Light Amplification in Derivatives of Pyrazoline-Based Systems Adam Szukalski, Lech Sznitko, Konrad Cyprych, Andrzej Miniewicz, and Jaroslaw Mysliwiec* Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland ABSTRACT: Here we report on experimental results characterizing stimulated emission (SE) and random lasing (RL) phenomena in two new pyrazoline derivatives (PRDs) in comparison with earlier studied 3-(1,1-dicyanoethenyl)-1- phenyl-4,5-dihydro-1H-pyrazole (DCNP) dye. Both phenom- ena were observed in thin films of poly(methyl methacrylate) containing selected dyes upon excitation with 6 ns light pulses delivered by a doubled in frequency Nd:YAG laser (λ = 532 nm, f = 10 Hz). We also report on photodegradation process taking place in the samples during observation of SE and RL and estimate average lifetimes in each case. The pumping threshold levels for observation of random lasing emission amounted from about 1.5 mJ/cm2 to nearly 70 μJ/cm2, depending on pyrazoline dye and for stimulated emission from 16 mJ/cm2 to 5 mJ/cm2, respectively. The resonator lengths related to the RL phenomenon were estimated using the power Fourier transformation method. ■ INTRODUCTION Pyrazoline derivatives are known not only from their medical applications,1,2 but also as materials showing luminescent and nonlinear optical properties.3−7 Structure of pyrazoline substituted with pendant groups and its capability of intra- molecular charge transfer through the pyrazole ring to surrounding, usually electron donor and acceptor groups, allows for manipulation of its physicochemical properties during molecular design or synthesis. New synthesized derivatives of pyrazoline (PRDs) exhibit high values of ground state dipole moments and due to the presence of π-conjugated bonds also high values of first hyperpolarizability which are desired attributes for nonlinear optics (NLO) chromo- phores.9,10 Nonlinear optical properties of organic com- pounds3−7 are being studied intensively by numerous scientific groups. Pyrazolines show fluorescence with a relatively high quantum yield11 as well as fluorescence excited via the two-photon absorption (TPA) process.3,8,12 It is interesting to study the photoluminescence spectral changes in organic nanocrystals due to presence of size effects.13 Organic pyrazoline nanocryst- als can easily be obtained with precipitation or reprecipitation methods13 allowing one to acquire nanocrystals of sizes ranging from nanometers to hundreds of nanometers. Second harmonic generation (SHG) was measured in nanocrystals of pyrazoline derivatives like 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro- 1H-pyrazole (DCNP).8,14 Changes in fluorescence excitation and emission spectra were observed with decrease of crystallite sizes.13,15 Easily tunable properties of pyrazoline derivatives favor their usage in a wide range of applications. For example, azo-dyes based on pyrazolines coupled with PMMA matrices have been investigated due to birefringence changes as a function of light- induced cis−trans isomerisations.16,17 Pyrazoline derivatives have been used in organic electroluminescent devices.18−21 There have been designed systems capable of selective detection of Fe3+ ions in various media. A receptor unit that consists of 1,3,5-triaryl-Delta(2)-pyrazoline and meso-substi- tuted boron-dipyrromethene (BDP) shows increase of fluorescence yield in the presence of Fe3+ ions.22 Pyrazoline derivatives were used as optical brighteners; furthermore, compounds synthesized for optical brightening like 1,3-diaryl-5- pyridyl-4,5-dihydropyrazole were also used for fluorescent PET (Photoinduced Electron-Transfer) sensors, where pH changes induce “on−off” switching of fluorescence.23 ■ MATERIALS We have previously reported observation of amplified spontaneous emission (ASE) based on DCNP molecules dispersed in PMMA matrices.24,25 DCNP molecule26 was our inspiration to conduct the research for other pyrazoline derivatives, and this compound served as the reference. At first we designed two chemical compounds roughly similar to the DCNP molecule, but differing in electron-acceptor groups. Their chemical structures are shown in Figure 1a−c. Mentioned derivatives of pyrazoline have been synthesized by the method of Knoevenagel and Fischer.27 Received: November 9, 2013 Revised: March 10, 2014 Published: March 25, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 8102 dx.doi.org/10.1021/jp411031b | J. Phys. Chem. C 2014, 118, 8102−8110 pubs.acs.org/JPCC ■ EXPERIMENTAL SECTION UV−Vis Characterization. The basic optical character- ization of the PRDs was performed by UV−vis spectroscopic measurements in order to obtain absorption and emission spectra of the compounds in solution. All spectra were measured for 0.004% w/w concentration of dye in THF solvent using a Hitachi F-4500 spectrofluorometer. Emission and absorption spectra after normalization are presented in Figure 2. One can observe that absorption