We show that mexylaminotriazine molecular glass functionalized with the azobenzene derivative Disperse Red 1 shows equally strong second-order nonlinear optical response as well-known polymers with the same photoactive component. Furthermore, even high chromophore loading does not adversely affect the nonlinear response. This suggests that chromophore-chromophore intermolecular interactions do not greatly limit the response of such molecular glasses, which therefore provide an excellent materials platform for nonlinear optical applications.
© 2016 Optical Society of America
Azobenzenes are a well-known trigger for controlling material properties with light thanks to their reversible photoinduced cis-trans isomerization . The isomerization process can drive photomechanical movement such as bending  and formation of surface relief gratings (SRGs)  as well as photoinduced molecular realignment into highly asymmetric structures , each controlled by the properties of the excitation light. Even the combination of SRGs and asymmetric molecular order has been proven effective [5, 6]. Azobenzene-based materials are therefore under intense study for applications in optical data storage, photonic circuitries and energy conversion. A particular example of photoinduced asymmetry is all-optical poling (AOP) , where a coherent superposition of a dual-frequency optical field leads to photoexcitation with polar selectivity. After repeated trans-cis-trans cycles, the process leads to broken inversion symmetry enabling second-order nonlinear optical (NLO) properties [7, 8].
For many applications, a stable amorphous phase of the photoactive moiety is preferred. Pure azobenzenes are brittle crystalline materials, which has lead to the practice of embedding them into a polymer forming an amorphous support structure . These azo-polymers inherit their photoactive properties from the azobenzene while maintaining the rigidity and ease or processing of the parent polymer. While azo-polymers have shown their value as a platform for light-controlled materials, a less used alternative strategy is provided by small molecules capable of forming stable amorphous structures. Such materials, referred to as molecular glasses, have the advantage that the molecular structure and mass are well known in contrast to polymers, which yields easier purification, characterization and better reproducibility [9, 10].
While some small molecules capable of forming amorphous phase have been known for decades, intense study of molecular glasses started in the 1990s focusing on organic charge-transporting materials for electroluminescence [10, 11]. Photochromic glassy materials using azobenzenes were soon demonstrated , allowing photoinduced birefringence and surface-relief grating formation . High groove depth and complex patterning  as well as efficient coupling into a waveguide  have been demonstrated with photoinduced SRGs in azobenzene molecular glasses. Attempts have been made to create molecular glasses with NLO response by combining push-pull type azobenzenes [15, 16], and other suitable nonlinear optical chromophores  with functional groups that promote the formation of an amorphous phase. To date, a single attempt of AOP in a non-polymeric organic glass has been reported . Molecular glasses functionalized with azobenzenes have not been studied by AOP although the method is known to be well-suited for azo-polymers [7, 19].
A few years ago, a glass-forming group named mexylaminotriazines was developed by Lebel and coworkers [20, 21]. This group can be easily modified [22, 23] allowing for facile control of its properties, most importantly the glass transition temperature Tg. The mexylaminotriazines possess relatively high symmetry, rigid structure and possibility to self-assemble through hydrogen bonding, all of which defy the traditional design features of molecular glasses. Yet their ability to form stable amorphous phases is remarkable even when functionalized with compounds that readily form crystalline structures when pure . Recently, a mexylaminotriazine functionalized with Disperse Red 1 (DR1) azobenzene was synthesized and shown to be equally good as DR1-PMMA functionalized polymer system for surface-relief grating inscription [25, 26].
In this study, we show that this new mexylaminotriazine functionalized with DR1 provides an ideal candidate for all-optical poling. We find that the DR1 glass reaches an NLO response that is as high as that of similar polymer counterparts while maintaining the benefits of the well-known structure and better repeatability inherent to molecular glasses. Furthermore, we show that chromophore-chromophore intermolecular interactions do not compromize the nonlinear properties with this material even at fairly high chromophore concentrations.
2. Materials and methods
The NLO response was studied with AOP using 1064 nm as the fundamental writing field and its second harmonic at 532 nm from a KDP crystal as the weak seeding field. The source for the fundamental field was an Ekspla PL2231 diode-pumped Nd:YAG solid-state laser producing 10 mJ, 28 ps pulses at 100 Hz repetition rate. Both fields were linearly polarized with pulse energies of 7 mJ and 1 μJ for the fundamental and seeding fields, respectively, at the sample plane. This seeding ratio of 7000:1 was chosen after careful optimization for the sample series. The optimization process has been explained in detail in . In the poling process, two-photon absorption of the writing field and one-photon absorption of the seeding field interfere creating polar selective excitation for the DR1 moieties which gradually results in an in-plane polar order. The increasing order is monitored by blocking the seeding field periodically and by measuring the second-harmonic produced by the sample as the writing field is still applied on the sample.
