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media. Also, the medium, such as copolymer micelles,13,14 ionic
liquids,15 nucleic acids, polypeptides, proteins, and carbon
nanotubes, is capable of influencing porphyrin aggregation
behavior.
porphyrin-based dyes, commercial TSPP (4) and an analogue
(5) in acidic aqueous solution, was investigated without and
with presence of the homopolymer template. The aggregation
properties were evaluated by UV−vis absorption, fluorescence,
and fluorescence lifetime decay studies, clearly demonstrating J-
aggregation was more pronounced in dyes in the presence of
the polymer, possibly due to enhanced stabilization of the
anionic dye’s periphery by the cationic nature of the pendant
Investigation of polymer-based self-assembled J-aggregation
of dyes is intriguing and of current interest. Santoro et al.
demonstrated that the tetraanionic meso-tetrakis(4-
sulfonatophenyl)porphine (H2TSPP) in the pH range 5−12
exists in a monomeric form, and its fluorescence is not pH-
dependent.16 However, in the presence of polylysine,
absorption, circular dichroism, and resonant light scattering
data indicate extensive polymer-induced self-aggregation of the
porphyrins. In particular, at low pH (<7), the protonated
polylysine promotes porphyrin binding and self-aggregation
with consequent strong quenching of fluorescence.16 Periasamy
et al. observed that poly-(L, D, or DL)-lysine, depending on
optical chirality, induced J-aggregation of TSPP more efficiently
than monomeric lysine.17a Only micromolar concentrations of
polylysine were required for complete conversion of the
porphyrin monomer to its J-aggregate.17 Quite impressively,
Whitten et al. demonstrated “superquenching” of polyelec-
trolytes containing cyanine pendant polylysines (repeat unit:
1−900) both in solution and after adsorption onto silica
nanoparticles.18 The self-assembled polymer-initiated surface
activated quenching led to formation of J-aggregates due to
enhanced binding with an increasing number of repeat units of
the polymer.18 Laponite clay exerted a similar effect with
cyanine dyes to induce J-aggregation.19 Zhao et al. recently
reported the micellization of poly(ethylene glycol)-block-
poly(4-vinylpyridine) (PEG114-b-P4VP61) induced by TPPS
in acidic solution, where the core contained TSPP/PV4P and
the shell was structured with PEG. TSPP formed J-aggregates
and H-aggregates in the micellar core at pH 1.5−2.5 and 3.0−
4.0, respectively.14 Kano et al. found that the TSPP-acid form
was stabilized to induce J-aggregation by binding with ferric
myoglobin (metMb) in water at neutral pH due to
encapsulation and fixation by the relatively rigid protein
molecules. The hydrophobic core of the J-aggregate caused
the deformation of the secondary structure of the metMb, and,
thus, denaturation of the protein.20 Chmelka et al. reported that
mesostructured silica-block copolymer thin films provided
orientationally ordered host matrixes for stable alignment of
coassembled porphyrin J-aggregates with anisotropic optical
properties.21 Smith et al. reported the induction of J- and H-
aggregation of TSPP by the cationic polyelectrolyte poly-
(diallyldimethylammonium chloride) (PDDA) on films depos-
ited on Si. The films were made by dipping in alternating
aqueous solutions containing film components (layer-by-layer
deposition).22 From these and other reports, it is evident that
there is substantial interest in inducing and controlling
porphyrin aggregation.
+
ammonium moieties (−NHMe2 ) of the polymer at low pH.
Finally, 2PA was determined for free (unaggregated) and
aggregated (without and with the polymer template)
porphyrins 4 and 5.
EXPERIMENTAL SECTION
■
Materials. 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin
(TSPP) and tetraphenylporphyrin (TPP) were purchased from
Strem and Aldrich, respectively. 6-N,N-Dimethylaminohexanol was
obtained from TCI America. Grubbs’ first generation catalyst and
norbornene carboxylic acid were purchased from Aldrich. The
norbornenyl acid chloride 1 was prepared as previously reported.23
All solvents were purified and dried according to standard procedures.
