4
11
Table 1. Photophysical data for M and P
Compound
abs/nm
Amaxa
emb/nm
Φfc
M
P
391
394
0.636
0.53
467
470
0.382
0.713
aMaximum absorbance. Emission maximum, excited at
b
c
3
91 nm for M and 394 nm for P. Fluorescence quantum yield.
Table 2. Photophysical properties of M and P in solvents with
various polarities
abs
emb
/nm /arb unit
Id
Compound Solvents
Amaxa
/
nm
Figure 1. The absorption and fluorescence spectra of M
¹6
¹1
M
Toluene
THF
391 0.595 444
391 0.636 467
2584
4761
3575
5152
2665
(solid) and P (dash dotted) in THF (8.24 © 10 g mL ).
Chloroform 395 0.641 477
Acetone
Ethanol
for 24 h, the solvent was removed, and the crude product was
purified by column chromatography over silica gel, using
petroleum ether/ethyl acetate (2/1) as the eluent, to give a
yellow solid. The procedure for the syntheses of P was similar to
that for M; the crude polymers were washed with ethanol,
methanol, and acetone. P dissolved easily in common organic
solvents such as chloroform and THF. The structures of M and P
391 0.584 490
397 0.585 517
P
Toluene
THF
Chloroform 395 0.53
Acetone
392 0.468 453
394 0.527 470
6537
8317
5981
7660
2876
487
1
1
were confirmed by H NMR analysis. In the H NMR spectrum
of P, residual aldehyde groups can hardly be observed, which
indicates that polymerization occurred. GPC analysis indicates
that the number-average molecular weight (Mn) of polymer P is
392 0.487 492
395 0.433 528
Ethanol
aMaximum absorbance. Emission maximum, excited at
91 nm for M and 394 nm for P. Fluorescence intensity.
b
d
3
3901 with a polydispersity index (PDI) of 2.316. The broad
peak in the XRD pattern of the powdery polymer P shows
low crystallinity, indicating no strong ³³ stacking between
polymer molecules; this explains the good solubility of polymer
implying that they have little dipole moments in the ground
state. However, the emission peak wavelengths red-shift
significantly as the solvent polarity increases. For example, the
emission peak positions are 444 nm for M and 453 nm for P in
toluene, and these are shifted to 517 and 528 nm, respectively, in
ethanol. These compounds become more dipolar upon excitation
1
P. All the spectra, including H NMR, GPC, and XRD patterns
1
6
of polymer P, can be found in the Supporting Information.
Both M and P are easily soluble in common solvents.
Additionally, M and P were tested at the same mass concen-
¹
6
¹1
14
tration (8.24 © 10 g mL ). The absorption and fluorescence
spectra of M and P in THF are shown in Figure 1. The
excitation wavelengths for the fluorescence spectra are 391 nm
for M and 394 nm for P. Their photophysical data are summa-
rized in Table 1. As observed in Figure 1 and Table 1, although
P is a much larger conjugated molecule than M, both its
absorption and emission peak wavelengths are only slightly
longer (3 nm) than those of M. This observation implies that not
all of the constituent M moieties in polymer P take part in
conjugation, which may be attributed to the presence of large
dihedral angles between some conjugated units. This supposition
is in agreement with the good solubility of P, which implies no
strong ³³ stacking between polymers.
because of intramolecular charge-transfer (ICT). Consequent-
ly, the interactions between the excited molecules and solvents
become stronger. The larger the solvent polarity is, the stronger
the solvation of the excited molecule becomes, and the lower
the energy level of the emission state is. Thus, the emission
wavelength becomes longer as the solvent polarity increases.
The fluorescence intensity does not change regularly. We are at
present unable to clarify the reason for the irregularity. However,
special solutesolvent interactions in some solvents may be
responsible.
The Stokes shift (ꢀ ¯~ ) is the difference between the emission
and absorption peak wavelengths. It is closely related to the
solvation of emission molecules. The Stokes shifts of M and P
versus the orientation polarizability (¦f) of various solvents are
shown in Figure 2. As observed, the Stokes shifts increases
significantly with increasing solvent polarity. This means that
the energy of the excited state decreases more remarkably than
that of the ground state, which reveals a stronger solutesolvent
interaction in the excited state relative to that in the ground state.
This result implies that the polarity of the excited state increases
upon excitation. From Figure 2, we also find that the slope of
ꢀ ¯~ versus ¦f for M is larger than that for P, indicating that M
However, there is great difference between the fluorescence
intensities of M and P. As shown in Figure 1, under the same
mass concentration, the fluorescence intensity of P is two times
higher than that of M. This may be attributed to intramolecular
rotation of constituent M in polymer P being more restricted
than that of the free molecule M in solution. This explanation is
in accordance with the quantum yield study. Using coumarin
3
07 in ethanol as a standard (ºf = 0.56),13 the fluorescence
quantum yield (º = 0.713) of P was found to be higher than
f
that of M (ºf = 0.382).
becomes more dipolar after excitation than P does. This suggests
that strong ICT occurred more easily in the small molecule,
which is responsible for the weaker fluorescence intensity of M
relative to that of P.
Table 2 lists the absorption and emission data of M and P
in solvents with various polarities. The absorption wavelengths
of M and P both show minor solvent polarity dependence,
Chem. Lett. 2013, 42, 410412
© 2013 The Chemical Society of Japan
Diao, Lixiao
