Ultrafast Dynamics of the 9,10-Bis[(triisopropylsilyl)ethynyl]anthracene Nanoaggregate and Thin Film: Influence of the Molecular Packing and Sample Morphology on Singlet Fission and Excimer Formation

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Photophysics and ultrafast exciton dynamics of 9,10-bis(triisopropylsilylethynyl)anthracene (TIPSAn) nanoaggregates and thin films are studied. The effect of molecular packing on singlet exciton relaxation is evaluated by comparing ultrafast dynamics in nanoparticles and vapor-deposited thin films. As compared to the strong emission of TIPSAn in solution, significant quenching of fluorescence in nanoaggregates and thin films suggests fast nonradiative relaxation of singlet excitons. In nanoaggregates, disordered molecular packing leads to long-lived excimer-state outcompeting singlet fission (SF)-mediated triplet formation. In microcrystalline thin films, exciton relaxation to the excimer state is inhibited due to ordered molecular packing and a significant fraction of singlet excitons undergo singlet fission, resulting in long-lived triplet excitons. However, the singlet fission yield in TIPSAn is much smaller than that of the structurally similar but strongly coupled anthracene derivative, namely 9,10-bis(phenylethynyl) anthracene (BPEA). The low yield of SF-mediated triplets in TIPSAn is attributed to weak intermolecular interactions due to larger intermolecular separation introduced by bulky triisopropylsilyl groups in crystalline thin films and additional competing exciton trapping to the excimer state in disordered nanoparticles.