“Vapochromic” Compounds as Environmental Sensors. 2. Synthesis and Near-Infrared and Infrared Spectroscopy Studies of [Pt(arylisocyanide)₄][Pt(CN)₄] upon Exposure to Volatile Organic Compound Vapors

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 -CNC 6 H 4 C n H 2 n +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, λ max abs = 744, λ max emit = 958; n = 6, λ max abs = 841, λ max emit = 910; n = 10, λ max abs = 746, λ max emit = 944; n = 12, λ max abs = 764, λ max emit = 912; n = 14, λ max abs = 690, λ max 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 (λ max abs (293 K) = 746; λ max emit (293 K) = 944; λ max abs (77 K) = 846; λ max 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 λ max 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 arise from hydrophobic blocking of the water at the solid/gas interface.