UV-induced fluorescence recovery and solubility modulation of photocaged conjugated oligomers

Article abstract of DOI:10.1039/c1jm12542g

This article describes photoactivatable conjugated oligomers that show both increased fluorescence quantum yield and pH-dependant solubility upon irradiation with UV light. Sonogashira coupling between ester-substituted phenylacetylenes and alkoxy-substituted diiodo phenylene-vinylenes yielded conjugated phenylene-ethynylene/phenylene-vinylene oligomers. Oligomers with nitrobenzyl ester moieties had quenched fluorescence in polar solvents; UV irradiation restored their quantum yield of fluorescence to that of corresponding alkyl ester-substituted oligomers. These photocaged oligomers also exhibited UV-induced changes in solubility consistent with photogeneration of carboxylic acids. This approach is therefore effective at tuning the properties of conjugated organics with light after traditional synthetic operations, and has potential for use in photoactivatable fluorophores or solution-processable multilayer devices.

Full text of DOI:10.1039/c1jm12542g

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PAPER
UV-induced fluorescence recovery and solubility modulation of photocaged
conjugated oligomers†
*
Robert H. Pawle, Victoria Eastman and Samuel W. Thomas, III
Received 3rd June 2011, Accepted 20th July 2011
DOI: 10.1039/c1jm12542g
This article describes photoactivatable conjugated oligomers that show both increased fluorescence
quantum yield and pH-dependant solubility upon irradiation with UV light. Sonogashira coupling
between ester-substituted phenylacetylenes and alkoxy-substituted diiodo phenylene-vinylenes yielded
conjugated phenylene-ethynylene/phenylene-vinylene oligomers. Oligomers with nitrobenzyl ester
moieties had quenched fluorescence in polar solvents; UV irradiation restored their quantum yield of
fluorescence to that of corresponding alkyl ester-substituted oligomers. These photocaged oligomers
also exhibited UV-induced changes in solubility consistent with photogeneration of carboxylic acids.
This approach is therefore effective at tuning the properties of conjugated organics with light after
traditional synthetic operations, and has potential for use in photoactivatable fluorophores or solution-
processable multilayer devices.
functional groups, such as substitution of terminal bromides with
Introduction
trimethylamine or esterification of carboxylic acids,5–7 is also
a common strategy. A valuable alternative strategy is to develop
materials that have tunable properties after traditional synthetic
operations or processing is complete. Successful approaches to
the post-processing tuning of conjugated polymers have included
thermally cleavable esters8–10 or acid-labile tetrahydropyran-
protected alcohols11,12 on poly(thiophene) backbones. Solution-
processed thin films of these materials become hard and insoluble
and display increased chromophore density and stability upon
thermolysis.13–15
This article describes conjugated oligomers that have both
increased fluorescence efficiency and pH-dependant solubility
upon irradiation with UV light. We anticipate this strategy will
be applicable to new caged fluorophores and solution-processed
multilayer organic devices. Conjugated materials behave as
organic semiconductors: they can conduct holes and electrons,
and have mobile excitons upon excitation. Such materials have
therefore become increasingly popular for a number of applica-
tions, such as photovoltaics,1 sensors,2 and light-emitting
devices.3 They offer several unique advantages over inorganic
semiconductors; for example, when substituted with solubilizing
groups, many conjugated materials are solution-processable,
enabling coating techniques that do not require high temperature
or vacuum, such as spin-casting and roll-to-roll processing.4 In
addition, properties such as intensity of fluorescence or solubility
depend on chemical structure, which chemists can tune through
rational design and the modularity of many of the chemical
transformations (such as cross-coupling) used in their syntheses.
