An example of an aggregation‐induced emission enhancement (AIEE)‐active new naphthalene appended Schiff base (H2L) as an Al3+ ion selective fluorescent sensor has been successfully designed and synthesized. H2L showed turn‐on fluorescence at high concentration in DMF only, and at low concentration in DMF‐H2O mixtures having high water content (more than 20%). But at very low concentration of H2L and in DMF/H2O solutions with low water fraction (fw ≤ 20%), the fluorescence was turned ON in presence of Al3+ ions, which helps to detect Al3+  ions as low as 4.9×10−7 M with high selectivity in an DMF solution without any interference of other competitive metal ions. The fluorescence of H2L in different conditions and in solid state due to AIE effect has been established with the help of detailed spectroscopic studies, dynamic light scattering (DLS), scanning electron microscope (SEM), life time using time‐resolved photoluminescence and optical fluorescence microscope. The experimentally observed H‐type aggregation nature of the probe H2L has also been supported by the theoretical (DFT studies and TDDFT) studies. This probe (H2L) has also been employed for on‐site Al3+ ions detection in solid state.

A newly designed multifunctional AIE‐coupled excited‐state intramolecular proton transfer (ESIPT)‐active asymmetric multichromicfluorophor (MCF) has been synthesized and characterized. From tedious experimental spectroscopic dissection of the photophysical properties of MCF and two supporting compounds, (3‐benzotriazol‐2‐yl‐2‐hydroxy‐5‐methyl‐benzaldehyde) (R1) and (2‐benzotriazol‐2‐yl‐4‐methyl‐phenol) (R2)with theoretical discussion, it has been established that ESIPT was driven exclusively towards triazole N centre involving a 6‐member H‐bonded network over imine‐N center in both solid state and solution phase. The competition of aggregation enhanced emissive feature and ACQ (aggregation caused quenching) nature of MCF were also studied thoroughly in the THF/water mixture. New classes of mechanisms are introduced regarding AIE/ACQ phenomenon, which will eradicate the limitation of designing and development of novel solid state emitters. To the best of our knowledge, this unique feature has never been achieved before. Furthermore, reversible mechanochromic fluorescence behavior upon external grinding and acidochromism nature of MCF as acid‐base switch has also been observed. In addition, MCF selectively senses Zn2+ ion through a colorimetric and fluorescence “turn‐on” route. The actual binding site of Zn2+ with MCF has also been established with the help of spectroscopic studies. The present study will enlighten for a new way to achieve new multifunctional organic luminescent solids.

A coumarin-based fluorescent compound, bilirubin fluorescent probe N-oxide (BFPNox), was successfully designed and synthesized for highly selective and sensitive detection of free bilirubin with short response time. The fluorescence "turn-on" response of the probe is based on the in situ generated Fe 2+mediated deoxygenation reaction of N-oxide from the diethylarylamine group of the probe, where the group attached to the coumarin π-conjugated system is responsible for the fluorescence quenching state of the probe, BFPNox. Here, the reaction of the added Fe 3+ ions with bilirubin produces Fe 2+ ions in situ in aqueous buffer. Fluorescence enhancement of BFPNox was achieved by more than 12-fold when a double equivalent of bilirubin solution was added in reaction buffer at pH 7.2 (50 mM HEPES, 5% DMSO) at 25 °C under excitation at 400 nm. It detected free bilirubin as low as 76 nM in an aqueous system without any interference of metal ions, anions, and other important biomolecules with a linear concentration range of 0−10 μM (R 2 = 0.991). The probe was also employed in the estimation of free bilirubin in human serum specimens to verify the efficacy of this probe. With these, it is revealed that this probe is a good candidate to be used as a powerful diagnostic tool for the assessment of free bilirubin with significant accuracy and reliability.

The excited-state intramolecular proton transfer (ESIPT)-assisted aggregation induced emission enhancement (AIEE) in an organic moiety (4-[(2,4-dihydroxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (HL)) has been established based on detailed experimental and theoretical studies by synthesizing and characterizing two imine-based compounds, 4-[(2,4-dihydroxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (HL) and its supporting analog 4-[(2-methoxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (L′). HL is nonemissive in the solution state, whereas in the solid state, it shows a bluish-green emission (λex: 360 nm) at room temperature, and on heating, it becomes nonemissive at ∼90 °C with a quick thermo “off–on” reversible phenomenon for a long period. The characterization studies including the single-crystal X-ray diffraction study explained the emissive feature of HL due to close packing and aggregation through multiple intermolecular H-bonding, C–H···π, π–π interactions, and nonemissive characteristics feature of HL in the twisted-keto form at elevated temperature. The theoretical support of ESIPT and water-mediated ESPT of HL is obtained by computing structural and energy parameters of enol, keto, and transition states in ground and excited states using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. To detect the molecular packing in the crystalline phase, all of the inter- and intramolecular noncovalent interactions (NCIs) were computed using the noncovalent interaction-reduced density gradient (NCI-RDG) method along with Bader’s quantum theory of atoms-in-molecules (QTAIM). Additionally, in both solid and aqueous phases, HL was found to perform as a dual-channel sensing probe for Al3+ and Zn2+ ions by tuning the λex and the pH of the aqueous medium. The present study will open a new way to explore novel organic luminescent solids as an off–on switch using external thermal stimuli.

A new benzorhodol-based fluorogenic probe with a ‘turn on’ mechanism, having a biofriendly excitation wavelength (580 nm), was used to analyze biologically toxic free bilirubin in aqueous buffer medium.