"Doping" pentacene with sp(2)-phosphorus atoms: towards high performance ambipolar semiconductors

Theoretical design of phosphorus-doped perylene derivatives as efficient singlet fission chromophores

Singlet fission (SF) is considered as a promising strategy to overcome the Shockley-Queisser limit of single-junction solar cells. However, only a handful of chromophores were observed to undergo SF to date. To broaden the number of SF chromophores, we designed a series of phosphorus-doped perylenes based on the diradical character strategy and examined their SF feasibility using theoretical calculations. By analysis of frontier orbitals, diradical character and aromaticity, SF-capable candidates were prescreened. These analyses reveal that the diradical character of perylene is effectively enhanced by P-doping at bay- and peri-positions of perylene, making SF more thermodynamically feasible. However, the diradical character remains nearly unchanged when P atoms are doped at ortho-positions because the spin center cannot be stabilized, leading to a more endothermic SF. This study shows how SF-related energies and diradical character of SF chromophores are altered by P doping, and extends the SF-capable molecular library.

Design and Synthesis of Far-Red to Near-Infrared Chromophores with Pyrazine-Based Boron Complexes

We synthesized new binuclear boron complexes based on pyrazine with ortho and para substitution patterns. It was demonstrated that the para-linked complexes possess a significantly narrow energy gap between highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), leading to their far-red to near-infrared emission properties. Meanwhile, the ortho-substituted complex showed orange emission. Considering the HOMO and LUMO distributions of pyrazine, the boron complexation to the nitrogen atoms would stabilize its LUMO more efficiently than its HOMO because a nodal plane in the HOMO passes through the two nitrogen atoms. The theoretical study suggests that the para-substitution would not significantly perturb such a characteristic HOMO distribution originating from pyrazine in stark contrast to the ortho-substituted one. As a result, the HOMO-LUMO gap of the para-linked complex is dramatically narrower than that of the ortho-linked one.

Phosphaacene as a structural analogue of thienoacenes for organic semiconductors

An air-stable λ³-phosphinine-containing polycyclic aromatic compound without steric protection was synthesized and its charge transport properties were evaluated, which revealed moderate hole mobility. This research is the first experimental demonstration of the organic electronic applications of low-coordinate phosphorus compounds.

Rapid Computational Approach Towards Designing Singlet-Fission Chromophores by Tuning the Diradical Character of Heteroatom-Doped Polycyclic Aromatic Hydrocarbons Using the Atom-Specific Fukui Function

Singlet fission (SF) is proposed as a promising method to circumvent the Shockley-Queisser threshold of single junction photovoltaics. Progress towards realizing efficient SF-based devices has been impeded by the fact that only a handful of molecules and their derivatives practically exhibit efficient SF. In the present work, we demonstrate a TDDFT-based rapid and cost-effective computational approach for designing SF chromophores by doping various atomic sites (substituting carbon atoms) of polycyclic aromatic hydrocarbons with nitrogen, phosphorus, and silicon. We establish a hitherto unexplored, direct correlation between the atom-specific chemical reactivity parameter─Fukui function─of these molecules with their frontier molecular orbital energies, diradical characters, and vertical singlet and triplet excitation energies. These quantitative correlations show exactly opposite trends for nitrogen-doped molecules and phosphorus- or silicon-doped molecules. The doped derivatives that have the Fukui function falling in a range of 0.03-0.14 possess the required intermediate diradical character and suitable singlet-triplet energies to qualify for SF candidature. Our findings enable one, at reasonable computational times and cost, to easily assess the doping criteria and to develop design rules for SF molecules in particular and for diradicaloids in general.

The chemical space of B, N-substituted polycyclic aromatic hydrocarbons: Combinatorial enumeration and high-throughput first-principles modeling

Combinatorial introduction of heteroatoms in the two-dimensional framework of aromatic hydrocarbons opens up possibilities to design compound libraries exhibiting desirable photovoltaic and photochemical properties. Exhaustive enumeration and first-principles characterization of this chemical space provide indispensable insights for rational compound design strategies. Here, for the smallest seventy-seven Kekulean-benzenoid polycyclic systems, we reveal combinatorial substitution of C atom pairs with the isosteric and isoelectronic B, N pairs to result in 7 453 041 547 842 (7.4 tera) unique molecules. We present comprehensive frequency distributions of this chemical space, analyze trends, and discuss a symmetry-controlled selectivity manifestable in synthesis product yield. Furthermore, by performing high-throughput ab initio density functional theory calculations of over thirty-three thousand (33k) representative molecules, we discuss quantitative trends in the structural stability and inter-property relationships across heteroarenes. Our results indicate a significant fraction of the 33k molecules to be electronically active in the 1.5-2.5 eV region, encompassing the most intense region of the solar spectrum, indicating their suitability as potential light-harvesting molecular components in photo-catalyzed solar cells.

Synthesis, Crystal Analysis, and Optoelectronic Properties of Diazole-Functionalized Acenes and Azaacenes

Doping heteroatoms into the skeletons of parent acenes can provide more opportunities to construct novel thermally and photostable organic π-conjugated semiconductors. Herein, a family of diazole-decorated acenes (APyS and APySe) and azaacenes (PyP, PyTh, PyPy, PyPh, and PyAP) have been successfully synthesized through the classical reactions. Single-crystal X-ray analyses showed that these as-formed diazole-modified derivatives adopted a twisted topology configuration, whereas the azaacenes display reclining-chair architectures, besides a twisted structure. All these compounds displayed yellow or red light in solution. Moreover, their electrochemical behaviors were also examined. We also found that the azaacenes exhibited a positive spectroscopic response to acid.

CF3 -Inspired Synthesis of Air-Tolerant 9-Phosphaanthracenes that Feature Fluorescence and Crystalline Polymorphs

9-Phosphaanthracene (dibenzo[b,e]phosphorin, acridophosphine) has attracted interest as one of the heavier acenes. Herein, we demonstrate an efficient synthetic process that provides air-tolerant 1,8-bis(trifluoromethyl)-9-phosphaanthracenes. The sterically encumbered and electron-withdrawing trifluoromethyl (CF₃ ) groups are quite advantageous not only to stabilize the intrinsically unstable heavier unsaturated phosphorus atom but also to facilitate construction of the phosphinine skeleton based on a putative increase in aromaticity. The isolated 9-phosphaanthracenes allowed characterization of their fluorescence functionality and planar heteroanthracene frameworks. The crystal structures of 9-phosphaanthracenes are remarkably dependent on the aryl substituents at the 10 position; anthryl-substituted 9-phosphaanthracene showed unique polymorphs that induced different-colored crystals.