Excited state intramolecular proton transfer induced fluorescence in triphenylamine molecule: Role of structural conformation and reversible mechanofluorochromism

Article abstract of DOI:10.1016/j.molstruc.2018.05.042

Triphenylamine (TPA) based Schiff base molecules (1–4) were synthesized and role of molecular structure and conformation on excited state intramolecular proton transfer (ESIPT) induced fluorescence in both solution as well as solid state has been explored

Full text of DOI:10.1016/j.molstruc.2018.05.042

Accepted Manuscript
Excited state intramolecular proton transfer induced fluorescence in triphenylamine
molecule: Role of structural conformation and reversible mechanofluorochromism
Anu Kundu, Subraminain Karthikeyan, Dohyun Moon, Savarimuthu Philip Anthony
PII:
S0022-2860(18)30608-2
10.1016/j.molstruc.2018.05.042
MOLSTR 25215
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 6 March 2018
Revised Date: 11 May 2018
Accepted Date: 11 May 2018
Please cite this article as: A. Kundu, S. Karthikeyan, D. Moon, S.P. Anthony, Excited state
intramolecular proton transfer induced fluorescence in triphenylamine molecule: Role of structural
conformation and reversible mechanofluorochromism, Journal of Molecular Structure (2018), doi:
10.1016/j.molstruc.2018.05.042.
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Graphical Abstract
Excited State Intramolecular Proton Transfer Induced Fluorescence in Triphenylamine
Molecule: Role of Structural Conformation and Reversible Mechanofluorochromism
Triphenylamine based Schiff base molecules showed structure and conformation dependent
ESIPT induced fluorescence and mechanofluorochromism.

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1
Excited State Intramolecular Proton Transfer Induced Fluorescence in Triphenylamine
Molecule: Role of Structural Conformation and Reversible Mechanofluorochromism
a)
b)
c)*
a)*
Anu Kundu, Subraminain Karthikeyan, Dohyun Moon and Savarimuthu Philip Anthony
a)
School of Chemical & Biotechnology, SASTRA Deemed University, Thanjavur-613401, Tamil
Nadu, India. Fax: +914362264120; Tel: +914362264101; E-mail: philip@biotech.sastra.edu
b)
Department of Chemistry, Kalasalingam Deemed University, Krishnan Kovil-626126, India
c)
Beamline Department, Pohang Accelerator Laboratory, 80 Jigokro-127beongil, Nam-gu,
Pohang, Gyeongbuk, Korea, Email: dmoon@postech.ac.kr
Abstract
Triphenylamine (TPA) based Schiff base molecules (1-4) were synthesized and role of molecular
structure and conformation on excited state intramolecular proton transfer (ESIPT) induced
fluorescence in both solution as well as solid state has been explored. 1 and 2, which are not
hydrogen bonding donor to form intramolecular H-bonding, basis for ESIPT, are non-fluorescent
in solution as well as solid. The incorporation of hydroxyl group in 3 and 4 promoted
intramolecular H-bonding that could facilitate ESIPT. Interestingly, 3 showed strong
fluorescence in solution (λmax = 489 – 540 nm, Φf = 0.337 – 0.018 compared to standard
fluorescein) as well as solid state (λmax = 553 nm, Φf = 4.84% (absolute quantum yield) whereas
structurally similar 4 did not show fluorescence. Further 3 also showed reversible
mechanofluorochromism (MFC) due to the incorporation of non-planar TPA. Solid state
structural analysis confirmed intramolecular H-bonding in 3 and 4 but 3 showed coplanar
conformation between TPA phenyl and imine group whereas one of the molecules in the
asymmetric unit of 4 revealed slightly twisted conformation. Powder X-ray diffraction studies of
3
indicated reversible conversion of crystalline to amorphous and vice versa. Computational
studies of 3 and 4 provided insight on the ESIPT properties of 3.
1

