Supramolecular Self-assembly and Functionalization of Porphyrin-based Systems①

XU Liang LI Yong-Jun (CAS Key Laoratory of Organic Solid, Beijing National Laoratory for Molecular Sciences(BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China) (University of Chinese Academy of Sciences, Beijing 100190, China)

1 INTRODUCTION

Supramolecular chemistry is concerned with manipulation of materials with structure features of nanometer size. To realize this goal, it is necessary to combine covalent chemical synthesis and programmed self-assembly together[1-4].

As we know, porphyrins are a class of organic compounds with a unique tetrapyrrole core and peripheral substituents[5]. The large two-dimensional π-conjugated framework is prone to aggregate by π-π stacking. Since different types of π-π interactions(H and J types) have distinctive absorption and emission properties, the optical features of porphyrins are also changed with the stacking types[6].The rigid, planar porphyrin ring could afford a variety of auxiliary motifs, such as hydrogen bonding and metal-ligand bonds[1]. Therefore, the π-π stacking,hydrogen bonds and metal ion coordination interactions are viewed as the main driver of selfassembly.

Generally, porphyrins have two types of absorption bands: one is Soret band between 400 and 430 nm and the other is Q-band between 500 and 650 nm[7]. The optical features make them better lightenergy collectors than other chromophores. And their electrical properties prove that they are fine candidates for electron-donor and photo-sensitizer[8].In addition to the tetrapyrrolic skeleton, the optical and electrical properties also depend on the exocyclic modifications and coordinated metal heart. Thus,porphyrins have rich and tunable properties[9]. In nature, plants use chlorophyll to collect photons and generate energy through photosynthesis[10]. As a structural analogue of chlorophyll, porphyrin has received a tremendous amount of attention in artificial light harvesting and converting systems due to their structural and functional similarities to chlorophyll[10-11].

It is expected that nano-scaled materials will have unique properties not obtainable by larger-scaled materials or by the molecules themselves[12-13]. Porphyrins have abundant photoelectronic and biochemical properties themselves. And the functional features of self-assembled porphyrin materials are influenced not only by the intrinsic properties of porphyrins, but also by the aggregation behavior including the orientation, overlap and distance of individual porphyrin chromophores. Thus, it is of great interest to achieve special qualities by preparing different porphyrin assemblies[14]. To date,various well-defined porphyrin architectures ranging from zero-, one- and two-dimension to more complex three-dimension have been generated under dynamic or thermodynamic equilibration[5].

2 SELF-ASSEMBLY OF PORPHYRINS

Self-assembly is a useful strategy for nanofabrication. It offers an easy way to access a variety of welldefined porphyrin nanostructures such as spheres[15],vesicles[16], tubes[17], wires, rods[18], sheets[19],wheels[20]and helical ribbons[21]. The driving forces required by self-assembly process are not only from the molecules themselves but also from the surroundings like the interactions between the organic compounds and solvents or auxiliaries[22-23]. Therefore, two routes are usually adopted to develop different porphyrin morphologies. One is to design novel porphyrin molecules with predefined geometries, and the other is to tune the condition of the self-assembly process.

2.1 Self-assembly of porphyrin monad molecules

Usually, highly ordered films could exhibit extremely attractive properties[24-25]. A novel porphyrin monad molecule TEOP has been designed and investigated for aggregation behavior[15]. Through a straightforward evaporation-driven self-assembly process, a film made up of well-ordered nanospheres was obtained, whose diameter could be adjusted by tuning the solution or atmosphere temperature.Lowering the temperature of the solution or the atmosphere could result in a longer growth period and a bigger diameter of the nanospheres (Fig. 1)[15].

Fig. 1. (a) Schematic outline. (b) SEM images of TEOP pattern prepared by thermal treatment mode I in 5 min, (c) 6 min,(d) 7 min, (e) 8 min, (f) 9 min,(g) 10 min. (h) Layer-by-layer assembly and (i) multilayer assembly of TEOP film, scale bars: 2 μm

The crystal structure (Fig. 2 a’ and b’) suggested that the quadrel-like TEOP molecules were inclined to self-assemble into J-type aggregates (Fig. 2)[15].Accordingly, the assemble mechanism was proposed.First, TEOP nanospheres were formed by the drive of porphyrin-porphyrin π-π stacking and solvent effects.Then, parts of the nanospheres assembled into a linear order, which can act as templates and tempted a small area of ordered arrays. Finally, the ordered arrays were extended and a layer-by-layer 3D ordered pattern was formed.

