Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Investigation of crystal structure, vibrational characteristics and molecular conductivity of 2,3-dichloro-5,6-dicyno-p-benzoquinone Poonam Rani a, Gunjan Rajput b, R.A. Yadav a,⇑ a b

Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The computed parameters of

Graphical representation of the vibronic coupling constant.

geometry agree with the experimental parameters.  The electronic absorption spectrum displays three bands at about 391, 354 and 288 nm.  Mainly the C@O and ring stretchings participate in the charge transfer process.

a r t i c l e

i n f o

Article history: Received 24 February 2014 Received in revised form 5 August 2014 Accepted 23 August 2014 Available online 1 September 2014 Keywords: Vibrational spectrum IR and Raman spectrum Charge transfer properties Vibronic coupling and electron–phonon coupling

a b s t r a c t Molecular geometries and vibrational spectra for the ground state of 2,3-dichloro-5,6-dicyno-p-benzoquinone (DDQ) and its anion (DDQ) were computed using DFT method at the B3LYP level employing 6-311++G(d,p) basis set whereas for the first excited state (DDQ⁄), these were calculated using TD-DFT at the B3LYP level employing the 6-311++G(d,p) basis set available with the Gaussian 09 package. The spectra have been experimentally investigated and the observed IR and Raman bands have been assigned to different normal modes on the basis of the calculated potential energy distributions (PEDs). XRD of single crystal has been investigated to determine molecular and crystal structures of DDQ. In order to elucidate the transfer of electrons, electronic structure and electronic absorption have been calculated with the TD-DFT method. Vibronic interaction and its role in the appearance of superconductivity in the DDQ, DDQ and DDQ⁄ molecules have been investigated. The present XRD, molecular, electronic and vibronic studies indicate that mainly the ag C@O stretching and ring stretching modes participate in the charge transfer process. Ó 2014 Published by Elsevier B.V.

Introduction

⇑ Corresponding author. Tel.: +91 5422368593; fax: +91 5422368468. E-mail addresses: [email protected], [email protected] (R.A. Yadav). http://dx.doi.org/10.1016/j.saa.2014.08.044 1386-1425/Ó 2014 Published by Elsevier B.V.

In search of the novel molecular materials based on radical donor and acceptor molecules, much effort have been made to introduce additional non-bonding interactions such as hydrogen bonding to orient and control the solid state association of radical

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molecules [1]. These efforts were concentrated mainly on cation radical salts of tetrathiafulvalene derivatives, obtained by electrocrystallization with various anions acting as hydrogen bond or halogen bond acceptors. Halogen bonding interactions were also explored in charge-transfer salts [2–4]. According to Lieffrig et al. [5] 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) acts as a powerful oxidant toward iodinated TTFs and the formation of novel charge-transfer salts with original halogen bonding patterns. DDQ is not only a strong oxidant but also it offers potentially three different halogen bonding acceptor sites, the carbonyl oxygen atoms, the nitrile nitrogen atoms and the chlorine atoms. 3,4-ethylenedithio-30 -iodo-tetrathiafulvalene (EDT-TTF-I) and 3,4-ethylenedithio-30 ,40 -diiodo-tetrathiafulvalene (EDT-TTF-I2) are reported to be engaged in a variety of cation radical salts upon electrocrystallization with anions acting as halogen bond acceptors such as halides [6,7], polyhalides [8], halometallates [9,10], cyanometallates [6,11,12] or thiocyanatometallates [13,14]. In p-conjugated systems, strong coupling exists between the geometrical and electronic structures which controls the transport properties. Addition and removal of an electron usually lead to marked geometry relaxations, which in turn modifies the electronic structure [15], as a result efficiency of the charge transport and ultimately the performance of electronic devices, are controlled to a large extent by the reorganization energies due to hole-vibration or electron-vibration interactions. Hence, for different molecular structures, charge transfer rates essentially depend on their reorganization energies (stabilization energy). For an efficient charge transport process, the magnitude of the reorganization energy should be low and therefore, such molecules are expected to be good conductors. The reorganization energy (k), is a purely intramolecular quantity and it measures the gain in energy due to electron–phonon coupling [16]. The reorganization energy DE plays an important role and it has been applied to investigate charge transport in hole-/electron-transport materials [17–22]. The stabilization energy can be written in terms of vibronic coupling constants (VCCs). A molecule with large/small VCCs has large/small DE [23], hence, an understanding of VCC ordering clarifies the reason for the size of the stabilization energy. The computed VCCs using the DFT/B3LYP method agree well with the new experimental constants, and thereby, the discrepancy persisting in earlier studies [24] is basically removed. As the mobility of the electrons is much higher than that of the nuclei, the electron transfer must occur with no exchange of energy with the external medium during the electron hopping. Consequently, vibrational excitations must take place before isoenergic electron transfer can occur [25]. By designing molecules with tunable HOMO and LUMO levels, one can achieve properties of materials and the performance of the electronic devices based on them. The molecules with low HOMO/LUMO gaps are of particular importance due to their ability to easily donate or accept an electron, which is the basic process in all organic electronic devices. The value of the band gap (in organic semiconductors) and the conductance band formation (in organic metals) are directly connected with the position of HOMO/LUMO gap which are of prime importance for molecular electronic applications [26]. In the present study, the molecular and electronic structures and the vibrational spectra of DDQ, DDQ⁄ and DDQ have been investigated in details. The X-ray crystal structure of DDQ has also been suggested that the carbonyl C and O and the ring atoms participate in the intermolecular interaction and help transfer the charge. The calculated vibrational frequencies have been correlated to the experimental observed IR and Raman bands. Normal mode assignments have been made on the basis of the computed potential energy distribution (PED). In addition, a comprehensive study of molecular geometries, atomic charges and vibrational fundamentals of these three molecules has been carried out. Vibronic

coupling in DDQ, DDQ and DDQ⁄ has been investigated and VCCs have been computed using the DFT/B3LYP method. Reorganization energies and electron–phonon coupling constants for the effective modes of DDQ, DDQ and DDQ⁄ have also been computed and compared. Theoretical background The theory of the vibronic effects is based on the consistent account of the interactions of the electrons with the nuclei [26–29]. The total Hamiltonian of the Schrödinger equation can be written as:

H ¼ Hr þ HR þ Vðr; RÞ

ð1Þ

where Hr, is the electronic component including the kinetic energy of the electrons and the interelectronic electrostatic interaction, HR is the kinetic energy of the nuclei, and V(r, R) is the energy due to interaction of the electrons with the nuclei and internuclear repulsion (r and R denote the whole set of coordinates of the electrons ri, i = 1, 2, . . ., n, and nuclei Ra, a = 1, 2, . . ., N, respectively). The operator V(r, R) can be expanded as a series of small displacements of the nuclei about the point Ra, = Ra,0 = 0 (chosen as origin):

Vðr; RÞ ¼ Vðr; 0Þ þ

X  @V 

ðHR þ ek ðRÞ  EÞvk ðRÞ þ

@Ra 0 X

1X @2V Ra þ 2 a;b @Ra @Rb 0

! Ra Rb þ . . .

ð2Þ

0

W km ðRÞvm ðRÞ ¼ 0

ð3Þ

m–k

where the expansion coefficients vk (R) are functions of the nuclear coordinates, Wkm(R) denotes the electronic matrix element of vibronic interactions (VI) i.e., that part of the electron-nuclear interaction V(r, R) which depends on R,

Wðr; RÞ ¼ Vðr; RÞ  Vðr; 0Þ ! X  @V  1 X @2V Ra þ Ra Rb þ . . . ¼ @Ra 0 2 a;b @Ra @Rb

ð4Þ

0

is the potential energy of the nuclei in the mean field of the electrons in state uk(r). The first part of the Eq. (4) describes the vibronic coupling (Vi). The strength of the VC corresponding to the vibrational mode i is given by [26–29],

Vi ¼

X@Eðr; RÞ uiA  pffiffiffiffiffiffiffi @RA MA R0 A

ð5Þ

where E(r, R) is the eigenvalue corresponding to the Hamiltonian He(r, R), RA is the position of Ath nucleus, uiA is the 3D component of the displacement of the Ath nucleus oscillating under the ith mode, and MA is the nuclear mass [24]. Vibronic coupling can be written in another way, i.e., the dimensionless vibronic coupling constant gi (which is a measure of the strength of the vibronic coupling) is defined by [28],

Vi g i ¼ qffiffiffiffiffiffiffiffiffiffiffiffi 2hx3i

ð6Þ

where xi is the frequency of the ith mode. Rys–Huang factor (Si), which is also a measure of the coupling strength, is defined by [28],

Si ¼

DEi V 2i ¼  xi 2hx3i h

ð7Þ

where DEi is the reorganization energy of the mode i, which is equal to the Jahn–Teller stabilization energy in the degenerate state. Si signifies the stabilization energy measured by the vibrational

