Singlet Fission in a Pyrrole-Fused Cross-Conjugated Skeleton with Adaptive Aromaticity

Abstract

Singlet fission (SF) materials hold the potential to increase the power conversion efficiency of solar cells by reducing the thermalization of high-energy excited states. The major hurdle in realizing this potential is the limited scope of SF-active materials with high fission efficiency, suitable energy levels, and sufficient chemical stability. Herein, using theoretical calculation and time-resolved spectroscopy, we developed a highly stable SF material based on dipyrrolonaphthyridinedione (DPND), a pyrrolefused cross-conjugated skeleton with a distinctive adaptive aromaticity (dual aromaticity) character. The embedded pyrrole ring with 4n+2 π-electron features aromaticity in the ground state, while the dipole resonance of the amide bonds promotes a 4n π-electron Baird's aromaticity in the triplet state. Such an adaptive aromaticity renders the molecule efficient for the SF process [E(S 1 ) ≥ 2E(T 1 )] without compromising its stability. Up to 173% triplet yield, strong blue-green light absorption, and suitable triplet energy of 1.2 eV, as well as excellent stability, make DPND a promising SF sensitizer toward practical applications.

Probing the Origin of Adaptive Aromaticity in 16‐Valence‐Electron Metallapentalenes

“…[8] Excited state (anti)aromaticity provides am anner to fine tunet he singlettriplet energy gap of compounds andt his is an important factor in the design of new materials that act as molecular motors, [9] high-spin organicm olecules, [10] photoluminescence materials, [11] or photovoltaic cells. [12] One way to design aromatic compounds with intermediate singlet-triplet energy gap is by making use of the adaptive aromaticityc oncept, which refers to the preservation of aromatic character after ac hange in the electronics tate (normally,f rom singlet to triplet). [13] Species with adaptive aromaticity do not follow Baird's rule.…”

Adaptive σ‐Aromaticity in an Unsaturated Three‐Membered Ring

“…[8] And a highly stable singlet fission material based on a polycyclic dipyrrolonaphthyridinedione skeleton was found to have adaptive aromaticity in the pyrrole ring ( Figure 1f). [9] Note that the adaptive aromatic species mentioned above are all π-aromaticity due to the delocalization of πelectrons in unsaturated rings. According to the type of delocalized electrons, aromaticity can be divided into πaromaticity and σ-aromaticity.…”

Effect on the aromaticity of heterocyclic ligands by coordination with ruthenium electron‐withdrawing metal centers

“…This photophysical behavior has been recently explored using Baird's rule for the excited‐state aromaticity to manipulate the singlet‐triplet energy gap, proposing a new family of derivatives of indolonaphthyridine thiophenes [ 52 ] and dipyrrolonaphthyridinedione compounds [ 53 ] , able to produce SF. The possibility to undergo an aromaticity in both ground and excited states (adaptive aromaticity) is an exclusive behavior demonstrated in a reduced number of molecules [ 53–55 ] . In this regard, the adaptive aromaticity has also been theoretically studied in a series of osmapyridine and osmapyridinium complexes to understand their triplet ground‐state character, using different aromaticity indices, including HOMA, ELFπ, and MCI, among others [ 56 ] .…”

“…[49] In addition, Barbasiewicz and co-workers have studied the substitution effect of the ruthenafurane ring on "Hoveyda-Grubbs"-based catalysts, demonstrating that the aromatic character causes a fine-tuning that modulates the catalytic activity. [50] Beyond Ru(II) complexes, the aromaticity studies have been also useful to evaluate the capability of some molecules undergoing singlet fission (SF) [51][52][53] . This photophysical behavior has been recently explored using Baird's rule for the excited-state aromaticity to manipulate the singlettriplet energy gap, proposing a new family of derivatives of indolonaphthyridine thiophenes [52] and dipyrrolonaphthyridinedione compounds [53] , able to produce SF.…”

“…[50] Beyond Ru(II) complexes, the aromaticity studies have been also useful to evaluate the capability of some molecules undergoing singlet fission (SF) [51][52][53] . This photophysical behavior has been recently explored using Baird's rule for the excited-state aromaticity to manipulate the singlettriplet energy gap, proposing a new family of derivatives of indolonaphthyridine thiophenes [52] and dipyrrolonaphthyridinedione compounds [53] , able to produce SF. The possibility to undergo an aromaticity in both ground and excited states (adaptive aromaticity) is an exclusive behavior demonstrated in a reduced number of molecules [53][54][55] .…”