2 research outputs found
Acrostichum flagelliferum
The synthesis, structures, optical and electrochemical
properties, and aromaticity of a series of 5,15-diaza-10,20-dimesitylporphyrins
(M–DAP; M = Pb, H<sub>2</sub>, Ni, Pd, Pt, Zn; mesityl = 2,4,6-trimethylphenyl)
are reported. Treatment of mesityl-substituted bisÂ(5,5′-dibromodipyrrin)
with sodium azide in the presence of leadÂ(II) acetylacetonate afforded
Pb–DAP, which was quantitatively converted to H<sub>2</sub>–DAP by acidolysis. The free base H<sub>2</sub>–DAP
reacted with palladiumÂ(II), platinumÂ(II), and zincÂ(II) salts to give
Pd–DAP, Pt–DAP, and Zn–DAP, respectively. The
crystal structures, optical and electrochemical properties, and aromaticities
of these β-unsubstituted M–DAPs were comprehensively
investigated by X-ray crystallography, UV–vis absorption/fluorescence
spectroscopy, nanosecond flash photolysis, cyclic and differential
pulse voltammetry, NMR spectroscopy, and density functional theory
calculations. The obtained data show that replacement of the 5- and
15-methine carbons with nitrogen atoms alters the intrinsic properties
of the porphyrin 18Ď€ system as follows: (i) the coordination
spheres at the N<sub>4</sub> core become contracted while keeping
high planarity; (ii) the Q bands are red shifted and largely intensified;
(iii) the electron-accepting ability is enhanced, whereas the electron-donating
ability is reduced; (iv) the radiative decay rates from the S<sub>1</sub> state are enhanced; and (v) the aromaticity of the 18Ď€
circuit is slightly reduced in terms of both geometric and magnetic
criteria. These optical and electrochemical properties of M–DAPs
stem from their characteristic frontier orbitals; two HOMOs and two
LUMOs are nondegenerate as a result of the incorporation of the electronegative
nitrogen atoms at the two meso positions. In addition, the group 10
metals incorporated at the core finely tune the fundamental properties
of DAP π systems through inductive effects as well as dπ–pπ
antibonding orbital interactions; the HOMO–LUMO gaps of the
group 10 metal complexes increase in the order Ni–DAP <
Pd–DAP < Pt–DAP
Synthesis and Photophysical Properties of Two Diazaporphyrin–Porphyrin Hetero Dimers in Polar and Nonpolar Solutions
Two diazaporphyrin (DAP)-porphyrin
hetero dimers, in β–<i>meso</i> and β–β
configurations, were prepared
to study their photoinduced intramolecular electron transfer properties.
The two <i>meso</i> nitrogen atoms in the porphyrin ring
of DAP change its redox potential, making DAP more easily reduced,
compared to its porphyrin counterpart. A charge-transfer from porphyrin
to DAP in both hetero dimers was verified by versatile optical spectroscopic
methods. The steady-state fluorescence spectra indicated an efficient
intramolecular exciplex formation for both dimers. For the β–<i>meso</i> dimer, ultrafast time-resolved spectroscopic methods
revealed the subpicosecond formation of two types of primary short-living
(1–18 ps) intramolecular exciplexes, which relaxed in toluene
to form a long-living final exciplex (1.4 ns) followed by a longer-living
charge transfer complex (>5 ns). However, in benzonitrile, the
lifetime
of the final exciplex was longer (660 ps) as was that of the charge
transfer complex (180 ps). The β–β analogue formed
similar short-living exciplexes in both solvents, but the final exciplex
and the charge transfer state had significantly shorter lifetimes.
The electrochemical redox potential measurements and density functional
theory calculations supported the proposed mechanism