Nonlinear Optical Switching in Regioregular Porphyrin Covalent Organic Frameworks

Heterocyclic Donor Moiety Effect on Optical Nonlinearity Behavior of Chrysene-Based Chromophores with Push-Pull Configuration via the Quantum Chemical Approach

Organic-based nonlinear optical (NLO) materials may be used in many optical-electronic systems and other next-generation defense technologies. With the importance of NLO materials, a series of push-pull architecture (D-π-A) derivatives (DTMD2-DTMD6) were devised from DTMR1 through structural alteration of different efficient donor heterocyclic groups. Density functional theory-based computations were executed at the MPW1PW91/6-31G(d,p) level to explore the NLO behavior of the derivatives. To investigate the optoelectronic behavior of the said compounds, various analyses like the frontier molecular orbital (FMO), global reactivity parameters, density of state (DOS), absorption spectra (UV-vis), natural bond orbital, and transition density matrix (TDM) were performed. The derivatives have a smaller band gap (2.156-1.492 eV) and a larger bathochromic shift (λmax = 692.838-969.605 nm) as compared to the reference chromophore (ΔE = 2.306 eV and λmax = 677.949 nm). FMO analysis revealed substantial charge conduction out of the donor toward the acceptor via a spacer that was also shown by TDM and DOS analyses. All derivatives showed promising NLO results, with the maximum amplitude of linear polarizability ⟨α⟩ and first (βtotal) and second (γtotal) hyperpolarizabilities over their reference chromophore. DTMD2 contained the highest βtotal (7.220 × 10-27 esu) and γtotal (1.720 × 10-31 esu) values corresponding with the reduced band gap (1.492 eV), representing potential futures for a large NLO amplitude. This structural modification through the use of various donors has played a significant part in achieving promising NLO behavior in the modified compounds.

Antiaromatic Covalent Organic Frameworks Based on Dibenzopentalenes

Despite their inherent instability, 4n π systems have recently received significant attention due to their unique optical and electronic properties. In dibenzopentalene (DBP), benzanellation stabilizes the highly antiaromatic pentalene core, without compromising its amphoteric redox behavior or small HOMO-LUMO energy gap. However, incorporating such molecules in organic devices as discrete small molecules or amorphous polymers can limit the performance (e.g., due to solubility in the battery electrolyte solution or low internal surface area). Covalent organic frameworks (COFs), on the contrary, are highly ordered, porous, and crystalline materials that can provide a platform to align molecules with specific properties in a well-defined, ordered environment. We synthesized the first antiaromatic framework materials and obtained a series of three highly crystalline and porous COFs based on DBP. Potential applications of such antiaromatic bulk materials were explored: COF films show a conductivity of 4 × 10^-8 S cm^-1 upon doping and exhibit photoconductivity upon irradiation with visible light. Application as positive electrode materials in Li-organic batteries demonstrates a significant enhancement of performance when the antiaromaticity of the DBP unit in the COF is exploited in its redox activity with a discharge capacity of 26 mA h g^-1 at a potential of 3.9 V vs. Li/Li⁺. This work showcases antiaromaticity as a new design principle for functional framework materials.

Covalent Organic Frameworks as Emerging Nonlinear Optical Materials

The vastness of organic synthetic strategies and knowledge of reticular chemistry have made covalent organic frameworks (COFs) one of the most chemically and structurally diverse class of materials with potential applications ranging from gas storage, molecular separation, and catalysis to energy storage and magnetism. Recently, this class of porous materials has garnered increasing interest as potential nonlinear optical (NLO) materials. Traditionally, inorganic crystals, small-molecule organic chromophores, and oligomers have been studied for their NLO response. Nevertheless, COFs offer significant advantages over existing NLO materials in terms of higher mechanical strength, thermochemical stability, and extended conjugation. Herein, we discuss crucial aspects, terminology, and measurement techniques related to NLO, followed by a critical analysis of the design principles for COFs with NLO response. Furthermore, we touch on selected potential applications of these NLO materials. Finally, future prospects and challenges of COFs as NLO materials are discussed.

