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Treatment of a κ1-P-monodentate bicyclic diphosphane iridium(iii) complex with a labile gold(i) precursor afforded an unusual IrIII/AuI complex in which the P-P single bond has been cleaved. This reaction was cleanly reversed upon addition of tertiary phosphine. Carbon-carbon bond activation, across neighbouring P2C2N rings of the coordinated bicyclic diphosphane, occurred upon thermolysis of the IrIII/AuI complex.

The ability to tune excited-state energies is crucial to many areas of molecular design. In many cases this is done based on the energies of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals. However, this viewpoint is incomplete neglecting the many-body nature of the underlying excited-state wavefunctions. Within this work we highlight the importance of two crucial terms, other than orbital energies, that contribute to the excitation energies and show how to quantify them from quantum chemistry computations: an attractive Coulomb and a repulsive exchange interaction. Using this framework, we explain under which circumstances the lowest excited state of a molecule, of either singlet or triplet multiplicity, is not accessed via the HOMO/LUMO transition and show two paradigmatic examples. In the case of the push-pull molecule ACRFLCN we highlight how the lowest triplet excited state is a locally excited state lying below the HOMO/LUMO charge transfer state due to enhanced Coulomb binding.In the case of the naphthalene molecule, we highlight how the HOMO/LUMO transition (the 1L_a state) becomes the second excited singlet state due to its enhanced exchange repulsion term. More generally, we explain why excitation energies do not always behave like orbital energy gaps, providing insight into photophysical processes as well as methodogical challenges in describing them.

Tuning the energies of molecular excited states is a central research theme in modern chemistry with high relevance for optoelectronic applications and chemical synthesis. Whereas frontier orbitals have proven to be an intuitive and simple model in many cases, they can only provide a very rough approximation of the underlying wavefunctions. The purpose of this Perspective is to explore how our qualitative understanding of electronic excitation processes can be promoted beyond the molecular orbital picture by exploiting methods and insights from modern quantum chemistry. For this purpose, the physics of a correlated electron-hole pair is analysed in detail to show the origin of exchange repulsion and a dynamic Coulomb attraction, which determine its energy aside from the orbital energies. Furthermore, we identify and discuss the two additional effects of secondary orbital relaxation and de-excitations. Rules for reconstructing these four contributions from general excited-state computations are presented and their use is exem- plified in three case studies concerned with the relative ordering of the singlet and triplet ππ∗ and nπ∗ states of uracil, the large energetic differences between the first singlet and triplet states of the polyacenes, and the assignment of plasmonic states in octatetraene. Finally, we lay out some general ideas for how the knowledge gained could ultimately lead to new design principles for tuning molecular excitation energies as well as for diagnosing possible shortcomings of commonly used electronic structure methods.

Thermally activated delayed fluorescence (TADF) is a current promising route for generating highly efficient light-emitting devices. However, the design process of new chromophores is hampered by the complicated underlying photophysics that requires a number of different pathways to be optimised simultaneously. In this work, four closely related donor-pi-acceptor-pi-donor systems have been investigated, two of which were synthesised previously, with the aim of elucidating their varying effectiveness for TADF. We, first, outline that neither the frontier orbitals nor the singlet-triplet gaps are sufficient in discriminating between the molecules. Subsequently, a detailed analysis of the excited states, performed at a correlated ab initio level, is shown highlighting the presence of a number of closely spaced singlet and triplet states of varying character. Five density functionals are benchmarked against this reference revealing dramatic changes in, both, excited state energies and wavefunctions following variations in the amount of Hartree-Fock exchange included. Excited-state minima are optimised in solution showing the crucial role of structural variations for stabilising locally excited and CT states and of symmetry breaking for producing a strongly emissive S1 state. More generally, this work shows how a detailed analysis of excited-state wavefunctions can provide critical new insight into excited-state electronic structure, helping to reveal the photophysics of existing push-pull chromophores and ultimately guiding the design of new ones.