Dye-quencher pair screening for efficient photo-CIDNP: The role of molecular diffusion

Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging are well-established techniques to acquire diverse molecular information, while their potential applications remain limited due to low sensitivity. Photochemically induced dynamic nuclear polarization (photo-CIDNP) is one of the promising methods to solve this issue, and numerous studies have been conducted to understand its physical mechanism using a particular combination of a dye and a quencher of interest. However, the research across multiple dye-quencher combinations remains largely unexplored. Here, we explore plenty of dye-quencher combinations and reveal that not only the electron transfer process but also the optimal value of the g-value difference (Δg), considerably affected by the escape process of a radical pair, plays a key role in maximizing the enhancement of the NMR signal by photo-CIDNP. The combinations of 115 quenchers with several dyes were experimentally investigated, and 36 photo-CIDNP active quenchers were revealed. Exploration of many different dye-quencher combinations with four dyes revealed that molecular diffusion significantly affects the Δg dependence of photo-CIDNP enhancement of each dye. These findings provide important insights into pioneering new dye-quencher combinations suitable for biological and medical applications.