1. Introduction

Novel organic compounds featuring large three-photon absorption (3PA) cross section have been of particular interest since many attractive applications are based on its high-order nonlinearity of media’s response to the exciting light. Molecules with large 3PA cross section can be widely used in the fields of ultrahigh-resolution biological imaging (three-photon confocal microscopy) [1–3], high-efficiency up-converted stimulated emission [4–7], optical limiting [8,9], biomedical [10], light-activated therapy field [11–13], three-dimensional optical data storage [14], microfabrication [15].

In many laser-based applications, such as optical data storage and biological imaging, a random intensity fluctuation is harmful. One of the best technical approaches to reduce such laser fluctuation is to make the laser beam simply pass through a nonlinear medium. The mechanisms include reverse saturable absorption, multiphoton absorption, nonlinear refraction, and optically triggered scattering. In multiphoton-absorption-induced optical stabilization, using materials with a larger 3PA cross section value will result in a better stabilization behavior. 3PA-induced optical stabilization has some salient features: (i) instantaneous response; (ii) high linear transmittance at low incident power and rapid attenuation at high incident power, which is based on the cubic dependence. However, the relatively small 3PA cross section values of present nonlinear molecules limit their practical applications.

Here, we designed and successfully synthesized a bifluorenylidene derivative with four branches as electron donors distributed on each corner. Carbon-carbon double bond units were utilized as the connecting spacers between two fluorenylidene (by C⁹ and C^9’) and between the central core and the peripheral groups, in order to ensure effective electronic conjugation between end groups and the core moiety, and allow large intramolecular charge transfer to take place within the chromophore. Hence, the overall molecular structure of the model compound is expected to simultaneously possess several potential 3PA-enhancing characters and cooperative enhancement including multibranched intermolecular charge transfer between core and molecular termini, increased π-electron number, elongated coplanarity of the conjugation system. The synthesized molecule with the symmetric 2D-π-2D conjugated structure, is named as 2,7,2’,7’-tetra(4-vinylanisole)-[9,9’]bifluorenylidene. D and π represent electron donor and conjugated π-electron bridge, respectively. Figure 1 shows the synthetic route of the compound.

 

Fig. 1 (a) Zn dust, AcOH, reflux; b) PBr₃, 150°C; (c) DBU, acetonitrile, 60°C; (d) DMF, 4-Methoxystyrene, Palladium acetate, K₂CO₃, TBAB, 110°C. ¹H-NMR(400MHz, CDCl₃):δppm 7.87(s, 4H), 7.64(d, 4H, J = 8Hz), 7.49(d, 8H, J = 8.4Hz),7.10(d, 8H, J = 18.4Hz), 6.92(d, 8H, J = 8.8Hz),6.16(s, 4H),3.85(s, 12H). MS(ESI) m/z: 896 [M + K]⁺.

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2. Experimental

The experimental setup for 3PA-induced fluorescence and nonlinear absorption effects is presented in Fig. 2 . In the measurement, the incident 1064 nm laser was provided by a Q-switched mode-locked Nd:YAG pulsed laser (Continuum, PY61-10) with pulse width of 38 ps, repetition rate of 10 Hz. After spatial filtering (lenses L1, L2, and the pinhole PH), the laser beam was directed to the sample, and focused inside the 10 mm cell filled with dye solution using lens L3 (focal length 25.6 cm). The focal plane is at the mid-point of the cell. The upconversion fluorescence light from the dye was collected with lens L4 perpendicular to the cell, and then coupled into the spectrometer. The laser beam was separated into two beams using a beam splitter. J3-05 probes (Molectron Co.), i.e. D1 and D2, were used to monitor the incident and transmitted laser pulse energy simultaneously, respectively. The beam waist radius at the focal plane was 26 μm (z = 0). The beam radius at the input and output plane are both 52 μm (z = −5 mm and + 5 mm, respectively). The measurement of the 3PA properties of the compound was done at 8.5 × 10⁻⁴ mol/L in CHCl₃.