The first sample series consists of the DR1-functionalized mexylaminotriazine molecular glass (DR1MG) and of two well-known reference polymers (Fig. 1). The first one is the homopolymer Poly(Disperse Red 1 acrylate) (PDR1A) and the second is the copolymer Poly[(methyl methacrylate)-co-(Disperse Red 1 acrylate)] (P(MMA-co-DR1A)). DR1MG was acquired from Solaris Chem and the polymers from Sigma-Aldrich. All components were used without further purification. The homopolymer represents a polymer material with one of the highest possible DR1 mass fractions (85 %) for an amorphous material. The copolymer, on the other hand, was chosen as its DR1 fraction is 47 wt%. This fraction is very close to the value of 50 wt% calculated for the molecular glass. The absorption spectra of the materials in 1,2-dichloroethane are shown in Fig. 2(a).
Thin film samples of the molecular glass and polymers were spin-coated on clean microscope glass slides. Solutions with 2 % of the compounds in 1,2-dichloroethane were prepared and filtered through 0.45 μm filter. The spinning rate was set to 1000 rpm for P(MMA-co-DR1A) and 3000 rpm for PDR1A and DR1MG in order to reach absorbance of 0.3 at 532 nm which is well suited for AOP. After the poling experiments, the thicknesses of the samples were measured with Dektak 6M stylus profiler. The absorption spectra of the samples are shown in Fig. 2(b). The thicknesses of the samples were 100 nm, 87 nm and 152 nm for DR1MG, PDR1A and P(MMA-co-DR1A), respectively. The homogeneity and stability of the samples was confirmed with polarized optical microscopy where little to no signs of crystallization were seen several months after the experiments. The density of DR1MG was determined using small flakes from a thick drop-cast film which were placed in a concentrated sucrose water solution where they float. The solution density was lowered in small steps by dilution with water until the flakes sink. The density of the solution was measured before and after the flakes sink which gives the DR1MG density 1.25 ± 0.04 g cm−3.
The evolution of second-harmonic (SH) signal during the poling process was monitored for 15 minutes after which all of the samples nearly reached saturation. Next, the decay of the signal was monitored for 15 minutes without further poling. In a film shorter than the coherence length for second-harmonic generation, the SH intensity scales quadratically with the sample thickness . Therefore, in order to access the material properties, the measured SH signals were normalized with the sample thicknesses squared. The evolution of the thickness-normalized SH intensities are shown in Fig. 3. From the results, it can be concluded that, within experimental uncertainties, the second-order response of DR1MG is identical to the reference polymers. It should be noted that the Tg-values of the reference polymers PDR1A (79 °C) and P(MMA-co-DR1A) (102 °C) are somewhat higher than 71 °C  found in the molecular glass. This could lower the stability of the asymmetric molecular order in the molecular glass compared to the polymers. However, in the case SRGs, it has been found that the thermal stability of DR1MG is similar to a reference polymer with 20 °C higher Tg possibly owing to the hydrogen bonding present in the glass . This makes the comparison with the chosen polymers well justified and the results in Fig. 3 hint towards slightly slower dynamics in the glass compared to the polymers.
Much of the work on polymers for NLO applications has focused on increasing the concentration of the nonlinear chromophores in order to reach higher NLO response. However, for polar push-pull type molecules, the interchromophore interactions start to play against this goal well before the mechanical properties become compromised [28, 29]. For DR1-PMMA guest-host system the optimum concentration is 20–30 wt% , while in a copolymer P(MMA-co-DR1MA), similar to the one used here, the maximum NLO response is reached at 30–40 wt% . Therefore, the optimum concentration for the DR1 molecular glass is also of great interest and was studied in our second series of experiments. In order to vary the DR1 concentration, DR1MG was mixed with another mexylaminotriazine acquired from Solaris Chem Inc. (see Fig. 1) with no absorption in the visible range. Otherwise, the properties of this non-absorbing molecular glass (NAMG) are expected to be close to DR1MG. For instance, the glass transition temperature of NAMG is 71 °C , equal to that of DR1MG . It should be noted that the pure DR1MG contains 50 wt% of DR1. This sets the upper limit for the chromophore loading in our experiments.
A series of molecular glass mixture (MGM) samples with the DR1 mass fraction varying from 0 to 50 % in 10 % steps was fabricated by mixing solutions of NAMG and DR1MG in appropriate ratios and spin coating. The spin coating was performed at varying rates between 600 and 5500 rpm to reach absorbance close to 0.3 at the 532 nm wavelength. This ensures that the AOP seeding ratio need not be changed from sample to sample. The thicknesses and absorbances of the molecular glass mixture samples at 532 nm are shown in Table 1. It should be noted that MGM 0.0 represents pure NAMG and MGM 0.5 pure DR1MG.
