Unique structural effect on the fluorosolvatochromism and dual fluorescence emission of D-π-A+ cationic chromophores with furyl bridge. An approach to white light emitters

Photophysical behavior of two D - π - A+ cationic compounds with the same furyl bridge and nicotinamidine group as an electron acceptor moiety and two electron donating groups, namely methoxy (I) and N,N-dimethylamino (II) groups was examined using steady-state and time-resolved techniques in variety of solvents. Time-dependent density functional theory (TDDFT) calculations were performed in some representative solvents and compared with the experimental results. Steady state and time-resolved studies in different solvents reveal that fluorescence emission of (I) is ascribed to an emission from an excited state (ICT) with higher dipole moment than the ground state while the emission of (II) is a dual emission from a state with high charge transfer nature (ICT) in addition to the locally excited state (LE). The fluorescence emission spectra of (II) were found to depend on the excitation wavelength and an increase in the excitation wavelength led to the formation of a longer wavelength emission band with lower quantum yield. It has also been found that the fluorescence excitation spectra were dependent on the emission wavelength. The effect of solvent on the nature of dual emission was examined. Correlation of the photophysical properties of the excited states of (I) and (II) with the solvent polarity, ε, reveals the charge transfer nature of (I) and the long wavelength emission band of (II), while their correlation with the solvent polarity parameter (ETN) shows two different trends when the solvents are divided to aprotic and protic solvents. For precise investigation of the impact of each solvent parameter on each photophysical property, Catalán's and Laurence's four parametric linear solvation energy relationships were studied. We have found that the non-specific interactions of the solvent are primarily responsible for controlling the photophysical properties, as demonstrated by Catalán's and Laurence's treatments. DFT and TDDFT calculations were used to anticipate the dipole moments in the ground and excited states and geometry of both states.

Substituents make a difference: 6,6″-modified terpyridine complexes with helix configuration and enhanced emission

A series of complexes L22-M (L2: 6,6″-bis(4-methoxyphenyl)-4'-phenyl-2,2':6',2″-terpyridine, M: Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) were synthesized by coordinating p-methoxyphenyl 6,6″-substituted terpyridine ligand with first-row transition metal ions and characterized by NMR, ESI-MS, and X-ray single crystal diffraction techniques. Single-crystal structures demonstrated that the steric hindrance of p-methoxyphenyl substituents endowed complexes L22-M with obvious longer coordination bond lengths and larger bond angles and dihedral angles compared with unmodified L12-M (L1: 4'-phenyl-2,2':6',2″-terpyridine). The chiral helix geometry was observed for L22-M, in which 2,2':6',2″-terpyridine moiety dramatically twisted to a spiral form in comparison to the nearly coplanar structure of the parent L12-M, resulting in plentiful intramolecular and intermolecular π-π interactions. Also, the appealing racemic (P and M) double helix packed structure for 6,6″-modified bisterpyridine complex L22-Cu was formed in the crystal. The consequent appealing charge transfer (CT) emission for L22-Zn in the solution and solid were investigated via UV-vis and fluorescence spectroscopy techniques and time-dependent density functional theory (TD-DFT) calculations. This work afforded a new method to achieve intriguing chiral geometry and CT optical properties via the subtle design and modification of terpyridine ligands.

Intermolecular Charge-Transfer Luminescence by Self-Assembly of Pyridinium Luminophores in Solutions

Designing a luminophore for application both in solution and in the solid state is a highly challenging task given the distinct nature of intermolecular interactions in these phases. In this context, we demonstrate that self-assembly of non-emissive charged pyridinium luminophores enables luminescence in solutions through a mechanism that is characteristic for the crystal state. Specifically, protonation of pyridine luminophore subunits in a solution promotes oligomer formation through intermolecular π+ -π interactions, leading to an intermolecular charge-transfer type luminescence. The luminescence turn-on by protonation is utilized for a highly efficient solution-state luminescent sensing of hydrogen chloride and sulfonic acids (TfOH, TsOH and MsOH) with detection limits spanning the range from 0.06 to 0.33 ppm. The protonation followed by self-assembly results in a bathochromic shift of the emission from 420 nm to 550 nm.