The most common approach for tuning the properties of
conjugated polymers is to design and synthesize different
monomers and corresponding polymers, with each polymer
prepared having a given set of properties. Post-polymerization
modification of conjugated polymers that have reactive
We are interested in developing photochemical strategies for
tuning the properties of conjugated materials after the comple-
tion of traditional synthetic steps. Photochemical processes have
the advantage that many of the properties of light are easy to
control. These properties include spatiotemporal distribution,
intensity, wavelength, and polarization. Much of the work in this
area has focused on photochemical control of the solubility of
specially designed conjugated polymers, akin to negative-tone
photolithography. These examples, to our knowledge, have
relied either on photoinduced heating16,17 or photoacid genera-
tors11 to promote loss of a labile solubilizing group. Other studies
have focused on photochemically controlling fluorescence effi-
ciency of conjugated polymers through switching of a photo-
chromic pendant group18–22 or photochemical synthesis of
a conjugated backbone from a precursor material.23,24 In this
paper, we report conjugated oligomers, substituted with nitro-
benzyl esters (NBEs), that show photoactivated changes in
fluorescence quantum yield and solubility.
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford,
MA, 02155, USA. E-mail: sam.thomas@tufts.edu; Fax: +1-617-627-
3443; Tel: +1-617-627-3771
† Electronic supplementary information (ESI) available: Full
experimental section, including synthetic procedures, absorbance and
fluorescence spectra, and additional experimental details. See DOI:
10.1039/c1jm12542g
The molecules discussed in this paper contain two key struc-
tural elements. One is a conjugated backbone that contains both
phenylethynyl (PE) and phenylvinyl (PV) linkages. There have
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been a number of reports of conjugated polymers that include
a combination of these two structural moieties.25–27 Such poly-
mers offer a unique combination of properties, such as high
quantum yield of fluorescence (FF), red-shifted absorbance and
emission spectra as compared to similarly substituted poly(phe-
nylene ethynylene)s (PPEs), and reduction potentials and
oxidative stabilities that are more favorable than those of poly-
(phenylene vinylene)s (PPVs).25 As such, they are a potentially
useful but underutilized class of conjugated materials. Moreover,
only several individual reports have been published concerning
the synthesis and properties of corresponding oligomeric struc-
tures,28–30 even though conjugated oligomers can be rigorously
purified and identified, and can also serve as active components
of devices. We chose PE/PV oligomers because they have
substantial absorbance in the visible region of the spectrum,
where photocleavable nitrobenzyl esters do not absorb light, but
still allow installation of these esters using the functional group-
tolerant Sonogashira reaction. Additionally, the five-arene
conjugated oligomer backbone we used has been shown to be
a suitable model for the photophysical properties of the corre-
sponding P(PE/PV) conjugated polymers.31
Fig. 1 Structures of conjugated oligomers described in this study, and
The second structural element of our design is the o-nitro-
benzyl ester group. This moiety is both photolabile upon irra-
diation with UV light32 and a fluorescence quencher by
photoinduced electron transfer (PET). Caged fluorophores have
become important tools in applications such as cellular
imaging.33 Although other novel photochemical strategies for
caged fluorophores have been reported,34–36 nitrobenzyl groups
remain a popular chemical strategy in the design of photo-
activatable fluorescent dyes and quantum dots.37–43 In addition
to fluorescence modulation by PET, nitrobenzyl groups can
‘‘cage’’ other properties such as drug release, electrostatic inter-
actions, or bioactivity.44–46
general reaction conditions for their preparation.
gave photolabile terminal alkyne 9. Sonogashira coupling
between 6b and 8 or 9 gave the target PE/PV oligomers 1 and 1Q.
This synthetic strategy was generally applicable; we prepared
1OMeQ, which is identical to 1Q except that each photolabile
ester contains two methoxy groups. Esterification of 4-iodo-
benzoic acid yielded aryl iodide 10, and subsequent Sonogashira
coupling with TMSA and deprotection of the silyl group with
tetrabutylammonium fluoride gave terminal acetylene 12.
Sonogashira coupling between 12 and diiodide 6b gave the target
bis(dimethoxynitrobenzyl ester)-substituted PE/PV oligomer
1OMeQ. In addition, as illustrated in Fig. 3, lowering the stoi-
chiometry of the terminal alkyne in the Sonogashira reaction
between 6b and 9 gave a separable mixture of the singly coupled
(13) and doubly coupled (1Q) products; subsequent coupling of
13 with 8 yielded unsymmetrically substituted 2, which had the
same conjugated backbone as the other derivatives, but only one
labile NBE quencher, as opposed to the symmetric derivatives,
which each had zero or two NBE groups.
