CuAAC-based assembly and characterization of a ruthenium-copper dyad containing a diimine-dioxime ligand framework.

The design of molecular dyads combining a light-harvesting unit with an electroactive centre is highly demanded in the field of artificial photosynthesis. The versatile Copper-catalyzed Azide-Alkyne Cycloaddition (CuAAC) procedure was employed to assemble a ruthenium tris-diimine unit to an unprecedented azide-substituted copper diimine-dioxime moiety. The resulting RuIICuII dyad 4 was characterized by electrochemistry, 1H NMR, EPR, UV-visible absorption, steady-state fluorescence and transient absorption spectroscopies. Photoinduced electron transfer from the ruthenium to the copper centre upon light-activation in the presence of a sacrificial electron donor was established thanks to EPR-monitored photolysis experiments, opening interesting perspectives for photocatalytic applications.

Molecular excited states: accurate calculation of relative energies and electronic coupling between charge transfer and non-charge transfer states.

We show for a series of six small donor-acceptor dyads that the energy difference between non-charge transfer (non-CT) and charge transfer (CT) excited states, as well as the squares of the electronic couplings between these states, can be predicted from first-principles using variational orbital adapted configuration interaction singles (VOA-CIS) theory. VOA-CIS correctly predicts the observed experimental trends in these values and provides quantitative accuracy roughly on par with a modern long-range corrected density functional, ωB97X. Using VOA-CIS and ωB97X, the experimental energy difference between the non-CT and CT excited states is predicted with root mean squared errors of 0.22 eV and 0.21 eV, respectively. The square of the electronic coupling between these states is predicted with root mean squared errors of 0.08 eV(2) and 0.07 eV(2), respectively. Orbital optimized CIS (OO-CIS) and CIS(D), two perturbative corrections to CIS, provide a significant correction to the errant relative energies predicted by CIS, but the correction is insufficient to recover the experimentally observed trend.

Singlet exciton fission in polycrystalline thin films of a slip-stacked perylenediimide.

The crystal structure of N,N-bis(n-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide), 1, obtained by X-ray diffraction reveals that 1 has a nearly planar perylene core and π-π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π-π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of 1 results in a 140 ± 20% yield of triplet excitons ((3*)1) in τ(SF) = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis.

Communication: Adjusting charge transfer state energies for configuration interaction singles: without any parameterization and with minimal cost.

In a recent article, we showed that configuration interaction singles (CIS) has a systematic bias against charge-transfer (CT) states: CT vertical excitation energies are consistently too high (by 1-2 eV) as compared with non-CT energies [J. E. Subotnik, J. Chem. Phys. 137, 071104 (2011)]. We now show that this CIS error can be corrected approximately by performing a single Newton-Raphson step to reoptimize orbitals, thus establishing a new set of orbitals which better balances ground and excited state energies. The computational cost of this correction is exactly that of one coupled-perturbed Hartree-Fock calculation, which is effectively the cost of the CIS calculation itself. In other words, for twice the computational cost of a standard CIS calculation, or roughly the same cost as a linear-response time-dependent Hartree-Fock calculation, one can achieve a balanced, size-consistent description of CT versus non-CT energies, ideally with the accuracy of a much more expensive doubles CIS(D) calculation.

Vibrational Dynamics of a Perylene-Perylenediimide Donor-Acceptor Dyad Probed with Femtosecond Stimulated Raman Spectroscopy.

The ultrafast vibrational dynamics of the photoinduced charge-transfer reaction between perylene (Per) and perylene-3,4:9,10-bis(dicarboximide) (PDI) were investigated using femtosecond stimulated Raman spectroscopy (FSRS). Specifically probing the structural dynamics of PDI following its selective photoexcitation in a covalently linked dyad reveals vibrational modes uniquely characteristic to the PDI lowest excited singlet state and radical anion between 1000 and 1700 cm(-1). A comparison of these vibrations to those of the ground state reveals the appearance of new (1*)PDI and PDI(-•) stretching modes in the dyad at 1593 and 1588 cm(-1), respectively. DFT calculations reveal that these vibrations are parallel to the long axis of PDI and thus then may be integral to the charge separation reaction. The ability to differentiate excited state from radical anion vibrational modes allows the evaluation of the influence of specific modes on the charge transfer dynamics in donor-bridge-acceptor systems based on PDI molecular constructs.