The well-ordered TEOP film possessed strong saturated absorption with the nonlinear absorption coefficient and nonlinear refraction coefficient to be β = –4.3×10−6m·W−1and n2= 2.8×10−13m2·W−1,respectively. While the TEOP solution exhibited reverse saturated absorption, having the following two corresponding values: β = 1.5×10−9m·W−1and n2= –2.53×10−16m2·W−1, respectively. The novel TEOP film displayed enormous potential applications for NLO devices (Fig. 2)[15].

Interestingly, highly ordered TEOP nanospheres film also exhibited efficient antibacterial activity(Fig. 3)[26]. As a common photosensitizer, phopyrins could produce singlet oxygen under visible light in the presence of oxygen. In comparison to the disordered spheres film, ordered spheres film prolonged the optical length of incident light and enhanced the photon harvesting owing to the properties of photonic band gap materials, which substantially increased the quantity of photons interacting with the TEOP molecules. A traditional surface plating technique was employed for bacterial survival experiments. The results showed that the killing efficiency for ordered TEOP film was over twice of that for disordered TEOP spheres and ITO glass. Additionally, the morphology of ordered spheres film was not disrupted upon light irradiation. In singlet oxygen detection experiment, a strong signal of the generated singlet oxygen was detected for ordered TEOP spheres film,while weak or absent signal was monitored for the disordered film or ITO glass. Thus, well-ordered TEOP nanospheres film could be used as a novel antibacterial film.

Fig. 2. (Left) (a΄–b΄) The crystal structure of ZnTEOP and (a–f) schematic outline of the growth and assembly procedure of TEOP film with ordered pattern. (Right) Nonlinear optical properties of TEOP films. (g) NLO absorptive properties of the TEOP well-ordered pattern under an open-aperture configuration and (h) the closed-aperture configuration. (i) NLO absorptive properties of the TEOP solution under an open-aperture configuration and (j) the closed-aperture configuration

Fig. 3. Number of colony forming units (cfu) for E. coli on an LB agar plate. (a) Cfu control without TEOP in the dark,(b) cfu control without TEOP irradiated with visible light (λ = 420 nm), (c) cfu of E. coli suspension incubated with TEOP in the dark, (d) cfu of E. coli suspension incubated with TEOP and irradiated with visible light. Diameter of the solid LB agar plates was 90 mm. (e) Biocidal activity of TEOP toward E. coli in the dark and under light illumination for 30 min.Dark and light control experiments were done with the cell suspensions irradiated or in the dark in the absence of photosensitizers. The visible light dose was 35 J·cm-2 (irradiation for 30 min at a fluence rate of 40 mW·cm-2). Values represent mean standard deviation of three separate experiments. Error bars represent standard deviations of data from three separate measurements. The inset shows the SEM images of the TEOP ordered film on ITO glass. The near-IR luminescence of singlet oxygen at 1268 nm (f) the disordered film, ITO glass and ordered film detected by NIR pmt at the same time, (g) the ordered film detected by NIR pmt at different time intervals

A novel porphyrin derivative dioctaoxacyloporphyrin zinc complex (DOCP-Zn) has been synthesized in our group (Fig. 4)[27]. With a hydrophobic porphyrin central ring and two hydrophilic glycol side chains, a new amphiphilic porphyrin was designed like a butterfly. Two styles of nanoarchitectures for DOCP-Zn were obtained from a solvent evaporation-driven route. In THF and methanol, nanorods with the diameter of 20–700 nm and the length of 2–20 μm were observed. However, in THF and isopropanol, long bandage-like slices with the width of 2–3 μm and the length of 5–15 μm were detected.

Interestingly, reversible nanostructure conversion occurred with 4,4΄-bipyridine coordinating (Fig. 5)[27]. In the mixed solvent of CHCl3and cyclohexane, we discovered the morphology of nanospheres, which reaggregated into nanorods by adding the mixed solvent of CHCl3and methanol, and nanoslices by adding isopropanol. Oppositely, by dropping the mixed solvent of CHCl3and cyclohexane onto the nanorods or nanoslices, we noticed that the initial nanospheres reappeared. And the DOCP-Zn nanorods also exhibited high active waveguide property[27].

Fig. 4. (Left) Schematic outline showing the aggregation procedure of nanostructures and the nanostructure conversion between spheres and slices or nanorods. (Right) SEM images in silicon slices of 7 prepared in (a)THF/MeOH (v/v 1:1), nanorods, and (b) THF/iPrOH (v/v 1:1), long thin slices at the temperature of 25 ℃(The insets of panels (a) and (b) show the corresponding TEM images). The microscope and CLSM (λex = 415 nm) images of DOCP–Zn 7 on the slide glass in (c,e) THF/MeOH (v/v 1:1) and (d,f) THF/i-PrOH (v/v 1:1)

