Biomimetic zinc chlorin-poly(4-vinylpyridine) assemblies: Doping level dependent emission-absorption regimes

Article abstract of DOI:10.1039/c3tc00499f

To develop biomimetic dye-polymers for photonics, two different types of Zn chlorin-poly(4-vinylpyridine) (P4VP) assemblies were prepared by varying Zn pyro-pheophorbide a methylester (ZnPPME) and Zn 3¹-OH-pyro- pheophorbide a methylester (Zn-3¹-OH-PPME) doping levels. ¹H NMR spectroscopy and diffusion ordered NMR spectroscopy (DOSY) studies revealed that a coordinative interaction between Zn chlorin and P4VP was predominant in solution (d₅-nitrobenzene). Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) characterization of bulk samples of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) doped with variable amounts of Zn chlorin showed that the pigment doping transformed the native cylindrical block copolymer nanostructures to lamellar morphologies. The result indicates that the pyridine moiety-Zn chlorin coordination is stronger than the aggregation tendency between the pigment molecules even in the solid state. UV-Vis absorption spectroscopy studies of a Zn chlorin-P4VP thin film showed characteristic monomeric chlorin spectra, while steady-state fluorescence spectroscopy displayed quenching of fluorescence and time-resolved studies indicated shortening of fluorescence lifetimes with an increasing chlorin doping level. Notably, time-resolved fluorescence spectroscopy revealed that the lifetime decay changed from monoexponential to biexponential above 0.5 wt% (ca. 0.001 equiv.) loadings. The Foerster analysis implies that excitonic chlorin-chlorin interactions are observed in the thin films when the distance between the pigment molecules is approximately 50 A. The Zn chlorin-P4VP solid films emit strongly up to 1 wt% (ca. 0.002 equiv.) doping level above which the chlorin-chlorin interactions start to linearly dominate with an increase of doping level, while with 10 wt% (ca. 0.02 equiv.) loading less than 10% of fluorescence remains. Doping levels up to 300 wt% (0.5 equiv.) can be used in absorbing materials without the formation of chlorin aggregates. These defined optical response regions pave the way for photonic materials based on biopigment assemblies.

Full text of DOI:10.1039/c3tc00499f

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Materials Chemistry C
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PAPER
Biomimetic zinc chlorin–poly(4-vinylpyridine)
assemblies: doping level dependent
Cite this: J. Mater. Chem. C, 2013, 1,
2166
emission–absorption regimes†
Ville Pale,‡a Taru Nikkonen,‡b Jaana Vapaavuori,c Mauri Kostiainen,c Jari Kavakka,b
a
a
b
*
*
Jorma Selin, Ilkka Tittonen and Juho Helaja
To develop biomimetic dye–polymers for photonics, two different types of Zn chlorin–poly(4-vinylpyridine)
(P4VP) assemblies were prepared by varying Zn pyro-pheophorbide a methylester (ZnPPME) and Zn 31-OH-
pyro-pheophorbide a methylester (Zn-31-OH-PPME) doping levels. 1H NMR spectroscopy and diffusion
ordered NMR spectroscopy (DOSY) studies revealed that a coordinative interaction between Zn chlorin
and P4VP was predominant in solution (d5-nitrobenzene). Small angle X-ray scattering (SAXS) and
transmission electron microscopy (TEM) characterization of bulk samples of polystyrene-block-poly(4-
vinylpyridine) (PS-b-P4VP) doped with variable amounts of Zn chlorin showed that the pigment doping
transformed the native cylindrical block copolymer nanostructures to lamellar morphologies. The result
indicates that the pyridine moiety–Zn chlorin coordination is stronger than the aggregation tendency
between the pigment molecules even in the solid state. UV-Vis absorption spectroscopy studies of a Zn
chlorin–P4VP thin film showed characteristic monomeric chlorin spectra, while steady-state fluorescence
spectroscopy displayed quenching of fluorescence and time-resolved studies indicated shortening of
fluorescence lifetimes with an increasing chlorin doping level. Notably, time-resolved fluorescence
spectroscopy revealed that the lifetime decay changed from monoexponential to biexponential above
¨
0.5 wt% (ca. 0.001 equiv.) loadings. The Forster analysis implies that excitonic chlorin–chlorin
interactions are observed in the thin films when the distance between the pigment molecules is
˚
approximately 50 A. The Zn chlorin–P4VP solid films emit strongly up to 1 wt% (ca. 0.002 equiv.) doping
level above which the chlorin–chlorin interactions start to linearly dominate with an increase of doping
level, while with 10 wt% (ca. 0.02 equiv.) loading less than 10% of fluorescence remains. Doping levels
up to 300 wt% (0.5 equiv.) can be used in absorbing materials without the formation of chlorin
aggregates. These defined optical response regions pave the way for photonic materials based on
biopigment assemblies.
