Room-Temperature Pentacene Fluids: Oligoethylene Glycol Substituent-Controlled Morphologies and Singlet Fission

Indications for a Direct Singlet Fission Mechanism in TIPS-Pentacene Crystals from Hybrid DFT/MRCI and Molecular Mechanics Studies

A hybrid quantum mechanics/molecular mechanics setup was used to model the singlet fission (SF) of electronically excited 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) in the crystalline phase. The optically bright S1bright state possesses nearly identical excitation energies and oscillator strengths in the two nonequivalent dimer units with large and small structural overlap, respectively. A shearing/tilting motion of the two slip-stacked TIPS-pentacene building blocks is the key for stabilizing the singlet-coupled triplet-pair state, 1(TT), in the large/small overlap situation. In both dimer models, the S1bright and 1(TT) states swap energetic order upon geometry relaxation, indicative of strong nonadiabatic coupling between these states and a direct SF mechanism in TIPS-pentacene crystals, at variance with unsubstituted pentacene where charge-transfer contributions are substantial in V-shaped substructures. The overall energy balance E(S1bright) - (E(T1) + E(T1)) remains positive at all investigated nuclear arrangements. The ferromagnetically coupled 5(TT) state lies energetically too high to take part in the SF mechanism.

Chirality in Singlet Fission: Controlling Singlet Fission in Aqueous Nanoparticles of Tetracenedicarboxylic Acid Ion Pairs

The singlet fission characteristics of aqueous nanoparticles, self-assembled from ion pairs of tetracene dicarboxylic acid and various amines with or without chirality, are thoroughly investigated. The structure of the ammonium molecule, the counterion, is found to play a decisive role in determining the molecular orientation of the ion pairs and its regularity, spectroscopic properties, the strength of the intermolecular coupling between the tetracene chromophores, and the consequent singlet fission process. Using chiral amines has led to the formation of crystalline nanosheets and efficient singlet fission with a triplet quantum yield as high as 133% ±20% and a rate constant of 6.99 × 109 s-1. The chiral ion pairs also provide a separation channel to free triplets with yields as high as 33% ±10%. In contrast, nanoparticles with achiral counterions do not show singlet fission, which gave low or high fluorescence quantum yields depending on the size of the counterions. The racemic ion pair produces a correlated triplet pair intermediate by singlet fission, but no decorrelation into two free triplets is observed, as triplet-triplet annihilation dominates. The introduction of chirality enables higher control over orientation and singlet fission in self-assembled chromophores. It provides new design guidelines for singlet fission materials.