New excited state intramolecular proton transfer (ESIPT) dyes based on naphthalimide and observation of long-lived triplet excited states

Article abstract of DOI:10.1039/c2cc35210a

A new excited state intramolecular proton transfer chromophore of naphthalimide (NI) conjugated 2-(2-hydroxyphenyl) benzothiazole (HBT) was prepared which shows red shifted absorption and long-lived triplet excited states. The Royal Society of Chemistry 2

Full text of DOI:10.1039/c2cc35210a

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New excited state intramolecular proton transfer (ESIPT) dyes based on
naphthalimide and observation of long-lived triplet excited statesw
Jie Ma,ab Jianzhang Zhao,* Pei Yang, Dandan Huang, Caishun Zhang and Qiuting Li
a
a
a
a
Received 19th July 2012, Accepted 9th August 2012
DOI: 10.1039/c2cc35210a
A new excited state intramolecular proton transfer chromophore of
naphthalimide (NI) conjugated 2-(2-hydroxyphenyl) benzothiazole
On the other hand, naphthalimide (NI) is a versatile chromo-
phore and has been widely used in fluorescent molecular probes
35
(
HBT) was prepared which shows red shifted absorption and
and molecular arrays. But NI was never used for ESIPT. Herein
we prepared new HBT derivatives with NI substituents (N-1–N-4,
Scheme 1). The compounds show strong absorption of visible
light. ESIPT was observed for N-1, which is different from the
long-lived triplet excited states.
Excited state intramolecular proton transfer (ESIPT) dyes
have attracted much attention due to the applications in
3
4
Bodipy-derived HBT. Furthermore, a long-lived triplet excited
state was also observed for the compounds. The present molecular
structural motif may be useful for design of new luminescent
1
–7
8–11
luminescence materials,
molecular probes,
molecular
ESIPT dyes
1
logic gates, biological imaging and labeling.
2
13–23
24
ESIPT molecular probes or triplet photosensitizers.
usually show large Stokes shifts and low fluorescence quantum
yields, which are different from the ordinary fluorophores.
Furthermore, the fluorescence of ESIPT dyes is usually due to
the keto form, which is not responsible for the steady state UV-vis
The starting material for the synthesis is 5-bromosalicyl-
aldehyde (see ESIw for details). A CRC linker was introduced
with Pd(0) catalyzed Sonogashira coupling to ensure efficient
electronic communication between the HBT and NI parts. N-2
and N-3 were prepared as controls to investigate the effect of a
HBT unit and alkylation of the phenolic group on the photo-
physical properties. The p-conjugation of N-4 is furthermore
extended with a carbazole moiety, to access longer absorption
wavelength. All the compounds were obtained in moderate to
satisfactory yields.
1–5,24
absorption.
The well-known ESIPT dyes are 2-(2-hydroxy-
phenyl) benzothiazole (HBT) and 2-(2-hydroxyphenyl) benzoxa-
2
5
zole (HBO). However, these dyes give weak absorption in the
visible range, which is a disadvantage for the applications with
visible light excitation. Derivatization of HBT and HBO
6
was reported, such as with 7-hydroxy-1-indanone, 2-phenyl-
28
1
7,20,26,27
phenol,
1,3-bis(imino)isoindole,
diphenyl-amino-
29
The absorption of the compounds is red-shifted compared
fluorene. However, these derivatives usually show absorption
1
1
to HBT (Fig. 1). N-1 gives absorption at 385 nm (e =
in the UV range.
À1 À1
4 000 M cm ), in comparison HBT gives maximal absorp-
3
A straightforward method to access red-shifted absorption/
À1
À1
tion at 337 nm (e = 20 100 M cm ). Alkylation of –OH
emission is to extend the p-conjugation of the chromo-
3
0,31
phore.
derivatives with a large p-conjugation framework is rare.
ESIPT dyes with naphthalene units or electron acceptors on
To our surprise, the report of HBT and HBO
2
9
2
5,32
the benzothiazole part were prepared,
2
is in the UV range.
but the absorption
5,32,33
Recently we reported HBT deriva-
tives with boron-dipyrromethene (Bodipy) substituents, which
gave much longer absorption/emission wavelength than HBT
3
and HBO, but no ESIPT was found for the new derivatives.
4
Therefore, much room is left for revealing the elusive photo-
physics of ESIPT chromophores with a large p-conjugation
framework.
2
4
a
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology, Dalian 116024,
P. R. China. E-mail: zhaojzh@dlut.edu.cn,
