that the incorporation of 1 into the PNIPAM structure has
thermal transitions of the polymer are tunable by such simple
external manipulations4 as changing its solution concentration.
The FL of P1a is temperature dependant but its thermal
response is non-monotonic, which is more informative than
the monotonic pattern afforded by other analytical techniques.4
Dynamic light scattering (DLS) and solution turbidity
measurements, for example, furnish curves that start to
monotonously increase from 28 and 31 1C, respectively
(Fig. 2B). These data imply that the PNIPAM chains start
to aggregate from the specific temperatures but fail to offer
detailed information about the transition processes because of
their lower sensitivity, in comparison to that of the FL
technique.
exerted little effect on the molecular weights and polydispersity
1
indexes (ESIw, Table S1). In the H NMR spectra of P1, the
protons of the aromatic rings resonate at d B7.0–7.8
(ESIw, Fig. S1–S3), confirming that the PNIPAM chain has indeed
been labelled by the TPE fluorogen by the simple procedure of
direct polymerization. Using the calibration curve drawn from
the UV absorption data (ESIw, Fig. S4 and S5), the degrees of
labelling in P1a, P1b and P1c are determined to be 0.27%,
0.58% and 1.12%, respectively (ESIw, Table S1).
The polymer with the lowest degree of labelling (P1a) is non-
emissive when dissolved in THF, a good solvent for both TPE
and PNIPAM (Fig. 1). The FL becomes visible when large
amounts ( Z 70%) of water, a poor solvent for TPE, are added
into the THF solution of P1a, indicating that the TPE label
maintains its AIE activity after being incorporated into the
PNIPAM structure. The polymer with the highest degree of
labelling (P1c) shows similar AIE behaviour (ESIw, Fig. S6).
The FL intensity of P1 is increased with increasing extent of
labelling (ESIw, Fig. S7) and concentration of polymer
(ESIw, Fig. S8).w This serves as another proof for the AIE
To collect more information about chain conformation of
P1a and to gain more insight into its thermal transitions, its 1H
NMR spectra are measured in different solvents at various
temperatures. The NMR spectrum taken at room temperature
in DMSO, a good solvent for both PNIPAM and TPE,
contains resonance signals of all the protons, as marked in
Fig. 3A. In water, however, the peaks for the aromatic protons
in the downfield disappear. As water is a polar solvent, the
hydrophobic TPE labels may have been wrapped by the
polymer coils or aggregated into tiny clusters. The wrapping
and aggregation must have been loose, because the P1a
solution is homogeneous and transparent. The loose wrapping
and aggregation partially hamper intramolecular rotation of
the aryl rings of the TPE label, which explains why P1a is
luminescent in water (cf., Fig. 1).
nature of the TPE label, because the emission of
a
‘‘conventional’’ fluorophore is normally weakened, rather than
strengthened, with increasing concentration.
The temperature effect on the FL behaviour of P1a in water
is shown in Fig. 2A. Little change in the emission intensity of
P1a is recorded when its aqueous solution is heated from 14 to
25 1C, while a small bump is observed in the temperature
region of 25 to 29 1C. Above 29 1C, the FL intensity of P1a
swiftly increases with increasing temperature and reaches a
maximum at 34.2 1C. Further heating leads to a continuing
decrease in the FL intensity. When the aqueous mixture of P1a
is cooled from 50 to 18 1C, a largely reversible FL intensity–
temperature curve is obtained, with the big peak and small
bump recorded in the similar temperature regions. When the
When P1a solution is heated to 25 1C, the PNIPAM chains
start to dehydrate, probably from the isopropyl pendants.4
This partially breaks the water cages surrounding the polymer
coils and promotes the dehydrated chain segments to undergo
a coil–globule transition. The change at this initial stage is too
small to be detected by DLS, UV and NMR analyses. The
volume shrinkage accompanying the coil–globule transition,
although small at this stage, is picked up by the TPE label. The
reduced volume impedes its intramolecular rotation, thus
making it more emissive (cf., Fig. 2A).
polymer concentration is changed from 1 to 0.1 mg mLÀ1
,
the starting point for the FL enhancement is increased
from B25 to B32 1C and the FL maximum is shifted
from B34 to B37 1C (ESIw, Fig. S9). This indicates that the
In the temperature region of 29 to 34 1C, coil–globule
transition becomes active. At this stage, not just the pendants
and segments but the whole polymer chains dehydrate. The
dehydration results in the formation of compact aggregates, as
evidenced by the decrease in the NMR peak intensities
(Fig. 3D). This greatly activates the RIR process of the TPE
label. The fluorogen thus becomes highly emissive,
manifestation of its AIE effect.
a
Further heating of the polymer mixture to above 34 1C may
cause little change in the compactness of the polymer
aggregates because the phase transition has already finished
at the LCST, although the aggregates continue to grow in size.
Indeed, the aggregates become so big at the high temperature
that NMR spectra cannot be taken (Fig. 3E). The TPE labels
buried in the big aggregates can hardly be reached by the UV
excitation beam. Meanwhile the high-temperature heating
activates the molecular motions such as intramolecular
rotation and vibration of the TPE fluorogen. These two effects
collectively make the polymer mixture less emissive, as can be
seen from the continuous decrease in the FL intensity in the
high temperature region (cf., Fig. 2A).
Fig. 1 (A) FL spectra of P1a in water–THF mixtures with different
water contents measured at 17 1C (lex = 322 nm, [P1a] = 1 mg mLÀ1).
(B) Change of FL maximum of P1a with water content of the
aqueous mixture. (C) Photographs of P1a solutions taken under UV
illumination.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4974–4976 | 4975
Tang, Li
