Synthesis of Strongly Fluorescent Imidazole Derivatives: Structure Property Studies, Halochromism and Fluorescent Photoswitching.

New series of methoxy and hydroxyl group substituted triphenylamine (TPA)-imidazole fluorescent molecules (5-(diphenylamino)-2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol (1), 5-(diphenylamino)-2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenol (2), 5-(diphenylamino)-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (3), 5-(diphenylamino)-2-(1,4,5-triphenyl-1H-imidazol-2-yl)phenol (4), N-(3-methoxy-4-(1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-N-phenylbenzenamine (5), N-(3-methoxy-4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-N-phenylbenzene amine (6), and N-(3-methoxy-4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl)-N-phenylbenzenamine (7)) have been synthesized that exhibited strong solution fluorescence and molecular structure and conformation controlled fluorescence photoswitching, solid state fluorescence and halochromism. Hydroxyl substituted molecules (1-4) showed moderate to strong fluorescence in solution depend on solvent polarity and very weak solid state fluorescence. Methoxy substituted molecules (5-7) displayed strong fluorescence both in solution and solid state. Solid state structural studies revealed strong intramolecular H-bonding in the crystal lattice. Interestingly, highly twisted structure (6) showed rare light induced reversible fluorescence switching in CHCl3. The observation of isobestic point in time dependent fluorescence photoswitching studies indicated structural isomer conversion. Further, acid sensitive imidazole nitrogen has been made use to demonstrate solid state fluorescence switching via halochromism. Thus the present studies attempted to develop new fluorescent molecules and establish structure-property relationship for designing fluorescence switching materials. Graphical Abstract Molecular structure controlled solid state fluorescence, halochromism and a rare fluorescence photoswitching in chloroform have been observed with triphenylamine-imidazole derivatives.

Unusual fluorescent photoswitching of imidazole derivatives: the role of molecular conformation and twist angle controlled organic solid state fluorescence.

Molecular photoswitching, light induced reversible color/fluorescence modulation, has mostly been realized in organic molecules via E/Z isomerization of azobenzenes and stilbenes and ring opening/closing reactions of spiropyrans and diarylethenes. We report here new fluorescent molecular photoswitches based on triphenylamine (TPA)-imidazole derivatives, N-phenyl-N-(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)benzenamine (NTPB) and N-phenyl-N-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)benzenamine (NPPB), that exhibited light induced reversible fluorescence switching via conformational change from a twisted molecular structure to more planar. NTPB and NPPB in CHCl3 showed red shift of absorption and fluorescence upon UV light irradiation whereas white light exposure reversed both absorption as well as fluorescence. The role of the TPA-imidazole twisted molecular structure in photoswitching was established based on structure property, computational and photophysical studies. The isobestic point observed in time dependent fluorescence change under UV light irradiation clearly demonstrated the presence of two different conformational isomers. Interestingly, polymorphism and torsion angle (τ) dependent fluorescence of NTPB and NPPB in the solid state also supported the role of the twisted molecular structure of TPA-imidazole in fluorescence switching/tuning. Interestingly, NTPB showed fluorescence photoswitching in the solid state also whereas rigid phenanthrene based NPPB did not show fluorescence photoswitching. Thus the present studies provide structural insight for designing a new type of fluorescent organic molecular photoswitches based on conformational modulation that could be of potential interest in optoelectronic devices.

Bay Functionalized Perylenediimide with Pyridine Positional Isomers: NIR Absorption and Selective Colorimetric/Fluorescent Sensing of Fe3+ and Al3+ Ions.

Bay functionalized perylene diimide substituted with pyridine isomers, (2-pyridine (2HMP-PDI), 3-pyridine (3-HMP-PDI) and 4-pyridine (4-HMP-PDI)) have been synthesized and explored for selective coloro/fluorimetric sensing of heavy transition metal ions. HMP-PDIs showed strong NIR absorption (760-765 nm) in DMF. The absorption and fluorescence of HMP-PDIs have been tuned by make use of pyridine isomers. Reddish-orange color was observed for 2-HMP-PDI (λmax = 437, 551, 765 nm) whereas 4-HMP-PDI exhibited light green (λmax = 432, 522, 765 nm). 3-HMP-PDI showed orange-yellow (λmax = 431, 524, 762 nm). The fluorescence spectra of 2-, 3- and 4-HMP-PDI showed λmax at 585, 538, 546 nm, respectively. Interestingly, HMP-PDI dyes showed selective color change (intense pink color) and fluorescence quenching for Fe3+ and Al3+ metal ions in DMF. Absorbance spectra revealed complete disappearance of NIR absorption and intensification/appearance of new peak at lower wavelength. The concentration dependent studies suggest that 4-HMP-PDI can detect up to 36.52 ppb of Fe3+ and 43.12 ppb of Al3+ colorimetrically. The interference studies in presence of other metal ions confirmed the good selectivity for Fe3+ and Al3+. The mechanistic studies indicate that Lewis acidic character of Fe3+ and Al3+ ions were responsible for selective color change and fluorescence quenching.

