Supramolecular Polymer of Near-Infrared Luminescent Porphyrin

Article pubs.acs.org/Macromolecules

Supramolecular Polymer of Near-Infrared Luminescent Porphyrin Glass Mitsuhiko Morisue,*,† Yuki Hoshino,† Masaki Shimizu,† Takayuki Nakanishi,§ Yasuchika Hasegawa,§ Md. Amran Hossain,‡ Shinichi Sakurai,‡ Sono Sasaki,‡ Shinobu Uemura,∥ and Jun Matsui⊥ †

Faculty of Molecular Chemistry and Engineering and ‡Faculty of Fiber Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, 606-8585 Kyoto, Japan § Graduate School of Engineering, Hokkaido University, North 13 West 8, Kita-ku, Sapporo 060-8628, Japan ∥ Department of Advanced Materials Science, Kagawa University, Hayashi-cho, Takamatsu 761-0396, Japan ⊥ Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Kojirakawa-cho, Yamagata 990-8560, Japan S Supporting Information *

ABSTRACT: A comprehensive study of supramolecular polymerization of ditopic zinc (2-pyridylethynyl)porphyrin dimer 1 in toluene and thin films was performed. A glass-forming porphyrin bearing 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups, named “porphyrin glass”, was introduced with the 2-pyridylethynyl group as a supramolecular organizing unit; two zinc (2-pyridylethynyl)porphyrins were held together by self-complementary pyridyl-tozinc coordination bonds to form a slipped-cofacial stack. Then, ditopic zinc (2-pyridylethynyl)porphyrin could be extended to a linear supramolecular polymer. The small binding constant limited the degree of supramolecular polymerization of 1 in toluene solution. In spin-cast film, on the other hand, 1 adopted a form of supramolecular polymer of porphyrin glass, which was effective enough to display a large bathochromic shift of the absorption bands exceeding the narrowest limit of the optical band gap extrapolated from the electronic structures in solution. The supramolecularly polymerized porphyrin glass formed excimer, which exhibited solid-state near-infrared (NIR) luminescence at approximately 1025 nm.

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implementation for the controlled the π-stacked morphology, as representatively exemplified by J-aggregates,9 although strong π-stacked interaction is an intrinsic disturbance. In this context, supramolecular polymers, which have emerged as a novel type of polymeric materials,10−13 could introduce a simple approach to solution-processable materials, including amorphous molecular glass. The current molecular design of supramolecular polymers introduces a slipped-cofacial stack of porphyrin planes by extracting the essential unit structure from natural photosynthetic light-harvesting antenna complexes, wherein circularly arranged bacteriochlorophyll pigments in a successive slippedcofacial stack efficiently capture sunlight and transport excitons quantitatively to the neighboring subunit.14−18 Aimed at supramolecular polymerization of porphyrin glass, the present molecular design employed self-complementary methodology; two zinc (2-pyridylethynyl)porphyrin planes are held together

INTRODUCTION The quest for solution-processable π-conjugated molecules that spontaneously order in thin films is the central subject in organic electronic materials because their geometric and energetic order/disorder in the π-stacked structures dominantly operates on the photoelectronic functionalities.1−5 Recently, we found “porphyrin glass”; porphyrins bearing 3,4,5-tri((S)-3,7dimethyloctyloxy)phenyl groups at the meso-positions form amorphous molecular glass and form intermolecular excimers which are luminescent at the near-infrared (NIR) wavelengths region (approximately 970 nm).6 The results highlighted a new approach to surpass general obstacles for NIR emission, such as energy gap law; nonradiative decay becomes fast as the optical band gap narrows.7,8 Our present challenge focuses on the supramolecular engineering π-system of porphyrin glass. Supramolecular interactions are advantageous in the direct fabrication of well-organized molecular assemblies and in the pursuit of excellent photoelectronic properties because one of the most distinct features of these reversible interactions lies in their ability to funnel thermodynamic structures to the most stable one. For instance, noncovalent interactions are possible © 2017 American Chemical Society

Received: February 14, 2017 Revised: March 26, 2017 Published: April 5, 2017 3186