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1
. Introduction
Organic molecules that can exhibit photoinduced excited state intramolecular proton
*
*
transfer (ESIPT) from enol (E ) to keto (K ) tautomers, corresponding to proton migration within
an intramolecular hydrogen bonding site, have attracted considerable attention because of their
potential applications in optical and optoelectronic devices [1]. The enol-keto photocycle (E-E*-
K*-K-E) offers a distinct four-level laser scheme to achieve population inversion; therefore,
ESIPT fluorophores are useful as laser dyes [2]. The formation of intramolecular H-bonding
between proton donor and acceptor within the molecule is pre-requisite for ESIPT [3]. The
drastic electronic and structural changes through ESIPT often produced strong fluorescence in
the solid state with a large Stokes shifts without self-absorption [4]. On the other hand, the
majority of the π-conjugated organic molecules that showed strong fluorescence in solution
becomes weak or non-fluorescence in the solid state due to aggregation caused quenching.
Special approaches such as aggregation enhanced emission (AEE) and supramolecular
interactions have often employed to generate strong solid state fluorescent materials [5]. Further,
the intramolecular H-bonding and fluorescence of ESIPT fluorophores are strongly influenced by
surrounding factors such as pH, solvent polarity and ions that could result in dual fluorescence
*
*
from ESIPT K and non-ESIPT E tautomers [6]. The subtle conformational and molecular
structure change has produced tunable solid state fluorescence [7]. The green emission of ESIPT
microcrystal laser was tuned to red by introducing intermolecular H-bonding functional group
[8]. Anthracene showed strong fluorescence in solution but weak/non fluorescence in the solid,
however, strong solid state fluorescent materials based on anthracene derivatives were
synthesized by attaching ESIPT unit [9]. The dual and tunable fluorescence and metal chelating
functionality of ESIPT molecules further used for fluorescent sensing and to realize white light
emission using single fluorophore [10,11]. External stimuli-responsive reversible fluorescence
switching has also been demonstrated by make use of halochromic functionality and
incorporation of AEE unit in the ESIPT compounds [12].
Schiff base organic molecules are very interesting because of their structural versatility,
simple synthetic method and easy tailorability, good biological activities, strong photophysical
properties and coordination ability with metal ions [13]. Moreover, hydroxyl functional group
incorporated Schiff base derivatives can exhibit strong intramolecular H-bonding and offered
opportunity to investigate ESIPT [14]. Nevertheless, only some of the Schiff base molecules
2

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exhibited fluorescence and most of the Schiff bases are non-fluorescent. For example, ESIPT
coupled AEE in poly(aryl ether) dendron based Schiff base exhibited tunable blue-green-white-
yellow emission by controlling aggregate [15]. Polymorph structure dependent yellow and
orange AEE was observed with (E)-4-chloro-2-(((4-(dimethylamino)phenyl)imino)methyl)
phenol [16]. Coumarin–based AEE Schiff base derivatives displayed multiple colours depend on
the substitution [17]. Recently, we have reported stimuli-responsive high-contrast reversible
fluorescence switching and nanofabrication induced white light emission via ESIPT in simple
aryl-ether based Schiff bases [18]. However, still it is not clear why only some of the Schiff base
molecules showed fluorescence via ESIPT and others are non-fluorescent. Herein, we have
attempted to derive some understanding by synthesizing new triphenylamine based Schiff base
molecules (1-4) and exploring molecular structure dependent ESIPT induced fluorescence both
in solution and solid state [Scheme 1]. 1 and 2 are non-fluorescence due to unavailability of
intramolecular H-bonding facilitating hydroxyl functional group in the structure. However, the
introduction of hydroxyl functionality in 3 facilitated intramolecular H-bonding and ESIPT that
lead to strong solution as well as solid state fluorescence. Absorption studies of 3 in different
solvent did not show solvatochromism but exhibited drastic modulation of fluorescence
wavelength and intensity. Further the temperature dependent fluorescence reduction and metal
ions induced quenching supported the role of ESIPT on the fluorescence of 3. Solid state
structural studies also unambiguously confirmed the formation of strong intramolecular H-
bonding. Structural studies of 4 also revealed strong intramolecular H-bonding but slightly
twisted non-coplanar conformation in the crystal lattice. But structurally similar 4 did not show
any fluorescence that might be due to photoinduced electron transfer effect of imino
nitrogen/aminophenyl. Interestingly, 3 showed stimuli-responsive reversible fluorescence
switching upon strong crushing and heating. PXRD studies revealed the conversion crystalline to
amorphous state upon crushing and reversing amorphous to crystalline by heating.
Computational studies of 3 and 4 have also been performed to gain insight on the ESIPT
properties of 3. Thus the present studies provide structural insight for developing Schiff base
based ESIPT fluorescent materials.
2
. Experimental section
Triphenylamine, 3-methoxy-N,N-diphenylaniline, dimethylformamide (DMF, HPLC grade), and
phosphorous oxychloride were purchased from Sigma-Aldrich and used without further
3

