Reduced and Oxidized Forms of the Pt-Organometallic Version of Polyaniline

This work represents an effort to
synthesize all four forms of
polyaniline (PANI) in its organometallic versions. Polymers containing
substituted 1,4-benzoquinone diimine or 1,4-diaminobenzene units in
the backbone exhibiting the general structure (CCC6H4–NC6X4N–C6H4CC-PtL2)n and (CCC6H4NH-C6X4-NHC6H4CC-PtL2)n along with the corresponding model compounds
(CCC6H4–NC6X4N–C6H4CC)­(PtL2Cl)2 and (CCC6H4NH-C6X4-NHC6H4CC)­(PtL2Cl)2 (L = PBu3; X = H, F, Cl) were synthesized.
The polymers and corresponding model compounds were characterized
(including 1H and 31P NMR, IR, mass spectra,
elemental analysis, and X-ray structure determinations) and investigated
for their redox properties in the absence and in the presence of acid.
Their optical properties, including ns transient spectroscopy were
also investigated. These properties were interpreted through density
functional theory (DFT) and time-dependent DFT (TDDFT) computations.
These materials are found to be oligomers (GPC) with thermal stability
(TGA) reaching 350 °C. The greatest stabilities were found in
the cases with X = F. Using a data bank of 8 X-ray structures of diimine
derivatives, a relationship between the CN bond distance and
the dihedral angle between the benzoquinone ring and the flanking
phenyl planes is noted. As the size of the substituent X on the benzoquinone
center increases, the degree of conjugation decreases as demonstrated
by the CN bond length. The largest dihedral angles are noted
for X = Cl. These polymers exhibit drastic chemical differences when
X is varied (X = H, F, Cl). The completely reduced polymer (CCC6H4NH-C6H4–NHC6H4CC-PtL2)n (i.e., X = H) was not chemically accessible whereas in the
cases of X = F, Cl, these materials were obtained and represent the
first examples of fully reduced organometallic versions of PANI (i.e.,
leucoemaraldine). For the (CCC6H4–NC6X4N–C6H4CC-PtL2)n polymers, the completely oxidized
form for X = H was isolated (pernigraniline), but for X = F and Cl,
only the largely reduced mixed-valence form (i.e., emaraldine) was
obtained via chemical routes. In acidic solutions, the chemically
accessible polymer for X = H, (CCC6H4–NC6H4N–C6H4CC-PtL2)n, exhibits two chemically reversible waves indicating that
the reduced form (CCC6H4NH-C6H4–NHC6H4CC-PtL2)n can be generated. The absorption
spectra of the highly colored diimine-containing species exhibit a
broad charge transfer band (assigned based on DFT calculations (B3LYP);
C6H4CC-PtL2-CCC6H4 → NC6X4N) in the 450–800 nm window red shifting according
X = H → Cl → F, consistent with their relative inductive
effect. The largest absorptivity is measured for X = H because this
polymer is fully oxidized whereas for the cases where X = F and Cl,
these polymers exists in the mixed valence form. The ns transient
absorption spectra of two polymers (X = F; reduced and mixed-valence
polymers) were measured. The triplet excited state in the mixed-valence
polymer is dominated by the reduced diamine residue and the T1-Tn absorption of the diimine
is entirely quenched.