Excited-state intramolecular proton transfer in a dendritic macromolecular system: Poly(aryl ether) dendrimers with phototautomerizable quinoline core

Article abstract of DOI:10.1021/ma011385o

Poly(aryl ether) dendrimers of three different generations (n = 1, 2, 3) that are cored with phototautomerizable quinoline (QGn) were synthesized to investigate the effect of dendritic architecture on the excited-state intramolecular proton transfer (ESIPT). It was deduced from the generation-dependent absorption spectra that the peripheral crowdedness arising from the dendritic structure not only decouples the ESIPT core from the molecular surrounding but also influences the core planarity. Static and picosecond kinetic studies on ESIPT emission provided further information that the ESIPT in the quinoline core is slowed down with increasing dendrimer generation via the reduced core planarity. However, it was also observed that the proton transfer is still so effective even in the highest generation dendrimer that the emission efficiency is largely increased with dendrimer generation through the enhanced isolation effect. Compared to the polystyrene blend film containing nondendritic model compound (MQ), the films of QGn were proven to be a truly single-component solid solution with better performances in terms of the emission efficiency and chromophore content.

Full text of DOI:10.1021/ma011385o

2748
Macromolecules 2002, 35, 2748-2753
Excited-State Intramolecular Proton Transfer in a Dendritic
Macromolecular System: Poly(aryl ether) Dendrimers with
Phototautomerizable Quinoline Core
Seh oon Kim , Don g Wook Ch a n g, a n d Soo You n g P a r k *
School of Materials Science and Engineering, Seoul National University, ENG 445,
San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, Korea
Hid ek i Ka w a i a n d Tosh ih ik o Na ga m u r a
Research Institute of Electronics, Shizuoka University, 3-5-1 J ohoku,
Hamamatsu 432-8011, Shizuoka, J apan
Received August 2, 2001; Revised Manuscript Received December 11, 2001
ABSTRACT: Poly(aryl ether) dendrimers of three different generations (n ) 1, 2, 3) that are cored with
phototautomerizable quinoline (QGn) were synthesized to investigate the effect of dendritic architecture
on the excited-state intramolecular proton transfer (ESIPT). It was deduced from the generation-dependent
absorption spectra that the peripheral crowdedness arising from the dendritic structure not only decouples
the ESIPT core from the molecular surrounding but also influences the core planarity. Static and
picosecond kinetic studies on ESIPT emission provided further information that the ESIPT in the quinoline
core is slowed down with increasing dendrimer generation via the reduced core planarity. However, it
was also observed that the proton transfer is still so effective even in the highest generation dendrimer
that the emission efficiency is largely increased with dendrimer generation through the enhanced isolation
effect. Compared to the polystyrene blend film containing nondendritic model compound (MQ), the films
of QGn were proven to be a truly single-component “solid solution” with better performances in terms of
the emission efficiency and chromophore content.
Ch a r t 1. Sem ir igid P olyqu in olin e (P QH) Exh ibitin g
ESIP T
In tr od u ction
Excited-state intramolecular proton transfer (ESIPT),
a phototautomerization occurring in the excited states
of the heterocyclic molecules possessing a cyclic in-
tramolecular hydrogen bond (H-bond), has been the
research subject intensively studied in photochemistry
and photophysics.1-6 ESIPT-exhibiting molecules exist
exclusively as enol form tautomers in the ground state,
but once excited, they are preferentially transformed
into the excited-state keto tautomers via an extremely
fast and irreversible ESIPT process occurring in the
subpicosecond time regime. After the excited keto forms
decay radiatively to the ground-state keto forms, the
energy-wasting backward proton transfer occurs very
readily restoring their initial enol forms, which is in
general very efficient and thus has been applied to UV
photostabilization.7 Moreover, the different dominant
species in the ground and the excited states via ESIPT
give rise to a large Stokes shifted fluorescence without
self-reabsorption and also to a population inversion of
the proton-transferred keto form in the lowest excited
state facilitating stimulated emission. This peculiar
characteristic has encouraged more novel applications
such as luminescent solar concentrator8 and proton-
transfer laser.9-12
blends or copolymers have been studied academically14
or with an aim to realize solid lasers9,11,12,15 or elec-
troluminescence,11 the tolerable contents of ESIPT-
chromophore were far too low in order to achieve an
aggregation-free and highly efficient solid solution in a
polymeric matrix. Very recently, however, we were able
to synthesize an extremely concentrated but still ESIPT-
active solid film consisting solely of a semirigid polyquin-
oline (PQH). We have already reported ESIPT kinetics
of PQH and its applicability to electroluminescence.16
PQH has two alternating structural units based on the
quinoline ring as shown in Chart 1, one of which with
the hydroxyl group is a novel ESIPT chromophore
responsible for the abnormally large Stokes shift. Pho-
tophysical and photochemical processes related to this
ESIPT chromophore are depicted for the simplified
quinoline structure in Chart 2.