Received 26th October 2012
Accepted 27th January 2013
DOI: 10.1039/c3tc00499f
www.rsc.org/MaterialsC
and organic solar cells, sensors, optical storage media and
organic lasers.1–9 The advantages of these materials are their
Introduction
Dye-doped organic polymeric materials have signicant
applications in functional materials and optoelectronics, such
as organic electronics, organic light-emitting diodes (OLEDS)
structural versatility, low cost and ease of fabrication, as well
as a strong and fast response to light stimuli.4 Various poly-
mers have been used as hosts in non-covalent anchoring of
the guest dye molecules into the polymer matrix.8 The main
advantages in the supramolecular approach are the avoidance
of synthetic steps required in covalent matrix build-up as well
as easily adjustable doping levels. However, the typical strong
aggregation tendency of dyes is a major drawback in a non-
covalent approach, which in practice oen sets upper limits
for usable doping levels. In turn, especially for thin lms, low
optical density reduces the efficiency of the systems, which is
critical, e.g., for organic lasers.10 Therefore, supramolecular
materials in which the host (e.g. polymer)–guest assembly
overcomes aggregative dye–dye interactions are highly desir-
able. Existing doping dyes cover a wide range of conjugated
molecules, aromatics and heterocycles.11 Nevertheless,
aDepartment of Micro and Nanosciences, Aalto University, P.O. Box 13500, FI-00076
Aalto, Finland. E-mail: ilkka.tittonen@aalto.; Fax: +35 8207227012; Tel: +35
8405437564
bLaboratory of Organic Chemistry, Department of Chemistry, University of Helsinki,
P.O. Box 55, FI-00014, Finland. E-mail: juho.helaja@helsinki.; Fax: +35
8919150466; Tel: +35 8919150430
cDepartment of Applied Physics, Aalto University, P.O. Box 15100, FI-00076 Aalto,
Finland
† Electronic supplementary information (ESI) available: Molecular modelling;
DOSY, SAXS, TEM and SEM measurement data; uorescence lifetime decays;
¨
intermolecular distance and Forster radius calculations. See DOI:
10.1039/c3tc00499f
‡ These authors contributed equally to this work.
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biological chromophores integrated in polymers are rare in
photonic and optoelectronic applications, even though bio-
logical systems have been extensively studied in non-polymeric
solar cell applications.12 We were interested in preparing and
studying chlorophyll based supramolecular dye materials,
which would have functional similarities with their biological
counterparts.
In the photosynthetic systems of green plants and algae,
chlorophylls (Chl) function in light absorption, excitation
energy transfer and charge separation processes.13 A single Chl
pigment can act in any of these roles depending on the
molecular environment and the mutual distance between the
chromophores. In photosynthetic protein complexes the Chl’s
are precisely organized by a protein scaffold to fulll these
tasks. One crucial interaction is the coordination between the
central metal (Mg) of Chl and protein amino acid residue.
Fig. 1a depicts a fragment of light-harvesting complex II
(LHC-II)14 associated with Photosystem II, in which the Mg's of
the Chl's are coordinated by various electron pair donors of
different amino acid residues. Overall, the pigment assembly is
seemingly random; the distances and angles between the Chl
planes vary substantially, and as a whole the system has a high
degree of complexity. Nevertheless, one distinctive feature of
LHC-II is the lack of any intermolecular coplanar plane to plane
interactions.