Synthesis. Synthesis of 6-(N,N-Dimethylamino)hexylbicyclo-
[2.2.1]hept-5-ene-2-carboxylate (2). Acid chloride (1) (10.8 g,
0.069 M) was dissolved in freshly dried THF. Then, a mixture of 6-
N,N-dimethylaminohexanol (14.22 mL, 0.086 M) and NaHCO3 (11.6
g, 0.14 M) was added to the solution at room temperature under N2
and refluxed overnight. After the reaction was complete, the mixture
was filtered to remove the salt, and THF was removed under reduced
pressure. This was followed by washing with water and extraction with
CH2Cl2, and then drying over anhydrous Na2SO4. Colorless oil was
obtained after column chromatography with 3:1 CH2Cl2/MeOH,
1
solvent removal, and vacuum drying (13.73 g, 75%). H NMR (500
MHz, CDCl3) δ: 8.98 (NH, 1H), 6.18−5.79 (m, 2H, HCCH),
4.09−3.82 (m, 2H), 3.13 (s, 0.5H), 3.04−2.68 (m, 2H), 2.42−2.06 (m,
8H), 1.92−1.73 (s, 1H), 1.64−1.06 (m, 11H). 13C NMR (125 MHz,
CDCl3) δ: 176.24, 174.69, (CO exo and endo) 138.02, 137.55,
135.74, 132. 36, 64.29, 63.38, 59.40, 46.74, 45.76, 43.07, 43.31, 30.40,
30.22, 29.27, 28.88, 28.34, 26.90, 25.79. HR-MS-ESI theoretical m/z
[M+H]+ = 266.21, found 266.21.
Synthesis of Polymer 3. ROMP of monomer 2 with Grubbs’ first
generation catalyst was performed as shown in Scheme 1. The
glassware was dried, evacuated under vacuum, and purged with N2 in a
Schlenk line several times prior to conducting the polymerization
reaction. A solution (0.2 M) of monomer 2 (265 mg, 1 × 10−3 M, 175
equiv) was prepared in dry CH2Cl2 under N2. The catalyst solution
was prepared by dissolving the catalyst in anhydrous CH2Cl2 under N2
in a glovebox. The catalyst solution (8.5 mg, 1 × 10−6 M in 0.5 mL
CH2Cl2, 1 equiv) was added to the reaction mixture and stirred for 1 h
at 30 °C. The polymerization reaction mixture was terminated with
excess ethyl vinyl ether (300 equiv relative to catalyst) and stirred for
another 1 h. The reaction mixture was then poured into cold
methanol, stirred, collected by filtration, and dried under vacuum,
yielding flaky white solid in 82% yield. 1H NMR (500 MHz, CDCl3) δ:
5.54−5.09 (b, −HCCH−), 4.22−3.84, 3.26−2.61, 2.58−2.22, 2.15−
1.85, 1.80−1.05(b). Mw =3400 (by NMR, see the Supporting
Information),24 n ∼ 13.
Herein, our main focus is to determine whether a functional
polymer can serve as the foundation to build a supramolecular
structure containing a porphyrin-based dye and facilitate J-
aggregation. Of particular interest is the potential modulation
and enhancement of two-photon absorption (2PA) by J-
aggregates relative to the corresponding unaggregated mono-
mers. We report the synthesis and characterization of a
norbornene-based monomer, containing dimethylamino pend-
ant groups, and the corresponding homopolymer by ring-
opening metathesis polymerization (ROMP). ROMP was
selected for its tolerance to a variety of functional groups and
the ability to control molecular weight. J-Aggregation of two
1
Characterization. H NMR and 13C NMR spectra were acquired
on a Varian NMR spectrometer at 500 and 125 MHz, respectively,
using CDCl3 as the solvent for all monomers and polymers. High
resolution mass spectrometry (HR-MS) analysis was performed in the
Department of Chemistry, University of Florida, Gainesville, FL.
Samples for the absorption spectroscopy measurements were prepared
by dissolving the dyes in ultrapure water and acidified with 0.2 M HCl
solution. Different buffer solutions (pH: 2.2−1.0) were prepared
according to the literature.25
Linear photophysical properties were investigated in spectroscopic-
grade solvents (DMSO and ultrapure water) at room temperature.
Absorption spectra were obtained with an Agilent 8453 UV−visible
1516
dx.doi.org/10.1021/la203883k | Langmuir 2012, 28, 1515−1522