Photoinduced Birefringence in PMMA Polymer Doped with

Apr 17, 2015 - In this work, we prepared poly(methyl methacrylate) thin films doped with PY-oCNNO2 and measured the characteristic for optical switche...
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Photoinduced Birefringence in PMMA Polymer Doped with Photoisomerizable Pyrazoline Derivative Adam Szukalski,† Karolina Haupa,‡ Andrzej Miniewicz,† and Jaroslaw Mysliwiec*,† †

Faculty of Chemistry, Advanced Materials Engineering and Modelling Group, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ‡ Chemistry Department, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland S Supporting Information *

ABSTRACT: Upon S0−S1 excitation, the pyrazoline derivative molecule (Z)-2(4-nitrophenyl)-3-(1-phenyl-4,5-dihydro-1H-pyrazol-3-yl)acrylonitrile, abbreviated as PY-oCNNO2, can be transformed from its ground state trans (E) form to bended cis (Z) form. Similar to the case of the well-known family of the photochromic azobenzenes, such a molecular property can be employed to fabrication of photochromic polymers by suitable doping of the chromophores into polymer matrix. In this work, we prepared poly(methyl methacrylate) thin films doped with PY-oCNNO2 and measured the characteristic for optical switchers dynamic and static photoinduced birefringence (PIB) phenomenon. Possible conformational states of PY-oCNNO2, energy barriers, and associated dipole moments were calculated using TD-DFT quantum chemical methods. The presented experiments show that pyrazoline derivatives constitute a prospective group of materials with a great potential for photonic applications.

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INTRODUCTION

In this work we present a synthesized by us pyrazoline chromophore molecule which serves as a photoactive and at the same time luminescent dye characterized by a push−pull electron donor−acceptor structure. Moreover, this chromophore changes its chemical structure and optical properties by light-induced conformational isomerization. Optical consequence of this conformational change in chromophore embedded in polymer can be observed as induction of macroscopic optical anisotropy (birefringence) that can be measured in the optical Kerr effect experimental setup. According to the best of our knowledge, it is shown for the first time that fully reversible photoinduced optical birefringence can be observed in the pyrazoline-based luminescent system.

Nowadays, there is an observed increasing interest of advanced technologies enabling processing of organic materials that can be used in various photonic applications. One of the topics of our interest has been devoted to exploring spectroscopic, optical, and nonlinear optical properties of pyrazoline derivatives, studied also by many scientific groups.1−4 Optical quality polymer films when doped with organic chromophores or luminescent dyes may exhibit, upon suitable excitation, a dynamic change of their refractive index (e.g., photoinduced birefringence, PIB), generation of light-induced diffraction gratings understood as refractive index, or absorption coefficient periodic changes in response to light interference field but also various light amplification phenomena, like amplified stimulated emission, random lasing, or distributed feedback lasing.5−10 Polymeric materials which have already found broad applications in photonics as optical storage devices, all optical switches, or modulators belong to the group of photochromic polymers. Among them the most interesting are azobenzene derivatives-based polymeric systems.11−13 Photoorientation and microscopic mass transport resulting from multiple light-induced trans−cis−trans photoisomerizations of azobenzenes in polymers doped or functionalized with them have been the subject of continuous investigations in the last 30 years.14−16 However, for using materials in advanced optical devices, there are many limitations and requirements especially regarding their properties but also costs, facility of fabrication, ease of processing, photostability, and biocompatibility. © 2015 American Chemical Society

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MATERIALS AND METHODS The pyrazoline derivative ((Z)-2-(4-nitrophenyl)-3-(1-phenyl4,5-dihydro-1H-pyrazol-3-yl)acrylonitrile, abbreviated as PYoCNNO2) has been synthesized by us before. The synthesis method is described elsewhere,17,18 and the PY-oCNNO2 molecular structure is shown in Figure 1. The PY-oCNNO2 molecule possesses an aromatic ring serving as the electrondonating group, a pyrazole ring, and two different acceptor groups substituted on its other side. One of them is a nitrile group connected in the middle part of the structure, and the second one is a nitro group located in para position of the Received: February 27, 2015 Revised: April 13, 2015 Published: April 17, 2015 10007