Fig. 5. (Left) SEM images of the nanostructures of DOCP-Zn-bpy complex 8 in (a) CHCl3/cyclohexane (v/v 1:1)and pattern conversion from nanospheres to nanorods in (b) CHCl3/MeOH (v/v 1:1) or (c) nanoslices in i-PrOH at the temperature of 30 ℃, scale bar: 5 μm. (d) Experiment procedure. (Right) (e) Microscope images and the corresponding PL microscopy images, (f) the spatial resolved spectra of the waveguided emission that is outcoupled by excitation at distances of 60, 50, 40, 30, 20, and 10 μm from the tip of a single nanorod (from 1 to 6 in panel e),and (g) the output intensity as a function of propagation length, respectively. The DOCP-Zn 7 was prepared in THF/MeOH (v/v 1:1) on slide glass, scale bar: 20 μm. The red area was excited by focused laser light (λex = 422 nm)

With regard to the unique physicochemical properties of organic-inorganic hybrid materials, functional gold nanoparticles have attracted much attention recently[28-32]. Porphyrin-oligo (p-phenylene vinylene, P-OPV) oligomers can serve as organic semiconducting materials owing to their physical properties[33]. Pyridyl (Py) is able to coordinate with gold nanoparticles (AuNPs). Taking advantage of the nature of P–OPV oligomer and the binding ability of Py, a kind of P–OPV–Py oligomer functionalized AuNPs has been synthesized in our group(Fig. 6)[34], in which the OPV unit performed as a charge- and energy-transfer bridge between the donor porphyrin and acceptor pyridyl[34].

Fig. 6. Molecular structure of P-OPV-Py AuNPs

A new technique combining the relative concentration and the molar ratio between the ligand and AuNPs has been developed to regulate the size and shape of P–OPV–Py protected AuNPs assemblies.Microscopic experiments demonstrated that different morphologies ranging from branched-rods, chainnetworks and uniform clusters to larger nanoparticle aggregates could be achieved by this methodology(Fig. 7)[34].

Fig. 7. (Left) (a) UV/Vis spectra of P–OPV–Py capped gold nanoparticles at different mole ratios (r) after being left for 24 h. The maximum absorption peak at 424 nm of P–OPV–Py is omitted for clarity in order to view the SP band. (Right)TEM images of the self-assembly behavior of P–OPV–Py capped gold nanoparticles at (b) r = 4 k, (c) r = 10 k, (d) r = 16 k,(e) and (f) r = 19 k, (g) r = 30 k in CHCl3/toluene (v/v 2:1). Inset in (d): high-magnification image of selected area

2.2 Self-assembly of porphyrin dyad molecules

As we know, in a bipyridyl group, the relative positions of the two pyridine moieties can be fixed by coordinating with palladium dichloride (PdCl2). And bipyridine-PdCl2units exhibit hydrophilic properties.On the foundation of this knowledge, an amphiphilic porphyrin dyad 1 (Fig. 8)[16]has been designed and synthesized by attaching two zinc porphyrins to the 4,4΄-position of the 2,2΄-bipyridine-PdCl2group.Consequently, the molecule conformation could be regulated by the addition-elimination equilibrium of Pd2+. The single-crystal data of bpy-ZnP-pyridine complex indicated that the Pd-free compound had a linear conformation. Calculation and NMR titration results showed that the Pd complex had a V-shaped structure[16].

Fig. 8. (a) Molecular structure of dyad 1. (b) Crystal structure of the bpy-ZnP-pyridine complex.Hydrogen atoms, 3,4,5-trimethoxyphenyl, and solvent molecules are omitted for clarity.(c) Space-filling models of bpy-ZnP from a top view, and (d) a top view and (e) side view of complex 1. The MM2 force field was used to calculate the minimum-energy conformation

A compatible solvent system chloroform-methanol was selected in the morphology study. The CHCl3solution of 1 was injected into CH3OH and equilibrated over one day before being dropped on the silicon slide. SEM images showed vesicles with a diameter of 200 nm. To investigate the self-assembling details, different heating temperatures and patterns for preparing samples were employed. The results indicated that hollow capsules and wormlike structures could be produced from vesicles. The probable mechanism was proposed as follows. In the above solvent system, the amphiphilic bisporphyrinbipyridinium-palladium complex assembled into a bilayer-structure membrane, which closed to form spherical vesicles. When coordinating with the zinc porphyrin, the methanol was encapsulated in the membrane, so the dyads were loosely packed. When heated, the methanol quickly escaped from the spherical vesicles and the molecular motion of 1 was accelerated, which resulted in the fusion of vesicles and the appearance of hollow capsules. When treated with heating pattern II, the dyad rearranged from loose packing into dense packing during the cooling process, and then shaped wormlike aggregates (Fig. 9)[16].

