Vandana Bhalla

Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters

Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔEST ) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with ΔEST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.

Highly Selective Fluorescence Turn-on Chemodosimeter Based on Rhodamine for Nanomolar Detection of Copper Ions

A highly selective fluorescent chemodosimeter based on rhodamine is synthesized which undergoes Cu(2+) driven hydrolysis in aqueous media to produce fluorescence turn-on changes with a detection limit up to the nanomolar range.

Self-assembled pentacenequinone derivative for trace detection of picric acid.

Pentacenequinone derivative 3 forms luminescent supramolecular aggregates both in bulk as well as in solution phase. In bulk phase at high temperature, long-range stacking of columns leads to formation of stable and ordered columnar mesophase. Further, derivative 3 works as sensitive chemosensor for picric acid (PA) and gel-coated paper strips detect PA at nanomolar level and provide a simple, portable, and low-cost method for detection of PA in aqueous solution, vapor phase, and in contact mode.

Fluorescent Nanoaggregates of Pentacenequinone Derivative for Selective Sensing of Picric acid in Aqueous Media

Novel pentacenequinone derivative 3 has been synthesized using the Suzuki-Miyaura coupling protocol which forms fluorescent nanoaggregates in aqueous media due to its aggregation-induced emission enhancement attributes and selectively senses picric acid with a detection limit of 500 ppb.

Tuning the Intramolecular Charge Transfer Emission from Deep Blue to Green in Ambipolar Systems Based on Dibenzothiophene S,S-Dioxide by Manipulation of Conjugation and Strength of the Electron Donor Units

The efficient synthesis and photophysical properties of a series of ambipolar donor-acceptor-donor systems is described where the acceptor is dibenzothiophene S,S-dioxide and the donor is fluorene, carbazole, or arylamine. The systems exhibit intramolecular charge transfer (ICT) states (of variable ICT character strengths) leading to fluorescence emission ranging from deep blue to green with moderate to high photoluminescence quantum yields. The emission properties can be effectively tuned by systematically changing the position of substitution on both donor and acceptor units (which affects the extent of conjugation) and the redox potentials of the donor units. The results are supported by cyclic voltammetric data and TD-DFT calculations.

A Molecular Keypad Lock Based on the Thiacalix[4]arene of 1,3-Alternate Conformation

A chemosensor based on the thiacalix[4]arene of 1,3-alternate conformation has been designed and synthesized. The binding behavior of this chemosensor has been studied toward different metal ions by fluorescence spectroscopy, and it was observed that the chemosensor selectively senses Cu(2+) ions. The chemical inputs of Cu(2+) and F(-) in a sequential manner generate an output which mimics the functions of a security keypad lock.

Attogram Detection of Picric Acid by Hexa-peri-Hexabenzocoronene-Based Chemosensors by Controlled Aggregation-Induced Emission Enhancement

Hexa-peri-hexabenzocoronene (HBC) based molecules 5 and 6 have been designed and synthesized. These planar coronenes are appended with rotors to invoke aggregation induced emission enhancement (AIEE) phenomenon by controlling the ratio of H2O in solutions of aggregates. These aggregates of HBC derivatives serve as highly selective chemosensors for picric acid (PA) in mixed aqueous solution. These aggregates are also able to detect PA in vapor phase. In addition, fluorescent test strips have been prepared by dip-coating the Whatman paper with aggregates of both compounds for trace detection of PA in contact mode with detection limits in attograms.

New Fluorogenic Sensors for Hg2+ Ions: Through-Bond Energy Transfer from Pentaquinone to Rhodamine

New pentaquinone derivatives 5 and 8 having rhodamine moieties have been designed and synthesized that undergo through-bond energy transfer (TBET) in the presence of Hg(2+) ions among the various cations (Cu(2+), Pb(2+), Fe(2+), Fe(3+), Zn(2+), Ni(2+), Cd(2+), Co(2+), Ag(+), Ba(2+), Mg(2+), K(+), Na(+), and Li(+)) tested in mixed aqueous media.

Recognition of adenosine monophosphate and H2PO4- using zinc ensemble of new hexaphenylbenzene derivative: potential bioprobe and multichannel keypad system.

Zinc ensemble of hexaphenylbenzene derivative 3 exhibits sensitive response toward adenosine monophosphate (AMP) and H(2)PO(4)(-) ions. Further, the application of derivative 3 as a multichannel molecular keypad could be realized in the presence of inputs of Zn(2+) ions, H(2)PO(4)(-) ions, and AMP.

Hexaphenylbenzene-based fluorescent aggregates for ratiometric detection of cyanide ions at nanomolar level: set-reset memorized sequential logic device.

A hexaphenylbenzene-based receptor 3 has been synthesized that forms a fluorescent spherical aggregate in mixed aqueous media due to its aggregation-induced emission enhancement attributes. These fluorescent spherical aggregates show ratiometric response toward cyanide ions via nucleophilic addition and undergo deaggregation to form smaller nanoaggregates. In addition, the solution-coated paper strips of 3 can detect cyanide ions in the range of ∼2.6 ng/cm(2), thus, providing a simple, portable, and low-cost method for detection of cyanide ions in aqueous media. Receptor 3 also behaves as a set-reset memorized sequential logic circuit with chemical inputs of CN(-) ions and trifluoroacetic acid or H(+) (pH ≤ 3).