Nonlinear optical properties and optimization strategies of D-π-A type phenylamine derivatives in the near-infrared region

Modifying simple molecular structures to significantly improve nonlinear optical (NLO) performance is a primary prerequisite for scientific research. Based on the four phenylamine derivatives reported in previous studies, we designed four organic nonlinear molecules by changing the acceptor group and π-linker. (Time-dependent) density functional theory (DFT/TD-DFT) was performed on molecular geometry optimization, the contribution of π electrons to the bond order, linear and two-photon absorption (TPA) spectra, the intra-molecular charge transfer matrix (CTM), and NLO coefficients. These aspects were considered to analyze in detail how the structural modification of acceptors and π-linkers affects NLO characteristics. The three modification methods were: adding a carbonyl group at the junction of the π-linker and the acceptor group, adding a carbonyl group and a nitrogen atom to the acceptor group, and replacing the quinolinone with a pyrenyl group as the π-linker. The latter two methods can significantly reduce the excitation energy and enhance the intensity of intra-molecular charge transfer during the two-photon transition. The maximum TPA cross-sections and wavelengths of the designed molecules are DPPM (84722.6 GM, 815.7 nm) and DDPM (21600.6 GM, 781.3 nm). These two molecules have large TPA cross-sections in the near-infrared region, which renders them as possible NLO materials with broad application prospects.

Fluorescence quenching, DFT, NBO, and TD-DFT calculations on 1, 4-bis [2-benzothiazolyl vinyl] benzene (BVB) and meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS) in the presence of silver nanoparticles

Steady-state fluorescence measurements were used to examine the fluorescence quenching of 1, 4-bis [-(2-benzothiazolyl) vinyl benzene (BVB) by sodium salt of meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS) in the presence and absence of silver nanoparticles (Ag NPs). The energy transfer (ET) process’s emission intensities and Stern–Volmer constants (KSV) showed that Ag NP’s presence increased ET’s efficiency. The molecular structures of TPPS, TPPS, and BVB/TPPS were optimized using the DFT/B3LYP/6-311G (d) technique to elucidate the mechanism. The discovered optimized molecular structure proved that whereas TPPS and BVB/TPPS MSs are not planar because the porphyrin group in TPPS is rotated out by phenyl sodium sulphate, the BVB MS is planer. All of the theoretical BVB results and the acquired experimental optical results were very similar.

Strong second order nonlinear optical properties of azulene-based porphyrin derivatives

The high stability, feasible modification, and good π-conjugation of porphyrin derivatives render these porphyrin-based nanomaterials suitable as potential third order nonlinear optical (NLO) materials. Introducing an azulene in pristine porphyrins can significantly improve the second order NLO properties of the system, and this is studied in the present work using density functional theory based methods and the sum-over-states model. The relative orientation of azulene plays a determinant role in the enhancement of the static first hyperpolarizability (〈β₀〉), e.g., the 〈β₀〉 per heavy atom increases from 0.31 × 10^-30 esu to 9.78 × 10^-30 esu. Further addition of metals (Mg and Zn) in these azulene-fused porphyrin systems leads to an even larger 〈β₀〉 per heavy atom of 41.59 × 10^-30 esu, much larger than that of a recently reported porphyrin derivative (26.47 × 10^-30 esu). A novel strategy to stabilize the electronic structures as well as maintain good second order NLO responses by introducing appropriate metals into the azulene-fused porphyrins is extendable to other similar systems. Strong sum frequency generation and different frequency generations of those azulene-fused porphyrins in visible and near-infrared regions may inspire experimental exploration and related applications of azulene-based porphyrins particularly in biological nonlinear optics.

