Photo-Physical Transformations in Pyrazoline Derivative Based Systems

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Photo-Physical Transformations in Pyrazoline Derivatives Based Systems Adam Szukalski, Andrzej Miniewicz, Karolina Haupa, Bartosz Przybyl, J. Janczak, Andrzej L. Sobolewski, and Jaroslaw Mysliwiec J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03915 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on July 4, 2016

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The Journal of Physical Chemistry

Photo-Physical Transformations in Pyrazoline Derivatives Based Systems

A. Szukalski,a,* A. Miniewicz,a K. Haupa,b B. Przybyl,c J. Janczak,c A. L. Sobolewskid and J. Mysliwieca

a

Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland

b

c

Chemistry Department, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland

Institute of Low Temperature and Structure Research, Polish Academy of Science, Okolna 2, 50-422 Wroclaw, Poland

d

Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland

* Adam Szukalski, corresponding author Advanced Materials Engineering and Modelling Group Faculty of Chemistry Wroclaw University of Science and Technology Wybrzeze Wyspianskiego 27 50-370 Wroclaw, Poland e-mail: [email protected] phone: (48 71) 320-2317 1 ACS Paragon Plus Environment


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fax: (48 71) 320-33-64 Abstract Discovery of E/Z or trans-cis photoisomerization in the azobenzenes and their derivatives had the tremendous impact on the whole domain of photochromic materials including photochromic polymers and liquid crystals. Here we show similar configurational photoinduced transformation in a simple derivative of pyrazoline. The X-ray crystallographic investigations of (E)-3-(4-nitrostyryl)-1-phenyl-4,5-dihydro-1H-pyrazole (abbreviated as PYpNO2) in grown crystals show two different structures comprising of either cis or trans molecules. The performed quantum chemical calculations confirm the existence of both configurations of PY-pNO2 at the room temperature. Photophysical properties of this compound derived from quantum chemical calculations predict possibility of trans to cis switching of PY-pNO2 by light. Indeed, molecules of PY-pNO2 embedded in PMMA polymeric matrix when illuminated with 532 nm linearly polarized laser light show the induced optical anisotropy, i.e. birefringence characteristic for photoisomerizable molecules similar like in the group of the azobenzene derivatives.

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1. Introduction Reversible phototransformation of a molecule between forms having different absorption spectra, especially those showing conformational changes, is a highly desired molecular property known as photochromism.1 Changes of molecular properties upon suitable light absorption can be utilized in photonic devices for information storage or processing. Among many nonbiological organic compounds like diarylethenes, fulgides, spiropyrans and spirooxazines, the family of azobenzene derivatives has drawn the greatest attention.1-4 Azobenzene derivatives have been used as photoalignment molecules for liquid crystals5-8, optical memories9-15 and when embedded in polymers for recording of polarization and surface relief gratings.16-22 The reason of their exceptionally wide usage was linked with the molecular configurational trans-to-cis transition leading to the several opto-mechanical effects.23-26 When azobenzene derivatives are attached to the polymer main chains they exhibit the Weigert effect27 which describes the light induced molecular orientation of the transstate azobenzene molecules with their long molecular axes set perpendicularly to the plane of linearly polarized light incident on the polymer film. This in turn leads to occurrence of photoinduced birefringence (PIB), dichroism, material directional shrinking or expansion.28-31 All these effects are possible due to the unique property of optically reversible photochromism occurring at the same wavelength of light in both ways i.e. trans-to-cis and cis-to-trans. In our studies we are searching for the molecular systems being an alternative to azobenzene derivatives family of compounds. In a recent paper we reported on the photoinduced birefringence effect observed in poly(methyl methacrylate) (PMMA) loaded with the pyrazoline derivative molecule ((Z)-2-(4-nitrophenyl)-3-(1-phenyl-4,5-dihydro-1H-pyrazol-3-yl)acrylonitrile

