Excited-State Intramolecular Proton-Transfer Properties of Three Tris(N-Salicylideneaniline)-Based Chromophores with Extended Conjugation

Article abstract of DOI:10.1002/chem.201604315

The synthesis and photophysical properties of three tris(N-salicylideneaniline) (TSA) compounds containing 1,3,5-triarylbenzene, -tristyrylbenzene, and -tris(arylethynyl)benzene core units are reported. The TSA compounds underwent efficient excited-state

Full text of DOI:10.1002/chem.201604315

A Journal of
Accepted Article
Title: The Excited State Intramolecular Proton Transfer Properties of
Three Tris(N-Salicylideneaniline)-Based Chromophores with
Extended Conjugation
Authors: Pradeepkumar Jagadesean, Tyler Whittemore, Toni Beirl,
Claudia Turro, and Psaras McGrier
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To be cited as: Chem. Eur. J. 10.1002/chem.201604315
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The Excited State Intramolecular Proton Transfer Properties of
Three Tris(N-Salicylideneaniline)-Based Chromophores with
Extended Conjugation
Pradeepkumar Jagadesan, Tyler Whittemore, Toni Beirl, Claudia Turro, Psaras L. McGrier*
Abstract: The synthesis and photophysical properties of three tris(N-
salicylideneaniline)
(TSA)
compounds
containing
1,3,5-
HO
triarylbenzene, -tristyrylbenzene and -tris(arylethynyl)benzene core
units are reported. The TSA compounds undergo efficient excited
state intramolecular proton transfer (ESIPT) in solution and in solid
state due to the preformed C=N---H-O hydrogen bonded motifs of the
structures. Steady state fluorescence emission spectra of the TSA
molecules revealed dual bands in only DMSO, and large Stokes
shifts in other polar aprotic and protic solvents. Femtosecond
transient absorption spectroscopic measurements in THF revealed
lifetime values in the range of 14-16 ps for the excited state keto-
tautomer. The TSA compounds are also responsive to metal ions
(Cu2+ and Zn2+) in DMSO, exhibit enhanced aggregate-induced
emission (AIE) properties in DMSO/water mixtures, and are highly
luminescent in the solid-state.
N
O
NH
E*
K*
ESIPT
O
Back proton
transfer
NH
HO
N
Introduction
keto tautomer (K)
enol tautomer (E)
Scheme 1. Schematic representation of the photoinduced four step (E-E*) –
Excited state intramolecular proton transfer (ESIPT) is
a
fundamental photophysical process that has been studied
extensively in solution[1] and the solid-state.[2] The efficiency of
this photoinduced process is highly dependent on the position of
the hydrogen bond donating and accepting substituents of a
particular system. A prominent feature of the ESIPT reaction is a
large Stokes shifted emission (100 - 200 nm) without self-
absorption and dual emission bands, which is often stimulated by
a rapid four-step enol (E-E*) – keto (K-K*) phototautomerization
process. As a consequence, this unique photochemical feature
has been exploited for a number of applications including
chemical sensors,[3] laser dyes,[4] photostabilizers,[5] molecular
recognition,[6] and optoelectronic devices.[7]
((K-K*) tautomerization process
novel salicylidene(aniline)-based (SA)[11] systems have received
little attention. In the ground state, the E form of SA is the most
stable species (Scheme 1). Upon photoexcitation, the proton on
the enol transfers to the imine generating the favored K form as
the final product, which can be converted back to the E form by a
photochemical or thermal process. On account of this reversible
process, SA molecules exhibit unique photochromic and
thermochromic properties making them useful for applications
related to optical data storage.[12,13] However, most of the
experimental[11] and theoretical[14] studies on SA molecules to
date have been restricted to small mono-substituted systems.
