Ultrafast fluorescence study of the effect of carboxylic and carboxylate substituents on the excited state properties of anthracene
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- 作者：William Rodr铆guez-C贸rdoba ; Raquel Noria-Moreno ; Pedro Navarro ; Jorge Peon
- 关键词：Ultrafast fluorescence up-conversion ; Anthracene-9-carboxylic acid ; Sodium anthracene-9-carboxylate ; Excited state dynamics ; State specific ; Linear response
- 刊名：Journal of Luminescence
- 出版年：January, 2014
- 年：2014
- 卷：145
- 期：Complete
- 页码：697-707
- 全文大小：2410 K
文摘
With the objective of understanding the interaction between carboxylic substituents and aromatic systems in electronically excited states, we have studied the photophysics of anthracene-9-carboxylic acid and its conjugate base through spectroscopic and computational approaches. We measured the emission spectrum evolution with femtosecond resolution observing that the formation of the relaxed fluorescent state of the acid corresponds to a red shifting of the emission which takes place within the first picosecond after excitation, a time-scale defined by the solvent response (acetone). For the case of the anthracene-9-carboxylate system, the spectral evolution is practically absent, indicating a lack of relaxation of the substituent orientation in the S1 state. Computational work at the time-dependent density functional theory level, considering the novel state-specific formalism, indicates that for anthracene-9-carboxylic acid, the first electronically excited state evolves from a structure with a nearly 60掳 dihedral angle between the carboxylic and aromatic systems, to a relaxed structure with a nearly 30掳 angle. On the other hand, the calculations show that for the salt, the carboxylate group remains decoupled from the aromatic system both in the ground and fluorescent state, remaining in both states at nearly 90掳. Our results elucidate that the emission spectra of the acid and conjugate base are defined by the degree of interaction between the carboxylic (or carboxylate) group and the aromatic system. Such interactions are drastically different from the formal charge present in the carboxylate ion.