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mixture of PY-oCNNO2:PMMA/THF was stirred, and after another 24 h it was deposited on the silica glass plate by drop casting technique. Then thin polymeric layer doped with the luminescent dye was left in solvent saturated atmosphere at room temperature for 48 h. The film thickness was measured by using a profilometer (Veeco Dektak 6 M Stylus Profiler), and typically films of thickness around 12 μm were obtained. Atomic force microscopy measurements confirmed that generally films obtained by this way were quite flat with small surface roughness of around 10 nm. Optical Kerr Effect. In order to examine the photoinduced anisotropy in the fabricated films, we employed the experimental setup used for measurements of optical Kerr effect (OKE). Optical Kerr effect belongs to the third order nonlinear optical phenomena and was observed usually in liquids, for example, nitrobenzene.19 In this effect, one can observe and measure a photoinduced optical birefringence Δn = n∥ − n⊥ due to the molecular reorientation effect toward polarization plane of an incident laser light. Based on that, the third order nonlinear averaged optical susceptibility χ(3) of a liquid can be calculated. Optical anisotropy created in the system by linearly polarized laser light (1) defines the so-called Kerr medium in which refractive index n is dependent on pumping light intensity I:

Figure 1. Chemical structure of (Z)-2-(4-nitrophenyl)-3-(1-phenyl4,5-dihydro-1H-pyrazol-3-yl)acrylonitrile (PY-oCNNO2). Dipole moment μg = 8.5 D in the ground state for PY-oCNNO2 in vacuum has been calculated using TD-DFT method.

aromatic ring in electron-acceptor region (cf. Figure 1). However, in this case, the π-electron bridge is preserved. Upon suitable molecular excitation, the dipole moment in the excited state increases due to intramolecular electron transfer. This molecule via the excited state can change its conformational structure from trans (E) form to cis (Z) form (cf. Figure 2). In Figure 2, the proper transformation between trans and cis isomers for PY-oCNNO2 are shown. The possibility of such an isomerization process including the transitions states was pointed out by quantum chemical calculations, which will be discussed later in this work. Commercially available poly(methyl methacrylate) in powder form (Mw = 966 000 Da, Sigma-Aldrich ) was added to tetrahydrofuran (THF) in 5% dry weight proportion. Solution was stirred and left until complete dissolution of polymer. After a few days, it was mixed with another THF solution of PY-oCNNO2 to obtain 3% solution (dry weight proportion of PY-oCNNO2 to PMMA). The final

n = n0 + n 2I

(1)

where n is the refractive index of the medium in the presence of pumping light, n0 is the low intensity refractive index, n2 is the nonlinear refractive index coefficient (proportional to the third order susceptibility χ(3)), and I is the pump light intensity. The quantity that characterizes the Kerr effect of the medium is known as the Kerr constant B in [mV−2] defined by B=

Δn λ⟨E2⟩

(2)

Figure 2. Scheme of the photoisomerization process of PY-oCNNO2 molecule. Conformational change (from trans to cis) is possible upon excitation of the molecule to the first excited state. Reverse reaction to the trans form is possible also by the light absorption with different wavelength or by simple thermal relaxation. All of the structures were calculated using TD-DFT method (DFT/B3LYP/6-311++G(d,p)). The most stable form is trans 1 (E). Atom colors: C, gray; H, green; N, blue; O, red. 10008

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Figure 3. (a) Experimental setup for measurements of the photoinduced birefringence. (b) Normalized absorption spectrum for PY-oCNNO2/ PMMA system where the pumping and reference beam are marked.

sample, an optical anisotropy is induced, which is observed by the appearance of a red light (reference beam) at the Si detector area (Thorlabs PDA55). Signals from the detector are read via digital oscilloscope (Tektronix). The signal amplitude and its shape correspond to the magnitude and kinetics of the photoinduced birefringence. The role of the half-wave plate in the pumping beam path is twisting a vertical linear light polarization of pumping beam by an angle 45° with respect to the He−Ne horizontal beam polarization. Experimental setup in described configuration can also be used to determine the socalled total (static) changes of refractive index. Optically induced anisotropy is probed by He−Ne laser beam, and this is schematically explained in Figure 4 where the optical indicatrix cross section of the polymer−chromophore system is shown prior to and under illumination by a pump beam.

