DFT Calculation of Static First Hyperpolarizabilities and Linear Optical Properties of Metal Alkynyl Complexes

Deciphering the Role of Alkali Metals (Li, Na, K) Doping for Triggering Nonlinear Optical (NLO) Properties of T-Graphene Quantum Dots: Toward the Development of Giant NLO Response Materials

Nanoscale nonlinear optical (NLO) materials have received huge attention of the scientists in current decades because of their enormous applications in optics, electronics, and telecommunication. Different studies have been conducted to tune the nonlinear optical response of the nanomaterials. However, the role of alkali metal (Li, Na, K) doping on triggering the nonlinear optical response of nanomaterials by converting their centrosymmetric configuration into noncentrosymmetric configuration is rarely studied. Therefore, to find a novel of way of making NLO materials, we have employed density functional theory (DFT) calculations, which helped us to explore the effect of alkali metal (Li, Na, K) doping on the nonlinear optical response of tetragonal graphene quantum dots (TGQDs). Ten new complexes of alkali metal doped TGQDs are designed theoretically. The binding energy calculations revealed the stability of alkali metal doped TGQDs. The NLO responses of newly designed complexes are evaluated by their polarizability, first hyperpolarizability (β_o), and frequency dependent hyperpolarizabilities. The Li@r8a exhibited the highest first hyperpolarizability (β_o) value of 5.19 × 10⁵ au. All these complexes exhibited complete transparency in the UV region. The exceptionally high values of β_o of M@TGQDs are accredited to the generation of diffuse excess electrons, as indicated by NBO analysis and PDOS. NCI analysis is accomplished to examine the nature of bonding interactions among alkali metal atoms and TGQDs. Our results suggest alkali metal doped TGQD complexes as potential candidates for nanoscale NLO materials with sufficient stability and enhanced NLO response. This study will open new doors for making giant NLO response materials for modern hi-tech applications.

Evolutionary search for (M©B16)Q (M = Sc-Ni; Q = 0/-1) clusters: bowl/boat vs. tubular shape

Herein, using universal structure predictor: evolutionary xtallography (USPEX) method, followed by density functional theory (DFT) calculations, we performed global searches for the most stable structures of (M©B₁₆)^Q (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni; Q = 0, -1) clusters. It was found that the obtained ground-state structures of (M©B₁₆)^Q clusters exhibited a distinct structural evolution as M changed from V to Ni: from bowl-shaped, to boat-shaped, to an M-centered tubular structure named wheel-shaped, to drum-shaped (the metal atom was adsorbed on top of the cross section of the B₁₆ species). Our analysis shows that hyper-coordination and the size of the metal atom are two competing factors determining the relative stability and topological properties of the (M©B₁₆)^0/-1 clusters, resulting in unprecedented structures for Sc, Ti, and Ni-doped clusters. The calculated binding energies for these new configurations are even larger than those of the previously synthesized B₁₆^-1, (Mn©B₁₆)^-1, and (Co©B₁₆)^-1 clusters, indicating their very good stability and possible experimental synthesis. A net charge transfer from the metal atom to the boron moiety occurs for all clusters, indicating that electrostatic interactions play an important role in the stability of these materials. Finally, the Sc©M₁₆ and Ti©B₁₆ clusters exhibit not only excellent thermal stability but also large first hyper-polarizability. Hence, they are expected to be potential innovative candidates for excellent electro-optical materials.

Redox-switching of ternary Ni(ii) and Cu(ii) complexes: synthesis, experimental and theoretical studies along with second-order nonlinear optical properties

The redox-switching behavior of the title compounds has been probed for the Ni(ii) derivatives using time-resolved spectroelectrochemistry under thin-layer conditions.