As an extension of this work, we report, in the present
paper, a novel class of solid medium for ESIPT, i.e.,
dendrimers covalently encapsulating the same quino-
line-based chromophore as in PQH. This dendritic
encapsulation strategy has successfully been applied as
a new methodology to alter properties of various func-
tional chromophores.17-24 Fre´chet’s archetypal poly(aryl
Despite these advantages, application of ESIPT has
largely been limited by the low efficiency and prominent
concentration quenching of keto emission.13 Accordingly,
most of ESIPT phenomena have been investigated
rather academically in a dilute solution system, such
as fluorescence probe for molecular environment. Al-
though several polymeric systems including simple
* Corresponding author. E-mail parksy@plaza.snu.ac.kr.
10.1021/ma011385o CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/19/2002

Macromolecules, Vol. 35, No. 7, 2002
Dendritic Macromolecular System 2749
Ch a r t 2. P h otop h ysica l a n d P h otoch em ica l P r ocesses
of P h otota u tom er iza ble Qu in olin e Com p ou n d
this optimized geometry, planarity-dependent transition ener-
gies and oscillator strengths between singlet states were
calculated by increasing the torsional angle (θ) around the
bond connecting quinoline and phenol units by 10° with the
configuration interaction calculation (90 configurations), utiliz-
ing the HyperChem 5.0 program (Hypercube).
Syn th esis. Bis(aminoketone) (1) is a known material and
was prepared following a literature method.16 The synthesis
of Fre´chet’s dendrons Gn-Br (number of generation, n ) 1-3)
followed the literature procedures.25,26 First, methyl 3,5-
dihydroxybenzoate was coupled with benzyl bromide in the
presence of anhydrous K2CO3 and 18-crown-6. The obtained
methyl ester was reduced to benzyl alcohol by LiAlH4 and
finally converted to G1-Br in the presence of CBr4 and PPh3.
The higher generation G2-Br and G3-Br were obtained in a
similar manner starting from the coupling of methyl 3,5-
dihydroxybenzoate with G1-Br and G2-Br, respectively.
Mod el Com p ou n d (MQ). 1.8 g of P2O5 was added to 3.6
mL of m-cresol at room temperature. The mixture was then
heated to 140 °C and stirred for 2 h. After cooling, 0.1 g of
bis(aminoketone) (1) and 0.07 g of 2′-hydroxy-4′-methoxy-
acetophenone (2-OCH3) were added and stirred overnight at
140 °C. The cooled reaction mixture was poured into excess
methanol, and the precipitate was collected by filtration. The
filtered product was purified by column chromatography on
silica gel, eluting with ethyl acetate/n-hexane (volume ratio
increasing from 1/2 to 1/1), to afford a yellow solid (0.047 g,
30% yield); mp 98 °C. 1H NMR (CDCl3, ppm): 3.84 (s, 6H),
3.87 (s, 6H), 6.51 (dd, J ) 8.79, 2.66 Hz, 2H), 6.60 (d, J ) 2.66
Hz, 2H), 7.00 (d, J ) 8.79 Hz, 4H), 7.40-7.50 (m, 8H), 7.82 (s,
2H), 7.84 (d, J ) 8.79 Hz, 2H), 8.02 (d, J ) 9.15 Hz, 2H). m/z
(EI) calcd for C46H36N2O7, 728.79; found, 728.
ether) dendron was chosen to suppress the concentra-
tion quenching of keto emission in the film state by
spatial isolation of ESIPT chromophore. A covalently
bonded and highly branched framework of the den-
drimer is also expected to provide multichannel paths
for the dissipation of heat generated in the core unit
during the high-power lasing experiment.12 Therefore,
we propose a “ESIPT dendrimer” as a promising solid
lasing material in the form of a single-component “solid
solution” with a large amount of active chromophore.