Gao, Li, and co-workers[15] recently revealed that SA
chromophores with extended conjugation exhibit interesting
optical and metal sensing properties in solution demonstrating
that a larger SA system does not inhibit the ESIPT properties of
the molecules. Yang and coworkers were successful in
constructing highly emissive bis(salicylidene) materials through
the formation of self-assembled aggregates.[16] Recently, Ouyang
and coworkers have also shown that varying the donor-
substitution of bis(salicylidene) chromophores can be used to
tune the aggregation-induced emission (AIE) and self-assembly
properties of the materials.[17] Inspired by these studies, we were
The ESIPT process typically requires an intramolecular
hydrogen bond (H-bond) between the proton donor (-OH, -NH2)
and proton acceptor (-C=O, -C=N-) groups within the same
vicinity of one another. In the excited-state, many chromophores
exhibit dual emission bands that cover the entire visible spectrum
on account of their combined short E* and long K* states. While
numerous systems highlighting the ESIPT induced fluorescent
properties of 2-(2’-hydroxyphenyl)benzimidazole (HBI),[8] 2-(2’-
hydroxyphenyl)benzoxazole
(HBO),[9]
and
2-(2’-
hydroxyphenyl)benzothiazole (HBT)[10] molecules in solution and
in the solid-state have been reported, experimental studies on
curious to examine if tris(salicylidene) chromophores
with
[a]
Dr. P. Jagadesan, T. Whittemore, T. Beirl, Prof. C. Turro, Prof. P. L.
McGrier
extended π-conjugation would display unique ESIPT properties
in solution and in the solid-state. We believe such studies are
useful for highlighting the potential of utilizing tris(salicylidene)
core units for the construction of functional two-dimensional
(2D)[18] polymeric systems and novel liquid crystalline
materials.[19]
Department of Chemistry & Biochemistry
The Ohio State University
100 W 18th Ave., Columbus, Ohio-43210, USA
E-mail: mcgrier.1@osu.edu
Supporting information for this article is given via a link at the end of
the document.
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Compound 11 was synthesized in 84% yield by carrying out a
Pd(PPh3)2Cl2/CuI-catalyzed Sonogashira coupling reaction
between 1,3,5-tribromobenzene and 9, followed by subsequent
addition of CF3COOH in DCM to produce the precursor 12 in
82% yield. Co-condensation of 12 with salicylaldehyde and
benzaldehyde afforded 13a and 13b in 64% and 66% yield,
R
R
R
N
N
N
1
respectively. All compounds were characterized by H, 13C NMR
N
N
R
N
R
R
N
R
N
N
R
and mass spectrometry (pgs. S4-S13, Supporting Information
(SI)).
R
R = -OH
R = -H
1a
1b
6a
6b
13a
13b
NHBoc
Figure 1. Structures of salicylideneaniline ESIPT compounds (1a, 6a and 13a)
and their corresponding benzylideneaniline analogs (1b, 6b and 13b).
BocHN
Br
9
PdCl2(PPh3
)
2
Br
Br
CuI
THF/NEt3
Herein, we present the synthesis and photophysical properties
10
BocHN
NHBoc
11
of three C3 symmetric tris(N-salicylideneaniline) compounds
1a,[20] 6a and 13a, containing 1,3,5-triarylbenzene,
-
CF3COOH
DCM
tristyrylbenzene and -tris(arylethynyl)benzene core units,
respectively. The ESIPT and chemosensing properties in the
presence of various metal ions such as Zn2+, Cu2+, Fe3+, Hg2+,
Ag+ and Li+ were investigated in solution using the steady state
UV-Vis and fluorescence spectroscopy. We have also examined
their AIE properties in DMSO/water mixtures, and their
luminescent properties in the solid state.
R
N
NH2
R
CHO
THF, reflux
H2N
NH2
N
N
R
12
R
Results and Discussion
13a,
13b,R = -H
R = -OH;
NO2
Scheme 3: Synthesis of 13a and 13b.
O
P
OEt
Br
OEt O2N
CHO
NaH/THF
P(OEt)3
O
P
The optical properties of 1a, 6a and 13a in different polar
aprotic and nonpolar solvents were examined using steady-state
UV-Vis spectra (Figure 2). The UV-Vis spectra of 1a, 6a and 13a
exhibit broad absorption bands ranging from λ= 356 to 378 nm
(Table 1). Solvent polarity had minimal effect on the absorbance
spectra for the E form of all three compounds including the
benzylideneaniline (BA) precursors 1b, 6b, and 13b which do not
contain any hydroxyl groups (Figure S2, SI). However, in
comparison to 1a and 13a, the absorption maxima of 6a exhibits
a 14-18 nm red shift in all polar aprotic and nonpolar solvents.
