Systematic investigation of the excited-state properties of anthracene-dicarboxylic acids

We report the photophysical properties of three anthracene-dicarboxylic acids - 1,4-anthracene dicarboxylic acid (1,4-ADCA), 2,6-anthracene dicarboxylic acid (2,6-ADCA) and 9,10-anthracene dicarboxylic acid (9,10-ADCA) - in a series of polar aprotic solvents using steady-state absorption spectroscopy, steady-state emission spectroscopy and time-correlated single photon counting emission lifetime spectroscopy. The addition of carboxylic acid functional groups on the anthracene ring perturbs the electronic transitions oriented along the longitudinal and transverse axes to varying degrees, resulting in differences in the photophysical properties. The three anthracene derivatives exhibit very different excited-state properties in basic solution compared to acidic and neutral solutions. Density functional theory (DFT) calculations reveal that the lowest-energy ground-state structures of both 2,6-ADCA and 1,4-ADCA have dihedral angles between the carboxylic acids and aromatic planes of theta = 0 degrees. For 9,10-ADCA, the same dihedral angle increases to theta = 56.6 degrees. Time-dependent DFT calculations suggest that the carboxyl groups of 1,4-ADCA and 2,6-ADCA remain coplanar with the anthracene ring system in the excited state. In contrast, the calculations reveal significant changes between the ground and excited geometries for 9,10-ADCA as the dihedrals decrease from 56.6 degrees to 27.7 degrees in the first excited state, and puckering of the anthracene moiety of 9,10-ADCA is observed. (C) 2016 Elsevier B.V. All rights reserved.