Rational Design of Carbazole- and Carboline-Based Ambipolar Host Materials for Blue Electrophosphorescence: A Density Functional Theory Study

Density functional theory has been employed to design 41 host molecules for blue electrophosphorescence by incorporating electron donor (carbazole (cbz)) and electron acceptor (α-carboline (Cb1)) units into <i>N</i>-phenylcarbazole (PhCbz). We have systematically investigated the influence of the number (mono-, di-, and trisubstituted) and positions of Cb1 and Cbz substitution on an array of electronic properties of the designed hosts. The results underline that the substitution of the <i>N</i>-phenyl ring with a carboline unit yields host molecules with low charge injection barriers, balanced charge transport, efficient charge separation, high triplet energy (<i>E</i>_T), and low singlet–triplet energy difference (Δ<i>E</i>_ST). For disubstituted hosts, the second subunit can either be Cb1 or Cbz substituted at the 2/7 position of PhCbz, while substituting the 2 and 3 positions of PhCbz with Cbz subunits generates trisubstituted hosts with efficient electronic properties. Thus, our results indicate that both number and position of subunit substitution in PhCbz play a decisive role in designing hosts with appropriate electronic properties. Among the 41 systems considered in the study, we have identified the two most efficient hosts, and their electronic properties are found to be very promising compared to some of the experimentally reported analogous hosts

White Light Emitting Polymers from a Luminogen with Local Polarity Induced Enhanced Emission

Aggregation induced enhanced emission (AIEE) is considered as an important tool to circumvent the aggregation caused quenching (ACQ) effect in organic light emitting diodes (OLEDs). Charge trapping and surplus long wavelength electroluminescence is a cause of concern in single polymer based white OLEDs. However, the potential of luminogens with AIEE property as a credible tool to offset the above problems in white light emitting single polymer is not properly explored. In this study design, synthesis and spectral characterization of a polymerizable luminogen, (2<i>Z</i>,2′<i>Z</i>)-6,6′-(2,7-dibromo-9<i>H</i>-fluorene-9,9-diyl)­bis­(hexane-6,1-diyl)­bis­(2-cyano-3-(10-hexyl-10<i>H</i>-phenothiazin-3-yl)­acrylate­(FCPA) with AIEE property and its copolymers is presented. Lippert-Mataga studies showed that reduced local polarity caused by aliphatic chains in condensed state of FCPA resulted in AIEE property. The copolymers P­(FCPA-1) and P­(FCPA-0.5) with 1% and 0.5% FCPA moieties showed white electroluminescence and enhanced thin film photoluminescence that matched very closely. The superior performance of OLEDs is attributed to the presence of a phenothiazine group in FCPA that resulted in nearly equal electron and hole injection barriers

Influence of Thiophenes on Molecular Order, Mesophase, and Optical Properties of π‑Conjugated Mesogens

Increasing interest in π-conjugated aromatic cores built essentially with thiophene rings is recognized owing to their applications in optoelectronics. In this investigation, an attempt is made to understand the influence of terminal thiophene rings on the molecular order, mesophase, and optical properties of mesogens in which phenyl benzoate is part of the core. Accordingly, mono-, di-, and terthiophene units are linked to two phenyl ring core by Suzuki cross coupling reaction. The synthesized thiophene-based π-conjugated mesogens exhibit enantiotropic nematic and smectic phases with excellent mesophase range. The tendency for smectic phases and the mesophase range enhanced with increased thiophene rings. The layer ordering in smectic A and smectic C phase is established by powder X-ray diffraction, while the orientational order of all the rings of core unit is accomplished by ¹³C NMR spectroscopy. Thus the ¹³C–¹H dipolar couplings determined from 2D separated local field NMR experiments show a very high value for terminal C–H of thiophene ring (∼9–11 kHz) irrespective of number of thiophenes in the mesogenic core. The density functional theory and time-dependent density functional theory calculations indicate the intramolecular charge-transfer transition between the phenyl-thiophene to phenyl benzoate unit. The solution absorption and fluorescence spectral studies reveal interesting features. The monothiophene-based mesogen is nonfluorescent, while those based on bithiophene and terthiophene show intense fluorescence. The well-resolved vibronic peaks observed in fluorescence spectra of mesogens are characteristic of oligothiophenes. Furthermore, the fluorescence excitation anisotropy measured by monitoring the vibronic features of the mesogens is found to be similar, signifying that the emission originates from the identical electronic energy level. Therefore, the investigation encompassing wide-ranging techniques manifests that the insertion of more thiophenes in the mesogenic core favors polymesomorphism and intense emission, enabling them for application in polarized emission

Three-Ring-Based Thermotropic Mesogens with a Dimethylamino Group: Structural Characterization, Photophysical Properties, and Molecular Order

Thermotropic liquid crystals exhibiting light-emitting properties are gaining popularity as functional materials in view of their application in organic light-emitting diodes. Such mesogens essentially require active chromophoric moieties in the mesogenic core so that the mutual light-emitting and liquid crystalline properties can be realized. In this work, three-ring-core-based mesogens with a terminal dimethylamino unit are subjected to structural characterization by various techniques. These mesogens exhibit enantiotropic nematic as well as smectic A phase with interdigitated layer organization (SmA_d). This is a surprising observation because the SmA_d organization is commonly observed for calamitic mesogens with terminal polar groups. Interestingly, the single-crystal structure of the C₆ homologue indicates antiparallel packing. Furthermore, the photophysical properties of a representative C₁₂ mesogen in solution disclose yet another exciting feature. The steady-state and time-resolved fluorescence studies indicate negative solvotochromism in solvents with differing polarity. To obtain greater insight, density functional theory (DFT)-based highest occupied molecular orbital–lowest unoccupied molecular orbital studies are carried out which support intramolecular charge-transfer interactions in this class of mesogens. Additionally, the DFT calculations also provide the ¹³C chemical shifts which are compared with the solution NMR values for the structural assignment of all carbons in the core unit. Furthermore, the two-dimensional separated local field measurements for the C₁₂ homologue in nematic and SmA_d mesophases offer ¹³C–¹H dipolar couplings from which the molecular order is determined to be 0.59 and 0.70, respectively

High-Resolution Solid State ¹³C NMR Studies of Bent-Core Mesogens of Benzene and Thiophene

Bent-core mesogens are an important class of thermotropic liquid crystals as they exhibit unusual properties as well as morphologies distinctly different from rodlike mesogens. Two bent-core mesogens with differing center rings namely benzene and thiophene are considered and investigated using high-resolution oriented solid state ¹³C NMR method in their liquid crystalline phases. The mesogens exhibit different phase sequences with the benzene-based mesogen showing a B₁ phase, while the one based on thiophene showing nematic and smectic C phases. The 2-dimensional separated local field (2D-SLF) NMR method was used to obtain the ¹³C–¹H dipolar couplings of carbons in the center ring as well as in the side-wing phenyl rings. Couplings, characteristic of the type of the center ring, that also provide orientational information on the molecule in the magnetic field were observed. Together with the dipolar couplings of the side-wing phenyl ring carbons from which the local order parameters of the different subunits of the core could be extracted, the bent angle of the mesogenic molecule could be obtained. Accordingly, for the benzene mesogen in its B₁ phase at 145 °C, the center ring methine ¹³C–¹H dipolar couplings were found to be significantly larger (9.5–10.2 kHz) compared to those of the side-wing rings (1.6–2.1 kHz). From the local order parameter values of the center (0.68) as well as the side-wing rings (0.50), a bent-angle of 130.3° for this mesogen was obtained. Interestingly, for the thiophene mesogen in its smectic C phase at 210 °C, the ¹³C–¹H dipolar coupling of the center ring methine carbon (2.11 kHz) is smaller than those of the side-wing phenyl ring carbons (2.75–3.00 kHz) which is a consequence of the different structures of the thiophene and the benzene rings. These values correspond to local order parameters of 0.85 for the center thiophene ring and 0.76 for the first side-wing phenyl ring and a bent-angle of 149.2°. Thus, the significant differences in the dipolar couplings and the order parameter values between different parts in the rigid core of the mesogens are a direct consequence of the nature of the center ring and the bent structure of the molecule. The present investigation thus highlights the ability of the ¹³C 2D-SLF technique to provide the geometry of the bent-core mesogens in a straightforward manner through the measurement of the ¹³C–¹H dipolar couplings

Monolayer to Interdigitated Partial Bilayer Smectic C Transition in Thiophene-Based Spacer Mesogens: X‑ray Diffraction and ¹³C Nuclear Magnetic Resonance Studies

Mesophase organization of molecules built with thiophene at the center and linked via flexible spacers to rigid side arm core units and terminal alkoxy chains has been investigated. Thirty homologues realized by varying the span of the spacers as well as the length of the terminal chains have been studied. In addition to the enantiotropic nematic phase observed for all the mesogens, the increase of the spacer as well as the terminal chain lengths resulted in the smectic C phase. The molecular organization in the smectic phase as investigated by temperature dependent X-ray diffraction measurements revealed an interesting behavior that depended on the length of the spacer <i>vis-a-vis</i> the length of the terminal chain. Thus, a tilted interdigitated partial bilayer organization was observed for molecules with a shorter spacer length, while a tilted monolayer arrangement was observed for those with a longer spacer length. High-resolution solid state ¹³C NMR studies carried out for representative mesogens indicated a U-shape for all the molecules, indicating that intermolecular interactions and molecular dynamics rather than molecular shape are responsible for the observed behavior. Models for the mesophase organization have been considered and the results understood in terms of segregation of incompatible parts of the mesogens combined with steric frustration leading to the observed lamellar order

A Highly Selective Chemosensor for Cyanide Derived from a Formyl-Functionalized Phosphorescent Iridium(III) Complex

A new phosphorescent iridium­(III) complex, bis­[2′,6′-difluorophenyl-4-formylpyridinato-<i>N</i>,<i>C</i>4′]­iridium­(III) (picolinate) (<b>IrC</b>), was synthesized, fully characterized by various spectroscopic techniques, and utilized for the detection of CN^– on the basis of the widely known hypothesis of the formation of cyanohydrins. The solid-state structure of the developed <b>IrC</b> was authenticated by single-crystal X-ray diffraction. Notably, the iridium­(III) complex exhibits intense red phosphorescence in the solid state at 298 K (Φ_PL = 0.16) and faint emission in acetonitrile solution (Φ_PL = 0.02). The cyanide anion binding properties with <b>IrC</b> in pure and aqueous acetonitrile solutions were systematically investigated using two different channels: i.e., by means of UV–vis absorption and photoluminescence. The addition of 2.0 equiv of cyanide to a solution of the iridium­(III) complex in acetonitrile (<i>c</i> = 20 μM) visibly changes the color from orange to yellow. On the other hand, the PL intensity of <b>IrC</b> at 480 nm was dramatically enhanced ∼5.36 × 10²-fold within 100 s along with a strong signature of a blue shift of the emission by ∼155 nm with a detection limit of 2.16 × 10^–8 M. The cyanohydrin formation mechanism is further supported by results of a ¹H NMR titration of <b>IrC</b> with CN^–. As an integral part of this work, phosphorescent test strips have been constructed by impregnating Whatman filter paper with <b>IrC</b> for the trace detection of CN^– in the contact mode, exhibiting a detection limit at the nanogram level (∼265 ng/mL). Finally, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed to understand the electronic structure and the corresponding transitions involved in the designed phosphorescent iridium­(III) complex probe and its cyanide adduct

A Highly Selective Chemosensor for Cyanide Derived from a Formyl-Functionalized Phosphorescent Iridium(III) Complex

A new phosphorescent iridium­(III) complex, bis­[2′,6′-difluorophenyl-4-formylpyridinato-<i>N</i>,<i>C</i>4′]­iridium­(III) (picolinate) (<b>IrC</b>), was synthesized, fully characterized by various spectroscopic techniques, and utilized for the detection of CN^– on the basis of the widely known hypothesis of the formation of cyanohydrins. The solid-state structure of the developed <b>IrC</b> was authenticated by single-crystal X-ray diffraction. Notably, the iridium­(III) complex exhibits intense red phosphorescence in the solid state at 298 K (Φ_PL = 0.16) and faint emission in acetonitrile solution (Φ_PL = 0.02). The cyanide anion binding properties with <b>IrC</b> in pure and aqueous acetonitrile solutions were systematically investigated using two different channels: i.e., by means of UV–vis absorption and photoluminescence. The addition of 2.0 equiv of cyanide to a solution of the iridium­(III) complex in acetonitrile (<i>c</i> = 20 μM) visibly changes the color from orange to yellow. On the other hand, the PL intensity of <b>IrC</b> at 480 nm was dramatically enhanced ∼5.36 × 10²-fold within 100 s along with a strong signature of a blue shift of the emission by ∼155 nm with a detection limit of 2.16 × 10^–8 M. The cyanohydrin formation mechanism is further supported by results of a ¹H NMR titration of <b>IrC</b> with CN^–. As an integral part of this work, phosphorescent test strips have been constructed by impregnating Whatman filter paper with <b>IrC</b> for the trace detection of CN^– in the contact mode, exhibiting a detection limit at the nanogram level (∼265 ng/mL). Finally, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed to understand the electronic structure and the corresponding transitions involved in the designed phosphorescent iridium­(III) complex probe and its cyanide adduct

A Highly Selective Chemosensor for Cyanide Derived from a Formyl-Functionalized Phosphorescent Iridium(III) Complex

A new phosphorescent iridium­(III) complex, bis­[2′,6′-difluorophenyl-4-formylpyridinato-<i>N</i>,<i>C</i>4′]­iridium­(III) (picolinate) (<b>IrC</b>), was synthesized, fully characterized by various spectroscopic techniques, and utilized for the detection of CN^– on the basis of the widely known hypothesis of the formation of cyanohydrins. The solid-state structure of the developed <b>IrC</b> was authenticated by single-crystal X-ray diffraction. Notably, the iridium­(III) complex exhibits intense red phosphorescence in the solid state at 298 K (Φ_PL = 0.16) and faint emission in acetonitrile solution (Φ_PL = 0.02). The cyanide anion binding properties with <b>IrC</b> in pure and aqueous acetonitrile solutions were systematically investigated using two different channels: i.e., by means of UV–vis absorption and photoluminescence. The addition of 2.0 equiv of cyanide to a solution of the iridium­(III) complex in acetonitrile (<i>c</i> = 20 μM) visibly changes the color from orange to yellow. On the other hand, the PL intensity of <b>IrC</b> at 480 nm was dramatically enhanced ∼5.36 × 10²-fold within 100 s along with a strong signature of a blue shift of the emission by ∼155 nm with a detection limit of 2.16 × 10^–8 M. The cyanohydrin formation mechanism is further supported by results of a ¹H NMR titration of <b>IrC</b> with CN^–. As an integral part of this work, phosphorescent test strips have been constructed by impregnating Whatman filter paper with <b>IrC</b> for the trace detection of CN^– in the contact mode, exhibiting a detection limit at the nanogram level (∼265 ng/mL). Finally, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed to understand the electronic structure and the corresponding transitions involved in the designed phosphorescent iridium­(III) complex probe and its cyanide adduct