Photocleavage of pyridyl-based aromatic polyureas

Article abstract of DOI:10.1021/ma034058k

The photophysical properties of a metal chelating pyridyl-based aromatic ureas and poly-(1,4-phenylene-2,6-pyridylurea) were investigated. In their solution state and upon exposure to 365 nm UV radiation, these low and high molecular weight compounds were found to cleave the urea linkage nearly quantitatively, generating the corresponding amine-terminated subunits and CO₂. Through a series of model compounds along with nuclear magnetic resonance (NMR), UV/vis absorption, and fluorescence spectroscopy characterization, the photocleavage process was delinated as a function of UV dose, O₂, and water. The presence of water was found crucial to this photocleavage process with O₂ playing a role only in subsequent oxidation of resulting amine functionalities. The experimental results support a light-assisted hydrolysis of the urea bond as the most plausible photocleavage mechanism. The photocleavage of such pyridyl-based ureas can potentially find usages in light-assisted base generation and/or for releasing metal cations.

Full text of DOI:10.1021/ma034058k

Macromolecules 2003, 36, 9775-9783
9775
Photocleavage of Pyridyl-Based Aromatic Polyureas
J er em ia h Mw a u r a ,† Ba och en g Ya n g,† Ron gfu Li, Ma r th a Mor ton , a n d
F otios P a p a d im itr a k op ou los*
Nanomaterials Optoelectronics Laboratory, Department of Chemistry, Institute of Materials Science,
University of Connecticut, Storrs, Connecticut 06269
Received J anuary 18, 2003; Revised Manuscript Received September 15, 2003
ABSTRACT: The photophysical properties of a metal chelating pyridyl-based aromatic ureas and poly-
(1,4-phenylene-2,6-pyridylurea) were investigated. In their solution state and upon exposure to 365 nm
UV radiation, these low and high molecular weight compounds were found to cleave the urea linkage
nearly quantitatively, generating the corresponding amine-terminated subunits and CO2. Through a series
of model compounds along with nuclear magnetic resonance (NMR), UV/vis absorption, and fluorescence
spectroscopy characterization, the photocleavage process was delineated as a function of UV dose, O2,
and water. The presence of water was found crucial to this photocleavage process with O2 playing a role
only in subsequent oxidation of resulting amine functionalities. The experimental results support a light-
assisted hydrolysis of the urea bond as the most plausible photocleavage mechanism. The photocleavage
of such pyridyl-based ureas can potentially find usages in light-assisted base generation and/or for releasing
metal cations.
Sch em e 1. Syn th esis of
P oly(1,4-p h en ylen e-2,6-p yr id ylu r ea ) I
In tr od u ction
Much attention has been recently focused on a variety
of conjugated polymers with good stability and high
electroluminescence efficiency.1-5 Besides improving
charge transport and quantum efficiency, prolonging
device lifetime presents also a major challenge. Unlike
inorganic light-emitting devices, which are very stable
under photoirradiation, polymers often tend to degrade
when exposed to long periods of absorptive radiation in
the presence of species such as O2, CO2, and moisture.6-9
To improve upon the stability of organic light-emitting
devices, a good molecular level understanding of the
sharp emission bands.28-30 Typically, thin films incor-
porating these chelates have been realized via vacuum
evaporation and spin-coating techniques.
degradation processes is of great importance.10,11
A
variety of spectroscopic methods have been used to
study such photodegradation processes with photooxi-
dation and photorearrangements dominating these light-
assisted degradation mechanisms.12-14 Certain photo-
degradation mechanisms of such polymers, although
undesirable for device configurations, can find alternate
usage in the field of micro/nanolithography, where light
and/or ionizing radiation patterning15-17 along with
photoacid18,19 and photobase19,20 generation are utilized
in polymer resist development.
Recently our group has reported on a novel layer-by-
layer film growth methodology where highly fluorescent
polyurethane-urea/lanthanide metal chelates (based on
2,6-diaminopyridine) thin films were assembled.31
A
related fully conjugated lanthanide chelating polymer
(poly(1,4-phenylene-2,6-pyridylurea)) (I) was synthe-
sized to improve charge mobility for OLED devices (see
Scheme 1).32 Surprisingly enough, this polymer was
found to undergo some interesting photophysical changes
upon exposure to 365 nm UV irradiation. In this paper,
we report on a detailed spectroscopic characterization
of these photophysical processes, where through a series
of model urea compounds the photocleavage mechanism
is elucidated. Although polymer I might prove unsuit-
able for OLED applications, photocleavage of 2,6-diami-
nopyridine-based ureas and their respective metal
chelates can potentially find usages in light-assisted
base generation and/or for releasing metal cations,
respectively.