All-optical poling was perfomed on the MGM samples as in the first measurement series and the SH signals after 15 minutes were recorded. From these values, the second-order susceptibilities (χ (2)) were calculated using 8] is the susceptibility of a Y-cut quartz reference, nω,S and nω,Q and are the refractive indices of the sample and quartz at the fundamental frequency and n 2ω,S and n 2ω,Q at the second-harmonic frequency, ΔkQ = 0.15 × 106 m−1  is the wave vector mismatch in quartz, A is the absorbance of the sample at the second-harmonic wavelength. The refractive indices of the molecular glass mixtures were approximated with values measured for 10 mol% DR1-PMMA , i.e. nω,S = 1.55 and n 2ω,S = 1.68. The χ (2) values are shown in Fig. 4(a). A more or less linear increase is seen in the studied mass fraction range.
Second-order susceptibility of 30 pm/V was reached for the DR1 molecular glass. This value is lower but comparable to ∼100 pm/V found for optimal DR1 polymer systems [7, 31]. The fact that the highest second-order susceptibility was reached with the pure DR1MG suggests that, for the molecular glass, the optimum chromophore loading is near or beyond 50 wt%. This is a high loading value compared to the optimum values experimentally found for similar polymer systems. However, it is peculiar that the NLO response only equals that found in the reference polymers as both of the polymers have higher than optimum DR1 loading which is known to decrease the achieved NLO response. With DR1MG density of 1.25 g cm−3, the number density of the DR1 moieties is 1.20×1021 cm−3. This value is in good agreement with the optimum value found in simulations taking into account the interchromophore interactions , which suggest that higher DR1 loading would not further increase the NLO response. The verification of this prediction is out of experimental reach with the studied molecular glass. However, a molecule with high hyperpolarizability is usually accompanied with high dipole moment. Higher dipole moment brings the optimum chromophore concentration down rapidly . Therefore, in order to estimate the practical limits of the functionalized mexylaminotriazine, substitution with a chromophore with higher hyperpolarizability would be required.
Examination of the spectral properties highlight the feasibility of mexylaminotriazine as a host material. As seen in Fig. 2(a), going from pure DR1 to the DR1 glass a 50 % drop is seen in the absorbance when studied in solutions with equal concentrations by weight. This is in excellent agreement with the fact that DR1 mass fraction is 50 % in this glass. In the polymers, on the other hand, the drop in absorption is much greater than the DR1 mass fraction would suggest. Therefore, it is evident that confinement into polymer side chains affects the properties of DR1 to much greater extent than substitution to the molecular glass. Also, the location of the absorption maximum in the glass at 484 nm wavelength is very close to that found for DR1 at 486 nm. For P(MMA-co-DR1A) and PDR1A, the absorption maxima are found at 476 nm and 471 nm showing much greater blue shift. This comparison is not totally fair as the polymeric structure forces some of the molecules close to one another even in solution. In the solid state, similar effect is to be expected for each material as the intermolecular distances inevitably decrease. Still, as seen in Fig. 2(b), the molecular glasses show less blue shift also in the solid state even when the DR1 concentration exceeds that found in P(MMA-co-DR1A). This is an indication of less pronounced chromophore-chromophore intermolecular interactions which are known to be detrimental to many optical properties .
In conclusion, we have studied the second-order nonlinear optical properties of mexylaminotriazine molecular glass functionalized with a photoactive nonlinear optical chromophore Disperse Red 1 using all-optical poling. The photoinduced nonlinearity of the molecular glass was found to be identical to that found in polymers functionalized with the same chromophore. Our results point that this new molecular glass is well-suited for a host structure for nonlinear optical materials. In addition, the optical properties of the chromophores are less affected by the molecular glass than by the polymers. The desired optical nonlinearity is reached with no adverse effect on the advantageous properties of molecular glasses. The results suggest that high chromophore loading with strongly dipolar chromophores can be reached in the molecular glass without compromised nonlinear optical response due to chromophore-chromophore intermolecular interactions. This leads to the conclusion that this platform would allow high optical nonlinearities to be reached by replacing Disperse Red 1 with a nonlinear optical chromophore with higher hyperpolarizability.
M.V. acknowledges the financial support from Tampere University of Technology Graduate School, the Väisälä Foundation and the Graduate School of Modern Optics and Photonics. A.P. acknowledges the financial support of the Academy of Finland (Academy Research Fellowship program, project number 277091) and the Emil Aaltonen Foundation. Semen Chervinskii is acknowledged for technical assistance.
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