By complexion of donor and acceptor using ionic interactions, the enhanced nonlinear optical responses of donor-acceptor ionic complexes in aqueous solution were studied with 7-ns laser pulses at 532 nm. The optical limiting performance of negatively charged gold nanoparticles or graphene oxide (Acceptor) was shown to be improved significantly when they were mixed with water-soluble, positively-charged porphyrin (Donor) derivative. In contrast, no enhancement was observed when mixing with negatively-charged porphyrin. Transient absorption studies of the donor-acceptor complexes confirmed that the addition of energy transfer pathway were responsible for excited-state deactivation, which results in the observed enhancement. Fluence, angle-dependent scattering and time correlated single photon counting measurements suggested that the enhanced nonlinear scattering due to faster nonradiative decay should play a major role in the enhanced optical limiting responses.
©2010 Optical Society of America
Optical limiters are devices consisting of materials that exhibit high transmittance for low-intensity light, and attenuate intense optical beams. They can be used to protect optical sensors (human eyes or CCD cameras) from damage caused by intense laser pulses [1,2]. Effective optical limiting (OL) of nanosecond laser pulses were observed in fullerenes , phthalocyanine complexes , carbon nanotubes [4–6], nanoparticles [7,8], metal nanowires  and organic chromophores . Most of the works have been carried out towards ideal OL materials by exploiting the underlying mechanisms including reverse saturable absorption , excited-state absorption , nonlinear scattering [5,12], and multi-photon absorption .
Donor-acceptor complexes offer great potential for the development of ideal OL materials. The enhanced OL mechanism involving photoinduced charge/energy transfer has been reported [14–20]. As shown by Dupuis and associates, photoinduced intramolecular charge transfer in classic donor-acceptor complexes offered significant enhancement in the OL response for nanosecond laser pulses . Mhuircheartaigh et al.  reported enhanced OL response of an non-covalently linked porphyrin and carbon nanotube (CNT) composite, as compared to their individual components. In addition, many research groups observed enhanced OL response in covalently linked porphyrin-CNT [17,18], porphyrin-C60 , and porphyrin-graphene composite materials . It was also demonstrated that CNTs, fullerenes and metal nanoparticles were excellent electron/energy acceptor materials [17,19,21] and porphyrins were superior electron/energy donor molecules [22,23]. It should be pointed out that most of the above-said studies were based on the covalent functionalization of donor and acceptor molecules.
Here we report a simple strategy to enhance OL responses in donor-acceptor complexes by utilizing ionic interactions between donor and acceptor materials. The donor-acceptor complexes were prepared simply by mixing oppositely charged donors and acceptors, which offers great advantages over covalent functionalization, where a complex chemistry is required for synthesizing such complexes. The enhancement in the OL response of negatively charged Gold nanoparticles (Au NPs) or graphene oxide (GO) (Acceptor) was observed when they were mixed with water-soluble, positively-charged porphyrin (Donor) derivative. But, no enhancement was observed when mixing with negatively-charged porphyrin. Time Correlated Single Photon Counting (TCSPC) measurements showed shortening of porphyrin emission lifetime when positively charged porphyrin bind to negatively charged Au NPs or GO due to either energy or/and electron transfer. Furthermore, pump-probe studies were performed for these donor-acceptor complexes. The transient absorption spectra confirmed that the energy transfer pathway should be responsible for the excited-state deactivation, which results in the enhanced nonlinearity. Fluence, angle-dependent scattering measurements were also performed, which suggested that nonlinear scattering should contribute to the enhanced nonlinearity of the donor-acceptor complexes.
The negatively-charged Au NPs were prepared by a previously reported method  and the negatively-charged graphene oxide (GO) was prepared by a modified Hummers method . Their TEM and AFM images are shown in Fig. 2 . The sizes of the obtained Au NPs and GO sheets were in the range of 13-15nm and 1-4 μm respectively. The positively-charged TMPyP (meso-Tetra (N-methyl-4-pyridyl) porphine tetra tosylate) (P+), and negatively charged T790 (meso-Tetra (4-carboxyphenyl) porphine) (P-) porphyrin were purchased from the Frontier Scientific and their structures are displayed in Fig. 1 . The stock solutions of porphyrin, Au NPs and GO were prepared in such a way that they have 50% absorbance in 1 cm quartz cell at 532 nm. The porphyrin-GO and porphyrin-Au complexes were prepared by mixing 1:1 and 1:2 volume ratios of their stock solutions, respectively. It was observed that the fluorescence quenching efficiency of Au was lower than that of GO when the Au NPs and GO of same linear transmittance were added into porphyrin solution. The nonlinear optical properties of porphyrin, GO, Au NPs and their complexes were measured with their linear transmittance fixed at 65%.