where the brackets represent a time average of pump light electric field E and λ is the light wavelength. For nitrobenzene at room temperature, B = 5.75 × 10−14 mV−2,19 and for CS2 the nonlinear refractive index coefficient n2 = 3 × 10−14 cm2/ W, which means that a laser beam of average intensity I = 1 MW/cm2 can produce a refractive index change of only 3 × 10−8. In the case of solid state such as a polymer with nonlinear chromophores dispersed in its bulk, the macroscopic phenomenon of light-induced refractive index anisotropy can be similar to that observed in liquids, but molecular mechanisms involved are quite different. The molecular reorientation in azo-polymers occurs mostly due to the trans−cis photoisomerization of embedded in polymer chromophores. Multiple acts of trans−cis−trans photoisomerizations due to angular selective absorption lead to slow, but continuous reorientation of trans molecules with their axes directed perpendicular to the polarization plane of the pumping laser light. High viscosity of the polymer helps to stabilize the molecular reorientation after removal of pumping beam. The exact mechanism of this process was described in several previous works mainly for azobenzenes.12,20−25 The process has a resonance character as a part of the incoming light is absorbed by chromophores. Therefore, the refractive index coefficient n2 is related to not only chromophore molecular second order hyperpolarizability, but also to the whole polymeric system doped with chromophores. Its large value observed in polymeric optical materials with photoisomers is enlarged with respect to the molecular reorientation process in liquids, because it is enhanced by the cumulative process of trans isomer reorientation and accompanying suitable reorientation of polymer chains. Therefore, one may expect different time scales of the photoinduced reorientations; the fast one is linked with molecular processes, whereas the slow one is linked with polymer chain reorientation and macroscopic stabilization of molecular reorientation by polymer. The experimental setup for measurements of photoinduced birefringence is schematically shown in Figure 3a. He−Ne laser (λ = 632.8 nm, cw) was used as a probe (reference) light beam, not absorbed by a material (Figure 3b.). A diode pumped solid state (DPSS) Nd:YAG laser (λ = 532 nm, cw) was used as a source of pumping beam. For observing dynamic changes of refractive index, it was necessary to use a mechanical chopper for periodic light modulation in the frequency range 10−400 Hz. The two beams intersect in a single spot at the sample (Figure 3), which is located between the crossed polarizer and analyzer. When the green light of pumping beam irradiates the

Figure 4. Cross section of the refractive index ellipsoid in exemplary azobenzene containing photochromic polymer under the influence of absorbed laser light irradiation (see the description in the text).

Due to the light-induced birefringence, the probing He−Ne light incident on the sample is split into two orthogonal polarization waves that propagate with different light velocities v given by the values of respective refractive indices. Lightinduced optical birefringence at λ = 632.8 nm Δn(I,t) is a function of pumping light intensity I532 and time t counted from the beginning of the beam opening (eq 3): Δn(I , t ) = n⊥(I , t ) − n (I , t ) =

λΔφ(I , t ) 2πd

(3)

where d is the sample thickness and Δφ is the phase change between orthogonally polarized beams. However, the phase change can be evaluated due to waves superposition which 10009