In contrast to that mentioned above, in green sulphur
bacteria the chlorosomal antennae structures are formed by
self-assembly of chlorosomal chlorophylls without any protein
guidance.15,16 While the former protein complexes are beyond
articial assembly, there are several examples of how to mimic
a chlorosomal J-aggregate assembly by chlorophyll derivatives
or even with porphyrins.17,18 It has been established that suit-
ably functionalized metallochlorins and porphyrins exhibit a
tendency to form self-assembled aggregates essentially by
metal–oxygen electron pair coordination (Fig. 1b).19 In these
tightly packed aggregates the distances of the chromophores
are small, and hence the excitonic couplings are strong
between the molecules. In this strong coupling regime excita-
tion energy is delocalized over multiple chromophores result-
ing in an efficient coherent energy transport that can enable
Fig. 1 (a) Fragment (amino acid residue numbers 215–163 from left to right) of
Spinach light-harvesting complex (LHC II). Chl's bound to Mg atoms are coordi-
close to unity quantum efficiencies in energy transfer nated by various electron pair donors such as the imidazole nitrogen of histidine
processes.20–22
(His or H, residue 212), amide oxygens of glutamine (Gln or Q, residue 197) and
asparagine (Asn or N, residue 183) and carboxylate oxygen of glutamic acid (Glu
In a pioneering chlorophyll–P4VP investigation, Seely has
demonstrated that chlorophylls (a and b) remain in a mono-
or E, residue 180). The figure was produced with a Discovery Studio protein
modeling program using the Protein Data Bank coordinates [PDB accession
meric form in solution with the aid of the polymer. This nding
number 1RWT]. (b) Sketch of self-assembled aggregates formed by Mg–oxygen
was evidenced by the measured absorbance and uorescence electron pair coordination and hydrogen bonding in BChl. (c) Illustration of
spectra in nitromethane solution.23 Recently, we found out that
Zn–pyridine interaction between Zn chlorin and P4VP.
Zn chlorins also tend to bind tightly with pyridine hosts in
solution.24,25 It has been shown in several solution studies that
metalloporphyrins have an affinity to P4VP moieties26 as well as
to other pyridines including supramolecular hosts.27–30 Yet this
topic has not been extensively studied in the solid state until
very recently; Krishnamoorthy and coworkers31 have prepared
Zn porphyrin–P4VP thin lms for eld effect transistors (FETs)
demonstrating that the porphyrin pigment becomes evenly
distributed into the polymer matrix even with high porphyrin–
pyridine moiety ratios.
In this work, we investigate the structural and optical prop-
erties of non-covalent supramolecular Zn chlorin–P4VP assem-
blies in solution and solid state thin lms as schematically
illustrated in Fig. 1c. For this purpose we have chosen Zn pyro-
pheophorbide a methylester (ZnPPME) and Zn 31-OH-pyro-
pheophorbide a methylester (Zn-31-OH-PPME), which are
chemically rather stable and straightforward to derivatise from
algae isolated pigments.
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Experimental section
General
All the reactions were performed under argon in the dark using
1
standard Schlenk techniques. H and DOSY NMR spectra were
recorded at 27 ꢀC using a Varian INOVA 500 MHz Spectrometer.
The DOSY spectra were measured using a Bipolar Pulse Pair
Stimulated Echo (BPPSTE) pulse sequence.32 The small angle
X-ray scattering (SAXS) measurements were performed at the
Dutch–Belgian beamline (BM26) of the European Synchrotron
Radiation Facility in Grenoble using a beam of 10 keV with an
area of 0.35 ꢁ 0.5 mm2 at the sample position. One-dimen-
sional SAXS data were obtained by azimuthally averaging the 2D
scattering data. The magnitude of the scattering vector was
4p
given by: q ¼
sin q, where 2q is the scattering angle. Bright-
l
eld TEM was performed on a FEI Tecnai 12 transmission
electron microscope operating at an accelerating voltage of
120 kV. The absorption and uorescence spectra of the poly-
mer–dye thin lms were measured with a Lambda 950 (Perkin-
Elmer) UV-Vis spectrometer and a Cary Eclipse (Varian) uo-
rescence spectrometer, respectively. For the uorometer, the
used photomultiplier voltage was 800 V and the mono-
chromator entrance and exit slits were 5 nm in all the
Fig. 2 (a) HBr/AcOH, (b) H2SO4/MeOH and (c) Zn(OAc)2/MeOH-DCM.
measurements. In the time-resolved measurements,
frequency-doubled mode-locked Ti:sapphire laser (Coherent,
a
Samples for small angle X-ray scattering (SAXS)
Samples were prepared by dissolving the polymer and chlorin in
chloroform yielding homogeneous mixtures with varying
compositions. The solutions were stirred for 24 h, aer which
the solvent was slowly evaporated at room temperature. The
samples were then dried under vacuum at 30 ꢀC and high-
vacuum annealed for several days.
