On-Surface Synthesis and Characterization of a High-Spin Aza-[5]-Triangulene.

Triangulenes are a class of open-shell triangular graphene flakes with total spin increasing with their size. In the last years, on-surface-synthesis strategies have permitted fabricating and engineering triangulenes of various sizes and structures with atomic precision. However, direct proof of the increasing total spin with their size remains elusive. In this work, we report the combined in-solution and on-surface synthesis of a large nitrogen-doped triangulene (aza-[5]-triangulene) on a Au(111) surface, and the detection of its high-spin ground state. Bond-resolved scanning tunneling microscopy images uncovered radical states distributed along the zigzag edges, which were detected as weak zero-bias resonances in scanning tunneling spectra. These spectral features reveal the partial Kondo screening of a high-spin state. Through a combination of several simulation tools, we find that the observed distribution of radical states is explained by a quintet ground state (S=2), instead of the quartet state (S=3/2) expected for the neutral species. This confirms that electron transfer to the metal substrate raises the spin of the ground state. We further provide a qualitative description of the change of (anti)aromaticity introduced by N-substitution, and its role in the charge stabilization on a surface, resulting in an S=2 aza-triangulene on Au(111).

Near-Unity Triplet Generation Promoted via Spiro-Conjugation.

An intersystem crossing (ISC) rate constant of 1.0×1011  s-1 was previously registered with a spiro-bis-benzophenone scaffold. Triplet generation efficiency could be further enhanced by stabilizing the spiro-charge-transfer (CT) state and rationally designing spiro-compounds (SCTs) that consist of electron-rich diphenyl ether as the spiro-CT donor and electron-deficient dinaphthyl ketone as the spiro-CT acceptor. Through fine-tuning of the energy level between the CT and high energy triplet states, near-unity triplet generation quantum yield was achieved and the underlying ISC mechanism is revealed by using ultrafast spectroscopy and quantum chemical calculations. Potential triplet sensitizing application was demonstrated in SCTs. Our findings suggest that a spiro-bichromophoric molecular system with an enhanced spiro-charge transfer warrants efficient triplet generation and is a powerful strategy of heavy-atom-free triplet sensitizers with predictable ISC properties.

Engineering the Charge-Transfer State to Facilitate Spin-Orbit Charge Transfer Intersystem Crossing in Spirobis[anthracene]diones.

Spiro conjugation has been proposed to dictate the efficiency of charge transfer, which could directly affect the spin-orbit charge transfer intersystem crossing (SOCT-ISC) process. However, this process has yet to be exemplified. Herein, we prepared three spirobis[anthracene]diones, in which two benzophenone moieties are locked in close proximity and differentially functionalized to fine-tune the charge transfer state. Its feasibility for SOCT-ISC was theoretically predicted, then experimentally evaluated. Through fine-tuning the spiro conjugation coupling and varying the solvent dielectric constants, ISC rate constants were engineered to vary in a dynamic range of three orders of magnitude, from 7.8×108  s-1 to 1.0×1011  s-1 , which is the highest ISC rate reported for SOCT-ISC system to our knowledge. Our findings substantiate the key factors for effective SOCT-ISC and offer a new avenue for the rational design of heavy atom free triplet sensitizers.

Triangular graphene nanofragments: open-shell character and doping.

In this work we study the intricacies of the electronic structure properties of triangular graphene nanofragments (TGNFs) in their ground and low-lying excited states by means of ab initio quantum chemistry calculations. We focus our attention on the radical and diradical characters of phenalenyl and triangulene, the smallest members of the TGNF family, and we describe the nature of their low-lying excited states. Moreover, we rationalize the modulation of the electronic and magnetic properties by means of selective boron or nitrogen substitution of carbon sites and by hydrogen saturation. The obtained results aim to guide future design of graphene-based materials with well-defined properties.

Singlet fission in spiroconjugated dimers.

Spiroconjugation results in a unique arrangement of conjugated fragments providing a novel way to chemically connect chromophoric units and control their electronic interaction, which is a key factor for the viability of the singlet fission photophysical reaction. In this study, we computationally explore the possibility of intramolecular singlet fission in spiroconjugated dimers by characterizing the nature of the low-lying excited electronic states, evaluating the magnitude of interstate couplings, describing possible singlet fission mechanisms, and investigating the potential role of low and high frequency vibrational modes in the exciton fission process. The spiro linkage of organic chromophores with the proper excited singlet and triplet energies favors the presence of low-lying charge resonance states, which play a major role in the formation of the triplet pair state. Overall, our results suggest that spiroconjugated dimers are potentially good candidates to efficiently generate independent triplet states through singlet fission.

Thieno[3,4-c]pyrrole-4,6-dione Oligothiophenes Have Two Crossed Paths for Electron Delocalization.

A new series of electron-deficient oligothiophenes, thieno[3,4-c]pyrrole-4,6-dione oligothiophenes (OTPDn ), from the monomer to hexamer, is reported. The optical and structural properties in the neutral states have been analyzed by absorption and emission spectroscopy together with vibrational Raman spectroscopy. In their reduced forms, these molecules could stabilize both anions and dianions in similar ways. For the dianions, two independent modes of electron conjugation of the charge excess were observed: the interdione path and the interthiophene path. The interference of these two paths highlighted the existence of a singlet diradical ground electronic state and the appearance of low-energy, thermally accessible triplet states. These results provide valuable insights into the device performance of TPD-based materials and for the rational design of new high-performance organic semiconductors.