EtO
Br
Br
EtO
EtO P O
OEt
O2N
2
3
4
NO2
Zn/aq.NH4Cl
Acetone
R
N
NH2
R
CHO
THF, reflux
Table 1. The absorption and emission maxima, molar extinction coefficients
and Stokes shift values for compounds 1a, 6a and 13a in various solvents.
H2N
N
NH2
R
N
R
5
6a,
6b,R = -H
R = -OH;
Scheme 2: Synthesis of 6a and 6b.
The synthetic routes for 6a, 6b and 13a, 13b are shown in
Scheme 2 and Scheme 3, respectively. Compound 4 was
obtained in 74 % yield by reacting 3 with 4-nitrobenzaldehyde
under Horner Wadsworth Emmons coupling conditions.
Reduction of 4 in the presence of Zn/NH4Cl afforded 5 in 89%
yield. Condensation of 5 with salicylaldehyde and benzaldehyde
afforded 6a and 6b in 49% and 25% yield, respectively.
[a] Excitation at λex= 356 nm. [b] Excitation at λex= 370 nm. [c] Excitation at λex
360 nm. ε – molar extinction coefficient.
=
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Figure 2. Normalized UV-Vis spectra of (a) 1a, (b) 6a and (c) 13a, and Normalized fluorescence emission spectra of (d) 1a (λex= 370 nm), (e) 6a (λex= 360 nm)
and (f) 13a (λex= 356 nm) in various solvents at 10 µM.
Figure 3. Fluorescence titration spectra of (i) 1a (λex= 356 nm), (i) 6a (λex= 370 nm) and (iii) 13a (λex= 360 nm) with CF3COOH in THF. [1a] = [6a] = [13a] = 10 µM,
3 mL.
This could be attributed to the restricted rotation of the alkene polar aprotic solvent DMSO, dual emission bands ranging from
C=C bonds, which leads to enhanced π-conjugation in 475-483 nm and 535-545 nm are observed, which is indicative of
comparison to 13a and 1a. It is well known that trans-stilbenes the E* and K* forms, respectively for all three compounds (Table
are nearly planar in solution at room temperature and alkene 1). The emission from E* arises from the suppression of the enol-
units help to maintain its planarity.[21] As a consequence, the keto tautomerization in the excited state on account of the ability
effect of extended conjugation is more pronounced in reducing of the enol moiety to interact with DMSO through hydrogen
the energy gap for π-π* transition of 6a. In the case of 13a, the bonding. This hydrogen bonding interaction stabilizes the excited
phenyl rings can undergo free rotation about alkyne-phenyl state of the E* form and slows the rate of ESIPT process. This
bonds in solution, which reduces the effect of π-conjugation phenomenon is not uncommon and has been observed for other
leading to a hypsochromic shift. [22]
ESIPT systems. Upon the addition of 0-6 µL of trifluoroacetic
acid (TFA) in THF, the broad emission bands of 1a, 6a and 13a
Figure 2d-2f shows the steady-state fluorescence emission undergo a large hypsochromic shifts (Figure 3) of ~ 80, 96, and
spectra of 1a, 6a and 13a in the polar aprotic and nonpolar 134 nm, respectively. The large hypsochromic shifts are
solvents. A broad and featureless K* emission band at ~ 535 nm attributed to protonation of the imine nitrogens, which completely
was observed for all three compounds in toluene, CHCl3, EtOAc, inhibits ESIPT process and gives rise to the predominant enol
and THF with large Stokes shifts ranging from 7763 to 9875 cm-1 emission. In addition, the emission bands for the BA precursors
(Table 1). We believe the large Stokes shifts are emanating from 1b, 6b, and 13b (Figure S2, SI) in polar aprotic and nonpolar
the K* form of the molecules as
a consequence of the solvents are very similar the E* emission of 1a, 6a, and 13a in
photoinduced ESIPT tautomerization process. However, in the
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DMSO further indicating that ESIPT is indeed occurring for these of the broad absorption profile (λex = 355 nm). The long-lived
compounds.
