Photon Upconversion in Crystalline Rubrene: Resonant Enhancement by an Interband State
2018
Cruz, Chad D. | Choi, Hyun Ho | Podzorov, Vitaly | Chronister, Eric L. | Bardeen, Christopher J.
Triplet–triplet exciton annihilation after sensitization of the triplet states by a near-infrared (NIR)-absorbing sensitizer enables rubrene to function as a photon upconversion (UC) material. In this paper, we demonstrate an alternate pathway to NIR upconversion in pristine rubrene crystals: resonantly enhanced two-photon absorption via a weakly allowed interband state. We find that all crystalline rubrene samples exhibit NIR-to-visible upconversion that can be easily observed by eye under low-intensity (20 W/cm²) continuous wave excitation. The amount of continuous wave photoluminescence (PL) is comparable to what is observed under femtosecond pulsed excitation with the same average intensity. A wide range of excitation intensities (I) for the PL power dependence are explored and careful fitting of the intensity dependence of the upconverted PL shows that it has an approximate I⁴ → I² transition. Moreover, there is a pronounced dependence of the per-pulse upconverted PL signal on the laser repetition rate. A four-state kinetic model with a long-lived (∼1 μs) interband state that takes into account fission and fusion dynamics can reproduce both the I⁴ → I² transition and the dependence of the PL on pulse repetition rate. The modeling suggests that this interband state arises from a low-concentration species, possibly a crystal defect or defective rubrene molecules. Several other polyacene crystals (tetracene, diphenylhexatriene, and perylene) measured under the same conditions did not exhibit similar behavior. The observation of resonantly enhanced upconverted PL without the addition of chemically distinct sensitizers suggests that interband states in organic molecular crystals can generate new and possibly useful photophysical behavior.
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