Structure-Property Relationship in Ï-Conjugated Bipyridine Derivatives

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C: Energy Conversion and Storage; Energy and Charge Transport

Structure-Property Relationship in #-Conjugated Bipyridine Derivatives: Effect of Acceptor and Donor Moieties on Molecular Behavior Nuriye Demir, Gül Yakal#, Merve Karaman, Yenal Gökpek, Serpil Denizalti, Hakan Bilgili, Bircan Dindar, #erafettin Demiç, and Mustafa Can J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05894 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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The Journal of Physical Chemistry

Structure-Property Relationship in πConjugated Bipyridine Derivatives: Effect of Acceptor and Donor Moieties on Molecular Behavior N. Demir, § G. Yakalı,# M. Karaman,# Y. Gökpek,† S. Denizaltı,ֆ H. Bilgili,‡ B. Dindar,§* Ş. Demiç,† M. Can #* §

Solar Energy Institute, Ege University, 35100 Bornova, Izmir, Turkey

#

Department of Engineering Sciences, Faculty of Engineering, Izmir Katip Celebi University, Cigli,

35620 Izmir, Turkey † Department of Material Sciences and Engineering, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey ֆ

Department of Inorganic Chemistry, Faculty of Sciences Department, Ege University, 35100

Bornova, Izmir, Turkey ‡

The Central Research Laboratory, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey

ABSTRACT: Here we reported the photophysical studies, single crystal X-ray crystallography (SCXRD) and Density Functionality Theory (DFT) calculations of two bipyridine derivative molecules. In addition, thermal gravimetric analysis (TGA) and Cyclic Voltammetry (CV) studies were also performed for both compounds. According to crystallographic data, the π-conjugated molecules have high-quality crystal structures as a result of intramolecular and intermolecular hydrogen bonds occuring through the molecules of the compound. It was determined that when the functional groups (F— and CH3O—) were introduced to the para positions, the molecules adopted slipped stacking (J-aggregate) and antiparallel cofacial stacking (H-aggregate). It was observed that these two bipyridine derivatives disclose the relationship between molecular conformation-molecular packing modes and photophysical behavior of organic solids. The results of DFT calculations supported the structural, spectroscopic and photophysical data and confirmed the compositions of frontier molecular orbitals in both molecules.

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INTRODUCTION The design and synthesis of π-conjugated organic electronic and optical materials have attracted remarkable interest in the area of their important applications in organic field effect transistors (OFET) 1

, organic light-emitting diodes (OLEDs) 2, organic light-electrochemical cells (OLECs) 3, organic thin-

film transistors (OTFTs) 4 and organic photovoltaics (OPV) 5,6. It is interesting to note that the optical properties of such structures comprising a combination of a donor and an acceptor groups can be adjusted between the classes of organic π-conjugates 7. In this respect, the symmetrical 2,2ʹ-bipyridine (bpy) ligand can be easily functionalized at the pyridine sites 8,9. On the other hand, the optical properties of these ligands can be tuned by the conjugation, donor end-capped functionalities, or a metal complexation

10,11

. The pyridine is a good electron accepting group, and therefore it can be easily

designed and derivitized by a closure of various donor portions introduced into the pyridine ring 12,13. The device performance and optoelectronic properties of π-conjugated materials are determined by the chemical structure of the comprised molecules and the investigation of intermolecular electronic interactions occurring between the molecules of the compounds. There are significantly four types of intermolecular interactions: hydrogen bonding, dispersion, electrostatic and attractive-repulsive orbital interactions 14–16. These interactions are ruled by the packing assembly of the π-conjugated molecules. In device applications of organic π-conjugated systems, the molecular arrangement and assembly of molecules contribute heavily to an enhance the device performance. Therefore, as well as increasing device performance, investigation of the relationship between molecular interactions and packing structures is also important for determining the ratio of charge transport in the medium 17,18. For instance, molecular packing through stacking interactions, which is extensively found in chemical, biological, and material systems creates one of the most significant charge transport channels for small molecules . An increased planarity in the π-conjugated materials and better overlapping between π-orbitals of

17

neighboring molecules induce the delocalization of the polarons or excitons, which provides a reduction in the activation barrier for charge transport and an increment in the device performance 19. The stacking interactions between two aromatic rings can be cofacial, parallel-displaced and edgeto-face stacking. The π-π stacking modes of conjugated molecules can be divided into four main types which are herringbone, slipped (J-aggregation), bricked layer and cofacial stacking modes (Haggregation) according to different packing geometries between the adjacent molecules 17,20. The optical properties of H- and J-aggregates correlate with their packing mode. Indeed, the difference in aggregated structure is due to the different slipping angles that are pitch and roll angles of the stacked molecules. The pitch angles describe the translation of adjacent molecules relative to one another in the direction of the long molecular axis while roll angles describe the translation of the molecules along the short molecular axis19,21. Therefore, partially large pitch angles preserve π-π interactions between adjacent molecules, whereas roll translations destroy π-π overlap between adjacent molecules. According to this information, the herringbone structure is formed by large roll translations. 2 ACS Paragon Plus Environment

