Molecular stacking controlling coherent and incoherent singlet fission in polymorph rubrene single crystals

Abstract

Singlet fission (SF) is an appealing process where one photoexcited singlet transforms to two triplets, which can overcome thermalization energy loss and improve solar cell efficiency. However, it remains unclear how intermolecular coupling, which is subject to molecular stacking, controls SF pathways and dynamics. Here, we prepared polymorph rubrene single crystals with different stacking geometries, including orthorhombic (Orth.), triclinic (Tri.), and monoclinic (Mono.) phases. By micro‐area ultrafast spectroscopy, we find that Orth. and Tri. phases with closer π‐π stacking exhibit co‐existing coherent and incoherent SF channels while loosely stacked Mono. phase shows only incoherent SF. Furthermore, incoherent SF is thermally activated in Orth. but barrierless in Mono. and Tri. phases. Quantum mechanical calculation reveals that different electronic coupling strength in different phases leads to different SF dynamics. This study demonstrates that molecular stacking governs SF dynamics through electronic coupling, providing guidance for designing efficient SF materials via crystal structural engineering.

Singlet fission is incoherent in pristine orthorhombic single crystals of rubrene: no evidence of triplet-pair emission

“…<25 fs) formation of triplet-pair signatures in rubrene single crystals. 21–23,27 The latter signatures are not predicted by the calculations 18,28 and current explanations for this surprising observation differ. The details of the proposed mechanisms and the discrepancies between them are nicely described in a recent review article.…”

“…17,29 Recent spectroscopic studies on single rubrene crystals, however, are contradictory and many do not obviously support this theoretical hypothesis. [10][11][12][13][14][15][16][18][19][20][21][22][23][24]27 For example, while some transient absorption 15,20 and transient grating pumpprobe experiments 24 of rubrene crystals appear to support the calculations, showing triplet-pair population increasing approximately exponentially following photo-excitation (∼2 ps time-constant), other studies instead report nearinstantaneous (instrument-limited, i.e. <25 fs) formation of triplet-pair signatures in rubrene single crystals.…”

“…SF and TTA are generally accepted to proceed via intermediate triplet-pair states ((TT), (T⋯T)) 5,6 but the details of their formation and evolution are not yet fully described, 7,8 even in archetypal SF/TTA materials such as crystalline pentacene and rubrene. 7–27…”

“…17,29 Recent spectroscopic studies on single rubrene crystals, however, are contradictory and many do not obviously support this theoretical hypothesis. 10–16,18–24,27…”

Structural Disorder at the Edges of Rubrene Crystals Enhances Singlet Fission

“…Such vibronic sidebands are common in absorption spectra of polycyclic aromatic hydrocarbons and are generally assigned to CC ring stretching modes that are displaced upon photoexcitation. Spectra measured both at the center and the edge of a crystal show a suppression of the 0–0 absorption band (533 nm) relative to the 0–1 vibronic sideband (496 nm), which is indicative of orthorhombic rubrene. , Notably, however, spectra measured near the crystal’s edge exhibit peaks that are broadened by ∼21% relative to those seen in spectra measured in the crystal’s center (Figure S4 and Table S1). This suggests that while rubrene molecules on average adopt an orthorhombic packing structure near crystal edges, enhanced structural disorder exists in this region relative to the crystal’s bulk.…”