Fig. 9. (a) SEM of 1 derived vesicles prepared in CHCl3/CH3OH (1:1) at room temperature. (b) SEM and (c) TEM of compound 1 vesicles derived from the heat treatment (pattern I at 70 ℃) of the vesicles prepared as in (a). The inset of(b) shows a close up of a vesicle, and the inset of (c) shows the membrane thickness, indicated by arrows. (d) Schematic representation of compound 1. (e) Schematic representation of a compound 1 vesicle formed in methanol with a close up of the vesicle membrane showing the proposed multibilayer structure. The blue dots represent the ligated CH3OH. (f)Schematic representation of an interdigitated bilayer structure. Morphology transition of the vesicle heated (g) at 60 ℃for 0.5 h (inset 2.6 × enlargement), (h) at 80 ℃ for 0.5 h (inset 2.6 × enlargement), and (i) SEM and (j) TEM of wormlike aggregates of 1 derived from the heat treatment (heating pattern II) of the vesicles prepared as (a)

Fig. 10. SEM and TEM images of TPDC2 prepared in CH2Cl2/CH3OH (v/v 1:1) at (a, b) 20 ℃for 30 min and 35 ℃ for (c, d) 30, (e, f) 45, (g, h) 60, (i, j) 90 min,and (k, l) more than 120 min. Scale bars: 500 nm.(m) Schematic growth process of the vesicles and their derivatives

Fig. 11. (a) Morphology and (b) current AFM image of the TPDC2 vesicles. (c) Schematic of the experimental setup. (d) Current-voltage (I-V) curves of the TPDC2 vesicles

2.3 Self-assembly of porphyrin triad molecules

Ordinarily porphyrin usually serves as an electron donor, but sometimes it acts as an electron acceptor as well. A U-shaped porphyrin triad TPDC2 has been synthesized by inserting two carbazole groups into three porphyrin fragments, in which porphyrin performed as an electron acceptor[17]. The function of intramolecular electron transfer not only offered TPDC2 with push-pull driving force for self-assembling, but also affected its conducting behavior.CH2Cl2and CH3OH were chosen as the solvent system for constructing nanoarchitectures. Tuning the evaporation conditions, mainly the growth time,different morphologies ranging from spherical vesicles, tadpole-like vesicles and vesicles-bearing tubes to Y-like vesicles-bearing tubes could be obtained (Fig. 10)[17]. UV-vis absorption spectra suggested J-type (edge-to-edge) interaction in the vesicles, according to which the mechanism was proposed. The formation of the spherical vesicles was familiar with the above. Prolongation of the growth time increased the length of the tails on the vesicle,so tadpole-like vesicles and vesicles-bearing tubes formed. When two tails came up to each other, Y-like vesicles-bearing tubes were shaped.

The conductive AFM showed the vesicles’ semiconducting properties with the I-V behavior are nearly exponential between –0.7 and +0.8 V (Fig. 11)[17].

2.4 Self-assembly of porphyrin-containing polymers

As mentioned above, porphyrins are attractive motifs for supramolecular assembling in the form of small molecules. In addition, porphyrin macromolecules are also capable of assembling into functional nanometer materials. Accordingly, a novel polymer 1 has been synthesized by incorporating both porphyrin and perylene bisimide groups with the aim of fabricating optoelectronic devices[35](Fig. 12).

Fig. 12. Molecular structures of polymers 1 and 2

By means of dropping a solution of 1 in CHCl3into C2H5OH, colloids with numerous nanoparticles were obtained. Compared to the solution state, the colloids state showed red-shifted Soret and Q bands of porphyrins due to more favorable π-π effects. To perform the optoelectronic measurements, we prepared the polymer film via casting the solution onto the ITO substrate, and it displayed a steady and rapid cathodic 13.6 µA·cm-2photocurrent response.

Design of polyacetylene-containing functional systems becomes a hot issue these years[36-37]. By introducing porphyrin and perylene bisimide units into the polyacetylene backbone, polymer 2 was derived, which showed different superstructures with the addition of bipyridine (Fig. 12). With the addition of 1 equiv of Bipy, a hierarchical structure was observed. With 2 equiv of Bipy, a uniform cottonball like superstructure with a mean diameter of 1µm was detected. With 4 equiv of Bipy, an agglomerate superstructure was monitored. Therefore, the axial coordination of zinc porphyrin and Bipy played an important part in modulating the morphologies.Irradiations of polymer 2 films with 70 mW·cm-2white light produced cathodic 1.72 µA·cm-2photocurrent responses[38].

