Stimuli-responsive fluorescence modulation of organic fluorophores is closely related to their structural organization, noncovalent interactions, ability to adopt different conformation, and phase change in the solid state. Herein, we have synthesized aggregation enhanced emissive fluorophores, (5 3)) and demonstrated molecular structure controlled tunable fluorescence (552 to 616 nm, Φ f = 14.6− 41.8%) and stimuli responses. One showed thermofluorochromism between 586 and 558 nm at room and liquid N 2 . 2 showed tunable fluorescence via polymorphism (2a (550 nm) and 2b (610 nm)). Interestingly, hard crushed −2b polymorph showed two different mechanofluorochromism (MFC) when heated at 80 and 180 °C as well as topochemical conversion from 2b to 2a. In contrast, 2a and 3 displayed usual MFC. Crystal structure, powder X-ray diffraction, and differential scanning calorimetric studies indicated conformational, structural, and phase changes with different stimuli which are responsible for fluorescence switching/tuning. Computational studies revealed that optical band gap modulation depend on the molecular conformation and support the fluorescence modulation.
Excited-state intramolecular proton transfer (ESIPT) compounds, 2-(2-((2-hydroxybenzylidine)amino)phenoxy)benzonitrile (1) and 2-(4-((2-hydroxy-4-methoxybenzylidine)amino)phenoxy)benzonitrile (2) exhibited aggregation induced enhanced emission (AIEE) in the solid state and nanofabrication mediated white light emission. The strong intermolecular interactions (H-bonding, C−H•••π and π••• π) restrict the free rotation and rigidify the fluorophores, 1 and 2, in the crystal lattice that lead to enhanced fluorescence in the solid state of 1 (yellow, λ max = 535 nm, Φ f = 38%) and 2 (greenish-yellow, λ max = 525 nm, Φ f = 21%). Surprisingly, nanofabrication of 1 and 2 produced metastable white light emitting nanoparticles (Φ f = 24% (1), 19% (2)) that slowly converted into stable yellow (1) or greenish-yellow (2) fluorescent nanoparticles with increasing time. The time dependent fluorescence, morphological, and metal ion interacting studies indicate the formation of soft nanoparticles in which both blue fluorescent cyanophenoxy and yellow/greenish-yellow fluorescent salicylidene group behave independently. 1 and 2 nanoparticles undergo morphological change from spherical to plates with increasing time. Thus, simple ESIPT molecules showed solid-state fluorescent and nanofabrication induced tunable fluorescence, particularly white fluorescent metastable nanoparticles.
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.
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