–



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abbreviated as PY-oCNNO2). We tentatively have explained the observed phenomenon by assuming the configurational photoisomerisation of trans-cis type in this molecule and supported the experimental findings by relevant quantum chemical calculations.32 In this paper we report a direct proof of the existence of two photoisomers in another member of the pyrazoline derivatives family, the compound named (E)-3-(4nitrostyryl)-1-phenyl-4,5-dihydro-1H-pyrazole (abbreviated as PY-pNO2). The proof is basing on growth and subsequent determination of two different crystallographic structures of PY-pNO2 compound: one composed of rod-like trans molecules and the second composed exclusively of bended cis molecules. The crystals were grown at room temperature from the same solvent (dichloromethane). Advanced quantum chemical calculations using DFT and CC2 methods have shown that trans-cis isomerization might be a common feature for the whole family of pyrazoline derivative compounds.

2. Physicochemical properties and crystal structures of PY-pNO2 compound Prior today, pyrazoline derivatives in the molecular as well as crystalline forms have been the subject of several studies including their fluorescence, nonlinear optical (NLO) properties, electrooptical effects and others.33-38 Majority of work in that respect has

been

done

on

3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole

compound, known as DCNP. Interestingly DCNP crystals are noncentrosymmetric showing second harmonic generation (SHG) accompanied by a strong, highly directional fluorescence.39-46 Mysliwiec et al.47 reported recently on the synthesis and optical properties (absorption, two-photon absorption and emission) of the group of five pyrazoline derivatives differing in their terminal groups R1, R2 and R3. The peculiarities of some recent physicochemical findings in this group of materials48


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prompted us to detailed quantum chemical calculations of these molecules and the verification of configurational transitions using the X-ray spectroscopy.

Similarly to

DCNP molecule the structure of (E)-3-(4-nitrostyryl)-1-phenyl-4,5-dihydro-1Hpyrazole is another example of the NLO push-pull type chromophore in which a single nitro group is located in the para position of an aromatic ring constituting an electronacceptor group. The π−electron bridge between donor (D) and acceptor (A) is preserved. Chemical structures of DCNP and PY-pNO2 are shown in Fig. 1. Single crystals of both compounds have been grown from the saturated solutions in the solvent/non-solvent mixture (dichloromethane/distilled water, respectively). The aim of this part of research was to check the possibility to obtain two different isomer forms - trans and cis, in a photostationary state in solution. For nonnegligible populations of both isomers at room temperature this can lead to growth of single crystals exhibiting polymorphic structures. The crystals of DCNP did not show any polymorphism and their crystallographic structure was exactly the same as published earlier33,34. However, the crystals of (E)-3-(4-nitrostyryl)-1-phenyl-4,5-dihydro-1Hpyrazole were found in two different polymorphic forms differing in configurational state of their molecules:

bended cis-type and the rod-like trans-type. This fact is

treated by us as a strong proof that the two ground state photoisomers of PY-pNO2 can be present in solution. Similar phenomenon was reported for diphenylstilbene containing α-cyanostilbenic moiety.50 The details of crystal’s structures measured by X-ray diffraction technique at T = 295(2) K are presented in Table 1. and in full extent in Supporting Information (see Table S1. and Figure S1.). PY-pNO2 compound crystallizes in the centrosymmetric P21/c space group of the monoclinic system in the case of both cis-isomer and transisomer, while DCNP crystallizes in the non-centrosymmetric Cc space group.



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Fig. 1. (a) Chemical structures of the investigated push-pull type compounds: DCNP (3-(1,1dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole) and PY-pNO2 ((E)-3-(4-nitrostyryl)-1phenyl-4,5-dihydro-1H-pyrazole). ORTEP shapes of (b) cis-PY-pNO2, (c) trans-PY-pNO2 and (d) DCNP molecule in a crystal lattice with labelling of atoms. Atoms displacement ellipsoids are drawn at 50% probability level.