In conjunction with conjugated polymers, various
metal chelates have been utilized as charge injectors
and emitters in the field of organic light-emitting diodes
(OLEDs).21-23 8-Hydroxyquinoline aluminum (Alq3) has
been the most widely used metal chelate in OLEDs, with
demonstrated bright green emission and good electron
transport properties.24,25 Recently, metal chelates based
on platinum and iridium complexes have been success-
fully employed in electrophosphorescent applications.26,27
One drawback with these materials (polymeric and
small molecule chelates) is that they have broad emis-
sion and hence difficult to obtain pure colors in EL
devices. This has led to an increase in research on
organo-lanthanide chelates due to their characteristic
Exp er im en ta l Section
Gen er a l Meth od s. Solvents and starting materials were
acquired from J .T. Baker and Aldrich, respectively, and used
without further purification unless otherwise stated.
UV absorptions of dilute solutions (10-4 M) of I and model
compounds were measured using a Perkin-Elmer Lambda 6
spectrometer with 1 cm quartz cells. Photoluminescence (PL)
spectra were recorded on a Perkin-Elmer LS-50 spectropho-
† Authors made equal contributions.
* Corresponding author. Tel 860-486-3447; Fax 860-486-4745;
e-mail papadim@mail.ims.uconn.edu.
10.1021/ma034058k CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/26/2003

9776 Mwaura et al.
Macromolecules, Vol. 36, No. 26, 2003
tometer. UV irradiation was performed using a high-pressure
long wave mercury UV lamp (UVP model B 100 AP) (21 700
µW/cm2), with emission envelope from 363 to 370 nm, peaking
at 365 nm (filtered UV light). The entire amount of the UV
irradiation of the lamp was directed into a highly reflective
enclosure that contained the sample, which was located at the
center, about 5 cm from the lamp. No provisions were made
to prevent the samples from heating, and temperature in-
creased no more than 80 °C after prolonged irradiation. 1H,
13C, 1D, and 2D NMR spectra were acquired using a Bruker
DRX 400 instrument. Both deuterated d-DMF and d-DMSO
solvents (Aldrich) (with water concentration on the order of
0.037% as measured for DMSO with GC-MS) were used to
make solutions (∼0.05 M) of I and other model compounds for
the NMR measurements. The solutions were UV-irradiated
with 365 nm filtered light (with UV light passing through the
glass NMR tubes) for 10, 20, 30, and 60 min durations followed
by recording both 1H and 13C NMR spectra. Samples of
terbium/polyurea I chelate were prepared by adding TbCl3 to
dilute solutions (∼10-4 M, based on polymer repeat unit) of I
in DMF to result in chelate solutions with a 1:2 (Tb3+/polymer)
molar ratio (based on polymer I repeat). UV irradiation was
performed, as described above for 10, 30, and 60 min, and PL
spectra were recorded for each sample. A variety of methods
like casting (slow drying), dip-coating, and spin-coating were
used to prepare solid samples of the polymer I. Following UV
irradiation, the films were redissolved in d-DMSO, and their
NMR spectra were collected.
GC-MS Ch a r a cter iza tion of th e UV P h otoclea ved
P r od u cts of Com p ou n d 1. 6 mg of 1 was dissolved in 2 mL
of DMSO, sealed in a screw-cap glass vial, and UV-irradiated
for given amount of times as described above. Following
irradiation, sample aliquots were quickly withdrawn and
injected into a Hewlett-Packard 5890 series II gas chromato-
graph equipped with a Hewlett-Packard 5971A mass selective
detector. Samples were analyzed immediately after irradiation
as well as after they were allowed to stand unsealed overnight.