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of 524 mW/cm2, the system is far from saturation, showing that for still higher illumination level birefringence can be larger. It is worth noting that the saturation value of static birefringence is built up in the time range of tens of seconds and after roughly 100 s it saturates. Optically induced birefringence in the PMMA thin film containing PYoCNNO2 is several times larger than those obtained in other PMMA systems containing different pyrazoline derivatives under the same experimental conditions. This suggests perhaps different mechanism of induction of optical anisotropy occurring in the investigated here case, when compared with other similar compounds.27 A plot of induced birefringence versus pumping light intensity shows almost linear dependence, suggesting that at still higher light intensities birefringence could be larger than Δn = 0.00042 obtained at I532 = 524 mW/ cm2. However, for high laser light intensity, there is a visible mechanism that diminishes the birefringence value with time what could be related with temperature effect on the PMMA polymer. In Figure 6 are shown decays of photoinduced optical birefringence during relaxation process in darkness (i.e., on

results in respective modulation of light intensity Itrans registered by the detector placed after analyzer: ⎛ πd ⎞ Itrans(t ) = I0 sin 2(2α) sin 2⎜ Δn(I , t )⎟ ⎝ λ ⎠

(4)

where I0 is the probe light intensity and α is the angle between linear polarizations of pump and probing beams. Setting α = 45°, one gets maximum transmittance value of Itrans for small values of light-induced phase changes. After suitable calibration of the detection system one can extract from eq 4 the value of Δn(I,t) at given time after opening the pump light and its intensity. Measurements of birefringence for different values of pumping light intensity I532 allowed for estimation of nonlinear refractive index coefficient n2 as defined by eq 1. Averaged value of the third order susceptibility χ(3) expressed in the SI system of units can be calculated (eq 5):26

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⎡ m2 ⎤ 4n n 2ε c χ (3) ⎢ 2 ⎥ = 2 0 0 [SI] 3 ⎣V ⎦

(5)

EXPERIMENTAL SECTION In this section we present experimental results divided into two parts. The first part concerns description of results obtained from Optical Kerr Effect measurements. The purpose of this section was to find answer whether molecules of PY-oCNNO2 are able to undergo conformational changes due to photoisomerization and how large optical birefringence can be induced in the host polymeric system. The second part refers to quantum chemical calculations of possible molecular structure changes under laser light excitation. Calculations of energy barriers between isomers and their energies should shed some light on possible events under light irradiation. Photoinduced Birefringence. Static Birefringence. In Figure 5, we show the induction of the static birefringence due

Figure 6. Changes of optical birefringence during dark/thermal relaxation process for PY-oCNNO2:PMMA after exposure to different intensities of pumping laser beam.

blocking the pump beam) after previous exposure of the samples to different light intensities of pumping laser. Characteristic for these decays is that all of them did not reach zero level after relaxation in darkness at room temperature, i.e. some quasi-permanent birefringence is left. Similar phenomenon was also observed for azopolymers for which long time (several months) is needed to restore the initial, that is, random orientation of molecules. Some of the molecules after the reorientation remain “frozen” in their positions,28 which depends on the magnitude of Tg temperature of the polymer. On the other hand, it is still possible to define time when the induced birefringence will drop to zero. As it is clearly seen, the time of the birefringence decay is much longer than the induction time for a given pumping light intensity. Birefringence decays were fit by a monoexponential function of the form:29,30

Figure 5. Static photoinduced changes of optical birefringence for PYoCNNO2 observed under different light intensities of pumping laser beam of 532 nm wavelength. Inset: Photoinduced birefringence measured for different pumping light intensities (DPSS, Nd:YAG, cw, λex = 532 nm) for PY-oCNNO2:PMMA.

Δn(t ) = Δn0 exp( −t /τd) + Δn∞

to constant (noninterrupted) laser light irradiation. It is clearly visible that the photoinduced optical birefringence in PYoCNNO2:PMMA is strongly dependent on the incident laser power and, as expected, increases with an increase in light intensity. In the inset to Figure 5, we show this dependence plotting saturation value of photoinduced birefringence in function of incident pump laser intensity. Even at the intensity

(6)

where Δn0 is the birefringence at time t = 0, Δn∞ is the remnant birefringence, and τd is a time constant of the decay process. In fact, the decays should be fit with biexponential function as a significant fall down of the birefringence occurs within first 5 s after switching off the beam. However, here we neglected the fast decays and focused attention to the slower 10010

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The Journal of Physical Chemistry C decay component and analyze it. The dependence of latter decay time constants on used pump light intensity in the range of 100−600 mW/cm2 seems to be correlated as it is shown in Figure 7. The increase in time constant can be approximated with function τd = 5.6 × 10−2 × I532 + 59 (when I532 is substituted in mW/cm2, the resulting time constant τd is in seconds).