Molecular shapes as they appear in crystallographic units cells are reported in Fig. 1b, c and d. Geometry of the cis-PY-pNO2 (cf. Fig. 1b) exhibits strong torsion of pnitrophenyl moiety in relation with pyrazoline ring (dihedral angle between planes determined by these rings is equal 75.23(6)°) due to steric hindrance around C(10)=C(11) double bond. The isomer trans-PY-pNO2 (cf. Fig. 1c) exhibits much smaller dihedral angle equal to 12.76(9)° due to the weaker geometrical obstacles. The phenyl and pyrazoline rings are almost coplanar, dihedral angles between the planes determined by them are equal 4.38(6)° and 3.61(10)°, respectively. The packing of the molecules cis-PY-pNO2 and trans-PY-pNO2 in the unit cells does not point on other important intermolecular interactions than van der Waals forces only. Owing to the


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bend structure of cis-PY-pNO2 isomer its molecular packing is looser (by approximately 5%) than that of trans-PY-pNO2 what is reflected in the volumes of crystallographic unit cells (Vcis = 1490.06 Å3 and Vtrans = 1425.1 Å3). This consequently leads to the weaker intermolecular π−π interactions in cis-PY-pNO2 isomer. We also confirmed that the molecule of DCNP in the crystal exhibits almost flat geometry, dihedral angles determined between pyrazoline and phenyl rings and between pyrazoline and dicyanomethylidene group are equal 3.93(6)° and 5.16(5)°, respectively. The packing of the molecules in the unit cell also does not indicate other important intermolecular interactions than the van der Waals ones (cf. Fig. S1. in Supporting Information, abbreviated further as SI).

Table 1. Basic crystallographic data for structures of cis-PY-pNO2, trans-PY-pNO2 and DCNP grown from solution at the room temperature. cis-PY-pNO2

trans-PY-pNO2

Empirical formula

C17H15N3O2

C17H15N3O2

Formula weight

293.32

293.32

222.25

Crystal system

monoclinic

monoclinic

monoclinic

Space group

P21/c

P21/c

Cc

a [Å]

14.1516 (10)

6.4715 (6)

11.8577 (6)

b [Å]

5.9117 (4)

16.5381 (16)

12.3476 (5)

c [Å]

18.1609 (14)

13.3805 (18)

7.8726 (4)

β [°]

101.267 (7)

95.670 (11)

90.463 (5)

1425.1 (3)

1152.62 (9)

V [Å3]

1490.06 (19)

DCNP C13H10N4

Dcalc /Dobs (g·cm–3)

1.308 /1.30

1.367 / 1.36

1.281 /1.28

Temperature (K)

295 (2)

295 (2)

295 (2)

Rint

0.0308

0.0746

0.0199 7

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R[F2>2σ(F2)]

0.0548

0.0781

0.0283

wR (F2 all reflections)

0. 1491

0.1542

0.0523

3. Quantum chemical calculations of PY-pNO2 and DCNP Very little is known about the photoisomerization process of pyrazoline derivatives. In view of experimental findings evidencing the existence of two PYpNO2 isomers of E and Z type at room temperature able to crystallize in two different structures we have undertaken preliminary quantum chemical calculation for this dye. The detailed quantum chemical calculations that have been reported for cis form of DCNP recently46 were directed toward its electronic and nonlinear optical properties only. We performed the DFT quantum chemical calculations for both DCNP and PYpNO2 isomers in gas phase including molecular geometry optimization, configurational analysis and transition barriers between stable ground states isomers. The results of these calculations are shown in Fig. 2. The main goal of the theoretical studies was devoted to showing that stable isomers of PY-pNO2 are energetically allowed and could be reached via suitable optical excitation of a molecule being in its ground state. From quantum chemical calculations it follows that DCNP has also trans and cis conformers (cf. Fig. 2. and Table S2. in SI) but PY-pNO2 has as much as four stereoisomers trans 1, trans 2, cis 1 and cis 2 (cf. Fig. 2. and Table S2. in SI). The molecular conformers and configurational isomers found by DFT methods for PYpNO2 (see Fig. S2. in SI) encourage us to use an advanced second-order coupled cluster (CC2) method and repeat above calculations. Using CC2 method we qualitatively confirmed the earlier results (see Tables S3. – S6. in SI) noting that differences between both methods are not meaningless (cf. Table S7.). In both DCNP and PY-pNO2 molecules the lowest energy ground state is always a trans one.