Long-Range Corrected DFT Calculations of First Hyperpolarizabilities and Excitation Energies of Metal Alkynyl Complexes

The performance of the CAM-B3LYP, ωB97X and LC-BLYP long-range corrected density functional theory methods in the calculation of molecular first hyperpolarizabilities (β) and low-lying charge transfer (CT) excitation energies of the metal alkynyl complexes M(C≡C-4-C₆ H₄ -1-NO₂ )(κ² -dppe)(η⁵ -C₅ H₅ ) [M=Fe (1), Ru (2), Os (3)] and trans-[Ru{C≡C-(1,4-C₆ H₄ C≡C)_n -4-C₆ H₄ -1-NO₂ }Cl(κ² -dppm)₂ ] [n=0 (4), 1 (5), 2 (6)] was assessed. The BLYP, B3LYP and PBE0 standard exchange-correlation functionals and the Hartree-Fock method were also examined. The BLYP functional was shown to perform poorly in the calculation of β and low-energy CT transitions. The hybrid functionals (B3LYP and PBE0) showed significant improvement over the pure functional BLYP, but overestimated the hyperpolarizability ratios and the wavelengths of the lowest energy metal-to-ligand CT transitions for 5 and 6. The effect of long-range corrections is noteworthy, particularly for the larger complexes, improving the calculation of β ratios for 4-6. However, CAM-B3LYP, ωB97X, and LC-BLYP considerably overestimated the low-lying CT energies. PBE0 was found to give the best transition energy match for 4. The influence of the phenylene ring orientation in the alkynyl ligand on the calculated properties is substantial, particularly for the larger complexes. For these types of calculations, a basis set with diffuse functions (at least 6-31+G(d)) for the heavy elements is recommended.

Quadratic and cubic hyperpolarizabilities of nitro-phenyl/-naphthalenyl/-anthracenyl alkynyl complexes

1-Nitronaphthalenyl-4-alkynyl and 9-nitroanthracenyl-10-alkynyl complexes [M](C[triple bond, length as m-dash]C-4-C10H6-1-NO2) ([M] = trans-[RuCl(dppe)2] (6b), trans-[RuCl(dppm)2] (7b), Ru(PPh3)2(η5-C5H5) (8b), Ni(PPh3)(η5-C5H5) (9b), Au(PPh3) (10b)) and [M](C[triple bond, length as m-dash]C-10-C14H8-9-NO2) ([M] = trans-[RuCl(dppe)2] (6c), trans-[RuCl(dppm)2] (7c), Ru(PPh3)2(η5-C5H5) (8c), Ni(PPh3)(η5-C5H5) (9c), Au(PPh3) (10c)) were synthesized and their identities were confirmed by single-crystal X-ray diffraction studies. Electrochemical studies and a comparison to the 1-nitrophenyl-4-alkynyl analogues [M](C[triple bond, length as m-dash]C-4-C6H4-1-NO2) ([M] = trans-[RuCl(dppe)2] (6a), trans-[RuCl(dppm)2] (7a), Ru(PPh3)2(η5-C5H5) (8a), Ni(PPh3)(η5-C5H5) (9a), Au(PPh3) (10a)) reveal a decrease in oxidation potential for ruthenium and nickel complexes on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. HOMO → LUMO transitions characteristic of MC[triple bond, length as m-dash]C-1-C6H4 to 4-C6H4-1-NO2 charge transfer red-shift and gain in intensity on proceeding to the ruthenium complexes; the low-energy transitions have increasing ILCT character on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. Spectroelectrochemical studies of the Ru-containing complexes reveal the appearance of low-energy bands corresponding to chloro-to-RuIII charge transfer that red-shift on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. Second-order nonlinear optical (NLO) studies at 1064 nm employing ns pulses and the hyper-Rayleigh scattering technique reveal an increase in quadratic optical nonlinearity upon introduction of metal to the precursor alkyne to afford alkynyl complexes and on proceeding from ligated-gold to -nickel and then to -ruthenium for a fixed alkynyl ligand. Quadratic NLO data of the gold complexes optically transparent at the second-harmonic wavelength reveal an increase in βHRS on proceeding from the phenyl- to the naphthalenyl-containing complex. Broad spectral range third-order nonlinear optical studies employing fs pulses and the Z-scan technique reveal an increase in two-photon absorption cross-section on replacing ligated-gold by -nickel and then -ruthenium for a fixed alkynyl ligand. Computational studies undertaken using time-dependent density functional theory have been employed to assign the nature of the key optical transitions and suggest that the significant optical nonlinearities observed for the ruthenium-containing complexes correlate with the low-energy formally Ru → NO2 band which possesses strong MLCT character, while the more moderate nonlinearities of the gold complexes correlate with a band higher in energy that is primarily ILCT in character.