Results and discussion
Synthesis of conjugated oligomers
We initially targeted symmetrically-substituted 5-ring PE/PV
derivatives (Fig. 1) that bore either fluorescence quenching,
photolabile nitrobenzyl esters (1Q, 1OMeQ, 3Q) or photoinert
alkyl esters (1, 3). The preparation of symmetrically substituted
benzoic ester-terminated PE/PV derivatives (1, 1Q, 1OMeQ)
followed a convergent pathway, as shown in Fig. 1 and 2. We
prepared the central diiodide components with either dime-
thoxyphenylene (6a) or dioctyloxyphenylene rings (6b) via
a modified literature procedure of Horner–Wadsworth–Emmons
olefination of the corresponding diphosphonate with 4-iodo-
benzaldehyde.29 We prepared the terminal alkynes by Sonoga-
We used our modular synthetic approach to prepare other
derivatives with different substituents at the terminal positions,
including alkoxy-terminated PE/PV analogs 3 and 3Q (Fig. 4).
Alkylation of 4-iodophenol with methyl 5-bromopentanoate,
followed by Sonogashira coupling with TMSA, gave common
precursor 15. Deprotection of the silyl group with KOH in THF/
methanol for 30 min yielded terminal unit 16, while deprotection
with NaOH in refluxing water/methanol resulted in both
deprotection of the silyl group and hydrolysis of the methyl ester,
yielding acid 17. DCC-mediated esterification of 17 with 2-
nitrobenzyl alcohol gave NBE-functionalized terminal acetylene
18. We used identical Sonogashira coupling conditions with
methoxy-substituted diiodide 6a and either 16 or 18, yielding
alkoxy-substituted PE/PV analogs 3 and 3Q, without and with
photolabile NBE groups, respectively. Additional examples of
the modularity of our synthetic approach are oligomers with
different terminal electron withdrawing groups (a benzonitrile
shira
coupling
of
methyl
4-iodobenzoate
and
trimethylsilylacetylene (TMSA), followed by simultaneous
deprotection of the silyl group and hydrolysis of the methyl ester
in a refluxing solution of sodium hydroxide to give the common
precursor 4-ethynyl benzoic acid (7).47 Acid-catalyzed esterifi-
cation of 7 with n-BuOH yielded butyl 4-ethynylbenzoate (8),
which we used because PE/PV derivatives with methyl esters
tended to be poorly soluble. N,N0-dicyclohexylcarbodiimide
(DCC)-mediated esterification of 7 with 2-nitrobenzyl alcohol
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Fig. 2 Synthesis of terminal alkynes for benzoic ester PE/PV oligomers.
derivative, 4) or terminal heteroaromatic rings (a thiophene
derivative, 5).
Absorbance and fluorescence of conjugated oligomers
Table 1 summarizes photophysical parameters of oligomers 1–5;
all of the PE/PV oligomers have absorption maxima around 420
nm with extinction coefficients of approximately 8 ꢀ 104
Mꢁ1cmꢁ1, and fluorescence maxima between 470 and 490 nm in
solvents such as THF and CH2Cl2 (DCM). As we expected from
our experimental design, the absorbance and fluorescence spectra
of 3, which lacks clear donor–acceptor character, are between
those of similarly substituted phenylene-ethynylenes48 (lmax
383 nm, lem ꢂ 430 nm) and phenylene-vinylenes49 (lmax
¼
¼
426 nm, lem ¼ 500 nm) that have the same number of arenes and
Fig. 3 Synthesis of unsymmetric oligomer 2.
exocyclic multiple bonds reported in the literature. The quantum
Fig. 4 Synthesis of alkoxy-terminated PE/PV oligomers.