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In optoelectronic device applications, emission characteristics of molecules can be adjusted by the modification of chemical structure 19. As known for many years, the π-conjugated molecules show strong emissions in dilute solutions, but partially or even completely quenched emissions which are attributed to intermolecular interactions leading to the formation of excimers and exciplexes in the solid state. But, this phenomenon has recently been reversed so that these molecules may have strong emissions with solid aggregate formation, while they may exhibit poor emission or non-emission properties in the solution phase19,20,22. In addition, several recent studies have indicated that molecular aggregation can lead to a significant increase in the luminescence efficiency. Such an aggregate-induced emission (AIE) has been attributed to variety of factors, including restricted molecular motion in the aggregates and planarization of the molecule

23

. Consequently, several design strategies have been

developed to alter organic molecular packing, such as an extension of π-conjugation to enhance π-π interactions between adjacent molecules 24–26, incorporation of heteroatoms into conjugated backbones to provide more noncovalent interactions and substitution with bulky groups. In this study, we incorporated fluorine and methoxy group to a molecule’s backbone. Fluorine atoms are frequently introduced into organic molecules to effectively lower their frontier orbital levels due to the strong electron withdrawing properties 27–29. Fluorination of organic molecules produces the rigid and planar backbone by forming intramolecular hydrogen bonds. In addition, it significantly contributes to intramolecules charge transport 30–32. However, electron donating groups such as methoxy have quite a different effect on π-delocalization of conjugated systems when bonded at the ortho-, para, or meta- position. The highest fluorescence quantum yield is obtained from substitution at the para position 33. This case can be much beneficial for the intermolecular electrostatic interaction or dipole– dipole interaction and possibly leading to the enhanced π–π attraction and strong π–π stacking. We mainly focus on the viewpoint of how molecular conformation and packing structures affects the photophysical properties based on π-conjugated 2,2ʹ-bipyridine derivatives structures in crystalline and solution state. The molecules were analyzed with photophysical studies, single crystal X-ray crystallography (SCXRD), TGA analysis, Cyclic Voltammetry (CV) measurement, and Density Functionality Theory (DFT) calculations. It is worth mentioning that, the single crystal structure of the optoelectronic materials are very important for the detailed investigation of molecular conformation and molecular arrangement modes. As well as the experimental studies, theoretical results which carried out by ORCA software on π-conjugated materials have contributed significant information toward the understanding of photophysical properties of our molecules.



Experimental Section Materials All of the chemicals were purchased from Sigma-Aldrich, TCI Chemical Co. or Riedel de Haen at the highest purity available. The solvents and reagents from commercial suppliers were used without further purification. The column chromatography was performed using Merck silica gels (230−400 mesh). Synthesis General procedure for the synthesis of ligands NM and NM1: The chemical structures of ancillary ligands (NM and NM1) are shown in Figure 1. 4-Fluoro(or methoxy)phenylboronic acid (2 mmol), 4,4'-dibromo-2,2'-bipyridine (0.8 mmol), dimethoxyethane (DME) (16 mL), and aqueous potasium carbonate (K2CO3) solution (2 M, 7 mL) were added to twoneck glass flask. After purging the mixture with argon for 10 minutes, [Pd(dppfCl2)] (5 mol % based on 4,4'-dibromo-2,2'-bipyridine 0.046 g) was added, and the whole mixture was refluxed for 24 h under agon atmosphere. Then the mixture was cooled to room temperature. The aqueous layer was separated from the organic one and then extracted with CH2Cl2 (3 x 10 mL). The combined organic layers were washed with pure water and the solvent was removed under reduced pressure. The crude product was purified by column chromatography (silica gel, CH2Cl2/MeOH; 15:1), and the pure product was given as off white powder. Then it was recrystallized from dichloromethane/hexane mixture. NM: Yield: 72%. 1H NMR (400 MHz, δ, CDCl3): 8.68 (4H, m), 7.75 (4H, m), 7.50 (2H, dd), 7.18 (4H, t). 13C NMR (100 MHz, δ, CDCl3): 164.69, 156.53, 149.66, 148.32, 134.34, 128.97, 121.53, 118.99, 116.15 (Figure S1and S2). NM1: Yield: 76%. 1H NMR (400 MHz, δ, CDCl3): 8.70 ppm (4H, td), 7.74 ppm (4H, ds), 7.52 ppm (2H, dd), 7.04 ppm (4H, ds) and 3.86 ppm (6H, s). 13C NMR (101 MHz, δ, CDCl3): 160.53, 156.67, 149.53, 148.93, 130.56, 128.32, 121.01, 118.56, 114.43, 55.61 (Figure S3 and S4).

Figure 1. Synthesis of 4,4'-bis(p-fluorophenyl)-2,2-bipyridine (NM) and 4,4'-bis(p-methoxyphenyl)2,2-bipyridine (NM1).


