Abstract
We present steady-state and time-resolved fluorescence spectroscopic data derived from coumarin 153 (C153) in a binary solution comprised of trihexyltetradecylphosphonium chloride ([P6,6,6,14]+Cl−) and supercritical CO2 (scCO2). Steady-state fluorescence of C153 was measured in neat scCO2 and ionic liquid (IL)-modified scCO2 solutions. The steady-state excitation and emission peak frequency data in neat scCO2 and IL/scCO2 diverge at low fluid density (ρr = ρ/ρc < 1). The prominent spectral differences at low fluid density provided clear evidence that C153 reports different microenvironments, and suggested that the IL is solubilized in the bulk scCO2 and heterogeneity of the C153 microenvironment is readily controlled by scCO2 density. C153 dimers have been reported in the literature, and this formed the basis of the hypothesis that dimerization is occurring in scCO2. Time-dependent density functional theory (TD-DFT) electronic structure calculations yielded transition energies that were consistent with excitation spectra and provided supporting evidence for the dimer hypothesis. Time-resolved fluorescence measurements yielded triple exponential decays with time constants that further supported dimer formation. The associated fractional contributions showed that the dominant contribution to the intensity decay was from C153 monomers, and that in high density scCO2 there was minimal contribution from C153 dimers.
Highlights
Room temperature ionic liquids (RTILs) are salts that consist of ions that are typically combinations of organic cations paired with inorganic anions that exist in the liquid state at room temperature
The data showed that the ionic liquid trihexyltetradecylphosphonium chloride, [P6,6,6,14]+Cl−, was dissolved into supercritical CO2 (scCO2) at 323 K
Low-density coumarin 153 (C153)/IL/scCO2 emission spectra showed significant differences compared to the neat scCO2 that were strongly dependent on choice of excitation wavelength
Summary
Room temperature ionic liquids (RTILs) are salts that consist of ions that are typically combinations of organic cations paired with inorganic anions that exist in the liquid state at room temperature. IInn tthhaatt rreeppoorrtt,, wwee aallssoo iinncclluuddeedd aann eemmiissssiioonn ssppeeccttrruumm ooff tthhee ddyyee ccoouummaarriinn 115533 ((CC115533)),, FFiigguurree 11,, iinn [[PP66,,66,,66,1,14]4+]C+Cl−/ls−c/CsOcC2 Oth2atthsautpspuoprpteodrttehdetahsesearstsieorntiothnatthtahtethILe hILadhaadmaeamseuarsaubrleabelfefeecftfeocnttohne tphreobpero’sbeem’siessmioisns.iTonh.atTphraetlipmreinliamryinsapreycstrpoescctoropsiccodpaitcadsahtoawsehdowusedthuast tthheatCth15e3Cw15a3s wabalseatbolerteoporerpt oornt othnethperepsreensceencoef oIfLILininscsCcCOO2 2anadndpprorommpptetdeduussttoo iinnvveessttiiggaattee tthhee [[PP66,6,6,6,,61,41]4+C]+lC−/ls−cC/sOc2CsOy2stseymstienmminorme odreetadiletgaiivlegnivthene gtheenegreanleirnatleirnetsetrienstILins aILnsdasncdCOsc2C. Upon connection to the syringe pump gaseous CO2 was flowed through the cell as a final step to displace nitrogen prior to heating and pressuring the sample for measurement. Because of physical size limitations in the instrument sample compartment, stirring was discontinued just prior to measurement and the solution was equilibrated for an additional 5 min. Time-resolved emission intensity decays were measured using the Fluorolog-3 instrument, modified with time-correlated single photon counting (TCSPC) components from Horiba Scientific as described elsewhere [59]. Instrument response was measured using an intensity matched scattering solution in a second, identically constructed stainless steel cell. The spectral intensity output for all results was plotted using the program’s default base-10 logarithmic scale that ranged from 1.0 × 10−6 to 1.0
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