Optically Multiresponsive Heteroleptic Platinum Dithiolene Complex with Proton-Switchable Properties

A Multiresponsive Calix[6]arene Pseudorotaxane Empowered by Fluorophoric Dansyl Groups

We report the synthesis and characterization, by means of NMR and UV-visible spectroscopy and electrochemical techniques, of a dansyl calix[6]arene derivative and of its pseudorotaxane complex with a bipyridinium-based axle. This novel macrocycle shows remarkable complexation ability, in analogy with parent compounds, while the dansyl moieties impart valuable features to the system. Indeed, these units: i) signal the state of the system by fluorescence; ii) can be reversibly protonated, enabling the modulation of the complexation abilities of the macrocycle; iii) participate in photoinduced electron transfer processes, which may be exploited to tune the stability of the supramolecular complex. Therefore, in this multiresponsive pseudorotaxane, the threading and de-threading motions of the molecular components can be modulated either by protonation of the calixarene host or by reduction of the bipyridinium guest, which can be accomplished both by electrochemical reduction and via photoinduced electron transfer. Overall, three orthogonal and reversible stimuli can be used to induce molecular movements of the pseudorotaxane components.

Halogen-Bonding-Mediated Radical Reactions: The Unexpected Behavior of Piperazine-Based Dithiooxamide Ligands in the Presence of Diiodine

N,N'-Dialkylpiperazine-2,3-dithiones (R2pipdt) were recognized as a class of hexa-atomic cyclic dithiooxamide ligands with peculiar charge-transfer donor properties toward soft electron-acceptors such as noble metal cations and diiodine. The latter interaction is nowadays better described as halogen bonding. In the reaction with diiodine, R2pipdt unexpectedly provides the corresponding triiodide salts, differently from the other dithiooxamides, which instead typically achieve ligand·nI2 halogen-bonded adducts. In this paper, we report a combined experimental and theoretical study that allows elucidation of the nature of the cited products and the reasons behind the unpredictable behavior of these ligands. Specifically, low-temperature single-crystal X-ray diffraction measurements on a series of synthetically obtained R2pipdt (R = Me, iPr, Bz)/I3 salts, complemented by neutron diffraction experiments, were able to experimentally highlight the formation of [R2pipdtH]+ cations with a -S-H bond on the dithionic moiety. Differently, with R = Ph, a benzothiazolylium cation, resulting from an intramolecular condensation reaction of the ligand, is obtained. Based on density functional theory (DFT) calculations, a reasonable reaction mechanism where diiodine plays the fundamental role of promoting a halogen-bonding-mediated radical reaction has been proposed. In addition, the comparison of combined experimental and computational results with the corresponding reactions of N,N'-dialkylperhydrodiazepine-2,3-dithione (R2dazdt, a hepta-atomic cyclic dithiooxamide), which provide neutral halogen-bonded adducts, pointed out that the difference in the torsion angle of the free ligands represents the structural key factor in determining the different reactivities of the two systems.

Insight into the Properties of Heteroleptic Metal Dithiolenes: Multistimuli Responsive Luminescence, Chromism, and Nonlinear Optics

A comprehensive investigation of the functional properties of heteroleptic donor-M-acceptor dithiolene complexes Bu₄N[M^II(L1)(L2)] is presented (M = Pd, Pt). The acceptor L1 consists of the chiral (R)-(+)α-methylbenzyldithiooxamidate ((R)-α-MBAdto), the donor L2 is 2-thioxo-1,3-dithiole-4,5-dithiolato (dmit) in 1 (Pd) and 2 (Pt), 1,2-dicarbomethoxyethylenedithiolate (ddmet) in 3 (Pd) and 4 (Pt), or [4',5':5,6][1,4]dithiino[2,3-b]quinoxaline-1',3'-dithiolato (quinoxdt) in 5 (Pd) and 6 (Pt). L1 is capable of undergoing proton exchange and promoting crystal formation in noncentrosymmetric space groups. L2 has different molecular structures while it maintains similar electron-donating capabilities. Thanks to the synergy of the ligands, 1-6 behave as H⁺ and Ag⁺ switchable linear chromophores. Moreover, the compounds exhibit a H⁺-switchable second-order NLO response in solution, which is maintained in the bulk for 1, 3, and 4 when they are embedded into a PMMA poled matrix. 5 and 6 show unique anti-Kasha H⁺ and Ag⁺ tunable colored emission originating from the quinoxdt ligand. A correlation between the electronic structure and properties is shown through density functional theory (DFT) and time-dependent DFT calculations.

Anti-Kasha Conformational Photoisomerization of a Heteroleptic Dithiolene Metal Complex Revealed by Ultrafast Spectroscopy

We investigated the anti-Kasha photochemistry and anti-Kasha emission of d⁸-metal donor-acceptor dithiolene with femtosecond UV-vis transient absorption spectroscopy and molecular modeling. Experimentally, we found a lifetime of 1.4 ps for higher excited states, which is exceptionally long when compared to typical values for internal conversion (IC) (10 s of fs or less). Consequently, a substantial emission originates from the second excited state. Molecular modeling suggests this to be a consequence of the spatially separated molecular orbitals of the first and second excited states, which gives a charge transfer character to the IC. More surprisingly, we found that the inherent flexibility of the molecule allows the metal complex to access different configurations depending on the photoexcited state. We believe that this unique manifestation of anti-Kasha photoinduced conformational isomerization is facilitated by the exceptionally long lifetime of the second excited state.

