Charge transfer state competing with excited state intramolecular proton transfer makes multiple emissions in salicylidene-2-anthrylamine

Abstract

Modulating anthracene substitution influences the radiative processes and the dynamics of excited-state intramolecular proton transfer (ESIPT) in salicylidene systems. Understanding the electronic interactions and structural changes resulting from anthracene substitution in salicylidene is vital for creating single molecule-based multilevel emissive materials for applications such as OLEDs and emission-based turn-on sensors. In this study, we identified a new charge transfer (CT) state, in addition to the well-known states derived from the ESIPT process, in the anthracene-based Schiff base salicylidene-2-anthrylamine (2ASD) and examined how their interplay influences the excited-state radiative decay processes of this molecule. The ESIPT process can be regulated by adjusting the pH of the solution, which subsequently leads to an enhancement of the CT emission. Theoretical studies further elucidated the structural and energetic plausibility of the CT state coexisting with the cis-keto state in the first excited state. On the other hand, an enhanced solid-state emission of 2ASD is observed, likely due to the formation of the cis-keto isomer, whose stability is attributed to both intermolecular and intramolecular hydrogen bonding, as evidenced by experimental and theoretical analyses. This study provides new insights into the photophysical behaviour of 2ASD in both solution and solid states, offering valuable guidance for designing anthracene–salicylidene-based white-light emissive devices and turn-on fluorescent sensors.