Photophysical study and theoretical calculations of an ionic liquid crystal bearing oxadiazole

Journal of Molecular Structure 1016 (2012) 76–81

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Photophysical study and theoretical calculations of an ionic liquid crystalbearing oxadiazole

Jorge A. Pedro, José R. Mora, Eduard Westphal, Hugo Gallardo, Haidi D. Fiedler ⇑, Faruk Nome ⇑Department of Chemistry, National Institute of Catalysis, Federal University of Santa Catarina, Florianópolis, SC 88040-900, Brazil

a r t i c l e i n f o

Article history:Received 8 November 2011Received in revised form 17 February 2012Accepted 20 February 2012Available online 27 February 2012

Keywords:Fluorescent probeLiquid crystalTheoretical calculationsOxadiazole

0022-2860/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.molstruc.2012.02.046

⇑ Corresponding authors. Tel.: +55 48 3721 6847x2E-mail addresses: [email protected] (F. Nome), fie

a b s t r a c t

We report a detailed photophysical study of 1-dodecyl-4-[5-(4-dodecyloxyphenyl)-1,3,4-oxadiazole-2-yl]pyridinium bromide (454Do), a cationic amphiphile that behaves as a fluorescent liquid crystal.Excitation and emission spectra of the probe in different environments result in significant changes inquantum yields which are correlated with changes in lifetimes and theoretical calculations.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction In this work we use as probe 1-dodecyl-4-[5-(4-dodecyloxyphe-

The study of fluorescent liquid crystalline materials has becomea popular research area due to their extensive industrial applica-tion in organic light-emitting diodes (OLEDs) [1]; dye-sensitizedsolar cells (DSSC) [2]; uni- and biaxially ionic conduction systems[3–5]; organized solvent for chemical reactions [6]; vectors forsmall interfering RNA (siRNA) transfection [7]; and as opticalrecording materials in laser disks [8]. Incorporation of an 1,3,4-oxadiazole heterocyclic moiety as core units in thermotropic liquidcrystals can result in large changes in their mesophases and phys-ical properties because of the notable increase in polarisable het-eroatoms such as nitrogen and oxygen [9,10]. Asymmetric (‘‘bentcore’’) nematic liquid crystals containing a nonlinear oxadiazoleunit show unambiguous biaxiality [9,10], a phenomenon whicheluded experimental demonstration for a long time. Furthermore,bent-core mesogens based on a low molar mass oxadiazole-con-taining nematic material have shown evidence of a ferroelectricresponse, which is probably connected with field-induced biaxial-ity, and also shows extraordinary magnetic field-induced effects[11,12]. The use of the 1,3,4-oxadiazole group in the liquid crystal-line core allowed the observation of good luminescent propertiesin chloroform solution as well as the building of interesting chiralliquid crystalline materials [13]. Furthermore, addition of a pyrid-inium bromide next to the 1,3,4-oxadiazole results in the forma-tion of a thermochromic ILCs where charge-transfer interactionplays an important role [14].

ll rights reserved.

27; fax: +55 48 3721 [email protected] (H. Fiedler).

nyl)-1,3,4-oxadiazole-2-yl]pyridinium bromide (454Do), a newfluorescent ionic liquid crystal and show that fluorescence excita-tion and emission spectra of the probe in different environmentsresult in significant changes in quantum yields that are correlatedwith changes in lifetimes. Experimental data are consistent withtheoretical calculations using Time Dependent Density FunctionalTheory (TD-DFT) [15–17].

2. Experimental section

2.1. Reagents

The solvents ethanol, methanol, acetonitrile (ACN), dimethyl-sulfoxide (DMSO) and dimethylformamide (DMF) were either spec-troscopic or HPLC grade. All other inorganic and organic reagentswere the best available analytical grade from commercial sources(Merck, Aldrich, Fluka and Across Organics) and used as received.Doubly deionized water with conductance <5.6 � 10�8 X�1 cm�1

and pH 6.0–7.0 from a NANOpure analytical deionization system(type D-4744) was used to prepare the standard and reagentsolutions.