state is attributed to the trans-isomer of SA that has been
previously reported and is known to exhibit a broad absorption
The fluorescence quantum yields of 1a, 6a, and 13a in DMSO profile centered at ~500 nm.1 The 14-16 ps time constants
were determined to be 0.13%, 0.15%, and 0.045%, respectively. measured for 1a and 13a are consistent with the excited state of
The low quantum yields in DMSO suggest that the twisting the keto form of the molecules,[24,25] expected to be populated
motions of the SA linkages and phenyl rings in solution provides following rapid ESIPT from the excited state of the corresponding
pathways for nonradiative deactivation of the E* and K* excited enol form, previously shown to occur in <50 fs.[24] Therefore, the
states that compete with their radiative decays. [23] Time resolved enol excited state in these compounds is expected to be too
spectroscopic studies of 1a, 6a and 13a in DMSO were carried short-lived for observation under our experimental conditions.
out using the time-correlated single photon counting technique. Vibrational cooling is not expected to be observed with excitation
The fluorescence (E*) decay profiles for 1a, 6a and 13a provided at 400 nm, to the red edge of the absorption peak.[24]
lifetimes values of 9, 14, and 9 ns, respectively, (Figure S6, SI).
Since the fluorescence decay of K* is too fast, we determined the
The ultrafast transient absorption spectrum of 6a in THF (Figure
lifetime values of the short-lived K* of 1a, 6a and 13a by ultrafast 4b) exhibits a positive signal with maximum at 510 nm which
transient absorption measurements.
decays biexponentially with time constants of 14 ps (57%) and
113 ps (43%), together with an absorption that persists to delay
times >3 ns. While the 14 ps lifetime is attributed to the keto state,
consistent with those of 1a and 13a, the 113 ps component is
inconsistent with the dynamics of structurally related complexes.
This 113 ps lifetime is attributed to the formation of the cis-trans
isomer of the C=C vinyl group, which competes with the
generation of the excited keto-state. The >3 ns lifetime is also
attributed to a trans-isomer due to the similar spectral features of
this compounds at long times when compared to those of 1a and
13a. In 6a, the absorption maximum of the keto form at 510 nm
is similar to that of 13a, but is red-shifted from that measured for
1a. This difference is attributed to the superposition of the
negative signal of the ground state bleach, which appears at
lower energy in 6a and 13a as compared to 1a. The ultrafast
transient absorption spectra of these complexes in THF show
absorption features that are inconsistent with the absorption
maxima and excited state lifetimes of the respective analogous
compounds 1b, 6b, and 13b that are not able to undergo ESIPT
(Figures S7 and S8).
SA ligands are well known for their ability to form complexes
with various transition metal ions in solution. [15,26] With this in
mind, we decided to evaluate the chemosensing ability of 1a, 6a,
and 13a towards Zn2+ metal ions in DMSO. Figure 5a shows the
UV-Vis titration experiment of 6a upon the addition of Zn2+ ions.
As the concentration of Zn(acetate)2 increased from 0 to 3
equivalents, the absorption band at 378 nm exhibited a 31 nm
bathochromic shift to 409 nm with isosbestic points at 349 and
405 nm. SA compounds 13a and 1a also displayed a large 47
nm bathochromic shift upon the addition of Zn(acetate)2 (Figures
6a and S13(i)). 13a displayed isobestic points at 333 and 391 nm,
while 1a exhibited isobestic points at 318 and 386 nm. The
absorption bands in the high energy region (λmax = 310-340 nm)
correspond to the ligand centered π-π* transition, whereas the
absorption band that red shifts (λmax = 405-420 nm) with respect
to the free SA ligand arises from either metal-to-ligand charge
transfer (MLCT) or ligand-to-metal charge transfer (LMCT)
transitions.31 The Job plots indicate that all three compounds
exhibit similar modes of binding in which the Zn2+ metal ions form
a 1:2 complex with the SA ligands (Figure S28). Non-linear curve
fitting of the UV-Vis titration data in DMSO revealed binding
Figure 4. fsTA of (a) 1a, (b) 6a and (c) 13a taken in THF (λex= 400 nm,
fwhm = 170 fs; 2.0 µJ).
