Enhancing NIR-to-visible photon upconversion in cast solid by introducing bulky substituents in rubrene and by suppressing back energy transfer

The efficiency of photon upconversion (UC) from near infrared (NIR) to visible emission by the triplet-triplet annihilation (TTA) mechanism in solids has been limited in spite of its importance. The singlet fission (SF) quenching of the fluorescence of rubrene, widely used annihilator and emitter, and back energy transfer (BET) from the emitter to sensitizer can be major reasons interfering with NIR TTA-UC. By introducing bulky substituents to the rubrene backbone and decreasing the sensitizer concentrations, we suppressed both SF and BET and achieved a high upconversion emission efficiency of eta(UC) = (4.0 +/- 0.3)% (in the 100%-scale) for the cast films from the mixed solution of the sensitizer and the rubrene derivatives with tert-butyl and alkoxy groups. The effects of position and the kind of substitution to the rubrene structure on the optical and UC properties were investigated for the new emitters in the solid phase. The detailed mechanisms of triplet-triplet energy transfer (TET) and BET dynamics were clarified by time-resolved emission measurements. The UC emission dynamics was characterized by the fast (nanoseconds) and slow (microseconds) TET from the sensitizer to the emitter in solid, resulting in high TET quantum efficiency, which also contributes to the high eta(UC).