Fig. 1. Textures observed by POM (100�) for compound 454Do. (a) Oily streak texture obtained for a thin film at 190 �C during heating at 10 �C/min; (b) formation of batonetsand fan-shaped texture with homeotropic areas, for a thick film at 179 �C during cooling.

300 350 400 450 500 550 600 650 7000

200

400

600

800

1000Emission

Rel

ativ

e Fl

uore

scen

ce In

tens

ity

Wavelength (nm)

Excitation

ACN

EtOH

HCCl3

MetOH

Fig. 2. Excitation (kEm at 581 nm) and emission (kEx at 360 nm) spectra of theoxadiazole 454Do, in different solvents, [454Do] = 5 � 10�6 mol L�1.

J.A. Pedro et al. / Journal of Molecular Structure 1016 (2012) 76–81 77

2.2. Solutions

Stock solutions of 1-dodecyl-4-[5-(4-dodecyloxyphenyl)-1,3,4-oxadiazole-2-yl]pyridinium bromide were 10�3 mol L�1 in a80:20% (v/v) mixture of ethanol and acetonitrile. A standardsolution of quinine sulfate (Fluka, purum for fluorescence)1 � 10 �3 mol L�1 in sulfuric acid 0.1 mol L�1 was prepared, anddilutions were made with nanopure water.

2.3. Spectroscopic measurements

Steady state fluorescence measurements were made on a CaryEclipse Varian spectrofluorimeter, with a 450 W Xenon arc lamp.The excitation and emission wavelengths for 454Do are 360 nmand 581 nm respectively, with slit widths of 10 nm and appliedvoltage on the PMT of 450 V. Absorbance was adjusted to0.25 ± 0.01 at the selected wavelength, with probe concentrationsbetween 10�5 and 10�4 M. The solutions were then excited at thiswavelength and the emission spectra obtained. Integration of theemission spectra and comparison to a quinine sulfate spectrum(obtained under identical conditions) allowed calculation of quan-

tum yields. Spectrophotometric measurements were carried out ona Cary 50 BIO model equipped with a xenon light source. Lifetimemeasurements were made with an Easy Life V instrument with a370 nm nanosecond pulsed LED excitation source. A 550 nm cutoff filter was used to control the excitation light. The data wereaccumulated in 100 channels with fixed parameters: start delay30 ns, end delay 450 ns and average 3. All spectroscopic measure-ments were made in a temperature controlled room, at 24 ± 1 �C.

Infrared spectra were recorded on a Perkin–Elmer model 283spectrometer using KBr discs. 1H and 13C NMR were recorded witha Varian Mercury Plus spectrometer operating at 400 and100.6 MHz respectively. XRD analysis is described in detail in theSupplementary information.

2.4. Synthesis

The fluorescent calamitic ionic liquid crystal (454Do) is an ori-ginal material and was prepared by a straight-forward and high-yield synthetic route, partially based in well known literature pro-cedures [18,19]. A detailed description of the synthesis proceduresand corresponding analytical data (melting point, IR, 1H, and 13CNMR spectra) are given in the Supplementary Information.

2.5. Thermal properties

The thermal properties of the fluorescent probe 454Do, investi-gated by DSC, POM and TGA, exhibit a complex thermal behavior(see Supplementary information). Comparison between DSC andPOM analysis shows that the compound has a Cr–Cr transition at80.2 �C; a transition to a kind of plastic crystal at 169.8 �C; and con-version to a liquid crystalline state at 178.0 �C. This mesophase wascharacterized by POM as a smectic A (SmA) phase with a tendencyof homeotropic alignment. As can be seen in Fig. 1, the textures ob-served were oily streaks and fan-shaped, depending of the filmthickness. The mesophase type was verified by XRD analysis withvariable temperature (see Supplementary information). With con-tinued heating, the compound decomposes around 220 �C, a resultwhich was verified by TGA (see Supplementary information).

2.6. Theoretical calculations

We have carried out calculations using Density Functional The-ory (DFT) and Time-Dependent Density Functional Theory TD-DFT

Table 1Quantum yields, emission and excitation maxima and fluorescence lifetimes of the oxadiazole 454Do in different solvents.