2.5 Self-assembly of porphyrin analogues

As a kind of porphyrin analogues, carbaporphyrinoid is the molecule which replaces pyrrolic nitrogen of porphyrin with carbocyclic C–H moieties. This modification changes the π conjugation degree and affords possibility of metal-carbon bond stabilization in the macrocycle[39-40]. A carbaporphyrinoid TMBPZnCl (Fig. 14)[41]has been synthesized with intent to construct interesting nanostructures[41].Adjusting the evaporation temperature and growth time, the morphologies of TMBPZnCl turned from spherical vesicles, protuberant open hole-bearing vesicles, aubergine-like vesicles and tubes-bearing vesicles to Ω-shaped vesicles. Remarkably, utilizing high concentration of TMBPZnCl solution, we produced different vesicle arrays (Fig. 13)[41]. The crystal analysis proved the L-shaped structure of TMBPZnCl. Accordingly, the assembly mechanism was presented, which was similar to that we presented before[16-17].

Fig. 13. SEM images of TMBPZnCl (10-4 M) prepared in CH2Cl2/CH3OH (v/v 1: 1) at (a) 30 ℃ for 4 min, and at 25 ℃ for (b) 4 min, (c) 8 min, (d) 12 min, (e) 16 min, and (f) in CH2Cl2/CH3OH (v/v 1:2)at 20 to 30 ℃over 12 min. SEM images of TMBPZnCl (10-3 M) prepared in CH2Cl2/CH3OH (v/v 1: 1)at 25 ℃ for (g, h) 4 min, (i, j) 8 min, and (k, l) 12 min. The scale bar is 1 mm

Fig. 14. Schematic representation of the TMBPZnCl block showing the stack of hollow structured membranes. The inset shows the crystal structure of TMBPZnCl

3 FUNCTIONAL SUPRAMOLECULAR SYSTEMS OF PORPHYRINS

Porphyrin-based small molecules, oligomers and polymers have been extensively designed and synthesized for the investigation of potential applications in catalyst[42-43], anion receptors[44], nonlinear optical materials[15], antibacterial film[26], organicinorganic films[45-46], optoelectronic gates[47], fluorescence quenching sensors[48], photoinduced picosecond molecular switches[49], artificial photosynthetic systems[50-52], photonic wires[53]and anticancer drugs[54]. In such multichromophoric systems, poly(phenylenevinylene) (PPV), oligo (p-phenylene vinylene) (OPV), fullerene, perylenetetracarboxylic diimides, triazolium, polyacetylene and ferrocene are also incorporated.

3.1 Supramolecular porphyrin anion receptors

The construction of host-guest supramolecular systems to mimic specific chemical process in nature has been a significant goal in chemistry[55]. A macrocycle based on porphyrin and on cationic triazolium and pyridinium units has been designed, which could act as a reactor for the dimerization of anion radicals of TCNQ (7,7,8,8-tetracyano-para-quinodimethane)(Fig. 15)[43].

Experiments showed that the dimerization rate was increased by over two orders of magnitude upon irradiation of the porphyrin part. The catalytic mechanism was proposed as follows. First, a [1-Me3-TCNQ]2+complex was formed through reduction and encapsulation of [TCNQ]–•anion radical. Then a porphyrin radical cation and a second [TCNQ]–•radial anion were created by a PET (photoinduced electron transfer) process from excited porphyrin to neutral TCNQ. The triazolium cations pulled the second [TCNQ]–•close to the cavity and the two[TCNQ]–•anion radicals produced the σ-dimer,which was replaced by new [TCNQ]–•. The porphyrin exhibited potential applications in supramolecular catalysis[43].

Fig. 15. Proposed mechanism of the photochemically induced dimerization of TCNQ by 1-Me33+ (the [TCNQ]–• might sit in or outside the cavity, but it is constrained around the pyridinium or triazolium centers by electrostatic interactions. Here, only the inside case is shown)

Past decades have witnessed a fast development of anion receptors due to their importance in chemical and biological processes. Many anion receptors based on triazole, tetrazine and porphyrin motifs have been reported[43,56-64]. Here a porphyrin cage 1 was synthesized, which showed high selectivity towards azide anions (Fig. 16)[44].

Fig. 16. (a) Molecular structure of porphyrin 1 and(b) crystal structure of porphyrin 1 with pyridine

The1H NMR titration of 1 with N3－provided structural information about the cage and its complexes. Model I presented the porphyrin cage 1 without N3－. When titrating from 0.4 to 1.6 equivalents of N3－, N3－entered into the cavity of 1 and coordinated with both Zn atoms to form a Zn–NNN–Zn unit as type II complex. In the process of titration from 1.6 to 30 equivalents of N3－, one of the Zn–N b onds was broken by the tension of the cage and the competition from the outside N3－. Then the type III complex formed (Fig. 17)[43]. UV-vis spectroscopy showed a red-shifted Q-band for the effect of N3－coordination. And the visible color changed from red-purple to deep green.