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Fig. 2. Scheme showing all possible ground states and transition states (TS) conformers of stereoisomers of DCNP (a) and PY-pNO2 molecule (b) basing on DFT and CC2 quantum chemical calculations. Relative energy differences are given in kJ/mol units.

Basing on the calculations of minimum energy profiles versus dihedral angle deformations we propose a photophysical scheme (cf. Fig. 3.) for the PY-pNO2 molecule characterized by

twisting either around C(7)-C(10) single bond or

C(10)=C(11) double bond. Configurational transitions between trans 1 and cis 1 and trans 2 and cis 2 occur via torsion around C(10)=C(11) double bond. Whereas conformational transitions between trans 1 and trans 2 and cis 1 and cis 2 occur via torsion around C(7)-C(10) single bond. The former can be treated as the photoinduced transitions and the latter rather as the thermo-physical ones. Energy profiles for torsional transitions in PY-pNO2 are shown in Fig. 4 for ground S0(S0) and excited states S1(S1), respectively. These profiles approach the intersection at 90o of C=C dihedral angle that may indicate a conical intersection. It is known that single reference methods such as ADC and CC2 broke in the proximity of the states degeneracy. Results were obtained at the MP2 (S0) and ADC(2) (S1) level of theory. Shaded ellipse in Fig. 4 denotes area where the S1/S0 conical intersection (CI) is expected to occur for PY-pNO2. For this transition in the ground state S0(S0) the energy barrier is around 1.4 9 ACS Paragon Plus Environment


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eV, a value typical for double C(10)=C(11) bond torsion. However, upon light absorption the transformation between trans 1 and cis 1 is possible.

Fig. 3 Scheme of photophysics of PY-pNO2. The transitions due to double bond twist are possible only via photophysical transformation after photon absorption while that due to single bond twist may occur thermally.

Fig. 4. Minimum-energy profiles for trans 1 - cis 1 reaction (left panel) and for trans 1 trans 2 reaction (right panel) of PY-pNO2 molecule in the ground state (circles) and the first excited singlet state (squares). Symbols connected by solid line denote energy profile


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optimized in a given state, and circles connected by dashed line denote ‘vertical’ energy of the ground state computed along the minimum reaction path in the S1 state.

Vertical excitation energies for S0-S1 and S0-S2 were calculated for the all stereoisomers. For the all isomers the light absorbing state S1 is of π−π* type and absorption is highly allowed (cf. SI Tables S3.-S6.) with the oscillator strengths of 1.25 and 0.99 for trans 1 and trans 2 conformers, and 0.72 and 0.35 for cis 1 and cis 2, respectively. In the first excited state S1 molecules are highly polar with dipole moments exceeding 20 D. The S2 states for all isomers are of n-π type. Absorption to these states S2(n-π*) is described by very low oscillator strengths 0.1 × 10-4 and 0.7 × 10-5 for both trans and 0.3 × 10-2 and 0.2 × 10-2 cis 1 and cis 2, respectively. These states can be regarded as dark states. From the Fig. 3. and Fig. 4. it follows that we deal with the PY-pNO2 molecular photo-switch that can be selectively photo-isomerized from the Z-isomer into the E-isomer and vice versa. We demonstrated by structural analysis of grown crystals that two Z (trans 1) and E (cis 1)-isomers exist in the electronic ground state. In the excited state S1 configurational isomers are separated by small energy barrier along the dihedral twisting coordinate. It would be interesting to know what fraction of the excited state population decays via the identified conical intersection and forms ground state E-isomers. Formally, that would require complex simulations of photodynamics or the study of fluorescence lifetimes and estimation of quantum yields of photo-switching phenomenon in PYpNO2. However, the elucidation of the discovered photoisomerisation mechanism will be the future task. Quantum mechanical methods allowed us to calculate the values of the dipole moments of stereoisomers. The knowledge of dipole moments values and directions can be very helpful in understanding the mechanism of the photoinduced molecular transformations for PY-pNO2



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molecules embedded in polymer matrix. The direct experiment of light-induced molecular orientation, the so called optical Kerr effect (OKE) will be reported in the following section. In Fig. 5. dipole moment vectors calculated with DFT (B3LYP/6-311++G(d,p)) in the gas phase and THF solution are plotted with respect to the molecular coordinates. The total dipole moment of the most stable trans DCNP conformer is 8.35 D and 9.41 D for cis form. Total dipole moment increases in THF solution when compared to the gas phase from 11.72 D to 12.27 D, respectively. The vector orientation is different in both conformers. It means that trans → cis transition occurs with substantial dipole moment change.