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purification. Solvents were obtained from Merck India. Hydrazine hydrate (99-100%), phenyl
hydrazine was obtained from fisher scientific. The metal ion solutions used for the colorimetric
sensor experiments were prepared in Mill-Q water. Aldehyde functional group into
triphenylamine and 3-methoxy triphenylamine was introduced by following reported procedure
[
19]. 4-(diphenylamino)-2-hydroxybenzaldehyde was synthesized by converting OCH group of
3
corresponding aldehyde to OH by stirring with BBr
20].
General procedure for 1-3 (Scheme S1)
3 2 2
in CH Cl following the reported procedure
[
Triphenylamine aldehyde (1.0 mmol) was dissolved in ethanol and stirred at room temperature
for 10 min. Then hydrazine hydrate (0.5 mmol, 99-100%) was added drop-wise under vigorous
stirring. The solution mixture was stirred at room temperature for 3 h and the resulting
precipitate was filtered, washed by cold ethanol and dried under vacuum.
°
1
1
: Yield = 77%. M.pt = 229 C. H NMR (300 MHz, CDCl ) δ 8.58 (s, 1H (imine)), 7.65 (td, J =
3
1
3
2
.2, 8.7 Hz, 2H), 7.33-7.27 (m, 4H), 7.16-7.11 (m, 4H), 7.09-7.03 (m, 4H). C NMR (75 MHz,
) δ 160.9, 150.4, 146.9, 129.5, 129.4, 127.3, 125.4, 123.9, 121.7. C38 (542.67):
calcd. C 84.10, H 5.57, N 10.32; found C 84.16, H 5.66, N 10.12.
CDCl
3
30 4
H N
°
1
2
8
1
1
7
3
: Yield = 75%. M.pt = 254 C. H NMR (300 MHz, CDCl ) δ 8.97 (s, 1H (imine)), 7.92 (d, J =
3
.4 Hz, 1H ), 7.33-7.26 (m, 4H), 7.16-7.13 (m, 4H), 7.11-7.06 (m, 2H), 6.62 (dd, J = 2.1, 8.5 Hz,
1
3
H ), 6.54 (d, J = 2.1 Hz, 1H), 3.67 (s, 3H). C NMR (75 MHz, CDCl
3
) δ 159.9, 156.5, 151.6,
47.0, 129.4, 127.9, 125.4, 123.9, 116.6, 114.7, 104.6, 55.5. C H N O (602.72): calcd. C
4
0
34
4
2
9.71, H 5.69, N 9.30; found C 79.56, H 5.72, N 9.22.
°
1
: Yield = 80 %. M.pt = 304 C. H NMR (300 MHz, CDCl
3
) δ 11.65 (s, 1H (OH)), 8.52 (s, 1H
(
(
imine)), 7.34-7.29 (m, 4H), 7.17 (dd, J = 1.2, 8.5 Hz, 4H), 7.13-7.11 (m, 2H), 7.08 (s,1H), 6.55
1
3
s, 1H), 6.51 (d, J = 2.4 Hz, 1H). C NMR (75 MHz, CDCl ) δ 162.0, 160.8, 152.2, 146.4,
3
1
9
32.8, 129.5, 126.3, 124.6, 112.3, 111.1, 107.4. C H N O (574.67): calcd. C 79.42, H 5.26, N
38 30 4 2
.75; found C 79.24, H 5.50, N 9.84.
Synthesis of 4
Aldehyde (1.0 mmol) was dissolved in ethanol and stirred at room temperature for 10 min. Then
phenyl hydrazine (1.0 mmol) was added drop-wise under vigorous stirring. The solution mixture
was stirred at room temperature for 3 h. The resulting precipitate was filtered, washed by cold
ethanol and dried under vacuum.
4

