A chiral bis(salicylaldiminato)zinc(ii) complex with second-order nonlinear optical and luminescent properties in solution

Whereas there is an increasing amount of reports on the second-order nonlinear optical (NLO) and luminescence properties of tetradentate [N2O2] Schiff base-zinc complexes, the study of zinc complexes having two bidentate [NO] Schiff-base ligands is relatively unexplored from an NLO point of view. This work puts in evidence that the known chiral bis(2-[(R)-(+)-1-phenylethyliminomethyl]phenolato-N, O)zinc(II) complex is a fascinating multifunctional molecular inorganic-organic hybrid material characterized by interesting second-order NLO and luminescent properties in solution. The emissive properties of the organic 2-(R)-(+)-1-phenylethyliminomethyl]phenol proligand are greatly enhanced upon coordination to the inorganic Zn(II) center

Intrinsic and Extrinsic Heavy-Atom Effects on the Multifaceted Emissive Behavior of Cyclic Triimidazole

Considering that heavy halogen atoms can be used to tune the emissive properties of organic luminogens, the understanding of their role in photophysics is fundamental for materials engineering. Here, the extrinsic and intrinsic heavy-atom effects on the photophysics of organic crystals were separately evaluated by comparing cyclic triimidazole (TT) with its monoiodo derivative (TTI) and its co-crystal with diiodotetrafluorobenzene (TTCo). Crystals of TT showed room-temperature ultralong phosphorescence (RTUP) originated from H-aggregation. TTI and TTCo displayed two additional long-lived components, the origin of which is elucidated through single-crystal X-ray and DFT/TDDFT studies. The results highlight the different effects of the I atom on the three phosphorescent emissions. Intrinsic heavy-atom effects play a major role on molecular phosphorescence, which is displayed at room temperature only for TTI. The H-aggregate RTUP and the I c5 c5 c5N XB-induced (XB=halogen bond) phosphorescence on the other side depend only on packing features

Ferrocene-quinoxaline Y-shaped chromophores as fascinating second-order NLO building blocks for long lasting highly active SHG polymeric films

The first example of a Y-shaped ferrocene quinoxaline derivative with a surprisingly high and stable second harmonic generation (SHG) response in composite polymeric films is reported. The interesting quadratic hyperpolarizability values of different substituted Y-shaped chromophores are also investigated in solution by the EFISH technique

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[MII(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

D-, L- and D,L-Tryptophan-Based Polyamidoamino Acids: pH-Dependent Structuring and Fluorescent Properties

Chiral polyamidoamino acids were obtained by polyaddition of N,N\u2019methylenebisacrylamide with D-, D,L- and L-tryptophan (M-D-Trp, M-D,L-Trp and M-L-Trp). L-tryptophan/glycine copolymers, M-G-L-Trp5, M-G-L-Trp10, M-G-L-Trp20 and M-G-L-Trp40, were prepared from L-tryptophan/glycine mixtures. These polymers were amphoteric, with acid-base properties similar to those of the parent amino acids. The L-tryptophan/glycine copolymers with high glycine content were water soluble in the pH range 2-12. M-G-L-Trp40 showed a solubility gap centred at pH 4.5 and all tryptophan homopolymers were soluble only at pH > 7. Dynamic light scattering measurements performed in their solubility ranges, namely 2-11 M-G-L-Trp5, M-G-L-Trp10 and M-G-L-Trp20 and 7-11 for M-G-L-Trp40, M-D-Trp, M-L-Trp and M-D,L-Trp, showed that the size of all samples did not significantly vary with pH. Both M-L-Trp and M-G-L-Trp copolymers showed pH-dependent circular dichroism spectra in the wavelength interval 200\u2013280 nm, revealing structuring. All samples were fluorescent. Their emission spectra were unstructured and, if normalized for their tryptophan content, almost superimposable at the same pH, providing evidence that only tryptophan governed the photoluminescence properties. Changing pH induced in all cases a slight shift of the emission wavelength maximum ascribed to the modification of the microenvironment surrounding the indole ring induced by different protonation degrees

Unravelling the intricated 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 photophyisical behaviour of 3-(pyridin-2- yl)triimidazotriazine (TT-Py) organic molecule, comprising excitation-dependent fluorescence and phosphorescence under ambient conditions in both blended film and 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 property is originated by the copresence, in the investigated system, of an extended polycyclic nitrogen-rich moiety (TT), strongly rigidified by p-p stacking interactions and short C\u2013H...N hydrogen bonds, and a fragment (Py) featuring partial conformational freedom

Tuning the Linear and Nonlinear Optical Properties of Pyrene-Pyridine Chromophores by Protonation and Complexation to d10 Metal Centers †