spectra for different compounds are quite similar as positions of absorption band maxima vary only by 17 nm, i.e., from λabs = 456 nm for PY- pNO2 to λabs = 473 nm for PY-oCNNO2. However, the positions of maxima of emission vary by 81 nm, i.e., from 542 nm for DCNP to 623 nm for PY-pNO2. Large Stokes shifts (from 79 to 168 nm) possibly result from the strong electron delocalization in the excited state, which is frequently observed in push−pull type molecules. It is possible that for a large Stokes shift, PY-pNO2 is responsible for conformational change of the molecule in its excited state with respect to the ground state. Large Stokes shift dyes are ideal for multicolor labeling applications, as they allow using the same excitation wave- length/source for two separate dyes with spectrally different emissions. Random Lasing and Light Amplification. Stimulated emission can occur in molecular systems for which excited state population inversion induced by absorbed pump light pulse can be achieved. This phenomenon is responsible for light amplification or stimulated emission. A random lasing occurs when an active material that spontaneously emits light and amplifies it via a stimulated emission process is also able to partially trap light within random cavities arising by multiple photon scattering processes.28−33 In some media and upon specific excitation conditions like various periods of laser pulse durations (shorter or longer), gain profile, excitation area, reabsorption process, light scattering, and leakage or material degradation, it is possible to distinguish between coherent and incoherent RL processes. Coherent random lasing is present when the interference of the scattered and amplified light is strongly allowed. We suppose that in our systems the second type of the two above-mentioned random lasing processes, namely, incoherent RL, is observed. To perform the laser-induced emission spectroscopy measurements, we have used a nanosecond pulsed laser (Nd:YAG) with doubled in frequency output, vertically polarized light (Surelite II, λ = 532 nm, pulse duration 6 ns, 10 Hz repetition rate) as a source of excitation. Generated radiation passed through the diaphragm and lens system, where the beam was spatially filtered and finally converted to a thin horizontal stripe by a cylindrical lens. The use of a movable beam blockade before the sample allowed controlling the size of the excited area, which is necessary for the net gain coefficient estimation in SE process. Emission generated from the sample layer was collected by an optical fiber from the edge of the sample and analyzed with either low resolution Ocean Optics 2000 (for SE) or high resolution Shamrock 163 (for RL emission) spectrometers. To observe random lasing emission, with its characteristic several narrow lines situated over SE spectrum and changing their positions and intensities at random, it is necessary to use a dynamic and high spectral resolution equipment. To measure the energy of the pumping beam, we used the calibrated measuring setup Coherent Field Max II coupled with a J-10MB-HE sensor. Experiments were conducted for dye-doped polymeric layers in which PRD acted as a luminescent dye and PMMA served as a matrix. At first, proper dye solutions were prepared. We added derivatives of pyrazoline to THF forming solution of 0.2% concentration by weight. Then, we mixed it and added it to another solution in 1:1 (V/V) proportion, which contained commercially available PMMA (Sigma Aldrich, Mw = 996 000) dissolved in THF of 5% by weight. Each of the final solutions has 2% weight proportion in dry mass (PRD to polymer matrix). After 1 day we applied PRD-PMMA/THF solutions onto silica glass plates using the drop-casting technique. Next, the samples were slowly dried under volatile THF by 24 h. The Figure 1. Molecular structures of (a) (Z)-2-(4-nitrophenyl)-3-(1- phenyl-4,5-dihydro-1H-pyrazol-3-yl)acrylonitrile (abbreviated name: PY-oCNNO2), (b) (E)-3-(4-nitrostyryl)-1-phenyl-4,5-dihydro-1H- pyrazole (abbreviated name: PY-pNO2), and (c) 3-(1,1-dicyanoethen- yl)-1-phenyl-4,5-dihydro-1H-pyrazole molecule (DCNP). Red color indicates the part of the chemical structure common to all of the investigated compounds; nitrile groups are shown in green, and p- nitrophenyl groups are in blue. Figure 2. Comparison between normalized absorption and emission spectra of PRDs in THF solution. Excitation wavelength λex = 480 nm. The molar absorption coefficient determined experimentally for these compounds is 49 200 dm3·mol−1·cm−1 for DCNP, 51 200 dm3·mol−1· cm−1for PY-oCNNO2 and 53 200 dm 3·mol−1·cm−1for PY-pNO2. The Journal of Physical Chemistry C Article dx.doi.org/10.1021/jp411031b | J. Phys. Chem. C 2014, 118, 8102−81108103 average thickness of the polymeric layers obtained with this method amounted to 3 μm. Optical Microscopy. The presence of microcrystals embedded in polymeric matrices was noticed using inverted microscope