Engineering TADF, mechanochromism, and second harmonic up-conversion properties in regioisomeric substitution space.

This research article explores the distinct TADF efficiency of three donor-acceptor based regio-isomers: DPAOCN (ortho-isomer), DPAMCN (meta-isomer), and DPAPCN (para-isomer). DPAPCN exhibits maximum TADF efficiency in both solution and solid-state with an impressive reverse inter-system crossing (RISC) rate of ∼106 s-1; the underlying cause being the minimum singlet-triplet splitting energy or ΔEST and maximum SOC (spin-orbit coupling) between the S1 & T1 states. Apart from TADF, differences in crystal packing of the regio-isomers result in intriguing bulk phase properties. DPAOCN, with its non-centrosymmetric P212121 space group and substantial crystal void volume, exhibits reversible tri-color mechanochromic luminescence behavior, while the meta and para isomers, due to their centrosymmetric packing and diminished crystal void volume, remain inert to mechanical pressure. Expanding the horizon of possibilities, the non-centrosymmetric nature of ortho-isomer further renders it an excellent SHG material, with a χ(2) value of 0.19 pm V-1 at 1220 nm and a laser-induced damage threshold (LIDT) value of 13.27 GW cm-2. Overall, a comprehensive investigation into the regio-isomers has been carried out, encompassing their TADF, SHG, and mechanochromic luminescent properties.

Sb3+ -Er3+ -Codoped Cs2 NaInCl6 for Emitting Blue and Short-Wave Infrared Radiation.

Cs2 NaInCl6 double perovskite is stable, environmentally benign and easy to prepare. But it has a wide band gap (5.1 eV), and therefore, does not show optical and optoelectronic properties in the visible and short-wave infrared (SWIR) region. Here we introduce such functionalities in Cs2 NaInCl6 by codoping Sb3+ (s-electron doping) and Er3+ (f-electron doping) ions. Sb3+ doping introduces optically allowed 5s2 → 5s1 5p1 electronic absorption at the sub-band gap level, which then emits blue photoluminescence with ≈93 % quantum yield. But f-f electronic absorption of Er3+ is parity forbidden. Codoping Sb3+ -Er3+ , leads to transfer of excitation energy from Sb3+ to Er3+ , yielding SWIR emission at 1540 nm. Temperature (6 to 300 K) dependent photoluminescence measurements elucidate the excitation and emission mechanism. A phosphor converted light emitting diode (pc-LED) fabricated by using the codoped sample emits stable blue and SWIR radiation over prolonged (84 hours) operation at 5.1 V.

Origin of Luminescence in Sb3+- and Bi3+-Doped Cs2SnCl6 Perovskites: Excited State Relaxation and Spin-Orbit Coupling.

Sb3+- and Bi3+-doped Cs2SnCl6 zero-dimensional perovskites are emerging as stable and nontoxic phosphors for light emitting diodes. The outermost s-electrons (ns2) of the dopants are responsible for both light absorption (ns2 to ns1np1) and emission (ns1np1 to ns2). At cryogenic temperatures, the Sb3+ dopant shows two emission peaks, but Bi3+ shows only one emission peak. Why? Here we address such questions, revealing the origin of luminescence in Sb3+- and Bi3+-doped Cs2SnCl6. We find that the emitting excited state ns1np1 is a triplet state 3T1u*. The notation "*" implies spin-orbit coupling between the 3T1u and 1T1u states. After light absorption, 3T1u* is occupied with one electron, which then undergoes Jahn-Teller distortion yielding a relaxed excited state (RES). For the Sb3+ dopant, the combination of Jahn-Teller distortion and spin-orbit coupling gives rise to two minima in RES 3T1u*, resulting in two emission peaks, whereas for the Bi3+ dopant, the spin-orbit coupling significantly dominates over the Jahn-Teller splitting yielding a single minimum in RES 3T1u* and, therefore, a single emission peak.

Bi3+ -Er3+ and Bi3+ -Yb3+ Codoped Cs2 AgInCl6 Double Perovskite Near-Infrared Emitters.

Bi3+ and lanthanide ions have been codoped in metal oxides as optical sensitizers and emitters. But such codoping is not known in typical semiconductors such as Si, GaAs, and CdSe. Metal halide perovskite with coordination number 6 provides an opportunity to codope Bi3+ and lanthanide ions. Codoping of Bi3+ and Ln3+ (Ln=Er and Yb) in Cs2 AgInCl6 double perovskite is presented. Bi3+ -Er3+ codoped Cs2 AgInCl6 shows Er3+ f-electron emission at 1540 nm (suitable for low-loss optical communication). Bi3+ codoping decreases the excitation (absorption) energy, such that the samples can be excited with ca. 370 nm light. At that excitation, Bi3+ -Er3+ codoped Cs2 AgInCl6 shows ca. 45 times higher emission intensity compared to the Er3+ doped Cs2 AgInCl6 . Similar results are also observed in Bi3+ -Yb3+ codoped sample emitting at 994 nm. A combination of temperature-dependent (5.7 K to 423 K) photoluminescence and calculations is used to understand the optical sensitization and emission processes.