The synthesis, characterization, and vis−NIR−IR
vapochromic/spectroscopic studies are
reported for isocyanide compounds of the form
[Pt(arylisocyanide)4][Pt(CN)4]
(where arylisocyanide =
p-CNC6H4C
n
H2n+1;
n = 1, 6, 10, 12, 14). The dark blue, solid materials
change
color in the NIR (near-infrared) spectral region upon exposure to the
ambient room-temperature vapor pressure of volatile organic compounds (VOCs). At
room temperature
the PtPt compounds exhibit strong solid-state absorption and emission
bands in the NIR
region of the spectrum that are red-shifted from similar bands in the
PtPd analogues; (n =
1, λmax
abs = 744,
λmax
emit = 958; n = 6,
λmax
abs = 841,
λmax
emit = 910; n = 10,
λmax
abs = 746,
λmax
emit = 944; n = 12,
λmax
abs = 764,
λmax
emit = 912; n = 14,
λmax
abs = 690,
λmax
emit = 876
nm). The positions of these broad bands depend on the number of
carbons in the alkyl
substituent. The absorption and emission bands for the solid
material (n = 10 compound)
also exhibit a substantial red-shift upon cooling to 77 K
(λmax
abs (293 K) = 746;
λmax
emit (293
K) = 944; λmax
abs (77 K) = 846;
λmax
emit (77 K) = 1094 nm) that is
consistent with an
alternating cation−anion stacked structure. Qualitatively,
compounds with n > 6 respond
well to nonpolar VOCs; the n = 1, 6 compounds respond
better to polar VOCs. The shifts
observed for λmax
abs (at 293 K) are on the
order of 700 cm-1 and are 2−3 times greater
than
those exhibited by the PtPd analogue compounds under identical
conditions. The n = 10
compound is the most responsive; the positions of the vis−NIR band in
the presence of several
solvent vapors are as follows: none, 746 nm; methanol, 757; ethanol,
782; 2-propanol, 782;
diethyl ether, 787; acetonitrile, 809; hexanes, 775; acetone, 800;
benzene, 801; dichloromethane, 811; chloroform, 837. No response was observed for water
vapor. IR studies of
films of the n = 10 compound on an ATR crystal show that
the sorption of VOC by the solid
causes no change in the ν(CN) isocyanide stretching frequency
but in some cases a substantial
shift (0−15 cm-1) in ν(CN) of the cyanide
stretch is observed. When the n = 10
compound
contacts VOCs capable of H-bonding with the
Pt(CN)4
2- anion, two cyanide stretches
are
observed. All the spectroscopic data suggest that the VOC
penetrates the solid and interacts
with the linear chain chromophore to cause the spectral shifts in the
vis−NIR−IR spectral
regions. The vapochromic shifts are suggested to be due to
dipole−dipole and/or H-bonding
interactions between the Pt(CN)4
2- anion
and polar VOCs. For nonpolar VOCs, lypophilic
interactions between the VOC and the isocyanide ligands that cause no
change in the ν(CN) stretching region must cause the NIR vapochromism observed.
The absence of a
vapochromic response for water vapor is suggested to a...
