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A novel glass matrix doped with phthalocyanine or naphthalocyanine is prepared by a modified sol-gel technique. The photophysical and optical limiting properties of the phthalocyanine compounds both in glass matrix and in THF solution were investigated. The obtained glass matrix is homogeneous and transparent, as well as mechanically and thermodynamically stable enough to withstand very high laser fluence; the optical limiting performances of these compound samples are better than that of benchmark materials like C60 in toluene, carbon black in water, and graphene oxide in water or ethanol under nanosecond pulsed laser at 532 nm. Two prototypes of optical limiters doped in the glass matrix have very good optical limiting performances, which may provide potential practical use for optical limiting materials in a near future.
© 2016 Optical Society of America
1. Introduction
Accompanying with the wide use of laser in many fields, the concept of laser protection has attracted much more attention in recent years. Especially, for the protection of human eyes and optical sensors against strong laser pulses, some optical limiting materials are then developed [1–3], which include of fullerenes, porphyrins, phthalocyanines(Pc) and some other organic-metallic compounds [4–8]. PcAlCl as Pc derivative was used to study its optical limiting behaviors in 1989 [9]. Pcs showed reverse saturable absorption (RSA) in organic solution and were proved to be good optical limiter. Perry et al. reported that indium tetra(tert-butyl)phthalocyanine chloride showed strong nonlinear absorptive response [10]. Shirk, Hanack et al. reported nonlinear optical properties of a series of soluble axially substituted or bridged gallium and indium Pcs [11–13]. Even now, Pc-based materials are still being investigated to improve their optical limiting performance and to expand their optical limiting window from visible to near IR range, in order to meet the practical applications [14,15]. Naphthalocyanines (Nc) with larger π-electron conjugation system are attracting broad interests as potential optical limiting materials because of their large optical nonlinearities on larger spectral window, RSA and ultrafast response times [16,17]. In our previous work, the effect of peripheral substituents and central metals on the photophysical and optical limiting properties of Pcs and Ncs were investigated [18–23], results showed that better optical limiters have weaker linear absorption, stronger excited absorption, higher triplet state formation quantum yield as well as longer triplet lifetime.
Up to now, however, most samples studied were in liquid solution. For the practical application of optical limiting behavior, solidified optical limiting materials dispersed in solid medium have received considerable attention. One of the fascinating methods is sol-gel technique which has been used to incorporate a variety of organic and metal-organic molecules in inorganic host [24–27]. However, the solidified samples may have poor physical properties such as frangibility, poor transparence as well as mechanical and thermodynamic instability and so on, which may be the great hindrance for their practical use as optical limiters.
We report here the preparation of novel glass matrix doped with axially substituted Pcs: (t-BuPhO)4PcAlCl (1), (t-BuPhO)4PcGaCl (2), (t-BuPhO)4PcInCl (3), and Nc: (t-BuPhO)8NcGaCl (4) (Fig. 1). The axially substituted Pcs (1-3) [23] and Nc (4) [20] were synthesized according to our published papers. Strong optical limiting behaviors and favorable mechanical properties were found in the obtained glass matrix. The photophysical and optical limiting properties of 1-4 both in glass matrix and in THF solution were investigated, in order to analyze the mechanism of the improved optical limiting behaviors.
2. Experimental section
2.1 Instruments and methods
UV-Vis absorption spectra were recorded on a Hitachi U-3010 spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. Transient absorption at nanosecond time scale was detected by an Edinburgh LP900 and recorded on a Tektronix TDS 3012B oscilloscope and computer. The excitation light was the harmonic of Nd:YAG laser (Continuum Surelite II, 355 nm and 7 ns FWHM). A pulsed xenon arc lamp was used to provide the analyzing light. The configuration of the monitoring light with respect to the excitation laser pulse was right-angle geometry. The liquid samples (1.0cm quartz cell) and solid matrix (0.2cm thickness) were settled on the platform at the intersection of monitoring light and excitation pulse. The triplet-minus-ground state extinction coefficients (∆εT) were calculated by the method of total depletion or saturation [18–23,28,29]. The quantum yields of the triplet state were determined by the comparative method [18–23,30], using unsubstituted ZnPc in 1-chloronaphthalene as reference standard (ФT = 0.65). The triplet lifetimes were obtained by kinetic analysis of the transient absorption. The optical limiting properties were measured by the standard setup of our previously reported method [18–23], in general, a nanosecond Nd:YAG laser (Continuum, 532nm, 7ns FWHM) was used as the laser source for the nonlinear transmission measurements, the laser beam was divided by a beam splitter: the reflected beam was used as reference and the transmitted one was focused onto the sample. The reference and transmitted laser pulses were monitored by energy detectors. The solid state of 1-4 in glass matrix were settled on the same position of the sample platform.
