The ultrafast excited-state dynamics of the carbonyl-containing carotenoid 12′-apo-carotenoic-12′-acid (12′CA) have been used for probing the microscopic environment in various ionic liquids (ILs). The following IL cations were investigated: 1,3-din -alkyl-imidazolium featuring different n-alkyl chain lengths and also additional methylation at the C2 position, triethylsulfonium, as well as two tetraalkylammonium ions. These were combined with different anions: [BF 4 ]-, [PF 6 ]-, ethyl sulfate ([EtOSO 3 ]-), and bis(trifluoromethylsulfonyl)amide ([Tf 2 N]-). The probe molecule was excited via the S 0 f S 2 transition at 425 or 430 nm, and the characteristic stimulated emission decay of the low-lying excited electronic S 1 /ICT (intramolecular charge transfer) state of 12′CA was monitored in the near IR (850 or 860 nm). Its lifetime τ 1 is sensitive to the micropolarity-induced stabilization of S 1 /ICT relative to S 0. The lifetime τ 1 of the S 1 /ICT state varies only moderately in all ionic liquids studied here (∼ 40-110 ps), which lies in the range between ethanol (109 ps) and methanol (49 ps). While organic solvents show an excellent correlation of τ 1 with the solvent polarity function ∆f) (-1)/(+ 2)-(n 2-1)/(n 2 + 2), where and n are the static dielectric constant and the refractive index of the solvent, respectively, this is not the case for ILs. This is due to dominant local electrostatic probe-cation interactions which cannot be easily quantified by macroscopic quantities. Methylation at the C2 position of 1,3-din -alkyl-imidazolium reduces the accessibility of the cation and therefore the electrostatic stabilization of the probe, resulting in an increase of τ 1. A similar increase is observed upon extension of one of the n-alkyl chains from ethyl to n-decyl. Tetraalkylammonium ILs show an increased τ 1 probably due to their more delocalized positive charge which cannot interact so favorably with the probe, in contrast to trialkylsulfonium ILs where the charge is more localized on the sulfur atom. The dependence of τ 1 on the IL anion is much weaker, the only notable exception being [EtOSO 3 ]-, where 12′CA experiences a less polar local environment than expected on the basis of extrapolated static dielectric constants. This is possibly due to the competition of the anion and probe for the cation interaction sites. Considerable electrostatic probecation interactions can be also introduced by addition of large amounts of LiClO 4 salt to ethanol and diethyl ether. In this case, τ 1 also strongly decreases, indicating an efficient coordination of Li + cation(s) with the carbonyl oxygen at the negative end of the probe molecule. The S 1 /ICT f S 0 internal conversion of the 12′CA probe in ILs accelerates with increasing temperature, which can be characterized by an apparent activation energy of a few kJ mol-1 , which is expected for energy-dependent nonradiative processes.

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Experiments employing pulsed high-pressure (< or = 90 bar) supersonic jet expansions into a laser REMPI time-of-flight mass spectrometer are presented. Due to the very short opening time of the high-pressure valve, a compact arrangement with moderate pumping speed requirements is sufficient, which can be used up to 50 Hz repetition frequency, still maintaining an operating pressure in the expansion chamber of ca. 10(-4) mbar. p-Fluorotoluene was used as a rotational and vibrational "molecular thermometer" to characterize the cooling capabilities of high-pressure pulsed argon and CO(2) expansions into vacuum, spanning a wide range of stagnation pressures into the supercritical regime. Rotational and torsional temperatures were deduced from the contour of the S(0) --> S(1)(0-0) transition by an asymmetric-top/free-internal-rotor simulation and compared with the results of a simpler rigid-rotor asymmetric-top fit. The experiments show that average rotational temperatures of ca. 1-2 K in argon and 13-15 K in CO(2) can be reached at the highest pressures studied. The population of higher |m| torsional levels of p-fluorotoluene is clearly demonstrated by the appearance of characteristic features on the blue edge of the contours, which are more pronounced in the warmer CO(2) expansions. While the rotational temperatures in argon expansions compare well with estimated gas dynamical terminal translational temperatures, there are considerable differences in the case of CO(2). Still, the degree of internal cooling reached with CO(2) is sufficiently low, so that a setup of this kind might provide good opportunities for future studies of thermally labile low-volatile molecules to cool them to cryogenic temperatures low enough to achieve a sufficient simplification of their spectra.