Solvents /0 kMax, nm kExc, nm kEm, nm s1, ns A1 s2, ns A2 v2 f1a f2

b sAvc

ACN 0.103 355.9 361.1 580.9 0.34 0.028 2.49 0.075 0.98 0.039 0.961 2.99EtOH 0.053 360.9 361.1 574.9 0.88 0.049 3.10 0.004 1.26 0.782 0.218 1.36EtOH/H2O (3:1) 0.026 358.0 359.1 579.9 0.37 0.024 2.80 0.002 1.27 0.616 0.383 1.30EtOH/H2O (1:1) 0.019 355.0 356.0 585.9 0.29 0.018 2.64 0.001 1.41 0.668 0.332 1.07MetOH 0.026 357.1 361.1 585.9 0.50 0.024 3.10 0.002 1.15 0.638 0.362 1.44CHCl3 0.022 380.9 380.0 561.0 0.16 0.019 3.88 0.003 0.93 0.410 0.589 1.67

a f1 = A1s1/(A1s1 + A2s2).b f2 = A2s2/(A1s1 + A2s2).c sAv = f1s1 + f2s2.

0 2 4 6 8 10 12 1410

100

1000

10000

Fluo

resc

ence

Inte

nsity

t (ns)

Ethanol

ACN

Fig. 3. Fluorescence decay of oxadiazole 454Do HNA in acetonitrile and ethanol at24 ± 1 �C with a timescale of 0.14 ns/channel.

Table 2Geometrical parameters of 454Et using acetonitrile as solvent at MPW1PW91/6-31++G(d,p) level of theory.

Atomic lengths (Å) GS ES Dihedral angles GS ES

C1–O2 1.435 1.452 C1–O2–C3–C4 179.22 179.41O2–C3 1.344 1.313C3–C4 1.406 1.425

C5–C6 1.407 1.422 C5–C6–C7–N8 �0.71 �0.18C6–C7 1.444 1.426C7–N8 1.309 1.314

N9–C10 1.298 1.340 N9–C10–C11–C12 �0.90 �0.40C10–C11 1.449 1.413C11–C12 1.400 1.427

C13–N14 1.348 1.369 C13–N14–C15–C16 88.31 87.94N14–C15 1.481 1.465C15–C16 1.517 1.521

78 J.A. Pedro et al. / Journal of Molecular Structure 1016 (2012) 76–81

[9–11] for the ground and excited state, respectively (consideringsinglets, number of states = 6 and root = 1). These calculations were

Fig. 4. 3-D Potential Energy Surface (PSE) relaxed scan for 454Et, in the ground state, at Mand C(4)–C(3)–O(2)–C(1)).

performed using the GAUSSIAN 09 program on Linux operative sys-tems [20]. In order to obtain the global minimum in the GroundState, an optimization with modred Keyword, at MPW1PW91/6-31G(d,p) level of theory was performed. A TD-DFT optimization

PW1PW91/6-31++G(d,p) level of theory. Dihedral angles (C(13)–N(14)–C(15)–C(16)

J.A. Pedro et al. / Journal of Molecular Structure 1016 (2012) 76–81 79

was made on the Excited State. To reduce the calculation time, themolecule model used in these calculations was 4-(5-(4-ethoxy-phenyl)-1,3,4-oxadiazol-2-yl)-1-ethylpyridin-1-ium (454Et). Twodihedral angles were varied for the Ground State (C(13)–N(14)–C(15)–C(16) and C(4)–C(3)–O(2)–C(1)). Convergence parameterson the density matrix, the maximum displacement and the maxi-mum force were 10�9 atomic units, 0.0018 Å and 0.00045 -Hartree/Bohr, respectively. The solvent effect was included usingthe SCRF keyword, and the Polarizable Continuum Model (PCM),and the Solvation Model Density (SMD) [21]. This calculation wasmade in two different solvents (acetonitrile and ethanol). The exci-tation and emission wavelength were determined taking intoaccount the non-equilibrium solvation calculation because theseprocesses are too rapid for the solvent to have time to fully respond.Thus, the calculations are realized using the ground state geometryand, to obtain the vertical excitation, the optimization in the groundstate proceeded considering NonEq = write and NonEq = read toobtain the vertical linear response [16,22]. These values were ob-tained at a MPW1PW91/6-31++G(d,p) level of theory.