Web: http://finechem.dlut.edu.cn/photochem
College of Chemistry and Chemical Engineering, Qiqihaer
University, Qiqihaer 161006, Heilongjiang, P. R. China
b
w Electronic supplementary information (ESI) available: Synthesis
details, characterization data and more spectra. See DOI: 10.1039/
c2cc35210a
Scheme 1 Molecular structures of N-1, N-2, N-3 and N-4. The model
compound HBT is also presented.
9
720 Chem. Commun., 2012, 48, 9720–9722
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a
Table 1 Photophysical properties of N-1, N-2, N-3, N-4 and HBT
f
t
(ms)
b
c
d
e
l
Solvents (nm) (M cm
abs
e
l
em
(nm)
F
F /
%
À1
À1
)
Air
Ar
N-1
PhCH
THF
MeOH
3
405
384
396
374
371
386
386
392
433
429
428
337
335
332
30 400
35 300
32 500
23 400
26 100
17 200
17 600
18 000
18 300
20 400
19 800
20 100
21 200
20 800
545
592
—
12.8 31.0
63.0
—
g
g
9.2
—
—
g
g
124.5 118.2
g
g
h
g
h
h
g
g
g
g
N-2
N-3
PhCH
THF
3
414
425
440
476
540
491
553
41.2
51.9
54.4
51.4
—
—
—
—
—
—
—
—
—
—
PhCH
THF
3
74.5
—
g
g
—
h
MeOH
—
—
—
—
h
N-4
PhCH
THF
3
40.1
16.7
g
g
—
g
g
MeOH
HBT PhCH
—
0.5
3
514
363
373
10.9
—
18.6
—
h
g
g
THF
MeOH
—
—
g
9.7
10.9
a
The excitation wavelength for N-1, N-2, N-3, N-4 and HBT was 370 nm,
À5
3
b
50 nm, 370 nm, 410 nm, 315 nm, respectively. 1.0 Â 10 M, 20 1C.
c
d
Absorption wavelength. Molar extinction coefficient. Fluorescence
e
emission wavelength. Fluorescence quantum yields determined with
Fig. 2 (a) Fluorescence emission spectra of N-1 (lex = 370 nm).
f
Bodipy (F = 72% in THF) as the standard. Lifetimes determined with
the transient absorption spectroscopy. Not determined. The signal was
too weak.
(
b) N-3 (lex = 370 nm). (c) N-4 (lex = 410 nm). (d) HBT (lex = 315 nm)
g
h
À5
in different solutions. c = 1.0 Â 10 M. 20 1C.
This is reasonable since ESIPT is completely prohibited in N-3
by alkylation of –OH (Fig. 2b). Similarly, the emission of N-4
indicated the absence of ESIPT, which is supported by the lack
of transient absorption (Fig. 2c). Dual emission was observed
for HBT in THF, acetonitrile, which is an indication of ESIPT
(
Fig. 2d). The calculated free energy changes for the support
3
4
that N-1 is more likely to undergo ESIPT (Fig. S27, ESIw).
The nanosecond time-resolved transient difference absorp-
tion spectra of N-1 were studied (Fig. 3a). Upon pulsed laser
excitation, bleaching at 380 nm was observed. Furthermore,
transient absorption (TA) at 328 nm and in the 400–750 nm
range was observed. The lifetime of the transient was determined
to be 63.0 ms.
Fig. 1 UV-vis absorption of N-1, N-2, N-3, N-4 and HBT. c = 1.0 Â
À5
10
M in toluene. 20 1C.
reduces the absorption intensity, but not the wavelength (N-1
and N-3). N-2 gives slightly blue-shifted absorption vs. N-1 and
À1
À1
N-3. N-4 gives absorption at 433 nm (e = 18300 M cm
Table 1). The absorption of these compounds in the visible
)
(
region is mainly attributed to the NI parts. Very few ESIPT
4
dyes give absorption in the visible range. Previously a HBT
derivative with a diphenylaminofluorene substituent was reported
2
9
to show similar absorption. Recently we reported Bodipy-
grafted HBT with strong absorption in the visible range, but the
34
compounds did not show any ESIPT effect.
N-1 gives strong emission in toluene (FF = 12.8%). The
emission intensity is reduced in polar solvents, such as CH Cl
2
2
and THF (Fig. 2a). Interestingly, dual emission was observed
for N-1 in THF and ethyl acetate, which is characteristic of
ESIPT. The enol form gives emission at shorter wavelength
3
2
and the keto form gives emission at longer wavelength. The
emission of N-1 is completely quenched in methanol, due to
the significant intramolecular charge transfer character (ICT).
Fig. 3 Nanosecond time-resolved transient difference absorption
spectra of N-1. (a) In deaerated benzene and (b) in aerated benzene,
À5
after pulsed laser excitation (lex = 355 nm). c = 2.0 Â 10 M. 20 1C.
3
2
This result is similar to a HBO derivative with an ICT effect.
DFT calculated absorption spectra of (c) the triplet state of N-1 and
(d) the Z-keto form of N-1. Calculated by TDDFT at the B3LYP/6-31G(d)
using Gaussian 09W.
Different from N-1, the quenching of N-3 fluorescence in polar
solvents is not significant, and no dual emission was observed.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9720–9722 9721

