Abstract
By using wavelength-tunable, low fluence picosecond pulses at a 1 kHz repetition rate in a transient grating pump and probe configuration, we show that photoexcitation to higher vibrational levels leads to slower singlet exciton fission rates in rubrene. After the 1 ps photoexcitation, the initial growth of the triplet exciton population is exponential, with its time-constant systematically varying from 2.5 ± 0.7 ps to 40 ± 15 ps as the excitation photon energy is increased from the lowest to the third main vibrational band in the vibrational progression of rubrene’s absorption spectrum. We also determine that short-pulse-induced fluorescence in amorphous rubrene films deposited by molecular beam epitaxy in high vacuum decays as a single exponential with a lifetime of 15.2 ns, close to rubrene’s radiative lifetime for molecules in solution. This shows that singlet fission is non-existent in fully amorphous rubrene films, despite the close molecular packing, which indicates that in these films there is no short range molecular order matching the molecular arrangement in orthorhombic rubrene crystals.
Highlights
The first electronic excited state of larger organic molecules can have a significantly lower energy in a triplet configuration than in the photoexcited singlet configuration
By using wavelength-tunable, low fluence picosecond pulses at a 1 kHz repetition rate in a transient grating pump and probe configuration, we show that photoexcitation to higher vibrational levels leads to slower singlet exciton fission rates in rubrene
This shows that singlet fission is non-existent in fully amorphous rubrene films, despite the close molecular packing, which indicates that in these films there is no short range molecular order matching the molecular arrangement in orthorhombic rubrene crystals
Summary
The first electronic excited state of larger organic molecules can have a significantly lower energy in a triplet configuration than in the photoexcited singlet configuration. A simple description of singlet fission starts from a ground-state and excited-state molecule pair 1S0 +1S1, and can be summarized as1–3 This is a multi-step process: the photoexcited pure singlet state initially becomes a spin-correlated, entangled pair of triplets in an overall singlet state, 1(T1T1), which can undergo a transition to a state 1(T1 ⋯ T1) with a larger distance between its triplet constituents.. In the second one we use the photons originating from triplet fusion and the dynamics of the decay of shortpulse-induced fluorescence to study the efficiency of singlet exciton fission in amorphous and partially crystalline rubrene thin films These experiments will be discussed below, after a short section that summarizes the properties of rubrene and its crystal structures
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