There are materials which absorb photons of shorter wavelength and emit photons of longer wavelength. Can there be stable substances which absorb photons of larger wavelength and emit light of shorter wavelength?

(N/A) No, in practice, such stable substances cannot be obtained. According to the law of conservation of energy, the energy of an emitted photon $(E_{emit} = hc / \lambda_{emit})$ must be less than or equal to the energy of the absorbed photon $(E_{abs} = hc / \lambda_{abs})$, plus any internal energy the substance might provide. Since $\lambda_{emit} < \lambda_{abs}$ implies $E_{emit} > E_{abs}$, the substance would need to continuously supply its own internal energy to the photons to facilitate this process. For a substance to remain stable, it cannot continuously lose its internal energy in this manner; therefore, such a process is not possible for stable substances.