3-Ethynyltriimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine Dual Short- and Long-Lived Emissions with Crystallization-Enhanced Feature: Role of Hydrogen Bonds and π-π Interactions.

Organic room temperature phosphorescent (ORTP) materials with stimuli-responsive, multicomponent emissive behaviour are extremely desirable for various applications. The derivative of cyclic triimidazole (TT) functionalized with an ethynyl group, TT-CCH, is isolated and investigated. The compound possesses crystallization-enhanced emission (CEE) comprising dual fluorescence and dual phosphorescence of both molecular and supramolecular origin with aggregation-induced components highly sensitive to grinding. The mechanisms involved in the emissions have been disclosed thanks to combined structural, spectroscopic and computational investigations. In particular, strong CH⋯N hydrogen bonds are deemed responsible, for the first time in the TT family, together with frequently observed π⋯π stacking interactions, for the aggregated fluorescence and phosphorescence.

Trapping an unprecedented octacoordinated iron(II) complex with neutral bis-tetrazolylpyridyl ligands and solvent molecules.

Iron(II) can show a very rich coordination chemistry with concomitant modulation of its properties as promising functional materials. Metalation of the neutral tridentate nitrogen-donor mer-coordinating ligand 2,6-bis(2-(methyl)-2H-tetrazol-5-yl)pyridine (Me2btp) with Fe(ClO4)2·6H2O through accurate solvent polarity control enables the selective crystallization of [FeHS/LS(Me2btp)2](ClO4)2·MeCN·2.75H2O (2HS/LS·MeCN·2.75H2O) as red rods, where half of the iron(II) centres resides in the low spin (LS, S = 0) state and the other half is in the high spin (HS, S = 2) state. The red rods spontaneously convert into yellow crystals once removed from the mother liquor and exposed to air due to solvent rearrangement within the crystal packing; these new crystals can be assigned to [FeHS(Me2btp)2](ClO4)2·solvent (2HS·solvent) where all the iron(II) centres are now blocked in the HS state, as confirmed by magnetic measurements. The polarity of the crystallization solvent, together with the maintenance of the crystals within the mother liquor, are pivotal for the reactivity and interconversion of different species. Indeed, upon long standing in solution, 2HS/LS·MeCN·2.75H2O converts to another form of red crystals belonging to [FeLS(Me2btp)2][FeHS(Me2btp)(MeCN)2(H2O)](ClO4)4·MeCN (2LS·3HS·MeCN), as confirmed by single crystal X-ray diffraction data. In this co-crystal, the iron(II) in 2 resides in the LS state at all temperatures while the iron(II) in 3 is blocked in the HS state. Well-formed yellow crystals could be also isolated among the red crystals of 2HS/LS·MeCN·2.75H2O, and they could be identified as the unprecedented octacoordinated species [Fe(Me2btp)2(MeCN)(H2O)](ClO4)2·H2O (1·H2O) by single-crystal X-ray diffraction. These yellow crystals are stable in the air, but slowly convert into 2LS·3HS·MeCN if kept in the mother liquor for about one week. 1·H2O can be considered the trapped intermediate in the solid state during the conversion of [FeHS(Me2btp)2]2+ into [FeHS(Me2btp)(MeCN)2(H2O)]2+ in solution, where the two tridentate ligands in the starting species can unfold to accommodate coordinated MeCN and H2O molecules, as confirmed by theoretical calculations, and eventually one of the two Me2btp is completely replaced by the solvent.

Nonlinear Optical Properties of Zn(II) Porphyrin, Graphene Nanoplates, and Ferrocene Hybrid Materials.

