Electron-donating porphyrins (Por), electron-accepting phthalocyanines (Pcs), and reduced graphene oxide (RGO) were integrated into a multicomponent nanoconjugate (Por-RGO-Pc). The donor-π-acceptor nanoconjugate Por-RGO-Pc was characterized using Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and ultraviolet–visible (UV–Vis) spectroscopy. Photoinduced cascading electron/charge transfer from Por to RGO and from RGO to Pc was established from fluorescence, electrochemical, and femtosecond transient absorption (fs-TA) spectroscopy studies. The increased distance between the electron donors and acceptors of the Por-RGO-Pc nanoconjugate compared to the parent materials and the intermediate RGO-Pc results in long-lived charge separation, and an enhancement in nonlinear optical (NLO) absorption (a large NLO coefficient of about 827.44 cm/GW) towards nanosecond laser irradiation at 532 nm.

Edge-functionalization of graphene is emerging as a powerful chemical method for the construction of π-delocalized highly-planar graphene nanoconjugates that are not accessible through surface-supported syntheses. Herein, a graphene-porphyrin nanoconjugate via a robust pyrazine (pz) linkage has been obtained by condensing 2,3-diamino-5,10,15,20-tetraphenylporphyrin (DA-TPP) with ortho-quinone (o-quinone) moieties at edge sites of graphene oxide (GO). The as-prepared GO-pz-TPP exhibits an intense absorption extending from 375 to 900 nm and a high quenching yield (98%) of fluorescence, indicating a strong electronic coupling effect between GO and TPP units. GO-pz-TPP displays strong nonlinear optical (NLO) absorption and giant NLO coefficients with 800 and 1,030 nm fs laser, in sharp contrast to traditional graphene-porphyrin nanohybrids only NLO-active towards ns laser. Such a dramatic NLO performance towards femtosecond pulsed laser has not been achieved in any carbon-chromophores nanohybridized materials to date. This work validates the π-extended edge-functionalization strategy as a means to tune the NLO properties of graphene, thereby providing a new paradigm for the assembly of versatile optoelectronic materials.

Covalent modification of graphene oxide (GO) with functional chromophores plays an important role in constructing various kinds of advanced optoelectronic materials for applications in molecular diagnosis, light-harvesting, photodynamic therapy, and optical limiting. Herein, a new approach to functionalizing GO with meso-substituted formylporphyrins at GO’s edge sites via imidazole condensation is developed, which affords a novel GO-imi-Por nanohybrid covalently-linked by imidazole rings between two components. The structure of the GO-imi-Por nanohybrid was thoroughly characterized by scanning electron microscopy (SEM), attenuated total reflectance-Fourier transform infrared (ATR-FTIR), Raman, and X-ray photoelectron spectroscopy (XPS). The red-shifted steady-state absorption, 95% quenched fluorescence, and largely enhanced nonlinear optical (NLO) properties through Z-scan studies at lower input energies demonstrate that this GO-imi-Por nanohybrid exhibits a more effective photoinduced energy/electron transfer between the intrahybrid two components and can be flexibly applied as an optical limiter candidate. This covalent edge-functionalization approach provides a new paradigm for constructing various edge-expanding GO nanohybrids with an efficient energy/electron transfer process and improved nonlinear optical effects, which would draw inspiration for engineering more adaptable optoelectronic devices.

Functional materials displaying large ultrafast third-order optical nonlinearities across a wide spectral region and broad temporal domain are required for all-optical signal processing. Particularly desirable is nonlinear optical (NLO) activity at near-infrared (NIR) wavelengths with femtosecond pulses. Herein the first triply-fused porphyrin dimer (TFP)-functionalized single-walled carbon nanotube (SWCNT) nanohybrid was successfully constructed by covalently grafting TFPs onto SWCNT. The results of Z-scan techniques demonstrate that the newly obtained TFP-SWCNT nanohybrid was found with a strong NLO performance under both nanosecond and femtosecond irradiation. In the nanosecond regime, an enhancement in optical limiting (OL) of the TFP-SWCNT nanohybrid is seen at 532 nm when compared with the performance of porphyrin monomer-functionalized SWCNT nanohybrid Por-SWCNT. Under femtosecond irradiation, the TFP-SWCNT nanohybrid exhibits a particularly strong OL effect with a giant two-photon absorption (TPA) cross section value (ca. 15,500 GM) at 800 nm pulses that mainly stems from intense TPA of TFP, in sharp contrast to the Por-SWCNT nanohybrid which exhibits only saturable absorption under identical irradiation. These results demonstrate that the newly-developed TFP-SWCNT nanohybrid is a very promising OL candidate for practical applications across wide spectral and temporal domains, and that covalently functionalizing carbon-based materials with triply-fused chromophores may be a useful approach to engineering adaptable photonic devices with broad-ranging NLO activity.

The on-demand modulation of defects in materials for the effective modulation of optical nonlinearity is desirable, while it remains a great challenge. In this work, we demonstrate that electrochemical activation is a facile and convenient approach to modulating the broadband third-order nonlinear absorption of nanoporous tungsten oxide (WO_3–x) thin film. The film does not exhibit optical nonlinearity at the initial state, while shows a distinct saturable absorption under an applied voltage of –2.5 V with the excitation of 515, 800, and 1, 030 nm laser. The nonlinear absorption coefficient (β_eff) is –766.38 ± 6.67 cm·GW^–1 for 1, 030 nm laser, –624.24 ± 17.15 cm·GW^–1 for 800 nm laser, and –120.70 ± 11.49 cm·GW^–1 for 515 nm laser, and the performance is competitive among inorganic saturable absorbers. The activation is accomplished in 2 min. The performance enhancement is ascribed to the formation of abundant in-gap defect states because of the reduction of the tungsten atoms, and a Pauli-blocking effect occurs during the excitation of in-gap defect states. The small feature size of WO_3–x (~ 12 nm) enables the effective and fast introduction and removal of the defects in porous film, and accordingly the fast and broadband modulation of optical nonlinearity. Our results suggest a controllable, effective, and convenient approach to tuning the nonlinear absorption of materials.