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energy [28]. Kato and Yamabe [30] proposed a relation between the dimensionless vibronic coupling constant (gi) and the electron– phonon coupling constant (li) given by,

gi ¼

sffiffiffiffiffiffiffiffiffi li hxi

ð8Þ

Experimental The DDQ compound forms yellow powder at room temperature. This compound was purchased from the Sigma Aldrich Chemical Co. (USA). Single crystals suitable for the X-ray diffraction studies were grown by slow evaporation from the saturated solution in dichloromethane: hexane mixture (80:20; V/V). Dark brown broad plates like single crystals were obtained after slow evaporation of the solution. Single crystal X-ray data for DDQ were collected on an Oxford diffraction X-caliber CCD diffractometer at 293 °K using the Mo Ka radiation k = 0.71073 Å. Data reduction for DDQ was carried out using the CrysAlis program [31]. The structure was solved by direct method using SHELX-97 [32] and refined F2 by full matrix least squares technique using SHELX-97. Diagram for DDQ molecule (Fig. 1) was prepared using ORTEP [33]. The electronic absorption spectrum of DDQ was recorded on a Shimadzu UV-1700 PharmaSpecUV–Vis Spectrometer. The IR spectrum of the sample in KBr pellet was recorded on a Varian FTIR-3100 spectrometer in the spectral range 400–2500 cm1. The experimental parameters for recording the IR spectrum were: resolution- 4 cm1, scan-100, gain-20. The Raman spectrum of this compound was recorded in powder form on a Jobin Yvon HORIBA HR800 Raman spectrometer in the spectral range 100–2500 cm1. The 633 nm radiation from an Ar+ laser was used as the exciting source. The following experimental parameters were used for recording the Raman spectrum: Scan speed: 100 cm1/min; resolution: 1 cm1; power of the laser beam: 10 mW; PMT voltage: 800 V; entrance slit-width: 200 mm; accuracy of the measurements: 2 cm1; sensitivity: 1010.

geometry optimization the initial parameters were taken from the final X-ray refinement cycle. In addition, all the above molecular properties and vibrational wavenumbers alongwith the corresponding intensities in IR spectrum for the first excited state of the DDQ molecule were calculated using the time dependent DFT (TD-DFT) at the same B3LYP/6-311++g⁄⁄ level. The computed IR and Raman spectra were obtained from the spectrum option in results of considered molecule with the help of GaussView5 software [35]. The electronic structure and electronic absorption spectrum of DDQ were calculated using the TD-DFT method. For the anion and first excited state of the neutral molecule, the optimized geometry of the neutral molecule was taken as the input structure and calculations were carried out. All the optimized geometries and frontier MOs were viewed with the help of the GaussView5 software [35]. The unscaled B3LYP/6-311++g⁄⁄ vibrational frequencies are generally larger than the experimental values. In order to obtain the reasonable frequency matching, scale factors proposed by Rauhut and Pulay [36] were employed. The Raman activities (Ai) calculated from the Gaussian 09 program were converted to Raman intensities (Ii) using the relation [37],

Ii ¼

f ðm0  mi Þ4 Ai mi ½1  expðhcmi =kTÞ

where m0 is the exciting frequency (in cm1); mi is the vibrational wavenumber of the ith normal mode; h, c, k and T are Planck’s constant, speed of light, Boltzmann’s constant and temperature, respectively and f is a suitably chosen common scaling factor for all the peak intensities. For the subsequent normal coordinate analysis (NCA), the force field obtained in the Cartesian coordinates and dipole derivatives with respect to the atomic displacements were extracted from the archive section of the Gaussian 09 output file and transformed to a suitably defined set of internal coordinates by means of GAR2PED software [38]. Calculations were performed with the B3LYP/6-311++g⁄⁄ method to obtain the optimized geometries, the normal modes uiA and the gradient of the total   electronic energy @Eðr;RÞ . VCCs of the following four systems were @R i

Theoreical computations The optimized molecular geometries, natural charges and fundamental vibrational wavenumbers alongwith the corresponding intensities in the IR and Raman spectra, Raman activities and depolarization ratios of the Raman bands for DDQ and DDQ have been computed using the DFT/B3LYP method employing the 6-311++g⁄⁄ basis set available with the Gaussian 09 package [34]. For the

R0

investigated and analyzed: (i) DDQ as the initial state and DDQ- as the final state of transition, (ii) DDQ- as the initial state and DDQ as the final state of transition, (iii) DDQ as the initial state and DDQ⁄ as the final state of transition and (iv) DDQ⁄ as the initial state and DDQ as the final state of transition. Results and discussion Charge distribution

Fig. 1. ORTEP diagram of DDQ.

The computed natural charges at various sites of the DDQ, DDQ⁄ and DDQ molecules are collected in Table 1. From the Table 1, it could be seen that the atomic sites C1, C2, C4, C5, O9, O10, N13 and N14 possess negative charges whereas the remaining atomic sites C3, C6, Cl7, Cl8, C11 and C12 possess positive charges. Large variation in natural charges is found at the atomic sites of all the six C atoms of the phenyl ring due to various attachments of groups containing different electronegative atoms at their positions. Due to addition of an electron to the neutral molecule, charge is redistributed and consequently, some atomic sites gain/loose charges. It is found that in going from DDQ to DDQ, the atomic sites C3, C6, Cl7, Cl8, C11 and C12 gain negative atomic charges whereas the atomic sites C1, C2, C4, C5, O9, O10, N13 and N14 gain negative charge. In going from DDQ to DDQ, the negative atomic charges at the sites (C3, C6), (Cl7, Cl8) and (C11, C12) are increased by 0.18, 0.11 and 0.12 a. u, respectively, whereas at the atomic sites (C1, C2), (C4, C5), (O9, O10) and (N13, N14) charges are decreased by 0.071, 0.087, 0.28 and 0.063 a.u. The natural charges at all the sites are

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P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347 Table 1 Calculated Natural chargesa at various atomic sites of DDQ, DDQ⁄ and DDQ.

a

Sr. No.

DDQ

DDQ⁄

DDQ

C1 C2 C3 C4 C5 C6 Cl7 Cl8 O9 O10 C11 C12 N13 N14

0.118 0.118 0.496 0.115 0.114 0.496 0.125 0.125 0.442 0.442 0.256 0.256 0.203 0.202

0.135 0.135 0.475 0.105 0.105 0.475 0.140 0.140 0.437 0.437 0.254 0.254 0.192 0.192

0.037 0.037 0.217 0.028 0.028 0.217 0.016 0.016 0.168 0.168 0.140 0.140 0.140 0.140

Table 2 Experimental and computed geometrical parametersa of DDQ, DDQ⁄ and DDQ. Definition

Experimental

(C1AC2) (C1AC6) (C1ACl8) (C2AC3) (C2ACl7) (C3AC4) (C3AO9) (C4AC5) (C4AC11) (C5AC6) (C5AC12) (C6AO10) (C11AN13) (C12AN14) a(C2vC1AC6) a(C2AC1ACl8) a(C6AC1ACl8) a(C1AC2AC3) a(C1AC2ACl7) a(C3AC2ACl7) a(C2AC3AC4) a(C2AC3AO9) a(C4AC3AO9) a(C3AC4AC5) a(C3AC4AC11) a(C5AC4AC11) a(C4AC5AC6) a(C4AC5AC12) a(C6AC5AC12) a(C1AC6AC5) a(C1AC6AO10) a(C5AC6AO10) d(C6AC1AC2AC3) d(C6AC1AC2ACl7) d(Cl8AC1AC2AC3) d(Cl8AC1AC2ACl7) d(C2AC1AC6AC5) d(C2AC1AC6AO10) d(Cl8AC1AC6AC5) d(Cl8AC1AC6AO10) d(C1AC2AC3AC4) d(C1AC2AC3AO9) d(Cl7AC2AC3AC4) d(Cl7AC2AC3AO9) d(C2AC3AC4AC5) d(C2AC3AC4AC11) d(O9AC3AC4AC5) d(O9AC3AC4AC11) d(C3AC4AC5AC6) d(C3AC4AC5AC12) d(C11AC4AC5AC6) d(C11AC4AC5AC12) d(C4AC5AC6AC1) d(C4AC5AC6AO10) d(C12AC5AC6AC1) d(C12AC5AC6AC10)

r r r r r r r r r r r r r r

Natural charges in e.

