Search

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 5.2×104 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. [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]

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. [ABSTRACT FROM AUTHOR]

Donor–acceptor (D–A) triplet photosensitizers have gained considerable attention in the field of photodynamic therapy. While numerous studies have been conducted on these photosensitizers, their energy transfer mechanism has not been completely elucidated. Herein, we prepared BODIPY-based D–A complexes (AM2–4) to investigate their photophysical and solvatochromic properties. In addition, the hypothesis that a decrease in the singlet and triplet energy gap (ΔE ST) can increase the intersystem crossing rate and singlet oxygen quantum yield (SOQY) was investigated. Moreover, by designing the experiments under more controlled conditions than those reported in previous studies, we obtained more accurate data. AM2–4 materials with the charge-transfer (CT) character exhibited higher SOQY (0.32–0.44) than AM1 without the CT character. Besides, as the electron-accepting power of the photosensitizers increased, ΔE ST between the first singlet excited state and triplet excited state decreased, resulting in an increased SOQY. This study elucidates the molecular mechanism of D–A triplet photosensitizers and contributes to the development of efficient photosensitizers for photodynamic therapy. Image 1 • Correlation between singlet-triplet energy gap and singlet oxygen quantum yield in controlled conditions was investigated. • A donor-acceptor (D-A) complex based on triphenylamine (TPA) and boron dipyrromethane (BODIPY) was used. • The singlet oxygen quantum yield (SOQY) increased as the singlet-triplet energy gap decreased. [ABSTRACT FROM AUTHOR]

We synthesised thionium-ion embedded aromatic porphyrinoids: a free-base 5-thiaporphyrinium cation and its zinc complex. The sulphur atom effectively participates in the macrocyclic π-conjugation. Fluorescence quantum yields of thiaporphyrinium cations were lower than 1% unlike oxaporphyrinium cations. Detailed photophysical analysis and DFT calculations clarified the vibrational mode regarding the out-of-plane motion of the sulphur atom induced ultrafast quenching of the excited state in comparison to the corresponding oxaporphyrinium cations. [ABSTRACT FROM AUTHOR]

This communication describes the photophysical behavior of three analogs of cyclophane bearing the dipyrrolonaphthyridinedione (DPND) core. In these molecules, intersystem crossing (ISC) can be successfully induced by distinct changes in the deviation from planarity within the DPND core, allowing at the same time the emission maximum to shift from the green to red region of the visible spectrum without any synthetic modifications of the chromophore structure. This finding may build the foundation for a new paradigm for inducing ISC-type transitions within other centrosymmetric and planar cross-conjugated chromophores. [ABSTRACT FROM AUTHOR]

Elucidating structural roles in photoinduced charge transfer is indispensable, as nuclear rearrangements are simultaneously usually involved in the dynamics. However, it is hard to evaluate whether the structural changes occur or not by using conventional time‐resolved electronic spectroscopy. Here, time‐resolved impulsive stimulated Raman spectroscopy is applied to record the evolution of vibrational snapshots during charge‐separation dynamics of donor–acceptor–donor‐type quadrupolar perylene bisimide in real time. Drastic frequency shifts were observed for several Raman bands with their population kinetics, thus symmetry‐breaking charge separation accompanies significant structural changes, as supported by (TD)‐DFT calculations. A comparison between time‐resolved Raman spectra of the neutral S1 state and the radical anion species shows that the spectral signatures, especially in high‐frequency regions, provide important clues to bond length alternation patterns in the PBI core. [ABSTRACT FROM AUTHOR]

Elucidating structural roles in photoinduced charge transfer is indispensable, as nuclear rearrangements are simultaneously usually involved in the dynamics. However, it is hard to evaluate whether the structural changes occur or not by using conventional time‐resolved electronic spectroscopy. Here, time‐resolved impulsive stimulated Raman spectroscopy is applied to record the evolution of vibrational snapshots during charge‐separation dynamics of donor–acceptor–donor‐type quadrupolar perylene bisimide in real time. Drastic frequency shifts were observed for several Raman bands with their population kinetics, thus symmetry‐breaking charge separation accompanies significant structural changes, as supported by (TD)‐DFT calculations. A comparison between time‐resolved Raman spectra of the neutral S1 state and the radical anion species shows that the spectral signatures, especially in high‐frequency regions, provide important clues to bond length alternation patterns in the PBI core. [ABSTRACT FROM AUTHOR]

