Exploring 12'-Apo--carotenoic-12'-acid as an Ultrafast Polarity Probe for Ionic Liquids
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- 作者：Peter W. Lohse ; Reinhard Bü ; rsing ; Thomas Lenzer ; Kawon Oum
- 刊名：Journal of Physical Chemistry B
- 出版年：2008
- 出版时间：March 13, 2008
- 年：2008
- 卷：112
- 期：10
- 页码：3048 - 3057
- 全文大小：152K
- 年卷期：v.112,no.10(March 13, 2008)
- ISSN：1520-5207
文摘
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 followingIL cations were investigated: 1,3-di-n-alkyl-imidazolium featuring different n-alkyl chain lengths and alsoadditional methylation at the C2 position, triethylsulfonium, as well as two tetraalkylammonium ions. Thesewere combined with different anions: [BF4]-, [PF6]-, ethyl sulfate ([EtOSO3]-), and bis(trifluoromethylsulfonyl)amide ([Tf2N]-). The probe molecule was excited via the S0 
S2 transition at 425 or 430 nm, and thecharacteristic stimulated emission decay of the low-lying excited electronic S1/ICT (intramolecular chargetransfer) state of 12'CA was monitored in the near IR (850 or 860 nm). Its lifetime 
1 is sensitive to themicropolarity-induced stabilization of S1/ICT relative to S0. The lifetime 1 of the S1/ICT state varies onlymoderately 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 1 with the solvent polarityfunction 
f = (
- 1)/( + 2) - (n2 - 1)/(n2 + 2), where and n are the static dielectric constant and therefractive index of the solvent, respectively, this is not the case for ILs. This is due to dominant local electrostaticprobe-cation interactions which cannot be easily quantified by macroscopic quantities. Methylation at theC2 position of 1,3-di-n-alkyl-imidazolium reduces the accessibility of the cation and therefore the electrostaticstabilization of the probe, resulting in an increase of 1. A similar increase is observed upon extension of oneof the n-alkyl chains from ethyl to n-decyl. Tetraalkylammonium ILs show an increased 1 probably due totheir more delocalized positive charge which cannot interact so favorably with the probe, in contrast totrialkylsulfonium ILs where the charge is more localized on the sulfur atom. The dependence of 1 on the ILanion is much weaker, the only notable exception being [EtOSO3]-, where 12'CA experiences a less polarlocal environment than expected on the basis of extrapolated static dielectric constants. This is possibly dueto the competition of the anion and probe for the cation interaction sites. Considerable electrostatic probe-cation interactions can be also introduced by addition of large amounts of LiClO4 salt to ethanol and diethylether. In this case, 1 also strongly decreases, indicating an efficient coordination of Li+ cation(s) with thecarbonyl oxygen at the negative end of the probe molecule. The S1/ICT S0 internal conversion of the12'CA probe in ILs accelerates with increasing temperature, which can be characterized by an apparentactivation energy of a few kJ mol-1, which is expected for energy-dependent nonradiative processes.
-carotenoic-12'-acid(12'CA) have been used for probing the microscopic environment in various ionic liquids (ILs). The followingIL cations were investigated: 1,3-di-n-alkyl-imidazolium featuring different n-alkyl chain lengths and alsoadditional methylation at the C2 position, triethylsulfonium, as well as two tetraalkylammonium ions. Thesewere combined with different anions: [BF4]-, [PF6]-, ethyl sulfate ([EtOSO3]-), and bis(trifluoromethylsulfonyl)amide ([Tf2N]-). The probe molecule was excited via the S0 S2 transition at 425 or 430 nm, and thecharacteristic stimulated emission decay of the low-lying excited electronic S1/ICT (intramolecular chargetransfer) state of 12'CA was monitored in the near IR (850 or 860 nm). Its lifetime 1 is sensitive to themicropolarity-induced stabilization of S1/ICT relative to S0. The lifetime 1 of the S1/ICT state varies onlymoderately 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 1 with the solvent polarityfunction f = ( - 1)/( + 2) - (n2 - 1)/(n2 + 2), where and n are the static dielectric constant and therefractive index of the solvent, respectively, this is not the case for ILs. This is due to dominant local electrostaticprobe-cation interactions which cannot be easily quantified by macroscopic quantities. Methylation at theC2 position of 1,3-di-n-alkyl-imidazolium reduces the accessibility of the cation and therefore the electrostaticstabilization of the probe, resulting in an increase of 1. A similar increase is observed upon extension of oneof the n-alkyl chains from ethyl to n-decyl. Tetraalkylammonium ILs show an increased 1 probably due totheir more delocalized positive charge which cannot interact so favorably with the probe, in contrast totrialkylsulfonium ILs where the charge is more localized on the sulfur atom. The dependence of 1 on the ILanion is much weaker, the only notable exception being [EtOSO3]-, where 12'CA experiences a less polarlocal environment than expected on the basis of extrapolated static dielectric constants. This is possibly dueto the competition of the anion and probe for the cation interaction sites. Considerable electrostatic probe-cation interactions can be also introduced by addition of large amounts of LiClO4 salt to ethanol and diethylether. In this case, 1 also strongly decreases, indicating an efficient coordination of Li+ cation(s) with thecarbonyl oxygen at the negative end of the probe molecule. The S1/ICT S0 internal conversion of the12'CA probe in ILs accelerates with increasing temperature, which can be characterized by an apparentactivation energy of a few kJ mol-1, which is expected for energy-dependent nonradiative processes.