1

454Et

N N

ONO

CH2H3C

CH2

CH3

23

4 56 7

8 9

10

11

1213

14

15

1617

1819

2021

22

Table 3The excitation and emission wavelength at MPW1PW91/6-31++G(d,p) level of theory.

Solvent kExc

Theoretical E (eV) Oscillator stren

Ethanol 371.47 (361.07) 3.337 (3.433) 0.3953Acetonitrile 368.80 (361.07) 3.361 (3.433) 0.3944

Values in parentheses are the experimental.

Table 4Charge distribution and dipole moment using acetonitrile and ethanol as solvents at MPW

Dipole moment (Debye)

GS9.5988 (9.5894)

Atom GS Charge ES Charge

C1 0.196 (0.204) 0.258 (0.265)O2 �0.194 (�0.214) �0.258 (�0.269)C3 �1.002 (�0.992) �0.935 (�0.933)C4 �0.353 (�0.382) �0.448 (�0.469)C5 0.322 (0.330) 0.119 (0.132)C6 0.707 (0.703) 0.633 (0.618)C7 0.309 (0.315) 0.728 (0.736)N8 �0.087 (�0.091) �0.458 (�0.455)N9 �0.260 (�0.260) 0.137 (0.135)C10 �0.453 (�0.450) �0.423 (�0.430)C11 �0.502 (�0.510) �0.616 (�0.608)C12 1.424 (1.463) 0.117 (0.118)C13 �0.955 (�1.011) 0.034 (0.024)N14 0.344 (0.338) 0.317 (0.311)C15 �0.084 (�0.090) �0.102 (�0.103)C16 �0.567 (�0.563) �0.544 (�0.543)C17 �0.864 (�0.870) �0.891 (�0.896)

Values in parenthesis are for ethanol as solvent.

3. Results and discussion

3.1. Spectroscopic experiments

Steady state fluorescence emission and excitation spectra of theoxadiazole 454Do, in solvents of different polarities [23], areshown in Fig. 2, and the excitation spectra are fully consistent withthe UV–vis spectra (see Supplementary information). The absor-bance, emission and excitation maxima and the quantum yields(/0) of the probe in each solvent are given in Table 1 (with allanalyses following previously published procedures [24–31]).

In ethanol/water mixtures, the fluorescence of the oxadiazole454Do was not affected significantly by changes in pH in the rangeof 2 to 12, indicating the absence of protonation of the probe with-in that range. It is interesting to note that while excitation andemission maxima show only rather small kMax shifts as a functionof the added solvent, the fluorescence quantum yields dependmarkedly on the nature of the solvent. In order to make reliablecomparisons, fluorescence quantum yield measurements weremade in triplicate using quinine sulfate as the standard. One cansee in Table 1 that the quantum yield in acetonitrile is highestamong the solvents, and that increasing water concentration inEtOH reduces the quantum yield. Clearly, the effect of solvent onthe spectral properties and quantum yield of the oxadiazole454Do is complex since compounds with widely different hydro-gen bond donor/acceptor capabilities (such as MeOH) reduce theintensity of the fluorescence emission, without a marked effecton the emission kMax.

Fluorescence lifetime measurements for the oxadiazole 454Dowere conducted in the same solvents. Fluorescence decay datawere fitted while considering that I(t) is the sum of the fluores-cence decays of the individual species in solution Ii(t), as given by:

kEm

gth Theoretical E (eV) Oscillator strength

562.70 (574.85) 2.203 (2.156) 0.5468568.07 (580.88) 2.182 (2.134) 0.5598

1PW91/6-31++G(d,p) level of theory.