 

Fig. 2 Experimental setup for 3PA induced fluorescence and input-output relation measurements. Two lenses (L1, L2) and a pinhole (PH) form a spatial filter. D1 and D2 are used to obtain the incident and transmitted intensity. The fluorescence light is collected by lens L4 and coupled into the spectrometer with a photomultiplier (D3).

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The linear absorption and steady fluorescence spectra of the compound were measured using a UV-VIS-NIR Cary5000 spectrophotometer and a Spex fluorescence spectrometer, respectively.

3. Result and discussion

Figure 3 shows the linear absorption and steady-state fluorescence spectra of the compound in CHCl₃ at a concentration of 2.5 × 10⁻⁶ mol/L. The influences from the quartz liquid cell and the solvent have been subtracted. The molecule has strong UV absorption in the spectral ranges of 310-470 nm. One can find that an interesting feature of the absorption spectra is the absence of linear absorption in the spectra range of 470-1200 nm. This indicates that excitation in that wavelength range can only occur through nonlinear (multiphoton) absorption process. The three-photon energy of the 1064 nm radiation just falls into the strong UV absorption band, hence very large 3PA cross section value in this compound may be expected.

 

Fig. 3 Linear absorption (solid line), steady-state fluorescence spectra (short dot line) and upconversion fluorescence spectra (scattered square) of the molecule in CHCl₃.

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Excited state absorption can be discarded because of three reasons: (1) The absence of one-photon (1064 nm) and two-photon (532 nm) absorption in the absorption spectrum; (2) Quantum chemistry computations have been carried by means of the TD-HF/6-31G method and the sophisticated polarized continuum model (PCM), and the data of the first six excited states are shown in Table 1 . The results indicate that the compound have no stepwise absorption (excited state absorption) channels such as 1 + 1 + 1, 2 + 1, or 1 + 2 photon absorption, only the one transition S₀→S₁ (375.35 nm) matches the 3PA rules for 1064nm wavelength laser; (3) In Fig. 4 , the upconversion fluorescence intensity exhibits a cubic dependence on incident intensity, which is characteristic of three-photon process. The shapes of the steady-state and upconversion fluorescence (in Fig. 3) are similar. The difference between the steady-state and upconversion fluorescence spectra is attributed to the reabsorption effect of the solution [8]. Propagating within the solution sample, the different spectral components of the fluorescence emission undergo different attenuation, and the attenuation in the shorter-wavelength range is much stronger than that in the longer-wavelength range [8]. One can be confident that the excitation process is induced by the simultaneous absorption of three photons.

Table 1. Electronic transition data obtained by the TD-HF/6-31G combined with PCM model

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Fig. 4 Measured upconversion fluorescence intensity as a function of the incident 1064 nm intensity. The solid lines is the best-fit curves based on the function y = axⁿ with n = 2.92

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Intensity-dependent transmittance measurements are utilized to obtain 3PA cross section. Neglecting the linear absorption at the pump wavelength, the beam attenuation due to three-photon absorption along the optical propagation path z is given by the following equation:

As we know, the irradiance of a Gaussian beam with no absorption or beam depletion can be written as

The sample passed by the laser is averagely divided into m cylinders along the z direction, and every cylinder is averagely divided into n annuluses along the r direction, hence there are m × n annuluses in the sample. The light intensity in a given annulus is deemed homogeneous along the r direction and parallel along the z direction, therefore Eq. (2) can be used for all the annuluses.

