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Self‐trapped exciton (STE) emission of low‐dimensional metal halides has witnessed explosive developments in both display and illumination, due to its intriguing photoluminescence properties. As one typical feature, STE emission energy is commonly independent of excitation wavelength. Herein, a rare phenomenon of inverse excitation‐dependent dual‐band emission is achieved on 0D Cs2InBr5·H2O. Under initial compression, the contraction of inhomogeneously coordinated InBr5O octahedra gives rise to blue‐shifted STE emission with the decreased Stokes shift. As the phase transition occurs under higher pressure, considerable octahedral distortions generate a new defect‐related localized exciton emission. Notably, the high‐energy emission from the intrinsic STE state is only observed under the low‐energy excitation, which is believed to originate from the excitation‐dependent multiple excited states in high‐pressure Cs2InBr5·H2O. [ABSTRACT FROM AUTHOR]

A first‐principles‐based effective Hamiltonian is developed and employed to investigate finite‐temperature structural properties of a prototype of perovskite halides, that is CsPbI3. Such simulations, when using first‐principles‐extracted coefficients, successfully reproduce the existence of an orthorhombic Pnma state and its iodine octahedral tilting angles around room temperature. However, they also yield a direct transformation from Pnma to cubic Pm3¯m upon heating, unlike measurements that reported the occurrence of an intermediate long‐range‐tilted tetragonal P4/mbm phase in‐between the orthorhombic and cubic phases. Such disagreement, which may cast some doubts about the extent to which first‐principle methods can be trusted to mimic hybrid perovskites, can be resolved by "only" changing one short‐range tilting parameter in the whole set of effective Hamiltonian coefficients. In such a case, some reasonable values of this specific parameter result in the predictions that i) the intermediate P4/mbm state originates from fluctuations over many different tilted states; and ii) the cubic Pm3¯m phase is highly locally distorted and develops strong transverse antiphase correlation between first‐nearest neighbor iodine octahedral tiltings, before undergoing a phase transition to P4/mbm under cooling. [ABSTRACT FROM AUTHOR]

Multicolor carbon dot (CD)‐based nanomaterials offer a variety of opportunities for potential applications in bioimaging, optoelectronic devices, and information security. However, it still remains a challenge to modulate the conjugated π‐structure of CDs to achieve multicolor room‐temperature phosphorescence (RTP). Herein, the authors present a strategy based on thermally driven amorphous−crystalline phase transition to achieve multicolor carbonized polymer dots (CPDs) with the emission color tunable from green to orange‐red. This is the first report on multicolor RTP emission from CDs by means of thermal stimulus. Further investigations reveal that the formation of self‐protective covalently crosslinked frameworks and codoping of multiple heteroatoms play a crucial role in the production of RTP. RTP color tunability can be attributed to different crystalline contents of the conjugated π‐domain within CPDs. Potential application of the developed CPDs as printable and writable security inks for advanced multilevel anti‐counterfeiting and encryption is demonstrated. This work paves a path for the development of multicolor RTP materials and suggests great potential of CDs in exploiting novel optical materials toward intriguing applications. [ABSTRACT FROM AUTHOR]

Multicolor carbon dot (CD)‐based nanomaterials offer a variety of opportunities for potential applications in bioimaging, optoelectronic devices, and information security. However, it still remains a challenge to modulate the conjugated π‐structure of CDs to achieve multicolor room‐temperature phosphorescence (RTP). Herein, the authors present a strategy based on thermally driven amorphous−crystalline phase transition to achieve multicolor carbonized polymer dots (CPDs) with the emission color tunable from green to orange‐red. This is the first report on multicolor RTP emission from CDs by means of thermal stimulus. Further investigations reveal that the formation of self‐protective covalently crosslinked frameworks and codoping of multiple heteroatoms play a crucial role in the production of RTP. RTP color tunability can be attributed to different crystalline contents of the conjugated π‐domain within CPDs. Potential application of the developed CPDs as printable and writable security inks for advanced multilevel anti‐counterfeiting and encryption is demonstrated. This work paves a path for the development of multicolor RTP materials and suggests great potential of CDs in exploiting novel optical materials toward intriguing applications. [ABSTRACT FROM AUTHOR]