Fig. 17. Models of I, II and III ((a) the addition of TBAN3＜1.6 eq., (b) the addition of TBAN3＞1.6 eq.)

3.2 Supramolecular porphyrin NLO materials

Porphyrins have been studied for desirable nonlinear optical materials due to their tunable optical properties, ultrafast response capability, large thirdorder nonlinearity and potential applications in all-optical signal processing[65-66]. Herein, porphyrin derivatives P2 (5,15-bimesitylene-10,20-bi(1,2-diphenylethene)-porphyrin) and its zinc analogue ZnP2 are synthesized (Fig. 18)[67].

Fig. 18. Molecular structures of P2 and ZnP2

Fig. 19[67]showed the nonlinear absorptive curves of P2 and ZnP2. ZnP2 showed a larger RSA (reverse saturated absorption) than P2. Additionally, coordinating Zn to P2 turned the nonlinear refraction from self-defocusing to self-focusing at 4 ns pulse duration, which was not observed in the case of 10 ns pulse duration. The results indicated the competition between the coordination and thermal effects. In the case of short pulse duration, the coordination effect played a dominant role, while at the long one the thermal effect did. These materials exhibited potential applications in optical-limiting and pulseshape devices.

Fig. 19. Normalized open-aperture Z-scan curves of P2 (squares) and ZnP2 (circles) with (a) 4 ns and (b) 10 ns pulse duration at 532 nm. Normalized closed-aperture Z-scan curves of P2 (squares) and ZnP2 (circles) in CH2Cl2 for the cases of (c) 4 ns and (d) 10 ns. The solid lines are the fitting curves

By fabricating composite and multilayer nanostructures with electrostatic interactions, the quality and performance of materials could be improved[24].With controlled supramolecular assemblies, solid film materials could display exceptional properties such as a large NLO susceptibility[68-69]. ELSA(Electrostatic layer-by-layer self-assembly) technique was widely used for easy layer-formation, good thickness-control, and broad applicability[70-71]. Poly(phenylenevinylene) (PPV) displayed a large thirdorder optical nonlinearity. Especially, highly π-conjugated PPV thin-films exhibited a large nonlinear refractive index and a two-photon absorb for all-optical switching[72]. Herein, an ultrathin multilayer film was fabricated using a water-soluble porphyrin derivative DHP and oppositely charged polyelectrolyte BH–PPV through ELSA methodology (Fig. 20)[73].

Nonlinear optical measurements showed that the 20-bilayer film of BH-PPV/DHP possessed saturated absorption and strong self-defocusing behavior, with the nonlinear absorption coefficient and refractive index to be –1.9×10-5m·W−1and –5.57×10-12m2·W−1, respectively. The prepared ultrathin film presented potential applications in solid-state NLO materials.

Fig. 20. Molecular structures of BH–PPV and DHP

3.3 Supramolecular porphyrin light energy conversion systems

From a long-term perspective, it is essential to find sustainable energy resources to replace fuels.Hence, the construction of light energy conversion systems has been a hot issue in the past decades[74-79].Such systems usually involve multiple chromophores with different functions in light-harvesting and converting processes. As an electron-donor and photo-sensitizer, porphyrins are amenable to fast electron transfer to an acceptor, and they have moderate absorption in the visible spectrum and tunable redox properties[9,53,80-83]. As a three-dimensional electronacceptor, fullerenes have exhibited enormous potentialities in photoinduced electron-transfer processes for their contribution to rapid charge-separation and slow charge-recombination[84-85]. Thus, porphyrinfullerene systems have been extensively employed in creating solar energy conversion systems[86-88]. As a kind of common red dyes, PDIs (perylenetetracarboxylic diimides) could significantly absorb solar energy between porphyrins’ Soret and Q-bands[89-90].And they could also serve as charge-transfer participants and charge carriers[91-92]. Additionally, baysubstituted PDIs are proved to be good electron acceptors for constructing photovoltaic devices[93].Therefore, in an integrated system, PDIs existed as an important complementary component. Herein, fulleropyrrolidine-perylenetetracarboxylicdiimide-porp hyrin triads FPP and its zinc analogue ZnFPP were synthesized (Fig. 21)[94]. These compounds were also assembled on indium tin oxide electrodes for photovoltaic devices[95].

Fig. 21. Molecular structures of FPP and ZnFPP

The photoinduced charge-separated process was experimentally confirmed by steady-state photolysis with sacrificed electron acceptor methyl viologen(MV2+) and sacrificed electron donor 1-benzyl-1,4-dihydronicotinamide (BNAH). The photovoltaic devices have been fabricated using the triads and moderate power conversion efficiencies (0.028% for FPP and 0.035% for ZnFPP) were obtained[95].

