Excited-state electronic structure of molecules

5 problems

• Nature of the dark $S*$ state in carotenoids

  Contested

  Carotenoids, the orange pigments in carrots and plant leaves, have excited states that do not absorb light directly but appear in fast laser experiments. One of them, called S*, has been assigned to at least four different origins.

• A practical TDDFT kernel that captures double excitations

  Open

  The standard fast method for excited states, time-dependent density functional theory (TDDFT), describes only states in which one electron is promoted, so states with two electrons promoted are missing. These doubly excited states matter in polyenes, carotenoids and many photochemical pathways.

• Size and sign of inverted singlet-triplet gaps in heptazine molecules

  Partially resolved

  In almost all molecules the lowest triplet excited state lies below the lowest singlet, a pattern usually attributed to Hund's rule. A few nitrogen-rich molecules built on heptazine (a flat ring system of carbon and nitrogen) reverse this order, which could make organic light-emitting diodes more efficient.

• Excitation energies of large chromophores to 0.05 eV

  Open

  Reference excitation energies accurate to a few hundredths of an electronvolt exist for small molecules. For the larger dye and protein pigment molecules that matter in practice, methods still disagree by more than that.

• Mechanism of singlet fission and triplet-pair separation

  Contested

  In some organic crystals one absorbed photon can create two energy packets called triplets, which could raise solar-cell efficiency. How the two triplets form and then separate into independent ones is not fully understood.