2.2 Preparation of glass matrix
The glass matrix was prepared by sol-gel technique with our modified method. A mixture of tetraethyl orthosilicate (TEOS) 1mL, 2,3-epoxypropoxy propyltrimethoxysilicane (KH560) 3mL, and distilled water 1mL (pH = 2) in a glass bottle was stirred at room temperature for 2 hours. Aqueous Na2SiO3 solution (wt, 5%) 0.1mL was added and kept on stirring for another 2 hours, and followed by addition of a selected volume of 4 × 10−3M Pc (or Nc) solution in CH2Cl2 and stirred for further 4 hours. After the mixture was homogeneous and transparent, it was transferred into a flat-bottomed quartz utensil and degassed at 60 °C under vacuum for 2 hours, then dried at room temperature for 2 days and at 120 °C for a week to give homogeneous and transparent Pc or Nc glass matrix.
3. Results and discussion
3.1 Solidified optical limiter
Traditionally, glass matrix is made from the catalytic hydrolysis of tetraethyl orthosilicate by sol-gel technique, which may suffer from the fragileness. As a modification for the traditional sol-gel technique (Fig. 2), 2,3-epoxypropoxy propyltrimethoxysilicane (KH560) and sodium silicate (Na2SiO3) are added as indispensable ingredients. Here KH560 with long alkyloxy group plays the role of intensifying the flexility of the glass matrix and improving the capability of conglutination for the organic and inorganic materials [31]. The sodium silicate (Na2SiO3) is filled in the pores which are formed at the aging process of the gel, making the glass matrix more compact [32]. On the other hand, the Na2SiO3 is a bonding agent which combines all the fraction of the matrix together and made matrix more difficult to crack. The glass matrix is transparent, mechanically and thermodynamically stable, and shown in Fig. 3, which is suitable for the investigation of photophysical and optical limiting properties. It should be pointed out that the diameter of the glass matrix shown in these photos was as large as 50 mm, and the glass matrix can be polished to optical grade just as general optical glass, which may be used as protection lens for human eyes and optical sensors. Moreover, the inorganic component of sodium silicate spread all over the interspace of the matrix would greatly increase the capability of heat conductibility, so as to stand against high energy of laser irradiation.
3.2 Optical limiting properties
On the base of favorable mechanical and optical capability, optical limiting behaviors were investigated for the samples 1-4 doped in glass matrix (Fig. 4). Figure 4 shows the optical limiting properties of 4 in glass matrix in contrast to that in THF solution. Every sample of 1-4 either in glass matrix or in THF solution has the same linear transmittance (Tlin) of 56%, 70%, 51% and 66%, respectively. At the highest fluence used in our experiments, the limiting saturated transmittances (Tlim) decrease to 2.7%, 3.1%, 1.9%, 2.6% for 1-4 in glass matrix, and 5.1%, 5.0%, 3.7%, 4.7% for 1-4 in THF solution, respectively. The values of nonlinear attenuation factor (NAF = Tlin/Tlim) for 1-4 are 20.7, 22.6, 25.9, 24.4 in glass matrix and 11.2, 14.0, 13.5, 13.7 in THF solution, respectively. Moreover, the optical limiting thresholds (defined as the incident fluence at which the transmittance falls to half of the linear transmittance) of 1-4 in glass matrix are smaller than that of in THF solution. For compound 4 doped in glass matrix, its optical limiting threshold decreased to 0.17 J·cm−2. The optical limiting threshold of these compounds samples is better than that of benchmark materials like C60 in toluene, carbon black in water and graphene oxide in water or ethanol under nanosecond pulsed laser at 532 nm [33–35]. The value is similar as that of the graphene oxide in bisphenol-A polycarbonate film, in which the optical limiting threshold is 0.08 J·cm−2 (using 3.5ns pulse laser at 532nm) [36]. The optical limiting properties of porphyrins in sol-gel and fulleropyrrolidines in Nafion membrane were studied before [37,38], the optical limiting thresholds of our results are similar with that of porphyrins in sol-gel(using 15ns pulse laser at 584nm) and that of fulleropyrrolidines in Nafion membrane(using 10ns pulse laser at 532nm), but our optical limiters can stand much higher laser incident intensity. And, the optical limiting threshold value of the compound 4 doped in glass matrix can reach or beyond the values in optical limiter patent [39], which indicates that these samples may be used as practical use for optical limiting materials. All of the optical limiting parameters (shown in Table 1) suggest that the optical limiting performance has been greatly improved for 1-4 doped in the glass matrix.
Table 1. Optical Limiting Parameters of 1-4 in THF Solution and in Glass Matrix
One of the most important features is that the glass matrix is stable enough to stand against high laser energy (the highest fluence of 30 J·cm−2 in our experiment setup). After the same scale of input laser fluence irradiation on the sample at different times, the detected output energies fluctuate very little in the glass matrix, comparing with that in THF solution. The experimental data lines in Fig. 4 are smoother and thinner in the glass matrix than that of in THF solution, which is also in supporting of noticeable mechanical and optical stability for the glass matrix.
3.3 Photophysical properties
The previous studies have proved that optical limiting performance can be improved by reverse saturable absorption (RSA) resulted from excited state absorption. The triplet parameters such as the quantum yield of the triplet state (ФT) and the triplet-minus-ground state extinction coefficients (∆εT) are important parameters to affect the optical limiting performance [18–23]. Thus, for a better understanding of the mechanism of the improved optical limiting behaviors, the photophysical properties of 1-4 in glass matrix were investigated, and the photophysical properties of 1-4 in THF solution can be found in our published papers [20,23].