Mixed superalkalis are a better choice than pure superalkalis for B12N12 nanocages to design high-performance nonlinear optical materials

Mixed superalkali clusters are a source of excess electrons, as their vertical ionization energies (2.81-3.36 eV) are much lower than those of alkali metals (even cesium (∼3.85 eV)) and the superalkali Li₃O (3.42 eV). In the present work, the geometric, electronic, and nonlinear optical (NLO) properties of mixed superalkali cluster-doped B₁₂N₁₂ nanocages are studied theoretically. All complexes, A-G, have very high interaction energies (-98.02 to -123.13 kcal mol^-1) and are thermodynamically stable when compared to previously reported Li₃O@B₁₂N₁₂ (-92.71 kcal mol^-1). The designed complexes have smaller HOMO-LUMO energy gaps (3.36-4.27 eV) than pristine B₁₂N₁₂ (11.13 eV). Charge transfer in the complexes is studied through natural population analysis and non-bonding interactions are evaluated through quantum theory of atoms in molecules (QTAIM) and non-covalent interaction analyses. These complexes have absorption maxima (1076-1486 nm) in the near-infrared region (NIR) and they are transparent in the UV region. The first hyperpolarizability of complex C is 1.7 × 10⁷ au, which is much higher than the value of 3.7 × 10⁴ au for a pure Li₃O superalkali-doped B₁₂N₁₂ complex calculated at the same level of theory, as reported by Sun et al. (Dalton Trans., 2016, 45, 7500-7509). The large second hyperpolarizability values also reflect the enhanced nonlinear optical response. The best computed values for the electro-optical Pockels effect, second harmonic generation, and hyper-Rayleigh scattering are 3.29 × 10¹⁰ au, 1.17 × 10¹⁰ au, and 6.71 × 10⁶ au, respectively. Furthermore, the electro-optic dc-Kerr effect and electric-field-induced second harmonic generation have maximum values of 3.96 × 10¹¹ au and 3.46 × 10¹⁰ au at 1064 nm. There are enhancements in the quadratic nonlinear refractive index (n₂) values for complexes A-G, with a highest n₂ value of 3.35 × 10^-8 cm² W^-1 at 1064 nm. These results suggest that mixed-superalkali-doped B₁₂N₁₂ nanoclusters are potential candidates when designing high-performance NLO materials.

Exploration of Nonlinear Optical Properties for the First Theoretical Framework of Non-Fullerene DTS(FBTTh2)2-Based Derivatives

Organic compounds having significant nonlinear optical (NLO) applications are being employed in the optoelectronics field. In the current work, a series of non-fullerene acceptor (NFA) based compounds are designed by modifying the acceptors with different substituents using DTS(FBTTh ₂ ) ₂ R1 as a reference compound. To study the NLO responses to the tuning of various acceptors, DFT and TD-DFT based parameters were calculated at the M06 level along with the 6-31G(d,p) basis set. The designed compounds (MSTD2-MSTD7) showed smaller values of the energy gap in comparison to the reference compound. The energy gaps of the title compounds were linked to global reactivity insights; MSTD7 provided a lower band gap, with smaller and larger quantities for hardness and softness characteristics, respectively. Further, UV-vis analyses were performed for all of the designed compounds, displaying wavelengths red-shifted from that of DTS(FBTTh ₂ ) ₂ R1 _. The intraelectron transfer (ICT) process and stability of the title compounds were explored via frontier molecular orbital (FMO) and natural bond orbital (NBO) studies, respectively. Out of all the designed compounds, the highest value of linear polarizability ⟨α⟩ of 3.485 × 10^-22 esu, first hyperpolarizability (β_total) of 13.44 × 10^-27 esu and second-order hyperpolarizability ⟨γ⟩ of 3.66 × 10^-31 esu were exhibited by MSTD7. In short, all of the designed compounds exhibited promising NLO properties because of their low charge transport resistance. These NLO properties may be useful for experimental researchers to uncover NLO materials for modern applications.

