Tetrabenzoporphyrin and -mono‑, -cis-di-
and Tetrabenzotriazaporphyrin Derivatives: Electrochemical and
Spectroscopic Implications of meso CH Group Replacement with Nitrogen
posted on 2015-06-01, 00:00authored byAdele van As, Chris C. Joubert, Blenerhassitt E. Buitendach, Elizabeth Erasmus, Jeanet Conradie, Andrew N. Cammidge, Isabelle Chambrier, Michael J. Cook, Jannie C. Swarts
Nonperipherally hexyl-substituted
metal-free tetrabenzoporphyrin (2H-TBP, 1a) tetrabenzomonoazaporphyrin
(2H-TBMAP, 2a), tetrabenzo-cis-diazaporphyrin
(2H-TBDAP, 3a), tetrabenzotriazaporphyrin (2H-TBTAP, 4a), and phthalocyanine (2H-Pc, 5a), as well
as their copper complexes (1b–5b),
were synthesized. As the number of meso nitrogen atoms increases from
zero to four, λmax of the Q-band absorption peak
becomes red-shifted by almost 100 nm, and extinction coefficients
increased at least threefold. Simultaneously the blue-shifted Soret
(UV) band substantially decreased in intensity. These changes were
related to the relative electron-density of each macrocycle expressed
as the group electronegativity sum of all meso N and CH atom groups,
∑χR. X-ray photoelectron spectroscopy differentiated
between the three different types of macrocyclic nitrogen atoms (the
Ninner, (NH)inner, and
Nmeso) in the metal-free complexes. Binding energies of
the Nmeso and Ninner,Cu atoms in copper chelates
could not be resolved. Copper insertion lowered especially the cathodic
redox potentials, while all four observed redox processes occurred
at larger potentials as the number of meso nitrogens increased. Computational
chemical methods using density functional theory confirmed 1b to exhibit a Cu(II) reduction prior to ring-based reductions, while
for 2b, Cu(II) reduction is the first reductive step
only if the nonperipheral substituents are hydrogen. When they are
methyl groups, it is the second reduction process; when they are ethyl,
propyl, or hexyl, it becomes the third reductive process. Spectro-electrochemical
measurements showed redox processes were associated with a substantial
change in intensity of at least two main absorbances (the Q and Soret
bands) in the UV spectra of these compounds.