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Table 1 Absorbance and fluorescence of conjugated oligomers
c
FF
l
max (abs)a/nm
l
max (em)a/nm
C6H6
THF
DCM
ACN
1
1Q
1OMeQ
2
3
3Q
4
423
424
423
424
419
418
426
422
491
494
493
494
472
472
496
482
0.71
0.73
0.69
0.72
0.74
0.71
0.77
0.77
0.75
0.59
0.64
0.65
0.74
0.66
0.71
0.72
0.75
0.25
0.28
0.31
0.68
0.52
0.73
0.77
0.72
0.09
0.10
0.13
b
b
0.77
0.75
5
a
In CH2Cl2. b Not determined due to solubility. c Reported relative to Coumarin 6 in ethanol (FF ¼ 0.78).
yields of fluorescence of all the oligomers that do not have NBE
show absorbance maxima under 420 nm, while the molecules
terminated with electron withdrawing esters (1, 1Q, 1OMeQ) or
nitriles (4) show absorbance maxima slightly above 420 nm.
These interactions have a more significant impact on emission
spectra, with a shift of 24 nm between 3 (472 nm) and 4 (496 nm)
in CH2Cl2. Molecules with stronger donor–acceptor character
are also solvatochromic: compound 1 has maximal fluorescence
intensity at 470 nm in hexanes and 497 nm in acetonitrile (DE ¼
0.14 eV).
moieties are high (0.7 < FF < 0.8) in all solvents investigated
5 shows
(benzene, CH2Cl2, THF, and acetonitrile). Fig.
normalized absorbance and emission spectra of compounds 1
and 3 in CH2Cl2.
Donor–acceptor
interactions
between
the
central
dialkoxyphenylene (a common structural component among all
oligomers studied here) and the terminal rings of these five-ring
systems result in a bathochromic shift of the optical spectra:
molecules terminated with electron donating alkoxy (3, 3Q) units
In the visible region of the spectrum, conjugated oligomers
that have bound NBE quenchers (1Q, 1OMeQ, 2, 3Q) have
solution absorption spectra that are nearly identical to analogous
derivatives that do not have covalently bound quenchers (1 and
3), which indicates that the presence of the quenchers has
minimal impact on the ground state conjugated chromophore. In
the UV portion of the spectrum, the quenched molecules 1Q and
1OMeQ absorb significantly more light than 1—up to 320 nm for
nitrobenzyl esters and 390 nm for dimethoxynitrobenzyl esters
(Supplementary Information†)—consistent with known absor-
bances of the NBE moieties.50 Excitation spectra of NBE-linked
fluorophores show spectra that have the same shape as absorp-
tion spectra of the analogous unquenched fluorophores.
Steady-state Stern–Volmer analysis of 2-nitrobenzylacetate as
a quencher of 3 resulted in a Stern–Volmer constant of 11–14
Mꢁ1, corresponding to bimolecular rate constant on the order of
1010 Mꢁ1sꢁ1 (the experimentally determined lifetime of 3 was 1.1
ns). Consistent with this observation, the molecules with cova-
lently bound NBEs had lower quantum yields of fluorescence
than the molecules without covalently bound NBEs. For
instance, the quantum yields of fluorescence of 1 and 1Q in
acetonitrile are 0.72 and 0.09, respectively. Monosubstituted 2
also showed quenched fluorescence, although it was 20–40%
more fluorescent than 1Q.
The difference in fluorescence efficiency between a conjugated
oligomer and its analog that bears NBEs depends heavily on the
polarity of the solvent: while FF of non-quenched molecules did
not depend strongly on solvent, FF of quenched molecules
decreased in solvents of higher polarity. In all examples studied
the ranking of quantum yield in solvents (numbers in parentheses
are ET(30) values) was benzene (34) > THF (37) > CH2Cl2 (41) >
acetonitrile (MeCN) (46). The effect is more pronounced in the
ester-terminated analog, probably due to the proximity of the
quencher to the fluorophore. In addition, the fluorescence
spectra of pairs of non-quenched and quenched chromophores
Fig. 5 Top: Absorbance (black lines on left) and emission spectra (blue
lines on right) of compounds 1 (solid lines) and 3 (dashed lines) in
CH2Cl2. Bottom Left: Stern–Volmer plot showing fluorescence quench-
ing of oligomer 3 by o-nitrobenzylacetate. Bottom Right: Dependence of
quantum yield of fluorescence of 1 (red diamonds) and 1Q (black circles)
on the solvent parameter ET(30).
