15 Search Results

Two-dimensional materials have unusual properties and promise applications in nanoelectronics, spintronics, photonics, (electro)catalysis, separations, and elsewhere. Most are inorganic and their properties are difficult to tune. Here we report the preparation of Zn porphene, a member of the previously only hypothetical organic metalloporphene family. Similar to graphene, these also are fully conjugated two-dimensional polymers, but are composed of fused metalloporphyrin rings. Zn porphene is synthesized on water surface by two-dimensional oxidative polymerization of a Langmuir layer of Zn porphyrin with K₂IrCl₆, reminiscent of known one-dimensional polymerization of pyrroles. It is transferable to other substrates and bridges μm-sized pits. Contrary tomore » previous theoretical predictions of metallic conductivity, it is a p-type semiconductor due to a predicted Peierls distortion of its unit cell from square to rectangular, analogous to the appearance of bond-length alternation in antiaromatic molecules. The observed reversible insertion of various metal ions, possibly carrying a fifth or sixth ligand, promises tunability and even patterning of circuits on an atomic canvas without removing any π centers from conjugation.« less

Symmetry breaking charge transfer (SBCT) is a process in which a pair of identical chromophores absorb a photon and use its energy to transfer an electron from one chromophore to the other, breaking the symmetry of the chromophore pair. This excited state phenomenon is observed in photosynthetic organisms where it enables efficient formation of separated charges that ultimately catalyze biosynthesis. SBCT has also been proposed as a means for developing photovoltaics and photocatalytic systems that operate with minimal energy loss. It is known that SBCT in both biological and artificial systems is in part made possible by the local environmentmore » in which it occurs, which can move to stabilize the asymmetric SBCT state. However, how environmental degrees of freedom act in concert with steric and structural constraints placed on a chromophore pair to dictate its ability to generate long-lived charge pairs via SBCT remain open topics of investigation. In this work we compare a broad series of dipyrrin dimers that are linked by distinct bridging groups to discern how the spatial separation and mutual orientation of linked chromophores and the structural flexibility of their linker each impact SBCT efficiency. Across this material set, we observe a general trend that SBCT is accelerated as the spatial separation between dimer chromophores decreases, consistent with the expectation that the electronic coupling between these units varies exponentially with their separation. However, one key observation is that the rate of charge recombination following SBCT was found to slow with decreasing interchromophore separation, rather than speed up. This stems from an enhancement of the dimers’ structural rigidity due to increasing steric repulsion as the length of their linker shrinks. This rigidity further inhibits charge recombination in systems where symmetry has already enforced zero HOMO-LUMO overlap. Additionally, for the forward transfer the active torsion is shown to increase LUMO-LUMO coupling, allowing for faster SBCT within bridging groups. Further, by understanding trends for how rates of SBCT and charge recombination depend on a dimer’s internal structure and their environment, we identify design guidelines for creating artificial systems for driving sustained light-induced charge separation. Such systems can find application in solar energy technologies and photocatalytic applications but can serve as a model for light-induced charge separation in biological systems.« less

Pyridine-terminated molecular rods and either (i) the cis-(dppp)(I)Pt(C≡C-triptycene-C≡C)Pt(I)(dppp) rod or (ii) the trans-(PEt₃)₂(I)Pt(C≡C-biphenyl-C≡C)Pt(I)(PEt₃)₂ rod assemble into macrocycles, characterized by NMR, ESI-IMS, and in two cases also single-crystal X-ray diffraction. The former form rectangles with bidentate phosphine-containing cis-coordinated Pt(II)-alkyne corners. In the latter, the preference of the Pt centers for a trans configuration overrules the preference of the triple bonds for linearity and NMR shows that they have oval structures with alternating bent rod and bent trans (C≡C)Pt(PEt₃)₂(C≡C) components, in agreement with density functional theory calculations.

Here, we report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well bemore » produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. Here, in order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model.« less

Diaminoquinones with a captodatively stabilized biradicaloid structure are options for singlet fission, but few such compounds are known. We report the solution spectroscopy and photophysics of 1,2,2,3-tetramethyl-2,3-dihydro-1H-naphtho[2,3-d]imidazole-4,9-dione (1): its steady-state and transient UV-visible absorption, linear dichroism in stretched poly(vinyl alcohol), and magnetic circular dichroism. We also describe the absorption spectra of the stable radical ions 1+ and 1- and of two parent structures, 2,3-diamino-1,4-naphthoquinone (2) and 2,3-bis(methylamino)-1,4-naphthoquinone (3). The spectra are interpreted and electronic transitions are assigned by comparison with the results of density functional theory and MS-CASPT2 calculations.

Here, a procedure is described for unbiased identification of all π-electron chromophore pair geometry choices that locally maximize the rate of conversion of a singlet exciton into a singlet biexciton (triplet pair), using a simplified version of the diabatic frontier orbital model of singlet fission (SF).

A simplified version of the frontier orbital model has been applied to pairs of C₂, C_2v, C_s, and C₁ symmetry 1,3-diphenylisobenzofuran rotamers to determine their best packing for fast singlet fission (SF). For each rotamer the square of the electronic matrix element for SF was calculated at 2.2 × 10⁹ pair geometries and a few thousand most significant physically accessible local maxima were identified in the six-dimensional space of mutual arrangements. At these pair geometries, SF energy balance was evaluated, relative SF rate constants were approximated using Marcus theory, and the SF rate constant k_SF was maximized by further optimizationmore » of the geometry of the molecular pair. The process resulted in 142, 67, 214, and 291 unique geometries for the C₂, C_2v, C_s, and C₁ symmetry molecular pairs, respectively, predicted to be superior to the C₂ symmetrized known crystal pair structure. Furthermore, these optimized pair geometries and their triplet biexciton binding energies are reported as targets for crystal engineering and/or covalent dimer synthesis, and as possible starting points for high-level pair geometry optimizations.« less

The previously reported (Duman et al., J. Org. Chem. 2012, 77, 4516) calculated state energies of monomeric difluoroborondipyrromethene (BODIPY) and its axial dimer would suggest that these dyes are promising candidates for singlet fission, and the dimer was computed to have an unusual low-lying doubly excited state. We find that these results were affected by the use of an imbalanced active space in multireference calculations and are not correct. Multistate complete-active-space second-order perturbation theory (MS-CASPT2/cc-pVDZ) calculations using an [8,8] (8 electrons in 8 orbitals) active space for the monomer and a [16,16] active space for the dimer reproduce quite wellmore » the observed excitation energies of the S₁ states of both, and yield T₁ excitation energies well in excess of half of the S₁ excitation energies. We conclude that neither BODIPY monomer nor its axial dimer would permit exothermic singlet fission and are not worthy of investigation as potentially useful candidates, and that the unusual low-energy doubly excited states of the dimer were artifacts.« less

We report the electron paramagnetic resonance spectra of the radical cation and radical anion of 1,2,2,3-tetramethyl-2,3-dihydro-1H-naphtho[2,3-d]imidazole-4,9-dione (1) and its doubly ¹³C labeled analogue 2, of interest for singlet fission. The hyperfine coupling constants are in excellent agreement with density functional theory calculations and establish the structures beyond doubt. Unlike the radical cation 1^•+, the radical anion 1^•– and its parent 1 have pyramidalized nitrogen atoms and inequivalent methyl groups 15 and 16, in agreement with the calculations. The distinction is particularly clear with the labeled analogue 2^•–.