Following some previous work by some of us on the second order nonlinear optical (NLO) properties of Zn(II) meso-tetraphenylporphyrin (ZnP), fullerene, and ferrocene (Fc) diads and triads, in the present research, we explore the NLO response of some new hybrids with two-dimensional graphene nanoplates (GNP) instead of a zero-dimensional fullerene moiety as the acceptor unit. The experimental data, collected by Electric Field Induced Second Harmonic generation (EFISH) technique in CH2Cl2 solution with a 1907 nm incident wavelength, combined with Coupled-Perturbed (CP) and Finite Field (FF) Density Functional Theory (DFT) calculations, show a strongly enhanced contribution of the cubic electronic term γ(-2ω; ω, ω, 0), due to the extended π-conjugation of the carbonaceous acceptor moiety.

Stimuli Responsive Features of Organic RTP Materials: An Intriguing Carbazole-Cyclic Triimidazole Derivative.

Stimuli responsive luminescent materials possessing room temperature phosphorescence (RTP) are extremely desirable for various applications. The here investigated derivative of cyclic triimidazole (TT) functionalized with carbazole (Cz), namely TT-Ph-Cz, belongs to this class. TT-Ph-Cz possesses high conformational freedom resulting in rigidochromic and multi-stimuli responsive emissive behavior. It has been isolated as MeOH-solvated and de-solvated forms characterized by distinctive emissive features. In particular, the solvated form, in which hydrogen bonds with MeOH inhibit competitive non-radiative deactivation channels, possesses a higher quantum yield associated with a strong phosphorescence contribution which is preserved in DMSO/water solutions.

Substituent-Guided Cluster Nuclearity for Tetranuclear Iron(III) Compounds with Flat {Fe4(µ3-O)2} Butterfly Core.

The tetranuclear iron(III) compounds [Fe4(µ3-O)2(µ-LZ)4] (1-3) were obtained by reaction of FeCl3 with the shortened salen-type N2O2 tetradentate Schiff bases N,N'-bis(salicylidene)-o-Z-phenylmethanediamine H2LZ (Z = NO2, Cl and OMe, respectively), where the one-carbon bridge between the two iminic nitrogen donor atoms guide preferentially to the formation of oligonuclear species, and the ortho position of the substituent Z on the central phenyl ring selectively drives towards Fe4 bis-oxido clusters. All compounds show a flat almost-symmetric butterfly-like conformation of the {Fe4(µ3-O)2} core, surrounded by the four Schiff base ligands, as depicted by both the X-ray molecular structures of 1 and 2 and the optimized geometries of all derivatives as obtained by UM06/6-311G(d) DFT calculations. The strength of the antiferromagnetic exchange coupling constants between the iron(III) ions varies among the three derivatives, despite their magnetic cores remain structurally almost unvaried, as well as the coordination of the metal ions, with a distorted octahedral environment for the two-body iron ions, Feb, and a pentacoordination with trigonal bipyramidal geometry for the two-wing iron ions, Few. The different magnetic behavior within the series of examined compounds may be ascribed to the influence of the electronic features of Z on the electron density distribution (EDD) of the central {Fe4(µ3-O)2} core, substantiated by a Quantum Theory of Atoms In Molecules (QTAIM) topological analysis of the EDD, as obtained by UM06 calculations 1-3.

Crystallization-Enhanced Emission and Room-Temperature Phosphorescence of Cyclic Triimidazole-Monohexyl Thiophene Derivatives.

The development of organic room-temperature phosphorescent (ORTP) materials represents an active field of research due to their significant advantages with respect to their organometallic counterparts. Two cyclic triimidazole (TT) derivatives bearing one and three hexyl-thiophene moieties, TT-HThio and TT-(HThio)3, have been prepared and characterized. Both compounds display enhanced quantum yields in their crystalline form with respect to those in a solution state, revealing crystallization-enhanced emissive (CEE) behavior. Importantly, while single fluorescence is observed in solution, crystalline powders also feature dual ORTP, whose respective molecular and aggregate origins have been disclosed through X-ray diffraction analysis and DFT/TDDFT calculations. The relation between the photophysical properties of TT-HThio and its crystallinity degree has been confirmed by a decrease in photoluminescent quantum yield (Φ) and loss of vibronic resolution when its crystals are ground in a mortar, revealing mechanochromic behavior and confirming CEE features.