found to vary due to transition of an electron from the DDQ with A1 symmetry of electronic wavefunction to its first excited state with B2 symmetry of electronic wavefunction. On the redistribution of charge in DDQ⁄, the atomic sites (C1, C2), (C3, C6) and (N13, N14) gain negative charge whereas the atomic sites (C4, C5) and (O9, O10) loose negative charges. There is no change in natural charges at the atomic sites C11 and C12 in going from DDQ to DDQ⁄. Molecular structure The results of our investigation for the crystal from single crystal X-ray diffraction are collected in Table 2. The optimized geometrical parameters for the DDQ, DDQ⁄ and DDQ molecules are also collected in the Table 2 with their optimized structures shown in Fig. 2. The present calculations predicted that all the three species of DDQ have planar structures whereas XRD molecular structural parameters suggest non-planarity of DDQ. The three species of DDQ belong to the C2v point group symmetry; the C2 axis is taken as the z-axis passing through the center of the C1@C2 bond and the molecular plane is taken to be the yz plane. As can be seen from the Table 2, the experimental bond lengths and bond angles agree nicely with their corresponding computed bond lengths and bond angles. Zanotti et al. [39] have also reported structural parameters for DDQ which are also in good agreement with the present computed parameters. The bond lengths of the bond pairs (C1AC6, C2AC3), (C5AC6, C3AC4) and (C5AC12, C4AC11) are found to be shorter than that expected for the CAC single bond length (1.54 Å) whereas the bond pairs (C2@C3, C4@C5), (C3@O9, C6@O10), (C11„N13, C12„N14) are found to be longer than their usual bond lengths (1.33, 1.20, 1.16) in the neutral DDQ molecule, which could be due to the antibonding character between the atom pairs (C5, C6), (C3, C4), (C5, C12), (C4, C11), (C2, Cl7), (C1, Cl8) and pbonding character between the atom (C6, C1), (C2, C3), (C3AO6), (C6, O10) which causes conjugation along all bond lengths of DDQ molecule. Conjugation provides a route to conduct charges in the DDQ molecule. Therefore, this molecule appears to be a good candidate for conducting the electronic charge. Due to addition/excitation of an electron in the neutral molecule, variation is found in the partial atomic charges (natural charges) at all the atomic sites. Bond lengths in a molecule strongly depend on charge distribution. Hence, variation in bond lengths is expected in going from DDQ to DDQ and DDQ⁄. The optimized bond lengths C1@C2, C4@C5, (C3@O9, C6@O10), (C11„N13, C12„N14) are increased by 0.011, 0.044, 0.032, 0.004 and by 0.044, 0.071, 0.005, 0.003 Å, respectively, whereas the bond lengths (C3AC4, C5AC6), (C4AC11, C5AC12) are decreased by 0.048, 0.001 and 0.073, 0.006 Å,

DDQ⁄

DDQ

a

Present

Ref. [39]

1.339(3) 1.479(3) 1.694(2) 1.484(2) 1.692(2) 1.492(3) 1.201(3) 1.343(3) 1.435(3) 1.486(3) 1.438(3) 1.210(3) 1.129(3) 1.127(4) 121.2(2) 122.7(2) 116.1(2) 121.9(2) 122.9(2) 115.2(2) 116.6(2) 123.5(2) 119.9(2) 121.7(2) 115.2(2) 123.0(2) 120.9(2) 122.3(2) 120.9(2) 117.6(2) 122.6(2) 119.8(2) 1.8 177.9 178.5 1.8 3.7 175.2 176.6 4.5 0.6 179.2 179.6 1.0 1.1 179.6 179.7 0.9 0.9 178.7 178.7 2.1 3.2 175.7 176.4 4.7

1.339(4) 1.483(4) 1.698(3) 1.481(4) 1.695(3) 1.502(4) 1.199(3) 1.343(4) 1.429(4) 1.491(4) 1.442(4) 1.206(3) 1.135(4) 1.133(4) 121.3(2) 122.8(2) 115.9(2) 122.1(2) 122.8(2) 115.1(2) 116.5(2) 123.7(2) 119.8(2) 121.6(2) 115.2(2) 123.7(2) 121.0(2) 122.4(2) 116.6(2) 117.4(2) 122.8(2) 119.8(3)  – – – – – – – – – – – – – – – – – – – – – – –

Computed

Computed

1.351 1.495 1.717 1.495 1.717 1.504 1.207 1.355 1.424 1.505 1.424 1.207 1.154 1.154 121.549 123.079 115.372 121.539 123.108 115.353 117.223 122.387 120.390 121.244 116.251 122.506 121.235 122.538 116.227 117.210 122.425 120.365 0.0 180.0 180.0 0.0 0.0 180.1 179.9 0.1 0.0 180.1 180.1 0.1 0.0 180.1 180.1 0.1 0.0 180.0 179.9 0.0 0.0 180.1 179.9 0.1

1.395 1.528 1.683 1.528 1.683 1.432 1.222 1.426 1.418 1.432 1.418 1.222 1.157 1.157 121.831 122.387 115.782 121.831 122.387 115.782 114.675 118.374 126.951 123.494 116.174 120.332 123.494 120.332 116.174 114.675 118.374 126.951 0.1 179.8 179.8 0.2 0.1 179.9 179.9 0.1 0.1 179.9 179.9 0.1 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0

DDQ

1.362 1.476 1.744 1.476 1.744 1.456 1.239 1.399 1.423 1.456 1.423 1.239 1.158 1.158 122.765 121.890 115.345 122.765 115.345 122.192 122.378 121.075 122.378 121.075 116.547 114.857 122.192 122.951 116.547 122.378 121.075 122.378 0.0 180.0 180.0 0.0 0.0 180.1 180.0 0.1 0.0 180.1 180.0 0.1 0.0 180.0 179.9 0.1 0.0 180.0 180.0 0.0 0.0 180.1 180.0 0.1

Bond lengths in Å, bond angles (a) and dihedral angles (d) in degrees (°).

respectively in going from DDQ to DDQ and DDQ⁄. The unusual variation in bond lengths is found for the bond lengths (C1@C2, C4@C5), (C2ACl7, C1ACl8) due to radicalization and excitation. All the angles are found to deviate from 120° within ±5°. In going from the DDQ to DDQ and DDQ⁄, the C@O bonds nearly bisect the bond angles a(C2AC3AC4) and a(C1AC6AC5). As can be seen from the Table 2, the observed dihedral angles of the DDQ molecule deviate within ±5° from 0°/180° whereas the calculated dihedral angles are 0°/180°. This difference between the experimental and calculated results could be due to the interaction of atoms of a single molecule with the neighboring

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P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347 Table 3 Short bond interaction of DDQ in unit cell with the molecules of neighboring unit cells and its own cell.

a b

Fig. 2. Front and side views of (a) DDQ (b) DDQ⁄ (c) DDQ.

molecules in the neighboring unit cells and the same unit cell leading to slight non-planarity of the molecule whereas in DFT calculations, one considers an isolated molecule. The calculated structural parameters are expected to agree with the experimental microwave structural parameters. The observed crystal structure pattern shows that all the atoms of the molecule interact except the C1, C2, C11 and C12 atoms. Table 3 and Fig. 3(a) show short bonding interaction in the crystal. In Fig. 3(a), (0) molecule represents as the reference molecule whereas the molecules (i)–(vii) represent seven molecules in the same unit cell, (I)–(VI) represent neighboring unit cells. The C5, C6, Cl7, and O9 atoms of the reference molecule interact with O10, O10, O9 and Cl7, respectively, of the IV unit cell whereas C3, C4 and O10 atoms interact with N13, N13 and (C5, C6) atoms of the same unit cell having different magnitudes of the bond lengths. The short distances corresponding to Cl8 (0)  N14 (II) and N14 (0)  Cl8 (V) have larger values as compared to the other short distances. Therefore, one could see that all the three carbonyl oxygen atoms, the nitrile nitrogen atoms and chlorine atoms participate in interaction which could act as three different halogen acceptor sites. This could be a possible reason that is why DDQ acts as a good acceptor in organic conducting molecules. Crystal structure The crystallographic data and refinement detail are listed in Table 4. See Supplementary Material for crystallography data of the DDQ molecule. The single crystal of the DDQ compound belongs to orthorhombic system with pbca point group. A unit cell of the DDQ crystal with dimensions, a = 16.6453 Å, b = 5.9211 Å and c = 17.8192 Å contains eight DDQ molecules (Fig. 3(b)). The molecules are arranged in herringbone structure which realizes two dimensional uniform charge transfers. As can be seen from the Table 2 the experimental dihedral angles of DDQ deviate from 0°/180° but in herringbone crystal structure, molecules look planar which helps to decrease reorganization energy and to increase the conductivity in electron transfer between neutral and anionic molecule. The Cl7, Cl8, O9 and N14 atoms of the DDQ molecule form hydrogen bonds with O9, N14, Cl7 and Cl8 of the neighboring molecules, respectively, having distance 3.221 Å (x, 1  y, 1  z), 3.213 Å (x, ½  y, ½ + z), 3.221 Å (x, 1  y, 1  z) and 3.213 Å (x, ½  y, ½ + z), respectively

Original atoms

Interacting atoms

Distances (Å)

Unit-cella

Moleculeb

C1 C2 C3 C4 C5 C6 Cl7 Cl8 O9 O10 C11 C12 N13 N14

– – N13 N13 O10 O10 O9 N14 Cl7 C5, C6 – – C3, C4 Cl8

– – 2.939 3.097 2.912 2.912 3.221 3.213 3.221 2.912 – – 2.939, 3.097 3.213

– – 0 0 IV IV IV II IV 0 – – IV V

– – iv iv iii iii v i v iii – – iv i

0 represents reference unit cell, I–VI represent neighboring unit cells. i–vii represent seven molecules in the reference unit cell.

