Two-photon absorption enhancement induced by aggregation due to intermolecular hydrogen bonding in V-shaped 2-hydroxypyrimidine derivatives

Article abstract of DOI:10.1039/b718147g

The aggregation of V-shaped hydroxypyrimidine derivatives is formed through an intermolecular hydrogen bond in chloroform that cannot form a hydrogen bond with the solute, giving rise to a large enhancement of the two-photon absorption and two-photon induced fluorescence. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718147g

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Two-photon absorption enhancement induced by aggregation due to
intermolecular hydrogen bonding in V-shaped 2-hydroxypyrimidine
derivativesw
Zijun Liu,a Pin Shao,a Zhenli Huang,*b Bo Liu,b Tao Chena and Jingui Qin*a
Received (in Cambridge, UK) 23rd November 2007, Accepted 22nd January 2008
First published as an Advance Article on the web 11th March 2008
DOI: 10.1039/b718147g
The aggregation of V-shaped hydroxypyrimidine derivatives is
formed through an intermolecular hydrogen bond in chloroform
that cannot form a hydrogen bond with the solute, giving rise to a
large enhancement of the two-photon absorption and two-photon
induced fluorescence.
cross-section in the J-aggregation of pseudoisocyanine was also
observed in water by Belfield.9 This enhancement is attributed to
the electronic cooperative effect in both cases. Recently, Prasad
has reported aggregation-enhanced fluorescence and two-
photon absorption in nanoaggregates due to the hindering of
molecular internal rotaion.10 All these interesting results are of
scientific significance and technical potential.
The promising applications of organic two-photon absorption
(TPA) materials in optical limiting, two-photon laser scanning
fluorescence imaging, microfabrication, 3-dimensional optical
data storage, photodynamic therapy and so on,1 have at-
tracted considerable attention in the last decade and stimu-
lated substantial research on structure–property relationships.
Although it is still under exploration, thanks to the effort of
scientists, some efficient molecular design strategies were put
forward to provide guidelines for the development of materials
with large TPA cross-sections, including the synthesis of
D–p–D molecules with symmetric intramolecular charge
transfer,2 D–p–A molecules with asymmetric intramolecular
charge transfer3 and muti-dimensional molecules.4 In these
strategies, adjusting the action of molecular p-electron
through changing the molecular substituents and/or structure
is the key to obtain large TPA cross-section.
In this communication, we wish to report aggregation-
induced enhancement of TPA and two-photon induced fluor-
escence (TPIF). To the best of our knowledge, this is the first
example of TPA and TPIF enhancement induced by aggrega-
tion due to hydrogen bonding. The materials of interest in this
work are V-shaped noncentrosymmetric D–p–A–p–D mole-
cules, where D is an electron-donating, A an electron-accept-
ing group and p a conjugating moiety (Scheme 1).
Both the UV-vis maximum absorption (lambax) of 1a and 1b
(at a concentration of about 1 Â 10À5 M) is red-shifted about
40 nm, respectively when the solvent of THF is replaced by
ab
CHCl3. This implies a strong dependence of l
solvent in the solution of 1a and 1b (Fig. 1).
