Intermolecular Hydrogen Bonding Modulates Triplet–Triplet Annihilation Upconversion in a Cationic Sensitizer as Revealed by Transient Electronic Spectroscopy

Abstract Metal-free triplet–triplet annihilation upconversion (TTA-UC) under long-wavelength excitation rules out metal toxicity and improves biocompatibility, showing great potential for biomedical use of photon upconversion materials. Intermolecular hydrogen bonding commonly occurs in biological environments, yet its effects on organic upconversion performance have rarely been clarified. Herein, we present a cationic Nile blue derivative (SeNB) as an effective sensitizer that produces far-red to visible upconversion luminescence, and adopt this molecule as a model to explore hydrogen-bonding impacts on organic upconversion. Transient electronic spectroscopy measurements show solvent hydrogen bonding weakens upconversion by disrupting excited states of both the sensitizer and annihilator, and by slowing intermolecular triplet energy transfer via the solvent cage effect. Such interactions directly quench the triplet state of SeNB, greatly reducing intersystem crossing rates and triplet quantum yields. The diketopyrrolopyrrole (DPP) annihilator carries hydrogen-bond accepting sites, and undergoes accelerated triplet quenching and shortened triplet lifetimes under hydrogen-bonding interactions. At low annihilator content, solvent cage effects driven by hydrogen bonding hinder Dexter-type triplet energy transfer from the sensitizer to the annihilator, which further lowers upconversion efficiency. This study demonstrates that Nile blue derivatives are effective sensitizers for TTA-UC, but also reveals that intermolecular hydrogen bonding severely suppresses their upconversion performance in protic solvents. These findings provide fundamental insights into solvent-regulated photophysical behaviors of ionic sensitizers and highlight the need for molecular engineering strategies to mitigate hydrogen-bonding effects for future applications in biological environments.