Fluorescent gold nanoclusters (Au NCs) with excellent one-photon and multiphoton properties have been demonstrated as promising candidates in many application fields. However, small multiphoton absorption (MPA) cross sections and weak multiphoton excitation (MPE) fluorescence impede their practical applications under near-infrared (NIR) excitation for biological imaging. Here, we report the regulated one-photon and multiphoton properties and mechanisms of arginine-stabilized 6-aza-2-thiothymine Au NCs (Arg/ATT-Au NCs) and the applications for MPE fluorescence imaging. The introduction of arginine into the capping layer of ATT-Au NCs significantly modifies the electronic structure, the absorption cross sections, and the relaxation dynamics of the lowest excited state, drastically reducing the nonradiative relaxation, suppressing the blinking, and greatly enhancing the fluorescence. Besides the improved one-photon properties, Arg/ATT-Au NCs demonstrate remarkable MPE fluorescence with a large MPA cross section. The two-photon (λex = 850 nm), three-photon (λex = 1400 nm), and four-photon (λex = 1700 nm) absorption cross sections have been determined to be 6.1 × 10–47 cm4 s1 photon–1, 1.5 × 10–78 cm6 s2 photon–2, and 5.5 × 10–108 cm8 s3 photon–3, respectively, much higher than those of conventional organic compounds and previously reported Au NCs. Moreover, Arg/ATT-Au NCs have been successfully applied in two-photon and three-photon excitation fluorescence imaging of living cells with NIR excitation. The manifold advantages of small size, high quantum yield, suppressed blinking, good photostability and cytocompatibility, large MPA cross sections, and excellent MPE fluorescence imaging performances make fluorescent Arg/ATT-Au NCs a great candidate of imaging probes with vis–NIR excitation.

Graphene-based materials have been attracted many attentions due to their excellent properties and potential applications in many fields. Graphene also provides a flexible substrate to develop novel functional materials by hybridizing with other organic or inorganic components. Herein, we report the functionalization of reduced graphene oxide (RGO) with an azobenzene derivative (BNB-t8) containing the π-conjugated moiety and hydrogen bonding groups, to improve the optical and nonlinear optical properties of RGO. With the introducing of BNB-t8, a new absorption band is formed and dominates the absorption spectrum, clearly demonstrates that the BNB-t8 has been hybridized with RGO, by combining the analysis of Raman and XRD data. Femtosecond Z-scan results present a highly enhanced saturable optical absorption of BNB-t8/RGO hybrid compared with that of RGO. By optimizing the hybridization ratio of BNB-t8 to RGO, the saturable absorption coefficient of BNB-t8/RGO hybrid reaches to −237 m/W, 38 times larger than that of RGO (−6.2 m/W). In the meantime, the third-order susceptibility χ(3) of BNB-t8/RGO hybrid is aslo enhanced by 8 times to be 5.18×10−13 esu. These enhancements of nonlinear optical properties of BNB-t8/RGO hybrid mainly arise from the charge transfer from RGO to BNB-t8. Femtosecond transient absorption measurements reveal that the charge separation takes place in 0.28 ps and the charge recombination in 2.0 ps, indicating a strong electron coupling and thus an enhanced electron delocalization in BNB-t8/RGO hybrid compared with those in RGO. We suggest that the noncovalent π-π interaction plays the dominant role for enhancing the electron delocalization of RGO after hybridizing with BNB-t8, while the hydrogen bonding interaction reinforce the coupling interaction between BNB-t8 and RGO moieties in the hybrid. The as-prepared BNB-t8/RGO hybrid with high saturable absorption coefficient with an ultrafast response presents a potential candidate as saturable absorber of mode-locked laser.

Carbon dots (CDs) have attracted extensive attention for their unique properties and promising applications in many fields. Many efforts have been made to improve the optical and physicochemical properties of...

Thiolate-stabilized Au nanoclusters (NCs) have attracted considerable interest because of their unique properties and promising applications. However, there are still challenges in fully understanding the involved mechanisms of their electronic structure and optical properties. Here, we have focused primarily on bovine serum albumin (BSA)-protected Au25(SR)18 NCs to gain deep insight into their electronic structure by creating quantum beats in fluorescence dynamics with multiple pulse excitations. The electronic oscillations among degenerate excited states have been successfully generated and systematically investigated. The visibility of quantum beats demonstrates an insensitivity to the excitation wavelength, excitation power, and solution temperature but is very sensitive to the emission wavelength, the population of triplet states, and the number of Au atoms. The generated oscillations in fluorescence dynamics by sequential pulse excitations clearly reveal the existence of long-lived degenerate triplet states in the electronic structure of Au25(SR)18 NCs. There are three typical frequencies of 1.79 ± 0.01 GHz, 2.64 ± 0.05 GHz, and 3.87 ± 0.03 GHz determined from the Fourier transform analysis of the oscillation profile, suggesting the electronic superimpositions and energetic differences among the degenerate triplet states. These findings are significant for deep understanding of the relationship between the electronic structure and photophysical properties of BSA-protected Au25(SR)18 NCs.

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