Optical limiters are a kind of materials which could help to protect naked eyes and photoelectric optical systems against the hostile lasers and attenuate the intensity of these lasers with their unique transient filtering actions (absorptive and refractive effects) [1] . Optical limiters could dramatically decrease intense incident laser beams and present weaker transmittance at high-intensity light while the irradiation pass through the optical limiters linearly under lower light intensities [2] . The nonlinear optical (NLO) performances of conjugated polymers (CPs) including polyaniline [3] , polythiophene [4] and polypyrrole [5] have been reported extensively in the past three decades, due to their high conductivity, extensive π electron delocalization and architectural flexibility. Polypyrrole and its derivatives do trigger greater attention due to their excellent processability, eco-friendly and eminent mechanical property [6] . However, the poor solubility and dispersion stability of polypyrrole materials occurred to be the first barrier for their application in nonlinear optical field [7] . Many efforts have been putted to solve this urgent issue, and one of the most effective methods is the functionalization of polypyrrole materials with soluble molecules [8] . In the present case, we synthesized modified polypyrrole (PPy) with a modification of previously reported method [9] .

Porphyrins have continued to be of considerable interest since they were discovered, because they possess many appealing chemical and photochemical performances, such as high thermostability, intense visible absorption bands, long-lived excited state, extensive π-electron delocalization, besides, they are ubiquitous in nature and they are non-toxic and harmless, commendably [10] [11] . Porphyrins have been promising materials for the applications in photodynamic therapy, electrochemical sensors, data storage, optical switching and nonlinear optics [12] [13] . The particular π-conjugated structure endows porphyrins great NLO properties, and the optical limiting behaviors of porphyrins has been widely studied and they have exhibited attractive results, what’s more, porphyrins are often used in supramolecular self-assembly [14] .

In this article, we firstly synthesized a novel PPy self-assembly with 4,4’,4”,4”’-(porphyrin-5,10,15,20-tetrayl)tetraphenol (Tpp(OH)₄) (PPy/Tpp(OH)₄) nanohybrid in accordance with the previously reported method [15] . The resultant PPy/Tpp(OH)₄ nanohybrid was characterized by Fourier-transform infrared, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy to ensure the successful formation, while Ultraviolet-visible absorption and steady state fluorescence were carried out to demonstrate the efficient electron and energy transfer between Tpp(OH)₄ moieties and PPy units, thermogravimetric analysis was conducted to study the thermostability of resultant samples. The NLO absorption properties of PPy, Tpp(OH)₄ and PPy/Tpp(OH)₄ nanohybrid were characterized by Z-scan measurements with a 4 ns (fwhm) 532 nm pulses. It turned out that PPy/Tpp(OH)₄ nanohybrid displayed an enhanced NLO property compared with the benchmark Tpp(OH)₄ moieties and PPy units.

Pyrrole and p-hydroxy benzaldehyde were purchased from Sinopharm Chemical Reagent Co. Ltd. China, ammonium persulfate (APS), trichloromethane, dichloromethane, tetrabutyl ammonium iodide (TBAI) as well as dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich Chemical Co. All chemicals were of analytical reagent grade and used without further purification, unless otherwise stated. Ultrapure water was used throughout the study. Tpp(OH)₄ was synthesis by literature procedures [16] .

Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet 5700 FT-IR spectrometer with KBr pellets in the 400 - 4000 cm⁻¹ region, and all the FT-IR samples were prepared as KBr discs using spectroscopic grade KBr at room temperature. Ultraviolet-visible (UV-Vis) spectra were recorded using a Shimadzu UV-2550 spectrophotometer in the range of 200 - 800 nm. Steady state fluorescence spectra were recorded on a Shimadzu RF-5300 PC fluorescence spectrophotometer using a Xe lamp as the light source, samples were dissolved in dry DMF, filtered, transferred to a long quartz cell, and then capped and deoxygenated by bubbling with N₂ before measurement. The surface composition was determined by X-ray photoelectron spectroscopy (XPS) using a Thermo ESCALAB 250 spectrometer with a monochromatic Al Ka X-ray source and a charge neutralizer. The decomposition patterns of samples were taken on a Q600 SDTTGA/DSC thermogravimetric analyzer under an N₂ flow rate of 100 mL/min at a heating rate of 10˚C/min from room temperature to 800˚C. Scanning electron microscopy (SEM) of sample was measured with FLA650F type of the FEI company. Raman spectra of resultant products were carried out with a Micro Raman System RM3000 spectrometer with excitation laser wavelength at 532 nm, the laser light was focused onto samples by using a microscope equipped with a 100× objective. The NLO performances of the as-prepared samples were measured by performing Z-scan measurements using a Nd:YAG laser (Continuum, Surelite II) with 4 ns (fwhm) 532 nm pulses, operating at repetition rates of 2 Hz [17] .

