This thesis consists of 8 chapters and is organized into 3 parts: 1, 2 and 3 unpaired spins. Chapter 1 recapitulates the fundamental concepts and equations for electron spin in chemistry. In Chapter 2, two structures BDPA and BDX are introduced to the existing toolbox of stable radicals and their extraordinary magnetic, optical and redox properties are highlighted. These tailor-made radicals are responsible for the unprecedented results detailed in the following chapters. Chapter 3 presents an example of adverse environment for an unpaired spin, which becomes a probe into the local magnetic environment and structural dynamics of the cation radical.

In the second part, three regimes of the interaction between two unpaired spins are considered. When they are accommodated by the same site, the anisotropic dipolar interaction is significant, and Chapter 4 provides a general approach termed ternary plot to handle all possible types of anisotropies. As the two spins separate in space, they form a spin-correlated radical pair. Chapter 5 discusses how their EPR spectra are modulated in both frequency and time domains by the hyperfine interaction with local nuclear spins. Chapter 6 covers the third regime, where the two unpaired spins are located at a long yet fixed distance, just as in the photosynthetic reaction centers. The donor-acceptor distance is precisely determined by a pulse EPR technique called electron spin echo envelope modulation (ESEEM), and is corroborated by X-ray crystallography.

The situation becomes much more complicated with three unpaired spins. In Chapter 7, a stable free radical TEMPO is attached to a spin-correlated radical pair. The charge recombination product, a local triplet state, not only switches the spin basis set but also induced a strongly emissive polarization on the third spin. The ultimate chapter employs EPR spectroscopy to test the EPR paradox. A donor-chromophore-acceptor-radical tetrad is built to perform a spin teleportation experiment, in which one of the spin-correlated radical pair recombines spin-selectively with the third spin, thus transmitting the spin information to the receiver spin over a distance of 35 Å. Experimentally, a negative, linear correlation between the polarization of the sender and receiver spins are observed.