Temperature-Controlled Locally Excited and Twisted Intramolecular Charge-Transfer State-Dependent Fluorescence Switching in Triphenylamine-Benzothiazole Derivatives.

Triphenylamine-benzothiazole derivatives, N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylbenzenamine (1), N-(4-(benzo[d]thiazol-2-yl)-3-methoxyphenyl)-N-phenylbenzenamine (2), and 2-(benzo[d]thiazol-2-yl)-5-(diphenylamino)phenol (3), showed unusual temperature-controlled locally excited (LE) and twisted intramolecular charge-transfer (TICT) state fluorescence switching in polar solvents. The detailed photophysical studies (absorption, fluorescence, lifetime, and quantum yield) in various solvents confirmed polarity-dependent LE and TICT state formation and fluorescence tuning. 1 and 2 exhibited strong fluorescence with short lifetime in nonpolar solvents compared to polar solvents. 1, 2, and 3 in dimethylformamide (DMF) at room temperature showed low-energy weak TICT state fluorescence, whereas high-energy strong LE state fluorescence was observed at -196 °C. Interestingly, further increasing the temperature from 20 to 100 °C, the DMF solution of 1 and 2 exhibited rare fluorescence enhancement with a slight blue shift of λmax via activating more vibrational bands of the TICT state. Thus, 1 and 2 showed weak TICT state fluorescence at room temperature, strong LE state fluorescence at -196 °C, and activation of TICT state at 100 °C. Moreover, molecular conformation and aggregation in the solid state influenced strongly on the fluorescence properties of 1, 2, and 3. Solid-state fluorescence and pH-responsive imidazole nitrogen have been exploited for demonstrating halochromism-induced fluorescence switching. Computational studies provided further insights into the fluorescence tuning and switching. The present studies provide understanding and opportunity to make use of D-A organic molecules in the LE and TICT states for achieving fluorescence switching and tuning.

Triphenylamine based reactive coloro/fluorimetric chemosensors: Structural isomerism and solvent dependent sensitivity and selectivity.

Triphenyl amine based chemosensors, (2-(((2-(9H-carbazol-9-yl)phenyl)imino)methyl)-5-(diphenylamino)phenol (ortho-CPDP) and 2-(((4-(9H-carbazol-9-yl)phenyl)imino)methyl)-5-(diphenylamino)phenol (para-CPDP), showed solvent and isomerism dependent selective coloro/fluorometric sensing of multiple metal ions (Fe3+, Al3+ and Zn2+) with distinguishable responses. In CH3CN, ortho and para-CPDP selectively produced yellow color upon addition of Al3+ and Fe3+ that was slowly disappeared. The yellow color of ortho and para-CPDP in DMF was decolourised selectively by adding Al3+ and Fe3+. Both ortho and para-CPDP in CH3CN showed nearly similar rate of decolourization for Fe3+ and Al3+. However, the rate of decolourization of ortho and para-CPDP in DMF was different for Fe3+ (10µM, 8min) and Al3+ (5×10-4M, 40min) ions. The limit of detection of para-CPDP for Fe3+ is 10µM and Al3+ 500µM. The mechanistic studies revealed the imine hydrolysis of ortho and para-CPDP in presence of Lewis acidic Fe3+ and Al3+. The reactivity based sensing lead to high selectivity for Al3+ and Fe3+ ions. Further, para-CPDP exhibited selective fluorescence turn-on for Zn2+ in DMF (λmax=513nm) and detection limit of 6.0µM. Thus, reactive chemosensors, ortho and para-CPDP, exhibited selective and distinguishable colorimetric sensing of Fe3+ and Al3+ ions and isomerism and solvent dependent fluorescence sensing of Zn2+.

Synthesis of new colori/fluorimetric chemosensor for selective sensing of biologically important Fe³âº, Cu²âº and Zn²âº metal ions.

New Schiff base chemosensors (1 and 2) based on aryl ether amine were synthesized and demonstrated positional isomer and functional group dependent colori/fluorimetric sensing of Fe(3+), Cu(2+) and Zn(2+) at ppm level. Methoxy salicylaldehyde based chemosensor 1 exhibited selective colorimetric sensing of Fe(3+) whereas 2-hydroxy naphthaldehyde based chemosensor 2 showed selective disappearance of yellow color for Cu(2+) ions. Interestingly, both 1 and 2 exhibited a highly selective strong turn-on fluorescence for Zn(2+). The significance of COOH group in 1 and 2 for Zn(2+) turn-on fluorescence sensing has been confirmed by structure-property studies. Concentration dependent studies of 1 and 2 indicate that Fe(3+), Cu(2+) and Zn(2+) can be detected up to 10 µM. The formation of 1:1 Zn(2+) and chemosensor (1 and 2) confirmed by NMR studies. High selectivity of 1 and 2 was demonstrated by interference studies in presence of different metal ions.