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.