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Low-spin ground states and low-lying excited states of higher spin were investigated for four pyrene oxoverdazyl monoradicals 14 and eight pyrene dioxoverdazyl diradicals 512. The ground states for quartet and quintet spin symmetries that are in reality excited states were found in the region of 565-775 nm above the respective electronic ground states. We calculated the adiabatic magnetic exchange coupling constant in the electronic ground state of each isolated biradical (5-12) by unrestricted density functional theory. A number of hybrid functionals such as B3LYP, PBE0, M06, and M06-2X were used. We also used range-separated functionals such as LC-omega PBE and omega B97XD to compare their effects on the coupling constant and the relative energy of the high-spin state. Molecular geometries were optimized for the doublet and quartet spin states of every monoradical (14), and the broken symmetry and triplet solutions were optimized for every biradical (512), by systematically using 6-311G, 6-311G(d,p), and 6-311++G(d,p) basis sets with each functional. The geometry of each quintet diradical (512) was optimized using 6-311G basis set. B3LYP produced the best spin values. The excited state (quartet or quintet)ground state energy difference (Delta E) increases in the presence of para-phenylene connectors. These energy differences were predicted here. The nature of spin coupling and consequently the ground state spin agree with spin alternation rule and the calculated atomic spin population. The adiabatic coupling constants were predicted for the biradicals (5-12) in their electronic ground states. Electron paramagnetic resonance parameters were determined at 6-311++G** level for the ground state and the quartet state of 1 and compared with the available experimental data. Low-lying excited states were found for the radical center (oxoverdazyl), pyrene, molecule 1, and diradical 5 by time-dependent density functional theory (TDDFT) method using B3LYP hybrid, 6-311++G(d,p) basis set, and the molecular geometry in the electronic ground state. Data from these calculations were used to discuss possible mechanisms for the achievement of the high-spin (excited) states in 1 and 5 and to predict a similar outcome for radicals 24 and 612 upon excitation. A comprehensive mechanism for the first excitation is proposed here. In particular, we show that the initial excitation of 1 involves large contributions from mixed transitions between pyrene and oxoverdazyl moieties, whereas the initial excitation of 5 is basically that of only the pyrene fragment. Subsequent internal conversion and intersystem crossing are likely to lead to the high-spin states of lower energy. Sample spin-flip TDDFT calculations were also done to confirm the energetic location and composition of the quartet state of 1 and the quintet state of 5.

Low sensitivity is the primary limitation to extendingnuclearmagnetic resonance (NMR) techniques to more advanced chemical andstructural studies. Photochemically induced dynamic nuclear polarization(photo-CIDNP) is an NMR hyperpolarization technique where light isused to excite a suitable donor-acceptor system, creating aspin-correlated radical pair whose evolution drives nuclear hyperpolarization.Systems that exhibit photo-CIDNP in solids are not common, and thiseffect has, up to now, only been observed for C-13 and N-15 nuclei. However, the low gyromagnetic ratio and naturalabundance of these nuclei trap the local hyperpolarization in thevicinity of the chromophore and limit the utility for bulk hyperpolarization.Here, we report the first example of optically enhanced solid-state H-1 NMR spectroscopy in the high-field regime. This is achievedvia photo-CIDNP of a donor-chromophore-acceptor moleculein a frozen solution at 0.3 T and 85 K, where spontaneous spin diffusionamong the abundant strongly coupled H-1 nuclei relays polarizationthrough the whole sample, yielding a 16-fold bulk H-1 signalenhancement under continuous laser irradiation at 450 nm. These findingsenable a new strategy for hyperpolarized NMR beyond the current limitsof conventional microwave-driven DNP.