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Despite the long history of spectroscopic studies of the C$$_2$$ molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C$$_2$$ is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here to disentangle the numerous configuration interactions within four symmetry manifolds of C$$_2$$ ($$^{1}\Pi_g$$, $$^{3}\Pi_g$$, $$^{1}\Sigma_u^+$$, and $$^{3}\Sigma_u^+$$). The key concept of our model is the existence of two "valence-hole" configurations, $$2\sigma_g^22\sigma_u^11\pi_{u}^33\sigma_g^2$$ for $$^{1,3}\Pi_g$$ states and $$2\sigma_g^22\sigma_u^11\pi_{u}^43\sigma_g^1$$ for $$^{1,3}\Sigma_u^+$$ states that derive from $$3\sigma_g\leftarrow2\sigma_u$$ electron promotion. The lowest-energy state from each of themore » four C$$_2$$ symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. Finally, as a result of their large binding energy (nominal bond order of 3) and correlation with the 2s$^2$2p$^2$+2s2p$^3$ separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the $global$ electronic structure and unimolecular dynamics of C$$_2$$.« less

The dicarbon molecule (C2) is found in flames, comets, stars, and the diffuse interstellar medium. In comets, it is responsible for the green color of the coma, but it is not found in the tail. It has long been held to photodissociate in sunlight with a lifetime precluding observation in the tail, but the mechanism was not known. Here we directly observe photodissociation of C₂. From the speed of the recoiling carbon atoms, a bond dissociation energy of 602.804(29) kJ∙mol^-1 is determined, with an uncertainty comparable to its more experimentally accessible N² and O₂ counterparts. The value is within 0.03more » kJ∙mol^-1 of high-level quantum theory. This work shows that, to break the quadruple bond of C₂ using sunlight, the molecule must absorb two photons and undergo two “forbidden” transitions.« less

Here, the performance of a perylene monoimide annihilator is evaluated in a photochemical upconversion composition. It is found to perform up to five times better than the commonly employed rubrene annihilator at low excitation intensity, but suffers from a low annihilation singlet yield which hinders its performance under strong excitation. Upconversion action spectroscopy under broadband bias reveals that under one sun illumination, an upconversion composition employing the perylene monoimide utilizes more than 12% of the generated triplet states to generate emissive, excited singlet states. In a suitable medium, this composition could enhance the energy conversion efficiency of high band gapmore » solar cells.« less

In this Viewpoint, we propose an updated labeling scheme for describing quantum yield in TTA-UC so as to better facilitate communication and comparisons across applications, materials, and intervening events and to introduce more consistency in the scientific literature.