DOI: 10.1021/acs.macromol.7b00316 Macromolecules 2017, 50, 3186−3192

Article

Macromolecules Scheme 1. Supramolecular Polymerization of 1

Figure 1. (A) Electronic absorption spectra of 1 upon increasing the concentration from 1.5 × 10−7 to 1.2 × 10−4 M (black lines, concentration increased according to arrows) at 25 °C in toluene and in spin-cast film on quartz substrate (red line). The inset shows a photograph of a solution of 1 in toluene (left) and its drop-cast film (right). (B) Plots of optical band gap (Eg) for Soret (open circles) and Q-band (filled circles) in toluene (black) and in spin-cast film (red), in which ⟨dp⟩w was estimated assuming K = 9.0 × 104 M−1 in the isodesmic supramolecular polymerization model.

effective in optical band gap engineering. Therefore, extending the slipped-cofacially stacked conformation to supramolecular polymerization would be a straightforward approach to controlling the molecular arrangement of polymeric structure in thin films, and the absorption and emission bands of supramolecular multiporphyrin array could thus shift from the visible region to the red. This study aimed to establish a supramolecular polymerization methodology of porphyrin glass for solution-processable photoelectronic materials.

by self-complementary pyridyl-to-zinc coordination bonds to form a slipped-cofacial dimer.19−21 Then, the ditopic zinc (2pyridylethynyl)porphyrin 1 is extended to give a slippedcofacial stack, which then forms a biomimetic successive πstacked system through supramolecular polymerization (Scheme 1), wherein the primary structural key to 1 was the use of meso-3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups to form amorphous molecular glass, the so-called porphyrin glass, under solvent-free conditions.6 As underpinned by Kasha’s molecular exciton theory,22,23 the longitudinal displacement of cofacially stacked chromophores is seminal in producing the lowest optically allowed electronic transition for long-range exciton transport. The shift in the optical energy gap (ΔEg) is tunable as a function of the number of chromophore units (N), the center-to-center distance (r), and the tilt angle (θ):22,23 ΔEg =

2 ⎛⎜ 1 ⎞ μ2 1 − ⎟ 3 (1 − 3 cos2 θ ) c⎝ N⎠r

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RESULTS AND DISCUSSION Comparative exploration of the supramolecular polymerization of 1 both in solution and in spin-cast films is the subject of this study. In the first stage, the supramolecular polymerization behaviors of 1 were analyzed based on the electronic absorption spectra by varying the concentration in toluene (Figure 1A, black line). Upon increasing concentration, the Soret and Q bands shifted to the red, which suggested an extension of the πsystem through a supramolecular polymer backbone, wherein step-growth of self-complementary pyridyl-to-zinc coordination bond formation of zinc (2-pyridylethynyl)porphyrin unit formed supramolecular backbone comprising a slipped-cofacial

(1)

where c is the velocity of the photon and μ is the transition dipole moment. In particular, the theory predicts that increased N narrows Eg, which indicates that propagation of the supramolecular polymer through the stacked π-system is 3187


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transition dipole of each chromophore unit, of the static supramolecular multiporphyrin array in spin-cast films compared with the dynamic structure in toluene solution. Differential scanning calorimetery (DSC) found an endothermic profile assigned to the glass transition at 34 °C (Figure 2A). Moreover, synchrotron microbeam glazing-incidence X-