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(1 and 1Q, for example) differ only in their fluorescence effi-
ciency; the shapes of the emission spectra are indistinguishable.
These observations support the conclusion that quenching
occurs via photoinduced electron transfer,41 as higher polarity
solvents stabilize the charge-separated state resulting from elec-
tron transfer.
Fluorescence photorecovery
Fig. 6 shows the photocleavage reaction of nitrobenzyl esters that
we anticipated upon irradiation of the quencher-linked oligomers
with UV light. Absorption of a photon gives a carboxylic acid
and 2-nitrosobenzaldehyde as the products. Our design of these
molecules was such that UV-induced photocleavage would break
a bond connecting the nitroaromatic quencher and the conju-
gated fluorophore. This reaction would shut down the PET
quenching pathway, because although quenching might still be
energetically favorable, it would be diffusion-limited (vide supra:
ksv ¼ 11–14 Mꢁ1). We therefore expected that irradiation of the
NBE-linked oligomers would enhance fluorescence, with the
product a highly fluorescent conjugated carboxylic acid.
Fig. 7 Emission intensity of monosubstituted conjugated oligomer 2
before and after 6 min of irradiation at l > 295 nm with a 200 W Hg/Xe
lamp. Inset: Quantum yield of fluorescence vs. irradiation time.
Fig. 7 shows that UV photolysis of a sample of 2 in acetonitrile
[with 0.03% butylated hydroxytoluene (BHT), vide infra] resulted
in emission spectra consistent with these expectations. To
perform these experiments, we used a 200 W Hg/Xe ozone-free
lamp with a 295 nm high-pass filter. The quantum yield of
fluorescence of the photolyzed sample reached a value similar
that of 1 (ꢂ0.7) after 4 min of these irradiation conditions, while
the shapes of both of emission and excitation spectra remained
unchanged during the experiment. The recovery of emission
efficiency also occurred in other solvents, such as THF or
CH2Cl2, although the contrast in FF before and after photolysis
was smaller because the fluorescence quenching of the unirradi-
ated oligomers was not as efficient in solvents less polar than
acetonitrile.
fluorescence recovery of 1OMeQ was approximately two times
faster than 1Q: dialkoxy nitrobenzyl groups have ꢂ5 ꢀ larger
absorption cross-sections at 365 nm than unsubstituted nitro-
benzyl groups, although they have lower quantum yields of
reaction.50 Control compound 1 did not show a statistically
significant change in quantum yield during the irradiation,
consistent with its lack of photocleavable moieties. The change in
absorbance of all compounds under these irradiation conditions
at 365 nm was 10% or less. Irradiation using shorter wavelength
light (such as with a 295 nm long-pass filter) also resulted in
a similar recovery of fluorescence quantum yield, but also
resulted in more significant decomposition as reflected in
decreases in absorbance for all compounds irradiated. Homolytic
cleavage of the C–O bond in alkyl ethers is known to occur
during irradiation of related materials in the absence of oxygen.51
Fig. 8 shows a comparison between the rates of recovery of
quantum yield of benzoic ester-based oligomers 1Q, 1OMeQ,
and 2 upon irradiation using a 365 nm interference filter. We
observed recovery of fluorescence quantum yield within 20 min
of irradiation of a 60/40 (v/v) THF/MeCN solution of disubsti-
tuted 1Q or 1OMeQ or monosubstituted 2. The disubstituted
molecules reach quantum yields (0.62–0.68) similar to that of the
independently synthesized 1Q-hn (0.7 ꢃ 0.07). The UV-induced
Fig. 8 Dependence of quantum yield of caged (1Q, 1OMeQ, 2) or non-
caged (1) conjugated oligomers as a function of UV irradiation time in 60/
40 THF/MeCN (v/v) using a 365 nm interference filter.
Fig. 6 Photocleavage of nitrobenzyl ester quenchers from 1Q to yield
‘‘unquenched’’ dicarboxylic acid 1Q-hn.
