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Triplet–triplet annihilation photon upconversion (TTA-UC) is a photophysical process in which the energy of two photons are combined into a single photon of higher energy. While this strategy has been recognized in applications ranging from bioimaging to solar energy conversion, its uses in synthetic organic chemistry have not been extensively developed. Here, we present a short tutorial on the theoretical underpinnings and the design principles of TTA-UC systems. Selected applications are then discussed to highlight key features of the TTA mechanism, along with a prospective discussion of the potential of these mechanisms to enable innovation in photocatalysis, methods development, and synthetic organic chemistry.

Previously we have employed the 7-hydroxyquinoline·(NH3)n ammonia-wire clusters to study excited state H-atom transfer along a sequence of hydrogen bonds. H-atom transfer experimentally occurs for n = 4 excited to its vibrationless S1 state, for n = 3 with additional excitation of ammonia-wire vibrations (~200cm–1). However, for the n = 2 cluster, no reaction is observed, even with additional excitation energy of 900cm–1. CIS calculations have been performed on the n = 2 and n = 3 clusters to understand the effect of the wire length on the energy profile of the reaction. The enol ? keto reaction is always exoergic, and the barrier for H transfer to the proximal ammonia controls the reaction kinetics.

The 7-hydroxyquinoline·(NH3)3 cluster has been used as a model to study excited state H-atom transfer along a hydrogen-bonded NH3…NH3…NH3 wire. Excitation of the supersonically cooled cluster to its vibrationless S1 state produces no reaction, whereas excitation of ammonia-wire vibrations induces fluorescence of the excited 7-ketoquinoline·(NH3)3 cluster, revealing the excited state enol ? keto tautomerization. This reaction is assumed to occur via successive H transfers along the ammonia wire in a Grotthus-like process. Ab initio calculations of the reaction path show that proton and electron movement along the wire are closely coupled. The rate-controlling S1 state barriers arise from crossing of a ??* and a Rydberg-type ??* electronic excited state.