Computer-Assisted Design of an ON/OFF Switch for ESIPT via Substituent Positioning for Tunable Low-Threshold Light Amplification

Organic chromophores exhibiting excited-state intramolecular proton transfer (ESIPT) are promising candidates for organic lasers due to their four-level systems and ability to deliver efficient, tunable light amplification. Here, we demonstrate the application of a priori computational modeling to design chromophores with switchable ESIPT properties through strategic substituent positioning on the molecular core. Specifically, quantum mechanical calculations predict that the attachment of a phenylfuranylphenyl (PFP) unit to the meta position of a hydroxyphenanthroimidazole (HPI) core (HPI-mPFP) renders ESIPT kinetically and thermodynamically unfavorable, whereas para substitution (HPI-pPFP) promotes efficient ESIPT. This ON–OFF switching behavior arises from differing substituent effects on the ESIPT energy landscape: para substitution stabilizes the transition state and product, while meta substitution destabilizes them. To validate these predictions, both analogues were synthesized and characterized. Consistent with theory, the substituent position exerts a dramatic influence on photophysical and light amplification properties. HPI-pPFP exhibits efficient ESIPT with keto-form emission at 506 nm (green), while HPI-mPFP is ESIPT inactive and instead emits via its enol form at 418 nm (blue). Incorporation of the PFP substituent also strengthens electronic transitions, leading to higher molar absorption coefficients, shortened excited-state lifetimes, increased photoluminescence quantum yields, and enhanced radiative decay rate constants compared with the parent HPI. Computational analysis further reveals that the higher ESIPT barrier of HPI-mPFP not only suppresses keto-form emission but also modulates the balance between optical gain and self-absorption losses. As a result, HPI-mPFP achieves a remarkably low solid-state amplified spontaneous emission (ASE) threshold of 1.1 μJ/cm2 at 462 nm, while HPI-pPFP exhibits a higher green ASE threshold of 5.2 μJ/cm2 at 522 nm. These findings highlight the predictive power of computational design, elucidate key structure–property relationships governing ESIPT dynamics, and establish a framework for developing next-generation organic gain media for advanced optoelectronic applications.