Herein are described the synthesis and also the static
optical properties of ESIPT dendrimers together with
the fluorescence kinetics.
Qu in olin e Cor e (3). Reaction of bis(aminoketone) (1, 0.59
g) and 2′,4′-dihydroxyacetophenone (2-OH, 0.38 g) by the
procedure described above gave 3. The crude product was
purified by silica gel column chromatography with the eluent
of ethyl acetate/n-hexane (the volume ratio increasing from
1/1 to 2/1), affording an orange solid (0.42 g, 48% yield); mp
83 °C. 1H NMR (CDCl3, ppm): 3.83 (s, 6H), 6.46 (dd, J ) 8.25,
2.01 Hz, 2H), 6.54 (d, J ) 2.01 Hz, 2H), 7.00 (d, J ) 8.22 Hz,
4H), 7.40-7.48 (m, 8H), 7.80 (s, 2H), 7.82 (d, J ) 8.25 Hz,
2H), 8.01 (d, J ) 9.33 Hz, 2H). m/z (EI) calcd for C44H32N2O7,
700.73; found, 700.
Exp er im en ta l Section
Ma ter ia ls. 18-Crown-6 (>99.5%), benzyl bromide (98%),
methyl 3,5-dihydroxybenzoate (97%), 2′-hydroxy-4′-methoxy-
acetophenone (99%), 2′,4′-dihydroxyacetophenone (99%), P2O5
(98+%), and m-cresol (99%) were purchased from the Aldrich
Chemical Co. All reagents were used without further purifica-
tion.
Mea su r em en ts. Chemical structures were identified by 1H
NMR (J EOL J NM-LA300, 300 MHz) and IR (Midac FT-IR
spectrophotometer) spectroscopy. Mass spectra were measured
by GC/mass in EI mode (J MS AX505WA) and matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF) mass
spectrometry (PerSeptive Biosystem Voyager-DE spectrom-
eter) using 1,8-dihydroxy-9[10H]-anthracenone in THF as a
matrix. Thermal analysis was performed on Perkin-Elmer
DSC7 at a rate of 20 °C/min. For the film state optical
measurements, QGn films and polystyrene-MQ blend films
with a thickness of 80-120 nm were prepared onto a glass
substrate by spin-casting from the chlorobenzene solution of
appropriate concentrations (3-10 wt %).
Absorption spectra were recorded on a HP 8452A diode
array spectrophotometer. Emission spectra were obtained on
a fluorescence spectrophotometer (Hitachi, F-4500) under the
identical conditions of excitation power and measurement
geometry for all the samples. Emission spectra were corrected
for the optical density variation (i.e., film thickness and dye
concentration) to give relative intensity by dividing absolute
emission intensity with absorbance at the excitation wave-
length.
Fluorescence kinetic profiles for the emission above 550 nm
passed through a cutoff filter were measured using an imaging
spectrograph (Hamamatsu, C5094) and a streak scope (Hama-
matsu, C2830) with a high-speed streak unit (Hamamatsu,
M2547). Samples were excited with the third harmonic (355
nm) of passively/actively mode-locked Nd:YAG laser (B.M.
Industries, 5022 D.PS. DP10) with the pulse duration of 33
ps.
Sem iem p ir ica l Ca lcu la tion Meth od . The ground-state
geometry of the simplified model structure of ESIPT chro-
mophore (Figure 3b) was optimized using the PM3 parameter
implemented in the Mopac 97 program (Fujitsu). Starting from
Gen er a l P r oced u r e for Cou p lin g of Qu in olin e Cor e (3)
w ith Gn -Br (QGn ). To a solution of 3 (1 equiv) and dendritic
benzyl bromide Gn-Br (2.1 equiv) in dry acetone or THF was
added 3 equiv of K2CO3 and 0.2 equiv of 18-crown-6. After 1
day of reflux, the reaction mixture was cooled to room
temperature and diluted with excess CH2Cl2. The solution was
then filtered, and the solvent was removed under reduced
pressure. The crude product was thoroughly purified by
column chromatography on silica gel to give an orange glass.