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Instrumentation 1

H and 13C NMR spectra of NM and NM1 ligands in chloroform (CDCl3) were recorded on a

Bruker AM-400 (400MHz) NMR spectrometer with tetramethylsilane ( TMS, δ = 0 ppm) as an internal standard. Electrochemical characterizations of the ligands were performed on Autolab 204 model electrochemical device with a three-electrode cell system (a glassy carbon working electrode, a platinum wire

counter-electrode,

and

Ag/AgNO3

reference

electrode).

0.1M

Tetrabutylammonium

hexafluorophosphate (TBAPF6) was used as supporting electrolyte after purification from pure dichloromethane. Thermogravimetric analysis of the samples was carried out using a thermal gravimetric analysis (TGA) instrument model Q600 from TA Instruments, under high purity nitrogen gas supplied at a constant 100 ml min−1 of flow rate. The UV-Vis absorption and photoluminescence (PL) spectra of ligands in dichloromethane (CH2Cl2) were carried out by Perkin Elmer Lambda 950 UV/VIS/NIR spectrometer and Edinburg Instruments FLS900P spectra photometers, respectively. The crystalline photographic was obtained by using the Olympus CH-2 Binocular Microscope and UV lamp with 365 nm UV irritation.

X-Ray Crystallographic Studies The single crystals of 4,4’-bis(4-fluorophenyl)-2,2’-bipyridine (NM) and 4,4’-bis(4methoxyphenyl)-2,2’-bipyridine (NM1) were selected for an X-ray crystallographic study. These crystals were recorded on a Rigaku-Oxford Xcalibur diffractometer with an Eos Charge Coupled Device (CCD) detector using graphite-monochromated Mo Ka radiation (k = 0.71073 A°) with CrysAlisPro software. The data reduction, scaling, and analytical absorption corrections were performed using CrysAlisPro 34. The structure solution and full-matrix least-squares refinement based on F2 for both compounds were performed by the direct methods with SHELXT

35

and SHELXL

36

, respectively,

incorporated into the OLEX2 program package 37. Anisotropic thermal parameters were applied to all of the non-hydrogen atoms. Hydrogen atom positions in both molecules were calculated geometrically and refined using the riding model. Crystallographic details are indicated in Table 1. Computational Studies The chemical interactions between molecules can be explained by the frontier orbitals. The term "frontier orbitals" refer both to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Most chemical reactions occur by giving or receiving electrons, thus HOMO and LUMO levels are under the direct effect managed by the molecular structures. These molecular orbitals can be calculated and visualized by some approximations and software. Calculations were performed on the ORCA version 4.0.1.2 program package. For the geometry optimization and excited states, B3LYP functional with 6-311G (d, p) basis set is used

38,39


. The first 50 excited states were


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calculated. Calculations were performed in gas phase and in dichloromethane solvent. All images of structures and molecular orbitals are generated by Chemcraft 1.8. Results and Discussion TGA Analysis The thermal properties of the ligands were investigated by the TGA. The TGA curves are shown in Figure S5. The thermal stabilities of NM and NM1 ligands were subjected by 10 °C min−1 heating rate and were characterized from 10 °C to 600 °C under nitrogen atmosphere. The results revealed that the ligands have exceptional thermal stability. The 5% mass loss was observed at 309 °C for NM, while this loss for NM1 was at 277 °C. The ligand with C—F groups is more stable than those with C—OMe groups (Td = 407 °C and 323 °C for NM and NM1, respectively).

Electrochemical Properties The electrochemical properties of the ligands NM and NM1 were studied by cyclic voltammetry (CV) measurements in solution of 0.1 M solution of TBAPF6 dissolved in CH2Cl2. The cyclic voltammograms of NM and NM1 are displayed in Figure S6 and the redox potentials are summarized in Table 2. The ligand NM showed single reduction peak at ‒ 1.24 V and the corresponding oxidation peak is near ‒ 0.39 V. The reduction peak of NM1 was found near ‒1.18 V with the corresponding oxidation peak is near ‒0.35 V. The LUMO energy levels of NM and NM1 are calculated to be ‒ 3.16 eV and ‒3.22 eV, respectively, according to Equations S1-S3. The HOMO energy levels of ligands were calculated by using their optical band gap (Egopt). One of the most common approaches used in the determination of the optical band gap of semiconductors in literature is the wavelength corresponding to the maximum absorption band edge

40,41

. This onset

wavelength value was used for the calculation and given in equation 4 of Supplementary Information. The optical band gap results for NM and NM1 are 3.80 eV and 3.73 eV, respectively. The HOMO energy levels of NM and NM1 ligands are calculated as ‒6.96 eV and ‒6.95 eV with Equation S3, respectively. From the electrochemical study of 4,4ʹ-bipyridine, it was only observed a reduction peak around ‒1.30 V as reported in the literature 42. The reduction potential of NM with an electron-withdrawing group was 0.06 V higher than that of NM1 having an electron-donating group substitution. This result can be explained by the fact that electron-withdrawing groups increase the electron affinity of the organic system and thus causes an increase in the redox potential 43.


