Nonlinear Optical Chromophores with Two Ferrocenyl, Octamethylferrocenyl, or 4-(Diphenylamino)phenyl Groups Attached to Rhenium(I) or Zinc(II) Centers

Substituent and redox effects on the second-order NLO response of Ru(ii) complexes with polypyridine ligands: a theoretical study

Modifying ligands in four studied Ru(II) complexes have slight influence on their static first hyperpolarizabilities (βtot) values. However, the βtot values of complexes 1+/−–4+/− are all enhanced and the largest βtot value is observed for complex 4−.

Hyperpolarizabilities of Push-Pull Chromophores in Solution: Interplay between Electronic and Vibrational Contributions

Contemporary design of new organic non-linear optical (NLO) materials relies to a large extent on the understanding of molecular and electronic structure-property relationships revealed during the years by available computational approaches. The progress in theory-hand-in-hand with experiment-has enabled us to identify and analyze various physical aspects affecting the NLO responses, such as the environmental effects, molecular vibrations, frequency dispersion, and system dynamics. Although it is nowadays possible to reliably address these effects separately, the studies analyzing their mutual interplay are still very limited. Here, we employ density functional theory (DFT) methods in combination with an implicit solvent model to examine the solvent effects on the electronic and harmonic as well as anharmonic vibrational contributions to the static first hyperpolarizability of a series of push-pull α,ω-diphenylpolyene oligomers, which were experimentally shown to exhibit notable second-order NLO responses. We demonstrate that the magnitudes of both vibrational and electronic contributions being comparable in the gas phase significantly increase in solvents, and the enhancement can be, in some cases, as large as three- or even four-fold. The electrical and mechanical anharmonic contributions are not negligible but cancel each other out to a large extent. The computed dynamic solute NLO properties of the studied systems are shown to be in a fair agreement with those derived from experimentally measured electric-field-induced second-harmonic generation (EFISHG) signals. Our results substantiate the necessity to consider concomitantly both solvation and vibrational effects in modeling static NLO properties of solvated systems.

Oxidative C-H Homocoupling of Push-Pull Benzothiazoles: An Atom-Economical Route to Highly Emissive Quadrupolar Arylamine-Functionalized 2,2'-Bibenzothiazoles with Enhanced Two-Photon Absorption

Copper(II)-catalyzed C-H/C-H coupling of dipolar 2-H-benzothiazoles end-capped with triphenylamine moieties affords highly fluorescent 2,2'-bibenzothiazoles with quadrupolar (D-π-A-π-D) architecture displaying large two-photon absorption (TPA) cross sections (543-1252 GM) in the near-infrared region. The notably higher TPA performance as compared to quadrupolar π-systems with a widely used 2,2'-bipyridine core, along with the ease of the synthesis and chelating N^N ability, makes the title biheteroaryl platform an attractive building block for a large scope of functional dyes exploiting nonlinear optical phenomena.

Excited-State Switching in Rhenium(I) Bipyridyl Complexes with Donor-Donor and Donor-Acceptor Substituents

The optical properties of two Re(CO)₃(bpy)Cl complexes in which the bpy is substituted with two donor (triphenylamine, TPA, ReTPA₂) as well as both donor (TPA) and acceptor (benzothiadiazole, BTD, ReTPA-BTD) groups are presented. For ReTPA₂ the absorption spectra show intense intraligand charge-transfer (ILCT) bands at 460 nm with small solvatochromic behavior; for ReTPA-BTD the ILCT transitions are weaker. These transitions are assigned as TPA → bpy transitions as supported by resonance Raman data and TDDFT calculations. The excited-state spectroscopy shows the presence of two emissive states for both complexes. The intensity of these emission signals is modulated by solvent. Time-resolved infrared spectroscopy definitively assigns the excited states present in CH₂Cl₂ to be MLCT in nature, and in MeCN the excited states are ILCT in nature. DFT calculations indicated this switching with solvent is governed by access to states controlled by spin-orbit coupling, which is sufficiently different in the two solvents, allowing to select out each of the charge-transfer states.

