Synthesis,
Spectroscopic Properties, and Photoconductivity of Black Absorbers
Consisting of Pt(Bipyridine)(Dithiolate) Charge Transfer Complexes
in the Presence and Absence of Nitrofluorenone Acceptors
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Abstract
The
diimine–dithiolato ambipolar complexes Pt(dbbpy)(tdt) and Pt(dmecb)(bdt)
(dbbpy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine;
tdt<sup>2–</sup> = 3,4-toluenedithiolate; dmecb = 4,4′-dimethoxyester-2,2′-bipyridine;
bdt<sup>2–</sup> = benzene-1,2-dithiolate) are prepared herein.
Pt(dmecb)(bdt) exhibits photoconductivity that remains constant (photocurrent
density of 1.6 mA/cm<sup>2</sup> from a 20 nm thin film) <i>across
the entire visible region of the solar spectrum</i> in a Schottky
diode device structure. Pt(dbbpy)(tdt) acts as donor when combined
with the strong nitrofluorenone acceptors 2,7-dinitro-9-fluorenone
(DNF), 2,4,7-trinitro-9-fluorenone (TRNF), or 2,4,5,7-tetranitro-9-fluorenone
(TENF). Supramolecular charge transfer stacks form and exhibit various
donor–acceptor stacking patterns. The crystalline solids are
“black absorbers” that exhibit continuous absorptions
spanning the entire visible region and significant ultraviolet and
near-infrared wavelengths, the latter including long wavelengths that
the donor or acceptor molecules alone do not absorb. Absorption spectra
reveal the persistence of donor–acceptor interactions in solution,
as characterized by low-energy donor/acceptor charge transfer (DACT)
bands. Crystal structures show closely packed stacks with distances
that underscore intermolecular DACT. <sup>1</sup>H NMR provides further
evidence of DACT, as manifested by upfield shifts of aromatic protons
in the binary adducts versus their free components, whereas 2D nuclear
Overhauser effect spectroscopy (NOESY) spectra suggest coupling between
dithiolate donor protons with nitrofluorenone acceptor protons, in
correlation with the solid-state stacking. The NMR spectra also show
significant peak broadening, indicating some paramagnetism verified
by magnetic susceptibility data. Solid-state absorption spectra reveal
further red shifts and increased relative intensities of DACT bands
for the solid adducts vs solution, suggesting cooperativity of the
DACT phenomenon in the solid state, as further substantiated by ν<sub>C–O</sub> and ν<sub>N–O</sub> IR bands and solid-state
tight-binding computational analysis