Mira; FWHM ¼ 150 ps, pulse repetition rate 76 MHz, lexc
¼
410 nm, average excitation intensity 5 W cmꢂ2) was used to
probe the samples. For the detection, a Peltier-cooled micro-
channel plate photomultiplier tube (Hamamatsu: Vbias ¼ 2.7 kV,
acquisition time 45 s) along with single-photon counting elec-
tronics was used. The used scanning electron microscope (SEM)
was Helios Nanolab 600 (FEI).
Samples for transmission electron microscopy (TEM)
The bulk samples prepared as described above were embedded
in epoxy and thin sections (ꢃ60 to 70 nm) were microtomed at
room temperature using a Leica Ultracut UCT-ultramicrotome
and a Diatome diamond knife. Samples were stained in I2 vapor
for several hours in order to improve contrast.
Materials
All the solvents used in the syntheses, sample preparation and
measurements were obtained as HPLC quality and used as
received. All commercial polymers were used without further
purication. Poly(4-vinylpyridine) (P4VP, Mw ꢃ 60 000 g molꢂ1
)
used in NMR measurements was purchased from Sigma
Aldrich. Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP,
Mw ¼ 47 600–20 900 g molꢂ1) used for TEM and SAXS
measurements and poly(4-vinylpyridine) (P4VP Mw ¼ 5100 g
molꢂ1) used for the optical measurements were supplied by
Polymer Source, Inc. The glass wafers which were used as
Samples for optical measurements
Standard solutions of P4VP (5 wt%) and Zn chlorin (0.1 wt%)
were prepared using THF as the solvent. The required amounts
of the P4VP and Zn chlorin standard solutions were mixed
together in order to obtain polymer–dye assemblies with vari-
able doping values (from 0.1 wt% up to 12 wt% for uorescence
measurements and higher doping levels for absorption
measurements). These solutions were le in the dark to stir for
24 h. The Zn chlorin–P4VP solutions were then spin-coated on a
glass substrate that was cleaned using acetone, isopropanol and
DI water, aer which the samples were heated for 2 h at 80 ꢀC in
an oven to remove any residual solvent.
¨
substrates in spin coating were purchased from Menzel-Glaser.
Synthesis
Chlorophyll a was extracted and puried from Spirulina pacica
and converted into pyro-pheophorbide a methylester (1),33
which was used as a starting material for the syntheses. Zn pyro-
pheophorbide
a
methylester24,25 and Zn 31-OH-pyro-pheo-
Results and discussion
phorbide a methylester (31-epimeric mixture, (31R)/(31S) ¼ 1/1)34
(Fig. 2) were synthesized according to literature procedures and The nature of the interaction between ZnPPME and P4VP was
their NMR spectra were in agreement with the published data. studied in solution using 1H NMR spectroscopy. The
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measurements were performed in a weakly coordinating 0.3 ꢁ 10ꢂ9 m2 sꢂ1, for the ZnPPME–P4VP assembly as for P4VP
solvent, d5-nitrobenzene, to make coordination favourable alone. This result shows that in solution, P4VP is capable of
between Zn and the pyridine electron pair. Fig. 3 shows that in tight coordinative Zn chlorin binding even with rather high
the 1H NMR spectrum, chlorin ring methine proton resonances pigment loading.