2-Azabutadiene complexes of rhenium(I): S,N-chelated species with photophysical properties heavily governed by the ligand hidden traits

The reaction of [Re(CO)3(THF)(μ-Br)]2 or [Re(CO)5X] (X = Cl, Br, I) with the diaryl-2-azabutadienes [(RS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CAr2] containing two thioether arms at the 4,4-position forms the luminescent S,N-chelate complexes fac-[(OC)3ReX{(RS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CAr2}] (1a-h). The halide abstraction by silver triflate converts [(OC)3ReCl{(PhS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2}] (1c) to [(OC)3Re(OS([double bond, length as m-dash]O)2CF3){(PhS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2}] (1j) bearing a covalently bound triflate ligand. The cyclic voltammograms reveal reversible S^N ligand-centred reduction and irreversible oxidation waves for all complexes. The crystal structures of nine octahedral complexes have been determined along with that of (NaphtylS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2 (L6). A rich system of weak non-covalent intermolecular secondary interactions through CHX(Cl, Br)Re, CHO, COπ(Ph), CHπCO, CHO and CHS contacts has been evidenced. The photophysical properties have been investigated by steady-state and time-resolved absorption (fs transient absorption, fs-TAS) and emission (ns-TCSPC and ps-Streak camera) spectroscopy in 2-MeTHF solution at 298 and 77 K. The emission bands are composed of either singlet (450 < λmax < 535 nm) and/or triplet emissions (at 77 K only, λmax < 640 nm, or appearing as a tail at λ > 600 nm), which decay in a multiexponential manner for the fluorescence (short ps (i.e. <IRF) < τF < 1.9 ns at 298 and 77 K) and monoexponentially for the phosphorescence (4.0 < τP < 7.0 ns at 77 K). The fs-TAS data reveal the presence of 2 to 4 transient species decaying in four narrow time windows (generally 125-165 fs, 370-685 fs, 3-6 ps, 30-45 ps). The complexity of these kinetics was explained by studying the photophysical behaviour of ligand L6. Its behaviour is the same as the complexes thus indicating that the ligand dictates the kinetic traits of the Re-species, except for the triplet emission as L6 is not phosphorescent. The triplet lifetime (4.0 < τP < 7.0 ns) is considered very short but not unprecedented. Furthermore, the nature of the lowest energy excited states of these chelate compounds and L6 has been addressed using DFT and TDDFT calculations and been assigned to metal-to-ligand (MLCT) and/or intraligand charge-transfer (ILCT).

Novel inorganic aromatic mixed-valent superalkali electride CaN3Ca: an alkaline-earth-based high-sensitivity multi-state nonlinear optical molecular switch

Focusing on innovative high-performance single-pole double-throw nonlinear optical (NLO) molecular switches, two C3v configurations (1 and 3) and one D3h configuration (2) of bipyramidal CaN3Ca have been obtained by using quantum mechanical methods. Not only are 1, 2, and 3 alkaline-earth-based aromatic superalkalis, but they are also interesting electrides. The salt-like electronic structures of e-Ca2+N33-Ca2+ (1) and Ca2+N33-Ca2+e- (3) with localized redox centres are rare inorganic Robin-Day class II-type structures, and e0.5-Ca2+N33-Ca2+e0.5- (2) with a delocalized structure is a class III-type mixed-valent superalkali electride. Under a small external electric field of ±0.0110 a.u. (0.565 V Å-1), the short-distance hopping of Ca atoms in CaN3Ca from the D3h configuration with in-plane aromaticity to each C3v configuration with out-of-plane aromaticity brings about the long-range transfer of half an electron from one Ca atom to another. And, subsequently, a large dipole moment (μ0) and remarkable static first hyperpolarizability (β0) occur. μz and βzzz range from 0 (D3h, off form) to -12.1 or 12.1 D (C3v, on forms) and from 0 (D3h, off form) to -19 428 or 19 428 a.u. (C3v, on forms), respectively. These extremely large differences in μz and βzzz values between the D3h and each of the C3v configurations confirm the potential of these inorganic aromatic Robin-Day-type superalkali electrides for applications in high-sensitivity multi-state nonlinear optical switches.

Palladium(ii) complexes of tetradentate donor–acceptor Schiff base ligands: synthesis and spectral, structural, thermal and NLO properties

Structural and NLO behavior of push–pull palladium(ii) complexes of metallocenyl-containing asymmetric Schiff base ligands.

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.

Spontaneous Symmetry Breaking Facilitates Metal-to-Ligand Charge Transfer: A Quantitative Two-Photon Absorption Study of Ferrocene-phenyleneethynylene Oligomers

Change of the permanent molecular electric dipole moment, Δμ, in a series of nominally centrosymmetric and noncentrosymmteric ferrocene-phenyleneethynylene oligomers was estimated by measuring the two-photon absorption cross-section spectra of the lower energy metal-to-ligand charge-transfer transitions using femtosecond nonlinear transmission method and was found to vary in the range up to 12 D, with the highest value corresponding to the most nonsymmetric system. Calculations of the Δμ performed by the TD-DFT method show quantitative agreement with the experimental values and reveal that facile rotation of the ferrocene moieties relative to the organic ligand breaks the ground-state inversion symmetry in the nominally symmetric structures.