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°
1
4
: Yield = 85 %. M.pt = 150 C. H NMR (300 MHz, CDCl ) δ 10.90 (s, 1H (OH)), 7.81 (s, 1H
3
(imine)), 7.35-7.25 (m, 7H), 7.16-7.05 (m, 6H), 6.98-6.87 (m, 4H), 6.63 (d, J = 2.1 Hz, 1H), 6.54
1
3
(
dd, J = 8.1, 8.4 Hz, 1H). C NMR (75 MHz, CDCl ) δ 158.1, 149.7, 147.0, 143.7, 141.5, 129.9,
3
1
7
29.5, 129.3, 125.5, 123.8, 120.5, 113.4, 112.6, 112.5, 109.2. C H N O (379.45): calcd. C
25 21 3
9.13, H 5.58, N 11.07; found C 79.32, H 5.38, N 11.22.
Characterization
NMR spectra were measured on a Bruker 300 MHz AVANCE-II. Absorption spectra were
recorded using Perking Elmer Lambda 1050. Fluorescence spectra and quantum yield were
recorded on Jasco fluorescence spectrometer FP-8300 instruments equipped with integrating
sphere and calibrated light source. Powder X-ray diffraction (PXRD) patterns were obtained
using an XRD-Bruker D8 Advance XRD with Cu Kα radiation (λ = 1.54050 Å) operated in the
2
θ range from 10° to 50°. Single crystals of 3 and 4 coated with paratone-N oil and diffraction
data was measured at 100 K with synchrotron radiation (λ = 0.62998 Å) on an ADSC Quantum-
10 detector at 2D SMC with a silicon (111) double crystal monochromator (DCM) at the
2
Pohang Accelerator Laboratory, Korea. CCDC Nos. – 1818752 (3) and 1818753 (4) contain the
supplementary crystallographic data for this paper. The HOMO, LUMO and band gap of 3 and 4
are studied using B3PW91/6-31+G(d,p) level theory (Gaussian 09 package).
3
. Results and Discussion
Compound 1-4 was synthesized by simple condensation reaction between different
triphenylamine aldehyde and hydrazine hydrate/phenyl hydrazine [Scheme 1 and S1].
Absorption spectra of 1 in DMF showed λmax at 406 nm that could be assigned for n-π*
transition [Figure S1]. Introducing electron donor methoxy group at ortho position (2) slightly
red shift the absorption to 418 nm whereas hydroxyl attached 3 exhibited further red shift of
absorption (λmax = 425 nm). In contrast, phenyl hydrazine based 4 showed blue shifted
absorption (λmax = 380 nm). Interestingly, 1 and 2 showed very weak fluorescence in DMF
whereas 3 exhibited highly enhanced emission [Figure S2]. The strong fluorescence of 3
compared to 1 and 2 could be attributed to the ESIPT since structure of 3 can form
intramolecular H-bonding between phenyl hydroxyl donor and imine acceptor but 1 and 2 could
not form such intramolecular H-bonding due to non-availability of hydroxyl functional group.
Absorption studies of 3 in different solvent did not show solvatochromism (λmax = 424 to 431
nm, Table 1) and suggest that there might not be any change in the dipole moment of molecular
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ground and excited states [21, Figure 1a]. However, fluorescence λmax and intensity of 3 was
strongly influenced by solvent polarity [Figure 1b, Table 1]. 3 in toluene showed strong
fluorescence at 489 nm (Φf = 0.107 compared to standard fluorescein) whereas red shifted very
weak fluorescence at 540 nm (Φf = 0.018) was observed in CH OH. The strongest fluorescence
3
for 3 was obtained from CH Cl (Φf = 0.337). In non-polar environment, intramolecular H-
2
2
bonding of 3 might be intact whereas polar solvents could form intermolecular H-bonding
between solvent and hydroxyl group of 3 and compete with intramolecular H-bonding that lead
to reduction of fluorescence intensity [14b]. Further, temperature dependent fluorescence change
also supported the role of intramolecular H-bonding on the fluorescence via restricting C=N
3
isomerization [Figure S3]. At 0 °C, 3 in CH CN showed strong fluorescence at 523 nm,
however, increase of temperature to 80 °C lead to three times reduction in the fluorescence
intensity with slight blue shift of λmax (513 nm). Similarly, 3 dissolved in CH CN and injected
3
into water also exhibited decrease of fluorescence intensity with temperature [Figure S4]. The
increase of temperature could reduce the molecular rigidity and might weaken the intramolecular
H-bonding. In contrast, 4 in which phenyl group was attached with other end of the hydrazine
amine did not show fluorescence in any solvent, probably because of the photoinduced electron
transfer (PET) from imino nitrogen or aminophenyl nitrogen [22]. The strong solution
fluorescence coupled with metal chelating functional group prompted us to explore metal
induced fluorescence changes since that can also support the role of ESIPT on the fluorescence
2
+
2+
2+
of 3 [Figure 2]. Interestingly, addition of Co , Ni and Cu metal ions quenched the
fluorescence of 3 whereas other ions did not show significant change. 3 showed absorption λmax
2
+
at 427 nm in DMF. Addition of Co showed disappearance absorption at 427 nm and
appearance of weak featureless broad absorption from 350 to 550 nm [Figure 2a]. On the other
2
+
hand, Ni addition exhibited decrease of absorption intensity with slight blue shift of λ to 408
max
2
+
nm whereas Cu addition showed decrease of intensity with broadening of absorption without