IX71 (Olympus) equipped with 10× microscope objective. Polymer layers were investigated both in transmitted and reflected light modes in order to facilitate the observation of superficial and bulk defects. Images of resolution 1600 × 1200 pixels were acquired with a Retiga-2000R (QImaging) camera. The occurrence of various kinds of disorders in matrices was monitored due to their relation to possible mechanisms of observed random lasing phenomenon. Acquired images were analyzed using ImageJ software for statistical description and size determination of observed objects. These microscopic measurements allowed us to describe the morphology of the luminescent dye in the matrix. It was possible to distinguish several similar areas on the sample. Data obtained from microscopic images (diameter of microcrystals), after a few simple calculations including refractive index of luminescent dye, confirmed the calculations of cavity length, which is shown later (Power Fourier Transform section). Images of the obtained thin films were more transparent in the central positions of the layers than at the edges, where group of microcrystals and, in one case, multiple cracks of the casted layer were noticed. In Figure 3 we present few exemplary microscope photographs for measured samples containing microcrystals (a−c), sometimes arranged into larger agglomer- ations (a, c). The sample area close to the glass edge showed also surface irregularities and cracks (d). What is important to note is that, in our measurements, the generated in stimulated emission or RL processes light was collected from the area located very close to the edge of the sample. Figure 3a−d shows only images from these areas. Different colors in the background in presented pictures result from used different intensities of light in microscopic observations. All of the samples were measured in transmitted and reflected light modes simultaneously. ■ RESULTS Stimulated Emission. The laser-induced fluorescence experiments were conducted for PRDs in THF solution with concentration equal to 0.004% placed in a quartz cuvette of 1 cm thickness. In this case, during experiments even for the maximum laser pulse energies of Nd:YAG laser, only fluorescence has been observed without any stimulated emission features. Then further studies were performed and are reported here only for dye-loaded polymeric layers. For all of the tested polymeric samples, we observed occurrence of stimulated emission above certain threshold energy density of pumping light. Threshold energies were estimated for each sample from SE intensity dependence on pumping light fluencies. In the case of DCNP-doped PMMA polymeric layer SE showed separate bands centered at two wavelengths of 586 and 626 nm. For the PY-oCNNO2-doped PMMA, the single SE band was observed at a wavelength of 673 nm, whereas for PY-pNO2-doped PMMA, the SE occurred at 683 nm. The peak intensities of SE of investigated systems versus energy density of pumping laser beam are plotted in Figure 4a,c,e. In Figure 4b,d,f, the SE threshold level has been determined for each sample. In any case, the SE was obtained in the most inhomogeneous places of obtained polymeric layers (i.e., close to its edges) using a circular area pump laser beam (diameter = 4 mm) incident normally to the sample. The observation of two peaks in the SE spectrum for DCNP was already reported.8 Their origin is attributed to the two distinct fluorescence species (traps) revealed in bulky DCNP crystal at Figure 3. Optical microscope photographs from (a) DCNP, (b) PY-oCNNO2 and (c), (d) PY-pNO2compounds. The large (a,c) and small (b) groups of little crystals suspended into volume of polymeric layer are clearly seen. Layer cracks (d) are caused by slow evaporation process of the solvent facilitating nucleation and growth of microcrystals. The Journal of Physical Chemistry C Article dx.doi.org/10.1021/jp411031b | J. Phys. Chem. C 2014, 118, 8102−81108104 low temperatures.30 The PY-oCNNO2SE shows also a tendency to emit light from two species like DCNP, but for some reason the longer wavelength band rises faster than the shorter one. We suppose that trapping of light in the microcrystals is responsible for the occurrence of band positioned at longer wavelengths. Furthermore, the maximum in the fluorescence band from the PY-oCNNO2 in THF solution, which is 585 nm, confirmed our supposition. PY- pNO2 shows only a single SE peak. Random Lasing. Using a cylindrical lens, we changed the beam’s shape to the narrow stripe (L = 22 mm). We manage to observe characteristic emission spectra for random lasing with multiple fine spikes on top of the remaining stimulated emission whose frequencies changed statistically. Examples of normalized RL spectra superposed over normalized SE spectra for different samples are presented in Figure 5. Characteristic for the studied systems is that in each case, the RL spectrum is located at the long wavelength side