The femtosecond transient absorption (fsTA) spectra of 1a and
13a in THF exhibit positive signals with maxima at 440 nm and
500 nm, respectively (λex = 400 nm) (Figures 4a and 4c). The
features decay monoexponentially with τ = 16 ps for 1a and τ =
14 ps for 13a, respectively. These excited state signals decay to
broad, long-lived absorption bands that remain present for the
duration of the experiment (>3 ns). Nanosecond transient
absorption spectroscopy on 1a and 13a reveals the persistence
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Figure 5. UV-Vis titration spectra of 6a with (a) Zn(acetate)2, (b) Cu(acetate)2 and (c) FeCl3 in DMSO. Fluorescence titration spectra of 6a with (d) Zn(acetate)2,
(e) Cu(acetate)2 and (f) FeCl3 in DMSO (λex= 370 nm). [6a] = 10 µM and [Mn+]= 0 to 30 µM.
Figure 6. UV-Vis titration spectra of 13a with (a) Zn(acetate)2, (b) Cu(acetate)2 and (c) FeCl3 in DMSO. Fluorescence titration spectra of 13a with (d) Zn(acetate)2,
(e) Cu(acetate)2 and (f) FeCl3 in DMSO (λex= 360 nm). [13a] = 10 µM and [Mn+]= 0 to 30 µM.
constant (Ka) values ranging from 1.38 x 104 M-1 to 4.6 x 104 M-1 Zn2+ ions over the SA ligands (Figure S16 in the SI). Similar
(Table S3, SI). Interestingly, the emission spectra of 1a, 6a, and fluorescence ‘turn-on-off-on’ events were also observed for 13a
13a exhibited an enhancement in fluorescence upon the addition and 1a upon the addition of equimolar amounts of EDTA and
of 0 to 3 equivalents of Zn2+ in DMSO (Figures S13(iv), 5d and Zn(acetate)2 (Figure S17 and S18 in SI).The addition of
6d, respectively).
increasing equivalents of Zn2+ to BA analogs 1b, 6b, and 13b in
DMSO did not produce any significant optical responses in their
UV-Vis and emission spectra. (Figures S9, S11 and S14).
It should be noted that the 10 µM solutions of 1a, 6a, and 13a
are weakly emissive in DMSO indicating that all three
compounds could potentially function as a turn on sensor for Zn2+
After examining the chemosensing properties of the 1a, 6a, and
ions in solution.29 The addition of ethylenediaminetetraacetic 13a towards Zn2+ ions in DMSO, we were interested in
(EDTA) to a DMSO solution containing a 6a-Zn2+ complex investigating their optical response in the presence of Cu2+, Fe3+,
immediately turns off the fluorescence and restores the original Hg2+, Ag+, and Li+ metal ions. Figure 5b, 6b, and S13(ii) shows
emission profile due to the stronger binding affinity of EDTA for the UV-Vis titration spectra of 1a, 6a, and 13a upon the addition
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of Cu(acetate)2, respectively. The absorption spectra is very
similar to the addition of Zn(acetate)2 in DMSO producing
absorption bands ranging from 356 to 378 nm and exhibiting 33-
49 nm bathochromic shifts upon the addition of 0 to 3 equivalents
of Cu(acetate)2. The Job plots indicate the formation a 3:1
binding complex of the SA ligands with Cu2+ (Figure S29). Non-
linear curve fitting of the UV-Vis titration data afforded binding
constant (Ka) values ranging from 2.5 x 104 M-1 to 3.0 x 104 M-1
(Table S3, SI). In contrast to the addition of Zn(acetate)2, 6a, and
13a displayed large hypsochromic shifts of ~ 58 nm and 78 nm
from 535 nm to 477 nm, and 545 nm to 467 nm, respectively,
upon the addition of 0 to 3 equivalents of Cu(acetate)2 in DMSO
(Figures 5e & 6e). The emission bands are low in intensity and
the fluorescence appears to be virtually quenched. In
comparison, 1a exhibited not only a similar 62 nm hysochromic
shift from 535 nm to 473 nm, but also an enhancement in
fluorescence (Figure S13(v)). The reason for such enhancement
is not well understood at the moment. Titration experiments of 1a,
6a, and 13a with increasing equivalents of FeCl3 in DMSO did
not show any significant changes in their absorption (Figures 5c,
6c and S13(iii)). However, the E* emission band disappears as a
result of the interaction of Fe3+ with the enol of the SA
compounds (Figures 5f, 6f and S13(vi)). Similar behavior was
observed in the absorption spectra upon the addition of
increasing equivalents of Hg2+, Ag+ and Li+ ions in DMSO
(Figures S10, S12 and S15). The addition of an excess of Hg2+,
Ag+, and Li+ (30 – 60 equivalents) in DMSO resulted in the
disappearance of the E* emission band which suggests weak
binding interactions with the enol moiety of the SA compounds
(Figures S10, S12, and S15). However, these interactions did not
lead to any noticeable changes in fluorescence intensity.
