A Theoretical Investigation on the Role of Primary, Secondary, and Tertiary Amine Substituents in Excited State Intramolecular Proton Transfer of p-Amino o-Hydroxy Benzaldehyde.

This article reports a comprehensive theoretical analysis of the dual fluorescence property of three derivatives of p-amino o-hydroxy benzaldehyde molecules, namely, para-N,N-dimethylamino orthohydroxy benzaldehyde (tertiary PAOHBA), para-N-methylamino orthohydroxy benzaldehyde (secondary PAOHBA), and para-amino orthohydroxy benzaldehyde (primary PAOHBA) through ab initio calculations and excited state molecular dynamics. The results revealed that excited-state intramolecular proton transfer is responsible for the dual emission properties of such molecules. The conclusions are made based on the computed vibrational frequencies, excited state antiaromaticity, potential energy surfaces, absorption and emission spectra, and finally, from the excited state dynamics. The calculations showed that the dual emission property is most effective for primary PAOHBA, followed by secondary PAOHBA, and tertiary PAOHBA. The rates of proton transfer reaction are observed to be (18.5 ± 0.06) × 10¹², (16 ± 0.04) × 10¹², and (16.7 ± 0.05) × 10¹² s^-1, respectively, for primary, secondary, and tertiary PAOHBA. The results also show that the proton transfer reaction is more reversible for secondary PAOHBA than for the other two compounds.