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Photophysical Studies Spectroscopic Properties Even though NM and NM1 ligands are known molecules, their optical properties have never been studied, probably due to the emission at the UV-violet border. According to the UV-vis absorption spectra of NM and NM1 ligands (5 × 10−6 M) in dichloromethane shown in Figure S7, while NM possess two intense absorption bands at 250 and 298 nm NM1 ligand has an absorption bands at 260 nm and 285 nm. These absorption bands are all attributed to π→π* transitions 44. As expected, NM emission has shifted to blue region with respect to NM1 by the effect of electron attractive atom fluorine. The luminescense behavior of NM and NM1 display emissions at 356 nm and 380 nm upon excitation at 298 and 285 nm, respectively. Upon illumination of a 365 nm UV-lamp, ligand crystals support emission in the UV-violet region (Figure 2). As shown in Table 3, the large Stokes shift of about 100 nm for all ligands is based on the large energetic difference between the excited and the ground state of the structures 45.

Figure 2. Photographic images of large amount of crystals under daylight (a and c for NM and NM1, respectively) and the 365 nm UV irritation under ambient conditions (b and d for NM and NM1, respectively).



The emission properties of the molecules in solid- and solution-states show the difference (Figure 3). It is shown that the absorption spectrum of NM exhibits absorption maxima at 250 and 300 nm in CH2Cl2. The absorption spectrum of NM at solid-state shows absorption maxima at 287 nm and 320 nm. The compound NM also exhibits similar behavior and its solution and solid-state emission peaks were observed at 354 and the 391 nm, respectively (Figure 3a). The compound NM1 has 2 distinct absorption peaks, both in solid (at 267 and 292 nm) and liquid phases (as CH2Cl2 solution at 330 nm and 361 nm). It also exhibits liquid and solidphase emission peaks at 380 and the 400 nm, respectively. From its absorption spectra recorded in solution and solid state, it is observed that there is about 20-40 nm redshift which signifies the presence of intermolecular interactions between the molecules (Figure 3b). The change in color of the emited light is attributed due to the aggregation effect. This also affects the fluorescence intensity as well as the efficiencies of these materials.

Figure 3. Solution phase and film phase UV−vis absorption and PL spectra of a) NM and b) NM1 ligands.

It is believed that NM ligand represents H-aggregation while NM1 has a J- aggregation as a result of the intermolecular and intramolecular interactions occurring between their individual molecules. In other words, the NM1 ligand has a J-type packing because of its planar structure . Its solid-state emission intensity is higher than the one observed in solution phase because of the aggregate-induced emission (AIE). The luminescent intensity in liquid phase is weak while the formation of aggregates in the solid state increases the emission intensity attributed to AIE. The PL intensity of NM in the solid state is decreased relatively the molecule NM in the solution (Figure S8). This is explained by the packing mode of the molecules of NM as H-aggregation or cofacial π…π stacking mode. Also, The PL of the ligands display single exponential decay in solid state with excited state lifetimes (τ) of 2.39 and 2.33 ns for NM and NM1, respectively (Figure S9-S10). The excited state lifetimes of both compounds are comparable due to their structural similarity. The detailed absorption, emission and lifetime characteristics of these two compounds are summarized in Table 3. 8 ACS Paragon Plus Environment

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Crystallographic Studies Single crystals of molecule NM and NM1 for X-ray diffraction were grown from dichloromethane/hexane mixture. The bulky groups bound to the extremity of the compounds resulted in significant changes in molecular packaging in solid state. Different types of crystal packaging are illuminated by single crystal analysis. The molecular views are demonstrated in Figure 4 and selected bond lengths and angles are listed in Table 4. For a deep understanding of the different PL behavior, crystal structures of the molecule NM and NM1 are analyzed by single crystal X-ray diffraction studies. SCXRD studies has revealed that the structure of NM crystallizes in monoclinic crystal system with P21/c space group, whereas the structure of NM1 crystallizes in orthorhombic crystal system with P21/c space group. Both of the molecules contain half individual molecule in the asymmetric unit. Another individual half molecule is generated by symmetry operator 1-x, -y, 1-z for NM and –x, 1-y, 1-z for NM1. All of the molecules lie in inversion centers, therefore the π-conjugated part of the molecule is seen to be highly planar.

a)

b)

Figure 4. Single Crystal X-ray Diffraction ORTEP image of a) NM and b) NM1 ligands.