Figure 8. (a) (Kubelka-Munk function) diffuse-reflectance spectra, and (b)
normalized fluorescence emission spectra of 1a, 6a and 13a in solid state.
DMSO. Compound 6a is also considerably more red-shifted than
1a and 13a in the solid-state.
In an effort to fully understand the drastic differences of their
fluorescent properties in solid-state, we attempted to grow single
crystals of 6a and 13a. The crystal structure of 1a has been
previously reported by Banerjee and coworkers revealing the
presence of preformed C=N---H-O bonds for all the three SA
units.[20] We observed a very similar C=N---H-O intramolecular
hydrogen bonded motif in the crystal structure of 13a (Figure
9).[30] Interestingly, the H-bond lengths for the three SA units vary
slightly (1.66(4) Å, 1.60(4) Å and 1.56(4) Å), and the dihedral
angles between the central benzene and the arylethynyl arms
were also unequal (40.35O, 14.68O and 9.31O) on account of the
twisted geometry of the phenyl rings. The existence of the
preformed intramolecular hydrogen bonds provides further
evidence for the ESIPT process in the solid state. Attempts to
grow crystals 6a were not successful thanks to its limited
solubility in organic solvents. However, we believe the restricted
rotation of the tristyrylbenzene arms of 6a leads to enhanced π-
conjuagtion for the coplanar structures in the solid-state, which
produces the observed red-shifted emission. Since the phenyl
rings of 1a and 13a have the ability to twist in the solid-state, the
fluorescent properties of their coplanar structures are expected to
Figure 7. Photographs of 1a, 6a and 13a taken under UV illumination (λex
365 nm).
=
In comparison to their low quantum yields in DMSO, 1a, 6a, and
13a are highly fluorescent in the solid-state (Figure 7). The BA
analogs 1b, 6d, and 13b are not fluorescent in the solid-state due
to intermolecular vibronic interactions which can prompt various
nonradiative deactivation processes (Figures S19, S20, S21).[27]
However, the highly fluorescent nature of their SA counterparts is
mainly attributed to their ESIPT products. UV-Vis diffuse
reflectance spectra (Figure 8a) shows 1a, 6a, and 13a absorb
within 400-540 nm range, which is considerably more red-shifted
compared to their absorption maximums in polar aprotic and
nonpolar solvents. The solid-state emission spectra of 13a and
1a (Figure 8b), exhibits λmax values of 555 and 542 nm,
respectively, which is slightly red-shifted by 7-10 nm from their K*
emission maximums in DMSO. However, the solid-state emission
maxima (λmax = 577 nm) of 6a (Figure 8b) displayed a large 42
nm bathochromic shift compared to it’s K* emission at 535 nm in
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minimize the extended conjugation of the system and generate a
hypsochromic shift in their solid-state emission spectra.
