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.

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.

A Novel Class of Cyclometalated Platinum(II) Complexes for Solution-Processable OLEDs.

Substitution of the chlorido ligand of cyclometalated [Pt (5-R-1,3-di(2-pyridyl) benzene)Cl] (R = methyl, mesityl, 2-thienyl, or 4-diphenylamino-phenyl) by 4-phenylthiazole-2-thiolate leads to related thiolato complexes, which were fully characterized. Their photophysical properties were determined in degassed dichloromethane solution. The emission color of the novel complexes can be easily tuned by the nature of the substituents on the terdentate ligand, as is the case for the parent chlorido complexes. Their luminescence Quantum Yield is high, with that of the compounds with the 2-thienyl or 4-diphenylamino-phenyl substituents being much higher than that of the related chloride complexes. The platinum complex with the cyclometalated 5-(2-thienyl)-1,3-di(2-pyridyl) benzene was used as the emitter for the fabrication of a yellow solution-processable OLED.

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.

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.

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.

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⋅⋅⋅N XB-induced (XB=halogen bond) phosphorescence on the other side depend only on packing features.

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.

Photochromic DTE-Substituted-1,3-di(2-pyridyl)benzene Platinum(II) Complexes: Photomodulation of Luminescence and Second-Order Nonlinear Optical Properties.

We disclose a new family of photochromic cyclometalated platinum(II) complexes (PtDTE1 and PtDTE2), where a dithienylethene (DTE) unit is connected at the para-position of the central phenyl ring of (N^C^N) cyclometalated ligand, through two different linkages. Their syntheses are presented along with the X-ray characterizations of both the open and closed isomers of PtDTE1. The investigation of their photophysical properties is made, including absorption, photochromism, emission, and second-order nonlinear properties. We report a quantitative photoisomerization for both PtDTE1 and PtDTE2, irrespective of the nature of the connecting mode between the DTE unit and the platinum(II) moiety. The efficient photochromism allows a significant NLO photomodulation, both in solution and in thin films. In addition, we show that the photoluminescence of the PtDTE1 and PtDTE2 can be controlled by the open/closed isomerization of the DTE unit.

Partial in Situ Reduction of Copper(II) Resulting in One-Pot Formation of 2D Neutral and 3D Cationic Copper(I) Iodide-Pyrazine Coordination Polymers: Structure and Emissive Properties.

On the way to copper(I) iodide coordination polymers with specific luminescent properties, the in situ reduction of Cu(II) in the presence of KI and bidentate N-heteroatomic ligand, either pyrazine (pyz) or 4,4'-bipyridine (bpy), resulted in one two-dimensional and two three-dimensional new coordination networks. Starting from Cu(NO3)2·3H2O in the presence of pyz, successive precipitation of known yellow [(CuII)2(pyz)]n, new orange [CuII(pyz)]n, and new dark blue {[CuI(pyz)2]·I5}n polymeric solids was observed. Starting from the same salt in the presence of bpy resulted in the successive precipitation of known yellow [(CuII)2(bpy)]n and new brown {[CuII(NO3)(bpy)2]·I3·(dmf·H2O)}n coordination polymers. By using either Cu(CH3COO)2·H2O or Cu(BF4)2 as starting materials, both known forms, yellow [(CuII)2(bpy)]n and orange [CuII(bpy)]n, precipitated successively. The new solids were characterized by IR spectroscopy and X-ray analysis. [CuII(pyz)]n represents the missing member in the row of two-dimensional coordination networks with general formula [CuIX(pyz)]n (X = Cl, Br, I). Its steady state and time-resolved characterization together with DFT and TDDFT calculations revealed that the emission at room temperature is mainly delayed fluorescence originating from mixed singlet metal-to-ligand charge transfer and halide-to-ligand charge transfer states, while that at 77 K is phosphorescence, associated with the small singlet-triplet energy differences (ΔE = 70 meV).

Cyclic Triimidazole Derivatives: Intriguing Examples of Multiple Emissions and Ultralong Phosphorescence at Room Temperature.

The performance of solid luminogens depends on both their inherent electronic properties and their packing status. Intermolecular interactions have been exploited to achieve persistent room-temperature phosphorescence (RTP) from organic molecules. However, the design of organic materials with bright RTP and the rationalization of the role of interchromophoric electronic coupling remain challenging tasks. Cyclic triimidazole has been shown to be a promising scaffold for such purposes owing to its crystallization-induced room-temperature ultralong phosphorescence (RTUP), which has been associated with H-aggregation. Herein, we report three triimidazole derivatives as significant examples of multifaceted emission. In particular, dual fluorescence, RTUP, and phosphorescence from the molecular and supramolecular units were observed. H-aggregation is responsible for the red RTUP, and Br substituents favor yellow molecular phosphorescence while halogen-bonded Br⋅⋅⋅Br tetrameric units are involved in the blue-green phosphorescence.

Second-order NLO switches from molecules to polymer films based on photochromic cyclometalated platinum(II) complexes.

Novel photochromic dithienylethene-based platinum(II) complexes (C^N^N)Pt(C≡C-DTE-C6H4-D) ((C^N^N) = 4,4'-di(n-hexyl)-6-phenyl-2,2'-bipyridine; D = H, NMe2) were prepared and characterized. Their excellent photochromic properties allow the photoinduced switching of their second-order nonlinear optical properties in solution, as measured by the EFISH technique, due to formation of an extended π-conjugated ligand upon suitable electromagnetic radiation. Insights into the electronic structures of the complexes and the nature of their excited states have been obtained by DFT and TD-DFT calculations. These novel Pt(II) complexes were nanoorganized in polymer films which were poled, affording new materials characterized by a good second-order NLO response that can be easily switched, with an excellent NLO contrast. To the best of our knowledge, our compounds allowed designing the very first examples of switchable NLO polymer films based on metal complexes.