QG1 a n d QG1-t. 3 (0.1 g) and G1-Br (0.12 g) were coupled
following the general procedure described above in acetone.
Column chromatography was carried out eluting with ethyl
acetate/n-hexane/CH2Cl2 (1/10/12), and the gather of the
second band gave QG1 (52 mg, 28% yield). The third band was
then collected eluting with ethyl acetate/n-hexane/CH2Cl2 (1/
6/12) to give QG1-t (43 mg, 19% yield).
1
QG1. Tg 105 °C. H NMR (CDCl3, ppm): 3.84 (s, 6H), 5.05
(s, 8H), 5.07 (s, 4H), 6.56-6.61 (m, 4H), 6.67 (d, J ) 2.01, 2H),
6.71 (d, J ) 2.19, 4H), 7.00 (d, J ) 8.61, 4H), 7.26-7.49 (m,
28H), 7.82-7.86 (m, 4H), 8.03 (d, J ) 8.97, 2H). m/z (MALDI)
calcd, 1305.47; found, 1303.21.
QG1-t. Tg 56.7 °C. 1H NMR (CDCl3, ppm): 3.81 (s, 6H),
5.02-5.05 (m, 24H), 6.56-6.82 (m, 13H), 6.98 (d, J ) 8.61,
4H), 7.25-7.51 (m, 38H), 7.80-8.01 (m, 6H). m/z (MALDI)
calcd, 1607.83; found, 1605.33.
QG2. The reaction between 3 (0.1 g) and G2-Br (0.25 g)
following the general procedure was performed in acetone. The
product was isolated by column chromatography using ethyl
acetate/n-hexane/CH2Cl2 (1/10/12) as the eluent, to afford QG2
(0.18 g, 59% yield). Tg 53.6 °C. 1H NMR (CDCl3, ppm): 3.83
(s, 6H), 4.93-5.13 (m, 28H), 6.55-6.74 (m, 22H), 7.00 (d, J )

2750 Kim et al.
Macromolecules, Vol. 35, No. 7, 2002
Sch em e 1. Syn th esis of Low Mola r Ma ss Qu in olin es
(3, MQ) a n d Den d r itic Molecu les (QGn )
F igu r e 1. MALDI-TOF mass spectra of QGn.
coupling reaction would selectively provide peripheral
disubstitution of Gn-Br at the outer hydroxy groups of
3. In the case of the first generation, however, a
considerable amount of trisubstitution as well as di-
substitution occurred during the coupling reaction
between 3 and G1-Br, as was detected by thin-layer
chromatography and the MALDI-TOF mass spectrum
of the crude product. The di- and trisubstituted products
(QG1 and QG1-t) were isolated successfully by silica gel
column chromatography with the yields of 28% and 19%,
respectively. This result indicates that the intramolecu-
lar H-bond of 3 in solution is not strong enough to give
a selective peripheral substitution. On going to the
higher generation, trisubstitution became negligible due
to the steric hindrance arising from the bulkiness of
benzyl bromide dendron. Accordingly, the yield of the
disubstituted product could be increased to 59% for QG2
and 77% for QG3.
All the isolated dendritic compounds by silica gel
column chromatography showed acceptable purity and
comparable mass to the theoretical value, as determined
by MALDI-TOF spectra (Figure 1). It must be noted,
however, that different isomers other than peripherally
substituted ones are not distinguished by the chroma-
tography and mass spectra. Careful examination of the
IR spectra in Figure 2 was useful for this purpose, which
strongly supported the peripheral substitution for all
the isolated compounds. By comparing the IR spectra
of 3 and MQ, we can assign the peak of inner hydroxy
group from that of the outer one. The inner hydroxy
peak of MQ is a weak and broad one centered at 3405
cm-1 due to the intramolecular H-bond, while the outer
one of 3 exhibits a relatively sharp peak with middle
intensity centered at 3360 cm-1. Accordingly, it can be
mentioned that all the isolated dendritic compounds
showing hydroxy peak at ca. 3405 cm-1 (Figure 2) are
isomers with peripheral substitutions. All the dendritic
compounds QGn are amorphous glass and well soluble
in common organic solvents. It is also noted that glass
transition temperatures (Tg) tend to decrease from 105
to 50 °C with increasing dendrimer generation.