Fig. 5. Optimized ground state structures of DCNP and PY-pNO2 isomers in the gas phase with orientation of their dipole moment vectors. DFT/B3LYP/6-311++G(d,p) methodology.

In the case of PY-pNO2 the total dipole moments of trans 1 and trans 2 conformers are almost identical and equal to 8.00 and 8.05 D in the gas phase and increase to 10.47 and 10.71 D in THF solution, respectively. However, the total dipole moment of cis 1 conformer is much lower amounting to 4.11 D, but the another one - cis 2 is again high - 8.80 D, both in the gas phase and 7.74 D and 11.77 in THF solution, respectively. It means that only trans 1/2 → cis 1 transition is accompanied by significant dipole moment change.


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4. Photoinduced birefringence in amorphous polymeric matrices containing studied pyrazoline derivatives compounds Basing on the structural findings and assuming that molecule of PY-pNO2 has two pairs of photoisomers that can be switched by light of nearly the same photon energy we performed a simple experiment of light-induced molecular reorientation (or photoinduced birefringence – PIB) in an amorphous polymeric matrix with embedded molecules. We prepared two thin films containing 3% w/w of either PY-pNO2 or DCNP in poly(methyl methacrylate). The absorption spectra of the films are shown in Fig. 6a. It is widely accepted that the molecular reorientation observed in azobenzene doped

polymers

originates

from

the

multiple

acts

of

trans-cis-trans

photoisomerizations.50 Using cw laser light of a single wavelength of 532 nm and of linear polarization we expected that an angular selective absorption of trans PY-pNO2 molecules will lead to slow, but continuous reorientation of part of trans molecules with their long axes directed perpendicular to the polarization plane of the incident laser light and stabilized by the polymer matrix. This should induce the macroscopic optical anisotropy that could be optically detected. Large viscosity of the polymer is beneficial for temporal stabilization of the molecular reorientation after removal of a “pumping” laser beam. The induced optical anisotropy could be measured by the experiment similar to the Optical Kerr effect (OKE) where the auxiliary He-Ne laser beam of 632.8 nm wavelength and outside the absorption range of PY-pNO2, is used to monitor the refractive index change occurring in the film.



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Fig. 6. (a) Normalized absorption spectra of thin PMMA films doped with DCNP and PYpNO2, respectively. Positions of pumping and reference beams used in PIB experiment are marked with arrows. (b) Reversibility of birefringence signal induced by 532 nm pump light in thin film of PY-pNO2/PMMA composite. Light intensity changes due to birefringence at 632.8 nm are directly related via equation (2) with transmittance of the sample under crossed polarizers.

Fig. 7. Photoinduced transmittance switching in thin films of PY-pNO2/PMMA (a) and DCNP/PMMA (b) composites resulting from pump light (532 nm) chopped with frequency of 75 Hz and observed as detector readings with the help of oscilloscope working in AC acquisition mode. The exact mechanism of this process was described for azobenzenes in polymers elsewhere.28 When pumping beam irradiates the sample an optical anisotropy is induced that 14 ACS Paragon Plus Environment

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enables temporal probe light transmission through the system. In-plane light-induced optical birefringence ∆n(I,t) at λ = 632.8 nm is a function of pumping light intensity I532 and time t through relation:

∆n(I,t) = n⊥(I,t) - n||(I,t) = λ∆ϕ(I,t)/2πd

(1)

where d is the polymer film thickness and ∆ϕ(I,t) is the phase change between two orthogonally polarized beams of He-Ne laser propagating through birefringent sample. Change of probe light intensity Itrans(t) registered by the detector is given as:

Itrans(t) = I0 sin2[(πd∆n(I,t))/λ]