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Addition of 0.03% BHT to the solutions significantly inhibited
this decomposition, and did not affect the photorecovery of
fluorescence efficiency significantly.
experiments support this conclusion: i) the solubility of an
identical biphasic sample of 1OMeQ or 1Q that is not irradiated
does not change; therefore, hydrolysis of the nitrobenzyl ester
was not occurring; ii) identical irradiation of a biphasic sample of
1, which has no NBE groups, showed no change in solubility;
therefore, any background photoreactions of the conjugated
oligomer do not contribute to the observed change in solubility;
iii) an independently synthesized sample of the diacid 1Q-hn (by
hydrolysis of 1 under basic conditions) shows the same depen-
dence of solubility on pH as the irradiated sample of 1Q or
1OMeQ.
Photomodulation of solubility
During some of our attempts to synthesize PE/PV oligomers
without long solubilizing alkoxy chains, we noted such deriva-
tives tended to be only sparingly soluble—for example, a deriv-
ative with two methoxy groups on the central ring and a methyl
ester on each terminal ring was not sufficiently soluble to obtain
a 1H NMR spectrum of high quality. We therefore reasoned that
photocleavage of nitrobenzyl ester groups could change the
solubility of these conjugated oligomers, particularly because the
photoproduct is an ionizable carboxylic acid. The photocleavage
of nitrobenzyl esters is known to affect the solubility of metal
Conclusion
Our work presents a novel strategy for altering the properties of
conjugated materials with light. Here we demonstrated recovery
of fluorescence and modulation of solubility with photolabile
nitrobenzyl esters. This approach is a new example of altering
key macroscopic properties of these technologically important
materials after synthetic and purification steps have been
completed. We anticipate that our strategy will be useful in
methods for constructing photopatterned multilayer films of
conjugated polymers that do not require materials with orthog-
onal solubility, and for increasingly efficient photoactivatable
fluorophores due to the amplification of fluorescence quenching
in conjugated materials. Future work in our laboratory will focus
on further tuning of the solubility of materials before and after
photolysis, incorporation of NBEs into conjugated polymers, as
well as addressing the current drawbacks of this approach, such
as incorporating NBE groups into more photostable conjugated
backbones,54 and removing photolysis byproducts from solid-
state samples.
complexes
and
polymer
films
for
photopatterning
applications.52,53
Fig. 9 shows the result of a photolysis experiment using dini-
trobenzyl ester 1OMeQ. We irradiated a biphasic mixture of
1OMeQ dissolved in CH2Cl2 and an aqueous solution of 0.1 M
NaOH with 365 nm light from a 200 W Hg/Xe for 45 min. After
shaking the irradiated sample, the conjugated oligomer switched
from being fully dissolved in the organic layer to being insoluble
in both the organic and basic aqueous layer; it precipitated as
a yellow solid. Acidification of the aqueous layer with HCl and
agitation caused the chromophore to dissolve in the organic
layer. Compound 1Q showed similar behavior (ESI).
These observations are consistent with Fig. 6: photolysis of 1Q
or 1OMeQ yields the dicarboxylic acid, which is soluble in
CH2Cl2. Deprotonation by the aqueous base, however, gives the
disodium salt of the dicarboxylate, which is insoluble in both the
aqueous and organic layers. The relative concentrations of diacid
and dicarboxylate subsequently depend on pH. Three control
Acknowledgements
The authors thank Tufts University for support of this research
and the U.S. Department of Education for a GAANN Fellow-
ship (R.H.P.), as well as Dr. Trisha Andrew and Prof. Timothy
Swager for assistance in obtaining the fluorescence lifetime of
compound 3.
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