Figure 8. Dynamics of the photoinduced reversible changes of birefringence in OKE experiment presented as the relative signal intensity of transmitted probe beam as a function of time for five different modulation frequencies for the PY-oCNNO2 doped PMMA thin film.

population of molecules; therefore, a higher value of the signal amplitude is observed. In Figure 9, we show all-optical switching in the studied sample for 75 Hz modulation frequency in a wider time span. The dynamic refractive index changes are stable with time.

Figure 7. Dependence of birefringence decay time constant τd on pumping beam intensity for PY-oCNNO2:PMMA.

Dynamic Birefringence. We used the same experimental setup for studies of the dynamic part of photoinduced birefringence. This can be measured using mechanical chopper inserted in the pump beam and allowing for laser light modulation in the form of rectangular pulses in the frequency range of 10−400 Hz. We observed the dynamic and reversible changes of photoinduced birefringence that can be treated in terms of OKE phenomenon. Dynamic process which is discussed here may be composed of two contributions. The first one could be related with selective angular removal, by linearly polarized and absorbed light, of trans molecules assuming that their transition dipole moment is parallel to the long molecular axis. The second contribution may arise from process of multiple photoisomerisations. Absorbed light can induce switching between trans and cis states, respectively. If the same light is able to perform reverse photoisomerization of cis−trans type, we may face the same photoorientation mechanism as described for azobenzenes.18−23 In the absence of light, we observe the birefringence decrease due to rotational diffusion of the trans molecules and their thermal relaxation. He−Ne light modulation signal seen by the detector (cf. Figure 3), due to rectangular pulse excitation of PY-oCNNO2:PMMA sample, has the rise time constant τr in the range of single milliseconds. So the dynamic part of birefringence must be described by different process and dynamics than the static birefringence part. Figure 8 shows the He−Ne light intensity modulations due to the rectangular pulses of pump light for few frequencies of chopper from 50 up to 175 Hz observed using an oscilloscope in ac mode. Signal rise time τr (around 2.7 ms at 50 Hz) seems to be weakly dependent on chopping frequency, whereas its amplitude is strongly dependent on chopping frequency, indicating that the phenomenon is cumulative in its nature. Signal decay times τd (around 4.4 ms at 50 Hz) are similar to rise times with the tendency of increase with chopping frequency. When the polymer containing photochromic molecules is exposed to light for a longer time, the photoisomerization process and related reorientation changes occur for a larger

Figure 9. Transmitted He−Ne laser intensity modulation in PYoCNNO2 polymeric system under chopped pump light with frequency of 75 Hz.

In Table 1, we gathered parameters characteristic for photoinduced birefringence effect and respective time constants for the static process of birefringence increase and the dynamic one. PMMA containing PY-oCNNO2 molecules system shows static photoinduced birefringence (Δn = 0.00042 at 524 mW/ cm2) and optical Kerr effect constant B = 6.47 × 10−6 mV−2 for dynamic birefringence, that is, 6 orders of magnitude higher than that measured in liquid nitrobenzene. This high value is obtained at the cost of slow response of the order of a few milliseconds in polymer when compared to 10−12 s response in PIB nitrobenzene. The Δndynamic constitutes 14% of the whole photoinduced optical birefringence, ΔnPIB total.

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THEORETICAL CALCULATIONS Computational Methodology. To confirm/predict possible structural changes in PY-oCNNO2 compound upon excitation, a conformational analysis of the investigated molecule was performed using the Gaussian09 software.31 The B3LYP functional and the 6-311++(d,p) basis set were used for the molecule in gas phase calculations. The geometry optimization in a THF solution was done using the PCM 10011

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The Journal of Physical Chemistry C Table 1. Physicochemical Properties of PY-oCNNO2, Kinetic Parameters of PIB Processes, and Nonlinear Optical Parameters of OKE

