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Boron-based donor-spiro-acceptor compounds as novel thermally activated delayed fluorescence (TADF) emitters

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2021

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Thermally activated delayed fluorescence (TADF) emitters played a pivotal role in the improvement of organic light-emitting diode (OLED) technology. Thanks to the participation of triplet excited states in the delayed emission, diodes based on TADF chromophores display high efficiencies comparable to those of diodes that make use of transition metal-based phosphorescent emitters. In addition, employing purely organic emitters presents additional advantages connected to the cost of the emitters and their environmental impact. The molecular architecture of purely organic TADF emitters is directly connected to the observation and efficiency of the delayed emission. In particular, adopting a donor-acceptor architecture where the two different fragments are tilted, one with respect to the other, has proven to be an efficient strategy for designing efficient TADF emitters. A way to guarantee this spatial disposition is introducing a spiro center between the donor and acceptor fragments, which forces the two moieties in an orthogonal arrangement. This thesis discusses the synthesis of two different classes of donor-spiro-acceptor compounds, which both include a boron atom in the molecular structure but at different positions and with a coordinations. The first part of the thesis discusses the synthesis and functionalization of emitters that contain a tetracoordinated boron atom as the spiro center. The investigated acceptor fragments were based on the phenylpyridine moiety, whereas the donors employed were based on a disulfide, di- or triarylamine fragment. By combining different donors and acceptors, nine different emitters were synthesized and analyzed, which allowed us to establish a correlation between the emission wavelength and the relative strength of the two fragments. Additionally, the characterization of the photophysical properties of these emitters showed that the adopted molecular architecture successfully induced the emission via thermally delayed fluorescence in most cases. Nonetheless, the emitters belonging to this class displayed a low quantum yield in solution, which could be caused by the relatively high flexibility of the donor moieties employed. Therefore, an improvement of the chromophores' performances can be potentially obtained by implementing a more rigid donor fragment in the molecular structure. Additionally, it was shown how the derivatization of the emitters' donor moiety can be used to synthesize different chiral emitters or to insert a transition metal center in the molecular structure, thereby expanding the potential applications of this class of compounds. The second part of the thesis discusses the development of a synthesis path toward a class of emitters in which structure the boron atom is tricoordinated and confers the electron-accepting nature to the 9-bora-9,10-dihydroanthracene-based acceptor fragment. In this class of compounds, the spiro center is constituted by a carbon atom and the donor fragment by a triphenylamine moiety. The developed synthesis path was designed with the aim of generating a common precursor which photophysical properties could be tuned by exchanging the third substituent at the tricoordinated boron center. With the developed synthesis pathway, the derivative with a 9-mesityl-9-bora-9,10-dihydroanthracene acceptor moiety was synthesized and the analysis of its photophysical properties confirmed the emission via TADF and, consequently, the potential of this class of compounds. The emission via TADF involves relatively long-lived triplet excited states, making TADF chromophores also potentially applicable in photocatalysis. To investigate the performance of the synthesized emitters in this field, one compound from each of the above-mentioned classes was tested in photosensitized and photoredox reactions. The outcome of the catalytic trial showed that both tested compounds do promote photosensitized reactions via the generation of singlet oxygen. However, the two chromophores did not display photocatalytic activity in any of the tested photoredox transformations.

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540 Chemie

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TADF, Boron, Spiro, Donor-Acceptor, Fluorescence, Organoboron

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ISO 690STANOPPI, Marco, 2021. Boron-based donor-spiro-acceptor compounds as novel thermally activated delayed fluorescence (TADF) emitters [Dissertation]. Konstanz: University of Konstanz
BibTex
@phdthesis{Stanoppi2021Boron-69393,
  year={2021},
  title={Boron-based donor-spiro-acceptor compounds as novel thermally activated delayed fluorescence (TADF) emitters},
  author={Stanoppi, Marco},
  address={Konstanz},
  school={Universität Konstanz}
}
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The first part of the thesis discusses the synthesis and functionalization of emitters that contain a tetracoordinated boron atom as the spiro center. The investigated acceptor fragments were based on the phenylpyridine moiety, whereas the donors employed were based on a disulfide, di- or triarylamine fragment. By combining different donors and acceptors, nine different emitters were synthesized and analyzed, which allowed us to establish a correlation between the emission wavelength and the relative strength of the two fragments. Additionally, the characterization of the photophysical properties of these emitters showed that the adopted molecular architecture successfully induced the emission via thermally delayed fluorescence in most cases. Nonetheless, the emitters belonging to this class displayed a low quantum yield in solution, which could be caused by the relatively high flexibility of the donor moieties employed. Therefore, an improvement of the chromophores' performances can be potentially obtained by implementing a more rigid donor fragment in the molecular structure. Additionally, it was shown how the derivatization of the emitters' donor moiety can be used to synthesize different chiral emitters or to insert a transition metal center in the molecular structure, thereby expanding the potential applications of this class of compounds.
The second part of the thesis discusses the development of a synthesis path toward a class of emitters in which structure the boron atom is tricoordinated and confers the electron-accepting nature to the 9-bora-9,10-dihydroanthracene-based acceptor fragment. In this class of compounds, the spiro center is constituted by a carbon atom and the donor fragment by a triphenylamine moiety. The developed synthesis path was designed with the aim of generating a common precursor which photophysical properties could be tuned by exchanging the third substituent at the tricoordinated boron center. With the developed synthesis pathway, the derivative with a 9-mesityl-9-bora-9,10-dihydroanthracene acceptor moiety was synthesized and the analysis of its photophysical properties confirmed the emission via TADF and, consequently, the potential of this class of compounds.
The emission via TADF involves relatively long-lived triplet excited states, making TADF chromophores also potentially applicable in photocatalysis. To investigate the performance of the synthesized emitters in this field, one compound from each of the above-mentioned classes was tested in photosensitized and photoredox reactions. The outcome of the catalytic trial showed that both tested compounds do promote photosensitized reactions via the generation of singlet oxygen. However, the two chromophores did not display photocatalytic activity in any of the tested photoredox transformations.</dcterms:abstract>
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November 11, 2022
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Konstanz, Univ., Diss., 2022
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