphyrinium cations are enhanced due to their lowered LUMO level. Stabilization of the LUMO energy enables regio- and stereoselective Diels-Alder reactions of the cationic diazaporphyrin with cyclopentadiene. N-Methylation also enhances the acidity of the inner NH protons, and thus, allows facile deprotonation to provide nitrogen-substituted isoporphyrin analogues with only one NH group in the central cavity., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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 S 1 -states of the dimers relax with structural changes, which are larger for the trans-ethylene bridged dimer., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)