are shied downeld and clearly broadened aer the addition
In order to probe how pyridine coordination can be utilized
of ꢃ3 equiv. P4VP (molar amount calculated from the amount in the construction of solid material assemblies, we used a PS-b-
of pyridine units in P4VP). Correspondingly, the signals for P4VP diblock copolymer to direct the assembly of ZnPPME. Our
pyridine protons are shied upeld due to being exposed to the main motivation herein was to nd out how well the pyridine
aromatic ring current of the chlorin ring. Together these effects coordination can compete with the Zn chlorin self-aggregation
indicate that the chlorin units are in close interaction with the tendency with high pigment loadings and whether the amor-
polymer in a similar manner to the one we have recently phous material becomes organized and has a homogeneous
measured for tightly bound pyridine derivative–Zn chlorin character. The pure block copolymer without ZnPPME self-
complexes.24,25
assembles into a cylindrical nanostructure, where P4VP alone
1
In addition to the H NMR studies, diffusion ordered NMR forms hexagonally packed cylinders in a PS matrix (Fig. 4a). The
spectroscopy (DOSY) provided us with a convenient tool to small angle X-ray (SAXS) scattering curve from a bulk sample
ꢂ1
˚
probe the formation of the ZnPPME–P4VP assembly, since the shows the most prominent Bragg peak at q* ¼ 0.0132 A
method provides information about molecular mobility in corresponding to the reection from the (hk) (10) plane and a
solution.35 In DOSY experiments, spectra of compounds in the calculated (2p/q*) long period (Lp) of 48 nm. Multiple less
sample are separated according to their diffusion coefficients, intense reections with peak position ratios qn/q*
z
which are proportional to their hydrodynamic volumes. Hence, O3 : O7 : O9 are also observed indicating a hexagonally packed
any monomeric small molecule, when binding to a macromol- cylindrical structure, which is also clearly visible in the trans-
ecule, will experience a signicant decrease in diffusion rate. mission electron microscopy (TEM) image. A cylindrical nano-
The separate DOSY measurements of ZnPPME and P4VP in d5- structure is to be expected as the P4VP weight fraction is 0.31.36
nitrobenzene yielded translation diffusion coefficients When Zn chlorin (0.5 equiv. per pyridine) is added to the
(D values) of 2.1 ꢁ 10ꢂ9 m2 sꢂ1 and 0.3 ꢁ 10ꢂ9 m2 sꢂ1
,
polymer, a clear change in the structure morphology from
respectively.† The obtained values indicate high mobility for the cylindrical to lamellar is observed (Fig. 4b). The ZnPPMEs can
chlorin and sluggish motion for the polymer. The addition of 1/3 be selectively coordinated by the pyridine moieties, thus
equivalents (with respect to pyridine units) of ZnPPME into increasing the volume fraction of the P4VP block to yield a
the polymer mixture gave approximately the same D value, lamellar morphology. The SAXS curve shows the rst peak at a
Fig. 3 1H NMR spectra of (a) P4VP (polymer 52.6 mM; the molarity of pyridine units in the polymer backbone is 30 mM), (b) 1 : 3 mixture of ZnPPME (10 mM) and P4VP
(polymer 52.6 mM; the molarity of pyridine units in the polymer backbone is 30 mM), (c) ZnPPME (10 mM) measured in d5-nitrobenzene.
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Fig. 5 The normalized absorption spectra of (a) P4VP(ZnPPME) and (b) P4VP(Zn-
31-OH-PPME) with different dye loadings. The black broken line shows the
reference spectra of polymer-free chlorin aggregates. The doping level is given as
equiv. (with respect to the pyridine units) of the dye in the polymer.
Fig. 4 Small angle X-ray scattering and transmission electron microscopy data of
(a) PS-b-P4VP(ZnPPME)0 and (b) PS-b-P4VP(ZnPPME)0.5 showing cylindrical and
lamellar morphologies, respectively.
magnitude of the scattering vector q* ¼ 0.0148 Aꢂ1 and multiple
˚
doping of ZnPPME, while the increase of the chlorin doping level
induced slight bathochromic shis for the Soret and Qy bands.
Similarly, the corresponding doping of P4VP with Zn-31-OH-
PPME resistsaggregation up to0.5equiv. doping level, while with
1 equiv. the spectrum shows clearly some characteristics of
aggregation. Overall, the absorption spectra prove that with the
assistance of P4VP, Zn chlorins display characteristic mono-
meric absorption spectra up to 0.5 equiv. chlorin loadings, i.e.,
spectra without notable aggregation characteristics.