The crystal NM: The single crystal X-ray structure of the molecule NM showed that the compound is composed of bipyridine molecule and fluorine atoms which are bonded at para position



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relative to the terminal site of the backbone of the molecule. The pyridine ring and benzene ring of the molecule are nearly planar with respect to the imaginary molecular plane, forming dihedral angles of 14.35 (11)° and 13.26 (11)°, respectively. In addition, the dihedral angle between the pyridine ring and the benzene ring is 27.60 (14)°. This value indicates that the NM molecule has a slightly twisted conformation (Figure 5a). The value of the torsional angle of C4-C3-C6-C78 is 26.6 (4). The crystal NM1: The single crystal X-ray structure of NM1 molecule indicates that the compound is composed of bipyridine molecule having dimethoxy groups which is bounded from the para position of the terminal region of the backbone of the molecule . According to similar reported structure, the highest PL intensity is obtained by substituting at para positions

46

. The pyridine and

benzene rings of the molecule are planar to molecule plane, forming dihedral angles of 1.46 (11)° and 3.46 (12)°, respectively. In addition, the dihedral angle of between the pyridine and benzene rings is 4.57 (14)°. The torsional angle value of C2-C3-C6-C7 is 4.6(5)° (Figure 5b). These values demonstrates that the molecule NM1 has planar conformation in contrast to molecule NM. All the geometrical parameters of both compounds are compared with similar reported structures 46,47. The structural features of the molecule were well reproduced in the theoretical models obtained by the density functional theory (DFT: B3LYP/6-311G(d,p) calculations. According to the theoretical studies, the largest difference between the experimental and calculated bond angles and bond lengths are 1.0° and 0.02 Å, respectively. These discrepancies can be explained by the fact that the calculations assume an isolated molecule.

Figure 5. Molecular conformations of the crystal (a) NM and (b) NM1 with their phenyl-phenyl rings dihedral angles.


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Relationship between the molecular conformation and packing modes and PL intensity: In this study, in order to understand the effect of molecular conformation and packing modes on the photophysical behaviors, the acceptor (F—) and donor (CH3O—) groups are introduced into the terminus of the molecular backbones of NM and NM1, respectively. These groups enhanced π-π attraction and strong π-π stacking through the molecules. In addition, the internal rotations are restricted by the occurance of hydrogen bonds and intramolecular interactions. Therefore the molecule adopt to nearly planar and rigid conformation. This conformation results in high interchain charge transport. In this case, the crystal structures of the molecules demonstrate different emission intensities based on the molecular conformation and packing structures. Understanding the different crystal packing modes (e.g. from herringbone to a cofacial π-stacking motif) is of importance to confer good electronic transfer properties 48. In both molecules, the packing structures of the crystals affected by pyridine ring, the position of the nitrogen atom within the pyridyl core, and the donor&acceptor atoms/groups (F— and CH3O—) play important role on their PL behavior. For NM crystal, we observed two distinct packing structures. It is worth mentioning that in the first packing structure, each individual molecule in crystal NM can generate very strong C—H…N nonclassical intermolecular hydrogen bonds, which helps forming the dimer structure of NM (C10H10…N1= 175.8(18)°, Table 5). Additionally, each molecule is connected to the adjacent molecule through C—H…π interactions (Cg2: C6\C11, H…Cg2=2.86(3) Å, C—H…Cg =126.9(19) , C…Cg=3.552(3) Å, symmetry code: 3/2-x, 1/2+y,z). These intermolecular interactions are formed 1D H-aggregation mode supramolecular structure along the b axis (Figure S11). The H-aggregation mode enhances π…π overlapping that would adversely affect the photophysical behavior 49. The other packing structure of the molecule NM is formed via face to face π…π (Cg1: N1-C1/C5; Cg1…Cg1=4.139(2); symmetry code: -x, 3-y, 1-z; centroid slippage=1.778 Å) stacking interactions and nonclassical strong intermolecular hydrogen bonds (C10-H10…N1) (Table 5). With these interactions, the crystal NM pack into 2D H-aggregation mode supramolecular helical stacking interactions along the bc plane (Figure 6c). For NM1 crystal, as similarly NM crystal, two different packing modes are occurred. The one is that 1D supramolecular packing structure is stabilized via strong intermolecular hydrogen bonds C— H…N (C4—H4…N1= 100.8(3)°, Table 5) which formed the dimeric structure as shown in Figure S12, nonclassical weak intramolecular interactions C—H…N (C4—H4…N1= 100.8(3)°, Table 5) and strong C—H…π (Cg2: C6\C11, H…Cg2=2.99(14)Å, C-H…Cg=139.5(3)°, C…Cg=3.747(4)Å, symmetry code: 1-x, 1/2+y, 3/2-z) interactions along the b axis. This packing mode occurs as antiparallel displaced configuration or closely edge to face interactions. The other packing mode is that 3D supramolecular packing structure is stabilised by O…H short interactions whose value is shorter than Van der Waals radii which is 2.72 Å (O1…H12B=2.675 (7)Å) and π…π stacking interactions (Cg1: N1-C1/C5; 11 ACS Paragon Plus Environment