Typical GC-MS experimental conditions involve the follow-
ing: injection port temperature, 225 °C; detector port temper-
ature, 280 °C; initial time, 2 min; injector ON time, 0.5 min;
injector OFF time, 0.01 min; oven temperature ramp, 35-325
°C at 15 °C/min. Control experiments involve the GC-MS
analysis of the solvent (DMSO) as well as dilute solutions of
phenyl isocyanate and 2-aminopyridine under similar operat-
ing conditions.
Syn th esis of I. 2,6-Diaminopyridine and 1,4-phenylene
diisocyanate were sublimed before use. 10 mmol of each was
dissolved in 10 mL of freshly distilled DMF, and the solutions
were bubbled with N2 for 30 min. Then the 2,6-aminopyridine
solution was cannulated into the diisocyanate solution, and
the mixture was stirred for 2 h while continuing bubbling N2.
The turbid suspension was then poured into 400 mL of
methanol to yield a white precipitate. This was then filtered,
washed several times with methanol, and vacuum-dried
overnight. The polymer molecular weight of this product was
found to be 18.7 kDa (with a polydispersity index of 1.15) as
determined by size exclusion chromatography in DMF using
polystyrene standards.
Gen er a l P r oced u r e for th e Syn th esis of Mod el Ur ea
Com p ou n d s 1, 3, 4, 5, a n d 6 (shown in Scheme 2). A stirred
solution of the corresponding amine (10 mmol) in dichlo-
romethane (DCM) was cooled to 0 °C. Phenyl isocyanate (10
mmol) in DCM was added dropwise over 20 min. The solution
was then allowed to warm to room temperature and stirred
for 1-3 h. Subsequently, the solvent was then evaporated in
vacuo, and the residue was recrystallized from ethanol. All
compounds are known, and the analytical data were in
accordance with literature values.33
Sch em e 2. Low Molecu la r Weigh t Mod el Ur ea
Com p ou n d s Em p loyed To Assist w ith NMR P ea k
Assign m en ts a n d Elu cid a te th e P h otoclea va ge
P r ocess of I
and 2 mL of dioxane was added; the suspension was then
mixed with the 2-aminopyridine solution. The resulting mix-
ture was warmed over a steam bath until a clear solution was
obtained and then set aside for 24 h. The solvent was removed
in vacuo, 20 mL of water added, and the suspension stirred
for 30 min. Then the aqueous solution was extracted with 3 ×
10 mL of DCM. The organic phase was washed with brine and
dried over Na2SO4, and the solvent was removed in vacuo. The
crude product was purified by preparative TLC using 5%
MeOH in DCM eluting solvent. The final yield was 40%. All
analytical data of compound 2 are in accordance with literature
values.34
Resu lts a n d Discu ssion
The polyurea I used in this study was synthesized by
a simple condensation of 2,6-diaminopyridine and 1,4-
phenylene isocyanate as shown in Scheme 1. To inves-
tigate the photochemistry of polymer I, solutions (10-4
M) in DMF were irradiated (365 nm UV) for increasing
durations while measuring their UV-vis absorbance
and the corresponding photoluminescence (PL) spectra
as illustrated in Figure 1. As shown in Figure 1A, the
absorbance peak at 325 nm of I decreases during the
course of irradiation, while a broad absorption peaking
at 460 nm emerges (see inset). The decrease in absorp-
tion intensity of I and the corresponding blue-shifting
(from 325 to 315 nm after 60 min of irradiation) is
reminiscent of reduced conjugation that might be as-
sociated with a possible breakdown of the polymer chain
to smaller molecules. As it will become apparent later
on in the discussion, the progressive increase of the
broad peak at 450 nm is associated with photooxidiza-
tion of the polymer’s photocleaved byproducts.
The photoluminescence behavior of a dilute DMF
solution of I shows a more complex behavior as shown
in Figure 1B. The PL intensity first increases, and its
peak maximum shifts from 360 to 385 nm within 30 min
of irradiation. Such behavior can be explained by the
continuous formation of chemical species that exhibit
red-shifted emission and higher photoluminescent quan-
tum efficiencies compared to I. Irradiation of the
polymer beyond 30 min results in a decrease of PL
intensity. This PL intensity decrease along with the
corresponding blue shift of the peak maximum from 385
to 380 nm might originate from photooxidation of the
photocleaved byproducts of I, as noted before in UV-
vis spectra of Figure 1A.