(Fig. 3(c)), whilst the O10 and N13 atoms of DDQ have no hydrogen bonding with other molecules. These prominent intermolecular interactions could provide a route to charge transfer. Electronic structures and absorption The DDQ molecule has 112 electrons which occupy 56 MOs. Each MO contains two electrons with opposite spins called a (up spin) and b (down spin). For the neutral molecule, G09 program provides electronic energy levels for a electrons whereas for anions there are two sets of energy levels for a and b electrons with opposite spins. The present calculations predict an alpha MO for the highest occupied molecular orbital (HOMO) and a beta MO for the lowest occupied molecular orbital (LUMO) for the anionic species. All the computed electronic energies for the electronic states of DDQ and DDQ are collected in Table 5. The LUMO of the DDQ molecule belongs to b2 species and strong p-bonding character between the C11, C4, C3, C2 and C12, C5, C6, C1 atoms and anti-bonding character between (C1, Cl8), (C2, Cl7), (C3, O9), (C6, O10), (C11, N13) and (C12, N14) atom pairs are found. In the HOMO of DDQ, surface charge density is strongly localized on the atoms C3, C1, C2, C3 (half of the ring of the phenyl group). A strong p-bonding character exists between C4 and C5 atoms and a strong anti-bonding character exists between the C2, Cl7 and C1, Cl8 atoms pairs. The electronic energy of the HOMO is 8.26 eV in DDQ. The electronic energy of LUMO is 5.39 eV in DDQ. The electronic energy level diagram of the molecular orbitals of DDQ has been shown in the Fig. 4. As could be seen from the Fig. 4, the electronic energy levels are not distributed uniformly. The electronic absorption spectrum of DDQ in the dichloromethane solution exhibits three bands, namely, 278 and 216 with a broad shoulder peak in the range 324–423 nm. The experimental and computed bands of electronic absorption of the DDQ molecule are compared in the Table 5 and shown in Fig. 5. The TD-DFT calculation yielded the lower band 389 nm with oscillator strength 0.0078 (Table 5) which could be correlated to the experimental shoulder peak. Since each absorption line in a TD-DFT spectrum is due to several single excitations, a description of the transition character is generally not straightforward. However, approximate assignments have been made, although they provide a simplified representation of the transitions. The energy bands 389 and 223 nm arise due to the HOMO ? LUMO and HOMO ? (L + 1)UMO transition, respectively and are attributed to the electron transfer from the lone pair of the Cl atoms to the C atoms of the cyno group. Also charge is redistributed on the phenyl ring due to the conjugation. The second computed lower energy band 280 nm which could

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P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347

Fig. 3. (a) Short bonding of DDQ. Plane abcd = (I), Plane adef = (II), Plane efgh = (II), Plane bchg = (IV), Plane abfg = (V), Plane cdeh = (VI). (b) Crystal packing structure of DDQ (c) Hydrogen of DDQ.

vibrations of DDQ, DDQ⁄ and DDQ molecules are active in both the IR and Raman spectra except those under the species a2 which are IR inactive. The modes under the a1 species are expected to appear as polarized Raman bands while those under the a2, b1 and b2 species would give rise to the depolarized Raman bands. Further, modes belonging to the a2 and b1 species are due to non-planar vibrations whereas those belonging to the a1 and b2 species result due to planar vibrations. Though we have studied IR and Raman spectra of DDQ molecule, for the sake of completeness we discuss the vibrational assignments for all the 36 normal modes of DDQ, DDQ and DDQ⁄ in light of the observed IR and Raman spectra and DFT calculations. The calculated vibrational frequencies, their IR intensities, Raman intensities of all the normal modes at the B3LYP/6311++G⁄⁄ level, observed frequencies in the IR and Raman spectra alongwith their intensities for DDQ are collected in Table 6. The calculated vibrational data for DDQ⁄ and DDQ are also collected in the Table 6. The potential energy distributions (PEDs) for the normal mode of vibration of DDQ , are included in Table 7. The computed and experimental IR and Raman spectra for the DDQ molecule are shown in Fig. 6. The DFT methods provide the vibrational frequencies which are overestimated due to neglect of anharmonicity in the real systems. However, inclusion of electron correlation in DFT makes the frequencies smaller than the corresponding observed ones to a certain extent. The unscaled B3LYP/6-311++g⁄⁄ vibrational frequencies are generally larger than the experimental values. In order to obtain the reasonable frequency matching, scale factors proposed

Table 4 Crystallographic data and structure refinements of DDQ. Wavelength (Å) Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a, b, c V (A3) Z Density (g/cm3) Absorption coefficient (mm3) F00

0.717 C8Cl2N2O2 227.0 Orthorhombic Pbca 16.6453 (9) 5.9211 (2) 17.8192 (8) 90°, 90°, 90° 1756.23 (14) 8 1.717 0.707 896.0

be correlated to the experimental band 281 nm is assigned to the transition (H  1)OMO ? LUMO and in this transition charge is transferred from the cyno group and lone pairs of the O atoms to the phenyl ring. The present study revealed that in DDQ the Cl, O atoms, cyno group and phenyl ring participate in the charge transfer process. Vibrational assignments The 36 normal modes of vibration of DDQ, DDQ⁄ and DDQ are distributed as: 13a1 + 5a2 + 6b1 + 12b2. All the 36 fundamental Table 5 Energya of Frontier MOs for DDQ and DDQ. HOMO/LUMO

DDQ

DDQ

a (H  4)OMO (H  3)OMO (H  2)OMO (H  1)OMO HOMO LUMO (L + 1)UMO (L + 2)UMO (L + 3)UMO (L + 4)UMO

a

9.85 9.47 9.47 9.25 8.65 5.39 3.38 1.87 1.55 0.98

(52) (53) (54) (55) (56) (57) (58) (59) (60) (61)

a

b

5.79 (53) 5.18 (54) 4.53 (55) 4.44 (56) 4.41 (57) 2.36 (58) 0.79 (59) 2.10 (60) 2.55 (61) 2.95 (62)

5.78 (52) 4.46 (53) 4.36 (54) 4.25 (55) 4.23 (56) 0.52 (57) 1.21 (58) 2.11 (59) 2.97 (60) 3.03 (61)

Excited state

Excited Energy (nm)

Oscillator strength

Composition (%)

1 2 3

389 278 223

0.0078 0.2738 0.2509

HOMO ? LUMO (H  1)OMO ? LUMO HOMO ? (L + 1)UMO

Energies are measured in eV. Each numerical value within the bracket represents the corresponding orbital number.

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P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347

Fig. 4. Pictorial separation of the electronic energy levels with frontier MOs for DDQ.

Fig. 5. (a) Calculated electronic absorption spectrum of DDQ (b) electronic absorption spectrum of DDQ.

by Rauhut and Pulay [36] were employed and values are included in the Table 6. Vector diagram of all the normal modes are shown in Fig. 7. Only those displacements are shown in vector diagram whose magnitudes are greater than 0.20 a.u.

Quinoid ring modes The quinoid ring of the DDQ molecule possesses 12 normal modes of vibration as: 6 m(ring), 3 a(ring) and 3 U(ring) modes. The quinoid ring contains two C@C bonds and four CAC bonds. The C@C stretching vibrations are pure and highly localized modes and are computed to have higher wavenumbers (1593 and 1644 cm1) as compared to the CAC stretching modes. The experimental spectra contain two medium intense Raman bands 1554 and 1622 cm1 and two very strong IR bands 1555 and 1674 cm1 which could be correlated to the computed wavenumbers 1593 and 1644 cm1, respectively. The CAC stretching vibrations of the quinoid ring are coupled with the b(C@O), b(CAC), b(CACl) and b(C„N) modes. Appreciable contributions of the CAC stretching coordinates to the PEDs are found for the computed frequencies 1241, 1168, 968 and 343 cm1. The lowest magnitude (343 cm1) is the most suitable candidate for the breathing mode which is coupled with the a(ring) mode. This mode is correlated with the observed Raman frequency 343 cm1 (w). The IR frequencies in the present case corresponding to the other three remaining computed mR(CAC) modes, namely, 968, 1168 and 1241 cm1 are observed at 1010 (vw), 1174 (s) and 1269 (m) cm1, respectively. It is to be noted that only one medium intense Raman band is observed at 1173 cm1 which could be correlated to the computed frequency1168 cm1. Assignment of the planar ring deformation mode is also complicated due to coupling of these modes amongst themselves and with the other modes. The present calculations place the three planar-ring deformation vibrations at the frequencies 319 (b2), 469 (a1) and 504 (b2) cm1. The experimental IR and Raman spectra contain only a very weak band at 457 and 461 cm1, respectively, which could be assigned to the planar-ring deformation under the species a1. The calculated IR and Raman frequencies for the remaining two planar-ring deformation modes are found to be very weak and these could not be observed in the IR and Raman spectra. The three out-of-plane ring deformation modes are computed to have frequencies 46, 57 and 80 cm1 (Fig. 7). These modes are strongly coupled with the non-planar bending modes of the C„N, C@O and CACl groups.