Figure 5 shows transmitted on-axis intensity vs. incident on-axis intensity curves of the compound in CHCl₃. Each data point represents an average over 10 laser pulses. The solid line represents the theoretical fitting with the best-fit parameter ${\alpha }_3$ = 11.9 × 10⁻²⁰ cm³/W². No nonlinear effect can be detected in pure solvent. One can see that the sample displays apparent optical limiting effect. The measured transmittance becomes low slowly as the incident radiance increases below 10 GW/cm². There is a dramatic drop of the transmittance at the range 10-150 GW/cm² of incident radiance. When the incident irradiance reaches ~150 GW/cm², the transmittance decreases to 0.8% or so. There is no detectable decomposition even as the incident radiance reaches 200 GW/cm².

 

Fig. 5 Transmitted on-axis intensity vs. incident on-axis intensity curves of the compound. The solid line represents the theoretical fitting curve. The best-fit parameter was $\gamma$ = 11.9 × 10⁻²⁰ cm³/W².

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Generally, resonant energy transfer may take place and play a major role as the dye concentration increases. It probably has an unusual concentration dependence of higher nonlinear effects [16]. In the used solution, the transition moment ${\mu }_{12}$≈3.36 D (11 × 10⁻³⁰ C﹒m), the molecular spacing r≈1.2 × 10⁻⁸ m, and the half width of the transition is $\hslash {\Gamma }_{1/2}$≈0.53 eV. We may assume the excited-state lifetime ${\tau }_2$≈1 ps. According to the reference [16], the multi-photon enhancement factor ${\gamma }^R/{\gamma }^{NR}$, which is from the resonant energy transfer, is estimated to be 1.4 × 10⁻⁷. Consequently, the concentration dependence of higher nonlinear effects can be ignored.

For a given solution sample, the 3PA coefficient ${\alpha }_3$value is related to the solute concentration d₀ (mol/L), and the value of 3PA cross section ${\sigma }_3^{\text{'}}$ (in the units of cm⁶ s²) can be determined by

From the characteristic curve in Fig. 5, one can see when the incident intensity increased from 20 to 150 GW/cm², the transmitted on-axis intensity approaches a constant. Even a very large fluctuation (between 20 and 150 GW/cm²) of the incident intensity can just lead to a very small fluctuation of the transmitted intensity. Therefore, this type of input-output relation can be expected to be used for optical pulse stabilization purposes.

The optical stabilization measurement results are shown in Fig. 6(a) for the incident laser pulse energy fluctuation at the input face of the cell, in Fig. 6(b) for the corresponding transmitted laser pulse energy fluctuation at the output face of the cell and in Fig. 6(c) for the corresponding calculated on-axis transmitted laser pulse intensity fluctuation at the output face of the cell. The apparent difference between Fig. 6(b) and Fig. 6(c) is ascribed to Gaussian distribution of energy of the incident laser, since for a Gaussian beam, with the radial r increasing, the optical intensity becomes lower and the transmittance becomes higher.

 

Fig. 6 Measured pulse energy fluctuation of incident laser pulses (a). Measured pulse energy fluctuation (b) and on-axis intensity fluctuation (c) of the corresponding transmitted laser pulses.

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From Fig. 6, one can see that the instantaneous energy fluctuation for the incident laser pulses is very severe, the maximum fluctuation can even be near 300%. However, after passing through the 3PA medium, the maximum fluctuation for the transmitted pulse energy can be reduced to less than 10%, and the on-axis transmitted intensity is almost a constant, which is an ideal optical stabilization.

Moreover, a 3PA-induced nonlinear absorptive system is one of the best technical approaches for optical stabilization and optical limiting with the advantages of (i) a fast temporal response, (ii) a higher initial transmittance for weak input signals and (iii) the threshold value of optical limiting and optical stabilization of the solution can be easily adjusted by changing its concentration.

4. Conclusions

We designed and synthesized a bifluorenylidene derivative with extended π-conjugated system. Ideal 3PA-induced optical limiting capability has been achieved. The measured 3PA cross section is as high as (81 ± 8) × 10⁻⁷⁶ cm⁶s². The optical stabilization performance of the compound is distinguished. The on-axis transmitted intensity approached a constant even though the incident laser pulse fluctuation was 300%.