on the
max
To check whether the shift is caused by solvatochromism,11
we measure the UV-vis absorption of 1a in a variety of solvents
ab
of 1a with the
max
(at B1 Â 10À5 M). The relationship of l
Molecular interaction is another efficient way to influence the
action of molecular p-electron and might induce the large TPA
cross-section.5 In contrast to the research on molecular design
strategy, little attention has been put into the influence of the
molecular interaction on TPA property. Aggregation is one of
the most common phenomenons of molecular interaction. It is
important to understand the influence of aggregation on the
molecular TPA property whether in research or in practical
applications. Enhancement of TPA in molecular aggregates has
been theoretically reported,6 but experimental confirmation of
aggregation-induced enhancement of TPA appeared only in the
last two or three years. A strong enhancement of the TPA cross-
section of porphyrin systems (symmetric7 or asymmetric8) form-
ing J-aggregates was reported, and an enhancement of the TPA
Snyder’s polarity index of solvent is illustrated in Fig. 2, where
two separate straight lines are drawn. In each line, lambax increases
with the increasing polarity of the solvents, implying the effect of
solvatochromism. Obviously, however, the relationship between
ab
the polarity of solvent and l
is not monotonic. The longest
max
lambax does not appear in the most polar solvent (DMSO), but in
CH2Cl2 and CHCl3, and the shortest lambax is not in CCl4, which
has the weakest polarity. So there must be another important
factor to affect the lambax of 1a in different solvents. After careful
examination of the solvent members in the individual line, it is
found that the solvents can be divided to two groups. One group
of solvents with higher ratio of lambax/polarity contains CCl4,
toluene, CH2Cl2 and CHCl3, and all are not hydrogen bonding
forming solvents, generally speaking. On the other hand, the
other group with lower ratio of lambax/polarity consists of ether,
THF, ethyl ester, acetone, DMF and DMSO, all of them are
a Hubei Key Laboratory on Organic and Polymeric Opto-electronic
Materials, Department of Chemistry, Wuhan University, Wuhan
430072, China. E-mail: jgqin@whu.edu.cn; Fax: +86-27-68756757;
Tel: +86-27-68756757
b Key Laboratory of Biomedical Photonics of Ministry of Education,
Huazhong University of Science and Technology, Wuhan, 430074,
China
w Electronic supplementary information (ESI) available: Preparation,
characterization and the single-photon spectra of the four compounds;
chemical shift of the hydroxyl in 1a and 1b vs. concentration; details of
TPA measurement. See DOI: 10.1039/b718147g
Scheme 1 The molecular structure of 1a, 1b, 2a and 2b.
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Scheme 2 The hydrogen bonding of 1a in CHCl3 and THF.
tions. Neither 2a nor 2b can form hydrogen bonds to each
other or to an aprotic solvent since the hydroxyl has been
replaced by alkoxyl. Unlike the large shift of lambax of 1a and 1b
ab
in different solvents, the l
of 2a and 2b have a very small
Fig. 1 The UV-vis absorption spectra of 1a and 1b in CHCl3 and THF.
max
red shift when the solvent is changed from THF to chloro-
form. This confirms further that the intermolecular hydrogen
bonding has an important influence on the UV-vis spectra of
1a and 1b in solution. It seems possible that the hydrogen
bonding may have favored the formation of molecular p–p
stacking among conjugated molecules, so as to promote the
large difference of the UV-vis spectra of 1a and 1b in solution.
All the four compounds display good single-photon fluor-
escence in both CHCl3 and THF. The single photon spectro-
scopic properties are listed in Table 1. The change trend of
fluorescent maximum (lemmax) is similar to UV-vis absorption.
The fluorescent quantum yields (F) of 1a or 1b in CHCl3 and
THF are almost the same, indicating that the effect of aggre-
gation on the intensity of fluorescent emission is slight.
hydrogen bond acceptors and can form strong hydrogen bonds
with hydroxyl in the compound 1a.
Therefore, it seems reasonable to propose that in the first
group of solvents, the solute 1a form aggregates due to strong
intermolecular hydrogen bonds so as to show the large red
shift of lambax, while in the second group of solvents, the solvent
can offer oxygen to form hydrogen bond with the hydroxyl of
1a and prevent the solute self-aggregation (Scheme 2).