Syn t h esis of 1-Met h yl-1-p h en yl-3-(2-a m in op yr id yl)-
u r ea (2). 940 mg (10 mmol) of 2-aminopyridine was dissolved
by warming in 2 mL of pyridine. 1.7 g (10 mmol) of N-methyl-
N-phenylcarbamoyl chloride was dissolved in 3 mL of pyridine,
but very soon crystals of the pyridine-acyl halide complex
began to separate, and in a few minutes the mixture had set
to a solid mass of crystals. The solid was reduced to powder,
The photocleavage mechanism was studied through
the use of several model urea compounds 1-6. These
compounds exhibit similar UV absorption and PL

Macromolecules, Vol. 36, No. 26, 2003
Pyridyl-Based Aromatic Polyureas 9777
F igu r e 1. Solution UV-vis (A) and photoluminescence (PL) (B) spectra of I (10-4 M solutions in DMF) upon exposure to 365 nm
UV irradiation for varying durations. The PL spectra were obtained from 340 nm excitation.
F igu r e 2. 1H NMR of model compound 1 (2-aminopyridylphenyl urea, 0.05 M in d-DMSO) before (a) and after (b-d) UV irradiation
for the indicated duration. The top spectrum (e) is the 1H NMR of an equimolar mixture of 2-aminopyridine and aniline.
characteristics as polymer I. Figures 2 and 3 illustrate
the 1H and 13C NMR spectra of model compound 1,
respectively, before and after 10, 30, and 60 min of UV
irradiation.
As seen in Figure 2, the photocleavage of I is evident
from the dramatic change in the chemical shifts upon
UV exposure. After 10 min irradiation, several peaks
at δ ) 5.8, 6.4, 6.5, and 7.9 ppm and a broad peak at
5.0 ppm (indicated with arrows) develop which, with the
exception of the 5.0 ppm peak, all increase in intensity
upon longer UV exposure.
as the top spectrum in Figure 2 and Figure 3, respec-
tively, to the spectra obtained from UV-irradiated 1
model compound. The closely matched spectrum of the
60 min irradiated 1 to the corresponding amines indi-
cates the near complete breakdown of the urea linkage.
Here it is noteworthy to mention that the proton peak
at 5.0 ppm for the irradiated model compound 1 is very
broad compared to the sharp peak of aniline proton
shown in Figure 2. This could be due to the different
water content in the NMR solvent, consequentially
leading to a different environment, causing a variance
in exchange rate between the amino proton and water
in the solvent. From the 1H NMR spectra (Figure 2),
we can also see that the photocleavage process was
completed within 1 h. This photocleavage process was
found to be much faster in d-DMSO than in d-DMF, and
consequently all the other NMR experiments were
carried out in d-DMSO.
In addition, from the 13C NMR spectra (Figure 3), we
observed the disappearance of the peak at 153 ppm
(carbonyl carbon) after an hour of UV irradiation,
implying the complete elimination of the carbonyl group
during the photocleavage process. The upfield shift of
aromatic protons on both pyridyl ring and phenyl ring
suggests a change in electron density of aromatic rings.
After carefully studying these NMR spectra, the pos-
sibility of urea bond breaking up to form two amines,
i.e., aniline and pyridine, became evident. This was
confirmed by comparing the proton and carbon NMR
spectra of pure amines (aniline and pyridine), shown
Ureas have been shown to photocleave through free
radical mechanism.14,35 Unlike the photo-Fries cleavage
mechanism, which usually gives several products,36,37
the urea system based on 2-aminopyridine as shown
above gives only two clearly definable products with

9778 Mwaura et al.
Macromolecules, Vol. 36, No. 26, 2003
F igu r e 3. 13C NMR of model compound 1 (0.05 M in d-DMSO) before (a) and after (b, c) UV irradiation for the indicated duration.
The top spectrum (d) is the 1H NMR of an equimolar the mixture of 2-aminopyridine and aniline.
nearly 100% conversion. The pyridine moiety seems to
play an important role in the photocleavage process.
When the presence of the pyridine ring is removed as
in model compound 4, even after 6 h of UV irradiation,
no photocleavage products (amines) are obtained (data
not shown). This could be due to the fact that, without
the pyridine ring, the diphenylurea 4 absorbs at 320 nm,
far below the wavelength (365 nm filtered light) at
which all the model compounds were irradiated. In
addition, photocleavage of model compound 2, which has
a methyl substitution on a phenylamino group, gave
2-aminopyridine and N-methylaniline while model com-
pound 3 gave 2,6-diaminopyridine and aniline (1H NMR
shown in Supporting Information, Figures S1 and S2).