Exploring Orthogonality between Halogen and Hydrogen Bonding Involving Benzene.

The concept of orthogonality between halogen and hydrogen bonding, brought out by Ho and coworkers some years ago, has become a widely accepted idea within the chemists' community. While the original work was based on a common carbonyl oxygen as acceptor for both interactions, we explore here, by means of M06-2X, M11, ωB97X, and ωB97XD/aug-cc-PVTZ DFT calculations, the interdependence of halogen and hydrogen bonding with a shared π-electron system of benzene. The donor groups (specifically NCBr and H2O) were placed on either or the same side of the ring, according to a double T-shaped or a perpendicular geometry, respectively. The results demonstrate that the two interactions with benzene are not strictly independent on each other, therefore outlining that the orthogonality between halogen and hydrogen bonding, intended as energetical independence between the two interactions, should be carefully evaluated according to the specific acceptor group.

Prompt and Long-Lived Anti-Kasha Emission from Organic Dyes.

Anti-Kasha behavior has been the subject of intense debate in the last few years, as demonstrated by the high number of papers appearing in the literature on this topic, dealing with both mechanistic and applicative aspects of this phenomenon. Examples of anomalous emitters reported in the last 10 years are collected in the present review, which is focused on strictly anti-Kasha organic molecules displaying radiative deactivation from Sn and/or Tn, with n greater than 1.

Tunable Linear and Nonlinear Optical Properties from Room Temperature Phosphorescent Cyclic Triimidazole-Pyrene Bio-Probe.

Organic materials with multiple emissions tunable by external stimuli represent a great challenge. TTPyr, crystallizing in different polymorphs, shows a very rich photophyisics comprising excitation-dependent fluorescence and phosphorescence at ambient conditions, and mechanochromic and thermochromic behavior. Transformation among the different species has been followed by thermal and X-ray diffraction analyses and the emissive features interpreted through structural results and DFT/TDDFT calculations. Particularly intriguing is the polymorph TTPyr(HT), serendipitously obtained at high temperature but stable also at room temperature, whose non-centrosymmetric structure guarantees an SHG efficiency 10 times higher than that of standard urea. Its crystal packing, where only the TT units are strongly rigidified by π-π stacking interactions while the Pyr moieties possess partial conformational freedom, is responsible for the observed dual fluorescence. The potentialities of TTPyr for bioimaging have been successfully established.

Nonlinear Optical Properties of Porphyrin, Fullerene and Ferrocene Hybrid Materials.

In this research, we investigated the second-order nonlinear optical (NLO) properties of multicomponent hybrid materials formed by meso-tetraphenylporphyrin P (both as free base and ZnII complex), carrying in 2 or 2,12 ß-pyrrolic position an electron donor ferrocene (Fc), and/or an electron acceptor fullerene (C60) moiety, connected to the porphyrin core via an ethynyl or an ethynylphenyl spacer. We measured the NLO response by the electric-field-induced second-harmonic generation (EFISH) technique in CH2Cl2 solution with a 1907 nm incident wavelength, recording for all the investigated compounds unexpected negative values of µß1907. Since density functional theory (DFT) calculations evidenced for P-Fc dyads almost null ground state dipole moments and very low values for P-C60 dyads and Fc-P-C60 triads, our EFISH results suggested a significant contribution to γEFISH of the purely electronic cubic term γ(-2ω; ω, ω, 0), which prevails on the quadratic dipolar orientational one µß(-2ω; ω, ω)/5kT, as confirmed by computational evidence.

Some Novel Cobalt Diphenylphosphine Complexes: Synthesis, Characterization, and Behavior in the Polymerization of 1,3-Butadiene.

