Luminescent organic radicals are an emerging class of molecular semiconductors which exhibit many unique properties attractive for optoelectronic and spintronic devices. In this thesis, we employ optical and spin-based probes to reveal the dynamics of photogenerated excitons in a selection of novel tris(2,4,6-trichlorophenyl)-methyl (TTM)-based radicals.

The first three chapters present a motivation, relevant theory and methodology.

In Chapter 4, we focus on the intrinsic properties of luminescent doublet (S=1/2) states. We find evidence of intermolecular charge transfer excitations which drastically alter the emission spectrum and lifetime in thin films.

In Chapter 5, we investigate solid state intermolecular interactions between radicals and triplet (S=1) states on closed-shell materials, and show their management can lead to improvements in Organic Light Emitting Diode (OLED) performance. For the first time we observe cycling between the triplet and doublet manifolds, and direct energy transfer on sub-nanosecond timescales.

In Chapter 6, we present the first organic molecules which can reversibly access a quartet (S=3/2) excited state. This is achieved by engineering strong exchange coupling between resonant radical and triplet manifolds in covalently linked structures. The resulting high-spin states are coherently addressable with microwaves even at 295 K, with optical read-out enabled by intersystem crossing to the energetically accessible radical state.

In Chapter 7, we extend these results to a luminescent biradical structure which supports a quintet (S=2) excited state. The light-induced cycling through this state drastically increases the strength of the exchange coupling between the two radical spins, and leads to a long-lived ground-state polarisation.

The findings and models developed in this thesis open a path to few functionalities for open-shell semiconductors, as outlined in Chapter 8, ranging from improved light emission to molecular quantum information science.