Water sensitive fluorescence tuning of V-shaped ESIPT fluorophores: Substituent effect and trace amount water sensing in DMSO

Highly sensitive nature of excited state intramolecular proton transfer (ESIPT) functionality in organic fluorophores made them potential candidates for developing environmental sensors and bioimaging applications. Herein, we report the synthesis of V-shaped Dapsone based Schiff base ESIPT derivatives (1-3) and water sensitive wide fluorescence tuning from blue to red in DMSO. Solid-state structural analysis confirmed the V-shaped molecular structure with intramolecular H-bonding and substituent dependent molecular packing in the crystal lattice. 1 showed strong solid-state fluorescence (λmax = 554 nm, Φf = 21.2 %) whereas methoxy substitution (2 and 3) produced tunable but significantly reduced fluorescence (λmax = 547 (2) and 615 nm (3), Φf = 2.1 (2) and 6.5 % (3)). Interestingly, aggregation induced emission (AIE) studies in DMSO-water mixture revealed water sensitive fluorescence tuning. The trace amount of water (less than 1 %) in DMSO converted the non-emissive 1-3 into highly emissive state due to keto tautomer formation. Further increasing water percentage produced deprotonated state of 1-3 in DMSO and enhanced the fluorescence intensity with red shifting of emission peak. At higher water fraction, 1-3 in DMSO produced aggregates and red shifted the emission with reduction of fluorescence intensity. The concentration dependent fluorescence study revealed the very low detection limit of water in DMSO. The limit of detection (LOD) of 1, 2 and 3 were 0.14, 1.04 and 0.65 % of water in DMSO. Hence, simple Schiff bases of 1-3 showed water concentration dependent keto isomer, deprotonated and aggregated state tunable fluorescence in DMSO. Further, scanning electron microscopic (SEM) studies of 1-3 showed water concentration controlled self-assembly and tunable fluorescence.

Carbazole fluorophore with an imidazole/thiazole unit: contrasting stimuli-induced fluorescence switching, water-sensing and deep-blue emission

Carbazole-based, π-conjugated donor-acceptor fluorophores were synthesized by integrating imidazole/thiazole units. Then, we investigated the impact of subtle structural changes on fluorescence properties. Carbazole integrated with imidazole (Cz-I) and carbazole integrated with thiazole (Cz-T) showed strong fluorescence in solution (quantum yield (Φ _f) = 0.18 (Cz-I) and 0.14 (Cz-T) compared with the standard quinine sulfate) and solid-state (Φ _f = 8.0% (Cz-I) and 14.6% (Cz-T)). Cz-I showed relatively more blue-shifted emission in solution compared with the solid-state (λ _max = 417 nm (CH₃CN) and 460 nm (solid)). Cz-T exhibited deep-blue emission in the solid-state compared with solution (λ _max = 455 nm (CH₃CN) and 418 nm (solid)). Interestingly, Cz-T exhibited a drastic change in fluorescence in organic solvents (CH₃CN, THF, CH₃OH, DMSO) with a low percentage (1%) of water. Cz-I showed reversible fluorescence switching between two fluorescence states upon exposure to trifluoracetic acid (TFA)/ammonia (NH₃). In contrast, Cz-T displayed reversible/self-reversible off-on fluorescence switching upon exposure to TFA or NH₃. Mechanofluorochromic studies of Cz-I showed a slight reduction in fluorescence intensity upon crushing and reversal to the initial state upon heating. Cz-T exhibited off-on reversible/self-reversible fluorescence switching upon crushing/heating. Computational studies indicated that thiazole integration improved the electron-withdrawing characteristics compared with imidazole and contributed to contrasting fluorescence responses. Thus, a simple change of nitrogen with sulfur produced contrasting self-assembly in the solid-state that led to different functional properties and stimuli-induced fluorescence switching.

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 λ_max 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.

Position- and region-isomerized derivatives of a V-shaped fluorophore: the unique solution-state dual emission and the unusual force-induced solid-state turn-on emission

A V-shaped parent fluorophore as well as its position- and region-isomerized derivatives with a phenyl introduced on either the electron-donating or the electron-withdrawing moiety in the ortho-, meta- or para-linkage is prepared. Compared with the parent, these derivatives exhibit unique solvent-dependent dual emission in solutions presumably due to the considerably enhanced rotation energy barrier triggered by the insertion of the phenyl, which results in competitive relaxation between the LE and the TICT states. The intrinsically differentiated electron effect induced by the linkage position, which is strengthened by the twisted conformations, should be responsible for the faint or fully quenched fluorescence of the ortho- and meta-isomers in both solution and solid states. The rare turn-on solid-state emission of such isomers under force stimuli is caused by the enhanced π-conjugation effect on excited molecules, which are released from broken lattices to possess more planarized geometries. Moreover, the red shifts of emission wavelengths before and after force application are remarkably reduced from the parent to the derivatives. X-ray crystallography helps us gain a deep insight on the reasons for such a reduction.

H-Bonding and C–H⋯π assisted mechanofluorochromism of triphenylamine-containing vinylheterocycles bearing cyano and methyl groups

The intermolecular interactions have affected the mechanofluorochromic performance of heterocycles.