Figure 10. Energy profiles of PY-oCNNO2 in a gas phase; DFT/ B3LYP/6-311++G(d,p).

than the trans 1 one by about 20.8 kJ/mol, and it has no contribution to population at room temperature. The energy barrier between cis and trans 1 conformers occurring via rotation along the C(12)−C(13) bond is small and equals to ca. 5 kJ/mol (cf. Figure 10). Calculations showed that irradiation by infrared light (4255 cm−1 for transformation trans 1 → trans 2 and 2148 cm−1 for trans 1 → cis) may cause conformational transitions. The relative energies, Gibbs free enthalpies, and abundance at room temperature for all of the conformers in the gas phase and in THF solution are collected in Table 2. The structures are similar in both environments. The abundance of less stable conformers, cis and trans 2, are higher in THF solution than their equivalents obtained for isolated molecule. Figure 11 and Table 3 present calculated dipole moment vectors. The total dipole moment of the most stable trans 1 conformer is 8.62 D, which is very close to 8.5 D obtained in our previous studies.18 The small difference is connected with the different basis set used for calculations. Total dipole moment increases in THF solution when compared to the gas phase. The dipole moment vector orientation is different in every conformer. It means that both trans 1 → trans 2 and trans 1 → cis transitions occur with substantial dipole moment change, mainly its direction. In contrast to the case of azobenzene molecules, the cis isomer cannot be regarded as almost isotropic in its shape, so it is expected that the mechanisms of PY-oCNNO2 photoorientation in polymeric matrices will be different from those known for simple azobenzene derivatives. To sum up, theoretical calculations confirmed that PY-oCNNO2 forms three stable conformers. The trans 1 → cis transition energy barrier is relatively low.

a

This paper. bStatic optical birefringence and n2 parameter evaluated for pumping beam: 524 mW/cm2. cTime constants evaluated for modulation frequency of pumping beam: 50 Hz. dTime constants evaluated for pumping beam power: 461 mW/cm2.

(Polarizable Continuum Model) model as implemented in the Gaussian09 package.32 The nature of the optimized geometries at the B3LYP level was checked with vibrational frequency calculations. The vibrational frequencies of PY-oCNNO2 were calculated for all minimum energies and transition states, which were confirmed to have zero and one imaginary frequency, respectively. Obtained relative energies include zero point vibrational energy (ZPVE) corrections. The abundance of the conformers at room temperature were calculated according to the Boltzmann energy distribution. For the electron absorption spectra, time-dependent B3LYP calculations were performed. Moreover, the solvent effects on the electron absorption spectra were evaluated by using the PCM model without optimization (single point TD-DFT). Conformational Analysis. The molecular structures of the all optimized conformers for PY-oCNNO2 have already been presented in Figure 2. The potential energy surface of PYoCNNO2 was scanned around torsion angles N(8)−C(9)− C(12)−H(23) and C(13)−C(24)−C(25) from 0° to 360° at 10° increments. The energy scans are shown in Figure 10. The three minima on the PES (potential energy surface) were found. Two of them correspond to trans isomers (trans 1 and trans 2), and a third one corresponds to cis conformation. The global energy minimum is for trans 1 for which abundance is over 99%. Trans 2 conformer minimum energy is situated 16.5 kJ/mol higher than that of the trans 1 conformer. The energy barrier between trans 2 and trans 1 conformers amounts to 34.4 kJ/mol, and the transition between these states is connected with the rotation of terminal group along the C(9)−C(12) bond. The structures of transition states are presented in Figure S1 (Supporting Information). The cis conformer is less stable

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CONCLUSIONS In summary, we have characterized in-depth a novel photochromic polymeric material: the host/guest system consisting of PMMA matrix doped with the PY-oCNNO2 chromophore. The material properties were analyzed by the photoinduced birefringence methods with emphasis on the optical Kerr effect (photoinduced dynamic change of birefringence in millisecond regime). We proved the occurrence of cooperative motions promoted by the molecular photoalignment that also led to static birefringence of 4 × 10−4, proving the material suitability for either long-term or reversible holography. All optical switching characteristic in PMMA doped with PY-oCNNO2 is reported for the first time. Photoinduced dynamic changes of birefringence proved that light can effectively switch between cis and trans isomers of PY-oCNNO2, which according to the theories developed for azobenzene family may lead to appearance of optical anisotropy. By treating this material as 10012