Some novel cobalt diphenylphosphine complexes were synthesized by reacting cobalt(II) chloride with (2-methoxyethyl)diphenylphosphine, (2-methoxyphenyl)diphenylphosphine, and 2-(1,1-dimethylpropyl)-6-(diphenylphosphino)pyridine. Single crystals suitable for X-ray diffraction studies were obtained for the first two complexes, and their crystal structure was determined. The novel compounds were then used in association with methylaluminoxane (MAO) for the polymerization of 1,3-butadiene, and their behavior was compared with that exhibited in the polymerization of the same monomer by the systems CoCl2(PnPrPh2)2/MAO and CoCl2(PPh3)2/MAO. Some significant differences were observed depending on the MAO/Co ratio used, and a plausible interpretation for such a different behavior is proposed.

Unravelling the intricate photophysical behavior of 3-(pyridin-2-yl)triimidazotriazine AIE and RTP polymorphs.

The development of purely organic materials showing multicolor fluorescent and phosphorescent behaviour represents a formidable challenge in view of practical applications. Herein the rich photophysical behaviour of 3-(pyridin-2-yl)triimidazotriazine (TT-Py) organic molecule, comprising excitation-dependent fluorescence and phosphorescence under ambient conditions in both blended films and the crystalline phase, is investigated by means of steady state, time resolved and ultrafast spectroscopies and interpreted on the basis of X-ray diffraction studies and DFT/TDDFT calculations. In particular, by proper excitation wavelength, dual fluorescence and dual phosphorescence of molecular origin can be observed together with low energy phosphorescences resulting from aggregate species. It is demonstrated that the multiple emission properties originate from the copresence, in the investigated system, of an extended polycyclic nitrogen-rich moiety (TT), strongly rigidified by π-π stacking interactions and short C-H···N hydrogen bonds, and a fragment (Py) having partial conformational freedom.

Electric-Field-Induced Second Harmonic Generation Nonlinear Optic Response of A4 ß-Pyrrolic-Substituted ZnII Porphyrins: When Cubic Contributions Cannot Be Neglected.

In this work, we have prepared a series of A4 ZnII porphyrins, carrying in the ß-pyrrolic-position one or two π-delocalized ethynylphenyl moieties with a -NO2 acceptor or a -NMe2 donor pendant, and measured their second-order NLO response in CHCl3 solution at 1907 nm via the electric-field-induced second harmonic generation (EFISH) technique. For some of these compounds, we have recorded an unexpected sign and/or absolute value of µß1907. Since their sterically hindered A4 structure should ensure the lack of significant aggregation processes in solution, we explain such anomalous EFISH results by invoking a non-negligible contribution of the electronic cubic term γ(-2ω; ω, ω, 0) to γEFISH, as supported by a qualitative evaluation of the third-order response through the measure of the cubic hyperpolarizability (γTHG) and by computational evidence.

The Origin of the σ-Hole in Halogen Atoms: a Valence Bond Perspective.

A detailed Valence Bond-Spin Coupled analysis of a series of halogenated molecules is here reported, allowing to get a rigorous ab initio demonstration of the qualitative models previously proposed to explain the origin of halogen bonding. The concepts of σ-hole and negative belt observed around the halogen atoms in the electrostatic potential maps are here interpreted by analysis of the relevant Spin Coupled orbitals.

Solid-State Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands.

