Spontaneous Formation of Lead‐Free Cs 3 Cu 2 I 5 Quantum Dots in Metal–Organic‐Frameworks with Deep‐Blue Emission

All‐inorganic lead‐free Cs 3 Cu 2 I 5 perovskite‐derivant quantum dots (QDs) have attracted tremendous attention due to their nontoxicity and unique optoelectronic properties. However, the traditional hot‐injection method requires high temperatures and multiple ligands to confine the growth of QDs. Herein, a strategy is reported to spontaneously synthesize ultrasmall Cs 3 Cu 2 I 5 QDs within metal–organic‐frameworks (MOFs) MOF‐74 at room temperature (RT) with an average diameter of 4.33 nm. The obtained Cs 3 Cu 2 I 5 QDs exhibit an evident deep‐blue emission with Commission Internationale de L'Eclairage coordinates of (0.17, 0.07), owing to the strong quantum confinement effect. Due to the protection of MOF‐74, the Cs 3 Cu 2 I 5 QDs demonstrate superior stability, and the photoluminescence quantum yield retains 89% of the initial value after the storage of 1440 h under the environment with relative humidity exceeding 70%. Besides, triplet–triplet annihilation upconversion emission is observed within the composite of Cs 3 Cu 2 I 5 @MOF‐74, which brings out apparent temperature‐dependent photoluminescence. This study reveals a facile method for fabricating ultrasmall lead‐free perovskite‐derivant QDs at RT without multiple ligands. Besides, the temperature‐dependent photoluminescence of Cs 3 Cu 2 I 5 @MOF‐74 may open up a new way to develop the applications of temperature sensors or other related optoelectronic devices.