Long-Range Exciton Diffusion via Singlet Revival Mechanism

Abstract

We clarify the mechanism that leads to extended exciton diffusion length in organic materials which exhibit a strong anisotropy of electronic coupling. We analyze the cooperative effects of singlet fission and triplet–triplet annihilation in the exciton diffusion by means of the dynamic Monte Carlo simulations. As a model system, we consider the rubrene crystal which exhibits a long-range exciton diffusion. The deexcitation of the singlet exciton is suppressed by singlet → triplet conversion via singlet fission. Even though the triplet exciton would hardly diffuse along the c-axis in the rubrene crystal (perpendicular to the high mobility plane) because of the small electronic coupling, the regeneration of the singlet exciton via triplet–triplet annihilation enables long-range exciton diffusion along the c-axis. This singlet revival mechanism can extend the overall lifetime and the diffusion length of the exciton, through back-and-forth transitions between an isotropically diffusing singlet exciton and a long-lived triplet exciton.

Ultrafast Tuning of Various Photochemical Pathways in Perylene–TCNQ Charge-Transfer Crystals

“…However, in the P3T1 crystal, we detected the long-lived (milliseconds) triplet excitons resulted from SF, nevertheless, still 4 times shorter compared with perylene triplets in solution and twice longer than in α-perylene crystals. It is in line with the literature data on tetracene and rubrene. − In crystals and films, the lifetime of triplet states and the diffusion length are strongly dependent on the molecular morphology. Moreover, drastic shortening of the triplet state lifetimes formed from SF due to polycrystalline random nonequilibrium assembly was observed in films but not in crystals.…”

Sweet Spot of Intermolecular Coupling in Crystalline Rubrene: Intermolecular Separation to Minimize Singlet Fission and Retain Triplet–Triplet Annihilation

“…Rubrene (Rub) is a simple molecule that in the solid state transforms into a multifunctional material enabling applications in near-infrared photon upconversion (NIR-UC), , organic light-emitting diodes (OLEDs), , and organic transistors. , Features such as fast charge transport, long-range triplet energy transport, − efficient singlet fission (SF), , and triplet–triplet annihilation (TTA) are related to the unique electronic structure of Rub molecules and their specific arrangement in crystals. In Rub singlet energy is nearly twice as large as triplet energy (S 1 ∼ 2 × T 1 ), allowing to double photon energy gain in the TTA emitter, yet also resulting in ultrafast SF.…”

Triplet and singlet exciton diffusion in disordered rubrene films: implications for photon upconversion

“…16,39 Although such short L S D is common for disordered films, much longer diffusion lengths of 180–390 nm can be obtained in highly ordered crystalline Rub films. 40 In the spin-coated neat Rub films studied in this work, a comparatively short τ S of ∼1.2 ns was obtained implying the presence of SF sites due to the emerged crystalline domains. Dispersion of Rub in the PS matrix at 80 wt% concentration led to slight prolongation of τ S (up to ∼1.5 ns) due to the suppressed Rub aggregation.…”

“…Hence, materials with a long triplet lifetime and high triplet diffusivity are desirable to attain efficient TTA. 35 Micron-range triplet exciton diffusion in Rub crystals has been previously reported; [40][41][42][43] however, there are no quantitative data on triplet diffusion in disordered Rub films. Taking into account that singlet and triplet diffusion are crucial for the design of highperformance solid-state TTA-UC systems, the key parameters determining exciton diffusivity, such as exciton lifetime, diffusion coefficient and length, need to be thoroughly assessed.…”