8.61, 4H), 7.26-7.49 (m, 48H), 7.78-7.82 (m, 4H), 8.01 (d, J
) 8.97, 2H). m/z (MALDI) calcd, 2154.44; found, 2152.51.
QG3. The coupling reaction between 3 (0.028 g) and G3-Br
(0.14 g) was performed in THF. The product was isolated by
column chromatography using ethyl acetate/n-hexane/CH2Cl2
(1/10/12) as the eluent, to afford QG3 (0.12 g, 77% yield). Tg
49.7 °C. 1H NMR (CDCl3, ppm): 3.80 (s, 6H), 4.89-5.05 (m,
60H), 6.51-6.69 (m, 46H), 6.97 (d, J ) 8.61, 4H), 7.24-7.41
(m, 88H), 7.74-7.79 (m, 4H), 7.97 (d, J ) 8.97, 2H). m/z
(MALDI) calcd, 3852.39; found, 3848.41.
Resu lts a n d Discu ssion
Syn th esis of Den dr itic Molecu les. Synthetic routes
of low molar mass quinolines (3, MQ) and dendritic
molecules (QGn) are depicted in Scheme 1. Benzyl
bromide-focused aryl ether dendrons (Gn-Br) are well-
known compounds in dendrimer chemistry and were
prepared by convergent strategy according to the lit-
erature procedures.25,26 The dendron synthesis was well
confirmed by comparable NMR data to the literature
reports for all the products in synthetic steps. Quinoline
core (3) and low molecular weight model compound
(MQ) were prepared by Friedla¨nder reaction between
bis(aminoketone) (1) and 2 equiv of OH-substituted
ketomethylene (2-X) in a manner similar to the litera-
ture.16,27,28 The dendritic product QGn were prepared
by coupling reactions between 3 and Gn-Br by a
standard method in the presence of anhydrous K2CO3
and 18-crown-6. As shown in Scheme 1, quinoline core
3 has four hydroxy groups capable of participating in
the coupling reaction, i.e., two inner ones adjacent to
quinoline ring and two outer ones opposite to it. The
inner one is known to form intramolecular cyclic H-bond
with nitrogen atom of quinoline and thus to show low
nucleophilicity.29 Hence, it can be assumed that the
Sta tic Op tica l P r op er ties. The effect of dendritic
framework on the optical properties of quinoline-based
ESIPT core was investigated by electronic absorption
and emission spectra. Figure 3a shows the absorption
spectra of MQ and QGn in CHCl3 solution, all of which
comprise four main absorption bands. The poly(aryl
ether) structure of dendron is known to have no elec-
tronic effect on core chromophore.24 Furthermore, in our
previous report, it was elucidated by quantum chemical
calculation for isolated gas phase that the conjugation

Macromolecules, Vol. 35, No. 7, 2002
Dendritic Macromolecular System 2751
F igu r e 2. IR spectra of the quinoline-based low molar mass
compounds (MQ, 3) and dendritic compounds QGn.
between quinoline and pendant phenyl ring attached
at its 4-position is broken owing to a large torsional
angle of ca. 67°.16 This theoretical consideration is
confirmed by the reported value of 49° in single cystals
of similar quinoline compounds.30 Thus, it can be
mentioned that the π-conjugated unit in the ESIPT core
is the 2-quinolin-2-yl-phenol moiety. From this consid-
eration, two large energy bands below 300 nm can be
assigned as transitions of poly(aryl ether) structure and
pendant methoxyphenyl substituent in the core unit.
The other two bands at ca. 378 nm and ca. 322 nm
would be from the transition of conjugated core unit,
each denoted as S1 and S2, respectively. As shown in
the inset of Figure 3a, it is remarkable that the
wavelength at absorption maximum (λmax) of S1 and the
absorbance ratio of S1 and S2 changes slightly but
meaningfully with a tendency depending on the den-
drimer generation, except for the trisubstituted QG1-t
(denoted as open symbols). Since the increment of
dendron generation means the growth of peripheral
steric hindrance without electronic interaction with core,
this spectral change may arise from the different
planarity of conjugated core unit governed by the
peripheral crowdedness. This assumption can be ratio-
nalized by the weak H-bond of the core chromophore in
solution or the low rigidity of core plane, as discussed
above. To be convinced of this idea, a semiempirical
molecular orbital calculation with the simplified model
structure shown in Figure 3b was carried out. The
optimized geometry of model structure preferred the
H-bonded conformation with perfect coplanarity be-
tween quinoline and phenol units (θ ) 0 in Figure 3b)
and gave the calculated transition energies comparable
to S1 and S2. Informatively, the larger energy band S2
is well accordant with the calculated transition energy
of 6-methoxyquinoline (334 nm), implying that S2 is
mainly from the quinoline segment. As the conjugation
plane was distorted by increasing the torsional angle
(θ) around the bond connecting quinoline and phenol
units, the calculated contribution of S2 showed growth
over that of S1. In other words, on going to a more
planar conformation, the calculated tendency for the
transition energy of S1 along with the oscillator strength
F igu r e 3. (a) Solution absorption spectra of MQ and QGn in
CHCl3 (10-5 M). The inset shows the tendency for λmax of S1
band and the absorbance ratio of S1 and S2 bands as a function
of the number of phenyl rings in dendron substituent (MQ, 0;
QG1, 6; QG1-t, 9; QG2, 14; QG3, 30). (b) Simplified model
structure for ESIPT chromophore and its planarity-dependent
tendency for the calculated λmax of S1 band and the oscillator
strength (f) ratio of S1 and S2 bands. The planarity was
distorted by changing θ by 10°.
(f) ratio of S1 and S2 is well consistent with the
experimental observation on increasing the generation
from MQ to QG3, as plotted in Figure 3. This calculation
result demonstrates that in the case of peripherally
disubstituted compounds the more sterically hindered
periphery of the higher generation gives rise to the more
planar core conformation in solution state. The deviation
of trisubstituted QG1-t from the above tendency of
disubstituted QGn’s can also be explained in the sense
of core planarity. The central steric hindrance from a
bulky dendritic substituent at the inner hydroxy group
would strongly distort the core plane and disturb the
conjugation, to result in the large hypsochromic shift
relative to other compounds.
Except for crystalline MQ, all dendritic compounds
QGn are glassy and formed a scattering-free transpar-
ent film when spin-coated. Figure 4 shows the absorp-
tion spectra of QGn films that display spectral charac-
teristics similar to the solution absorption. Trisubstituted
QG1-t showed hypsochromically shifted S1 also in the
film state, indicating that the central steric hindrance
is still significant in the condensed phase. For all

2752 Kim et al.
Macromolecules, Vol. 35, No. 7, 2002
F igu r e 5. Relative emission intensities of QGn films and
MQ-polystyrene blend films (3, 10 wt %), each spectrum
obtained by excitation at 370 nm. The inset is the excitation
spectra for the emission of QGn at 580 nm.
F igu r e 4. Film absorption spectra of QGn. The inset shows
the tendency for λmax of S1 band and its bathochromic shift
(∆λ) relative to the solution absorption as a function of the
number of phenyl rings in dendron substituent (defined in
Figure 3).
tion. Similar to PQH film, QGn films emitted no
detectable fluorescence from the enol form in 360-500
nm, suggesting that the effective H-bond and the fast
proton transfer are operating also in QGn films. Impor-
tantly, the relative emission intensity of QGn films,
which was normalized by the absorbance at excitation
wavelength of 370 nm, was increased with increasing
number of phenyl rings in dendron shell, indicative of
the improved efficiency of keto emission by the enhanced
shell effect. Moreover, all the dendritic compounds QGn,
whose chromophore content in total weight is from 18
wt % (QG3) to 54 wt % (QG1), showed dramatically
enhanced emission intensity relative to that of nonden-
dritic MQ in more dilute solid solution (3 and 10 wt %
in polystyrene blend film). Thus, it can be concluded
that the dendritic architecture shows enhanced perfor-
mance for the “solid solution” with high chromophore
content as well as improved emission efficiency. This is
primarily due to the cooperative effect of dendritic shell
that can actually separate the emitting core from each
other to suppress the fluorescence quenching via mo-
lecular association.
Contrary to the film, the QGn solution shows no
detectable emission from either the keto or enol form.
It is not the case for PQH solution, giving dual emission.
One possible explanation for this fluorescence quenching
in solution can be given in the sense of twisted intramo-
lecular charge transfer (TICT) state connected with a
large-amplitude torsional vibration of core plane around
the average planarity, which has been discussed in the
literature for the similar ESIPT system of 3-hydroxy-
and 3,3′-dihydroxy-2,2′-bipyridyl.8 In the case of PQH,
this quenching motion in solution might be constrained
to some extent due to the reduced degree of freedom
along the polymer backbone.
compounds except QG3, the λmax values of S1 in film
were bathochromically shifted relative to that in solu-
tion. The aggregation effect can be disregarded because
no aggregation and no excimeric behavior were observed
for this quinoline-based ESIPT chromophore even in the
more concentrated film of PQH.16 Thus, this spectral
difference between solution and film is most probably
attributed to the different core planarity between the
two molecular states. This means that the core plane
with low rigidity is affected not only by the intramo-
lecular peripheral crowdedness but also by the inter-
molecular steric effect. Importantly, the bathochromic
shift is the largest for QG1 that possesses the least
hindered periphery and shows a tendency to decrease
with increasing generation, i.e., increasing peripheral
hindrance. This generation dependency in S1 λmax is
thus opposite to that in solution, as shown in the inset
of Figure 4. Resultantly, on going to the higher genera-
tion, the core planarity in film is less different from that
in solution such that eventually in the third generation
dendrimer QG3 S1 λmax is the same in both states. Here
we note that solution and film are extreme cases of
molecular states with different molecular surroundings,
where the former is a solvated state with a large amount
of kinetic freedom but the latter has kinetic constraint
due to the intermolecular proximity. This spectral
tendency thus means that the crowded periphery of
dendritic shell not only determines the core planarity
intrinsically but also decouples it from the molecular
surroundings, forming a solid solution in the geometrical
and spectral senses.
The generation effect on the emission behavior was
studied for the film samples of QGn. As shown in Figure
5, all the QGn films emitted abnormally large Stokes
shifted (∼200 nm) orange fluorescence without spectral
overlap between absorption and emission. From the
comparison with PQH of similar structure, it is unam-
biguous that this orange emission is attributed to the
characteristic fluorescence from the proton-transferred
excited keto form of quinoline-based core.16 As reported
in our previous work,16 PQH solution gave dual emission
from both enol and keto forms (ca. 400 and 590 nm,
respectively) while the film gave only keto emission
because the excited-state intramolecular proton transfer
(ESIPT) in film was much faster than in solution by
effective H-bond due to the suppressed molecular mo-
The excitation spectra for keto emission from QGn
films are very similar to the absorption bands, S1 and
S2, of the conjugated core, as shown in the inset of
Figure 5. It is noted that the absorption of dendron shell
below 300 nm has no contribution to fluorescence.
Moreover, since the absorption is from the ground state
mainly existing in enol form, its good accordance with
the keto excitation strongly supports that the precursor
of the excited keto form is the Franck-Condon excited
state of enol form.
F lu or escen ce Kin etics. Figure 6 shows the pico-
second kinetic profiles of the emission above 550 nm

Macromolecules, Vol. 35, No. 7, 2002
Dendritic Macromolecular System 2753
Con clu sion s
Poly(aryl ether)-type dendrimers cored with ESIPT-
active quinoline were synthesized and identified. It was
founded that the dendritic shells are able to decouple
the core planarity from the molecular surroundings and
to enhance ESIPT emission by the spatial isolation of
ESIPT cores. It was demonstrated for the first time that
the ESIPT dendrimers are a new class of solid ESIPT
medium with large content of active chromophores and
efficient ESIPT emission.
Ack n ow led gm en t. This research was supported in
part by CRM-KOSEF.
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