Exploring 12 '-apo-beta-carotenoic-12 '-acid as an ultrafast polarity probe for ionic liquids

Abstract

The ultrafast excited-state dynamics of the carbonyl-containing carotenoid 12'-apo-beta-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-di-n-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: BF4, PF6, ethyl sulfate (EtOSO3), and bis(trifluoromethylsulfonyl)amide (Tf2N). The probe molecule was excited via the S-0 -> 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 ITCA was monitored in the near IR (850 or 860 nm). Its lifetime tau(1) is sensitive to the micropolarity-induced stabilization of S-1/ICT relative to S-0. The lifetime tau(1) of the S-1/ICT state varies only moderately in all ionic liquids studied here (similar to 40-110 ps), which lies in the range between ethanol (109 ps) and methanol (49 ps). While organic solvents show an excellent correlation of tau(1) with the solvent polarity function Delta f = (epsilon - 1)/(epsilon + 2) - (n(2) - 1)/(n(2) + 2), where epsilon 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-di-n-alkyl-imidazolium reduces the accessibility of the cation and therefore the electrostatic stabilization of the probe, resulting in an increase of tau(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 tau(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 tau(1) on the IL anion is much weaker, the only notable exception being EtOSO3, 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 LiClO4 salt to ethanol and diethyl ether. In this case, tau(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 -> 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.