Lattice dynamics are often regarded as signatures of the underlying crystal structure. Here, a first‐principle‐based effective Hamiltonian method combined with molecular dynamics simulations is used to study dynamical behaviors of CsPbI3 perovskite across temperature and structural phase transitions. A single (short‐range tilting) parameter in this effective Hamiltonian is varied in order to make the temperature range of the intermediate tetragonal P4/mbm phase, existing in‐between the cubic Pm3¯m and orthorhombic Pnma phases, either broader than observed or completely disappearing. Comparing the dynamics of these different cases allows one to conclude that real CsPbI3 perovskite should have i) two iodine‐octahedral‐tilt related modes that differ in frequency but both significantly soften as the temperature decreases within the cubic phase toward the Pm3¯m‐to‐P4/mbm transition; and ii) one mode that maintains a very low frequency (of the order of 1.0 cm−1) in the entire region of P4/mbm stability, as a result of the temporal exploration of various structural states. Such latter sub‐THz mode mixes fluctuations of antiphase iodine tiltings and Cs antipolar motions because of a trilinear energetic coupling. [ABSTRACT FROM AUTHOR]

Thermally Driven Amorphous-Crystalline Phase Transition of Carbonized Polymer Dots for Multicolor Room-Temperature Phosphorescence (Advanced Optical Materials 16/2021) Anti-counterfeiting, carbonized polymer dots, phosphorescence, thermally stimulated, topological transition Keywords: anti-counterfeiting; carbonized polymer dots; phosphorescence; thermally stimulated; topological transition EN anti-counterfeiting carbonized polymer dots phosphorescence thermally stimulated topological transition 1 1 1 08/23/21 20210818 NES 210818 Multicolor carbon dots-based materials offer a variety of opportunities for potential applications in bioimaging, optoelectronic devices, and information security. [Extracted from the article]

Keywords: anti-counterfeiting; carbonized polymer dots; phosphorescence; thermally stimulated; topological transition EN anti-counterfeiting carbonized polymer dots phosphorescence thermally stimulated topological transition 1 1 1 02/15/22 20210815 NES 210815 Multicolor carbon dots-based materials offer a variety of opportunities for potential applications in bioimaging, optoelectronic devices, and information security. Thermally Driven Amorphous-Crystalline Phase Transition of Carbonized Polymer Dots for Multicolor Room-Temperature Phosphorescence (Advanced Optical Materials 16/2021) Anti-counterfeiting, carbonized polymer dots, phosphorescence, thermally stimulated, topological transition. [Extracted from the article]

A luminescent cocrystal system is reported to undergo crystal‐to‐crystal phase transformation from yellow‐emitting polymorph I to green‐emitting polymorph II, triggered by THF fuming or heating, and the green emission can recover to the initial yellow emission by grinding. The established spectroscopic and crystallographic analyses demonstrate that the phase transition occurred and benefits from the combined effect of similar molecular arrange sequence and unique alteration of intermolecular interactions from halogen/hydrogen bonds in I to π–π stacking in II. Furthermore, I and II exhibit red‐shift emission under hydrostatic pressure. The emission of I and II shows a red‐shift and recovers towards the initial emission upon acid–base fuming. This is a rare example of reversible luminescent switching of cocrystal based upon crystal‐to‐crystal phase transition, and provides an alternative strategy to develop multi‐stimuli responsive materials. [ABSTRACT FROM AUTHOR]

A luminescent cocrystal system is reported to undergo crystal‐to‐crystal phase transformation from yellow‐emitting polymorph I to green‐emitting polymorph II, triggered by THF fuming or heating, and the green emission can recover to the initial yellow emission by grinding. The established spectroscopic and crystallographic analyses demonstrate that the phase transition occurred and benefits from the combined effect of similar molecular arrange sequence and unique alteration of intermolecular interactions from halogen/hydrogen bonds in I to π–π stacking in II. Furthermore, I and II exhibit red‐shift emission under hydrostatic pressure. The emission of I and II shows a red‐shift and recovers towards the initial emission upon acid–base fuming. This is a rare example of reversible luminescent switching of cocrystal based upon crystal‐to‐crystal phase transition, and provides an alternative strategy to develop multi‐stimuli responsive materials. [ABSTRACT FROM AUTHOR]

Aggregation-induced emission, crystal-to-crystal phase transition, piezochromic fluorescence switching, polymorph crystals The feather demonstrates the ultrahigh pressure sensitivity of TPE-AQ (~60 kPa, which is a state-of-the-art performance), and the emitted colorful light represents the crystal's unique piezochromic properties. Keywords: aggregation-induced emission; crystal-to-crystal phase transition; piezochromic fluorescence switching; polymorph crystals EN aggregation-induced emission crystal-to-crystal phase transition piezochromic fluorescence switching polymorph crystals 1 1 1 08/29/23 20230825 NES 230825 B The shiny diamond-like object b represents the reported TPE-AQ crystal, with the crystal formation process from the TPE-AQ molecule illustrated in the top-left corner. [Extracted from the article]