2
+
2+
2+
changing λmax. Fluorescence spectra of 3 with Co , Ni and Cu showed complete
disappearance of peak at 516 nm whereas other metals did not show any significant change in the
2+
2+
2+
fluorescence [Figure 2b]. The coordination of 3 with Co , Ni and Cu metal ions might lead
to loss of intramolecular H-bonding and ESIPT and hence the fluorescence was quenched. These
studies clearly indicated that ESIPT played important role in the fluorescence of 3.
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The intramolecular H-bonding facilitated ESIPT in 3 also lead to good solid state
fluorescence [Figure 3, = 4.84% (absolute quantum yield)]. AEE property of 3 has also been
Φ
f
demonstrated by increasing water fraction in methanol solution. Addition of water into 3
dissolved in methanol leads to decrease of fluorescence intensity initially before exhibiting a new
peak at 537 nm [Fig. S5a]. The green fluorescence of DCM droplet of 3 was converted to
yellowish green upon solvent evaporation [Fig. S5b]. Further addition of water reduced the
fluorescence intensity due to precipitation of formed colloidal particles. The non-planar
triphenylamine unit coupled with solid state fluorescence of 3 expected to exhibit mechanical
stimuli-induced fluorescence switching since triphenylamine based derivatives is known to
exhibit AEE and mechanofluorochromism (MFC) [23]. The crystals of 3 showed fluorescence at
5
53 nm and slight breaking lead to slight blue shift of fluorescence to 540 nm that might be due
to the disruption of long range molecular ordering and generating defects [24]. Hard crushed
crystals of 3 exhibited complete fluorescence quenching whereas heating lead to turn-on
fluorescence lλmax = 537 nm). In contrast, structurally similar 4 did not show fluorescence in the
solid state as well. PXRD studies of crystals showed clear sharp diffraction peaks that perfectly
matched with pattern simulated using single crystals data of 3 [Figure 4, S6]. The perfect
matching of experimental and simulated pattern confirmed the phase purity of the compound.
The hard crushed solids showed very weak diffraction peaks that indicated the conversion of
crystalline to amorphous or partially amorphous phase. However, heated solids showed
recovering of clear sharp diffraction peaks that matched with initial diffraction peaks of 3
crystals. The re-appearance of clear diffraction suggested the formation of crystalline phase from
amorphous phase and perfect matching with initial crystals peak indicated that there was no
structural change. Thus the solid state reversible fluorescence switching of 3 was due to the
reversible conversion of crystalline to amorphous and vice-versa by hard crushing followed by
heating. FTIR peaks of 3 before and after grinding did not show significant change in the peak
position except reduction of intensity after crushing [Fig. S7]. Hard crushing can cause the
molecules to have close packing in the powders that could lead to loss of molecular vibrations
and peak intensity [25].
Solid state structural analysis has been performed to confirm the presence of
intramolecular H-bonding and get the insight on the molecular packing induced solid state
fluorescence. Single crystals of 3 were grown by diffusing diethyl ether into CH Cl whereas 4
2
2
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were grown from ethanol (EtOH) by slow evaporation, but 1 and 2 did not produce quality
crystals. 3 revealed P21/n space group with one molecule in the asymmetric unity [Table S1-4].
As expected, 3 showed strong intramolecular H-bonding between hydroxyl donor and imine
acceptor [Figure 5a, Table 2]. Further the symmetric structure showed same intramolecular H-
bonding on the both end of the hydrazine. The hydroxyl group attached phenyl ring in both
triphenylamine along with hydrazine imine adopted coplanar geometry [Figure 5b, S8, torsion
3 4 7 2
angle (τ) of C -C -C -N = 0.12]. The weak intermolecular interaction between hydroxyl oxygen
and phenyl hydrogen and C-H…π interactions lead to the formation of ladder like structural
arrangement in the crystal lattice of 3 [Figure 5c, Table 2]. The asymmetric unit of 4 contain two
molecules that also showed strong intramolecular H-bonding in the crystal lattice but with
different H-bonding distances due to conformational differences [Figure 6a,b, Table 2].
Compared to 3, higher torsional angle between hydroxyl phenyl group of triphenylamine and
phenyl hydrazine was observed for both molecules in the crystal lattice of 4. One of the molecule
showed relatively more coplanar conformation compared to another molecule in the asymmetric
unit [Figure S9, τ = 2.38 and τ = 3.86]. The slightly higher twisted molecule formed dimer via
1
2
π…π interactions between hydrazine amine and imine carbon and the dimer on both end was
connected with more planar molecule through C-H…π interactions [Figure 6c, Table 2]. Thus