Salen-type metal complexes have been actively studied for their nonlinear optical (NLO) properties, and push-pull compounds with charge asymmetry generated by electron releasing and withdrawing groups have shown promising results. As a continuation of our research in this field and aiming at solid-state features, herein we report on the synthesis of mononuclear copper(II) derivatives bearing either tridentate N2O Schiff bases L(a-c)- and pyridine as the forth ancillary ligand, [Cu(La-c)(py)](ClO4) (1a-c), or unsymmetrically-substituted push-pull tetradentate N2O2 Schiff base ligands, [Cu(5-A-5'-D-saldpen/chxn)] (2a-c), both derived from 5-substituted salicylaldehydes (sal) and the diamines (1R,2R)-1,2-diphenylethanediamine (dpen) and (1S,2S)-1,2-diaminocyclohexane (chxn). All compounds were characterized through elemental analysis, infrared and UV/visible spectroscopies, and mass spectrometry in order to guarantee their purity and assess their charge transfer properties. The structures of 1a-c were determined via single-crystal X-ray diffraction studies. The geometries of cations of 1a-c and of molecules 2a-c were optimized through DFT calculations. The solid-state NLO behavior was measured by the Kurtz-Perry powder technique @1.907 µm. All chiral derivatives possess non-zero quadratic electric susceptibility (χ(2)) and an efficiency of about 0.15-0.45 times that of standard urea.

Impact of Singly Occupied Molecular Orbital Energy on the n-Doping Efficiency of Benzimidazole Derivatives.

We investigated the impact of singly occupied molecular orbital (SOMO) energy on the n-doping efficiency of benzimidazole derivatives. By designing and synthesizing a series of new air-stable benzimidazole-based dopants with different SOMO energy levels, we demonstrated that an increase of the dopant SOMO energy by only ∼0.3 eV enhances the electrical conductivity of a benchmark electron-transporting naphthalenediimide-bithiophene polymer by more than 1 order of magnitude. By combining electrical, X-ray diffraction, and electron paramagnetic resonance measurements with density functional theory calculations and analytical transport simulations, we quantitatively characterized the conductivity, Seebeck coefficient, spin density, and crystallinity of the doped polymer as a function of the dopant SOMO energy. Our findings strongly indicate that charge and energy transport are dominated by the (relative) position of the SOMO level, whereas morphological differences appear to play a lesser role. These results set molecular-design guidelines for next-generation n-type dopants.

Solid State Room Temperature Dual Phosphorescence from 3-(2-Fluoropyridin-4-yl)triimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine.

Organic room temperature persistent luminescence is a fascinating but still largely unexplored phenomenon. Cyclic-triimidazole and its halogenated (Br, I) derivatives have recently revealed as intriguing phosphors characterized by multifaceted emissive behavior including room temperature ultralong phosphorescence (RTUP) associated with the presence of H-aggregates in their crystal structure. Here, we move towards a multicomponent system by incorporating a fluoropyridinic fragment on the cyclic-triimidazole scaffold. Such chromophore enhances the molecular properties resulting in a high photoluminescence quantum yield (PL QY) in solution but preserves the solid-state RTUP. By means of X-ray diffraction (XRD) analysis, theoretical calculations, steady-state and time-resolved spectroscopy on solutions, polymethylmethacrylate (PMMA) blends and crystals, the nature of the different radiative deactivation channels of the compound has been disclosed. In particular, the molecular fluorescence and phosphorescence, this latter observed in frozen solution and in PMMA blends, are associated to deactivation from S1 and T1 respectively, while the low energy RTUP, observed only for crystals, is interpreted as originated from H aggregates.

Push-pull unsymmetrical substitution in nickel(ii) complexes with tetradentate N2O2 Schiff base ligands: synthesis, structures and linear-nonlinear optical studies.

