3,6,13,16‐Tetrabromo‐2,7,12,17‐tetrapropylporphycene

The title compound [systematic name: 3,10,13,20‐tetra­bromo‐4,9,14,19‐tetrapropyl‐21,22,23,24‐tetraazapentacyclo[16.2.1.1^2,5.1^8,11.1^12,15]tetracosa‐2(22),3,5,7,9,11,13,15(24),16,18,20‐undecaene], C₃₂H₃₄Br₄N₄, crystallizes in two distinct crystalline forms, viz. monoclinic prisms and triclinic plates, and the first of these is described here. The molecule of the prismatic form has a centre of symmetry and a more warped structure than that of the triclinic plate‐like form. The shape of the central N₄ cavity is rectanglar, enlarged in the direction of the methine‐bridge C atoms, and the N?N distances are 2.713 (3) and 2.818 (3) Å.     

Tautomerization in 2,7,12,17‐Tetraphenylporphycene and 9‐Amino‐2,7,12,17‐tetraphenylporphycene: Influence of Asymmetry on the Direction of the Transition Moment

Femtosecond transient absorption anisotropy studies have been performed for two porphycenes of different symmetry. In 2,7,12,17‐tetraphenylporphycene, the chemical identity of two trans forms implies a change in the S₀–S₁ transition‐moment direction upon tautomerization. Exploiting this phenomenon, the rates of double hydrogen transfer in both the S₀ and S₁ states (1.4×10¹² s^?1 and 2.7×10¹¹ s^?1, respectively) have been determined by performing time‐resolved anisotropy studies. In the asymmetric 9‐amino‐2,7,12,17‐tetraphenylporphycene, tautomerization occurs with a similar rate in the ground state. In the S₁ state, the reaction is hindered in its vibrationally relaxed form, but the excitation spectra suggest that it may occur for an unrelaxed molecule. Unlike all porphycenes that have been studied so far, 9‐amino‐2,7,12,17‐tetraphenylporphycene does not reveal significant changes in anisotropy owing to intramolecular double hydrogen transfer; rather, the transition‐moment directions are similar in the two tautomeric forms. Analysis of the molecular orbitals allows for an explanation of the “locking” of the transition moments: it is due to a large splitting of the two HOMO orbitals, which retain the order of their energies in the two tautomers.     

Thermodynamique des complexes metalliques ternaires des amino-acides. II. Etude des systemes Ni(II)—glycine—valine Ni(II)—alanine—valine

Standard enthalpies and entropies of formation for binary and ternary Ni(II) complexes with pairs of the following amino acids as ligands: glycine, DL--alanine and DL-valine, were calorimetrically determined at 25°C in aqueous solution using 1 M ionic strength (NaClO₄).

The results are discussed according to every possible pathway for mixed ligand complex formation and also using the classical statistical methods. Temperature-dependent and temperature-independent components of the thermodynamic data are calculated. In all cases with these ligands involving identical coordination sites, the temperature-independent component of the enthalpy change is closely constant for binary as well as for ternary complexes. All the data show that the stabilization of mixed ligand complexes with respect to the parent binary complexes arises from the entropy term and is maximum for the Ni(II)—glycine—valine system.