To the best of our knowledge, connecting electrondonor porphyrin and electron-acceptor PIm (perylenetetracarboxylic diimide) moieties through short N-phenyl linker or long linker has made encouraging progress in light-energy transfer systems. In these systems, electron transfer from porphyrin to PIm could be easily accomplished by exciting porphyrin or PIm groups[96-100]. Herein, some novel dyads and triads were synthesized by attaching porphyrin (H2P and ZnP) to PIm units via short ether bonds (Fig. 22)[101].

Fig. 22. Molecular structures of H2P–PIm, ZnP–PIm,H2P–PIm–H2P and ZnP–PIm–ZnP

The optimized molecular structures revealed that the porphyrin and PIm planes were almost in the same plane in H2P-PIm and ZnP-PIm dyads, while those of H2P-PIm-H2P and ZnP-PIm-ZnP triads were almost perpendicular to each other. And the HOMO was located on the H2P entity, while the LUMO was located on the PIm entity (Fig. 23)[101].

Fig. 23. (a) LUMO of H2P–PIm, (b) the HOMO of H2P–PIm, (c) the LUMO of H2P–PIm–H2P,and (d) the HOMO of H2P–PIm–H2P

The electrochemistry and absorption spectral investigations confirmed that no ground-state interactions exist between the porphyrin (H2P and ZnP)and PIm moieties. The CS (charge separation) process from the porphyrin moieties to the PIm moiety in their excited singlet states was demonstrated by nanosecond transient absorption spectra. And the lifetimes of CS states were estimated to be 7～14 ns.With methyl viologen (MV2+) or octyl viologen(OV2+) as an external sacrificial electron acceptor and 1-benzyl-1,4-dihydronicotinamide (BNAH) as an external electron donor, the charge-separated processes were proved by steady-state photolysis experiment.been extensively investigated in photoinduced electron-transfer process. And polyacetylenes possessed exceptional properties such as NLO and semiconductivity properties[102]. In addition, attaching side groups to polyacetylene backbone would contribute to the light-harvesting and charge separation[103-104].Thus, some conjugated polyacetylenes having pendant porphyrin groups were synthesized[105](Fig. 24).The photochemical and electrochemical properties of the polymers indicated that the nature of pendants was influenced by the conjugated backbone. The photoelectrochemical experimental results showed a steady and rapid cathodic 2.5 µA·cm-2photocurrent response for poly (1a0.2–co–50.8) at the irradiation of 21.2 mW·cm-2white light. However, the photocurrent of poly (50.2–co–20.8) monolayer film was one order lower than that of poly (1a0.2–co–50.8).

Fig. 24. Molecular structures of poly (1a0.2–co–50.8), poly (50.2–co–20.8),poly (60.05–co–50.95), poly (60.2–co–50.8) poly (60.1–co–50.9) and poly 5

In order to enhance the photocurrent generated by porphyrin-containing polyacetylene systems, poly(60.05–co–50.95), poly (60.2–co–50.8), poly (60.1–co–50.9)and poly 5 were designed and synthesized[106](Fig. 24). The photocurrent of the polymers was measured at 20.0 mW·cm-2of white-light irradiation.A steady and rapid cathodic 3.6 µA·cm-2response was monitored for poly (60.2–co–50.8), and the same order of photocurrent was detected for poly(60.05–co–50.95), poly (60.1–co–50.9) and the mixture of poly 5 and fullerene. While, the photocurrent response for poly 5 was two orders lower than that of poly (60.2–co–50.8), which indicated that the fullerene moiety could greatly contribute to the enhancement of the polymer photocurrent response. Additionally,poly (60.2–co–50.8) could form ellipse-shaped nanorods with a diameter of about 100 nm and a length of about 300 nm in CHCl3.

Besides being pendant groups, porphyrins have also been incorporated into the main chains of polymers. Several polymers containing porphyrin,perylenediimide and fullerene units were designed[107](Fig. 25). Experimental results showed that the fullerene-containing copolymer polySPeZnP2F generated the largest photocurrent of 2.2 µA·cm-2under the irradiation of 20.0 mW·cm-2white light.Compared to the free-based one, zinc porphyrin exhibited a higher donating ability, which resulted in a lower energy level of photoinduced charge-separated states[95]. And consequently, the photocurrent of polySPeZnP2was stronger than that of polyMPeP1and polySPeP1. The SEM and TEM images showed that polySPeZnP2F could form a large quantity of spherical nanoparticles with the size distribution range of 25～100 nm in chloroform and nonpolar toluene, probably because fullerenes were apt to gather together through the π-π stacking interaction of carbon cage[108].

