Near-Infrared-to-Visible Photon Upconversion Enabled by Conjugated Porphyrinic Sensitizers under Low-Power Noncoherent Illumination

We report four supermolecular chromophores based on (porphinato)­zinc­(II) (PZn) and (polypyridyl)metal units bridged via ethyne connectivity (Pyr 1 RuPZn 2 , Pyr 1 RuPZnRuPyr 1 , Pyr 1 RuPZn 2 RuPyr 1 , and OsPZn 2 Os) that fulfill critical sensitizer requirements for NIR-to-vis triplet–triplet annihilation upconversion (TTA-UC) photochemistry. These NIR sensitizers feature: (i) broad, high oscillator strength NIR absorptivity (700 nm < λmax(NIR) < 770 nm; 6 × 104 M–1 cm–1 < extinction coefficient (λmax(NIR)) < 1.6 × 105 M–1 cm–1; 820 cm–1 < fwhm < 1700 cm–1); (ii) substantial intersystem crossing quantum yields; (iii) long, microsecond time scale T1 state lifetimes; and (iv) triplet states that are energetically poised for exergonic energy transfer to the molecular annihilator (rubrene). Using low-power noncoherent illumination at power densities (1–10 mW cm–2) similar to that of terrestrial solar photon illumination conditions, we demonstrate that Pyr 1 RuPZn 2 , Pyr 1 RuPZn 2 RuPyr 1 , and Pyr 1 RuPZnRuPyr 1 sensitizers can be used in combination with the rubrene acceptor/annihilator to achieve TTA-UC: these studies represent the first examples whereby a low-power noncoherent NIR light source drives NIR-to-visible upconverted fluorescence centered in a spectral window within the bandgap of amorphous silicon.