β-Tetracyanobutadiene-Appended Porphyrins: Facile Synthesis, Spectral and Electrochemical Redox Properties, and Their Utilization as Excellent Optical Limiters

A series of β-TCBD (1,1,4,4-tetracyano-buta-1,3-diene)-appended porphyrins, M-TCBD (M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)), was synthesized from 2,3-diphenylethynyl-12-nitro-meso-tetraphenylporphyrin, H₂-PE₂, and characterized by various spectroscopic techniques and electrochemical studies. The reaction proceeds via [2 + 2] cycloaddition and retroelectrocyclization reactions of tetracyanoethylene (TCNE) with H₂-PE₂. The observed unusual reduction potentials in the cyclic voltammograms of the synthesized porphyrins in the range of -0.06 to -0.10 V are the consequence of the TCBD moiety present at the β-position of the porphyrin macrocycle. Notably, these porphyrins exhibited three porphyrin ring-centered reductions due to extended π-conjugation. The higher nonlinear optical response exhibited by the M-TCBD series as compared to the precursor (H₂-PE₂) was attributed to the existence of intramolecular charge transfer and enhanced polarization in the M-TCBD series. The single-beam femtosecond Z-scan measurements were performed to elucidate the third-order nonlinear optical properties, and the temporal response of these porphyrin molecules was investigated using optical pump-probe spectroscopy to study the excited state absorption dynamics. Z-scan measurements revealed that Co-TCBD exhibited a higher nonlinear optical response as compared to free base porphyrins. The two-photon absorption coefficient (β) and the imaginary part of third-order nonlinear optical susceptibility (χ⁽³⁾) were obtained from the open aperture experiment, whereas the close aperture experiment delivered the magnitude and the sign of the nonlinear refractive index (n₂) and the real part of χ⁽³⁾. Furthermore, the femtosecond transient absorption spectroscopy revealed a faster relaxation dynamics of various absorption processes in a picosecond timescale. The excellent optical limiting threshold (1.90-2.33 × 10¹⁵ W/m²) of the synthesized porphyrins makes them good materials for laser protection and high-power laser operation.

Editing Light Emission with Stable Crystalline Covalent Organic Frameworks via Wall Surface Perturbation

The ordered π skeletons of covalent organic frameworks make them viable light-emitting materials but their limited tunability has precluded further implementation. Here we report the synthesis of hydrazone-linked frameworks which are stable in water, acid, and base, and demonstrate their utility as a platform for light emission. The polygonal backbone is designed to be luminescent and partially π conjugated while the pore wall is docked with single atom or unit to induce resonance, hyperconjugation, and tautomerization effects. These effects can be transmitted to the backbone, so that the framework can emit three primary colors of light. The wall can be perturbated with multiple surface sites, rendering the material able to edit diverse emission colors in a predesignable and digital way. The systems show high activity, stability, tunability, and sensibility: a set of features attractive for light-emitting and sensing applications.

An Ultrastable Crystalline Acylhydrazone-Linked Covalent Organic Framework for Efficient Removal of Organic Micropollutants from Water

As an important member of crystalline porous polymers, acylhydrazone-linked covalent organic frameworks (COFs) have gained much attention in recent years. However, the low structural stability imparts a limit on their practical applications. To tackle this problem, we report a simple strategy to increase the chemical stability of acylhydrazone-linked COFs by incorporating azobenzene groups in the conjugated framework. Through reinforcing the π-π stacking interactions between the adjacent layers with increased π-surface, it is surprising to find that the resulting materials exhibit extreme stability in harsh environments, such as in strong acid, strong base, aqueous educing agent and boiling water, even exposed to air for one year. As a proof-of-concept, such frameworks have been used to remove various organic micropollutants such as antibiotics, plastic components, endocrine disruptors, and carcinogens from water with high capacity, fast speed and excellent reusability over a wide pH range at environmentally relevant concentrations. The results provide a new avenue to significantly enhance the stability of COFs for practical applications.