of the SE spectrum, possibly due to the lower losses experienced by light of longer wavelengths. For illustration of RL phenomenon, we present characteristic “sharp” spectra of RL for PY-oCNNO2 in PMMA as a function of pumping energy density (cf. Figure 6). The energy thresholds (ρth) required to obtain random lasing phenomenon are presented in Figure 7; they were estimated for each compound from integrated emission intensity dependence on pumping beam fluence. From every single measurement, the maxima of RL band were identified and then the amount of intensity integrated over whole spectrum was assigned to the applied pumping energy density. Then we approximated Figure 4. Characteristic for stimulated emission band narrowing and intensity rise for investigated compounds as a function of pumping energy density (a) DCNP, (c) PY-oCNNO2, and (e) PY-pNO2 in PMMA. Panels b, d, and f show estimations of the SE energy density threshold from plots of fwhm as a function of pumping energy density for DCNP (here for the band with the maximum at 628 nm), PY-oCNNO2, and PY-pNO2 compounds, respectively. Figure 5. Stimulated emission and random lasing spectra obtained for investigated PRDs. Red color represents the PMMA doped with DCNP, black color represents PMMA doped with PY-oCNNO2, and the blue color represents PY-pNO2-doped PMMA film. The Journal of Physical Chemistry C Article dx.doi.org/10.1021/jp411031b | J. Phys. Chem. C 2014, 118, 8102−81108105 experimental points by two linear functions, the intersection of which directly indicates the respective RL energy thresholds. Power Fourier Transform. Power Fourier transform (PFT) is a powerful tool for emission spectra analysis, especially for indicating the free space frequency of longitudinal cavity modes. When the Fourier transform is applied to emission spectra from a well-defined resonator expressed in wave vector k (2π/wavelength), the PFT spectra manifests itself as a set of spikes, indicating the certain path length for a given mode that integer multiplication restore the cavity length.34 For a Fabry−Perot cavity type, cavity length is related with particular Fourier harmonics by the following equation:35 π =L pm nmC (1) where LC is the cavity length, pm is the Fourier parameter, n is the refractive index of a gain medium (nPMMA = 1.48), and m is the order of Fourier harmonic component. For the ring cavity, the geometrical diameter can be found from the expression36 =D pm nm 2 (2) where D is the cavity diameter, pm is the Fourier parameter, n is the refractive index of a gain medium, and m is the order of Fourier harmonic component. In order to perform PFT, all of obtained RL spectra were changed into relation intensity versus k-vector value (2π/λ) with λ in micrometers. The results of this conversion (shown as insets in each case) are presented in Figure 8a−c. Then PFT has been made for each of measured spectra obtained for all of the samples, for the energy density of pumping laser above the threshold, and then averaged over 100 of PFT spectra. An example of averaged PFT spectra for a PMMA layer containing PY-oCNNO2 is presented in Figure 9. One can observe the characteristic for cavity modes, with Fourier harmonics showing rather complicated distribution. This characteristic position of Fourier harmonics determine particular random cavity longitudinal mode distribution and is unique for each random resonator. By indicating the following Fourier harmonics in PFT spectra, we were able to calculate the smallest cavity size (smallest repeatable gain unit) and its distribution using eqs 1 or 2 for different types of cavities. For the Fabry−Perot type of resonators, such distribution is presented in Figure 8a−c. One can see that calculated cavity sizes are ranging from dozen micrometers to few-tens of micrometers, pointing that light localization take place on such a length scale. We obtained the following values of cavity lengths: in DCNP compound Lc Figure 6. Characteristic “sharp” lines of random lasing for one of the investigated compounds (PY-oCNNO2) as a function of pumping energy density from 0.25 to 0.40 mJ/cm2. Figure 7. The RL normalized integrated intensity dependence on pumping light energy density for PY-pNO2 (blue curve), PY-oCNNO2 (black curve), and DCNP (red curve), in PMMA. Figure 8. Cavity length distributions and RL spectra versus 2π/λ (insets) for the investigated systems, respectively: DCNP (a), PY- oCNNO2 (b), and PY-pNO2 (c) in PMMA. The Journal of Physical Chemistry C Article dx.doi.org/10.1021/jp411031b | J. Phys. Chem. C 2014, 118, 8102−81108106 lies in interval between 15 to 21 μm, in PY-oCNNO2 from 23 to 26 μm and in PY-pNO2 from 17 to 22 μm. For the ring cavity-type resonator mentioned, distribution refers to the following ring diameters distribution D: for DCNP between 9.5 to 13.4 μm, for PY-oCNNO2 from 14.7 to 16.6 μm and for PY- pNO2 from 10.8 to 14.0 μm. These results are comparable to values obtained in