The fluorescence quenching of coumarin dyes by N -alkylpyridinium (Py n + ) ionic liquids with different alkyl-chain lengths ( n = 4, 8, and 12) and the same anion (bis(trifluoromethanesulfonyl)amide, NTf 2 − ) was studied by using transient absorption spectroscopy. Two groups of coumarin dyes (fast and slow electron-transfer (ET) groups) were chosen based on a previous study on the fluorescence quenching in Py 4 NTf 2 by time-resolved fluorescence measurements (Saladin and Maroncelli, J. Phys. Chem. B, 2020, 124, 11431–11445). For the slow group (coumarin 151 (C151) and coumarin 152 (C152)), the induced emission (IE) of the coumarin dyes showed a dynamic Stokes shift and decay, which were dependent on the alkyl-chain length. In contrast, the IE for the fast group (C460 and C480) decayed rapidly, and the absorption of the cation radical appeared. The ET rate was determined from the decay profile of IE, which became slower with an increasing alkyl-chain length. The ET rate constants of coumarin dyes in all Py n NTf 2 ( n = 4, 8, and 12) were estimated based on Marcus theory, which predicted that the rate decreases with increasing alkyl-chain length, primarily due to the decrease in the redox potential of N -alkylpyridinium with increasing alkyl-chain length. While the alkyl-chain length dependence of the slow group (C151 and C152) was almost explained by Marcus theory, that of the fast group (C460 and C480) showed a more enhanced effect experimentally. Electronic-state calculations on the pair of a coumarin dye and N -alkylpyridinium indicated a significant structural difference between the ground and excited states of the complexes. Whereas the π–π stacking structure was the most stable in the excited state, a nearly T-shaped structure between aromatic rings was preferable in the ground state. For the fast ET group, the alkyl-chain dynamics was comparable to the ET reaction times, which may result in an enhanced effect of the alkyl chain.
Sakamaki et al. (Fri,) studied this question.