Daniel G. Oblinsky

Exciton delocalization drives rapid singlet fission in nanoparticles of acene derivatives.

We compare the singlet fission dynamics of five pentacene derivatives precipitated to form nanoparticles. Two nanoparticle types were distinguished by differences in their solid-state order and kinetics of triplet formation. Nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal and exhibit nonexponential triplet formation kinetics (Type I), while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal and triplet formation kinetics associated with the intrinsic singlet fission rates (Type II). In the highly ordered nanoparticles, singlet fission occurs most rapidly. We relate the molecular packing arrangement derived from the crystal structure of the pentacene derivatives to their singlet fission dynamics and find that slip stacking leads to rapid, subpicosecond singlet fission. We present evidence that exciton delocalization, coincident with an increased relative admixture of charge-transfer configurations in the description of the exciton wave function, facilitates rapid triplet pair formation in the case of single-step singlet fission. We extend the study to include two hexacene derivatives and find that these conclusions are generally applicable. This work highlights acene derivatives as versatile singlet fission chromophores and shows how chemical functionalization affects both solid-state order and exciton interactions and how these attributes in turn affect the rate of singlet fission.

Single-residue insertion switches the quaternary structure and exciton states of cryptophyte light-harvesting proteins.

Significance There is intense interest in determining whether coherent quantum processes play a nontrivial role in biology. This interest was sparked by the discovery of long-lived oscillations in 2D electronic spectra of photosynthetic proteins, including the phycobiliproteins (PBPs) from cryptophyte algae. Using X-ray crystallography, we show that cryptophyte PBPs adopt one of two quaternary structures, open or closed. The key feature of the closed form is the juxtaposition of two central chromophores resulting in excitonic coupling. The switch between forms is ascribed to the insertion of a single amino acid in the open-form proteins. Thus, PBP quaternary structure controls excitonic coupling and the mechanism of light harvesting. Comparing organisms with these two distinct proteins will reveal the role of quantum coherence in photosynthesis. Observation of coherent oscillations in the 2D electronic spectra (2D ES) of photosynthetic proteins has led researchers to ask whether nontrivial quantum phenomena are biologically significant. Coherent oscillations have been reported for the soluble light-harvesting phycobiliprotein (PBP) antenna isolated from cryptophyte algae. To probe the link between spectral properties and protein structure, we determined crystal structures of three PBP light-harvesting complexes isolated from different species. Each PBP is a dimer of αβ subunits in which the structure of the αβ monomer is conserved. However, we discovered two dramatically distinct quaternary conformations, one of which is specific to the genus Hemiselmis. Because of steric effects emerging from the insertion of a single amino acid, the two αβ monomers are rotated by ∼73° to an “open” configuration in contrast to the “closed” configuration of other cryptophyte PBPs. This structural change is significant for the light-harvesting function because it disrupts the strong excitonic coupling between two central chromophores in the closed form. The 2D ES show marked cross-peak oscillations assigned to electronic and vibrational coherences in the closed-form PC645. However, such features appear to be reduced, or perhaps absent, in the open structures. Thus cryptophytes have evolved a structural switch controlled by an amino acid insertion to modulate excitonic interactions and therefore the mechanisms used for light harvesting.

Designs for molecular circuits that use electronic coherence

The mounting evidence of recent years regarding long-lived coherent dynamics of electronic excitations in several light-harvesting antenna proteins suggests the possibility of realizing and exploiting light-initiated quantum dynamics in synthetic molecular devices based on electronic energy transfer. Inspired by the field of molecular logic, we focus this discussion on the prospect of using quantum coherence to control the direction of energy flow in a molecular circuit. As a prototype system we consider a circuit consisting of three chromophores that deliver energy to two trap chromophores. Our aim is to control to which trap the energy is more likely to be delivered. This is achieved by switching one of the circuit chromophores ON and OFF from the system, such that the direction of energy flow substantially changes from the ON and OFF states of the circuit. We find that quantum coherence can allow a significant ability to direct energy transfer in the circuit. However, when realistic levels of noise are added, quantum coherence only slightly improves the ability to direct electronic energy in comparison to a classical hopping mechanism.

Charge Localization after Ultrafast Photoexcitation of a Rigid Perylene Perylenediimide Dyad Visualized by Transient Stark Effect

The intramolecular charge-transfer (CT) dynamics of a rigid and strongly conjugated perylenediimide-bridge-perylene dyad (PDIPe) has been investigated in dichloromethane using ultrafast transient electronic absorption spectroscopy and quantum chemical calculations. The strong electronic coupling between the dyad units gives rise to a CT band. Its photoexcitation forms a delocalized CT state with well-preserved ion bands despite the strong coupling. In the dyad, the electronic transition dipole moment of the electron donor perylene is aligned along the axis of the electric field vector with respect to the CT species. This alignment makes the donor sensitive to the Stark effect and thus charge density fluctuations in the CT state. Charge localization on the picosecond time scale manifests as a time-dependent Stark shift in the visible region. Quantum chemical calculations reveal a twist around the acetylene bridging unit to be the responsible mechanism generating a partial to an almost complete CT state. An estimate of the electric field strength in the CT state yields approximately 25 MV/cm, which increases to around 31 MV/cm during charge localization. Furthermore, the calculations illustrate the complexity of electronic structure in this strongly delocalized superchromophore and reflect the complications in the interpretation of transient absorption results when compared to steady-state approaches such as spectroelectrochemistry and model chromophore experiments such as photoinduced bimolecular charge transfer.