PPy was prepared from pyrrole monomer and p-hydroxy benzaldehyde by chemical oxidative method at room temperature in the presence of APS (Scheme 1). A typical procedures as follows: 1.26 mL of pyrrole monomer, 365.30 mg of p-hydroxy benzaldehyde and 100.00 mg of TBAI were distilled in 30.00 mL of icy trichloromethane ultrapure water solution (volume rate of trichloromethane:ultrapure water = 1:1) in a beaker under magnetic stirring with ice-water bath, and then 2.00 g of APS dissolved in 10.00 mL of icy trichloromethane ultrapure water solution was added dropwise into the system, and then stirred at 3˚C with ice-water bath for another 24 h. After the addition of APS solution, the initial colorless solution became darker and eventually precipitated as a dark sediment at the bottom of the beaker. The solution was filtered and washed with ultrapure water and CH₂Cl₂ several times until the filtrate was clear. The residual black solid was dried under vaccum for 48 h at room temperature to afford dry PPy powder.

PPy/Tpp(OH)₄ nanohybrid was prepared with Tpp(OH)₄, pyrrole monomer and p-hydroxy benzaldehyde by a self-assemble method. A typical synthesis routes as follows: 30.00 mg of TBAI was dissolved in 60.00 mL of icy trichloromethane ultrapure water solution in a beaker under magnetic stirring with ice-water bath (solution A). Solution B was prepared by adding 0.10 mL of pyrrole monomer, 30.34 mg of p-hydroxy benzaldehyde and 25.26 mg of Tpp(OH)₄ dissolved into 30.00 mL of icy solution A in a beaker under magnetic stirring with ice-water bath (solution B). 170.00 mg of APS was dissolved in 15.00 mL of icy solution A in a beaker under magnetic stirring with ice-water bath to procedure solution C. The B and C solutions were cooled to 3˚C and then mixed B and C quickly in a 100.00 mL beaker with magnetic stirring at 3˚C with ice-water bath. After stirring for 24 h, the solution was filtered and washed with ultrapure water (20.00 mL × 4) and CH₂Cl₂ (10.00 mL × 6) and then dried under vaccum for 48 h at room temperature to afford dry PPy/Tpp(OH)₄ nanohybrid powder.

Figure 1 represents the FT-IR spectra of as-synthesized PPy, Tpp(OH)₄ and PPy/Tpp(OH)₄ nanohybrid in the range of 4000 to 400 cm⁻¹, which can correspondingly characterize their chemical structures. In the FT-IR spectrum of PPy, all the characteristic peaks of polypyrrole and phenolic hydroxyl group were observed which confirmed the successful formation of PPy, where the peak at 797.73 cm⁻¹ can be attributed to C-H wagging and the 925.08 cm⁻¹ is assigned to out of plane ring deformation [18] . The peak around 1048.62 cm⁻¹ is related to = C-H in plane vibrations, 1159.19 cm⁻¹ for C-N bond and 1688.41 cm⁻¹ for C=N bond, 1548.10 cm⁻¹ for stretching vibrations of C=C bond, the peak at 3115.18 cm⁻¹ is ascribed to N-H stretching vibrations [19] . Where the peak at 1281.20 cm⁻¹ is due to the successful function of phenolic hydroxyl group [20] . Compared to PPy, the FT-IR spectrum of PPy/Tpp(OH)₄ nanohybrid displayed two

new peaks at 1712.05 and 2358.71 cm⁻¹ that are coincident with those displayed by the porphyrin unit [21] , conforming the successful functionalization of PPy with porphyrin via self-assemble.

To further characterize the structure features of pristine PPy and PPy/Tpp(OH)₄ nanocomposite, Raman spectrum was carried out with excitation laser wavelength at 532 nm. In Figure 2, a typical intense tangential resonance absorption band (G band) at 1559 cm⁻¹ and a defect band (D band) at 1375 cm⁻¹ was found in the spectrum of pristine PPy, which is corresponding to the C=C backbone stretching and the ring stretching mode of PPy, respectively [22] . The Raman spectrum of PPy/Tpp(OH)₄ nanocomposite gives rise to two principal spectral bands of interest, observed at Raman shift of 1578 cm⁻¹ (G band) and 1356 cm⁻¹ (D band), respectively. A comparison between the observed D and G bands for PPy before and after modification with Tpp(OH)₄ is given in Table 1, compared with PPy, the blue-shift of the D band and the red-shift of the G band for PPy/Tpp(OH)₄ nanohybrid are similar to previous reported functionalization of RGO by porphyrins, may result from the electron transfer from Tpp(OH)₄ to PPy [23] . In the present case, the intensity ratio of the D band to that of the G band (I_D/I_G) for PPy/Tpp(OH)₄ nanohybrid of 0.69 is significantly decreased than that of PPy (0.79), which is corresponding to the previous work by Wang et al. [24] .

The TGA spectrum was carried out to investigate the thermal stability of