Introduction of a triphenylamine substituent on pyridyl rings as a springboard for a new appealing brightly luminescent 1,3-di-(2-pyridyl)benzene platinum(II) complex family.

The preparation and characterization of three new complexes, namely [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-4,6-difluoro-benzene)Cl] ([PtL1Cl]), [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-5-triphenylamine-benzene)Cl] ([PtL2Cl]), and [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-5-mesityl-benzene)Cl] ([PtL3Cl]), is reported. All of them are highly luminescent in dilute deaerated dichloromethane solution (Φlum = 0.88-0.90, in the yellow-green region; the λmax,em in nm for the monomers are: 562, 561 and 549 for [PtL1Cl], [PtL2Cl] and [PtL3Cl], respectively).[PtL1Cl] is the most appealing, being characterized by a very long lifetime (103.9 µs) and displaying intense NIR emission in concentrated deaerated solution (Φlum = 0.66) with essentially no "contamination" by visible light < 600 nm. This complex allows the fabrication of both yellow-green and deep red/NIR OLEDs; OLED emissions are in the yellow-green (CIE = 0.38, 0.56) and deep red/NIR (CIE = 0.65, 0,34) regions, for [PtL1Cl] 8 wt% (with 11% ph/e EQE) and pure [PtL1Cl] (with 4.3% ph/e EQE), respectively.

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.

NLO-active Y-shaped ferrocene conjugated imidazole chromophores as precursors for SHG polymeric films.

New Y-shaped ferrocene conjugated imidazole chromophores were prepared and fully characterized. The Y-shaped structure was confirmed by the single crystal X-ray diffraction technique. The chromophores show interesting second-order nonlinear optical (NLO) properties in solution, as determined by the Electric-Field Induced Second Harmonic generation (EFISH) technique. Remarkably, the trifluoro substituted compound 3 is characterized by a high µßEFISH value and has good potential as a molecular building block for composite films with Second Harmonic Generation (SHG) properties. For all compounds, the dipole moments and frontier orbital energies were calculated by the Density Functional Theoretical method.

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'-methylenebisacrylamide 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 200⁻280 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.

Extrinsic Heavy Metal Atom Effect on the Solid-State Room Temperature Phosphorescence of Cyclic Triimidazole.

Four coordination compounds [Zn3 (CH3 COO)6 (H2 O)2 ](TT)2 [Cd(H2 O)6 ](ClO4 )2 (TT)2 , [Cd(H2 O)6 ](BF4 )2 (TT)2 , [Zn(H2 O)6 ](BF4 )2 (TT)2 (1-4) accommodating the crystallization induced emissive triimidazo[1,2-a:1',2'-c:1'',2''-e][1,3,5]triazine (TT) as a guest in their crystal lattice are isolated and fully photophysically and structurally characterized. Their emission properties are compared with those of afterglow TT and interpreted taking into account the heavy atom effect and crystal packing similarities and differences. In the case of 1, due to the closeness of the TT H-aggregates arrangement with that of the phosphor's pure phase, the observed intensification of the phosphorescent emission at the expense of the prompt one is attributed to the extrinsic heavy atom effect of Zn. In 2 and 3, the heavier Cd atom is responsible for a decrease in the lifetimes of the afterglow emission, despite the presence of tightly overlapped H-dimers in the crystal structure. Finally, for 4, isostructural with 3, the Zn atom reveals in RTUP lifetime comparable with that of 1.

H-Aggregates Granting Crystallization-Induced Emissive Behavior and Ultralong Phosphorescence from a Pure Organic Molecule.

Solid-state luminescent materials with long lifetimes are the subject of ever-growing interest from both a scientific and a technological point of view. However, when dealing with organic compounds, the achievement of highly efficient materials is limited by aggregation-caused quenching (ACQ) phenomena on one side and by ultrafast deactivation of the excited states on the other. Here, we report on a simple organic molecule, namely, cyclic triimidazole (C9H6N6), 1, showing crystallization-induced emissive (CIE) behavior and, in particular, ultralong phosphorescence due to strong coupling in H-aggregated molecules. Our experimental data reveal that luminescence lifetimes up to 1 s, which are several orders of magnitude longer than those of conventional organic fluorophores, can be realized under ambient conditions, thus expanding the class of organic materials for phosphorescence applications.

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.

Nonlinear optical properties of intriguing Ru σ-acetylide complexes and the use of a photocrosslinked polymer as a springboard to obtain SHG active thin films.

This work reports on the design, synthesis and photo-physical properties of two ruthenium σ-alkynyl complexes. It is shown that, despite similar optical absorption features recorded in solution, the introduction of a benzaldehyde moiety leads to an improved non-linear optical (NLO) response as measured by Electric Field Induced Second Harmonic (EFISH) generation and Third Harmonic Generation (THG) at 1.907 µm, both related to the second order hyperpolarizability. These structure-property relationships are rationalized based on few state modelling. Complex is subsequently processed to afford composite films that demonstrate a χ(2) of 1.4 pm V(-1), quite remarkable given the ease of film processing implemented in this work.