Abstract
Singlet fission, the process of forming two triplet excitons from one singlet exciton, is a characteristic reserved for only a handful of organic molecules due to the atypical energetic requirement for low energy excited triplet states. The predominant strategy for achieving such a trait is by increasing ground state diradical character; however, this greatly reduces ambient stability. Herein, we exploit Baird's rule of excited state aromaticity to manipulate the singlet-triplet energy gap and create novel singlet fission candidates. We achieve this through the inclusion of a [4n] 5-membered heterocycle, whose electronic resonance promotes aromaticity in the triplet state, stabilizing its energy relative to the singlet excited state. Using this theory, we design a family of derivatives of indolonaphthyridine thiophene (INDT) with highly tunable excited state energies. Not only do we access novel singlet fission materials, they also exhibit excellent ambient stability, imparted due to the delocalized nature of the triplet excited state. Spin-coated films retained up to 85% activity after several weeks of exposure to oxygen and light, while analogous films of TIPS-pentacene showed full degradation after 4 days, showcasing the excellent stability of this class of singlet fission scaffold. Extension of our theoretical analysis to almost ten thousand candidates reveals an unprecedented degree of tunability and several thousand potential fission-capable candidates, while clearly demonstrating the relationship between triplet aromaticity and singlet-triplet energy gap, confirming this novel strategy for manipulating the exchange energy in organic materials.
Highlights
Phenomena that permit the generation of multiple excitons from the absorption of one photon have received large attention recently due to their potential in photovoltaic cell efficiency enhancement. One such process is singlet fission, whereby two low-energy triplet excited states are generated from one high-energy singlet excited state, allowing quantum efficiencies up to 200% and offering the potential to surpass the single junction limit in solar cells.[1−8] Recent advancements in singlet fission have been materials-limited due to the rarity of molecules that meet the essential energetic requirement for the process, namely, that the energy of the lowest triplet excited state E(T1) be on the order of half the energy of the lowest sdiynegsl,e9t−1e2xcpiotelydensetas,t1e3−E15(Sd1)o.noKr−noawccnepstyosrtepmoslyminecrlsu,d16e−1r8ylaennde derivatives of the linear acenes, tetracene and pentacene, which have become the focus of the field.[2,19−31] The low triplet energies of the acenes are a function of their migratory Clar’s sextets resulting in their large localized
Current established rationales behind synthesizing organic molecules with large singlet (S1)−triplet (T1) energy gaps such as inducing biradicaloid character offer the synthetic chemist little assistance in identifying or developing new chromophores for this purpose, these rules are of very good use to check whether a structure is likely to fulfill the energetic criteria for singlet fission.[34−36] Merely focusing on increasing the amount of biradical character can often involve cost to chemical stability as it can result in highly reactive localized unpaired electrons, this can potentially be addressed with careful protection of the radical sites.[37,38]
We have designed a family of materials (Figure 1a) to investigate: (a) whether the indolonaphthyridine chromophore does undergo singlet fission; (b) whether the optical properties and singlet fission ability could be tuned by chemical functionalization; and (c) whether contributions from an aromatic triplet state enhance chemical stability
Summary
Phenomena that permit the generation of multiple excitons from the absorption of one photon have received large attention recently due to their potential in photovoltaic cell efficiency enhancement One such process is singlet fission, whereby two low-energy triplet excited states are generated from one high-energy singlet excited state, allowing quantum efficiencies up to 200% and offering the potential to surpass the single junction limit in solar cells.[1−8] Recent advancements in singlet fission have been materials-limited due to the rarity of molecules that meet the essential energetic requirement for the process, namely, that the energy of the lowest triplet excited state E(T1) be on the order of half the energy of the lowest sdiynegsl,e9t−1e2xcpiotelydensetas,t1e3−E15(Sd1)o.noKr−noawccnepstyosrtepmoslyminecrlsu,d16e−1r8ylaennde derivatives of the linear acenes, tetracene and pentacene, which have become the focus of the field.[2,19−31] The low triplet energies of the acenes are a function of their migratory Clar’s sextets resulting in their large localized. Lesser known is Baird’s Rule, which states that in the triplet state, cyclic systems with [4n] electrons are aromatic and stabilized.[39,40] Following confirmatory modern theoretical analysis,[41−43] the rule has been explored in atypical conjugated systems in the past few years.[44−47] Here, we demonstrate the synthesis and characterization of highly stable, tunable organic materials that undergo singlet fission through exploitation of Baird’s aromaticity of the triplet excited state
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