Cg1…Cg1=4.161(9) Å; symmetry code:2-x, 1/2+y, 3/2-z; centroid slippage=4.116) along the abc plane (Figure 6d). The mode of 3D supramolecular packing structure is named J- aggregation mode due to forming antiparallel slipped stacking. It is known that the face-to-face π–π interactions, which is called H-aggregation that results in cofacial configurations, increases the amount of π–π orbital overlap and thus are detrimental to the luminescence of the organic structures whereas J-aggregation mode enhances the fluorescence intensity. In other words, the rigid planar structure together with the J-type packing makes the crystal highly PL. The optical properties of NM and NM1 compounds were measured and the values are listed in Table 3. The similar absorption bands in solution phase demonstrate that introducing fluorine atom and methoxy groups at the terminal site of the molecule's backbone has negligible effect on their UV absorption characteristics. Comparing the emission behaviors of the structures, NM1 showed quenched emission behavior in contrast to NM in solution phase (Figure S8 ). However crystal states of these molecules show different emission behavior in the solid state (Table 3). In addition, the PL obtained from the samples in crystalline form is largely red-shifted when compared to those ones corresponding to the solution sample. This observation demonstrated the great impact of aggregation in the PL behavior of these materials. The PL intensity of the molecule NM1 is very low, because it is nearly quenched in the solution phase whereas this intensity dramatically increases in the solid state as shown in Figure S8. But the PL intensity of the molecule NM in the solution is higher than in the solid state. These discrepancies are attributed to the packing mode of molecule NM1 (J-aggregation or slipped π…π stacking) while packing mode of molecule NM is H-aggregation or cofacial π…π stacking. Another important discrepancy is related to molecular planarity, which occurs when the different bulky groups are incorporated into the molecular backbones, that results as the conformational change of both molecules. This indicates that introducing different substituent ot the molecule's backbone has an important effect on the molecular conformation and packing structure of the formed crystal samples. The differences dramatically affect emission behaviors of the molecules. The twisted molecular structure character is not beneficial for luminescent materials. The more planar conformation the individual molecule adopts, the brighter emission the crystal sample has. In the crystal of NM1, the molecule arranges itself in 3D supramolecular network with a pitch distance of 0.12 Å and the roll angles of 4.15° between adjacent molecules with a slipping angle of 50.3°. In the crystal of NM, the molecule arranges itself in 2D supramolecular network with a pitch distance of 3.96° and the roll distance of 1.17 Å between adjacent molecules with a big slipping angle of 75° (Figure 6d-h). Thiee pitch and roll distances are calculated according to one of the models of Janzenet et al. and Curtis et al.(2004)

. Thus, moderately large pitch distortions preserve π…π

19

interactions between adjacent molecules, whereas roll translations greater than 2.5 Å, which is approximately equal to a width of a benzene ring, essentially nearly destroy π…π overlap between 12 ACS Paragon Plus Environment

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adjacent molecules and thus the packing mode change from H-aggregation to J-aggregation. Although the interactions of NM are not strictly cofacial, it can be stated that they allow substantial-overlap between adjacent molecules, since the pitch distance (or angle) of interactions are longer than their corresponding roll distances (or angles). According to crystallographic data, it can be briefly said that these π-conjugated molecules have high-quality crystal structure because of intramolecular and intermolecular hydrogen bonds in the structures. We demonstrated that if different groups like fluorine (F) and methoxy (CH3O) at a para position are introduced on the backbone of the molecules, structures may adopt either slipped stacking or antiparallel cofacial stacking, therefore subtly changing the molecular structure and affecting the molecular packing in the crystal. In the light of this knowledge, the crystal structures of the molecules demonstrated different PL behaviors which were merely associated to their molecular conformations and packing structures with stacking interactions on the basis of photophysical data, crystallography, and Density Functional Theory.


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Figure 6. Demonstration of molecular structures (a, e), pitch angles (P) and intermolecular distance (d) (b, f), pitch (dP) and roll distances (dR) (h, g), packing modes (d, c) of the compounds. 14 ACS Paragon Plus Environment

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Theoretical Calculations The theoretical calculations for the organic structures are carried out with popular and wellstudied basis set, which has been proven to be both reliable and cost-effective. The results of the calculations showed that the stable geometries of the molecules and their geometrical parameters are in good accordance with the single crystal X-ray studies (Figure 7a-b). The biggest difference in bond lengths are 0.013 and 0.012 Å for NM and NM1, respectively. Similar results were obtained when the bond angles of the structures are compared 0.94° and 1.41° bond angle differences for NM and NM1, respectively, are to be found the biggest difference compared to the crystallographic results. These differences are both acceptable when the intermolecular interactions are neglected in single molecule calculations (Table 4).

a)

b)