porphyrin dimer as a repeat unit (Figures S1 and S2). The sigmoidal spectral change through several isosbestic points upon concentration variation suggested that the supramolecular polymerization obeyed an isodesmic mechanism (Figure S3), assuming an identical binding constant (K ≈ 9 × 104 M−1) for step-growth associating equilibria.24−26 The magnitude of the K value ranged in an insignificant order, as the binding constant is usually desired to be greater than 106 M−1 to obtain sufficiently extended supramolecular polymers under relatively diluted conditions.26 Hence, 1 afforded only approximately heptamers of the weight-average degree of polymerization (⟨dp⟩w) at approximately 1 × 10−4 M near the detection limit of our spectrometer.24 The linear relationship between the optical energy gap (Eg) and the reciprocal number of repeating units is empirically valid to describe the effective conjugation length of π-conjugated polymers,27,28 including ethynylene-linked porphyrin arrays29 and through-space conjugated polymers.30 This law was applied for the first time to supramolecular polymers, in which the Eg and the ⟨dp⟩w values were derived from the absorption maxima and the K value, respectively. The moderately linear Eg−⟨dp⟩w−1 relationships for both the Soret and Q bands (Figure 1B) indicated that the supramolecular porphyrin array firmly extended the π-system through the biomimetic π-stacked array, when supramolecular backbone was extended enough to become ignorable the effect of terminal groups. The Eg−⟨dp⟩w−1 relationship is the same as the Eg−N−1 relationship in molecular exciton theory (eq 1),22,23 which indicates exciton coupling over the stacked π-system of supramolecular polymer. Although even insignificant binding constants restricted the polymerization degree of 1 in solution, the prerequisite no longer retarded supramolecular polymerization when the solution was condensed to prepare thin films. The restricted polymerization degree is superior for obtaining solubility of the prepolymeric material in toluene, and the solution was appropriate for the solution-processing of thin film. As described above, the bulky 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups did not interfere with the selfcomplementary pyridyl-to-zinc coordination bond to form the slipped-cofacial dimer unit in toluene solution. Under solventfree conditions, 1 is expected to adopt the amorphous solid due to the elastic nature of the 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups, which could offer an appropriate environment for supramolecular polymerization under condensed conditions. We observed a dramatic color change from dark green to dark brown after drying droplets from the solution of 1 (Figure 1A, inset), which suggests a considerable extension of the πsystem in the thin-film form. In the second stage, we explored spin-cast films of 1 on solid substrates from toluene solution at approximately 5 × 10−4 M. A supramolecular porphyrin array on a quartz substrate showed remarkable bathochromic shifts of both the Soret and Q bands (red line in Figure 1A), shifting the Q-band from 717 nm for monomeric 1 in toluene to 785 nm for supramolecular polymer in a spin-cast film. The finely structured electronic absorption bands suggested that the supramolecular multiporphyrin arrays were highly ordered. Of note is the extended π-system that provided an optical band gap significantly narrower than the limit extrapolated from the linear Eg−⟨dp⟩w−1 relationship in toluene solution, the so-called “maximum conduction chain length” (the intercepts in Figure 1B).31 The discontinuous extension of the π-system indicated an enlarged coherent segment length, i.e., enhanced oscillator strength related to the

Figure 2. (A) DSC thermogram of 1 with a 10 °C min−1 heating rate. (B) GIXS profile and pattern of spin-cast film of 1 on a silicon wafer.

ray scattering (GIXS) found characteristic amorphous halo patterns at approximately q (q = 2π/d, wherein d refers to spacing) of 13.5 nm−1 (Figure 2B). Therefore, amorphous structure of a supramolecular polymer, namely “porphyrin glass”, was confirmed. Microscopic structures of supramolecular multiporphyrin arrays were revealed by atomic force microscopy (AFM). Fibrillar structures formed over the whole area of the surface (Figure 3A), with a relatively uniform height and roughness of approximately 3 nm inside the fibrillar structures (Figure 3A). In a typical AFM image of thinner parts of the spin-cast film (Figure S4), the difference in height between the first and the second layers was 2.8 ± 0.3 nm. Based on the geometryoptimized structure (Figure 4), the uniform fibrillar structures might be a bundle composed of two supramolecular strands.24 Probably, the bulky 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups might be interdigitated to assemble two supramolecular strands into a bundle, and the paraffinic corona hampers assembling further strands (Figure 4B). The sharp signal in a small-angle region of GIXS (Figure 2B) should include further microscopic structural information about molecular arrangement in nanometers scale. Synchrotron glazing-incidence small-angle X-ray scattering (GISAXS) elucidated the highly oriented internal structure in the spincast films. Diffraction peaks emerged dominantly in the out-of plane (qz) direction (Figure 3B,C).24 The unambiguous out-ofplane diffraction peak at approximately 3 nm of d spacing suggested that laterally disordered bundles were randomly accumulated on the silicon wafer (Figure 4). The result was in agreement with the height profile observed by AFM. 3188


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Figure 3. (A) AFM image of supramolecular multiporphyrin array of 1 on a silicon wafer. Inset shows cross section along the blue line. (B) Out-ofplane GISAXS pattern (A) and one-dimensional profile along qz direction integrated azimuthal angles of 178°−182° (C) of supramolecular multiporphyrin array of 1 on a silicon wafer observed by X-ray with 0.15° of incident angle.

