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.

Self-Assembly of N-Rich Triimidazoles on Ag(111): Mixing the Pleasures and Pains of Epitaxy and Strain.

In the present report, homochiral hydrogen-bonded assemblies of heavily N-doped (C9H6N6) heterocyclic triimidazole (TT) molecules on an Ag(111) substrate were investigated using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) techniques. The planar and prochiral TT molecules, which exhibit a threefold rotation symmetry and lack mirror symmetry when assembled on the substrate, carry multiple hydrogen-bonding donor and acceptor functionalities, inevitably leading to the formation of hexameric two-dimensionally extended assemblies that can be either homo- (RR/SS) or heterochiral (RS). Experimental STM data showing well-ordered homochiral domains and experimental LEED data are consistent with simulations assuming the R19.1° overlayer on the Ag(111) lattice. Importantly, we report the unexpected coincidence of spontaneous resolution with the condensation of neighboring islands in adjacent "Janus pairs". The islands are connected by a characteristic fault zone, an observation that we discuss in the context of the fairly symmetric molecule and its propensity to compromise and benefit from interisland bonding at the expense of lattice mismatches and strain in the defect zone. We relate this to the close to triangular shape and the substantial but weak bonding scheme beyond van der Waals (vdW) of the TT molecules, which is due to the three N-containing five-membered imidazole rings. Density functional theory (DFT) calculations show clear energetic differences between homochiral and heterochiral pairwise interactions, clearly supporting the experimental results.

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.

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.

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.

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.

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.

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.

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.

Chemical shift assignment of mono- and di-bromo triimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine derivatives by DFT/NMR integrated approach.

Mono- and di-bromo derivatives of triimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine have been proposed as new organic molecules presenting a very rich and complex photophysical behavior. Thus, we afforded the correct chemical shift assignment by integrating the experimental data with DFT calculation of NMR parameters. Our findings lay foundation for a structural reference in the organic synthesis and characterization of new congeners of this intriguing class of molecules.

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.

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.

In Situ Electroluminescence Color Tuning by Thermal Deprotonation Suitable for Thermal Sensors and Anti-fraud Labels.

A novel and versatile approach to tune photoluminescence and electroluminescence by in situ controlled thermal deprotonation is presented. This methodology, based on a single organic π-conjugated material (pyrene derivative), allows the manufacturing of organic light-emitting diode (OLED) prototypes by solution methods with controlled tunable emission ranging from the orange (protonated form of the dye) to the blue (pristine or deprotonated form). While several protonation/deprotonation cycles can be performed on thin films, for the devices only one cycle is possible so that their use as anti-fraud labels can be envisaged. OLEDs exhibit daylight visible brightness of 150 cd m-2 and device lifetime exceeding 30 hours of continuous operation. Thanks to the simplicity of both material design and device fabrication our approach opens new perspectives in the wide field of thermal sensors for customer care or risk perception.

Tuning the dipolar second-order nonlinear optical properties of 5-π-delocalized-donor-1,3-di(2-pyridyl)benzenes, related cyclometallated platinum(ii) complexes and methylated salts.

The synthesis and characterization of three 5-π-delocalized-donor-1,3-di(2-pyridyl)benzenes is reported along with that of their related cyclometallated platinum(ii) complexes and N,N-dimethylated iodide salts. The second-order nonlinear optical (NLO) properties of all the compounds have been determined by the Electric Field Induced Second Harmonic generation technique, showing how the µßEFISH absolute value of 1,3-di(2-pyridyl)benzenes can be tuned by the nature of the substituent on position 5 of the central benzene ring, and greatly increased by cyclometallation to Pt or by N-methylation.

An acido-triggered reversible luminescent and nonlinear optical switch based on a substituted styrylpyridine: EFISH measurements as an unusual method to reveal a protonation-deprotonation NLO contrast.

Diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (DPVPA) constitutes a novel acido-triggered reversible luminescent and nonlinear optical switch. Remarkably, for the first time the Electric-Field Induced Second Harmonic generation (EFISH) technique is used to reveal a protonation-deprotonation NLO contrast.

A Re(CO)5 fragment bonded to a polyhedral oligomeric hydroxysilsesquioxane as a model which gives further evidence to the existence of an unexpected Re(CO)5 surface species.

Reaction of Re(CO)5O3SCF3 with (c-C6H11)7Si8O12O-Li+ at 273 K under a CO atmosphere affords the [Re(CO)5OR] (R = (c-C6H11)7Si8O12) derivative (1). 1 is the first example of a rhenium pentacarbonyl bearing an OR ligand (R = alkyl, aryl, or silyl) stable enough to be characterized, and it represents also the first molecular model of the surface [Re(CO)5OSi] species formed by reductive carbonylation of silica-supported [Re(CO)3OH]4. At room temperature, 1 loses one carbonyl ligand and dimerizes to afford {Re(CO)4[(mu-O)O12Si8(c-C6H11)7]}2 (2), which has been characterized by X-ray diffraction and is the first reported example of a rhenium tetracarbonyl mu-oxo-bridged dimer of the type [Re(CO)4(mu-OR)]2.