Furthermore, their photocleavage rates were on the
same order of magnitude with model compound 1.
F igu r e 4. Percent conversion vs irradiation duration for
model compound 1 (0.05 M in d-DMSO) as a function of
different water contents.
To determine whether external reagents such as O2
and H2O played any significant role in the photocleav-
age of these ureas, NMR experiments were also carried
out using oxygen degassed (via multiple freeze-thaw-
pump cycles followed by vacuum-sealing) d-DMSO
solutions. No significant differences in photocleavage
rate between the degassed and nondegassed samples
were observed, which suggests that oxygen plays no
obvious role in the process. This is contrary to the
reported photooxidative degradation of aromatic poly-
urethanes to quinoids.12,13 On the other hand, the
amount of water in the d-DMSO was found to be crucial
in this photocleavage process. NMR samples with dif-
ferent water content (0.5, 1.5, 2.5, and 5 equiv to the
model compounds) were prepared, sealed in NMR tubes,
and similarly irradiated for different durations while
recording NMR spectra. Figure 4 illustrates the percent
conversion of disappearing 1 for different water content
as a function of UV exposure duration. The percent
conversion was calculated by comparing integrated
proton NMR peak values (specifically the proton R to
the pyridyl nitrogen) from the starting compound with
those from the photocleavage products. From the initial
slope of Figure 4, it is evident that the photocleavage
rate increased with water content. It should also be
noted that for the sample with the lowest water content
(0.5 mol equiv of H2O to 1) the photocleavage reaction
stopped nearly at 50% after all water was consumed,
as indicated by the disappearance of water peak in the
NMR spectra, even after prolonged exposure to UV
irradiation. This implies that water acts as a reactant
and not as a catalyst, in this cleavage process. With
samples containing more water, the photocleavage
reaction proceeded to completion with prolonged expo-
sure to UV irradiation. The kinetics of this process is
under investigation and will be the study of a separate
paper.
To study in more details the effect of the neighboring
pyridyl moiety to the photocleavage, model compounds
5 and 6 (shown in Scheme 2) were synthesized and their
NMR spectra (d-DMSO solutions) also investigated as
a function of exposure to UV irradiation. Here, as
compared to 1, model compounds 5 and 6 have their
amino groups on 3-position and 4-position of the pyri-
1
dine rings, respectively. Shown in Figure 5 are the H
NMR spectra of 5 and 6 before and after 1 h of UV
irradiation. Figure 5A indicates that for the 3-aminopy-
ridylphenylurea (5) little change was observed as indi-

Macromolecules, Vol. 36, No. 26, 2003
Pyridyl-Based Aromatic Polyureas 9779
F igu r e 5. 1H NMR of (A) model compound 5 (3-aminopyridylphenyl urea) and (B) model compound 6 (4-aminopyridylphenyl
urea) before and after 1 h irradiation (0.05 M in d-DMSO).
cated by the appearance of small intensity peaks at δ
6.5, 7.1, and 7.7 ppm. In contrast to 5, 6 (4-aminopy-
ridylphenylurea) exhibited an almost 50% conversion.
Upon UV irradiation, the new set of chemical shifts
(peaks marked in primed numbers) generated closely
match the corresponding pure amines, i.e., 4-aminopy-
ridine and aniline. From these observations it can be
concluded that compounds with the urea groups ortho
and para to the pyridyl nitrogen are much more prone
to water-assisted photocleavage as opposed to the meta-
substituted ones.
Sch em e 3. P r op osed P h otoclea va ge Mech a n ism of
th e Ur ea Com p ou n d s
It has been proposed that intramolecular hydrogen
bonding plays an important role in photoinduced proton-
transfer processes.38 In addition, Fiksdahl et al.39
reported that N-(2-pyridyl)amides can go through a
concerted proton-transfer mechanism involving a six-
membered intermediate to form 2-aminopyridines and
ketenes. Compounds 1 and 3 in Scheme 2 exhibit this
kind of intramolecular hydrogen bonding.40,41 This was
also confirmed by NMR where a NOE was detected
between protons 1 and 9, indicating the presence of a
hydrogen bond (see Supporting Information, Figure S3).