the structural comparison of 3 and 4 suggest that although both exhibit strong intramolecular H-
bonding the higher twisting along with PET effect from imine nitrogen/phenyl hydrazine amine
in 4 might lead to non-fluorescence in the solid state. However, the role of twisting on the solid
state fluorescence of Schiff base ESIPT molecules need further exploration with more examples.
The entire set of calculations have been performed using DFT-B3PW91 method with 6-
3
1+G(d,p) basis set using Gaussian 09 program package [26]. Computational studies were
performed using the crystal structure of 3 and 4 without optimization to get insight on the
fluorescence [27, Figure 7].The bond length, bond angle of structure 3 and 4 are given in the
supporting information [Figure S10, Table S5]. Figure 7 shows the isodensity surface plot
(isodensity contour =0.02) of highest occupied molecular orbitals (HOMOs), and lowest
unoccupied molecular orbital (LUMO) for structure 3 and 4. The HOMOs of 3 indicated that
electron density was predominantly localized on both TPA donor phenyl group as well as
slightly over the hydrazine imine nitrogen. The electron density in the LUMO was mostly
localized on the hydrazine imine nitrogen acceptor. HOMO-LUMO calculation for 4 was
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performed for both unique molecules observed in the asymmetric unit. Similar to 3, electron
density in the HOMO of 4-I (less twisted) and 4-II (more twisted) was mainly occupied on the
triphenylamine donor, however, in LUMO electron density was occupied on the imine nitrogen
and adjacent phenyl ring. The less twisted 4-I LUMO was more stabilized compared to 4-II and
lead to lower band gap (3.65 eV). 3 showed the lowest band gap of 3.09 eV. Hence, although
both 3 and 4 showed intramolecular H-bonding interactions, the band gap was strongly
influenced by molecular conformation and packing.
4
. Conclusion
In conclusion, we have synthesized new ESIPT fluorophore, 3, based on triphenylamine
Schiff base that showed strong fluorescence both in solution as well as solid state due to
hydroxyl functionality facilitated intramolecular H-bonding. Structural comparison revealed the
lack of intramolecular H-bonding site in 1 and 2 makes them non-fluorescent. Solvent polarity
and temperature dependent fluorescence modulation in solution indicated that ESIPT played
significant role for the fluorescence of 3. Solid state structural studies also confirmed the
formation of strong intramolecular H-bonding between the hydroxyl donor and imine nitrogen
acceptor. Incorporating non-planar triphenylamine unit was exploited for demonstrating
reversible MFC in the solid state. In contrast, the structurally similar Schiff base 4 that also
exhibited strong intramolecular H-bonding in the solid state did not show any fluorescence. This
could be attributed to the PET effect of imino nintrogen/aminophenyl and non-coplanar
molecular conformation in the solid state. PXRD studies suggested that conversion of crystalline
to amorphous and vice versa upon crushing and heating was responsible for fluorescence
switching of 3. Computational studies further provided insight on the ESIPT fluorescence of 3.
Thus the present studies indicate that even though the Schiff base molecule can form strong
intramolecular H-bonding, the molecular conformation and presence of other functionality plays
significant role on the fluorescence.
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Acknowledgement
Financial support from the Science and Engineering Research Board (SERB), New Delhi, India
SERB No. EMR/2015/00-1891) is acknowledged with gratitude. "X-ray crystallography at the
(
PLS-II 2D-SMC beamline was supported in part by MSIP and POSTECH.
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Figure Captions
Scheme 1.
Figure 1.
Molecular structure of 1-4.
(a) Absorption and (b) fluorescence spectra of 3 in different solvent. λexc = 360
nm.
Table 1.
Figure 2.
Absorption and fluorescence data of 3 in different solvents.
-3
(a) Absorption and (b) fluorescence responses of 3 (10 M) to different metal ions
-3
(
10 M). λexc = 360 nm.
Figure 3.
Figure 4.
Figure 5.
Solid state and mechanofluorochromism of 3. λexc = 360 nm.
PXRD pattern of 3.
(a) Intramolecular H-bonding, (b) molecular conformation and (c) weak
intermolecular interactions in the crystal lattice of 3. C (grey), N (blue), O (red),
H (white). H-bonding, C-H…O and π…π interactions (broken line) distances are
marked in Å.
Table 2.
Figure 6.
Inter and intramolecular H-bonding, C-H...π and π...π interactions in the crystal
lattice of 3 and 4.
Intramolecular H-bonding, (b) molecular conformation and (c) weak
intermolecular interactions in the crystal lattice of 4. C (grey), N (blue), O (red),
H (white). H-bonding, C-H…π and π…π interactions (broken line) distances are
marked in Å.
Figure 7.
Molecular orbital plots of the HOMOs and LUMOs of 3 and 4.
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Scheme 1. Molecular structure of 1-4.
2
0