Figure 4. Schematic illustration of supramolecular polymer of porphyrin glass based on geometry-optimized structures produced using the MM+ force field (the HyperChem Ver. 8.0 software). (A) A single supramolecular strand in toluene, including (i) a transoid-type and (ii) cisoid-type stacked conformations omitting the (S)-3,7-dimethyloctyloxy side chains for visual clarity. (B) Plausible bundle structure composed of two supramolecular strands and assembled bundles.

supramolecular strands were further assembled into bundles, over the course of solvent evaporation (Figure 4). It is noteworthy that the spin-cast film of 1 showed with a markedly red-shifted emission at approximately 1025 nm in NIR wavelength region (Figure 5), even without any dilution at ambient temperature. The emission wavelength of 1 was longer than the previous porphyrin glasses, e.g., 970 nm for nonsupramolecular butadiyne-linked porphyrin dimer.6 Such unusual NIR luminescence is certainly originating from the photoexcited state of the supramolecular multiporphyrin array, as elucidated by clear correlations between emission and excitation wavelengths. Since none of the electronic absorption bands longer than 850 nm were found, the NIR luminescence should originate from an optically forbidden state, such as an excimer site in the

The photophysical and structural observations revealed that supramolecular polymer 1 was highly extended in thin films. Solvent evaporation immobilized thermodynamic structures converged nearly to the most stable one, though the condensation process shouldbe in the most precise sense, under nonequilibrium conditions. Supramolecular polymerization could proceed through a highly ordered fluidic transition state during evaporation-induced propagation of supramolecular polymerization. In this supramolecular polymer design, a rigid core composed of a slipped-cofacial stack of porphyrin and flexible bulky side chains achieved a highly ordered polymeric structure on the surface. Eventually, the supramolecular multiporphyrin arrays were exceptionally extended across a micrometer order of length, and the 3189


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Figure 5. Emission−excitation correlated spectra of 1 in the spin-cast film. The upper panel shows emission spectrum upon excitation 520 nm, and the left panel shows absorption spectrum (gray line) and excitation spectrum monitored at 1025 nm (solid line).

same analogy in porphyrin glass in our preceding report.6 It is known that exciton−phonon coupling, such as Herzberg− Teller-type intensity borrowing from the Soret band, enhances optical transition of the Q-band.32,33 We propose that exciton− phonon coupling in the porphyrin glass surpassed the energy gap law; the radiationless deactivation rate of excitons becomes faster as the optical band gap narrows, and they thus usually struggle for NIR luminescene.7,8 For instance, the vibronic spacing of the NIR luminescence was reminiscent of resonance Raman scattering (Figure 6).34 It is known that extended πconjugation increases the probability of the statistical deactivation of a photoexcited singlet during energy migration along the conjugated backbone, which is known as the so-called “molecular wire effect”.35,36 The enlarged coherent domains of the supramolecular array could facilitate exciton migration over a long range, and thus excimer site effectively trap exciton, similarly to the primary photosynthetic events.37,38 The radiative decay profile of photoluminescence from the supramolecular multiporphyrin array in spin-cast films elucidated the existence of a markedly long-lived emission with a time constant of 184 μs (97%) and 91 ns (3%) at 1025 nm and 451 ns (59%) together with 71 ns (41%) at 800 nm (Figure S5). The exciton lifetimes were a thousand times longer than for the previously reported organized porphyrin films.22 These results ruled out the possible existence of undesired structural defects or disorder as an energy sink, as expected for porphyrin glass.

Figure 6. (A) Deconvoluted emission peaks with spacings of approximately 1400 and 1600 cm−1 as colored in green and blue, respectively. (B) Resonance Raman spectrum of 1 obtained by excitation at 532 nm.

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CONCLUSION This study has demonstrated that the supramolecular polymerization of amorphous molecular glass could be an easy way to provide highly ordered molecular organization in amorphous

molecular glass. Under solvent-free conditions, the porphyrins embedded in the 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl 3190


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Macromolecules groups bound to the meso-positions were readily self-organized. Exceptional extension of the stacked π-systems resulted in outstanding photophysical properties, pointing to potential photoelectronic applications in NIR-photoluminescent film electronics. In conclusion, the biomimetic design of supramolecular multiporphyrin arrays is a fascinating route to longlasting solid-state NIR photoluminescence in thin-film materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.M.). ORCID