New push-pull (A-D) nickel(ii) compounds of general formula [Ni(5-A-5'-D-saltn)] (3a-3l) with unsymmetrically-substituted N2O2 tetradentate Schiff base ligands are reported here. The ligands 5-A-5'-D-saltn2- (H2saltn = N,N'-bis(salicylidene)diaminopropane) possess differently-substituted salicylaldehyde (A/Dsal) moieties condensed to 1,3-diaminopropane (tn), and carry either an electron acceptor (A = H, Br, or NO2) or donor (D = H, Me, or OMe) group in the para position with respect to the coordinated phenoxido oxygen atoms. These compounds could be obtained by template synthesis involving derivatives [Ni(Gsal)2(H2O)2], 1a-e (G = NO2, Br, H, Me and OMe, respectively) and [Ni(GL)2], 2a-d (GL- = (E)-2-((3-aminopropylimino)methyl)-4-G-phenolate, G = NO2, Br, H and Me, respectively). Scrambling of the ligands and condensation to compounds 3 was suitably achieved by refluxing compounds 1 and 2 that carry the G groups suitable for the desired final A-D combination. Dinuclear intermediates [Ni2(µ-GL)2(G'sal)2] (4a,b,e,f,g) were also detected and isolated. The single-crystal X-ray diffraction structures of [Ni(5'-OMe-saltn)]·CHCl3 (3c·CHCl3), [Ni(5-Br-5'-OMe-saltn)]·EtOH (3g·EtOH) and [Ni(5-NO2-saltn)] (3j) show different degrees of distortion around the central core, leading to saddle-like (3c), planar (3g) and step-like (3j) molecular conformations. DFT geometrical optimization of the compounds showed that, for isolated molecules, the saddle-like conformation is slightly more stable with respect to the other conformations. UV-visible absorption spectra showed structured absorption profiles at about 320-440 nm, whose intensity was amplified by the presence of the nitro group, and this was assigned to a convolution of one metal-to-ligand charge transfer and two intra-ligand charge transfer transitions by TDDFT computations. Surprisingly, UV-visible spectra of the derivatives with Br were comparable to the derivatives with Me, suggesting that in this case the behaviour of the halogen was as a weak electron donor group. The experimental investigation (through electric-field-induced second-harmonic and solvatochromic measurements) of the second-order NLO responses of compounds 3, in conjunction with the theoretical calculations, revealed that the observed NLO efficiency is determined by the combined effect of two almost orthogonal charge transfer directions within the molecules, one along the axis approximately bisecting the donor and the accepting moieties and the other along the A-D axis.

Novel Cobalt Dichloride Complexes with Hindered Diphenylphosphine Ligands: Synthesis, Characterization, and Behavior in the Polymerization of Butadiene.

Two novel cobalt diphenylphosphine complexes were synthesized by reacting cobalt(II) chloride with tert-butyl(diphenyl)phosphine (PtBuPh2) and (S)-(+)neomenthyldiphenylphosphine [(S)-NMDPP]. The crystal structure of the former was determined by single-crystal X-ray diffraction studies. The two complexes were then used in combination with methylaluminoxane (MAO) for the polymerization of 1,3-butadiene: crystalline highly syndiotactic 1,2 poly(1,3-butadiene)s were obtained, with a 1,2 content and a syndiotactic index (percentage of syndiotactic triads [rr]) up to 95% and 85%, respectively. The results obtained further support and confirm what was already observed in the polymerization of 1,3-butadiene with CoCl2(PRPh2)2-MAO (R = methyl, ethyl, normal-propyl, iso-propyl, and cyclohexyl): the nature of the phosphine ligand strongly affects the polymerization stereoselectivity, the polymer syndiotacticity increasing with increasing phosphine ligand steric hindrance.

Evaluation of In-Batch and In-Flow Synthetic Strategies towards the Stereoselective Synthesis of a Fluorinated Analogue of Retro-Thiorphan.

A stereoselective synthetic strategy for the preparation of trifluoromethylamine mimics of retro-thiorphan, involving a diastereoselective, metal-free catalytic step, has been studied in batch and afforded the target molecule in good yields and high diastereoselectivity. A crucial point of the synthetic sequence was the catalytic reduction of a fluorinated enamine with trichlorosilane as reducing agent in the presence of a chiral Lewis base. The absolute configuration of the key intermediate was unambiguously assigned by X-ray analysis. The synthesis was also investigated exploiting continuous flow reactions; that is, an advanced intermediate of the target molecule was synthesized in only two in-flow synthetic modules, avoiding isolation and purifications of intermediates, leading to the isolation of the target chiral fluorinated amine in up to an 87:13 diastereoisomeric ratio.