Fluorescence properties of the spin-coated thin lms were
studied to nd out how solid-state chlorin emission depends on
the doping level. As a default in the solid state, plain chlorophyll
compounds have a strong tendency to form aggregates which
exhibit very short excited state lifetimes, and therefore negli-
gible uorescence.37 The steady-state uorescence spectra of
spin coated Zn chlorin–P4VP thin lms studied with variable
chlorin doping levels show expectedly that higher dye concen-
trations lead to stronger excitonic chlorin–chlorin interactions
and thus to the quenching of uorescence (Fig. 6a and b).
Additionally, for both dyes the emission maximum wavelength
displays a slight bathochromic shi (ca. 2 nm) with increasing
pigment concentration. The quenching strength for ZnPPME
and Zn-31-OH-PPME in P4VP assemblies was quantied from
Stern–Volmer type plots; the uorescence intensity normalized
with the initial uorescence intensity F0 was plotted as a func-
tion of dye loading. The Stern–Volmer plot in Fig. 6c shows that
uorescence quenches linearly with increasing dye loading;
reections at 2q*, 3q* and 4q* corresponding to a lamellar
morphology with Lp ¼ 42 nm. The change in morphology can
also be clearly observed by TEM, which now shows a lamellar
morphology.
Even with 1.0 equiv. loading of Zn chlorin (with respect to
pyridine units), the SAXS data indicate a dominant lamellar
morphology.† Semiempirical molecular modelling of a P4VP
fragment with the PM6 method conrms that a full, 1.0 equiv.,
loading level would be spatially favourable, i.e., geometry opti-
mized systems show that each pyridine unit coordinates to
ZnPPME.†
With the knowledge of tight coordinative binding at hand, the
next stage was to study the inuence of doping level on the
optical properties. The Zn chlorin–P4VP assemblies (ZnPPME–
P4VP and Zn-31-OH-PPME–P4VP) were spun on glass substrates.
Scanning electron microscope (SEM) and ellipsometer
measurements showed comparatively smooth surfaces, with the
average thickness being ca. 200 nm for the spun lms.† The UV-
vis absorption spectra of Zn chlorin aggregates and Zn chlorin–
P4VP complex lms are presented in Fig. 5a and b. Distinctively,
the Zn chlorins alone form aggregates, the UV-vis spectra of
which display characteristic line broadening and strong bath-
ochromic shis for the Qy band. For ZnPPME and Zn-31-OH-
PPME, the Qy band absorption shis from the monomer to
aggregate are 666 / 677 and 655 / 699 nm, respectively. In the
presence of P4VP the aggregation is prevented with 0.5 equiv.
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<s> ¼ x1s1 + x2s2,
(2)
The results of the calculated uorescence lifetimes are pre-
sented in Table 1 and plotted in Fig. 7 as a function of dye
loading. In accordance with the uorescence intensity data,
averaged uorescence lifetimes hsi decrease when the dye
loading increases. This, together with the observed uorescence
intensity quenching, indicates that the quenching mechanism
is purely dynamic. The quenching results from the energy
transfer between identical molecules, also referred to as homo-
FRET. Analysis of the uorescence lifetimes with low dye load-
ings (up to 0.5 wt%) reveals that the uorescence intensity decay
exhibits essentially monoexponential decay (Table 1). This is
explained by the large relative separation of the chromophores,
low loadings resulting in a weak coupling between the mole-
cules. Interestingly, when the dye loading is increased, the
uorescence intensity starts to show a biexponential decay,
indicating a pronounced intermolecular interaction between
the chromophores. The change from mono- to biexponential
decay takes place between 0.5 and 1.0 wt% doping levels,
indicating that above this region the chlorin–chlorin interac-
tion based quenching mechanism starts to dominate.