ES17.4693 (17.4318)

Atom GS Charge ES charge

C18 0.213 (0.243) 0.155 (0.175)C19 �0.313 (�0.315) �0.247 (�0.240)O20 �0.227 (�0.221) �0.224 (�0.219)C21 0.256 (0.248) 0.612 (0.650)C22 �0.838 (�0.803) �0.838 (�0.870)H23 0.185 (0.185) 0.203 (0.199)H24 0.231 (0.199) 0.244 (0.241)H25 0.200 (0.285) 0.227 (0.226)H26 0.285 (0.209) 0.281 (0.280)H27 0.255 (0.255) 0.222 (0.222)H28 0.253 (0.252) 0.225 (0.224)H29 0.210 (0.210) 0.193 (0.190)H30 0.202 (0.203) 0.189 (0.188)H31 0.285 (0.283) 0.265 (0.261)H32 0.161 (0.165) 0.118 (0.124)H33 0.207 (0.209) 0.256 (0.260)H34 0.216 (0.216) 0.243 (0.241)

Fig. 5. Change in dipole moment and HOMO–LUMO molecular orbitals.

80 J.A. Pedro et al. / Journal of Molecular Structure 1016 (2012) 76–81

IðtÞ ¼X

IiðtÞ ¼X

Aieð�t=siÞ ð1Þ

where si is the fluorescence decay time and Ai the preexponentialcoefficient of each individual fluorescent species [24–28]. Datatreatment initially used a procedure known as iterative reconvolu-tion to remove the convolution distortion imposed by the finitetemporal width of the excitation pulse and, therefore, allowingthe generation of the actual I(t) decay curve. Then the fitting pro-gram, based on the Marquadt algorithm [32], analyzed the I(t) decaywith the possibility of up to two individual exponential compo-nents. Typical examples of the fluorescence decay curve areillustrated in Fig. 3. Calculated values of fluorescence lifetime andpre-exponential factors obtained from data similar to that inFig. 3, are also given in Table 1.

The lifetimes in Table 1 show that in all cases, although we havea single fluorophore, bi-exponential decays are observed (Eq. (1)).The average lifetimes sAv, also included in Table 1, show that thechanges in quantum yields are accompanied, in all cases, by aconsistent decrease in lifetime. In acetonitrile, a dipolar aprotic sol-vent, the component with the shorter fluorescence lifetime (s1) isresponsible for a fractional contribution f1 = 0.039 to the observedsteady state fluorescence intensity, while the component with alonger lifetime, s2, corresponds to a fractional contributionf2 = 0.961 (see Table 1). Since we have a single fluorophore, mostprobably the results obtained in acetonitrile reflect different con-formations in the ground state. In the presence of ethanol andmethanol, a significantly smaller quantum yield is observed and,in both cases, the fractional contribution to the observed steadystate fluorescence of the component with shorter lifetime contrib-utes 63–65% of the observed intensity. Clearly, hydrogen bondingaffects significantly the quantum yield, probably due solvent rear-rangement in the solvation shell (vide infra). Addition of water toethanol (see Table 1) results in the decrease of both the longerand shorter lifetime components of the observed decay, whileincreasing the water content results in a significant decrease ofthe quantum yield. Obviously, in mixed solvents, the actual mean-ing of the pre-exponential factors A1 and A2 may well be related topreferential solvation with different degrees of exposures to waterin the first solvation shell. In any event, the increase in hydrogenbonding ability of the solvent opens new routes for deactivationof the excited state.

In order to understand better the observed solvent effect onthe quantum yields and lifetimes of the probe in different sol-vents, we analyzed the structure of 454DO as will now bediscussed.

3.2. Theoretical calculations

In order to obtain the global minimum for this compound, weperformed the Potential Energy Surface (PSE) for 454Et as shownin Fig. 4. The global minimum in the GS as well as in the ES, corre-spond to dihedral angles values of 90� and 180� for (C(1)–N(2)–C(3)–C(4) and C(5)–C(6)–O(7)–C(8)), respectively (Fig. 4, Table 2).Additionally, dihedral angles C5–C6–C7–N8 and N9–C10–C11–C12near zero show planarity and a resonance effect among these threerings (Table 2).