Figure 7. The optimized geometries of the molecules (a) NM and (b) NM1 The HOMO and LUMO frontier molecular orbitals of all the ligands are shown in Figure 8, which presented that the HOMO orbitals are localized at the donating groups and LUMO orbitals are closer to the acceptor groups. For NM molecule HOMO is localized at the pyridine rings, contrary to the HOMO localization of NM1, which is closer to the methoxy groups. This result also provides great insight that changing substituted groups (F— and CH3O—) affects the orbital localization dramatically. LUMO 15 ACS Paragon Plus Environment


orbital of NM is localized mostly on the pyridine ring, rather than F—atom part, similar to NM1 structure. The reason for this unexpected result can be seen clearly after the excited state calculations. For NM1 structure, the contribution to the first excited state is mostly HOMO → LUMO, whereas the contribution to the first excited state of NM is mostly HOMO−3 → LUMO. This result may explain the unexpected LUMO localization on the molecule of NM. The dominant contribution of HOMO → LUMO transition appears at a fifth excited state. It is also worth noting that the energy difference between HOMO and HOMO−3 is relatively small (0.39 eV). The calculated gas and solution phase HOMO and LUMO values are larger than the experimental values. Also notice that most of the calculated gaps are larger than the experimental gaps, which indicates a systematic difference that might be correlated 50.

Figure 8. Theoretical calculations of HOMO, LUMO distributions on the ground state for a) NM and b) NM1 in the gas phase. In order to investigate the solvent effect, the geometry of the given structures is also optimized in 16 ACS Paragon Plus Environment

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dichloromethane solvent, which was the solvent where the experimental absorbance measurements were aslo carried out. It is clearly seen from the energy values of the optimized structures that the total energy of the NM1 in a solvent (‒1186.136 Hartree) is lower than the total energy of the same structure in the gas phase (‒1186.117 Hartree). The same situation is also true for NM molecule (‒1155.440 Hartree in a solvent and ‒1155.338 Hartree in the gas phase). Therefore, it is safe to assume that the molecules are more stable in dichloromethane solvent than in the gas phase. Conclusion In this study; the electrochemical, thermal and optical properties of previously synthesized molecules (NM and NM1) were investigated. In addition, these structures are presented with a single crystal X-ray and illuminated with theoretical computational studies. The larger emission peak of the NM1 can be explained by the rigid planar structure. The torsion angle of the NM1 molecule is smaller than the NM molecule, which means that the molecule has a more planar structure. The NM ligand has higher thermal stability than NM1 ligand, due to its intermolecular interaction. The fluorine atom, bound to the bipyridine structure in the current composition, led to an increase in the acceptor property of the ligand, thereby shifting the reduction potential to a more negative value than the bipyridine ligand. If there was a stronger electron-withdrawing group instead of fluorine, the difference of reduction potential is expected to be higher. Due to the inductive effect of the methoxy group, the ligand (NM) bound to the fluorine atom has a greater potential for reduction. The theoretical and experimental data of synthesized molecules were found very similar to each other. According to photophysical properties, both molecules showed similar absorption bands in the solution phase but taking the emission behavior into account, the NM1 showed quenched emission in contrast to NM in the solution (Figure S8 ). However crystal states of these molecules show different emission behavior from in the solid state. The PL intensity of the molecule NM1 is nearly quenched in the solution, whereas this intensity dramatically increases in the solid state. In contrast, the PL intensity of the molecule NM in the solution is higher than in the solid state. These discrepancies are attributed to different planar conformation and different aggregation mode of the molecules. The twisted molecular structural character is not beneficial for luminescent materials. Higher planarity provides brighter emissions. In addition, this observation demonstrated the great impact of J aggregation in fluorescence efficiency of NM1 crystal. We believe that these results may have a significant impact on the structureproperty relationship in material science and provide a good way for constructing an optoelectronic device.



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Table 1. Crystallographic details for the compounds. Name

Molecule NM

Molecule NM1

C22H14F2N2

C24H20N2O2

Formula weight

344.35

368.44

Temperature (K)

296(2)

296(2)

orthorhombic

monoclinic

Pbca

P21/c

a (Å)

13.7193(18)

10.870(2)

b (Å)

6.5431(6)

5.3493(5)

c (Å)

18.296(5)

15.953(2)

Empirical formula

Crystal system Space group Unit cell dimensions

99.205(16)

β (⁰) Volume (Å3)

1642.3(5)

915.6(2)

4

2

Dcalc (g/cm-3)

1.393

1.3362

Absorption coefficient (mm-1)

0.098

0.086

F (000)

712.0

388.2

0.501 × 0.234 × 0.136

0.544 × 0.388 × 0.096

h ranges

‒8→16

‒8→16

k range

‒7→4

‒7→4

l range

‒22→8

‒22→8

Reflections collected/unique

3296/1543

3080/1743

Data / restrains / parameters

1543/0/146

1743/0/128

1.030

0.977

R1 = 0.0597

R1 = 0.0597

wR2 = 0.1057

wR2 = 0.1057

R1 = 0.1287

R1 = 0.1287

wR2 = 0.1405

wR2 = 0.1405

0.14/‒0.16

0.36/‒0.37

Z

Crystal size (mm)