Seely has shown in the classic solution studies of P4VP–
chlorin, with several chlorin derivatives, that with low pigment
loadings, i.e. pyridine/Chl > 500, the quantum yields of uo-
rescence become constant.23,38,39 We presume that in our case
the situation is essentially similar; below 1 wt% chlorin loading
levels uorescence lifetimes remain rather constant (Table 1
and Fig. 7), implying that below this level the quantum yields
resemble that of the monomeric pigment. Recently, it has been
shown in a zinc tetraphenylporphyrin study that the excited
state dynamics changed slightly upon the ligation of pyridine as
the uorescence lifetime shortened from 1.95 ns to 1.58 ns in
Fig. 6 The fluorescence spectra of (a) P4VP(ZnPPME) (lexc ¼ 437 nm) and (b)
P4VP(Zn-31-OH-PPME) (lexc ¼ 432 nm) with variable dye loadings. (c) Stern–
Volmer type plots of P4VP(ZnPPME) and P4VP(Zn-31-OH-PPME) assemblies display
quenching of fluorescence upon increase of dye loading. Values obtained for
P4VP(ZnPPME) at lem ¼ 671 nm and for P4VP(Zn-31-OH-PPME) at lem ¼ 660 nm.
Table 1 The fluorescence lifetimes hsi of chlorin–P4VP assemblies with different
doping levels (lexc ¼ 410 nm, values obtained for P4VP(ZnPPME) at lem ¼ 671 nm
and for P4VP(Zn-31-OH-PPME) at lem ¼ 660 nm)
Wt%
s1, ns (x1)
s2, ns (x2)
hsi, ns
P4VP(ZnPPME)
roughly 50% of the emission is quenched at 4 wt% loading,
while 10 wt% quenches 80% of the emission. Nevertheless, at
high doping levels of 6–10 wt% the quenching becomes
increasingly more efficient for Zn-31-OH-PPME–P4VP, indi-
cating that the 31-hydroxyl group starts to compete with P4VP,
leading to enhanced excitonic chlorin–chlorin interactions.
Additionally, time-resolved uorescence measurements were
carried out to study the connection between the uorescence
lifetime and the doping level. The measured decay of the uo-
rescence intensity was tted using a double-exponential func-
tion in eqn (1)
0.1
0.25
0.5
1
2
4
6
8
10
3.68 (98%)
3.63 (99%)
3.14 (94%)
1.78 (67%)
1.31 (84%)
0.36 (60%)
0.37 (80%)
0.30 (69%)
0.10 (97%)
0.45 (2%)
0.72 (1%)
0.85 (6%)
0.69 (33%)
0.46 (16%)
0.13 (40%)
0.12 (20%)
0.08 (31%)
0.50 (3%)
3.62 ꢄ 0.12
3.61 ꢄ 0.16
3.00 ꢄ 0.26
1.42 ꢄ 0.07
1.17 ꢄ 0.16
0.27 ꢄ 0.01
0.32 ꢄ 0.02
0.23 ꢄ 0.03
0.11 ꢄ 0.01
P4VP(Zn-31-OH-PPME)
0.1
0.25
0.5
1
2
4
6
8
10
3.66 (99%)
0.24 (1%)
0.25 (2%)
0.78 (10%)
0.76 (21%)
0.39 (16%)
0.20 (25%)
0.12 (20%)
0.12 (22%)
0.24 (26%)
3.61 ꢄ 0.07
3.45 ꢄ 0.09
2.9 ꢄ 0.05
3.51 (98%)
3.04 (90%)
2.31 (79%)
1.13 (84%)
0.50 (75%)
0.40 (80%)
0.37 (78%)
0.08 (74%)
ꢀ
ꢁ
ꢂ
ꢁ
ꢂꢃ
t
t
1.99 ꢄ 0.19
1.01 ꢄ 0.11
0.43 ꢄ 0.02
0.37 ꢄ 0.02
0.32 ꢄ 0.01
0.12 ꢄ 0.01
IðtÞ ¼ I0 x1exp ꢂ
þ x2 exp ꢂ
;
(1)
s1
s2
where I0 is the initial uorescence intensity aer the excitation
and xi is the amplitude of the exponent term i. The average
uorescence lifetime was approximated using s1 and s2 by
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spectral region, homogeneous lms are easy to prepare, and
moreover, unlike nitrocellulose, P4VP is chemically stable in
material applications.