The ground state absorptions of Q bands for 1-4 change little between glass matrix and THF solution (Fig. 5), both have an intense S0-S1 transition. However, the B bands display much stronger absorption at about 300nm in glass matrix. In addition, the shape of Q bands in glass matrix is almost the same as in THF solution, suggesting no aggregation occurred in glass matrix, for aggregation may decrease optical limiting performance [21]. According to the wavelength at the point of intersection between normalized absorption and fluorescence spectra, the S1 state energy levels were estimated (Table 2). Samples 1-4 in glass matrix have a bit lower S1 state energy level, which is beneficial to the intersystem crossing from S1 to T1 state in glass matrix.
Table 2. S1 State Energy Levels of 1-4 in THF Solution and in Glass Matrix
Transient absorptions of 1-4 in glass matrix were investigated (Fig. 6). In THF solution, the transient absorptions clearly show a positive transient absorption from 500 to 700nm with two negative bleaching peaks at the Q and B bands of ground state absorption region. But in the glass matrix, the bleaching absorption is untidy and splits into two or several peaks. The reason could be ascribed to the less probability of nonradiative transition of excited states in glass matrix than that in THF solution, resulting in stronger absorption of excited state at the Q and B band regions, then disorder the corresponding bleaching peak of excited state, and causes the bleaching peak disordered to be unable identified. In addition, the molecules distributed in different part of the glass matrix may cause different extent for bleaching and absorption of excited states, and this is also one of the reasons to explain the disordered bleaching spectra observed for Pcs(Nc) doped in the glass matrix [40,41].
The λmax of triplet state absorption for 1-4 are 590, 590, 600, 640 nm in glass matrix and 560, 570, 590, 610 nm in THF solution [20,23], respectively. Obvious bathochromic shifts observed in glass matrix, suggest lower energy gap between T1 and Tn states in glass matrix than that of in THF solution. In addition, the triplet-minus-ground state extinction coefficients (∆εT) are investigated in glass matrix and in THF solution (Table 3), all the data locate in the range of 104-105 M−1·cm−1, suggesting that the Pcs may achieve obvious absorption from T1 to Tn, which will effectively cause the enhancement of optical limiting effect.
Table 3. Parameters of Triplet State for 1-4 in THF Solution and in Glass Matrix
Moreover, the quantum yields of the triplet state ФT of the compounds are large, all the values are more than 0.5, both in the glass matrix and in THF solution. And the Pc (Nc) molecules present higher values of ФT in the glass matrix than that of in THF solution (except for the compound 1.). The ФT values for 1-4 are 0.55, 0.83, 0.94, 0.94 in glass matrix and 0.74, 0.76, 0.90, 0.70 in THF solution [20,23], respectively (Table 3). Decays of triplet state for 1-4 were obtained as shown in Fig. 7. It is obvious that 1-4 display much longer triplet lifetimes (τT) in glass matrix (Table 3). In the glass matrix, the decays of 1-4 completely obey the first-order exponential kinetics, while in THF solution the triplet states decay much faster, especially at high concentrations, and do not obey the first order exponential decay very well. In THF solution, the high concentration causes high probability of triplet interaction such as triplet-triplet annihilation, which may introduce of triplet self-quenching. While in the glass matrix the rigid surrounding effectively reduces the probability of nonradiative transition and the molecular π-π interaction, resulting in longer lifetime of triplet state relative to that of in THF.
For the Pc-based materials, longer triplet lifetime and higher triplet quantum yield could accelerate the triplet molecules to achieve RSA process, thus the optical limiting effect which directly related to RSA is accordingly enhanced. The rigid surroundings of the glass matrix beside the Pc/Nc molecules effectively decrease the probability of molecular interaction, in resulting of higher values of τT and ФT, and leading to better optical limiting behaviors observed for the Pc (Nc) molecules doped in glass matrix.
4. Conclusions
In summary, a novel glass matrix doped with phthalocyanine or naphthalocyanine prepared by the modified sol-gel technique displays much better optical limiting performance than that of in THF solution. The rigid matrix effectively decreases the probability of nonradiative transition of S1 and T1 states, which result in higher probability of intersystem crossing from S1 to T1 state, achieving higher triplet quantum yield ФT and triplet lifetime τT. The obtained glass matrix are homogeneous and transparent, as well as mechanically and thermodynamically stable enough to stand against very high laser fluence, which is a new approach to the practical use for optical limiting materials. Compounds 2 and 4 as prototype of optical limiters doped in the glass matrix have very good optical limiting performances, which may provide potential practical use for optical limiting materials in a near future.
Acknowledgments
We are grateful for the financial supports of the National Natural Science Foundation of China (NSFC) (Nos. 21373240, 21233011, 21273252, 21205122) and the National Basic Research Program of China (Nos. 2013CB834703, 2013CB834505).
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