Organic Phosphorescence Hot Paper DOI: 10.1002/anie.201402199 Metal-Free Triplet Phosphors with High Emission Efficiency and High Tunability** Michael Koch, Karthikeyan Perumal, Olivier Blacque, Jai Anand Garg, Ramanathan Saiganesh, Senthamaraikannan Kabilan, Kallupattu Kuppusamy Balasubramanian, and Koushik Venkatesan* Abstract: Design of highly efficient phosphorescent emitters based on metal- and heavy atom-free boron compounds has been demonstrated by taking advantage of the singlet fission process. The combination of a suitable molecular scaffold and appropriate electronic nature of the substituents has been utilized to tailor the phosphorescence emission properties in solution, neat solid, and in doped PMMA thin films. Molecules that display phosphorescence play a significant role in light- and energy-harvesting systems. [1] This is because the spin statistics theoretically allow for achieving high efficiencies in these systems by harnessing the triplet exci- tons. [2] In this context, search for new triplet emitters with high quantum yields and tunable emission properties is of major interest for numerous applications such as solar cells, organic light-emitting diodes (OLEDs), photocatalysis, sen- sors etc. [3] Most of the triplet emitters contain transition metals or heavy atoms that have large spin–orbit coupling to enable efficient intersystem crossing to the triplet states. [4] In contrast to the extensive number of examples of transition metal based triplet phosphors, purely organic materials that show room temperature phosphorescence are quite rare, particularly in solution. [2b, 5] Although it has long been established that phosphorescence can be achieved through singlet fission process in purely organic molecules, [6] the design and experimental investigations on new molecular scaffolds are limited. [7] Singlet fission is a process that involves sharing of energy between a singlet-excited state and a ground-state molecule to produce a correlated pair of triplet-excited molecules. In theory, it allows for quantum yields (QY) up to 200 % to be achieved. [5a] Recently, an external quantum efficiency of 160 % for an organic solar cell device employing pentacene molecules was demonstrated and also a first observation of efficient singlet fission in solution based on a bis(triisopropylsilylethynyl)-pentacene excimer was reported. [8] Boron b-diketonate compounds incorporated into a polylactide polymer were also reported to show phosphorescence, albeit only in the solid-state and the origin for the emission was ascribed to singlet fission. [9] Therefore, until now most investigations on well-defined small molecules have been limited to the acene family and the design of novel chromophores that show singlet fission induced phosphorescence based on purely organic com- pounds and a systematic tuning of their photophysical and chemical properties remains a major challenge. [5b] A key factor to the successful realization of singlet fission based materials relies on the design of molecules with appropriate electronic structure requirements that enables judicious disposition of the ground state as well as the excited state. [7] The rich photoluminescent properties of organoboron com- pounds prompted us to pursue them as molecules of choice for our investigations. [10] Moreover, the chemical and the electronic versatility of the molecular scaffold amenable to suitable modifications allows for achieving superior photo- luminescent behavior both in the solid state as well as in solution. [11] This work purposedly combines the electronic and structural requirements favoring singlet fission by satisfying different criteria such as presence of strong dipole moments, p–p interactions (see Scheme 1) and appropriate functional groups such as an aromatic nitro group as a part of the conjugate unit that can to some extent exhibit radical character in the excited state. [7, 12] Herein, we report the synthesis, structural and detailed photophysical investigations of a novel family of b-hydroxy- vinylimine boron compounds that show singlet fission induced room temperature phosphorescence emission with high efficiency both in solution and when doped in PMMA (poly(methyl methacrylate)) matrix. Two different classes of molecules were chosen and synthesized through a common approach as potential ligand candidates. The first (1–3) consists of b-hydroxyvinylimines derived from a 3,4-dihydro- naphthalene unit with scope for additional modulation of electronic (and dipole moment) changes made possible by varying the R 1 and R 2 substituents. The second set (4–7) involves b-hydroxyvinylimines derived from a 2H-chromene unit in which the donor moiety is already incorporated into the structure (oxa group). [*] M. Koch, [+] Dr. O. Blacque, Dr. J.A. Garg, Dr. K. Venkatesan Department of Chemistry, University of Zurich Winterthurerstrasse 190, CH-8057 Zurich (Switzerland) E-mail: venkatesan.koushik@chem.uzh.ch K. Perumal, [+] Dr. R. Saiganesh, Prof. Dr. K. K. Balasubramanian Shasun Research Centre 27 Vandaloor-Kelambakkam Road, Keelakottaiyur, Chennai-600048 (India) K. Perumal, [+] Dr. S. Kabilan Department of Chemistry, Annamalai University Annamalai Nagar, Chidambaram-600002 (India) [ + ] These authors contributed equally to this work. [**] This work was supported by the Swiss National Science Foundation NRP 62 Smart Materials Program (Grant No. 406240-126142). Support from the University of Zurich, Prof. em. Dr. Heinz Berke and Prof. Dr. Roger Alberto are also gratefully acknowledged. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201402199. . Angewandte Communications 6378 # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2014, 53, 6378 –6382