After examining the ESIPT properties of 1a, 6a, and 13a in the
solid-state, we were curious to determine if all three compounds
would exhibit enhanced AIE properties in solution. AIE is
attributed to a combination of intra- and intermolecular effects,
which are often associated with chromophore aggregation.[28] The
intramolecular effects on enhanced fluorescence intensity is
correlated with the suppression of the molecular rotation and
conformational changes of chromophores in the solid-state, while
the intermolecular effects can be attributed to the formation of
head-to-tail J-type aggregates in the solid state. In effort to
investigate this phenomenon, we first prepared 10 µM solutions
of 1a, 6a, and 13a in DMSO and gradually increased the amount
of H2O content to induce aggregation until the ratio of DMSO:H2O
reached 3:1 (Figure 10 and Figure S22, SI). The emission
spectrum of 6a in DMSO and DMSO/water mixtures is shown in
Figure 10a. Compound 6a exhibits a weak green emission in
DMSO. However, upon the addition of up 25% water, the green
emission gradually changes to a bright yellow emission while
concomitantly undergoing a 5-fold enhancement in fluorescence
intensity along with a bathochromic shift of ~ 13 nm. The
disappearance of E* emission band of 6a suggests that the
addition of water promotes a conformational isomerization to
attain an entropically favorable geometry involving the C=N---H-O
bond formation as a result of aggregation. In addition, K*
emission band reveals that the rotation around the single bonds
is restricted in DMSO/water mixtures by inducing the formation of
J-type aggregates. Similar results were observed for 1a (Figure
S22(ii)) and 13a (Figure 10b), which implies that the 1,3,5-
triarylbenzene, and -tris(arylethynyl)benzene core units have no
significant effect on their AIE properties.
Figure 9. Crystal structure (ORTEP diagram) of 13a (a) Top view along the
central axis, and (b) Side view along the molecule axis. Molecular formula
C51H33N3O3, Space group – Pbca, a = 34.252(7) Å, b = 7.6199(15) Å, c =
Z =8, 35745 reflections collected, 5984
independent reflections, R1 = 0.1361, wR2 = 0.3000.
35.197(7) Å, V = 9186(3) Å3,
Conclusions
In conclusion, we have synthesized and evaluated the ESIPT
properties of three C3 symmetric tris(N-salicylideneaniline)
compounds containing 1,3,5-triarylbenzene, -tristyrylbenzene and
-tris(arylethynyl)benzene core units. The introduction of alkene
and alkyne linker units between the central benzene and the SA
units has a considerable effect on their absorbance spectra, but a
minimal effect on their ESIPT properties in solution. We have
shown that all three compounds could potentially function as a
turn on sensor for Zn2+ ions in solution. In addition, all three
compounds are highly emissive in the solid-state, and exhibit
strong AIE characteristics in DMSO/water mixtures. We believe
these tris(N-salicylideneaniline) based materials, could serve as
useful monomers for the construction of functional 2D
polymers,[18] liquid crystalline materials,[19] or novel self-
assembled AIE structures.[17] Such explorations could be
beneficial for discovering other novel ESIPT systems with unique
optical properties.
Figure 10. Emission spectra and photographs of (a) 6a (λex= 370 nm, [6a] = 10
µM) and (b) 13a (λex= 360 nm, [13a] = 10 µM) taken in different DMSO-water
mixtures.
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Experimental Section
[10]
[11]
[12]
See supporting information (SI) for experimental details.
Acknowledgements
P.L.M acknowledges the National Science Foundation (NSF) and
Georgia Tech Facilitating Academic Careers in Engineering and
Science (GT-FACES) for a Career Initiation Grant, and funding
from The Ohio State University. C.T. thanks the NSF (CHE-
1465067) and the use of the Center for Chemical and Biophysical
Dynamics (CCBD) at OSU for ultrafast measurements.
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Keywords: conjugation • fluorescence • metal sensing • proton
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Pradeep Jagadesan, Tyler Whittemore,
Toni Beirl, Claudia Turro, Psaras L.
McGrier*
Page No. – Page No.
The Excited State Intramolecular
Proton Transfer Properties of Three
Tris(N-Salicylideneaniline)-Based
Text for Table of Contents
Chromophores
Conjugation
with
Extended
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