The proposal is confirmed by the NMR spectroscopy. The
signals of the OH in the 1H NMR spectra of 1a (5.9 Â 10À3 M)
and 1b (6.2 Â 10À3 M) in CDCl3 appears at 12.37 and 13.01 ppm,
respectively. Both signals upfield shift continuously upon gradual
dilution and appear at a position close to 10 ppm (B5.0 Â 10À4
M) (ESIw). This indicates that there are strong intermolecular
hydrogen bonds in 1a or 1b in CDCl3.12 No obvious chemical
shift of OH (at 10.78 ppm) is observed in the 1H NMR spectra of
1a in THF-d8 when the concentration is diluted from 7.4 Â 10À3
to 2.9 Â 10À3 M, implying strong hydrogen bonds between THF
and 1a. The dependence of the UV-vis absorption on the
concentration of 1a in CHCl3 also suggests that the intermole-
Two-photon absorption properties of the four compounds
are measured between 720–930 nm in both CHCl3 and THF
by TPIF technique with a mode-locked Ti:sapphire laser
(100 fs, 80 MHz).13 The reason for choosing CHCl3 and
THF as solvents is that they have a similar polarity so as to
exclude the effect of the solvent polarity on the TPA property.5
All the TPA measurement of these compounds are performed
with a concentration of about 1.0 Â 10À4 M. The output
intensity of TPIF of 1a in CHCl3 vs. the square of the input
laser (850 nm) intensity has been calculated and the linear
dependence indicates the occurrence of nonlinear absorption
(See ESIw). The TPA spectra of the four compounds are
shown in Fig. 3 and summarized in Table 2.
cular hydrogen bonding in aggregates is very strong since no
ab
obvious change of l
is observed when the concentration is
max
reduced gradually from 6.3 Â 10À5 to 2.4 Â 10À7 M. On the other
hand, when THF is added gradually into the solution of 1a in
ab
CHCl3, the l
is blue-shifted continuously, implying that the
max
addition of THF could suppress the self-aggregation of 1a (see
ESIw). Both the results indicate that the self-aggregation exists in
1a in CHCl3 at the concentration of 6.3 Â 10À5 M.
For 1a and 1b, the maximum TPA cross-section (s) values are
located at 820 nm in THF, but in CHCl3 it appears at 920 nm for
1a and 930 nm for 1b. The large red shift is consistent with that
observed in the UV-vis absorption spectra. However, for 2a and
2b, the maximum values are at 810 nm in CHCl3 and at 800 nm in
THF, which is also consistent with them having similar values of
lambax. Each of the maximum s values of 1a and 1b in CHCl3 are
more than two times larger than that in THF. However, for 2a
and 2b, only a negligible change exists in different solvents, and
the smax of 1a and 1b in CHCl3 are threefold as large as those of
2a and 2b in CHCl3 or THF. This indicates that the aggregation
of 1a and 1b has enhanced their two-photon absorption largely.
Both 1a and 1b have two TPA bands in CHCl3, but one in
THF. Since the optimal wavelength used for two-photon excita-
tion is roughly twice the linear absorption maximum wavelength,2
the TPA band of 1a and 1b in CHCl3 at a wavelength longer than
900 nm should be attributed to the lowest two-photon state of
aggregates. The source of the band at less than 900 nm is
complicated, although it appears at 850 nm the main contribution
To further examine the above explanation of the aggrega-
tion of 1a and 1b, we measured the UV-vis spectra of 2a and 2b
in chloroform and THF, respectively, under the same condi-
ab
Fig. 2 The l
of 1a against the Snyder’s polarity of the solvents.
max
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Table 1 The single photon optical property of the four compounds
photon excited action cross-section (dmax) increases by about
3-fold in aggregates in comparison with the monomer.
lambax/nm
lemmax/nm
Fa
In conclusion, the aggregation of V-shaped molecules contain-
ing 2-hydroxypyrimidine has been formed due to the intermole-
cular hydrogen bonding in the solvents that cannot form
hydrogen bonding with the solute as evidenced by UV-vis
CHCl3
THF
CHCl3
THF
CHCl3
THF
1a
1b
2a
2b
a
491
480
435
432
449
436
431
430
585
595
515
521
550
536
516
524
0.30
0.39
0.45
0.67
0.27
0.33
0.46
0.75
1
absorption and H NMR. And an aggreagation-enhanced two-
photon absorption has been observed in CHCl3. As a result, an
emission enhancement of two-photon excited fluorescence arises.
This provides some important and new informations for potential
application based on two-photon absorption.
F is the fluorescence quantum yield (reference: fluorescein pH = 11,
F = 0.9).
This work was supported by the National Natural Science
Foundation of China (No. 90201002 and 50673076).
Notes and references
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2262 | Chem. Commun., 2008, 2260–2262