3. Results and Discussion
The UV-Vis absorption spectra of P+, P- and their complexes (Au + P+, Au + P-, GO + P+ and GO + P-) in water solution are shown in Fig. 2. The Au NP solution shows a surface plasmon resonance peak at 525 nm and the GO solution possesses a broad absorption continuously decreasing to 800 nm. The P+ and P- solutions exhibit two main groups of transitions, corresponding to the Q band (in the visible) and the B (Soret) band (in the near UV). There is no shift of the bands observed in the donor-acceptor complexes, which suggests that the donor-acceptor mixing causes no alteration of the electronic states of donor and acceptor.
3.1 Excited-State Interactions
The excited-state interactions were probed by steady state and time-resolved fluorescence spectroscopy. The fluorescence spectra of P+, P-, Au + P+, Au + P-, GO + P+ and GO + P- complexes are shown in Fig. 3 . Upon excitation of the porphyrin (P+ & P-) at 430 nm, the solution of Au + P+ and GO + P+ complexes exhibited 60% and 73% quenching of the emission at 640 nm respectively. But there was no sign of quenching in the Au + P- and GO + P- complexes, demonstrating that the opposite charges are prerequisite for the formation of donor-acceptor complexes. The observed fluorescence quenching upon forming donor-acceptor complexes by ionic interactions indicates that there should be a strong interaction between the excited state of P+ and Au NP or GO in the donor-acceptor complexes. The possible pathways for the deactivation of excited P+ could be attributed to photoinduced electron or/and energy transfer (PET). Previously, Heeger et al.  reported possibility of electron/energy transfer when conjugated polymers formed ionic complexes with Au NPs.
Furthermore, the fluorescence lifetimes of porphyrin and their complexes with Au NPs, GO were measured by using TCSPC (Horiba Jobin Yvon IBH ltd) by exciting with NanoLED of wavelength 438 nm (pulse width of 250ps) and probing at 640 nm for the emission. The measurements are shown in Fig. 4a and Fig. 4b. The positively charged porphyrin alone shows monoexponential fluorescence decay with a life time of ~20 ns in water. The emission decays of positively charged porphyrin complex with Au NPs or GO exhibit an additional fast deactivation pathway. The Au + P+ complex decays with a fast component of 250ps followed by a slower component of 15 ns and the GO + P+ complex decays with a fast component of 1.2ns followed by a slower component of 6ns, which are faster than the pure P+. But, there was no change in the fluorescence lifetime of the negatively charged porphyrin (P-) when it was mixed with either Au NPs or GO in water as shown in Fig. 4b, where there was no interaction due to the repulsion between similar charges. The reduction in the fluorescence lifetime of the P+ is an indicative of the electron and/or energy transfer process from P+ to Au NPs or GO.
Femtosecond transient absorption spectroscopy was carried out for the better understanding of the mechanism of the quenching process. The transient absorption spectra of pure P+ and their complexes with Au NPs and GO were measured by exciting with 400 nm femtosecond laser pulses at a time delay of one nanosecond and those are shown in Fig. 5 . P+ shows strong excited sate absorption at 460 nm and a negative absorption at 650 nm. The negative ΔA/A arises from the stimulated emission of the excited P+. When the suspension of Au + P+ or GO + P+ was excited with a 400 nm femtosecond laser pulse, we observed the similar transient absorption spectra as pure P+. We did not observed any new absorption bands in the transient absorption spectra of Au + P+ and GO + P+ because neither reduced nor oxidized form of the porphyrin occurred. The lower absorption values of the spectrum (b) and (c) in Fig. 5 confirmed the quenching of the excited state within the laser pulse duration and the quenching is more efficient for GO + P+ complex, as compared with the Au + P+ complex. On the basis of these absorption and emission studies, we can infer that the energy transfer pathway is the major contribution in the excited state deactivation pathway. Previously it has been reported that, when chromophores bind to Au NPs of size less than 8 nm in diameter, an electron transfer pathway dominates [26–28]. Another possible pathway for the quenching of the fluorescence involves direct energy transfer to Au NPs or GO and this is dominant especially when the metal nanoparticles of larger size.