Influence of a Substituted Methyl on the Photoresponsive Third-Order Nonlinear-Optical Properties Based on Azobenzene Metal Complexes

For studying the effect of a substituted group on the photoresponsive third-order nonlinear-optical (NLO) properties, photosensitive azobenzene derivative H₂L1 was first selected to construct metal complexes {[Zn₂(L1)₂(H₂O)₃]·2DMA)}_n (1) and {[Cd(L1)(4,4'-bpy)H₂O]·H₂O}_n (2). Then H₂L2 with a substituted methyl on the azobenzene ring was used to construct complexes {[Zn(L2)(4,4'-bpy)(H₂O)]}_n (3) and {[Cd(L2)(4,4'-bpy)(H₂O)]}_n (4). When the azobenzene moiety of the complexes is trans, the NLO behaviors of the complexes are the same. However, after the azobenzene moiety is excited by ultraviolet (UV) light to change from trans to cis, the substituted methyl increases the repulsion between two azobenzene rings in 3 and 4, thereby affecting their NLO behaviors. Therefore, the nonlinearity of the two types of complexes is different after UV irradiation. Density functional theory calculations support this result. The substituted methyl has a significant influence on the nonlinear absorption behaviors of 3 and 4. This work not only reports the examples of photoresponsive NLO materials based on metal complexes but also provides a new idea to deeply explore NLO properties.

Metallated Graphynes as a New Class of Photofunctional 2D Organometallic Nanosheets

Two-dimensional (2D) nanomaterials are attracting much attention due to their excellent electronic and optical properties. Here, we report the first experimental preparation of two free-standing mercurated graphyne nanosheets via the interface-assisted bottom-up method, which integrates both the advantages of metal center and graphyne. The continuous large-area nanosheets derived from the chemical growth show the layered molecular structural arrangement, controllable thickness and enhanced π-conjugation, which result in their stable and outstanding broadband nonlinear saturable absorption (SA) properties (at both 532 and 1064 nm). The passively Q-switched (PQS) performances of these two nanosheets as the saturable absorbers are comparable to or higher than those of the state-of-the-art 2D nanomaterials (such as graphene, black phosphorus, MoS₂ , γ-graphyne, etc.). Our results illustrate that the two metallated graphynes could act not only as a new class of 2D carbon-rich materials, but also as inexpensive and easily available optoelectronic materials for device fabrication.

Phenothiazine functional materials for organic optoelectronic applications

Phenothiazine (PTZ) is one of the most extensively investigated S, N heterocyclic aromatic hydrocarbons due to its unique optical, electronic properties, flexibility of functionalization, low cost, and commercial availability. Hence, PTZ and its derivative materials have been attractive in various optoelectronic applications in the last few years. In this prospective, we have focused on the most significant characteristics of PTZ and highlighted how the structural modifications such as different electron donors or acceptors, length of the π-conjugated system or spacers, polar or non-polar chains, and other functional groups influence the optoelectronic properties. This prospective provides a recent account of the advances in phenothiazine derivative materials as an active layer(s) for optoelectronic (viz. dye sensitized solar cells (DSSCs), perovskite solar cells (PSCs), organic solar cells (OSCs), organic light-emitting diodes (OLEDs), organic field-effect transistor (OFETs), chemosensing, nonlinear optical materials (NLOs), and supramolecular self-assembly applications. Finally, future prospects are discussed based on the structure-property relationship in PTZ-derivative materials. This overview will pave the way for researchers to design and develop new PTZ-functionalized structures and use them for various organic optoelectronic applications.

Vertical Heterostructure of SnS-MoS2 Synthesized by Sulfur-Preloaded Chemical Vapor Deposition

We synthesize a vertical heterostructure (HS) between tin sulfide (SnS) and molybdenum sulfide (MoS₂) by chemical vapor deposition based on the preferential adsorption of sulfur on MoS₂. Most of the SnS grains nucleate on MoS₂ nanosheets, formatting partially stacked HS with large overlapping regions. Photoluminescence quenching of MoS₂ is observed and illustrates effective charge separation in HS. The HS shows increased reverse saturable absorption relative to MoS₂ and SnS. The preferential adsorption of sulfur powders on MoS₂ and HS growth reported herein will provide a promising approach to the synthesis of other two-dimensional HS.