C@O modes The C@O stretching frequency absorbs strongly in the IR spectrum in the range 1645–1750 cm1. The experimental IR spectrum shows three unresolved bands with a peak at 1674 cm1, and two shoulders at 1701 and 1740 cm1. As the above three bands are not properly resolved, intensities could not be taken as the guiding factor for assigning the C@O stretching mode. The frequency 1674 cm1 has already been assigned to the mR(C@C) mode and therefore, the frequencies 1701 and 1740 cm1 are possible candidates for the C@O stretching modes. As shown in the Fig. 7, the above two frequencies involve C@O stretching motions and are calculated to be 1749 and 1753 cm1. However, the latter arises due to the out-of-phase coupled (opc) stretching mode and the former one due to the in-phase coupled (ipc) stretching vibrations of the two C@O bonds. In uracil and its derivatives [40] the b(C@O) modes were assigned at much lower frequencies (400 cm1) as compared to the c(C@O) modes (700–800 cm1). The present investigation suggests that the modes involving planar and non-planar C@O bending motions give rise to the ipc b(C@O) at 777 cm1 (a1 species) and opc coupled b(C@O) mode at 380 cm1 (b2 species) and similarly the ipc c(C@O) mode (b1 species) is found to be 749 cm1 and the opc c(C@O) mode (a2 species) as 766 cm1. The observed frequencies corresponding to the IR active c(C@O) (b1) and b(C@O) (b2) modes are identified as the frequencies 723 and 764 cm1, respectively.

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P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347 Table 6 Calculated and experimental fundamental frequencies of DDQ, DDQ⁄ and DDQ and normal mode analysis. Sr no

DDQ c

Experimental IR 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

Calculated Raman

126 w

210 w 254 w 290 w 343 w

457 w

461 w

421 w

516 m

723 s 764 w 803 s 897 s 1010 w 1055 w 1174 s 1269 m 1555 s 1674 s 1701 w 1740 w 2233 m

785 w

1173 m 1331 m 1554 m 1622 m 1683 vs 2236 m 2343 vw

DDQ⁄

DDQ

Modesd

55 (1) 30 (0) 66 (0) 123 (1) 145 (2) 154 (18) 221 (0) 269 (0) 265 (0) 310 (0) 322 (5) 345 (19) 367 (2) 447 (2) 480 (7) 383 (0) 517 (2) 518 (1) 524 (1) 626 (0) 720 (5) 771 (0) 774 (1) 820 (40) 900 (66) 1023 (1) 975 (419) 1201 (2) 1249 (32) 1388 (24) 1423 (10) 1457 (14) 1681 (208) 1684 (196) 2310 (5) 2319 (0)

64 (1, 9) 0.75 61 (0, 3) 0.75 96 (0, 1) 0.75 129 (1, 19) 0.70 151 (2, 4) 0.75 160 (19, 1) 0.75 210 (0, 1) 0.70 267 (1, 0.2) 0.75 262 (2, 1) 0.75 333 (0, 0) 0.75 324 (0, 5) 0.75 345 (0.4, 13) 0.35 369 (0.1, 5) 0.39 436 (2, 0.2) 0.75 484 (14, 6) 0.24 460 (0, 0.2) 0.75 529 (2, 0.5) 0.75 505 (1, 0) 0.749 536 (1, 5) 0.12 623 (0, 0.3) 0.75 736 (7, 0.7) 0.75 765 (0, 0.1) 0.75 778 (0.2, 0.3) 0.745 779 (68, 1) 0.28 867 (74, 0.2) 0.75 1006 (23, 1) 0.75 1034 (124, 1) 0.14 1187 (137, 0.2) 0.749 1208 (15, 60) 0.34 1367 (0.1, 7) 0.75 1462 (97, 2) 0.14 1560 (48, 24) 0.29 1607 (1, 53) 0.48 1573 (313, 0.4) 0.7499 2300 (195, 53) 0.14 2307 (32, 22) 0.75

U (R) b1 U (R) a2 U (R) a2

b

Unscaled Freq.

Scaled Freq.a

46 (0.1, 2) 0.75 57 (0, 2) 0.75 80 (0, 0.1) 0.75 120 (2, 24) 0.72 144 (3, 2) 0.75 148 (21, 2) 0.75 207 (0.0, 2) 0.71 255 (1, 3) 0.75 257 (1, 1) 0.75 292 (0, 0.1) 0.75 319 (0.3, 1) 0.75 343 (0, 4) 0.47 380 (10, 0.2) 0.55 428 (1, 0) 0.7495 469 (5, 5) 0.14 475 (0, 0.2) 0.75 490 (0, 0) 0.75 504 (1, 0.2) 0.7498 526 (0.1, 2) 0.08 638 (0, 0.2) 0.75 749 (8, 0.1) 0.75 766 (0, 0) 0.75 777 (1, 0) 0.7499 796 (156, 0.3) 0.23 887 (28, 1) 0.75 968 (2, 0.1) 0.75 1051 (0.4, 0.2) 0.21 1168 (239, 2) 0.75 1241 (154, 0.1) 0.23 1320 (4, 9) 0.75 1593 (177, 2) 0.23 1644 (1, 19) 0.16 1749 (5, 17) 0.18 1753 (362, 0.2) 0.46 2336 (8, 23) 0.10 2347 (6, 13) 0.75

45 56 78 117 141 145 203 250 252 286 312 336 372 419 459 465 480 493 515 624 733 750 760 779 868 947 1004 1115 1185 1261 1521 1570 1670 1674 2231 2241

b(CACN) a1 b(CACN) b2 c(CACN) b1 b(CACl) a1 c(CACl) b1 b(CACl) b2 c(CACN) a2 a(ring) b2 m(ring) a1 b(C@O) a1 b(CAN) b2 a(ring) a1 c(CACl) a2 c(CAN) b1 a(ring) b2 b(CAN) a1 c(CAN) a2 c(C@O) b1 c(C@O) a2 b(C@O) b2 m(CACl) a1 m(CACl) b2 m(ring) b2 m(CACN) a1 m(ring) b2 m(ring) a1 m(CACN) b2 m(ring) a1 m(ring) a1 m(C@O) a1 m(C@O) b2 m(CAN) a1 m(CAN) b2

a

Calculated wave numbers below 1000 cm1 were scaled by the scale factor 0.9786 and those above 1000 cm1 by the scale factor 0.9550 for larger wave numbers. The number before each bracket represents the corresponding calculated frequency (cm1). The first and second numbers within each bracket represent IR intensity (km/ mol) and Raman intensity, the first number after each bracket represents corresponding depolarization ratio of the Raman band. c vs: very strong, s: strong, m: medium, w: weak, vw: very weak, sh: shoulder. d m = stretching, a = angular deformation, b = planar deformation, c = non-planar deformation, U = non-planar ring deformation. a1, a2, b1 and b2 are species of the point group C2v. b

CACl modes In the DDQ molecule there are two CACl bonds whose corresponding vibrations are shown in the Fig. 7. The PEDs of the CACl stretching vibrations indicate that these are strongly coupled with the m(CACN) and b(C@O) modes. In benzene derivatives the CACl stretching mode is observed in the range 700–1050 cm1 [40]. As shown in the Fig. 7 and given in the Table 7, appreciable contribution due to the m(CACl) mode is found to the two modes corresponding to the calculated frequencies 796 (a1) and 887 (b2) cm1. The intensities of the CACl stretching vibrations have been found to be very strong in the IR spectrum and the two bands corresponding to these two m(CACl) modes are found to be 803 and 897 cm1. In the present study, only a weak Raman band is found at the frequency 785 cm1 which could be correlated to the computed band 796 (a1) cm1. The frequencies of the planar and non-planar CACl deformation modes usually appear in close proximity. In the present calculation, the two planar and one nonplanar CACl deformation modes are found at (207, 257 cm1) and 255 cm1, respectively. As PEDs calculations, the remaining one non-planar CACl deformation is mainly involved in frequency corresponding to 475 cm1. The latter mode is coupled with the

non-planar ring deformation mode only whereas, the former c(CACl) mode is coupled with the c(C„N) and c(CACN) modes. In the Raman spectrum two bands at the frequencies 210 and 254 cm1 are observed with weak intensity which are assigned to the b(CACl) (a1) and c(CACl) (b2) modes, respectively. CACN modes Assignment of the CACN stretching modes is very difficult as these modes are strongly coupled with the m(ring), a(ring) and b(CACN) modes (Table 7). The CACN bonds bear partial double bond character. Therefore, its frequency should lie between the m(CAC) and m(C@C) modes. The Raman band 1331 (m) cm1 is assigned to the m(CACN) mode which could be correlated to the computed wavenumber 1320 cm1 whereas the ipc m(CACN) mode is calculated to be 1051 cm1 and is observed at 1055 cm1 in the IR spectrum. The two CACN in-plane bending vibrations are calculated to be 120 and 144 cm1, the former one of which is observed at 126 cm1 with weak intensity in Raman spectrum. The non-planar CACN deformation modes are calculated to be 148 and 292 cm1 and the latter one is correlated to the observed band corresponding to the frequency 290 cm1.