(2)

where I0 is the incident probe light intensity. By chopping input light intensity I0 with mechanical chopper one can observe both growth of static birefringence in the sample as well as dynamic (reversible) changes of birefringence. Despite of the differences in molecular structures between DCNP and PY-pNO2 both molecules when doped to PMMA showed similar PIB effects. The process of PIB (cf. Fig. 6b) is quite reversible but a remnant birefringence seems to growth with time of the experiment. Static birefringence was measured separately and is shown in Fig. S3. of SI. In response to chopped pumping light for both PY-pNO2/PMMA and DCNP/PMMA composites a similar behavior has been observed and registered using an AC oscilloscope mode (cf. Fig. 7). The dynamic birefringence growth and decay have been captured at frequency of beam chopping equal to f = 75 Hz. It is clear from Fig. 7 that amplitudes of dynamic birefringence changes at this measurement regime are quite similar but signal kinetics are quite 15 ACS Paragon Plus Environment


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different. Response time shown by PY-pNO2/PMMA is much faster than this shown by DCNP/PMMA system under exactly the same pump light intensity conditions. The characteristic signal build-up for PY-pNO2/PMMA is about 0.4 ms and for DCNP/PMMA it is about 2.5 ms (dynamic birefringence signals in function of chopping frequency are shown in Fig. S4 of SI). Results of PIB measurements obtained for both samples of PY-pNO2/PMMA and DCNP/PMMA are summarized in Table S8 of Supplementary Information. Exemplary nonlinear optical coefficients, i.e. n2 parameter extracted from the experiments and averaged 3rd order susceptibility χ(3) are dependent on the dye concentration in the polymeric matrix, its aggregation form, used light wavelength and light pulse duration and intensity, therefore they may differ from that obtained in similar experiments performed under different experimental conditions. However, the main purpose of the presented experiments was to prove that pyrazoline derivatives are characterized by two different conformational ground states. Thus, there is a supposition that both compounds exhibit light-induced trans-cis-trans isomerizations and hold promise that optical switchers, diffraction gratings, surface relief gratings and polarization gratings could be observed for the whole family of pyrazoline derivatives when attached to the polymer backbones. Particularly interesting is that pyrazoline derivatives are luminescent species so the luminescence changes are expected to show up together with conformational changes.49 5. Conclusions In summary, we have demonstrated that derivative of pyrazoline PY-pNO2 exhibits two ground state configurational isomers trans 1 and cis 1 differing in molecular shape (their mass and elemental compositions are identical). Both isomers crystallize at room temperature (295 K) in the same space group P21/c. The CC2


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quantum chemical calculations predicted four ground state conformers trans 1, trans 2, cis 1 and cis 2 of PY-pNO2 all differing in their ground state energies. Transitions trans 1 → cis 1 and trans 2 → cis 2 can be realized by absorption of a photon with a suitable energy exceeding the difference between S1 and S0 electronic states. The existence of at least two switchable by light configurational isomers in the group of pyrazolines similarly to the well-known derivatives of azobenzenes could result in numerous applications in the field of photonics, opto-mechanics, nanophotonics, etc. An additional asset of this family of compounds is their solid-state fluorescence. We believe that this insight may stimulate syntheses and research of new chromophores based on pyrazoline derivatives that can be as useful as azobenzenes in multitude of applications. Supporting Information Structure analysis of the two investigated compounds DCNP and PY-pNO2, details of quantum chemical calculations of PY-pNO2 molecule in its ground and excited states, computational methodology and some supplementary data concerning the photoinduced birefringence experiments performed for the DCNP and PY-pNO2 molecules embedded in PMMA matrices. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements A. S. would like to thank 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. M. thank OPUS grant of National Science Centre, Poland 2014/15/B/ST8/00115 and statutory fund of Faculty of Chemistry at Wroclaw University of Technology, 2015. A grant of computer time from



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the Wroclaw Centre for Networking and Supercomputing (WCSS) is also gratefully acknowledged.

References

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■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +48 (71) 3203197.

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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Photo-Physical Transformations in Pyrazoline Derivative Based Systems

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