Tuning Near-Infrared Absorbing Donor Materials: A Study of Electronic, Optical, and Charge-Transport Properties of aza-BODIPYs

The class of 4,4′-difluoro-4-bora-3a,4a,8-triaza-s-indacenes (aza-BODIPYs) are promising near-infrared absorber materials which are successfully used in organic solar cells to extend their absorption to the near-infrared regime. We computationally studied electronic properties, internal reorganization energies, and the optical properties of more than 100 promising candidates and derived design principles, including novel functionalization routes, to improve their performance as donor materials. We synthesized and characterized several of the promising molecules, confirming the predicted trends. The best charge transport properties and absorption characteristics are obtained for naphthalene-annulated molecular cores due to optimally delocalized frontier molecular orbitals. Further optimization can be achieved by α-functionalization with fluorinated groups, β-functionalization with accepting substituents, and modification of the borondifluoride group. For such molecules, we predict a bathochromic shift in the absorption, which should not significantly reduce the open-circuit voltage. Torsional restriction of α-substituents by carbon bridges can further improve both charge transport and absorption. The theoretically and experimentally observed independence of most of the functionalization strategies makes BODIPYs an ideal material class for tailor-made absorber materials that can cover a broad range of absorption, charge transport, and energetic regimes, calling for further exploration in organic solar cell applications, fluorescence microscopy, and photodynamic therapy

Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif

A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen–boron–nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized. Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilic bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP <b>5a</b> was nearly quantitatively converted to σ-dimer <b>5a-2</b> through an open-shell intermediate, as indicated by UV–vis–NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as well as 2D NMR spectra analyses. In situ spectroelectrochemistry was used to confirm the formation process of the dimer radical cation <b>5a-2</b>^•+. Finally, the developed new synthetic strategy could also be applied to obtain π-extended NBN-dibenzoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons

Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif

A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen–boron–nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized. Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilic bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP <b>5a</b> was nearly quantitatively converted to σ-dimer <b>5a-2</b> through an open-shell intermediate, as indicated by UV–vis–NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as well as 2D NMR spectra analyses. In situ spectroelectrochemistry was used to confirm the formation process of the dimer radical cation <b>5a-2</b>^•+. Finally, the developed new synthetic strategy could also be applied to obtain π-extended NBN-dibenzoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons

Absorption Tails of Donor:C₆₀ Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)–acceptor (A) interfaces. Weak CT absorption bands for D–A complexes occur at photon energies below the optical gaps of both the donors and the C₆₀ acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C₆₀ CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D–A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes