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Umamahesh B

VIT University, Organic Chemistry Division, SENIOR RESEARH FELLOW

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Highly skilled material chemist with intense experience in design strategy, synthesis, purification, and characterization of materials, photophysical properties, and OLED device fabrication accompanied by excellent scientific knowledge and research publications. Great interest and strong abilities to adapt to novel technologies, generating research reports, and providing scientific solutions.
Phone: 09790115974
Address: TT 730
VIT University, Vellore

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Bejoymohandas K S

Consiglio Nazionale delle Ricerche (CNR)

Hameed Al Attar

Macquarie University

Academia Sinica

California Institute of Technology

Arunandan Kumar

Université de Bourgogne

Frédéric Dumur

Aix-Marseille Université

Marie Cifuentes

The Australian National University

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Papers by Umamahesh B

An insight into the photophysical properties of amide hydrogen bonded N-(benzo[d]thiazol-2-yl) acetamide crystals

Three distinct, hydrogen bond associated N-(benzo[d]thiazol-2-yl) acetamides were synthesized by ... more Three distinct, hydrogen bond associated N-(benzo[d]thiazol-2-yl) acetamides were synthesized by refluxing benzothiazoles with acetic acid. The nature of the assemblies was characteristic to the substituent in the benzothiazole moiety. In N-(benzo[d]thiazol-2-yl)acetamide, water acts as a bridge for forming three hydrogen bonds, as an acceptor to amide N\ \H, and donors to carbonyl of amide and thiazole nitrogen assembles of three different N-(benzo[d]thiazol-2-yl)acetamide molecules. The N-(6-methylbenzo[d]thiazol-2-yl)acetamide formed a (amide) N-H…N (thiazole) bonded R 2 2 (8) molecular dimers by two homo-intermolecular hydrogen bonding interactions. N-(6-methoxybenzo[d]thiazol-2-yl)acetamide formed (amide)N-H…O (acid) & (acid)O-H…N (thiazole) interactions with the acetic acid, forming a R 2 2 (8) hydrogen-bonded ring by two hetero-intermolecular hydrogen bonding interactions.

Electroluminescence of iridium(III) complexes containing F or CF 3 substituents

Electron withdrawing groups F or CF 3 were attached to 6-methyl-4-diphenylquinoline (medpq) ligan... more Electron withdrawing groups F or CF 3 were attached to 6-methyl-4-diphenylquinoline (medpq) ligand. The effect of this substitution on photophysical and electroluminescent properties of four phosphorescent iridium(III) complexes containing the two upgraded medpq ligands and 2-(2-pyridyl) benzimidazole or 2-(1H-tetrazol-5-yl) pyridine ligand was comparatively investigated. All complexes emitted strong phosphorescence with absolute quantum efficiency of deoxygenated toluene solutions higher than 50% and the lifetime of 1.068-1.88 microseconds observed for neat films at 77 K. The orange phosphorescence of CF 3-substituted complexes was slight shifted to higher energy region in comparison to that of the corresponding F-containing complexes. The ionization potentials detected by photoelectron emission spectroscopy for solid-state samples of the complexes were practically the same (5.91-6.04 eV). Exploiting the same device structure, phosphorescent organic light-emitting diodes (PhOLEDs) containing a CF 3-substituted complex exhibited maximum external quantum efficiency of 10.33% in contrast to 5.36% observed for PhOLEDs based on a F-containing complex.

Enantioselective fluorescent sensing of chiral carboxylic acid by engaging boronic acid and BINOL

Two BINOL based boronate esters, namely (S)-2 and (S)-3, were designed and synthesized. Fluoresce... more Two BINOL based boronate esters, namely (S)-2 and (S)-3, were designed and synthesized. Fluorescent recognition studies of (S)-2 towards enantiomers of mandelic acid revealed that sensor (S)-2 exhibited high fluorescent enantioselective response towards mandelic acid. The other boronate ester (S)-3 which did not have hydroxyl group in the minor groove of BINOL did not show any fluorescent chiral discrimination. Their enantioselective recognition was also studied by means of 1 H NMR spectroscopy. The results indicated that the sensor (S)-2 was very promising as a chiral fluorescent sensor for the recognition of chiral carboxylic acid in THF solvent. Theoretically predicted hydrogen bonding interactions revealed a favorable chiral recognition between the receptor and l-mandelic acid, which is well correlated with experimentally observed enantioselective recognition phenomenon.

Highly emissive, naked-eye solvatochromic probe based on styryl tetrahydrodibenzo[a,i] phenanthridine for acidochromic applications

A new series of 5-styryl tetrahydrodibenzo[a,i]phenanthridines was readily synthesized from b-tet... more A new series of 5-styryl tetrahydrodibenzo[a,i]phenanthridines was readily synthesized from b-tetralone, ammonium acetate and cinnamaldehydes. These dyes emitted light with large Stokes shift values of 3894-7265 cm À1 , depending on the nature of the substituent. The dyes displayed positive solvatochromism in their emission spectra, suggestive of a more polar excited state. Dye 3c exhibited aggregation-induced emission (AIE) property in 10% THF-water mixture besides a ratiometric response to pH from 10.75 to 2.16 with blue shift of 84 nm in emission. The emission ratio (I 526 : I 442) further displayed a good linearity with pH in the range of 4.1 to 6.8 and the corresponding pK a value was found to be 4.55 AE 0.03. The addition of TFA led to a prominent bathochromic shift in spectra along with clear colour changes owing to protonation at the phenanthridine segment. Further, a second protonation of the phenanthrinium segment could take place based on the pK a strength of the acid added. The pH dependent spectral changes were successfully applied to quantitatively detect pH in biological fluids and acid impurities in solvents.

SnCl 2-catalyzed synthesis of dihydro-5Hbenzo[f]pyrazolo[3,4-b]quinoline and dihydroindeno[2,1-b]... more

Tetrazole iridium(III) complexes as a class of phosphorescent emitters for high-efficiency OLEDs

We show that iridium complexes incorporating a tetrazolate ligand can be used as new highly phosp... more We show that iridium complexes incorporating a tetrazolate ligand can be used as new highly phosphorescent emitters in the fabrication of high-efficiency organic light emitting devices (OLEDs). The nature of the excited states of these Ir(III) complexes was probed by means of electrochemical analysis, absorption and photoluminescence spectroscopies and with the aid of TD-DFT calculations. To demonstrate the high stability of these classes of complexes we used TGA and DSC characterizations. These complexes are used as dopant emitters in the emissive layers. The as-fabricated OLEDs display high-efficiency and high-brightness with blue, green and yellow/orange emission.

Journal of Molecular Catalysis A: Chemical

Nano-ZnAl 2 O 4 was synthesized by blending metal-citrate complex-acrylamide polymer using the pr... more Nano-ZnAl 2 O 4 was synthesized by blending metal-citrate complex-acrylamide polymer using the precursor method. Powder X-ray diffraction analysis of the catalyst affirmed that the spinel oxide was phase pure. Elemental analysis was confirmed by Energy Dispersive X-ray analysis. Transmission Electron Microscopic (TEM) analysis and Scherrer's formula revealed that the particles were found to be in the order of 20 nm in size. Thermal stability of the catalyst was analyzed by thermogravimetric and differential thermal analysis (TG-DTA). Total acidity of the nano-ZnAl 2 O 4 evaluated by NH 3-TPD was 8.8 mmol NH 3 /g. Diversity in the synthesis of xanthene derivatives was investigated using nanozinc aluminate as a catalyst. Nanoform of the catalyst showed better activity than the bulk form due to its large surface area 50 m 2 /g. The 2,2-arylmethylene bis (3-hydroxy-2-cyclohexene-1-one), 2,2-arylmethylene bis (3hydroxy-5,5-dimethyl-2-cyclohexene-1-one) and 1-Oxo-hexahydroxanthenes were synthesized rapidly using ZnAl 2 O 4 and the catalyst was successfully recycled without any loss of activity.

Suppression of external quantum efficiency rolloff in organic light emitting diodes by scavenging triplet excitons

Large external quantum efficiency rolloff at high current densities in organic light-emitting dio... more Large external quantum efficiency rolloff at high current densities in organic light-emitting diodes (OLEDs) is frequently caused by the quenching of radiative singlet excitons by longlived triplet excitons [singlet-triplet annihilation (STA)]. In this study, we adopted a triplet scavenging strategy to overcome the aforementioned STA issue. To construct a model system for the triplet scavenging, we selected 2,6-dicyano-1,1-diphenyl-λ 5 σ 4-phosphinine (DCNP) as the emitter and 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) as the host material by considering their singlet and triplet energy levels. In this system, the DCNP's triplets are effectively scavenged by BSBCz while the DCNP's singlets are intact, resulting in the suppressed STA under electrical excitation. Therefore, OLEDs with a 1 wt.%-DCNP-doped BSBCz emitting layer demonstrated the greatly suppressed efficiency rolloff even at higher current densities. This finding favourably provides the advanced light-emitting performance for OLEDs and organic semiconductor laser diodes from the aspect of the suppressed efficiency rolloff.

Direct anti and regio-specific aldol reactions of cyclododecanone catalyzed by alkali metal hydroxides: implications for supramolecular helical designw

The direct aldol condensation of benzaldehyde to cyclododecanone (CDD), catalyzed by NaOH is expe... more The direct aldol condensation of benzaldehyde to cyclododecanone (CDD), catalyzed by NaOH is expected to yield bis-benzylidene cyclododecanone, but we interestingly ended up with novel b-hydroxy carbonyl compounds and monobenzylidene cyclododecanone derivatives in the cases of 2-halo substituted benzaldehydes. This exceptional behaviour is due to the capability of the CDD ring to exist in the zwitter ionic form. The formation of monobenzylidine derivatives can be due to less stable hydrogen bonds between-OH and-CQO groups, electrostatic interaction between aldehyde substituents and metal enolates and the bulkiness of the benzaldehyde ring substituents. We extensively optimized the reaction conditions using different solvents and alkali metal hydroxide catalysts and we are getting high yield, regio-specificity and high diastereoselectivity (anti-aldol) in b-hydroxy carbonyl compounds. The representative crystal structures were examined and they suggest the tendency of anti-aldol compounds to form supramolecular helical motifs. The b-hydroxyl carbonyl derivatives form supramolecular helices via O-HÁ Á ÁO hydrogen bonds with two anti-aldol units per turn and a pitch of B6 Å. The cyclododecanone ring adopts the minimum energy [3333] square conformation. The nucleophile will attack only from the less hindered side of the carbonyl group in the CDD ring.

Tetrahydrodibenzo[a,i]phenanthridin-5-yl)phenol as a Fluorescent Probe for the Detection of Aniline

Two (tetrahydrodibenzo[a,i]phenanthridin-5yl)phenols that differ in their substituents at the par... more Two (tetrahydrodibenzo[a,i]phenanthridin-5yl)phenols that differ in their substituents at the para position (P1, R = H and P2, R = NEt 2) were designed and synthesized. The presence of a −NEt 2 group in probe P2 facilitates the twisted intramolecular charge transfer (TICT) process, making P2 emissive, which distinctly coordinated with boron trifluoride in the presence of different amines with different electronic properties. A substantial increase in emission intensity with increasing viscosity of the surrounding environment and smooth formation of a planar complex with boron and Zn 2+ ions concluded the presence of a TICT process. The selective reactivity of P2 toward a tetracoordinated boron complex has been explored as a potential tool for colorimetric and fluorescent discrimination of aromatic primary amines, i.e., anilines. Selective detection of aniline with probe P2 can be viewed through the naked eye, and the corresponding fluorescence turn-on detection limit was found to be 12.65 nM. In addition, the detection of aniline on precoated aluminum-backed thin-layer chromatography plates and Whatman filter paper strips was found to be in good agreement with the color change of P2 in solution and in vapor phase.

Human-eyes-friendly white electroluminescence from solution-processable hybrid OLEDs exploiting new iridium (III) complex containing benzoimidazophenanthridine ligand

This paper primarily deals with the development of highly efficient phosphorescent iridium (III) ... more This paper primarily deals with the development of highly efficient phosphorescent iridium (III) complex based on benzoimidazophenanthridine as main ligand. The potential of this complex, as phosphorescent emitter for optoelectronic applications, was screened by different methods, including luminescence spectrometry, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry as well as photoelectron emission spectrometry. The possible application of the developed complex in single-colour vacuum-deposited organic light emitting diodes (OLEDs) exhibiting warm orange electrophosphorescence was demonstrated. The device was characterized with CIE1931 colour coordinates of (0.539, 0.459), colour temperature of 2088 K and maximum external quantum efficiency of 17.5%. Good performance in OLEDs can be partly explained by efficient orange phosphorescence with quantum yield of 60% at room temperature. This paper also deals with the development of white hybrid solution-processable organic light-emitting devices, exploiting ultralow concentration of newly synthesized iridium (III) complex as phosphorescent dopant in combination with blue fluorescencent emitter(host), and green emitter exhibiting thermally activated delayed fluorescence emitter. It was designed and fabricated by spin coating white hybrid devices, and was characterized by high quality (human-eyes-friendly) electroluminescence achieving CIE1931 coordinates of (0.335, 0.392), colour temperature of 2910 K and colour rendering index of 72 as well as high maximum external quantum efficiency of 8.7%.

Rational Synthesis of Tetrahydrodibenzophenanthridine and Phenanthroimidazole as Efficient Blue Emitters and their Applications

Ten luminophores based on tetrahydrodibenzophenanthridine (THDP) and phenanthroimidazole (PI) wer... more Ten luminophores based on tetrahydrodibenzophenanthridine (THDP) and phenanthroimidazole (PI) were designed, synthesized and characterized for their thermal, electrochemical, electro-optical, charge-transporting characteristics and electroluminescent properties. The blue luminophores exhibited high photoluminescence quantum yields of 66-93 % in toluene solutions and of 5-59 % in solid films. The highest values were observed for the derivative of THDP and PI containing methoxy group. The compounds showed close values of ionization potentials (5.74-6.11 eV) and electron affinities (2.71-3.06 eV). The selected compounds were tested in electrolumi-[a]

Synthesis, photophysical and acidochromic properties of a series of tetrahydrodibenzo[a,i]phenanthridine chromophores

A series of luminescent 5-aryl-tetrahydrodibenzo[a,i]phenanthridines was developed through modifi... more A series of luminescent 5-aryl-tetrahydrodibenzo[a,i]phenanthridines was developed through modified synthetic protocol using SnCl 2-acetic acid as a catalyst. The synthesis was consistent with various functionalities and significantly improves the yields. These derivatives exhibit large molar absorption coefficients and higher Stokes shift values in the range 9739e2628 cm À1. The donor and acceptor groups significantly induced the charge transfer character leading to widespread emission properties in the range of 390e531 nm with moderate quantum yields. The protonation and quaternization studies of these derivatives revealed the formation of a new red shifted absorption band followed by hyperchromic effect in emission, which could be used as an acidochromic sensor.

One-Pot Synthesis and Photophysical Studies of Styryl-Based Benzo[f]pyrazolo[3,4-b]quinoline and Indeno[2,1-b]pyrazolo-[4,3-e]pyridines

A new family of extended π-conjugated dihydrobenzopyrazoloquinoline (DBPQ) and dihydroindenopyraz... more A new family of extended π-conjugated dihydrobenzopyrazoloquinoline (DBPQ) and dihydroindenopyrazolopyridine (DIPP) molecules were synthesized, and their photophysical and electrochemical properties were studied. The DBPQ derivatives showed significantly higher molar absorption coefficients, and their absorption and emission wavelengths were red-shifted with respect to those of the corresponding DIPP derivatives in chloroform solution. Dyes with no substitution and methoxy, nitro, and fluoro-substituted derivatives showed poor dependence on solvent polarity. At the same time, DBPQ and DIPP derivatives with dimethylamino groups exhibited strong solvatochromism in the emission spectra. Due to the presence of a vinyl link with a cylindrical π-cloud on the

Synthesis and Optical Properties of a Series of Green-Light-Emitting 2-(4-Phenylquinolin-2-yl)phenol-BF 2 Complexes (Boroquinols

A series of 2-(4-phenylquinolin-2-yl)phenol derivatives were efficiently synthesized and complexe... more A series of 2-(4-phenylquinolin-2-yl)phenol derivatives were efficiently synthesized and complexed with boron trifluoride to give fluorescent organoboron complexes. All the synthesized compounds were characterized by 1 H, 13 C, 11 B, 19 F NMR, FTIR, and HRMS analyses, and single-crystal structures of representative compounds were determined to examine the conformations of these compounds. The electronic properties of 2-(4-phenylquinolin-2-yl)phenol-BF 2 com

Photophysical studies of donor, acceptor substituted tetrahydrodibenzo[a,i]phenanthridines

Highly luminescent, 5ÀarylÀ7,8,13,14-tetrahydrodibenzo[a,i]phenanthridines (THDPs) in Donor(D)À p... more Highly luminescent, 5ÀarylÀ7,8,13,14-tetrahydrodibenzo[a,i]phenanthridines (THDPs) in Donor(D)À pÀAcceptor(A), AÀpÀA, AÀpÀAÀpÀA 1 configurations were synthesized in good to moderate yields by simple, two step reactions. All the phenyl rings were found to be in near-coplanar configuration due to the enforced planarity by the ethylene bridge. As a result, efficient p-conjugation was maintained throught the molecule. UVevisible absorption and fluorescence spectra of THDP became shifted to longer wavelengths upon substitution of different moieties at 5-Aryl position. A maximum of 200 nm redshifted fluorescence with respect to parent compound was observed for the nitro substituent. However, tradeoffs in spectral shifts were noticed when two strongly electron withdrawing groups such as nitro and cyano groups were present simultaneously. Presence of one or more acceptors induced the intramolecular electron transfer processes, and the photophysical properties of THDP core was tuned by different substituents. Application of this fluorophore has also been demonstrated by means of DNA interaction studies.

CuOeCuAl 2 O 4 and D-glucose catalyzed synthesis of a family of excited state intramolecular proton transfer imidazo[1,2-a]pyridine analogues and their optical properties

A simple and efficient route to luminescent imidazo[1,2-a]pyridines from 2-aminopyridines, phenyl... more A simple and efficient route to luminescent imidazo[1,2-a]pyridines from 2-aminopyridines, phenylacetylene and arylaldehydes was achieved by an one-pot, tandem process starting with an A3-coupling followed by 5-exo-dig cyclization. Copper(II)oxide/copper aluminate composite (5 wt% CuOeCuAl 2 O 4 composite) and D-glucose catalyzed optimal conditions for the reaction were established after examining various reaction parameters such as solvent, catalyst and temperature. The synthesis was much compatible with various functionalities. 2-Aryl-imidazo[1,2-a]pyridines not capable of excited state intramolecular proton transfer emission displayed moderate fluorescence quantum yields in the bluegreen region due to intramolecular charge transfer process. In addition, they exhibited peculiar solvatochromic behaviour in chlorinated solvents like chloroform and dichloromethane. Products containing 2-(2 0-hydroxyphenyl) substituent underwent excited state intramolecular proton transfer in non polar and polar-aprotic solvents. Although such type of emission in solvents was very weak, the Stokes shifts values were high (14,638e11,448 cm À1) with strong solid-state emission in green to yellow region.

SnCl 2-catalyzed synthesis of dihydro-5Hbenzo[f]pyrazolo[3,4-b]quinoline and dihydroindeno[2,1-b]... more

Dyes and Pigments

First of its kind inkjet-printed phosphorescent Iridium(III) complex [Ir(TPQ) 2 (4-EO 2-pic)] pap... more

Luminescent tetrahydrodibenzo[a,i]phenanthridin-5-yl)phenol-boron complexes (borophenanthridines

A series of weakly emissive, exited state intramolecular proton transfer active tetrahydrodibenzo... more A series of weakly emissive, exited state intramolecular proton transfer active tetrahydrodibenzo[a,i] phenanthridin-5-yl)phenols were efficiently synthesized using 2-tetralone and substituted salicylaldehydes. These dyes were smoothly coordinated with boron trifluoride diethyletherate resulting in pconjugated and rigid tetracoordinated boron-phenanthridine complexes. The complexes were highly emissive in solution and solid states with large Stokes shift values ranging from 6190 to 10410 cm À1 and moderate fluorescence quantum yields. A systematic photophysical and solvatochromism studies revealed that the intramolecular charge transfer process was more effective in borophenanthridines than with others, which was supported by DFT study. Further, the electrochemical studies testified the high electron-accepting ability of these boron complexes.

An insight into the photophysical properties of amide hydrogen bonded N-(benzo[d]thiazol-2-yl) acetamide crystals

Three distinct, hydrogen bond associated N-(benzo[d]thiazol-2-yl) acetamides were synthesized by ... more Three distinct, hydrogen bond associated N-(benzo[d]thiazol-2-yl) acetamides were synthesized by refluxing benzothiazoles with acetic acid. The nature of the assemblies was characteristic to the substituent in the benzothiazole moiety. In N-(benzo[d]thiazol-2-yl)acetamide, water acts as a bridge for forming three hydrogen bonds, as an acceptor to amide N\ \H, and donors to carbonyl of amide and thiazole nitrogen assembles of three different N-(benzo[d]thiazol-2-yl)acetamide molecules. The N-(6-methylbenzo[d]thiazol-2-yl)acetamide formed a (amide) N-H…N (thiazole) bonded R 2 2 (8) molecular dimers by two homo-intermolecular hydrogen bonding interactions. N-(6-methoxybenzo[d]thiazol-2-yl)acetamide formed (amide)N-H…O (acid) & (acid)O-H…N (thiazole) interactions with the acetic acid, forming a R 2 2 (8) hydrogen-bonded ring by two hetero-intermolecular hydrogen bonding interactions.

Electroluminescence of iridium(III) complexes containing F or CF 3 substituents

Electron withdrawing groups F or CF 3 were attached to 6-methyl-4-diphenylquinoline (medpq) ligan... more Electron withdrawing groups F or CF 3 were attached to 6-methyl-4-diphenylquinoline (medpq) ligand. The effect of this substitution on photophysical and electroluminescent properties of four phosphorescent iridium(III) complexes containing the two upgraded medpq ligands and 2-(2-pyridyl) benzimidazole or 2-(1H-tetrazol-5-yl) pyridine ligand was comparatively investigated. All complexes emitted strong phosphorescence with absolute quantum efficiency of deoxygenated toluene solutions higher than 50% and the lifetime of 1.068-1.88 microseconds observed for neat films at 77 K. The orange phosphorescence of CF 3-substituted complexes was slight shifted to higher energy region in comparison to that of the corresponding F-containing complexes. The ionization potentials detected by photoelectron emission spectroscopy for solid-state samples of the complexes were practically the same (5.91-6.04 eV). Exploiting the same device structure, phosphorescent organic light-emitting diodes (PhOLEDs) containing a CF 3-substituted complex exhibited maximum external quantum efficiency of 10.33% in contrast to 5.36% observed for PhOLEDs based on a F-containing complex.

Enantioselective fluorescent sensing of chiral carboxylic acid by engaging boronic acid and BINOL

Two BINOL based boronate esters, namely (S)-2 and (S)-3, were designed and synthesized. Fluoresce... more Two BINOL based boronate esters, namely (S)-2 and (S)-3, were designed and synthesized. Fluorescent recognition studies of (S)-2 towards enantiomers of mandelic acid revealed that sensor (S)-2 exhibited high fluorescent enantioselective response towards mandelic acid. The other boronate ester (S)-3 which did not have hydroxyl group in the minor groove of BINOL did not show any fluorescent chiral discrimination. Their enantioselective recognition was also studied by means of 1 H NMR spectroscopy. The results indicated that the sensor (S)-2 was very promising as a chiral fluorescent sensor for the recognition of chiral carboxylic acid in THF solvent. Theoretically predicted hydrogen bonding interactions revealed a favorable chiral recognition between the receptor and l-mandelic acid, which is well correlated with experimentally observed enantioselective recognition phenomenon.

Highly emissive, naked-eye solvatochromic probe based on styryl tetrahydrodibenzo[a,i] phenanthridine for acidochromic applications

A new series of 5-styryl tetrahydrodibenzo[a,i]phenanthridines was readily synthesized from b-tet... more A new series of 5-styryl tetrahydrodibenzo[a,i]phenanthridines was readily synthesized from b-tetralone, ammonium acetate and cinnamaldehydes. These dyes emitted light with large Stokes shift values of 3894-7265 cm À1 , depending on the nature of the substituent. The dyes displayed positive solvatochromism in their emission spectra, suggestive of a more polar excited state. Dye 3c exhibited aggregation-induced emission (AIE) property in 10% THF-water mixture besides a ratiometric response to pH from 10.75 to 2.16 with blue shift of 84 nm in emission. The emission ratio (I 526 : I 442) further displayed a good linearity with pH in the range of 4.1 to 6.8 and the corresponding pK a value was found to be 4.55 AE 0.03. The addition of TFA led to a prominent bathochromic shift in spectra along with clear colour changes owing to protonation at the phenanthridine segment. Further, a second protonation of the phenanthrinium segment could take place based on the pK a strength of the acid added. The pH dependent spectral changes were successfully applied to quantitatively detect pH in biological fluids and acid impurities in solvents.

SnCl 2-catalyzed synthesis of dihydro-5Hbenzo[f]pyrazolo[3,4-b]quinoline and dihydroindeno[2,1-b]... more

Tetrazole iridium(III) complexes as a class of phosphorescent emitters for high-efficiency OLEDs

We show that iridium complexes incorporating a tetrazolate ligand can be used as new highly phosp... more We show that iridium complexes incorporating a tetrazolate ligand can be used as new highly phosphorescent emitters in the fabrication of high-efficiency organic light emitting devices (OLEDs). The nature of the excited states of these Ir(III) complexes was probed by means of electrochemical analysis, absorption and photoluminescence spectroscopies and with the aid of TD-DFT calculations. To demonstrate the high stability of these classes of complexes we used TGA and DSC characterizations. These complexes are used as dopant emitters in the emissive layers. The as-fabricated OLEDs display high-efficiency and high-brightness with blue, green and yellow/orange emission.

Journal of Molecular Catalysis A: Chemical

Nano-ZnAl 2 O 4 was synthesized by blending metal-citrate complex-acrylamide polymer using the pr... more Nano-ZnAl 2 O 4 was synthesized by blending metal-citrate complex-acrylamide polymer using the precursor method. Powder X-ray diffraction analysis of the catalyst affirmed that the spinel oxide was phase pure. Elemental analysis was confirmed by Energy Dispersive X-ray analysis. Transmission Electron Microscopic (TEM) analysis and Scherrer's formula revealed that the particles were found to be in the order of 20 nm in size. Thermal stability of the catalyst was analyzed by thermogravimetric and differential thermal analysis (TG-DTA). Total acidity of the nano-ZnAl 2 O 4 evaluated by NH 3-TPD was 8.8 mmol NH 3 /g. Diversity in the synthesis of xanthene derivatives was investigated using nanozinc aluminate as a catalyst. Nanoform of the catalyst showed better activity than the bulk form due to its large surface area 50 m 2 /g. The 2,2-arylmethylene bis (3-hydroxy-2-cyclohexene-1-one), 2,2-arylmethylene bis (3hydroxy-5,5-dimethyl-2-cyclohexene-1-one) and 1-Oxo-hexahydroxanthenes were synthesized rapidly using ZnAl 2 O 4 and the catalyst was successfully recycled without any loss of activity.

Suppression of external quantum efficiency rolloff in organic light emitting diodes by scavenging triplet excitons

Large external quantum efficiency rolloff at high current densities in organic light-emitting dio... more Large external quantum efficiency rolloff at high current densities in organic light-emitting diodes (OLEDs) is frequently caused by the quenching of radiative singlet excitons by longlived triplet excitons [singlet-triplet annihilation (STA)]. In this study, we adopted a triplet scavenging strategy to overcome the aforementioned STA issue. To construct a model system for the triplet scavenging, we selected 2,6-dicyano-1,1-diphenyl-λ 5 σ 4-phosphinine (DCNP) as the emitter and 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) as the host material by considering their singlet and triplet energy levels. In this system, the DCNP's triplets are effectively scavenged by BSBCz while the DCNP's singlets are intact, resulting in the suppressed STA under electrical excitation. Therefore, OLEDs with a 1 wt.%-DCNP-doped BSBCz emitting layer demonstrated the greatly suppressed efficiency rolloff even at higher current densities. This finding favourably provides the advanced light-emitting performance for OLEDs and organic semiconductor laser diodes from the aspect of the suppressed efficiency rolloff.

Direct anti and regio-specific aldol reactions of cyclododecanone catalyzed by alkali metal hydroxides: implications for supramolecular helical designw

The direct aldol condensation of benzaldehyde to cyclododecanone (CDD), catalyzed by NaOH is expe... more The direct aldol condensation of benzaldehyde to cyclododecanone (CDD), catalyzed by NaOH is expected to yield bis-benzylidene cyclododecanone, but we interestingly ended up with novel b-hydroxy carbonyl compounds and monobenzylidene cyclododecanone derivatives in the cases of 2-halo substituted benzaldehydes. This exceptional behaviour is due to the capability of the CDD ring to exist in the zwitter ionic form. The formation of monobenzylidine derivatives can be due to less stable hydrogen bonds between-OH and-CQO groups, electrostatic interaction between aldehyde substituents and metal enolates and the bulkiness of the benzaldehyde ring substituents. We extensively optimized the reaction conditions using different solvents and alkali metal hydroxide catalysts and we are getting high yield, regio-specificity and high diastereoselectivity (anti-aldol) in b-hydroxy carbonyl compounds. The representative crystal structures were examined and they suggest the tendency of anti-aldol compounds to form supramolecular helical motifs. The b-hydroxyl carbonyl derivatives form supramolecular helices via O-HÁ Á ÁO hydrogen bonds with two anti-aldol units per turn and a pitch of B6 Å. The cyclododecanone ring adopts the minimum energy [3333] square conformation. The nucleophile will attack only from the less hindered side of the carbonyl group in the CDD ring.

Tetrahydrodibenzo[a,i]phenanthridin-5-yl)phenol as a Fluorescent Probe for the Detection of Aniline

Two (tetrahydrodibenzo[a,i]phenanthridin-5yl)phenols that differ in their substituents at the par... more Two (tetrahydrodibenzo[a,i]phenanthridin-5yl)phenols that differ in their substituents at the para position (P1, R = H and P2, R = NEt 2) were designed and synthesized. The presence of a −NEt 2 group in probe P2 facilitates the twisted intramolecular charge transfer (TICT) process, making P2 emissive, which distinctly coordinated with boron trifluoride in the presence of different amines with different electronic properties. A substantial increase in emission intensity with increasing viscosity of the surrounding environment and smooth formation of a planar complex with boron and Zn 2+ ions concluded the presence of a TICT process. The selective reactivity of P2 toward a tetracoordinated boron complex has been explored as a potential tool for colorimetric and fluorescent discrimination of aromatic primary amines, i.e., anilines. Selective detection of aniline with probe P2 can be viewed through the naked eye, and the corresponding fluorescence turn-on detection limit was found to be 12.65 nM. In addition, the detection of aniline on precoated aluminum-backed thin-layer chromatography plates and Whatman filter paper strips was found to be in good agreement with the color change of P2 in solution and in vapor phase.

Human-eyes-friendly white electroluminescence from solution-processable hybrid OLEDs exploiting new iridium (III) complex containing benzoimidazophenanthridine ligand

This paper primarily deals with the development of highly efficient phosphorescent iridium (III) ... more This paper primarily deals with the development of highly efficient phosphorescent iridium (III) complex based on benzoimidazophenanthridine as main ligand. The potential of this complex, as phosphorescent emitter for optoelectronic applications, was screened by different methods, including luminescence spectrometry, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry as well as photoelectron emission spectrometry. The possible application of the developed complex in single-colour vacuum-deposited organic light emitting diodes (OLEDs) exhibiting warm orange electrophosphorescence was demonstrated. The device was characterized with CIE1931 colour coordinates of (0.539, 0.459), colour temperature of 2088 K and maximum external quantum efficiency of 17.5%. Good performance in OLEDs can be partly explained by efficient orange phosphorescence with quantum yield of 60% at room temperature. This paper also deals with the development of white hybrid solution-processable organic light-emitting devices, exploiting ultralow concentration of newly synthesized iridium (III) complex as phosphorescent dopant in combination with blue fluorescencent emitter(host), and green emitter exhibiting thermally activated delayed fluorescence emitter. It was designed and fabricated by spin coating white hybrid devices, and was characterized by high quality (human-eyes-friendly) electroluminescence achieving CIE1931 coordinates of (0.335, 0.392), colour temperature of 2910 K and colour rendering index of 72 as well as high maximum external quantum efficiency of 8.7%.

Rational Synthesis of Tetrahydrodibenzophenanthridine and Phenanthroimidazole as Efficient Blue Emitters and their Applications

Ten luminophores based on tetrahydrodibenzophenanthridine (THDP) and phenanthroimidazole (PI) wer... more Ten luminophores based on tetrahydrodibenzophenanthridine (THDP) and phenanthroimidazole (PI) were designed, synthesized and characterized for their thermal, electrochemical, electro-optical, charge-transporting characteristics and electroluminescent properties. The blue luminophores exhibited high photoluminescence quantum yields of 66-93 % in toluene solutions and of 5-59 % in solid films. The highest values were observed for the derivative of THDP and PI containing methoxy group. The compounds showed close values of ionization potentials (5.74-6.11 eV) and electron affinities (2.71-3.06 eV). The selected compounds were tested in electrolumi-[a]

Synthesis, photophysical and acidochromic properties of a series of tetrahydrodibenzo[a,i]phenanthridine chromophores

A series of luminescent 5-aryl-tetrahydrodibenzo[a,i]phenanthridines was developed through modifi... more A series of luminescent 5-aryl-tetrahydrodibenzo[a,i]phenanthridines was developed through modified synthetic protocol using SnCl 2-acetic acid as a catalyst. The synthesis was consistent with various functionalities and significantly improves the yields. These derivatives exhibit large molar absorption coefficients and higher Stokes shift values in the range 9739e2628 cm À1. The donor and acceptor groups significantly induced the charge transfer character leading to widespread emission properties in the range of 390e531 nm with moderate quantum yields. The protonation and quaternization studies of these derivatives revealed the formation of a new red shifted absorption band followed by hyperchromic effect in emission, which could be used as an acidochromic sensor.

One-Pot Synthesis and Photophysical Studies of Styryl-Based Benzo[f]pyrazolo[3,4-b]quinoline and Indeno[2,1-b]pyrazolo-[4,3-e]pyridines

A new family of extended π-conjugated dihydrobenzopyrazoloquinoline (DBPQ) and dihydroindenopyraz... more A new family of extended π-conjugated dihydrobenzopyrazoloquinoline (DBPQ) and dihydroindenopyrazolopyridine (DIPP) molecules were synthesized, and their photophysical and electrochemical properties were studied. The DBPQ derivatives showed significantly higher molar absorption coefficients, and their absorption and emission wavelengths were red-shifted with respect to those of the corresponding DIPP derivatives in chloroform solution. Dyes with no substitution and methoxy, nitro, and fluoro-substituted derivatives showed poor dependence on solvent polarity. At the same time, DBPQ and DIPP derivatives with dimethylamino groups exhibited strong solvatochromism in the emission spectra. Due to the presence of a vinyl link with a cylindrical π-cloud on the

Synthesis and Optical Properties of a Series of Green-Light-Emitting 2-(4-Phenylquinolin-2-yl)phenol-BF 2 Complexes (Boroquinols

A series of 2-(4-phenylquinolin-2-yl)phenol derivatives were efficiently synthesized and complexe... more A series of 2-(4-phenylquinolin-2-yl)phenol derivatives were efficiently synthesized and complexed with boron trifluoride to give fluorescent organoboron complexes. All the synthesized compounds were characterized by 1 H, 13 C, 11 B, 19 F NMR, FTIR, and HRMS analyses, and single-crystal structures of representative compounds were determined to examine the conformations of these compounds. The electronic properties of 2-(4-phenylquinolin-2-yl)phenol-BF 2 com

Photophysical studies of donor, acceptor substituted tetrahydrodibenzo[a,i]phenanthridines

Highly luminescent, 5ÀarylÀ7,8,13,14-tetrahydrodibenzo[a,i]phenanthridines (THDPs) in Donor(D)À p... more Highly luminescent, 5ÀarylÀ7,8,13,14-tetrahydrodibenzo[a,i]phenanthridines (THDPs) in Donor(D)À pÀAcceptor(A), AÀpÀA, AÀpÀAÀpÀA 1 configurations were synthesized in good to moderate yields by simple, two step reactions. All the phenyl rings were found to be in near-coplanar configuration due to the enforced planarity by the ethylene bridge. As a result, efficient p-conjugation was maintained throught the molecule. UVevisible absorption and fluorescence spectra of THDP became shifted to longer wavelengths upon substitution of different moieties at 5-Aryl position. A maximum of 200 nm redshifted fluorescence with respect to parent compound was observed for the nitro substituent. However, tradeoffs in spectral shifts were noticed when two strongly electron withdrawing groups such as nitro and cyano groups were present simultaneously. Presence of one or more acceptors induced the intramolecular electron transfer processes, and the photophysical properties of THDP core was tuned by different substituents. Application of this fluorophore has also been demonstrated by means of DNA interaction studies.

CuOeCuAl 2 O 4 and D-glucose catalyzed synthesis of a family of excited state intramolecular proton transfer imidazo[1,2-a]pyridine analogues and their optical properties

A simple and efficient route to luminescent imidazo[1,2-a]pyridines from 2-aminopyridines, phenyl... more A simple and efficient route to luminescent imidazo[1,2-a]pyridines from 2-aminopyridines, phenylacetylene and arylaldehydes was achieved by an one-pot, tandem process starting with an A3-coupling followed by 5-exo-dig cyclization. Copper(II)oxide/copper aluminate composite (5 wt% CuOeCuAl 2 O 4 composite) and D-glucose catalyzed optimal conditions for the reaction were established after examining various reaction parameters such as solvent, catalyst and temperature. The synthesis was much compatible with various functionalities. 2-Aryl-imidazo[1,2-a]pyridines not capable of excited state intramolecular proton transfer emission displayed moderate fluorescence quantum yields in the bluegreen region due to intramolecular charge transfer process. In addition, they exhibited peculiar solvatochromic behaviour in chlorinated solvents like chloroform and dichloromethane. Products containing 2-(2 0-hydroxyphenyl) substituent underwent excited state intramolecular proton transfer in non polar and polar-aprotic solvents. Although such type of emission in solvents was very weak, the Stokes shifts values were high (14,638e11,448 cm À1) with strong solid-state emission in green to yellow region.

SnCl 2-catalyzed synthesis of dihydro-5Hbenzo[f]pyrazolo[3,4-b]quinoline and dihydroindeno[2,1-b]... more

Dyes and Pigments

First of its kind inkjet-printed phosphorescent Iridium(III) complex [Ir(TPQ) 2 (4-EO 2-pic)] pap... more

Luminescent tetrahydrodibenzo[a,i]phenanthridin-5-yl)phenol-boron complexes (borophenanthridines

A series of weakly emissive, exited state intramolecular proton transfer active tetrahydrodibenzo... more A series of weakly emissive, exited state intramolecular proton transfer active tetrahydrodibenzo[a,i] phenanthridin-5-yl)phenols were efficiently synthesized using 2-tetralone and substituted salicylaldehydes. These dyes were smoothly coordinated with boron trifluoride diethyletherate resulting in pconjugated and rigid tetracoordinated boron-phenanthridine complexes. The complexes were highly emissive in solution and solid states with large Stokes shift values ranging from 6190 to 10410 cm À1 and moderate fluorescence quantum yields. A systematic photophysical and solvatochromism studies revealed that the intramolecular charge transfer process was more effective in borophenanthridines than with others, which was supported by DFT study. Further, the electrochemical studies testified the high electron-accepting ability of these boron complexes.