Cyclometalated platinum(ii) complexes of 2,2'-bipyridine N-oxide containing a 1,1'-bis(diphenylphosphino)ferrocene ligand: structural, computational and electrochemical studies

The preparation and characterization of new heteronuclear-platinum(ii) complexes containing a 1,1'-bis(diphenylphosphino)ferrocene (dppf) ligand are described. The reaction of the known starting complex [PtMe(κ²N,C-bipyO-H)(SMe₂)], A, in which bipyO-H is a cyclometalated rollover 2,2'-bipyridine N-oxide, with the dppf ligand in a 2 : 1 ratio or an equimolar ratio led to the formation of the corresponding binuclear complex [Pt₂Me₂(κ²N,C-bipyO-H)₂(μ-dppf)], 1, or the mononuclear complex [PtMe(κ¹C-bipyO-H)(dppf)], 2, respectively. According to the reaction conditions, the dppf ligand in 1 and 2 behaves as either a bridging or chelating ligand. All complexes were characterized by NMR spectroscopy. The solid-state structure of 2 was determined by the single-crystal X-ray diffraction method and it was shown that the chelating dppf ligand in this complex was arranged in a "synclinal-staggered" conformation. Also, the occurrence of intermolecular C-H_CpO_bipyO-H interactions in the solid-state gave rise to an extended 1-D network. The electronic absorption spectra and the electrochemical behavior of these complexes are discussed. Density functional theory (DFT) was used for geometry optimization of the singlet states in solution and for electronic structure calculations. The analysis of the molecular orbital (MO) compositions in terms of occupied and unoccupied fragment orbitals in 2 was performed.

Ferrocene amphiphilic D–π–A dyes: synthesis, redox behavior and determination of band gaps

We reported the synthesis, optical and redox behavior of six ferrocenyl D–π–A dyes. Optical and electrochemical band gaps were determined and corroborated by TD-DFT calculations. Compounds4exhibit the smallest band gap of this series.

Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores

Three new dipolar cations have been synthesised, containing ferrocenyl (Fc) electron donor groups attached to helquat (Hq) acceptors. These organometallic Hq derivatives have been characterised as their TfO^- salts by using various techniques including NMR and electronic absorption spectroscopies and electrochemical measurements. UV-vis spectra show multiple intense low energy absorptions attributable to intramolecular charge-transfer (ICT) excitations. Each compound displays a reversible Fc^+/0 redox process, together with two reversible one-electron reductions of the Hq fragment. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering at 1064 nm, and Stark (electroabsorption) spectroscopic studies on the visible absorption bands. The obtained first hyperpolarizabilities β are moderate, consistent with the relatively short π-conjugation lengths between the Fc and attached pyridinium group. A single-crystal X-ray structure has been solved for one of the complexes as its PF₆^- salt, revealing a centrosymmetric packing in the triclinic space group P1[combining macron]. Density functional theory (DFT) and time-dependent DFT calculations indicate that the lowest energy absorption bands have mainly metal-to-ligand charge-transfer character. The donor orbitals involved in the electronic transitions forming the next lowest energy ICT band also have substantial contributions from the Fe atom. Good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the PBE0 functional with the 6-311++G(d)/LANL2DZ mixed basis set, and the theoretical β values are reasonably large. Oxidation of the Fc unit is predicted to cause the β_tot value to decrease by more than 80% in one of the complexes.

Thermally stable ferrocene-α-cyanostilbenes as efficient materials for second order nonlinear optical polarizability

Thermally stable ferrocene-α-cyanostilbenes exhibit non-zero dipole moments as well as good structure dependent second order nonlinear optical response.

1,1′-Diacetyloctamethylferrocene: an overlooked and overdue synthon leading to the facile synthesis of an octamethylferrocenophane

Surprisingly easy access to versatile synthons: high yielding acylation, chloroformylation and intramolecular cyclisation reaction to a ferrocenophane.

Four-coordinate nickel(ii) and copper(ii) complex based ONO tridentate Schiff base ligands: synthesis, molecular structure, electrochemical, linear and nonlinear properties, and computational study

The properties of NLO active ternary Ni(ii)- and Cu(ii)-unsymmetrical Schiff base complexes were explored in concert with DFT calculations.