Conclusions
In the visible spectral region, the polymer host (P4VP) was found
to be an optically transparent scaffold for Zn chlorins which are
analogous to chlorophyll–protein complexes. The spin coated Zn
chlorin–P4VP thin lm assemblies indicated that coordinative
pyridine moiety–Zn chlorin bonding was dominating over
chlorin–chlorin assembling tendency up to 0.5 equiv. doping
levels. Upon further increase of Zn chlorin doping, linear
quenching was observed for steady-state uorescence. The
quenching process was identied to be dynamic quenching
resulting from the energy transfer (FRET) between identical
Fig. 7 The fluorescence lifetimes for P4VP(ZnPPME) and P4VP(Zn-31-OH-PPME)
assemblies given as a function of dye weight percent.
¨
molecules. The approximated Forster distance R0 correlated well
with the literature values for related dyes. Doping levels for
absorbing materials lie at ca. 0.5 equiv. (ca. 300 wt%) and for
emitting materials at ca. 0.002 equiv. (ca. 1 wt%).
toluene and from 2.05 ns to 1.61 ns in polystyrene/toluene
mixtures respectively.40 Yet these uorescence lifetime changes
are rather small and thus have only a minor effect on the
quantum yield. We expect that for our chlorin system the
behaviour is similar.
The photonic application potential of the studied assemblies
lies in the light absorbing–harvesting realm that was initially
anticipated from the antennae biomimicry. The small Stokes
shi (ꢃ6 nm) makes these dye assemblies alone less attractive
for emission applications. To overcome this limitation, Tamiaki
et al. showed in chlorin aggregate studies that a bacteriochlorin,
i.e., a more red-shied chlorin, integrated into an aggregate was
able to receive and emit the light harvested by the chlorin
assemblies.43 We propose that a similar strategy would also be
viable for the studied assembly to strengthen its emission effi-
ciency, e.g., for solid state lasers, single photon emitters, optical
switches and OLEDs. Another material development option
would be in the high doping absorption regime; Zn chlorin–
P4VP assemblies could be built in contact with electron accep-
tors to construct optoelectronic devices such as solar cells
and FETs.
Finally, to gain further insight into Zn chlorin–P4VP
assemblies, we calculated intermolecular chlorin–chlorin
distances at different doping average values (from 0.1 to 10 wt
%) using both microscopic and macroscopic approximations
assuming homogeneous chlorin distribution in P4VP.† The
microscopic approximation was obtained using the Connolly
solvent excluded volumes for the pigment and the polymer 4-VP
unit. The macroscopic approximation was calculated using the
material density to obtain the chromophore number density in
a square lattice. The uorescence lifetimes of ZnPPME–P4VP
and Zn-31-OH-PPME–P4VP assemblies were plotted as a func-
tion of intermolecular distances to reveal a shortening of the
uorescence lifetimes upon decrease of chlorin–chlorin dis-
¨
tances.† The calculated Forster distance (R0) using the micro-
1
˚
scopic approximation for ZnPPMEs was 47.8 A and for Zn-3 -
˚
OH-PPMEs it was 48.3 A, whereas the corresponding values for
Acknowledgements
˚
˚
the macroscopic approximation were 44.7 A and 45.2 A for
ZnPPMEs and Zn-31-OH-PPMEs, respectively. Noticeably, both
approximations yielded similar results. These values are in good
accordance with Forster distances, 47 A and 57 A, reported
between Chl b–Chl a and Chl b–Chl b, in a light-harvesting a/b
complex (LHCIIb).41
For comparison, van Zandvoort et al. have successfully
prepared nitrocellulose–Chl a assemblies on lms with rela-
tively high doping levels by casting the lms from DMSO solu-
tions.42 Consistently, photophysical studies of these lms
showed similar optical properties for the studied chlorins
including monomeric absorption and emission spectra of Chl a
at low concentrations. Additionally, the measured uorescence
lifetime decay was monoexponential at low concentrations and
biexponential at higher concentrations. Also, the calculated
Dr Marco Mattila is kindly acknowledged for his assistance with
the time-resolved uorescence measurements. We gratefully
acknowledge the Academy of Finland [project no. (I.T.) 129043
and (J.H.) 135113], the Graduate School of Organic Chemistry
and Chemical Biology (GSOCCB) and the Finnish National
Graduate School for Materials Physics for nancial support. The
National Centre for Scientic Computing (CSC) is acknowl-
edged for computational resources.
˚
˚
¨
Notes and references
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2172 | J. Mater. Chem. C, 2013, 1, 2166–2173
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Paper
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