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Photochemical upconversion (UC) via triplet-triplet annihilation (TTA) has attracted considerable attention for its potential applications in solar energy conversion, photocatalysis, and bioimaging. Achieving a large anti-Stokes shift in photochemical UC is appealing but still a great challenge, especially for purely organic sensitizers. Here, we develop solid-state TTA UC systems with metal- and heavy atom-free dyes as the sensitizers, which sensitize the 9,10-diphenylanthracene acceptor through thermally activated triplet-triplet energy transfer. Solid-state UC emission with remarkable anti-Stokes shifts up to 1.10 eV is achieved owing to an evident enthalpy gain by the endothermic sensitization. The solid upconverter shows air-stable UC emission and potentials in dual-mode anti-counterfeiting and encryption applications. The present UC approach involving thermally activated sensitization enabled by purely organic dyes provides a versatile strategy to develop TTA UC materials with large anti-Stokes shift, air-tolerant emission, and environmental compatibility, which would have promising applications in information encryption, photochemical conversion, and bioimaging.