Sequential Multistep Excited-State Structural Transformations in N,N'-Diphenyl-dihydrodibenzo[a,c]phenazine Fluorophores.

We demonstrate that a single polycyclic π-scaffold can undergo sequential multistep excited-state structural evolution along the bent, planar, and twisted conformers, which coexist to produce intrinsic multiple fluorescence emissions in room-temperature solution. By installing a methyl or trifluoromethyl group on the ortho-site of N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC), the enhanced steric effects change the fluorescence emission of DPAC from a dominant red band to well-resolved triple bands. The ultra-broadband triple emissions of ortho-substituted DPACs range from ≈350 to ≈850 nm, which is unprecedented for small fluorophores with molecular weight of <500. Ultrafast spectroscopy and theoretical calculations clearly reveal that the above dramatic changes originate from the influence of steric hindrance on the shape of excited state potential energy surface (S1 PES). Compared to the steep S1 PES of parental DPAC, the introduction of ortho-substituent is shown to make the path of structural evolution in S1 wider and flatter, so the ortho-substituted derivatives exhibit slower structural transformations from bent to planar and then to twisted forms, yielding intrinsic triple emission. The results provide the proof of concept that the bent, planar, and twisted emissive states can coexist in the same S1 PES, which greatly expand the fundamental understanding of the excited-state structural relaxation.

Bright Triplet Self-Trapped Excitons to Dopant Energy Transfer in Halide Double-Perovskite Nanocrystals.

For inorganic semiconductor nanostructure, excitons in the triplet states are known as the "dark exciton" with poor emitting properties, because of the spin-forbidden transition. Herein, we report a design principle to boost triplet excitons photoluminescence (PL) in all-inorganic lead-free double-perovskite nanocrystals (NCs). Our experimental data reveal that singlet self-trapped excitons (STEs) experience fast intersystem crossing (80 ps) to triplet states. These triplet STEs give bright green color emission with unity PL quantum yield (PLQY). Furthermore, efficient energy transfer from triplet STEs to dopants (Mn2+) can be achieved, which leads to white-light emitting with 87% PLQY in both colloidal and solid thin film NCs. These findings illustrate a fundamental principle to design efficient white-light emitting inorganic phosphors, propelling the development of illumination-related applications.

Carrier Multiplication and Hot-Carrier Cooling Dynamics in Quantum-Confined CsPbI3 Perovskite Nanocrystals.

Carrier multiplication (CM) is an effective mechanism that makes it possible to use hot carriers (HCs) to bypass the Shockley-Queisser limit for solar-cell efficiency. In this paper, we present a detailed study of both CM and HC cooling dynamics in quantum-confined CsPbI3 perovskite nanocrystals (NCs), using femtosecond transient absorption spectroscopy. Our results show that barrierless CM, with an efficiency exceeding 90%, can be achieved in strongly confined NCs on a time scale of ≪200 fs. A low CM efficiency (∼40%), however, is observed in weakly confined NCs. HC cooling dynamics suggests the absence of an intrinsic phonon bottleneck in strongly confined NCs. Furthermore, the biexciton Auger rate increased 4-fold in strongly confined NCs compared to that in weakly confined NCs. These results suggest that the enhanced CM in strongly confined NCs likely originates from enhanced Coulomb coupling and relaxed momentum conservation.

Self-trapped exciton engineering for white-light emission in colloidal lead-free double perovskite nanocrystals.

Intrinsic broadband photoluminescence (PL) of self-trapped excitons (STEs) are systematically studied in lead-free double perovskite nanocrystals (NCs). It is clarified that bandgap (direct/indirect) has important influence on the PL properties of STEs: indirect bandgap NCs exhibit strong exciton-phonon coupling which results in non-radiative STEs, while direct bandgap NCs exhibit moderate exciton-phonon coupling, inducing bright STE PL. Furthermore, by alloying K+ and Li+ ions in Cs2AgInCl6 NCs, the NCs exhibit broadband white-light emission. Charge-carrier dynamics study indicates that the efficient white-light emission originates from the further suppressed non-radiative processes of the STEs in the direct bandgap structure. This work may deepen the understanding of STEs and guide the design of high-performance lead-free perovskites.