EXPERIMENTAL SECTION

Mitsuhiko Morisue: 0000-0002-7783-2492 Shinichi Sakurai: 0000-0002-5756-1066

The synthetic procedure of 1 has been reported elsewhere.19 NMR spectra were recorded on a Brüker AV-300. UV/vis absorption spectra were recorded on a spectrophotometer (SHIMADZU, UV-3100PC and UV-1800) equipped with a Peltier thermoelectric temperature controlling unit (SHIMADZU, TCC-240A). A quartz substrate (15 × 35 mm, 1 mm thickness, purchased from Sendai Sekiei, Co.) and silicon wafer (Si(100), Mitsubishi Materials Trading Co.) were treated with UV-ozone. The surface of the silicon wafer for AFM observation was then soaked in a solution of ∼0.1 vol % of trichloro(octyl)silane in chloroform overnight. The substrate was thoroughly rinsed with chloroform and then ethanol. A spin-cast film was then prepared on the substrate by casting from a solution of ca. 0.5 mM of 1 in toluene. The substrate was mounted on a spin-coater (MS-A100, Mikasa) and rotated at a speed of 500 rpm (30 s) followed by 1000 rpm (60 s). The spin-cast film on the hydrophobized silicon wafer was observed by AFM. AFM investigations were conduced under ambient conditions using a Multimode Nanoscope IV (Veeco Instruments, Santa Barbara, CA) in tapping mode. Silicon cantilevers with a spring constant of 40 N/m and a resonance frequency of 300 kHz (OMCLAC160TS, Olympus, Japan) were used. The scan rate was varied from 1 to 2 Hz. The NIR steady-state photoluminescence spectrum was measured using a Fluorolog 3 ps NIR spectroscopy system (HORIBA, Kyoto, Japan) and corrected for the response of the detector system. The following measurement conditions were employed: excitation wavelength 350 nm, step 1 nm, int. time 0.1 s, slit Ex/Em 5 nm/6 nm. Emission lifetimes (τ) of spin-cast film were measured by nano-LEDs (N-355, response time ≤1.2 ns HORIBA, Kyoto, Japan) and a photomultiplier (R5108, response time ≤1.1 ns, Hamamatsu photonics, Hamamatsu, Japan). Emission lifetimes were determined from the slope of logarithmic plots of the decay profiles. Microbeam GIXS was measured for spin-cast films of porphyrin on a silicon wafer utilizing synchrotron radiation at the BL45XU beamline in SPring-8 (RIKEN SPring-8 Centre Hyogo, Japan). The X-ray microbeam of 1 Å in wavelength (approximately 6 μm in diameter) was directly irradiated on the films at incident angle of 0.11°, and X-ray scattering from the films (irradiated area was approximately 0.01 mm2) was detected with the combination of an image intensifier and a CMOS camera (Hamamatsu Photonics K. K.) or a PILATUS 300 KW (Dectris Ltd.) at 330 mm of the sample-to-detector distance. When a sample is irradiated by the incident X-rays at a grazing angle lower than the critical angle of total reflection of the sample, total reflection of the incident X-rays occurs and only evanescent X-rays permeate the sample. The critical angle of total reflection of the Si substrate surface is ca. 0.14°. Therefore, the incident beam reaches the substrate and its total reflection also occurs at the substrate surface. GISAXS was also performed at the BL45XU beamline in SPring-8. The wavelength and incident angle, αi, of the direct X-ray beam, not microbeam in this case, were 1 Å and 0.12°, respectively. Scattering from the films was detected with the combination of an image Intensifier and a CMOS camera (Hamamatsu Photonics K. K.) or a PILATUS 300 K-W (Dectris Ltd.). The sample-to-detector distance was 983 or 1557 mm.

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Full NMR characterization of supramolecular polymer in toluene-d8, analysis of supramolecular polymerization in toluene, emission decay profiles, and GISAXS (PDF)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Harry L. Anderson (University of Oxford) for helpful discussion. The synchrotron radiation experiments were performed at BL45XU in SPring-8 with the approval of RIKEN (Proposal No. 20150076, 20160041). Resonance Raman spectroscopic analysis was conducted by the technical support by Kyoto Integrated Science & Technology Bio-Analysis Center (KIST-BIC). This work was financially supported by a Grant-in-Aid for Scientific Research (M.M., JP15H00741; H.Y., No. 24102012; S.S., JP15H00742; J.M., JP15H00720) in the Innovative Area “New Polymeric Materials Based on Element-Blocks (No. 2401)” and KAKENHI (M.M., JP16K05749) from JSPS.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00316. 3191


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