Theoretical investigation on second-order nonlinear optical properties of ruthenium alkynyl-dihydroazulene/vinylheptafulvene complexes

Ru metal acetylide electron donor-acceptor complexes have important applications in the field of nonlinear optics. Herein, in this work, a series of half-sandwich ruthenium-based Cp*(dpe)Ru ([Ru*]) metal complexes with the dihydroazulene/vinylheptafulvene (DHA/VHF) have been investigated by density functional theory (DFT) calculations. The results showed that the position of the [Ru*] acetylide functionality, either para or meta on the phenylene ring to the DHA/VHF core (1c/1o and 2c/2o), and additional a p-phenylene spacer (3c/3o) had a great influence on the second-order nonlinear optical (NLO) responses. The systems 1 and 3 can significantly increased second-order NLO responses compared with system 2. It was attributed to the more obvious charge transfer along y-axis, which is from [Ru*] acetylide functionality to DHA, accompanied by a significant decrease of the transition energy according electron density difference maps and time-dependent DFT calculations. The β_vec values of the open-ring complexes were larger than the corresponding closed-ring complexes owing to the smaller HOMO-LUMO gap in the open-ring complexes. It was also because of the smaller BLA values in open-ring complexes, which had stronger π-conjugation. Especially, the change ratio of β_vec value of system 2 was the largest due to the fact that their charge transfers degree varied greatly. In addition, the frequency-dependent NLO properties of the studied complexes were evaluated at 0.0239 a.u. and 0.0340 a.u. The calculation results demonstrated that the magnitude of the frequency-dependent first hyperpolarizability increased with the increasing frequency. We believe that our present work will be beneficial for further theoretical and experimental studies on large second-order NLO responses of metal complexes.

Second-order NLO responses of two-cavity inorganic electrides Lin@B20H26 (n = 1, 2): evolutions with increasing excess electron number and various B-B connection sites of B20H26

Confining excess electrons in a specific space is an effective strategy to design nonlinear optical (NLO) molecules. The complexants with excess electrons are usually organic compounds, but these compounds are thermally unstable and thus hardly meet the processing requirements of NLO materials. To obtain better thermostability and NLO response molecules, in this work, inorganic compounds of B₂₀H₂₆ isomers containing two cavities were proposed. With the two included cavities, B₂₀H₂₆ can be doped by one or two Li atoms to form electrides of Li@B₂₀H₂₆ and Li₂@B₂₀H₂₆. These electrides show larger NLO responses, with respect to the corresponding undoped complexant of B₂₀H₂₆. Particularly, Li₂@B₂₀H₂₆ has the largest β₀ value of 108 846 a.u. (MP2/6-31+G(d) level) that is 850 times as large as that of corresponding B₂₀H₂₆. Moreover, the change of β₀ values with excess electron number is remarkable for two of the isomers, and differences between the β₀ values among those isomers are also significant owing to various B-B connection sites between the two cavities. Therefore, the present inorganic electrides have not only better performance due to the magnitude of their β₀ values but also better behavior on the molecular-level modulation of NLO.

Fe(dppe)(η5-C5Me5)-Based Phenylalkynyl Complexes Featuring an NO2 End Group: A Theoretical Analysis

Electronic structures and optical properties of a series of compounds Fe(dppe)(η5-C5Me5){[C≡C(1,4-C6H4)]nNO2} (1–3; n = 1–3, dppe = 1,2-bis(diphenylphosphino)ethane) were investigated with the aid of density functional theory and time-dependent density functional theory computations. The results reveal that the extension of the carbon-rich chain has a moderate influence on the electronic properties of the studied compounds in their ground state, but affects notably their spectroscopic properties, with some unexpected hypsochromic shift of the lower energy absorption bands upon carbon chain lengthening. The appropriate excitations responsible for the low-energy absorption bands involve mainly HOMO–LUMO transitions dominated by important metal-to-ligand charge transfer. A functional including long-range corrections is necessary to reproduce the experimental results.