Giant Enhancement of Second Harmonic Generation Accompanied by the Structural Transformation of 7‐Fold to 8‐Fold Interpenetrated Metal–Organic Frameworks (MOFs)

Abstract

Interpenetration in metal–organic frameworks (MOFs) is an intriguing phenomenon with significant impacts on their properties, and functional applications. Herein, we show that a 7‐fold interpenetrated MOF (1) is transformed into an 8‐fold interpenetrated MOF by the loss of DMF in a single‐crystal‐to‐single‐crystal manner. This is accompanied by a giant enhancement of the second harmonic generation (SHG ca. 125 times) and two‐photon photoluminescence (ca. 14 times). The strengthened π–π interaction between the individual diamondoid networks and intensified oscillator strength of the molecules aid the augment of dipole moments and boost the nonlinear optical conversion efficiency. Large positive and negative thermal expansions of 1 occur at 30–150 °C before the loss of DMF. These results offer an avenue to manipulate the NLO properties of MOFs using interpenetration and provide access to tunable single‐crystal NLO devices.

Efficient and Anisotropic Second Harmonic Generation in Few‐Layer SnS Film

“…[57,59] Nonetheless, when normalization with monolayer SnS thickness of 0.54 nm, [22] the effective nonlinear second-order susceptibility of few-layer SnS is 1.37 pm V −1 , which is comparable to the traditional nonlinear crystals such as BBO (2.2 pm V −1 ), [60] KTP (2 pm V −1 ), [61] KDP (0.8 pm V −1 ), [62] LiNbO 3 (−31.8 pm V −1 ), [60] and Urea (1 pm V −1 ). [55] In order to further explore the correlation between SHG effect and the crystal structure, we monitored SHG signals as a function of the laser polarization in a reflection geometry with a normal incidence excitation. [63][64][65] During the measurement, the excitation polarizer was firstly parallel to the emission polarizer, and the orientation of the sample was rotated inplane to vary the azimuthal angle between incident laser beam and crystal lattice.…”

“…Our results suggest that SHG intensity of monolayer MoS 2 is about five times bigger than that of few‐layer SnS film (thickness: 30 nm, Figure 3c). The SHG intensity can be expressed as below: [ 55 ] …”

“…[ 57,59 ] Nonetheless, when normalization with monolayer SnS thickness of 0.54 nm, [ 22 ] the effective nonlinear second‐order susceptibility of few‐layer SnS is 1.37 pm V −1 , which is comparable to the traditional nonlinear crystals such as BBO (2.2 pm V −1 ), [ 60 ] KTP (2 pm V −1 ), [ 61 ] KDP (0.8 pm V −1 ), [ 62 ] LiNbO 3 (−31.8 pm V −1 ), [ 60 ] and Urea (1 pm V −1 ). [ 55 ]…”

In-Plane Anisotropic Nonlinear Optical Properties of Two-Dimensional Organic–Inorganic Hybrid Perovskite

“…Abdelwahab and Dichtl et al characterized third-harmonic generation (THG) and third-order susceptibility χ (3) of layered Ruddlesden–Popper halide perovskites (RPPs) (CH 3 (CH 2 ) 3 NH 3 ) 2 PbBr 4 ( n = 1) and (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n −1 Pb n I 3 n +1 ( n = 1–4), which shows that THG is significantly enhanced at excitonic resonances and varied with the flake thickness because of signal depletion and phase-matching conditions. , Saouma et al explored similar THG enhancement of (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n −1 Pb n I 3 n +1 ( n = 1–4), and the obtained χ (3) ranges from (2.6 ± 0.5) × 10 –11 to (5.6 ± 1.0) × 10 –11 esu in the IR window . Actually, NLO parameters, e.g., two-photon absorption (2PA) cross-section (σ 2 ), three-photon absorption cross-section (σ 3 ), and third-order susceptibility (χ (3) ), are known as strongly wavelength-dependent and scale with excitation intensity within the bandgap. − Inherent multiple-quantum well (MQW) structure of excitons also reinforces the NLO properties of the 2D OIHPs . Li et al investigated 2PA of single-crystal hybrid perovskite (C 4 H 9 NH 3 ) 2 (NH 2 CHNH 2 )­Pb 2 Br 7 and observed a prominent 2PA coefficient of 5.76 × 10 3 cm/GW, while Liu et al characterized giant 2PA properties of (C 6 H 5 (CH 2 ) 2 NH 3 ) 2 PbI 4 .…”

“…41 Combining the exciton lifetimes of the three perovskite flakes (Figure S4) and their respective quantum yields, we can determine the radiative decay rate of the three flakes (k r PEPI = 0.20 ns −1 ; k r C6H11 = 0.16 ns −1 ; k r C4PI = 0.12 ns −1 ). On the basis of the theoretical Strickler−Berg equation, k r = 0.688⟨v⟩ av 2 n 2 f, the radiative decay rate (k r ) is proportional to the oscillator strength (f) of the pertinent electronic transition, 28,47 where n is the refractive index of the host material and ṽis the transition frequency in the unit of wavenumbers. Our results indicate that PEPI exhibits the largest oscillator strength and transition dipole moment which would give rise to the strongest multiphoton emission among the three studied crystals.…”