Fig. 25. Molecular structures of PolyTPeP1, PolySPeP1, PolySPeZnP2, PolyMPeP1 PolySPeZnP2F, PP, P4P5F1, P1P2F1, ZnP4P5F1 and CoP4P5F1

PPV has been widely studied in diode due to their high electroluminescence[109], easy chemical modification[110-114], and exceptional optoelectronic properties[115]. Some polymers containing porphyrin,PPV, and pendant fullerene moieties were synthesized[116](Fig. 25). The polymer PP, P4P5F1, P1P2F1,ZnP4P5F1 and CoP4P5F1 films engendered cathodic photocurrent of 0.086, 0.149, 0.152, 0.196, and 0.095 µA·cm-2, respectively. The photocurrent could be enhanced with the content of electron acceptor fullerene unit in the polymers.

In the former studies, porphyrin and fullerene were connected by different linkers such as perylenediimide and polyacetylenes in supramolecular systems. As a good electron donor, ferrocene has been employed for the multi-step charge-separation systems[117]. Furthermore, disubstituted ferrocene has flexible conformations. Hence, a system of porphyrin-ferrocene-fullerene was designed with intent to find the relationship between the molecular topology and photoinduced electron transfer process(Fig. 26)[118].

Fig. 26. Molecular structures of H2P–Fc,ZnP–Fc, H2P–Fc–C60 and ZnP–Fc–C60

1H NMR spectra manifested that the porphyrin and C60moieties adopt gauche type conformation. Photophysical and electrochemical studies displayed that substantial interactions exist between the two units in the ground state and excited singlet state. Fluorescence lifetime measurements were also carried out.For H2P–Fc and ZnP–Fc, the short lifetime of fluorescence might be due to the quenching processes intersystem crossing and electron transfer. In the case of H2P–Fc–C60and ZnP–Fc–C60, increase in the decay rates was observed, indicating that electron or energy transfer occurred from the excited singlet state of porphyrin to fullerene.

Antenna supramolecules usually have one chromophore absorbing photon and the other emitting photon. Usually, porphyrin performed as an electron donor in donor-acceptor systems; however, it could also act as an electron acceptor when linked with strong donor such as oligo OPV. OPV derivatives have found broad applications in amphiphilic materials, supramolecular assembly, light-emitting materials, and field-effect transistors[119-120]owing to their thermal and photochemical stability, high molar extinction coefficient, and outstanding donating ability[53,121-124]. Thus, some antenna supramolecules were designed by incorporating electron acceptor porphyrins and electron donor OPVs with different lengths[125].

P–OPVn showed broadened and slightly redshifted porphyrin emission peak and decreased OPV emission peak, indicating that intramolecular energy transfer from OPV parts to the porphyrin core occurred. Upon 365 nm irradiation, P–OPV2 exhibited a dramatic red-shifted color from blue of OPV2 to red, and P–OPV4 displayed a tiny blue-shifted color from green OPV4 to indigo. P–OPV2–Zn emitted nacarat color and P–OPV4–Zn manifested bottle green (Fig. 27)[125].

The energy transfer performance had been demonstrated by steady-state and time-resolved fluorescence measurements. Steady-state fluorescence spectra showed that the emission intensity of the porphyrin increased, while those of OPVn were notably quenched. Time-resolved fluorescence spectra exhibited that the lifetime values of P–OPVn were decreased and those of P–OPVn–Zn were decreased much further. These results fully demonstrated the excited energy transfer from OPVn to the porphyrin (or Zn–porphyrin). The efficient light-harvesting antenna effect of OPV units was proved with these porphyrin-containing supramolecules.

Fig. 27. Molecular structures P–OPVn and P–OPVn–Zn (n = 2, 4). And normalized emission spectrarecorded in chloroform solutions of P–OPVn (n = 2, 4), P–OPVn–Zn, and TPP recorded at 5 nm slitswith 422 nm excitation and OPVn with 333 and 380 nm excitations, respectively. Inset: fluorescencephotographs of P–OPVn, P–OPVn–Zn, OPVn, and TPP recorded in chloroform solutions with 365 nm excitation

4 CONCLUSION AND OUTLOOK

In the middle of 20th century, Richard Feynman pointed out “I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have”[126].New century has witnessed and will keep witnessing the rapid development of self-assembly and selforganization.

This review described recent progress in developing controllable nanoarchitectures of porphyrin and functional nanostructured porphyrin materials. Here we were trying to understand two relations. One was the relation between the assembling behavior and the molecular structures as well as the environmental factors, and the other was the relation between the unique properties and the molecule themselves as well as the nanostructures. How to precisely control the assemblies of nanoscopic and microscopic dimensions and how to greatly promote the performance of the self-assembled structures are still big challenges.

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