Molecular Insights into the Ligand-Based Six-Proton- and Six-Electron-Transfer Processes Between Tris-ortho-Phenylenediamines and Tris-ortho-Benzoquinodiimines

The global demand for energy and the concerns over climate issues renders the development of alternative renewable energy sources such as hydrogen (H₂ ) important. A high-spin (hs) Fe^II complex with o-phenylenediamine (opda) ligands, [Fe^II (opda)₃ ]²⁺ (hs-[6R]²⁺ ), was reported showing photochemical H₂ evolution. In addition, a low-spin (ls) [Fe^II (bqdi)₃ ]²⁺ (bqdi: o-benzoquinodiimine) (ls-[0R]²⁺ ) formation by O₂ oxidation of hs-[6R]²⁺ , accompanied by ligand-based six-proton and six-electron transfer, revealed the potential of the complex with redox-active ligands as a novel multiple-proton and -electron storage material, albeit that the mechanism has not yet been understood. This paper reports that the oxidized ls-[0R][PF₆ ]₂ can be reduced by hydrazine giving ls-[Fe^II (opda)(bqdi)₂ ][PF₆ ]₂ (ls-[2R][PF₆ ]₂ ) and ls-[Fe^II (opda)₂ (bqdi)][PF₆ ]₂ (ls-[4R][PF₆ ]₂ ) with localized ligand-based proton-coupled mixed-valence (LPMV) states. The first isolation and characterization of the key intermediates with LPMV states offer unprecedented molecular insights into the design of photoresponsive molecule-based hydrogen-storage materials.

Structural diradical character

A reliable first-principles description of singlet diradical character is essential for predicting nonlinear optical and magnetic properties of molecules. As diradical and closed-shell electronic structures differ in their distribution of single, double, triple, and aromatic bonds, modeling electronic diradical character requires accurate bond-length patterns, in addition to accurate absolute bond lengths. We therefore introduce structural diradical character, which we suggest as an additional measure for comparing first-principles calculations with experimental data. We employ this measure to identify suitable exchange-correlation functionals for predicting the bond length patterns and electronic diradical character of a biscobaltocene with the potential for photoswitchable nonlinear optical activity. Out of four popular approximate exchange-correlation functionals with different exact-exchange admixtures (BP86, TPSS, B3LYP, TPSSh), the two hybrid functionals TPSSh and B3LYP perform best for diradical bond length patterns, with TPSSh being best for the organometallic validation systems and B3LYP for the organic ones (for which the D3 dispersion correction was included). Still, none of the functionals is suitable for correctly describing relative bond lengths across the range of molecules studied, so that none can be recommended for predictive studies of (potential) diradicals without reservation. © 2018 Wiley Periodicals, Inc.

Design of nickel donor–acceptor dithiolenes for 2nd order nonlinear optics: an experimental and computational study

This study, concerning the influence on the NLO properties of a remote functionality in the donor of D–Ni–A dithiolenes 1–4, allows to point out the relationship between NLO properties and the donor capability in modulating the low-energy absorption.

Redox-triggered switch based on platinum(ii) acetylacetonate complexes bearing an isomeric donor–acceptor conjugation ligand shows a high second-order nonlinear optical response

Pt(ii) acetylcetonate complexes with an isomeric D–A ligand exhibiting exceptionally large hyperpolarizabilities are promising redox-triggered NLO switches.

Uncommon Optical Properties and Silver-Responsive Turn-Off/On Luminescence in a PtII Heteroleptic Dithiolene Complex

Complex [Pt(iPr₂ pipdt)(Quinoxdt)] (iPr₂ pipdt=1,4-diisopropyl-piperazine-2,3-dithione; Quinoxdt=[1,4]dithiino[2,3-b]quinoxaline-2,3-dithiolate) exhibits a remarkable green emission at 570 nm (room temperature), which is above the lowest excited state. The complex is characterized by negative solvatochromism as well as a high second-order polarizability. Addition of Ag^I ions induces 1) hypsochromic shift of the lowest frequencies and 2) reversible quenching of luminescence. The corresponding Ni and Pd complexes have also been prepared and investigated to assist interpretation of optical properties within the triad. Computational studies based on DFT and time-dependent DFT highlight the electronic properties of [Pt(iPr₂ pipdt)(Quinoxdt)]. The preferential site of interaction between the Pt complex and incoming Ag^I is evidenced by the shape of the Fukui functions, pointing to the thiolic sulfur and platinum atoms as the most reactive sites towards a soft cation. Calculated optical properties are in agreement with experimental findings. This study sheds light on the structure-property relationship for this class of compounds.