Spectroscopy of Resonant Intermediate States for Triplet–Triplet Annihilation Upconversion in Crystalline Rubrene: Radical Ions as Sensitizers
Photoluminescence upconversion in crystalline rubrene can proceed without an added sensitizer, but the mechanism for this process has not been well-understood. In particular, the species responsible for photon absorption has not been identified to date. To gain insight into the identity of the intermediate state, we measured the near-infrared (NIR) upconversion photoluminescence (UCPL) excitation spectrum of rubrene crystals and found three distinct spectral features. The UCPL yield has a quartic dependence on the laser intensity, implying a four-photon process. On the basis of electronic spectra of radical cations and anions of rubrene, we propose a mechanism in which photoexcited radical anions and cations undergo recombination, forming an excited neutral triplet while conserving spin. The triplets formed this way ultimately undergo triplet–triplet annihilation, resulting in the observed photoluminescence. This mechanism explains the origin of the NIR absorption as well as the four-photon nature of the UCPL process.
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The Journal of Physical Chemistry Letters
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