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Table 7 PEDs of the DDQ molecule. Potential energy distributiona

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

U(ring)(60) + U(ring)(36) U(ring)(67) + U(ring)(12) + c(C11AN13)(6) + c(C12AN14)(6) U(ring)(79) + c(C1ACl8)(4) + c(C2ACl7)(3) + U(ring)(3) b(C5AC12)(26) + b(C4AC11)(26) + b(C12AN14)(22) + b(C11AN13)(22) b(C4AC11)(22) + b(C5AC12)(22) + b(C11AAN13)(21) + b(C12AN14)(21) + b(C1ACl8)(4) + b(C2-Cl7)(4) c(C5AC12)(25) + c(C4AC11)(25) + c(C1ACl8)(9) + c(C2ACl7)(9) + U(ring)(7) + c(C12AN14)(5) + c(C6AO10)(5) + c(C11AN13)(5) + c(C3AO9)(5) + U(ring)(4) b(C1ACl8)(45) + b(C2ACl7)(45) + a(ring)(3) c(C1ACl8)(35) + c(C2ACl7)(35) + c(C11AN13)(7) + c(C12AN14)(7) + c(C4AC11)(5) + c(C5AC12)(5) a(ring)(23) + b(C2ACl7)(17) + b(C1ACl8)(17) + a(ring)(8) + b(C6AO10)(8) + b(C3AO9)(8) + m(C5AC6)(3) + m(C3AC4)(3) c(C4AC11)(22) + c(C5AC12)(22) + U(ring)(20) + c(C12AN14)(11) + c(C11AN13)(11) + U(ring)(9) a(ring)(25) + m(C2AC3)(11) + m(C1AC6)(11) + b(C2ACl7)(10) + b(C1ACl8)(10) + a(ring)(9) + b(C11AN13)(5) + b(C12AN14)(5) + m(C1ACl8)(4) + m(C2ACl7)(4) a(ring)(27) + m(C2ACl7)(12) + m(C1ACl8)(12) + m(C1AC6)(10) + m(C2AC3)(10) + a(ring)(8) + m(C5AC6)(4) + m(C3AC4)(4) + m(C5AC12)(3) + m(C4AC11)(3) b(C6AO10)(28) + b(C3AO9)(28) + m(C5AC6)(9) + m(C3AC4)(9) + a(ring)(5) + m(C4AC5)(4) + m(C1AC2)(4) b(C12AN14)(18) + b(C11AN13)(18) + b(C3AO9)(14) + b(C6AO10)(14) + m(C3AC4)(8) + m(C5AC6)(8) + m(C1ACl8)(4) + m(C2ACl7)(4) + b(C5AC12)(3) + b(C4AC11)(3) a(ring)(22) + b(C11AN13)(13) + b(C12AN14)(13) + a(ring)(7) + b(C4AC11)(5) + b(C5AC12)(5) + m(C2AC3)(5) + m(C1AC6)(5) + m(C3AC4)(4) + m(C5AC6)(4) + m(C2ACl7)(4) + m(C1AC2)(4) + m(C1ACl8)(4) c(C2ACl7)(35) + c(C1ACl8)(35) + U(ring)(12) + U(ring)(7) c(C11AN13)(27) + c(C12AN14)(27) + c(C5AC12)(12) + c(C4AC11)(12) + c(C3AO9)(5) + c(C6AO10)(5) + c(C1ACl8)(5) + c(C2ACl7)(5) a(ring)(42) + m(C1ACl8)(11) + m(C2ACl7)(11) + m(C5AC12)(9) + m(C4AC11)(9) b(C12AN14)(14) + b(C11AN13)(14) + b(C5AC12)(13) + b(C4AC11)(13) + a(ring)(12) + m(C4AC5)(11) + a(ring)(4) + m(C1AC2)(3) c(C4AC11)(21) + c(C5AC12)(21) + U(ring)(14) + c(C1ACl8)(11) + c(C2ACl7)(11) + c(C11AN13)(8) + c(C12AN14)(8) c(C6AO10)(30) + c(C3AO9)(29) + U(ring)(9) + c(C5AC12)(7) + c(C4AC11)(7) + U(ring)(5) + c(C1ACl8)(5) + c(C2ACl7)(5) U(ring)(33) + c(C3AO9)(15) + c(C6AO10)(15) + c(C4AC11)(11) + c(C5AC12)(11) + c(C2ACl7)(7) + c(C1ACl8)(7) b(C6AO10)(22) + b(C3AO9)(21) + b(C1ACl8)(15) + b(C2ACl7)(15) + b(C5AC12)(9) + b(C4AC11)(9) m(C1ACl8)(22) + m(C2ACl7)(22) + m(C4AC11)(11) + m(C5AC12)(11) + b(C3AO9)(10) + b(C6AO10)(10) m(C1ACl8)(22) + m(C2ACl7)(22) + a(ring)(13) + m(C2AC3)(8) + m(C1AC6)(8) + a(ring)(7) + m(C4AC11)(4) + m(C5AC12)(4) m(C5AC6)(18) + m(C3AC4)(17) + b(C5AC12)(12) + b(C4AC11)(12) + m(C4AC11)(9) + m(C5AC12)(9) + a(ring)(5) + b(C1ACl8)(3) + b(C2ACl7)(3) m(C5AC12)(16) + m(C4AC11)(16) + m(C2AC3)(14) + m(C1AC6)(14) + m(C2ACl7)(9) + m(C1ACl8)(8) + m(C3AC4)(6) + m(C5AC6)(6) a(ring)(23) + m(C2AC3)(17) + m(C1AC6)(17) + m(C1ACl8)(7) + m(C2ACl7)(7) + m(C5AC12)(5) + m(C4AC11)(5) + b(C2ACl7)(4) + b(C1ACl8)(4) m(C5AC6)(19) + m(C3AC4)(19) + m(C1AC6)(15) + m(C2AC3)(14) + b(C6AO10)(10) + b(C3AO9)(10) + m(C5AC12)(4) + m(C4AC11)(4) m(C3AC4)(14) + m(C5AC6)(14) + a(ring)(14) + m(C4AC11)(11) + m(C5AC12)(10) + m(C2AC3)(6) + m(C1AC6)(5) + a(ring)(5) + b(C3AO9)(5) + b(C6AO10)(5) + a(ring)(4) m(C1AC2)(68) + m(C4AC5)(10) m(C4AC5)(63) + m(C1AC2)(10) + a(ring)(4) + m(C5AC12)(4) + m(C4AC11)(4) m(C3AO9)(64) + m(C6AO10)(24) + a(ring)(4) m(C6AO10)(63) + m(C3AO9)(23) + a(ring)(7) m(C11AN13)(44) + m(C12AN14)(44) + m(C4AC11)(6) + m(C5AC12)(6) m(C12AN14)(44) + m(C11AN13)(43) + m(C5AC12)(6) + m(C4AC11)(6)

The mode in bold letters in the last column corresponds to the assigned mode. a Same as in Table 6

P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347

Mode No.

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P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347 Table 8 Calculated frequencies (xi), Vi, gi, Si, DEi and li for totally symmetric modes of neutral and anionic state of DDQ.a Mode No.

m4 m7 m12 m13 m15 m19 m24 m27 m29 m31 m32 m33 m35 a

Neutral state of DDQ

Anionic state of DDQ

Modes

Frequency

Vi

gi

Si

DE i

li

Frequency

Vi

gi

Si

DE i

li

120 207 343 380 469 526 796 1051 1242 1593 1644 1749 2336

0.05 0.06 0.75 0.23 0.56 0.95 1.57 2.41 1.85 0.52 3.25 5.05 1.26

0.39 0.19 1.22 0.32 0.57 0.81 0.72 0.73 0.43 0.08 0.50 0.71 0.11

0.07 0.02 0.74 0.05 0.16 0.33 0.26 0.27 0.09 0.00 0.13 0.25 0.01

1.10 0.49 31.46 2.36 9.44 21.21 25.59 34.59 14.49 0.70 25.62 54.59 1.90

0.35 0.16 9.99 0.75 3.00 6.74 8.13 10.99 4.60 0.22 8.14 17.34 0.60

129 210 345 369 484 536 778 1034 1208 1463 1560 1607 2300

0.03 0.05 0.50 0.33 0.90 1.08 1.34 1.50 2.16 0.76 1.13 6.17 1.39

0.22 0.17 0.81 0.48 0.87 0.90 0.63 0.46 0.53 0.14 0.19 0.99 0.13

0.02 0.01 0.32 0.11 0.38 0.40 0.20 0.11 0.14 0.01 0.02 0.49 0.01

0.39 0.37 13.88 5.19 22.89 26.84 19.41 13.75 21.02 1.78 3.43 96.72 2.39

0.13 0.12 4.41 1.65 7.27 8.52 6.16 4.37 6.68 0.56 1.09 30.72 0.76

b(CAC) b(CACl) m(ring) b(C@O) a(ring) b(C„CN) m(CACl) m(CAC) m(ring) m(ring) m(ring) m(C@O) m(C„CN)

xi in cm1, Vi in 104 a.u., gi, Si, DEi, li in meV.