similar materials, where random lasing phenomenon was observed.34−39 Other groups estimated cavity length from 23 to 37 μm in different lasing systems containing polymers34−38 or biopolymers39 doped with luminescent dyes. On the basis of microscopic images of the investigated polymer films (cf. Figure 3), one can estimate the average distance between microcrystals as 6.5 μm for DCNP, 24.9 μm for PY- oCNNO2, and 7.9 μm for PY-pNO2, respectively. Moreover, it was possible to estimate microcrystals diameters amounting to 2.9 μm for DCNP, 3.0 μm for PY-oCNNO2 and 3.0 μm for PY- pNO2, respectively. On the microscopic images it is clearly visible that microcrystals, which have diameter of roughly 3.0 μm, are dominant; however, distribution of size is quite broad. In two cases (DCNP and PY-pNO2) in microscopic images there are also visible larger microcrystals (with diameter roughly 20 μm), but their number is much lower than for smaller ones. The rich microscopic phenomena leading to random lasing have not been well understood so far. The detailed origin of the random lasing phenomenon in the case of pyrazoline dyes in PMMA layers is unknown; however, three mechanisms of feedback for the photons should be considered and discussed here. The first one can be related to the dye microcrystals formation in the polymer bulk during the slow solvent evaporation process. The solubility of PRDs in the matrix and solvent mixture decreases while the solvent evaporates. When the solubility limit is reached, the microcrystals are emerging from oversaturated solution, forming nano- and microcrystal suspension in solid state polymeric matrix. Around each microcrystal or nanocrystal there is a polymer matrix depleted from the dye. These forms are a some kind of planar ring or disc cavity centered around the crystal with diameter larger than the crystal itself (cf. Figure 10). The light closed loop can be formed within these more or less circular cavities. However, lasing will occur only for cavity length Lγ large enough to provide a net gain.34 On the other hand large cavities of this type are much less numerous, so in any system with random cavities only selected part of them will result in lasing. These particular cavities are probably observed in our random lasing experiments. The presence of microcrystals in inves- tigated samples was clearly proven under an optical microscope. Because the SE has not been observed for liquid solutions of PRDs, we conclude that the gain mechanism may by related to the formation of aggregates and microcrystals. The second hypothesis is that gain can be achieved between scattering centers (microcrystals), which is quite common for most known molecular systems for which aggregation leads to the quenching effect, and thus microcrystals become efficient Mie scatterers, but not gain centers. In such case, light is amplified when it is propagating between scattering centers, and the feedback for coherent RL emission is based on formation of closed loops by light during multiple scattering events. The third hypothesis of the origin of light scattering takes into account the surface irregularities (corrugation) present on top layer of the investigated samples. The scattering of light from surface irregularities can be very efficient. It is interesting that random lasing was also observed in samples regions containing cracks. Such cracks form a set of island-type forms in micrometer scale and if the shape and dimension of such objects are proper the light can be confined inside them, as in a ring cavity, which considerably elongates the photon path in a gain medium and the conditions for laser emission are met. All of the hypotheses are equally probable, thus further studies should be carried out to distinguish which type of scattering is dominating in the studied systems. However, the main goal of present paper was to introduce new pyrazoline derivatives with interesting optical features for multiple optical applications. Moreover, another purpose of this work is to find explanation how different substituent to pyrazoline ring influence absorption and luminescence spectra of the dyes and dye−polymer system. At this level of study, we are unable to distinguish whether localization takes place inside the microcrystals or around them; however, calculated size of cavities directly shows that the feedback mechanism can be built upon light scattering from microcrystals. Moreover, even if light localization can take place inside the PRD microcrystals (the mean diameter of which in our case is roughly 3 μm), the majority of such microcavities due to the their size are unable to produce multimode laser operation, and these types of modes are invisible for PFT as well-defined Fourier harmonics. Figure 9. Averaged Fourier analysis of 100 spectra for the chosen example (PY-oCNNO2). Figure 10. Picture of microcrystals formed in a polymeric matrix and surrounded by ellipsoids and circles