To confirm that the model employed can be used to describethis system, calculations of the excitation wavelength were per-formed, employing both 454Do and 454Et. For the calculationsusing water as solvent, the excitation wavelengths for 454Do and454Et were 356.4 nm and 357 nm, respectively. Similarly, whenacetonitrile was examined, the excitation wavelengths for 454Doand 454Et are 369.1 nm and 369.5 nm, respectively. The similari-ties in these values suggest that the model molecule is adequateto this study. In this sense, the TD-DFT optimization on the excitedstate was made using 454Et.

Our calculations by TD-DFT at MPW1PW91/6-31++G(d,p) levelof theory, are shown in Table 3, and showed that the electronictransitions are allowed by orbital symmetry, with both HOMOand LUMO in group A in terms of symmetry. The oscillatorstrengths for both excitation and emission were found to be great-er than zero. Reasonably good agreement was found between the-oretical and experimental emission and excitation wavelengths,with difference in energy <0.1 eV (<3% error). The results clearlyshow that the changes in quantum yields are not related to anyfundamental change in the structure of the probe, but are mostprobably related to specific interactions between solvent andprobe which are transparent to the theoretical analysis using TD-DFT at MPW1PW91/6-31++G(d,p) level of theory. It is importantto remark that the PCM method does not consider any specificinteractions between the probe and the solvent; the calculatedresults are consistent with the small difference in dielectric con-stant between acetonitrile and ethanol. In fact, a kinetic/statisticanalysis using explicit solvent molecules in the calculation wouldbe necessary to fully understand this system.

The charge distribution was performed using Mulliken analysis.The greater change was observed over the following atoms N8, N9,C12 and C13. These changes in charge distribution cause an impor-tant change in magnitude and direction of dipole moment of ex-cited state (le) with respect to ground state (lg), (Table 4, Fig. 5).Clearly, this change in dipole will result in a significant solvent

J.A. Pedro et al. / Journal of Molecular Structure 1016 (2012) 76–81 81

reorganization in the excited state, which should follow immedi-ately after the Frank Condon transition in a fast process whicheffectively depopulates the excited state. As discussed by others,the depopulation results from solvent reorientation around the ex-cited state dipole, coupled with electron redistribution in the sol-vent provoked by the increase in the dipole moment of theexcited fluorophore that alters fluorophore-solvent interactions(including hydrogen bonding) [24]. The hydrogen bond donor sol-vents, which are effectively interacting with the 454Do probe, willbe submitted to a stronger solvent relaxation process. The HOMOmolecular orbital shows that the preferential interaction of hydro-gen-bond donors will most probably be in the direction of theplane containing the N9 atom in the oxadiazole ring. In the LUMOmolecular orbitals, the larger dipole and changes in charge distri-bution promote an increase in negative charge at N8. Certainly,the increase in dipole moment will promote solvent redistributionand relaxation and, as a consequence, decrease the experimentallyobserved quantum yield [24,33], a theoretical expectation which isconsistent with the observed experimental findings.

4. Conclusions

The liquid crystalline probe 1-dodecyl-4-[5-(4-dodecyloxyphe-nyl)-1,3,4-oxadiazole-2-yl] pyridinium bromide shows interestingphotophysical properties. Excitation and emission spectra of theprobe in different environments result in significant changes inquantum yields which are correlated with changes in the dipoleof the probe when promoted from the ground to the excited state.The fluorescence of the oxadiazole 454Do is independent of pH inethanol/water mixtures, in the range of 2–12, indicating the ab-sence of protonation of the probe in that particular range. Theoret-ical calculations are consistent with the experimental results andindicate a big change in the dipole between ground and excitedstate. Theoretical calculations are consistent with the experimentalresults and indicate that the large change in dipole moment willpromote a significant solvent reorganization and relaxation inthe excited state, immediately after the Frank Condon transition,in a fast process that effectively depopulates the excited state.

Acknowledgements

This work was supported financially by CAPES, INCT-Catalysis,FAPESC, CNPq and FAPERGS. Special thanks to Professor F. Menger,Emory University, for his help with the final version.

Appendix A. Supplementary information

Synthesis and characterization of the liquid crystal 454Do.Tables of cartesian coordinates and data from computational calcu-lations. Liquid Crystal Characterization. Supplementary data asso-ciated with this article can be found, in the online version, atdoi:10.1016/j.molstruc.2012.02.046.

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