Goodness of fit on F2 Final R indices [I > 2σ(I)]

R indices (all data) Largest difference peak and hole (e Å-3)


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Table 2. Frontier orbital energy levels and band gap values (eV) Ligand

HOMO (cal)

LUMO (cal)

HOMO (cal)

LUMO (cal)

HOMO (exp)

LUMO (exp)

Eg (cal)

Eg (cal)

Egopt (exp)

NM

Gas ‒6.59 ‒1.74

CH2Cl2 ‒6.61 ‒1.80

CH2Cl2 ‒7.03 ‒3.23

Gas 4.85

CH2Cl2 4.81 3.80

NM1

‒5.94

‒6.02

‒7.06

4.50

4.34

‒1.44

‒1.68

‒3.33

3.73

Table 3. Optical data of compounds NM1 and NM at different phases. Ligand λabs/nm(a) NM1 260, 285

λemis/nm(a) λabs/nm(b) λemis/nm(b) Stokes Shift(a) 380 330 370, 390 120, 95

Stokes Shift(b) 40,60

τ/ns 2.33

NM

356

62, 90

2.39

250, 298

306

368, 396

144, 58

a) in CH2Cl2 solution (5 × 10−6 M); b)in solid state;



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Table 4. Selected experimental and calculated parameters of the molecules. Bond Lengths (Å) Experimental NM

Calculated

NM1

NM

NM1

F1-C9

1.364(4)

O1-C9

1.359(4)

F1-C9

1.350(6)

O1-C9

1.361(8)

N1-C1

1.343(4)

O1-C12

1.430(5)

N1-C1

1.342(3)

O1-C12

1.422(0)

N1-C5

1.333(4)

N1-C1

1.336(4)

N1-C5

1.332(7)

N1-C1

1.332(8)

C3-C6

1.483(4)

N1-C5

1.340(4)

C3-C6

1.483(5)

N1-C5

1.343(0)

C3-C4

1.493(4)

C3-C4

1.481(6)

Bond Angles( °) Experimental NM

Calculated

NM1

NM

NM1

F1-C9-C10

118.5(3)

C9-O1-C12

118.0(3)

F1-C9-C10

119.0(9)

C9-O1-C12

118.5(5)

F1-C9-C8

118.2(3)

C1-N1-C5

116.0(3)

F1-C9-C8

119.0(6)

C1-N1-C5

117.4(4)

N1-C1-C2

122.6(2)

N1-C1-C2

124.2(3)

N1-C1-C2

122.6(3)

N1-C1-C2

123.9(9)

N1-C5-C4

124.2(3)

O1-C9-C10

124.6(3)

N1-C5-C4

123.8(8)

O1-C9-C10

124.8(4)

C2-C3-C6

121.8(3)

O1-C9-C8

116.6(3)

C2-C3-C6

121.4(4)

O1-C9-C8

115.8(4)

C3-C6-C11

120.7(2)

C2-C3-C6

122.7(3)

C3-C6-C11

120.8(7)

C2-C3-C6

121.7(1)

C1-N1-C5

116.6(2)

C11-C6-C3

121.1(3)

C1-N1-C5

117.5(6)

C11-C6-C3

121.1(7)

Table 5. Interactions geometry for the molecules (Å, °). Ligand D–H∙∙∙A

D–H

H∙∙∙A

D∙∙∙A

D–H∙∙∙A

C2-H2…N1

0.93(3)

2.45(2)

2.794(4)

101.9(16)

C10-H10…N1

1.02(3)

2.50(3)

3.514(4)

175.8(18)

C4-H4…N1

0.931(5)

2.481(4)

2.789(4)

100.8(3)

C12-H12…N1

0.960(15) 2.549(13) 3.434(5)

NM

NM1


153.3(12)

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ASSOCIATED CONTENT Supporting Information Crystallographic data as .cif files for the structures reported in this paper have been deposited at the Cambridge Crystallographic Data Center with CCDC 1903451 for the NM molecule and 1903449 for the NM1 molecule. Copies of the data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Center, 12, Union Road, Cambridge CB2 1EZ, UK). Email: [email protected]. Detail of 1H-NMR, 13CNMR and optical spectra of NM and NM1 ligands are given.

AUTHOR INFORMATION *

Corresponding author. Tel: +90-232-3293535; Fax: +90-232-3293999

*

Corresponding author. Tel: +90-232-3111246; Fax: +90-232-3438625

*

E-mail address: [email protected] (M. Can)

*

E-mail address: [email protected] (B. Dindar) The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported at Ege University, İzmir Katip Çelebi University and Dokuz Eylül University. The authors acknowledge İzmir Katip Çelebi University Central Research Laboratory for the Cyclic Voltameter measurement and TGA analysis, Ege University for NMR measurement and photophysical studies (Grant No: 17 GEE 004) and Dokuz Eylül University for the use of the Agilent Xcalibur Eos diffractometer (purchased under University Research Grant No: 2010.KB.FEN.13),



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