Visible light-harvesting naphthalenediimide (NDI)-C60 dyads as heavy-atom-free organic triplet photosensitizers for triplet–triplet annihilation based upconversion

Two naphthalenediimide (NDI)-C60 dyads that show strong absorption of visible light were prepared as heavy atom-free organic triplet photosensitizers. Alkylamino substituted NDI was used as the light-harvesting antennas and the C60 unit was used as spin converter for the intersystem crossing (ISC) from singlet excited state to triplet excited state. Upon visible light photoexcitation, triplet excited states of the dyads are populated (lifetime is up to 90.1 μs). Nanosecond time-resolved transient difference absorption spectroscopy indicated that the triplet excited state of the dyads is localized on either the NDI or the C60 unit. As a proof of concept, the dyads are used as heavy-atom-free organic triplet photosensitizers for triplet–triplet annihilation based upconversion. C60-organic chromophore dyads can be used as a general molecular structural motif of heavy-atom-free organic triplet photosensitizers, for which the photophysical properties can be readily changed by using different light-harvesting antennas, and the ISC property of these dyads is predictable.

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► Naphthalenediimide (NDI)-C60 dyads were prepared as heavy atom-free organic triplet photosensitizers.
► Strong absorption of visible light and long-lived triplet excited states were observed for the C60-NDI dyads.
► C60 dyads were used as triplet photosensitizer for triplet–triplet annihilation upconversion.
► Different localization of the triplet excited states of the dyads were observed.