To study the role of this intramolecular hydrogen bond,
during the photoirradiation process, compound 2, which
does not possess such hydrogen bond, was synthesized
and exposed to UV irradiation in a manner similar to
the other model compounds. As shown in the Supporting
Information, Figure S1, compound 2 exhibited similar
water-assisted photocleavage behavior to 1, clearly
suggesting that intramolecular H-bonding is not man-
datory for this photocleavage process. This is also
reinforced by the photocleavage behavior of 6, which also
is not capable of intramolecular hydrogen bonding. On
the basis of the results described above, a photoassisted
hydrolysis mechanism of the ortho- and para-substitut-
ed pyridyl ureas (shown only for the ortho position) is
proposed in Scheme 3. When such pyridyl ureas are
exposed to UV irradiation, it appears that their excited
states results in weakened amide bond, probably due
to resonance with the electron-withdrawing pyridine
nitrogen. A trace amount of water in the system
nucleophilically attacks the carbonyl carbon to initiate
such hydrolysis process. The equilibrium state is deter-
mined by the stability of the leaving groups. For the
phenyl amide moiety, the leaving group is the carbamic
acid, which most of the time is quite unstable and yields
the corresponding amine (Ph-NHR) plus CO2. For the
pyridylamine moiety, the 2-aminopyridine anion is
stabilized through tautomerization42,43 which leads to
efficient electron delocalization and further assists in
shifting the equilibrium to the right. Tautomerization
of the 2-aminopyridine anion explains the profound
reactivity deference between the ortho- and para-
pyridyl-based model ureas, 1, 2, 3, and 6. Similarly, the
meta-substituted pyridylurea (i.e., 5) cannot be stabi-
lized through tautomerization, a fact that adds weight
to the proposed photocleavage mechanism.

9780 Mwaura et al.
Macromolecules, Vol. 36, No. 26, 2003
confirmed that the solubility of CO2 in these media is
negligible. Moreover, immediately after addition of
water to a DMSO solution of phenyl isocyanate (known
to produce the unstable phenylcarbamic acid), the
GC-MS trace recorded abundant CO2 and phenylamine.
Upon letting that solution to stay unsealed overnight,
only phenylamine was detected, which confirms that the
phenylcarbamic acid is not stable at room temperature,
let alone the elevated heating within the GC oven. In
accordance to the proposed mechanism, no CO was
detected in all GC-MS experiments, and the very small
peak (near the prominent CO2 peak) results from air,
whose MS shows two prominent peaks at 28 and 32 Da
with intensity ratio always near 3/1 (i.e., N2 and O2).
As shown in Figure 6 and Scheme 4, on the basis of
the described detailed study on various urea model
compounds, it became apparent that polymer I photo-
cleaves in a manner similar to form, among other
photocleavage products, amine-terminated aromatic
moieties and ultimately 2,6-diaminopyridine and p-
phenylenediamine along with releasing of CO2. Several
new peaks (labeled by 1, 2, 3, and 4 in the 1H NMR
spectra of Figure 6) emerged upon UV irradiation. By
comparing the NMR spectra of pure diamines (i.e., 2,6-
diaminopyridine and p-phenylenediamine), a clear match
between peaks 1, 2, and 3 and protons from 2,6-
diaminopyridine is evident. In addition, peak 4 was at
exactly the same chemical shift as the phenyl protons
in p-phenylenediamine. However, the plurality of ad-
ditional peaks generated during prolonged UV radiation
may arise from oligomeric fragments along with further
photoassisted processes that take place in parallel to
photocleavage. These processes have been linked to
photooxidation of the amine-terminated moieties as well
as the two low molecular weight diamine byproducts
mentioned above.
F igu r e 6. H NMR spectra of polymer (I) (a) before irradiation
and (b, c, d) after 10, 30, and 60 min irradiation, respectively.
The top two spectra are the H NMR traces of pure (e) 2,6-
diaminopyridine and (f) phenylenediamine for comparison
purposes (0.05 M in d-DMSO).
To further reinforce this mechanism, we have con-
ducted a series of GC-MS experiments on the photoir-
radiated compound 1 along with two additional model
compounds (phenyl isocyanate and 2-aminopyridine).