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Figure 1. (a) Absorption and (b) fluorescence spectra of 3 in different solvent. λexc = 360 nm.
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1

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Table 1.
Absorption
Fluorescence
Φf (compared to
-
1
-1
Solvents
Toluene
CH Cl
THF
ε (Lmol cm )
λ
max (nm)
λ
max nm
fluorescein
0.107
2
1
1
1
1
1
1
1
1
1
.45 x 10
.50 x 10
.41 x 10
.32 x 10
.32 x 10
.32 x 10
.43 x 10
.41 x 10
.34 x 10
432
489
2
2
2
2
2
2
2
2
2
0.337
0.105
0.094
0.088
0.176
0.057
0.050
0.018
0.060
2
2
432
424
424
425
423
425
425
424
507
495
495
513
513
517
532
540
530
EtOAc
DMF
CH CN
3
DMSO
EtOH
MeOH
Isopropylalcoho
l
1.39 x 10
4
24
2
2

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-3
-3
Figure 2. (a) Absorption and (b) fluorescence responses of 3 (10 M) to different metal ions (10
M). λexc = 360 nm.
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3

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Figure 3. Solid state and mechanofluorochromism of 3. λexc = 360 nm.
Figure 4. PXRD pattern of 3.
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Figure 5. (a) Intramolecular H-bonding, (b) molecular conformation and (c) weak intermolecular
interactions in the crystal lattice of 3. C (grey), N (blue), O (red), H (white). H-bonding, C-H…O
and π…π interactions (broken line) distances are marked in Å.
Figure 6. Intramolecular H-bonding, (b) molecular conformation and (c) weak intermolecular
interactions in the crystal lattice of 4. C (grey), N (blue), O (red), H (white). H-bonding, C-H…π
and π…π interactions (broken line) distances are marked in Å.
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Table 2.
Hydrogen bonding
C-H…π
interactions
π …π
interactions (Å)
D-H...A
d(D...A) (Å)
<(DHA) (°)
(
Å)
O -H ...N
2
2.646
3.484
2.664
2.651
146.0
153.9
146.0
142.9
3.272
3.204
1
1
3
4
C
O
19-H19…O
1A-H1A...N2A
1
3.493
3.502
O -H ...N
2B
1B
1B
Figure 7. Molecular orbital plots of the HOMOs and LUMOs of 3 and 4.
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Highlights
•
•
•
•
Schiff base molecules for ESIPT induced fluorescence.
Molecular structure dependent ESIPT.
Role of molecular conformation and substituent on the ESIPT fluorescence.
Stimuli-responsive high contrast off-on fluorescence switching.