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Interestingly, the transient was not completely quenched
in aerated solution (Fig. 3b), thus the long-lived transient
observed in aerated solution can be attributed to the keto
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5
form (i.e. the product of ESIPT). We propose that the triplet
state feature is more significant for the TA measured in
deaerated solution (Fig. 3a), whereas the keto-form based
absorption is dominant in the TA spectra measured in aerated
solution (Fig. 3b). This postulation is supported by the DFT
calculations on the TA spectra (Fig. 3c and d). The TA of the
previously reported Bodipy-conjugated HBT was completely
quenched in aerated solution, which indicated the triplet
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The TA of N-1 in methanol was also studied. We found a
long-lived transient species (t = 118.2 ms), which was not
quenched in aerated solution (see ESIw, Fig. S20 and S21),
1
1
2
5
thus the transient was attributed to the keto form of N-1, i.e.
the product of the ESIPT of N-1. Similarly HBT was studied
with the TA spectra. ESIPT was confirmed in both benzene
and methanol solution (see ESIw, Fig. S22ÀS25).
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For N-3, transient with t
T
of 74.5 ms was observed in
1
1
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(
see ESIw, Fig. S26). Therefore the transient of N-3 is due to
the triplet excited state. This observation is in agreement with
the results of the emission of N-3, as well as the molecular
structure of N-3 (–OH group is alkylated). For N-4, no
transient was observed, indicating the absence of ESIPT.
In summary, naphthalimide conjugated-2-(2-hydroxyphenyl)
benzothiazole (HBT) compounds were prepared as a new excited
state intramolecular proton transfer (ESIPT) chromophore. These
compounds have a larger p-conjugation framework and show
strong red-shifted absorption vs. HBT. ESIPT was observed for
the new compound. This result is in contrast to the previously
reported Bodipy-conjugated HBT derivatives (for which no
ESIPT was observed) and demonstrated the elusive properties of
the HBT derivatives with a large p-conjugation framework, e.g.
the unpredictable ESIPT property of these derivatives. Further-
more, a long-lived triplet excited state was populated upon
photooxidation of the compounds. The new ESIPT chromophore
can be used in luminescent molecular probes, and will inspire more
investigations into this fascinating area.
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We thank the NSFC (20972024 and 21073028), Royal
Society (UK) and NSFC (China-UK Cost-Share program-
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This journal is c The Royal Society of Chemistry 2012