Figure S4 in the Supporting Information depicts the GC
traces of photocleavage products of compound 1, injected
from its DMSO solution immediately after irradiation
(A), and after they were allowed to stand unsealed
overnight (B), along with the respective MS assignment
(C) of the corresponding elution peaks of the total ion
chromatogram (TIC). As evident, CO2 was abundantly
detected in the samples immediately after irradiation.
However, for the same samples that were allowed to
stay unsealed overnight, only a very weak CO2 signal
was detected in the TIC, while the remaining byprod-
ucts remain unchanged. This might arise from (a) weak
solubility of CO2 in DMSO, (b) rapid decomposition of
the phenylcarbamic acid, and (c) weak basicity of the
byproduct phenylamine and 2-aminopyridine (also ob-
served in the TIC trace of Figure S4). To test (a) and
(c), GC-MS traces of CO2-bubbled DMSO in the presence
and absence of phenylamine and 2-aminopyridine,
respectively, after having been left unsealed overnight,
On the basis of the above-described NMR investiga-
tion, we now shift our attention to explain the UV-vis
and PL behavior of polymer I upon irradiation, shown
in Figure 1A,B. Figure 7A show the UV-vis absorption
spectral change of a mixture of the polymer I photo-
cleave amine products, 2,6-diaminopyridine, and p-
phenylenediamine as a function of UV irradiation (365
nm). The UV irradiation was carried out on DMF
solutions of the diamines mixture. The absorption at 310
nm decreases slightly while a broad absorption peak
around 410 nm increases with prolonged exposure to
F igu r e 7. UV-vis (A) and PL (B) spectra of mixture of pure 2,6-diaminopyridine and 1,4-phenylenediamaine upon exposure to
UV irradiation (365 nm) for varying periods of time. The PL spectra were obtained from 340 nm excitation for 10-4 M solutions
of each compound in DMF.

Macromolecules, Vol. 36, No. 26, 2003
Pyridyl-Based Aromatic Polyureas 9781
Sch em e 4. P r op osed P h otoclea va ge P r ocess for th e P olym er I
Sch em e 5. P ossible P h otooxid a tion P r od u cts fr om p-P h en ylen ed ia m in e44
Ta ble 1. Rep or ted p Ka Va lu es in CH3CN a n d H2O a lon g w ith Rela tive Ba sicity w ith Resp ect to P yr id in e of Va r iou s
Am in es Gen er a ted fr om th e P h otoclea va ge of Mod el Com p ou n d s 1, 2, 3, a n d 6
UV irradiation. Aromatic diamines, especially p-phe-
nylenediamine, have been shown to form dimerized and
trimerized quinoids when photooxidized.44 The forma-
tion of such quinoid-type compounds as illustrated in
Scheme 5 would explain the evolution of the broad red-
shifted absorption peak, which could also be a result of
formation of charge-transfer complexes between the
amines (donor) and quinoids (acceptor). From reports
in the literature45 and our own experiments with
photoluminescence of mixture of the diamines, it was
found they exhibit intense emission centered ∼390 nm
(when excited at 315 nm). As can be seen from PL
spectra in Figure 7B, of a mixture of 2,6-diaminopyri-
dine and p-phenylenediamine, emission intensity de-
creases as a function of duration of exposure of the
mixture to UV irradiation. This can be explained by the
fact that formation of quinoid compounds, which are
nonemissive, progressively quenches the PL.
Incorporating the photophysical results from the
diamines described above, we can explain the polymer
behavior illustrated in Figure 1. As the polymer pho-
tocleaves, less conjugated, smaller molecular weight
moieties are formed, and this explains the decrease in
absorption at ∼325 nm in Figure 1A. The photocleaved
products, especially p-phenylenediamine, get photooxi-
dized on prolonged irradiation to form quinoids (see
Scheme 5), among other products, that account for the
increasing broad absorption centered at 450 nm. The
initial relative increase and red shift (from 360 to 390
nm) in PL intensity observed (Figure 1B) is attributed
to the steady formation of the photocleavage products,
i.e., 2,6-diaminopyridine and p-phenylenedamine, as
illustrated in Figure 7B. Subsequent reduction in PL
intensity as the polymer sample is irradiated beyond
30 min is attributed to quenching by the nonemissive
photooxidation products formed on prolonged irradia-
tion, as also illustrated in Figure 7B. These observa-
tions, combined with the described NMR data from
model compounds, strongly support the proposed pho-
tocleavage mechanism.
