German Edition: DOI: 10.1002/ange.201911529 Polycyclic Aromatic Compounds Hot Paper International Edition: DOI: 10.1002/anie.201911529 Singlet Fission in Pyrene-Fused Azaacene Dimers Juan P. Mora-Fuentes + , Ilias Papadopoulos + , Dominik Thiel, Roberto lvarez-Boto, Diego Cortizo-Lacalle, Timothy Clark, Manuel Melle-Franco,* Dirk M. Guldi,* and Aurelio Mateo-Alonso* Abstract: Singlet fission has emerged as a promising strategy to avoid the loss of extra energy through thermalization in solar cells. A family of dimers consisting of nitrogen-doped pyrene- fused acenes that undergo singlet fission with triplet quantum yields as high as 125 % are presented. They provide new perspectives for nitrogenated polycyclic aromatic hydrocar- bons and for the design of new materials for singlet fission. Singlet fission (SF) has emerged as a promising strategy to overcome the Shockley–Queisser limit that predicts a max- imum power conversion efficiency of 33.7 % for a single junction device. [1] This is because, in SF, a singlet excited state (S 1 ) splits into two triplet excited states (T 1 ) and hence the generation of the extra exciton per absorbed photon avoids the loss of extra energy through thermalization. SF has been observed in several families of conjugated molecules and polycyclic aromatic hydrocarbons, [2] among which the acene family have shown a prominent position. For instance, acene dimers have become key to understanding how structural and electronic factors influence the dynamics of the formation and decay of triplet excitons. [3] Since SF efficiencies are known to increase with the number of linearly fused rings and pentacene is the most stable of the higher acenes, the synthesis and study of pentacene dimers have received a lot of attention, [2a,b] among which maximum triplet quantum yields (TQYs) of 200 % have been reported. However, the triplet energy of pentacene is not well- matched for use in solar cells, [4] and also, the stability of pentacene derivatives is limited in comparison to the shorter acenes. [5] On this regard, tetracene has been investigated as an alternative since it shows triplet energies above the band gap of silicon [4a,b] and is known to be more stable than pentacene. [5] Furthermore, dimers constituted by tetracene have been recently reported with TQYs  100 %. [6] Substitutional nitrogen-doping is another potential approach to improving the properties of acenes for SF, since the resulting azaacenes are more stable than the correspond- ing acenes and also their energy levels can be tuned with the number and the position of the nitrogen atoms. [7] Never- theless, azaacenes remain practically unexplored in SF and only a few studies about monomeric azaacenes in thin films have been reported. [8] Herein, we report a family of dimers constituted by nitrogen-doped pyrene-fused acenes that undergo SF (Figure 1). This series of regioisomeric dimers are constituted by two dibenzodiazahexacene cores bridged by a phenylene linker in an ortho-, meta-, or para- substitution pattern (o- DAD, m-DAD, and p-DAD), which show the electronic structure of two diazatetracenes (highlighted in pink) as the result of the localization of sextets in the off-linear pyrene rings (highlighted in sky blue) that compartmentalize the acene backbone in smaller and more stable tetracenic residues. The combination of ultrafast transient absorption measurements with global and target analyses reveal TQYs as high as 125 % and a detailed SF mechanistic insight for pyrene-fused azaacenes, which altogether provide new per- spectives not only for azaacenes and nitrogenated pyrene- fused acenes, but also for the design of new materials with enhanced properties for SF. Dibenzodiazahexacene dimers o-DAD, m-DAD, and p- DAD are members of the pyrene-fused azaacene [9] family that combine linear annulations (acene backbone) with periodic off-linear bisannulations (pyrene residues). Pyrene-fused azaacenes have shown a high stability as illustrated by synthesis of monodisperse nanoribbons with up to 30 linearly fused rings [10] and of polydisperse nanoribbons with approx- imately 80 linearly fused rings. [11] We selected dibenzodiaza- hexacene derivatives with TIPS-acetylene substituents because of their tetracene electronic structure, their stability, [*] J. P. Mora-Fuentes, [+] Dr. D. Cortizo-Lacalle, Prof. Dr. A. Mateo-Alonso POLYMAT, University of the Basque Country UPV/EHU Avenida de Tolosa 72, 20018 Donostia-San Sebastian (Spain) E-mail: amateo@polymat.eu I. Papadopoulos, [+] D. Thiel, Prof. Dr. D. M. Guldi Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials Friedrich-Alexander-Universität Erlangen-Nürnberg Egerlandstr. 3, 91058 Erlangen (Germany) E-mail: dirk.guldi@fau.de Dr. R. lvarez-Boto, Prof. Dr. M. Melle-Franco CICECO—Aveiro Institute of Materials, Department of Chemistry University of Aveiro, 3810-193 Aveiro (Portugal) E-mail: manuelmelle.research@gmail.com Prof. Dr. T. Clark Computer-Chemistry Centre, Department of Chemistry and Phar- macy, Friedrich-Alexander-Universität Erlangen-Nürnberg Naegelsbachstr. 25, 91052 Erlangen (Germany) Prof. Dr. A. Mateo-Alonso Ikerbasque, Basque Foundation for Science Bilbao (Spain) [ + ] These authors contributed equally to this work. Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201911529.  2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. A ngewandte Chemi e Communications 1113 Angew. Chem. Int. Ed. 2020, 59, 1113 –1117  2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim