A 3-Pyridyl-5,15-Diazaporphyrin Nickel(II) Complex as a Bidentate Metalloligand for Transition Metals

Easily Switchable 18π-, 19π-, and 20π-Conjugation of Diazaporphyrin Double-Pincer Bispalladium Complexes

Ni^II 3,7,13,17-tetrapyridyl-5,15-diazaporphyrin serves as a double tridentate ligand to Pd^II ions to provide a pincer-type bispalladium complex. Electrochemical analysis revealed that the bispalladium complex shows excellent ability to accept electrons and reversible redox properties due to the coordination of the two cationic Pd^II centers to the meso-nitrogen atoms. We isolated and characterized one- and two-electron reduction species of the bispalladium complex. The 20π antiaromatic nature of the two-electron reduction species was confirmed by ¹ H NMR spectroscopy, UV/Vis-near-IR (NIR) absorption spectra, and density functional theory (DFT) calculations. X-ray diffraction revealed highly twisted structures for the bispalladium complexes regardless of the oxidation state.

Synthesis and Characterization of 16π Antiaromatic 2,7-Dihydrodiazapyrenes: Antiaromatic Polycyclic Hydrocarbons with Embedded Nitrogen

We describe the two-electron reduction of N,N'-dimethyl-2,7-diazapyrenium dications (MDAP²⁺ ), which afforded the corresponding reduced form (MDAP⁰ ) as a highly electron-rich 16π antiaromatic system. A single-crystal X-ray diffraction analysis of MDAP⁰ revealed a distorted quinoidal structure with high bond-length alternation. The ¹ H NMR spectrum of MDAP⁰ exhibited a diagnostic proton signal (4.6 ppm) that is distinctly upfield shifted compared to that of aromatic diazapyrene (8.3 ppm). Theoretical calculations supported the existence of a paratropic ring current. These results indicate that MDAP⁰ exhibits antiaromatic character derived from its peripheral 16π-electron conjugation.

Novel insights on the vibronic transitions in free base meso-tetrapyridyl porphyrin

We present novel results on the free base 5,10,15,20-meso-tetra(pyridyl)-21H,23H-porphyrin (H₂TPyP). This molecule presents complex electronic and vibrational properties and despite the vast literature reporting the transitions observed in its absorption and fluorescence spectra, a more accurate interpretation has been kept elusive. In particular, we show that the molecule's Q-band develops into many electronic and vibronic transitions, whose the well-known "four orbital model" finds it difficult to reconcile. Using distinct spectroscopy techniques, we conclude that both Q_x- and Q_y-bands comprise, in fact, two quasi-degenerated electronic states together with their respective vibronic progressions each. The analysis of the Huang-Rhys factors and complementary time- and polarization-resolved measurements reinforce the need for the proposed Q-band multi features remodeling.

Unsymmetrically Substituted Donor-π-Acceptor-Type 5,15-Diazaporphyrin Sensitizers: Synthesis, Optical and Photovoltaic Properties

The first examples of unsymmetrical β-substituted donor-π-acceptor (D-π-A)-type 5,15-diazaporphyrin (DAP) sensitizers with both p-aminophenyl and p-carboxyphenyl groups at their peripheral 3-, 7-, 13-, and/or 17-positions have been synthesized for use in dye-sensitized solar cells (DSSCs). UV/Vis absorption and emission spectroscopy, electrochemical measurements, and DFT calculations revealed that these D-π-A dyes exhibit high light-harvesting properties over the whole visible range because of the intrinsic charge-transfer character of their electronic transitions. The cell performances of TiO₂ -based DSSCs fabricated with the newly prepared DAP derivatives were evaluated under standard AM1.5 conditions. Among the four dyes examined, 13,17-bis(p-carboxyphenyl)-3,7-bis[p-(N,N-dimethylamino)phenyl]-DAP showed the highest power conversion efficiency (2.0 %), which was 20 times larger than that obtained with 3-(p-carboxyphenyl)-DAP. These results show that the DAP chromophore could be used as the electron-accepting π unit in various types of functional dyes.

Chemo- and Regioselective Reduction of 5,15-Diazaporphyrins Providing Antiaromatic Azaporphyrinoids

Reagent-controlled chemo- and regioselective reduction of 5,15-diazaporphyrins has been developed. The selective reduction of carbon-carbon double bonds of diazaporphyrins provides 18 π aromatic isobacteriochlorin-type products, whereas the reduction of carbon-nitrogen double bonds leads to selective formation of 20 π N,N'-dihydrodiazaporphyrins in excellent yields. The distinct antiaromatic character of N,N'-dihydrodiazaporphyrins has been revealed. The free-base N,N'-dihydrodiazaporphyrin exhibits slower inner NH tautomerism than that in the corresponding 18 π porphyrins.