All the described results presented so far were for
liquid samples. Here we shift our attention to solid
samples of polymer I. Films of the polymer were
photoirradiated for similar durations as in solution
experiments and then solubilized in d-DMSO for proton
NMR analysis. The resulting chemical shifts pattern did
not change, and no generation of peaks was observed
(data not shown), suggesting that such a photocleavage
process is not favored in thin films of I. This could be
due to the fact that in solids no water is available for
the photocleavage reaction to occur, and this is consis-
tent with the high degree of hydrogen bonding which
leads to a very crystalline structure.46 In solution,
particularly in polar solvents, the hydrogen bonding is

9782 Mwaura et al.
Macromolecules, Vol. 36, No. 26, 2003
Sch em e 6. Relea se of Tb3+ Ion s a fter P h otoclea va ge
of th e Meta l-Ch ela tin g Ur ea Liga n d
In conclusion, the photocleavage of poly(1,4-phe-
nylene-2,6-pyridylurea) under 365 nm UV irradiation
has been spectroscopically studied through the use of
several urea compounds. The photocleavage was found
to occur through a photoassisted hydrolysis mechanism
in ortho- and para-substituted pyridine ureas. The
amount and chemical accessibility (solution vs solid
phase) of water to these pyridyl urea moieties were
found crucial for this photocleavage process, with O2
playing a role only in subsequent oxidation of the
resulting amine functionalities. Polymeric or oligomeric
analogues of this system might find a number of possible
applications where light-induced generation of bases
and/or release of certain metal cations are desired.
F igu r e 8. PL spectra (excited at 340 nm) of terbium/polyurea
I chelate for various exposure times to UV irradiation. Inset
depicts a closer view of the PL spectra between 400 and 650
nm, where the characteristic Tb3+ emission peaks at 490, 545,
585, and 620 nm gradually disappear with prolonged exposure
time [1:2 (Tb3+/polymer I) molar ratio in DMF (10-4 M of I
based on its monomer repeat)].
disrupted, providing water with adequate volume to
successfully allow the nucleophilic attack at the excited
urea linkage.
Ack n ow led gm en t. The authors thank Dr. Theodore
Goodson, III (Wayne State University), and his research
group for investigating the ultrafast dynamics of this
system (subject of a separate paper) that motivated us
to study the photoinduced transformations of this class
of compounds. We also thank Dr. Xiangqun Xie (Uni-
versity of Connecticut) for additional help with the NMR
experiments. Financial support by AFOSR (Wayne
State PO # Y-30173), NSF CAREER Grant DMR-
970220, and Connecticut Innovations Inc., Critical
Technologies Program, State of Connecticut, is greatly
appreciated.
The ease of formation of amine moieties upon irradia-
tion of these type of ureas, in the presence of H2O, opens
up the opportunity for exploiting these compounds as
(i) photobase generators and (ii) photoassisted metal
releasing agents. As shown in Table 1, the reported pKa
values and relative basicity of the various photocleaved
amines,47,48 stronger bases are formed after the sever-
ance of the urea linkage. Here, because of the insolubil-
ity of urea compounds in water, pyridine is selected to
compare the basicity of the starting urea compounds.
The actual basicity of the starting urea compounds is
much less than that of pyridine due to the fact that the
relatively acidic nature and electron-withdrawing ability
of the urea moiety reduces the basicity of the nearby
pyridine group. It is interesting to note that the basicity
of the photogenerated amines can be controlled by
utilizing different isocyanate compounds for synthesis
of the aminopyridyl-based ureas and polyureas. Also,
the solubility of these kinds of polyureas (in the current
study, soluble only in solvents like DMF and DMSO due
to high crystallinity) can be improved by using mono-
mers with bulky side chain substitutes (preferably
ethylene oxide based, to further increase water uptake)
This is expected to make such polyureas more amor-
phous, leading to higher solubility in solvents amenable
for spin-coating thin films.
Su p p or tin g In for m a tion Ava ila ble: Figures S1, S2, S3,
and S4. This material is available free of charge via the
Internet at http://pubs.acs.org.
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