Singlet fission (SF) has the potential to significantly enhance the photocurrent in single-junction solar cells and thus raise the power conversion efficiency from the Shockley-Queisser limit of 33% to 44%. Until now, quantitative SF yield at room temperature has been observed only in crystalline solids or aggregates of oligoacenes. Here, we employ transient absorption spectroscopy, ultrafast photoluminescence spectroscopy, and triplet photosensitization to demonstrate intramolecular singlet fission (iSF) with triplet yields approaching 200% per absorbed photon in a series of bipentacenes. Crucially, in dilute solution of these systems, SF does not depend on intermolecular interactions. Instead, SF is an intrinsic property of the molecules, with both the fission rate and resulting triplet lifetime determined by the degree of electronic coupling between covalently linked pentacene molecules. We found that the triplet pair lifetime can be as short as 0.5 ns but can be extended up to 270 ns.

We report three characteristics of ideal thermally activated delayed fluorescence molecular systems apparent in carbene-metal-amides: (a) an exceptionally small singlet-triplet gap that effectively eliminates the thermal activation barrier to reverse intersystem crossing; (b) significant singlet oscillator strength promoting fluorescence in the region of this small barrier; and (c) enlarged spin-orbit coupling driving reverse intersystem crossing in this region. We carry out highly correlated quantum-chemical calculations to detail the relative energies of and spin-orbit couplings between the singlet and triplet states, finding that they fall closer together in energy and couple more strongly in going from the singlet ground-state to the triplet optimized geometry. This structural reorganization is defined not by rotation of the ligands but by a nontrivial bending of the carbene-metal-amide bond angle. This bending reduces carbene-metal-amide symmetry and enhances singlet-triplet interaction strength. We clarify that the reverse intersystem crossing triggering delayed fluorescence occurs around the coplanar triplet geometric optimum.

We report quantum chemical calculations using multireference perturbation theory (MRPT) with the density matrix renormalization group (DMRG) plus photothermal deflection spectroscopy measurements to investigate the manifold of carotenoid excited states and establish their energies relative to the bright state (S 2 ) as a function of nuclear reorganization. We conclude that the primary photophysics and function of carotenoids are determined by interplay of only the bright (S 2 ) and lowest-energy dark (S 1 ) states. The lowest-lying dark state, far from being energetically distinguishable from the lowest-lying bright state along the entire excited-state nuclear reorganization pathway, is instead computed to be either the second or first excited state depending on what equilibrium geometry is considered. This result suggests that, rather than there being a dark intermediate excited state bridging a non-negligible energy gap from the lowestlying dark state to the lowest-lying bright state, there is in fact no appreciable energy gap to bridge following photoexcitation. Instead, excited-state nuclear reorganization constitutes the bridge from S 2 to S 1 , in the sense that these two states attain energetic degeneracy along this pathway.

We analyze correlated-triplet-pair (TT) singletfission intermediates toward two-triplet separation (T...T) using spin-state-averaged density matrix renormalization group electronic-structure calculations. Specifically, we compare the triplet− triplet exchange (J) for tetracene dimers, bipentacene, a subunit of the benzodithiophene−thiophene dioxide polymer, and a carotenoid (neurosporene). Exchange-split energy gaps of J and 3J separate a singlet from a triplet and a singlet from a quintet, respectively. We draw two new insights: (a) the canonical tetracene singlet-fission unit cell supports precisely three lowlying TT intermediates with order-of-magnitude differences in J, and (b) the separable TT intermediate in carotenoids emanates from a pair of excitations to the second triplet state. Therefore, unlike with tetracenes, carotenoid fission requires above-gap excitations. In all cases, the distinguishability of the molecular triplets that is, the extent of orbital overlapdetermines the splitting within the spin manifold of TT states. Consequently, J represents a spectroscopic observable that distnguishes the resemblance between TT intermediates and the T...T product.

The nature of carbene-metal-amide (CMA) photoluminescence in the solid state is explored through spectroscopic and quantum-chemical investigations on a representative Au-centred molecule. The crystalline phase offers well-defined coplanar geometries-enabling the link between molecular conformations and photophysical properties to be unravelled. We show that a combination of restricted torsional distortion and molecular electronic polarisation blueshift the charge-transfer emission by around 400 meV in the crystalline versus the amorphous phase, through energetically raising the less-dipolar S 1 state relative to S 0 . This blueshift brings the lowest charge-transfer states very close to the localised carbazole triplet state, whose structured emission is observable at low temperature in the polycrystalline phase. Moreover, we discover that the rate of intersystem crossing and emission kinetics are unaffected by the extent of torsional distortion. We conclude that more coplanar triplet equilibrium conformations control the photophysics of CMAs.

The nature of intramolecular charge transfer (ICT) and the mechanism of intramolecular singlet fission (SF) in peridinin remain open research questions. Obtaining an understanding of the population evolution from the bright state to dark state following a photoinduced electronic transition is critical. Unambiguously describing this evolution in peridinin, and light-harvesting carotenoids in general, has proven elusive experimentally and computationally. To offer a balanced description of the bright- and dark-state electronic structures, we here apply ab initio multireference perturbation theory quantum chemistry-the density matrix renormalization group self-consistent field and complete-active-space self-consistent field with second-order N-electron valence perturbation theory. At traditional bright- (S) and dark-state (S) optimized geometries, we find that an additional correlated triplet pair state and ICT state are derived from the canonical polyene B (S) and 3A (S) dark singlet excited states, respectively. Whereas the S state's physical properties are insensitive to peridinin's allene-tail donor and lactone ring acceptor functionalization, the S state exhibits a markedly enhanced oscillator strength and highest occupied molecular orbital-lowest unoccupied molecular orbital transition density. These changes suggest that the ICT character stems from mixing between the bright S and putatively dark S.

It has long been recognized that visible light harvesting in Peridinin–Chlorophyll–Protein is driven by the interplay between the bright (S 2 ) and dark (S 1 ) states of peridinin (carotenoid), along with the lowest-lying bright (Q y ) and dark (Q x ) states of chlorophyll- a . Here, we analyse a chromophore cluster in the crystal structure of Peridinin–Chlorophyll–Protein, in particular, a peridinin–peridinin and a peridinin–chlorophyll- a dimer, and present quantum chemical evidence for excited states that exist beyond the confines of single peridinin and chlorophyll chromophores. These dark multichromophoric states, emanating from the intermolecular packing native to Peridinin–Chlorophyll–Protein, include a correlated triplet pair comprising neighbouring peridinin excitations and a charge-transfer interaction between peridinin and the adjacent chlorophyll- a . We surmise that such dark multichromophoric states may explain two spectral mysteries in light-harvesting pigments: the sub-200-fs singlet fission observed in carotenoid aggregates, and the sub-200-fs chlorophyll- a hole generation in Peridinin–Chlorophyll–Protein.

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