The linear and second-order nonlinear optical (NLO) properties of two pyrene-pyridine chromophores, namely, 4-(pyren-1-yl)pyridine (L1) and 4-(2-(pyren-1-yl)ethyl)pyridine (L2), were investigated and modulated by performing protonation/deprotonation cycles or by complexation to d10 metal centers such as Zn(II) and Cu(I) to form the monomeric [Zn(CH3CO2)2(L1)2] complex and the [CuI(L2)]n coordination polymer, respectively. The structures of L1, L2, [Zn(CH3CO2)2(L1)2] and [CuI(L2)]n were determined by means of single-crystal X-ray diffraction studies. The NLO response, measured by the electric-field-induced second harmonic generation (EFISH) technique, was positive for both chromophores and showed an inversion of the sign after exposure to HCl vapors. This process was completely reversible and the original values were restored by simple exposure to NH3 vapors. Coordination of L1 to Zn(II) also resulted in a negative NLO response, although smaller in magnitude compared to the protonated form, due to the weak Lewis acidity of the \u201cZn(CH3CO2)2\u201d fragment. The results were also interpreted on the basis of DFT/TDDFT calculation

pH-Dependent structuring and fluorescence properties of tryptophan-derived polyamidoamino acids

The binding sites of proteins often contain tryptophan residues, whose fluorescent properties may be altered upon ligand binding. Conformational changes within the binding site can result in either fluorescence quenching or enhancement, which may be utilized to quantitatively investigate protein-ligand interactions. Amphoteric fluorescent (L)-, (D)- and (D,L)-tryptophan-deriving polyamidoamino acid (PAAC) homopolymers and their copolymers with glycine and arginine PAAC copolymers were synthesized by the polyaddition of N,N'-methylenebisacrylamide (MBA) with (L)-, (D)- and (D,L)-tryptophan and, respectively, glycine- and (L)-arginine/(L)-tryptophan mixtures. The polymerization reaction was run at 50\ub0C and pH>10 for 7 days under nitrogen atmosphere. All polymers were characterized by NMR spectrometry, UV-Visible spectroscopy, dynamic light scattering (DLS), steady state and time-resolved fluorescence (TRF) spectroscopy. Solubility in aqueous systems at different pH\u2019s was determined by measuring the transmittance at 450 nm. Homopolymers showed similar pH-dependent solubility patterns, with steep solubility drop at pH < 8. Volumetric particle size distribution obtained by DLS analysis showed hydrodynamic radii (Rh) 48 100 nm and negligible changes with pH. Fluorescence quantum yield of the MBA-L-tryptophan measured at 10-5 M repeating unit concentration decreased from 6 to 4 % from pH 11 to 8 respectively. Excitation and emission spectra showed maxima at 279 nm and 370 nm respectively, the latter showing a 20 nm blue shift by decreasing pH from 11 to 8. The excited-state lifetimes of the MBA-(L)-tryptophan at the maximum emission wavelength decreased from \u3c41 = 0.90 ns, \u3c42 = 4.40 ns to \u3c41 = 0.81 ns, \u3c42 = 3.49 ns passing from pH 11 to 8, with a population redistribution in favour of the shortest time. PAAC copolymers were synthesized under the same conditions of the homopolymers. The solubility curves in water of copolymers showed complete solubility for tryptophan content up to 20 % on a molar basis. For tryptophan content up to 30 and 40%, transmittance dropped down, respectively, to 60 % and 5 % in the pH range 2-7. DLS measurements at different pH\u2019s showed average volume particle size 2.5 nm in the range considered. Preliminary TRF spectroscopy data proved that PAAC copolymers were endowed with pH-dependent emission properties. Moreover, significant fluorescence quantum yield was detected at the tryptophan content as low as 5 %. Finally, preliminary circular dichroism measurements showed pH-dependent patterns. In conclusion, tryptophan-based PAAC polymers displayed tunable amphiphilicity, chirality and self-assembly properties

Highly efficient acido-triggered reversible luminescent and nonlinear optical switch based on 5-&#960;-delocalized-donor-1,3-di(2-pyridyl)benzenes

The acidochromic behaviour of trans-5-(p-(N,N-diphenylamino)styryl)-1,3-di(2-pyridyl)benzene (1) and 5-(p-(N,N-diphenylamino)phenylethynyl)-1,3-di(2-pyridyl) benzene (2) was investigated, both in solution and doped into films, studying their linear and nonlinear optical properties. Remarkably, the efficient luminescence of 1 drops drastically upon protonation and is restored upon deprotonation - both in solution and in the solid state - leading to an excellent on/off emissive acido-triggered reversible switch. In parallel, protonation leads to a large enhancement of the second-order NLO response, as determined by the EFISH technique, which is reversible upon deprotonation. The related methylated salts of 1 and 2 are similarly characterized by a high NLO response

Solid State Room Temperature Dual Phosphorescence from 3-(2-Fluoropyridin-4-yl)triimidazo[1,2-a:1&#8242;,2&#8242;-c:1&#8243;,2&#8243;-e][1,3,5]triazine

Organic roomtemperature 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) associatedwith the presence ofH-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