Photophysical properties and fluorosolvatochromism of D-π-A thiophene based derivatives

Solvation-dependent photophysical properties of two push-pull thiophene-based compounds with donor-π-acceptor (D-π-A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models. Intramolecular charge transfer characteristics of the structurally similar 2-fluoro-4-(5-(4-methoxyphenyl)thiophen-2-yl)benzonitrile (MOT) and 4-(5-(4-(dimethylamino)phenyl)thiophen-2-yl)-2-fluorobenzonitrile (DMAT) were investigated. Significant enhancement of intramolecular charge transfer strength has been observed through molecular structure modification of the electron donating group from a methoxy to dimethylamine group. Ground state absorption spectra show a small red shift of about 10 nm and 18 nm while the fluorescence emission spectra show a large red shift of about 66 nm and 162 nm on changing from the nonpolar cyclohexane to the aprotic polar DMSO for MOT and DMAT, respectively. Dipole moment change from the ground state to the charge transfer excited state is calculated to be 6.6 D in MOT and 9.0 D in DMAT. The fluorescence quantum yield, fluorescence lifetime and the derived radiative and non-radiative rate constants were found to be better correlated to the emission energy rather than any of the solvent properties. Three multi-parametric relationships were used in the interpretation of the specific versus non-specific solute-solvent interactions, namely, Kamlet-Taft, Catalán and Laurence et al. models. The findings of these approaches are used to extract useful information about different aspects of solvent effects on the photophysical properties of the two studied compounds. Kamlet-Taft solvatochromic model indicates that non-specific interactions are dominant in controlling the photophysical properties. Catalán's solvent dipolarity/polarizability parameter is found to play a significant role in solvatochromic behaviour which is also designated by the Laurence model.

TADF and exciplex emission in a xanthone-carbazole derivative and tuning of its electroluminescence with applied voltage

Materials showing white light emission have found applications in a variety of solid state devices especially in display technology. For white light emission, doping of red (R), green (G) and blue (B) emitters in a host matrix is commonly practised. However, finding RGB emitters of similar stability with homogenous doping is challenging. Furthermore, such devices suffer from color purity in the long run. Small organic light emitters, capable of colour tuning and having a broad emission spectrum are in high demand as they provide colour stability, reproducibility, a simple device geometry and high efficiency. Recently, it has been shown that the efficiency of OLEDs can be enhanced by employing thermally activated delayed fluorescence (TADF) materials. Here, we designed and synthesised a xanthone-carbazole based D-A-D material (Xan-Cbz) for TADF properties. Blue TADF emission, in neat thin films, at 470 nm was observed and further investigated by studying delayed fluorescence and lifetime measurements. In addition, a blend of Xan-Cbz with NPD shows exciplex emission at 525 nm in thin film. OLEDs based on Xan-Cbz were fabricated using several device configurations. OLEDs having the device configuration ITO/PEDOT:PSS/NPD/Xan-Cbz/Bphen/LiF-Al showed a luminance of 1.96 × 104 Cd m-2 (at a current density of 50 mA cm-2) and V ON at ∼6 V. Electroluminescence showed the features of both neat emission (470 nm) of Xan-Cbz and its exciplex (525 nm) with NPD. Further, colour tuning was observed as a function of applied voltage and the ratio of light intensity (I 525/I 470) of neat and exciplex emission was found to decrease with increasing voltage. Greenish-blue emission (CIE coordinates: 0.202, 0.382) from Xan-Cbz OLEDs was obtained. Xan-Cbz showed its neat emission (at 470 nm) in ITO/PEDOT:PSS/CBP/Xan-Cbz/Bphen/LiF-Al and pure exciplex emission (at 525 nm) in ITO/PEDOT:PSS/NPD:Xan-Cbz/Bphen/LiF-Al device configurations. Thus in this article we showed blue TADF emission, exciplex emission and voltage dependent color tuning in OLEDs based on a small organic emitter.