3.2 Nonlinear Optical Properties
The nonlinear optical properties of these materials were measured by using fluence dependent transmittance measurements at 532-nm with 7-ns laser pulses with repetition rate of 10-Hz. Figure 6 shows that the OL performance of the P+, P-, Au + P+, GO + P+, Au + P- and GO + P- complexes at 65% linear transmittance. These measurements demonstrated that the OL performance was enhanced for the Au + P+ and GO + P+ complexes as compared to their individual P+, Au NPs, or GO. However, there was no enhancement for the Au + P- and GO + P- complexes. The limiting threshold of the GO + P+ and Au + P+ complexes in 1 cm quartz cell were ~1.9 and 4.3 J/cm2, respectively, much less than pure P+, Au NPs, or GO solutions as shown in Fig. 6, indicating that the energy transfer enhances the nonlinear optical response. Although the GO + P+ complex exhibits lower threshold than the Au + P+ complex, but the enhancement of nonlinear optical response for Au + P+ was higher than the GO + P+ complex as compared to their individual P+, Au NPs, or GO. The strong enhancement for Au + P+ complex is due to the strong surface plasmon resonance of Au NPs at 532nm, which can enhance the nonlinear optical response of Au + P+.
The enhanced nonlinear scattering behavior was also observed for these donor-acceptor complexes. Figure 7 shows the fluence-dependent scattering measurement at an angle of 20° to the transmitted laser beam. As shown in Fig. 7(a) and 7(b), the nonlinear scattering was enhanced for the Au + P+ and GO + P+ complexes as compared to their individual P+, Au NPs, or GO. But there was no enhancement for the Au + P- and GO + P- complexes. The angle dependent scattering measurements are shown in the insets of Fig. 7(a) & 7(b). The scattering signals are in excellent agreement with the nonlinear optical transmittance measurements.
So the nonlinear scattering plays an important role in enhancement of nonlinear optical properties of donor-acceptor complexes. The nonlinear scattering arises partially due to thermal expansion of complexes; and partially due to bubble formation in the solution via energy transfer from the complexes to the solvent. The lifetimes of the Au + P+ and GO + P+ complexes are shorter than those for the pure P+. Hence, energy transfer is more efficient in the Au + P+ and GO + P+ complexes, resulting in efficient nonlinear scattering. Between the Au + P+ and GO + P+ complex, the former is less efficient than the later, consistent with their lifetime measurements. The major contribution for enhancement of nonlinear optical properties of the complexes is due to the fast deactivation due to energy transfer, thus most of the energy is dissipated through non-radiative decay, and more heat is generated. The generated heat transfers to the solvent will result in a formation of microbubbles, which can act as scattering centers and lead to the increase of optical limiting. Therefore donor-acceptor complexes like Au + P+ and GO + P+ can serve as good nonlinear scatterers and effective OL materials. It should be emphasized that these composites are attainable by simple mixing of oppositely charged donor and acceptor and no complicated chemistry is required.
The nonlinear optical properties of donor-acceptor ionic complexes have been studied in two model systems using positively-charged porphyrin derivative as donor and negatively-charged Au NPs or graphene oxide as acceptor. The results show that porphyrin-gold nanoparticle or porphyrin-graphene oxide ionic complexes exhibits enhanced nonlinear optical properties as compared with the individual porphyrin, Au NPs and graphene oxide. But, there is no enhancement when negatively-charged porphyrin is mixed with negatively-charged Au NPs or graphene oxide. Enhanced nonlinear scattering is observed in the donor-acceptor ionic complexes, which contributes to the enhanced nonlinear optical response. Transient absorption studies of the donor-acceptor complexes confirm that the energy transfer pathway is mainly responsible for the excited-state deactivation which results the enhanced OL performance.
The authors thank the National University of Singapore and the DSTA, Singapore (Grant # R-143-000-432-422) for financial support.
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