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. [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]

A family of tunable halochromic switches is developed using a naphthalimide‐fused dipyrrin as the core π‐conjugated motif. Electronic properties of these dipyrrins are tuned by substitution of their alpha and meso positions with aryl groups of variable donor–acceptor strength. The first protonation results in a conformational change that enhances electronic coupling between the dipyrrin chromophore and the meso substituent, leading to halochromic effects that occasionally exceed 200 nm and switch the absorption between the near‐infrared (NIR)‐I and NIR‐II ranges. A NIR‐II photothermal effect, switchable by acid–base chemistry is demonstrated for selected dipyrrins. Further protonation is possible for derivatives bearing additional amino groups, leading to up to four halochromic switching step. The most electron‐rich dipyrrins are also susceptible to chemical oxidation, yielding NIR‐absorbing radical cations and closed‐shell dications. [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]

Zeitaufgelöste impulsive stimulierte Raman‐Spektroskopie, eine ultraschnelle Raman‐Technik, ermöglicht es, Momentaufnahmen von Molekülen im angeregten Zustand mit hoher Empfindlichkeit und Zeitauflösung zu gewinnen. Durch Anwendung dieser Technik demonstrieren D. Kim et al. in ihrem Forschungsartikel auf S. 8649, dass struktureller Symmetriebruch eine wichtige Voraussetzung für eine Ladungstrennungsreaktion in einem Donor‐Akzeptor‐Donor‐Perylenbisimid ist. [ABSTRACT FROM AUTHOR]

Time‐resolved impulsive stimulated Raman spectroscopy, which is an ultrafast time‐domain Raman technique, facilitates capturing vibrational snapshots of excited‐state molecules with high sensitivity and time resolution. By using this technique, D. Kim and co‐workers demonstrate in their Research Article on page 8571 that structural symmetry breaking is an important prerequisite for a charge separation reaction in a donor–acceptor–donor perylene bisimide. [ABSTRACT FROM AUTHOR]

Xylem sap plays a major role in long-distance transport of water, nutrients, and metabolites. However, there is little information on the behavior of metabolites in mineral-deficient xylem sap. For this reason, the time-dependent changes in selected metabolites (amino acids, organic acids, and soluble sugars) from tomato xylem sap in response to nitrogen (N), phosphorus (P), or potassium (K)-deficient condition were investigated. Tomato plants ( Solanum lycopersicum L.) were grown hydroponically in liquid culture under three different mineral regimes: N-deficient [0.5 mM Ca(NO3)2 and 0.5 mM KNO3], P-deficient (0.05 mM KH2PO4), and K-deficient (0.5 mM KNO3), respectively. Xylem sap was collected at 10:00 am after 1, 5, 15, and 30 d, and the selected metabolites were analyzed with liquid chromatography. All N, P, or K deficiencies led to a substantial increase in metabolites in the xylem sap. The predominant amino acid in the xylem sap was glutamine and, interestingly, all mineral deficiencies resulted in a substantial amount of γ-aminobutyric acid (GABA). Additionally, organic acids (citrate and malate) and soluble sugars were strongly increased in all mineral deficiencies, and, in particular, the level of shikimate was greatly affected by N deficiency. Based on these data, it is necessary to clearly elucidate an unknown event taking place in xylem loading in a variety of environmental impacts, and we are now studying to expand our knowledge on metabolic and proteomic responses using GC-MS and LC-MS. [ABSTRACT FROM AUTHOR]

Keywords: energy transfer; porphyrinoids; self-assembly; structure elucidation; zinc EN energy transfer porphyrinoids self-assembly structure elucidation zinc 3888 3888 1 02/26/21 20210224 NES 210224 B A light-harvesting antenna model b made of three mutually axially coordinated zinc porphyrin molecules is reported in this work. With an energy gradient incorporated, energy transfer only occurs from porphyrin molecules specializing in light absorption to a porphyrin molecule as a charge separation center. Cover Feature: A Light-Harvesting/Charge-Separation Model with Energy Gradient Made of Assemblies of meta-Pyridyl Zinc Porphyrins (Chem. Eur. J. 12/2021). [Extracted from the article]