showing possible ring cavity formation. The cavities are formed due to reduction of the refractive index of the polymer around the microcrystal caused by depletion of a dye and higher refractive index of a single microcrystal. Large cavities (D ∼ 20 μm) are rare. The Journal of Physical Chemistry C Article dx.doi.org/10.1021/jp411031b | J. Phys. Chem. C 2014, 118, 8102−81108107 Temporal Stability. The last step of our studies was devoted to checking temporal stabilities of investigated dye− polymeric layers on high fluency laser light treatment. We performed experiments in order to achieve stimulated emission and random lasing phenomenon, and then we collected the spectra as a function of time or number of laser pulses at repetition frequency equal to 10 Hz. Then we recalculated time for number of exciting laser pulses and normalized intensity of SE/integrated intensity of RL spectra for investigated systems. We present the obtained correlations in Figures 11 and 12. Finally, we could estimate the photodegradation half-life of the dye−polymer system as a function of the number of laser shots. Results of these measurements, which are presented in Figures 11 and 12, indicate that photodegradation rates are quite typical for laser dye-doped polymeric matrices under nanosecond pulse laser excitation.40−43 All of the optical parameters that were investigated in this work are gathered in Table 1. It should be noticed that during temporal stability measure- ments for the SE process, energy used was just above the threshold. However in RL measurements, pumping energy was (relatively) much higher than that needed to observe this kind of emission, which resulted in faster signal decay in comparison to SE process (cf. Figures 11 and 12). Although the absolute value of the energy used in the second case was less about 1 order of magnitude, it was caused by two different kinds of emission and excitation mechanisms. Furthermore, during measurements in the RL case, high pumping energy can cause, e.g., mechanical destructions of the layer, or micro- crystals size changes due to the laser ablation process. This may result in amplification losses and change the RL emission character from coherent to incoherent (inset in Figure 12), and finally cause the emission decay. Excitation of laser light causes dye photodegradation, which is facilitated by the presence of oxygen in a matrix and local temperature increase due to radiationless photon energy dissipation. The differences in kinetics of photodegradation process depend on microscopic structure of the dye itself but also on its state in the matrix, i.e., molecular, aggregate, or microcrystalline. ■ CONCLUSIONS We presented two new dyes of pyrazoline derivatives and characterized their luminescent properties in the context of stimulated emission and random lasing processes. We showed that modification of chemical structure just by placing additional strong acceptor group into the ethenyl chain can significantly change the emission spectra of such modified dyes. Here it was shown that using molecules containing similar structures, it is possible to excite them with the same wavelength and finally receive emission in different spectral range. By showing that new dyes placed in a solid matrix of PMMA can exhibit both stimulated emission as well as random lasing, we proved that this class of materials is worth further studies, as these dyes show great potential in nanophotonics. An open question remains which mechanism among the two considered in this paper is responsible for random lasing in the studied systems. Therefore, further studies in this field are still required. ■ AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Figure 11. The SE normalized intensity dependency on the number of exciting laser pulses. Solid lines are biexponential decay functions fit to the experimental data for DCNP (red points), PY-oCNNO2 (black points), and PY-pNO2 (blue points), respectively. Figure 12. The RL normalized integrated intensity dependency on the number of exciting laser pulses. Solid lines are biexponential decay functions fit to experimental data for DCNP (red points), PY- oCNNO2 (black points), and PY-pNO2 (blue points), respectively. Table 1. Summary of Random Lasing Related Optical Properties of PY-oCNNO2, PY-pNO2, and DCNP Compounds compound PY-oCNNO2 PY-pNO2 DCNP λmax ABS [nm] 473 456 463 λmax LUM [nm] 588 623 542 Stokes Shift [nm] 115 167 79 λmaxSE [nm] 675 679 586 and 630 SE threshold [mJ/cm2] 6 5 16 N0.5SE [pulse number] a,b ∼6.00 × 102 ∼1.90 × 103 ∼5.50 × 103 λmax RL [nm] 694 679 639 RL threshold [mJ/cm2] 0.27 0.069 1.48 N0.5RL [pulse number] a,c ∼500 ∼1000 ∼2000 aNumber of laser shots that ASE/RL signal decreases by half. bExcitation: 19 mJ/cm2. cExcitation: 4 mJ/cm2. The Journal of Physical Chemistry C Article dx.doi.org/10.1021/jp411031b | J. Phys. Chem. C 2014, 118, 8102−81108108 mailto:
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