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Table 2. Calculated Relative Energies, Gibbs Free Enthalpies, and Abundance at Room Temperature for PY-oCNNO2 Conformersa gas phase

a

THF

PY-oCNNO2 conformers

ΔEZPVE

ΔG (298 K)

% (298 K)

ΔEZPVE

ΔG (298 K)

% (298 K)

trans 1 trans 2 cis trans 2/trans 1 TS cis/trans 1 TS

0.0 16.5 20.8 50.9 25.7

0.0 16.0 21.2 53.6 28.0

99.8 0.2 0.0

0.00 11.7 16.9

0.00 13.1 17.9

99.4 0.5 0.1

ΔEZPVE and ΔG(298 K) are given in kJ/mol.

Figure 11. Optimized structures of PY-oCNNO2 in the gas phase with orientation of dipole moment vectors; DFT/B3LYP/6-311++G(d,p).

Table 3. Calculated Ground State Dipole Moments for PY-oCNNO2 Conformersa gas phase

a

THF

PY-oCNNO2 conformers

μx

μy

μz

μtot

μx

μy

μz

μtot

trans 1 trans 2 cis trans 2/trans 1 TS cis/trans 1 TS

8.61 7.32 6.30 3.44 6.99

0.29 6.03 3.42 4.12 0.06

0.20 0.41 0.63 1.58 0.23

8.62 9.49 7.20 5.60 7.00

11.58 9.54 7.99

0.48 8.34 4.74

0.25 0.50 0.74

11.60 12.68 9.32

Values are given in D.

an optical Kerr medium, we have calculated for it the Kerr constant and third order susceptibility, respectively. The system studied here proves the principle of all-optical switching, but it deserves improvement in several aspects: by changing dopant concentration and method of film preparation.33,34 This work provides also a solid knowledge on the photochromic properties of the PY-oCNNO2 chromophore by identification with quantum chemical DFT methods of its conformers: two trans (E) and one cis (Z) type. Quantum chemical calculations point to the existence of bent cis-isomers that could be reached via optical excitation of molecules in its ground trans 1 state. Energies of ground molecular states and associated dipole moments were calculated.

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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.

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ACKNOWLEDGMENTS A.S. is thankful for financial support for dissertation preparation under ETIUDA II program which is financed by Polish National Science Centre (Doctoral Scholarship No. Dec-2014/ 12/T/ST4/00233). A grant of computer time from the Wroclaw Center for Networking and Supercomputing (WCSS) is gratefully acknowledged.

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ABBREVIATIONS PIB, photoinduced birefringence; DPSS, diode pump solid state; OKE, optical Kerr effect; THF, tetrahydrofuran; PMMA, poly(methyl methacrylate); PCM, Polarizable Continuum Model; ZPVE, zero point vibrational energy; PES, potential energy surface

ASSOCIATED CONTENT

S Supporting Information *

Optimized molecular structures of transition states between trans 2−trans 1 and cis−trans 1 conformers of PY-oCNNO2. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Barbera, J.; Clays, K.; Gimenez, R.; Houbrechts, S.; Persoons, A.; Serrano, J. L. Versatile Optical Materials: Fluorescence, Non-linear Optical and Mesogenic Properties of Selected 2-pyrazoline Derivatives. J. Mater. Chem. 1998, 8, 1725−1730. (2) Vishnumurthy, K. A.; Adhikari, A. V.; Sunitha, M. S.; Philip, R. Synthesis and Characterization of a New Conjugated Polymer Bearing

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +48 (71) 3203197. 10013

DOI: 10.1021/acs.jpcc.5b01947 J. Phys. Chem. C 2015, 119, 10007−10014

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DOI: 10.1021/acs.jpcc.5b01947 J. Phys. Chem. C 2015, 119, 10007−10014