Table 9 Calculated frequencies (xi), Vi, gi, Si, DEi and li for totally symmetric modes of ground and first excited state of DDQ.a Mode no.

m4 m7 m12 m13 m15 m19 m24 m27 m29 m31 m32 m33 m35 a

Ground state of DDQ

First excited state of DDQ

Modes

xi

Vi

gi

Si

DEi

li

Frequency

Vi

gi

Si

DE i

li

120 208 343 380 467 526 796 1051 1242 1593 1643 1749 2336

0.05 0.01 1.23 0.06 0.55 0.95 0.94 0.32 5.58 1.92 5.61 2.71 1.30

0.40 0.03 2.00 0.08 0.56 0.81 0.43 0.10 1.31 0.31 0.87 0.38 0.12

0.08 0.00 1.99 0.00 0.16 0.33 0.09 0.00 0.86 0.05 0.38 0.07 0.01

1.17 0.01 84.77 0.14 9.20 21.42 9.12 0.61 132.16 9.52 76.48 15.72 2.04

0.37 0.00 26.92 0.04 2.92 6.80 2.90 0.19 41.97 3.02 24.29 4.99 0.65

123 221 345 367 480 524 820 975 1249 1423 1457 1681 2310

0.03 0.08 0.68 0.40 0.79 1.04 0.93 2.09 1.33 1.38 1.64 6.07 1.42

0.24 0.25 1.08 0.58 0.78 0.89 0.41 0.71 0.31 0.26 0.30 0.91 0.13

0.03 0.03 0.59 0.17 0.30 0.40 0.08 0.25 0.05 0.04 0.05 0.41 0.01

0.45 0.85 25.14 7.71 17.97 25.99 8.52 30.23 7.49 6.19 8.29 85.37 2.47

0.14 0.27 7.98 2.45 5.71 8.25 2.71 9.60 2.38 1.96 2.63 27.11 0.79

b(CAC) b(CACl) m(ring) b(C@O) a(ring) b(C„CN) m(CACl) m(CAC) m(ring) m(ring) m(ring) m(C@O) m(C„CN)

xi in cm1, Vi in 104 a.u., gi, Si, DEi, li in meV.

C„N modes The C„N stretching vibrations gives rise to its characteristics frequency in the narrow spectral range 2220–2240 cm1 and is highly localized within the C„N group. In the present case, the m(C„N) modes are observed as the medium Raman band 2236 cm1 and weak IR band 2233 cm1, with the corresponding calculated frequencies 2336 and 2347 cm1 having very strong intensities in the Raman spectrum and weak intensities in the IR spectrum. The planar bending vibrations of the C„N groups have been calculated to have magnitudes 428 (b2) and 526 (a1) cm1 whereas the non-planar bending vibrations have been calculated to be 490 (b2) and 638 (a1) cm1. Only one weak IR band is observed at 421 cm1 which could be correlated to the computed b(C„N) frequency 428 (b2) cm1 and a medium Raman band at 516 cm1 could be correlated to the computed b(C„N) frequency 526 (a1) cm1. The non-planar C„N bending modes could not be observed. Comparative study of DDQ with DDQ⁄ and DDQ Under this section, the vibrational modes of DDQ, DDQ⁄ and DDQ molecules Table 6 have been compared. Quinoid ring modes Addition of an electron to the DDQ molecule as well as electronic excitation to its first excited state lead to significant changes in the m(ring). The ring frequencies corresponding to the C1@C2 and

C4@C5 bonds shift toward the lower side by 170 and 131 cm1 in going from DDQ to DDQ⁄ and by 190 and 184 cm1 in going from DDQ to DDQ. However, the frequencies corresponding to the ring breathing mode is not affected in going from DDQ to DDQ⁄ and from DDQ to DDQ whereas, frequencies of the two m(ring) modes under the b2 species are enhanced by 50 and 30 cm1 in going from DDQ to DDQ⁄ and 30 and 20 cm1 in going from DDQ to DDQ due to the shortening of the CAC bond lengths of quinoid ring. The remaining m(ring) under the species a1 exhibits a totally different behavior in going from DDQ to DDQ. The wavenumber corresponding to this mode is decreased by  130 cm1. Significant changes in the frequencies for the planar ring deformation and the non-planar ring deformation modes are not observed. CAX modes (X = O, Cl, C, N) As could be seen from the Table 2, the C@O bond lengths are enhanced by 0.015/0.030 Å in going from DDQ to DDQ⁄/DDQ. Due to this reason the frequencies corresponding to the m(C@O) modes under the species a1/b2 are reduced by 70/80 cm1 and 140/180 cm1 in going from DDQ to DDQ⁄ and DDQ, respectively. The frequencies for one of the two b(C@O) modes under the species b2 and c(C@O) modes under the species a2 are not changed significantly whereas the other b(C@O)/c(C@O) mode is enhanced/ reduced by 10/30 and 10/7 cm1 in going from DDQ to DDQ⁄ and DDQ. All the studied molecular species have two m(CACl) modes whose frequencies are enhanced by 30 and 20 cm1 in going from

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Fig. 6. (a) Experimental IR spectrum of DDQ (b) theoretical IR spectrum of DDQ (c) Experimental Raman spectrum of DDQ (d) theoretical Raman spectrum of DDQ.

DDQ to DDQ⁄ and are reduced by 20 and 20 cm1 in going from DDQ to DDQ due to the decreased/increased bond length CACl is in going from DDQ to DDQ⁄/DDQ. Only the CACl non-planar deformation under the species a2 is found to decrease in going from DDQ to DDQ⁄/DDQ. Unusual variations of frequencies corresponding to the m(CACN) modes are noticed on ionization and excitation. The higher frequency mode corresponding to the m(CACN) (b2) is increased whereas the lower frequency corresponding to the m(CACN) (a1) is decreased. Here, again only the CACN non-planar deformation under the species a2 is found to increase in going from DDQ to DDQ⁄/DDQ. The two C„N stretching frequencies decrease by 25 and 30 cm1 in going from DDQ to DDQ⁄ whereas, these decrease by 35 and 40 cm1 in going from DDQ to DDQ. For the above vibrational modes, both the IR intensity and Raman activity increase significantly in going from neutral species to its anion. It could also be seen (Table 6) that the calculated wavenumber for the out-of-plane deformation of C„N under the species b1 is increased by 30/ 40 cm1 in going from DDQ to DDQ⁄/DDQ. Vibronic and electron–phonon interaction The strength of the VC corresponding to the vibrational mode i (Vi) in 104 a.u., the dimensionless vibronic coupling constant (gi) in meV, Rys–Huang factor (Si) in meV, the reorganization energy (DEi) in meV and the electron–phonon coupling constant (li) in meV for the totally symmetric modes for transition from DDQ to DDQ and from DDQ to DDQ are listed in Table 8 whereas those from DDQ to DDQ⁄ and from DDQ⁄ to DDQ are listed in Table 9. The present results indicate that the contribution to the vibronic coupling comes almost from all the modes; however, the C@O stretching vibrational modes has relatively larger VCCs as compared to the other vibrational modes of DDQ. The contribution to the vibronic coupling is also noticed for the vibrational modes which include m(CAC), m(ring), m(CACl) and b(C„CN) vibrations in case of transition from DDQ to DDQ as the atoms involved in the above modes are participating in intermolecular interaction, discussed in the Section ‘Molecular structure’. Charge transport