A rationally designed vapoluminescent compound with adsorptive channels and responsive luminophores for volatile organic compounds (VOCs)

A novel cuprous complex bearing two functional parts, i.e. a luminophoric part and a structural part, exhibits distinct luminescence responses to a variety of volatile organic compounds of different polarities in the solid state.

Synthesis and structural analysis of dimethylaminophenyl-end-capped diketopyrrolopyrrole for highly stable electronic devices with polymeric gate dielectric

The synthesis and structural analysis of DPP(PhNMe₂)₂, a stable diketopyrrolopyrrole derivative end-capped with a strongly electron-donating dimethylaminophenyl moiety is reported and the origin of ambient stability is analyzed in detail.

A solvatochromic cyanostilbene derivative as an intensity and wavelength-based fluorescent sensor for water in organic solvents

A new solvatochromic fluorescent cyanostilbene derivative was developed for the qualitative and quantitative detection of water in THF and dioxane, respectively.

Benzobisoxazole cruciforms as fluorescent sensors

CONSPECTUS: Cross-conjugated molecular cruciforms are intriguing platforms for optoelectronic applications. Their two intersecting π-conjugated arms allow independent modulation of the molecules' HOMO and LUMO levels and guarantee a well-defined optical response to analyte binding. In addition, the rigid cross-conjugated geometries of these molecules allow their organization in two- and three-dimensional space with long-range order, making them convenient precursors for the transition from solution-based to the more practical solid-state- and surface-based devices. Not surprisingly, a number of molecular cruciform classes have been explored because of these appealing properties. These include tetrakis(arylethynyl)benzenes, tetrastyrylbenzenes, distyrylbis(arylethynyl)benzenes, tetraalkynylethenes, biphenyl-based "swivel" cruciforms, and benzobisoxazole-based cruciforms. In this Account, we summarize our group's work on benzobisoxazole molecular cruciforms. The heterocyclic central core of these molecules forces their HOMOs to localize along the vertical bisethynylbenzene axis; the HOMO localization switches to the horizontal benzobisoxazole axis only in cases when that axis bears electron-rich 4-(N,N-dimethylamino)phenyl substituents and the vertical axis does not. In contrast, the LUMOs are generally delocalized across the entire molecule, and their localization occurs only in cruciforms with donor-acceptor substitution. Such spatially isolated frontier molecular orbitals (FMOs) of the benzobisoxazole cruciforms make their response to protonation very predictable. Benzobisoxazole cruciforms are highly solvatochromic, and their fluorescence quantum yields reach 80% in nonpolar solvents. Solutions of cruciforms in different solvents change emission colors upon addition of carboxylic and boronic acid analytes. These changes are highly sensitive to the analyte structure, and the emission color responses permit qualitative discrimination among structurally closely related species. In self-assembled complexes with boronic acids, benzobisoxazole fluorophores switch their analyte preferences and become responsive to Lewis basic species: phenoxides, amines, ureas, and small organic and inorganic anions. These sensing complexes allow the decoupling of the sensor's two functions: a nonfluorescent boronic acid does the chemistry through the exchange of its labile B-O bonds for other nucleophiles, and it can be optimized for solubility and analyte specificity; the benzobisoxazole fluorophore senses the electronic changes on the boron and reports them to the operator through changes in its emission colors, allowing this sensing element to be kept constant across a broad range of analytes. We have recently expanded our studies to benzimidazole-based "half-cruciforms", which are L-shaped rigid fluorophores that maintain most of the spatial separation of FMOs observed in benzobisoxazole cruciforms. Unlike benzobisoxazoles, benzimidazoles are acidic on account of their polar N-H bonds, and this feature allows them to respond to a broader range of pH values than their benzobisoxazole counterparts. The deprotonated benzimidazolate anions maintain their fluorescence, which makes them promising candidates for incorporation into solid-state sensing materials known as zeolithic imidazolate frameworks.