High‐Resolution Electronic Spectroscopy Studies of <i>meta</i>‐Aminobenzoic Acid in the Gas Phase Reveal the Origins of its Solvatochromic Behavior

Abstract

Abstract A theoretical and experimental investigation of meta ‐aminobenzoic acid (MABA) in the gas phase is presented, with the goal of understanding counterintuitive observations on the solvatochromism of this “push‐pull” molecule. The adiabatic excitation energies, transition moments, and excited‐state structures are examined using the complete active space self‐consistent field approach (CASSCF and CASPT2), which shows the first excited electronic state of MABA to be of greater charge transfer character than was found in the para ‐isomer (PABA). The rotationally resolved electronic spectrum of MABA reveals the existence of two rotamers, owing to asymmetry in the carboxylic acid functional group. Stark measurements in a molecular beam show the change in permanent dipole moment upon excitation to be Δ μ ≈3.5 D for both rotamers, more than three times larger than that found in PABA. The excited state measurements reported here, along with supporting data from theory, clearly demonstrate how the meta ‐directing effects of asymmetric substitution in aniline derivatives can drive charge transfer pathways in the isolated molecule.

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Affiliated Institutions

• University of Pittsburgh US

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