along the stacks of conducting organic molecules involves electron transfer from a neutral molecule to an adjacent charged ion. In going from a neutral molecule to its negatively charged ion and vice versa the geometry is modified which involves relaxation energy. During the m(CAC), m(ring), m(CACl) and b(C„CN) normal modes of vibration under the a1 species the molecular geometry changes continuously between the two extreme configurations. During certain vibrational modes the geometries corresponding to the neutral molecule and its radical ion would lie between these two extreme configurations. The neutral molecule during such a vibrational mode may undergo electron transfer and yields its radical ion and vice versa without any excess requirement of activation/relaxation energy as and when its geometry becomes identical to the geometry of its radical ion. The dimensionless vibronic coupling constant (gi), calculated using the Eq. (6) (Table 8), is a measure of the strength of the vibronic coupling. The magnitude of gi is noticed to be larger for the ring stretching mode (breathing mode @m12) as compared to the other modes. The vibronic coupling strength is found negligible for the vibrational modes corresponding to the m7, m31 and m35 modes. As mentioned in the introduction section, Si (Rys–Huang factor) is another measure of the coupling strength and calculated in the present work by using Eq. (7). The contribution of modes in Si is found to be the same as that for gi. The reorganization of the mode i is calculated with the help of Eq. (7). The contribution to DEi from the C@O stretching vibration is larger in transition from DDQ to DDQ than the remaining transitions. One can estimate electron–phonon coupling constants li from the dimensionless vibronic coupling constants by using the Eq. (8). The calculated electron–phonon coupling constants in transition from DDQ to DDQ (Table 8) are shown in Fig. 8(a). One can see from the Fig. 8(a) that the a1 C@O stretching mode (1749 cm1) is strongly coupled to the HOMO of DDQ whose electronic orbital symmetry is A1. This can be understood in view of the orbitals patterns of the A1 HOMO and vibrational modes of DDQ. When DDQ is distorted along the a1 mode (C@O stretching) in the direction shown in the Fig. 7, the antibonding interaction between (C3 and O9) and (C6 and O10) becomes stronger and antibonding interaction between (C2 and C3) and (C1 and C6) and bonding interaction between (C3 and C4) and (C5 and C6) become weaker. Therefore,

P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347 o

o o o

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120 cm-1

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57 cm-1

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144 cm-1

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257 cm-1

292 cm-1

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428 cm-1

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469 cm-1

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526 cm-1

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766 cm-1

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1750 cm

-1

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1753 cm

2336 cm-1

Fig. 7. Pictorial representations of normal modes of DDQ.

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1644 cm-1

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1593 cm-1

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1320 cm-1

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1242 cm-1

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1168 cm-1

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1051 cm-1

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796 cm-1

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777 cm-1

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749 cm-1

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638 cm-1

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504 cm-1

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490 cm-1

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475 cm-1

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319 cm-1

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255 cm-1

2347 cm-1

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1345

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P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347

Fig. 8. Calculated electron–phonon coupling constant in (a) DDQ ? DDQ, (b) DDQ ? DDQ, (c) DDQ ? DDQ⁄ and (d) DDQ⁄ ? DDQ.

the HOMO is stabilized in energy. On the other hand, when the DDQ molecule is distorted in the opposite direction along the arrow of this mode, the HOMO is significantly destabilized in energy. This is the reason, why the C@O stretching vibration under the a1 symmetry strongly couples to the A1 HOMO electronic orbital in DDQ [34]. The C@O stretching vibrations have relatively larger VCCs in transition from DDQ to DDQ. The magnitudes of Vi corresponding to m(CAC), m(ring), m(CACl) and b(C„CN) are smaller and that corresponding to m(C@O) mode is larger in transition from DDQ to DDQ than in transition from DDQ to DDQ. Again the behavior of the change of magnitude of gi is same as that of Si with respect to change of frequencies. The main contributions to gi and Si come from the C@O stretching, ring breathing, planar ring deformation and b(C„CN) vibrational modes. The contribution of m(CAC) mode in vibronic coupling parameters becomes weaker in going from DDQ ? DDQ to DDQ ? DDQ transition. In reorganization energy, the C@O stretching mode is dominated as compared to the other symmetric vibrational modes in DDQ ? DDQ transition. Significantly, the large electron–phonon coupling constant for the DDQ ? DDQ transition comes from the contribution from the C@O stretching mode, for which the VCC is large as shown in Fig. 8(b). It could be explained with the help of molecular orbitals. When the anion molecule is distorted along the C@O stretching mode in the direction, as shown in Fig. 7 the antibonding interaction between (C3 and O9) and (C6 and O10) becomes stronger and bonding interaction between (C2 and C3) and (C1 and C6), (C3 and C4) and (C5 and C6) becomes weaker. Therefore, the B2 HOMO of

DDQ is significantly stabilized in energy. On the other hand, when the DDQ molecule is distorted in the opposite direction along the arrow of this mode, the antibonding interaction between (C3 and O9) and (C6 and O10) becomes weaker and bonding interaction between (C2 and C3) and (C1 and C6), (C3 and C4) and (C5 and C6) becomes stronger and thereby destabilizating the energy of the B2 HOMO of DDQ. This is the reason why the a1 modes strongly couple to the B2 HOMO in the species DDQ. The behavior of vibronic coupling is noticed completely different for transition DDQ ? DDQ⁄ and DDQ⁄ ? DDQ from the above two vibrations. The VCCs is dominant for the C@O stretching 1749 cm1 and the ring stretching mode 1242 cm1 for the DDQ ? DDQ⁄ transition, whereas, only for the C@O stretching vibration 1681 cm1 for DDQ⁄ ? DDQ. The dimensionless vibronic coupling constant and Rhys–Huang factor are noticed to be higher for the ring breathing stretching mode 343 and 345 cm1 for the DDQ ? DDQ⁄ and DDQ⁄ ? DDQ, respectively. In the DDQ ? DDQ⁄ transition, reorganization energy and electron phonon coupling come mainly from the ring stretching mode 1242 cm1, whereas, the C@O stretching vibration participates strongly in reorganization energy and electron–phonon coupling constant for the DDQ⁄ ? DDQ transition as shown in Fig. 8(c) and (d). Conclusions All of the studied molecules possess planar structure with C2v point group of symmetry. The structural parameters derived from the experimental XRD data agree quite well with the DFT

P. Rani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1334–1347

optimized structural data. The differences between the experimental and calculated dihedral angles could be due to the interaction of molecule with the molecules in neighboring unit cells and same unit cell in experimental treatment whereas in DFT calculations, molecule is treated as an isolated entity. From the present study one could conclude that in the DDQ molecule the Cl and O atoms, cyno group and phenyl ring participate in the charge transfer process. The electron-molecular-vibration and electron–phonon coupling and their roles in occurrence of the conductivity in the DDQ, DDQ and DDQ⁄ molecules have been investigated. The vibrational modes m(CAC), m(ring), m(CACl) and b(C„CN) under the species a1 of the DDQ molecule result geometry identical to DDQ ion and vice versa. Therefore, large vibronic coupling constants of these modes (Table 8) show that addition of an electron to DDQ or tendency of DDQ to donate an electron is much easier during above mentioned vibrational modes. Thus electronmolecular vibration interaction and electron–phonon interaction provide a direct link between the geometric and electronic structures of the molecule and its transport properties in the material. Acknowledgements The authors would like to express their deepest gratitude to Dr. Naoya Iwahara, Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan, for helpful discussion with him. The authors are also thankful to the Head, Department of Chemistry, Banaras Hindu University, for giving permission to use the FTIR, the Oxford Diffraction X-calibur CCD differactomater for recording single crystal data and the Shimadzu UV-1700 PharmaSpecUV/Vis spectrophotometer for recording electronic absorption spectrum. They are also grateful to Dr. V.G. Sathe, UGC-DAE Consortium for Scientific Research, Indore Centre (India), for giving permission to use the Raman spectrometer. One (Poonam Rani) of the authors is thankful to the Banaras Hindu University and UGC, New Delhi for providing financial support. Gunjan Rajput is thankful to the Banaras Hindu University and CSIR, New Delhi for providing financial support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.08.044. References [1] M. Fourmigué, P. Batail, Chem. Rev. 104 (2004) 5379–5418. [2] M. Iyoda, Y. Kuwatani, E. Ogura, K. Hara, H. Suzuki, T. Takano, K. Takeda, J. Takano, K. Ugawa, M. Yoshida, H. Matsuyama, H. Nishikawa, I. Ikemoto, T. Kato, N. Yoneyama, J. Nishijo, A. Miyazaki, T. Enoki, Heterocycles 54 (2001) 833–848. [3] A.S. Batsanov, M.R. Bryce, A. Chesney, J.A.K. Howard, D.E. John, A.J. Moore, C.L. Wood, H. Gershtenman, J.Y. Becker, V.Y. Khodorkovsky, J. Mater. Chem. 11 (2001) 2181–2191. [4] M. Iyoda, H. Suzuki, S. Sasaki, H. Yoshino, K. Kikuchi, K. Saito, I. Ikemoto, H. Matsuyama, T. Mori, J. Mater. Chem. 6 (1996) 501–503. [5] J. Lieffrig, O. Jeannin, K-S. Shin, P. Auban-Senzier, M. Fourmigué, Crystals 2 (2012) 327–337.

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