Journal of Molecular Structure 1054–1055 (2013) 199–208

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Journal of Molecular Structure

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UV–Vis, IR spectra and thermal studies of charge transfer complexesformed in the reaction of 4-benzylpiperidine with r- and p-electronacceptors

0022-2860/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.molstruc.2013.09.007

⇑ Corresponding author. Tel.: +974 4423 0018; fax: +974 4423 0060.E-mail address: bazzi@tamu.edu (H.S. Bazzi).

Adel Mostafa, Nada El-Ghossein, G. Benjamin Cieslinski, Hassan S. Bazzi ⇑Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar

h i g h l i g h t s

� Three CT-complexes of donor 4BPwith the acceptors DDQ, TBCHD andiodine are obtained.� An adduct 7,7,8-tricyano-8-

benzylpiperidinylquinodimethane[TCBPQDM] is produced with TCNQ.� The CT-complexes are characterized

through FTIR and UV Vis, elementaland thermal analysis.� The values of KCT, eCT, ECT, DG� and Ip

are calculated.

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

32

3

3

23

3

• Donor : 4-Benzylpiperidine (4BP)

• Acceptors : DDQ, TCNQ, TBCHD and I3

• Adduct : TCBPQDM

a r t i c l e i n f o

Article history:Received 16 June 2013Received in revised form 5 September 2013Accepted 5 September 2013Available online 11 September 2013

Keywords:4-BenzylpiperidineCharge-transferSpectraTCNQThermal

a b s t r a c t

The reactions of the electron donor 4-benzylpiperidine (4BP) with the r-acceptor iodine and p-acceptors7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), and2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) were studied spectrophotometrically in chloroformat room temperature. The electronic and infrared spectra of the formed molecular charge-transfer (CT) com-plexes were recorded. Based on the obtained data, the charge-transfer complexes were formulated as½ð4BPÞI�þI�3 , [(4BP)(DDQ)2], and [(4BP)(TBCHD)] for the donor (4BP) and the acceptors I2, DDQ and TBCHD.In the 4BP-TCNQ reaction, a short-lived CT complex is formed followed by rapid N-substitution by TCNQforming the final reaction product 7,7,8-tricyano-8-benzylpiperidinylquinodimethane [TCBPQDM]. Theseproducts were isolated as solids and have been characterized through electronic and infrared spectra as wellas elemental and thermal analysis measurements. The formation constants (KCT), charge transfer energy(ECT), molar extinction coefficients (eCT), free energy change DG� and ionization potential IP of the formedCT-complexes ½ð4BPÞI�þI�3 , [(4BP)(DDQ)2] and [(4BP)(TBCHD)] were obtained.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The donor molecule used in this study was 4-benzylpiperidine(4BP), a pharmaceutical research drug. 4BP acts as a monoaminereleasing agent with 20- to 48-fold selectivity for releasing dopa-mine versus serotonin. It has a fast onset of action and a short

duration. The charge-transfer (CT) complexes formed in the reac-tions of r- and p-electron acceptors with organic electron donorshave garnered attention for non-linear optical materials and elec-trical conductivities owing to their significant physical and chem-ical properties [1–4]. The chemical and physical properties of CT-complexes formed in the reactions of p- and r-electron acceptorswith different donors (e.g., amines, polysulfurs, crown ethers basesand oxygen–nitrogen mixed bases) have been the subjects of manystudies both in solution and in solid state [5–14]. The formation of

200 A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208

a particular polyiodide species depends on the nature of the donorbase and in some cases on the method of preparation [15–18]. Thep-electron acceptors 7,7,8,8-tetracyanoquinodimethane (TCNQ)and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) are known toform stable colored CT-complexes with many donor bases. The in-creased interest in the study of charge-transfer interactions stemsfrom the various applications of CT-complexes, including solarcells, electronics, and optical devices [19]. CT-complexes also actas an intermediate in a wide variety of reactions involving nucleo-philes and electron deficient molecules, playing very importantroles in biological systems [20]. In the paper herein, we reportthe formation of three new CT-complexes produced from the reac-tion of 4BP with the r-acceptor iodine (I2)and the p-acceptors DDQand 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) in CHCl3 asthe solvent. With the acceptor TCNQ, a short-lived CT- complex isformed followed by rapid N-substitution by TCNQ creating the fi-nal reaction product 7,7,8-tricyano-8-benzylpiperidinylquino-dimethane [TCBPQDM]. The aim of this work is to make anassessment of the correct nature and stoichiometry of each of theresulting new CT-complexes formed with each acceptor.

Table 1Spectrophotometric data for the formed 4-benzylpiperidine (4BP) acceptorcomplexes.

Complex Color Absorptiona (nm) Stoichiometry

(Donor:acceptor)½ð4BPÞI�þI�3 Dark brown 388s, 264s 1:2[(4BP)(DDQ)2] Dark red 727s, 491m 1:2[(4BP)(TBCHD)] Brownish green 557s, 442m 1:1

a The reactants 4BP, I2, TCNQ, DDQ and TBCHD have no measurable absorptionsin the region of study with used concentrations; m, medium; s, strong; sh, shoulder.

2. Experimental

2.1. Materials and measurements

Reagent grade chemicals were used in this study and purchasedfrom Sigma–Aldrich, USA, and used as received. The adductTCBPQDM was checked using Thermo Scientific LC MS/MS ModelTSQ. The electronic absorption spectra of the CHCl3 solutions of thesolid CT-complexes formed in the reactions of the donor 4BP andthe acceptors I2, TCNQ, DDQ, and TBCHD as well as the reaction prod-ucts were checked in the 1200–250 nm regions using a Lambda 950Perkin Elmer UV–Vis spectrometer with a quartz cell of 1.0 cm path

length. Elemental analysis was done using a Perkin Elmer CHNSOelemental analyzer Model 2400 Series II. The infrared spectra ofthe reactants, 4BP, TCNQ, DDQ and TBCHD (I2 has no infrared activ-ity) and the obtained CT-complexes (KBr pellets) were recorded on aSpectrum One Perkin Elmer FTIR spectrometer while thermogravi-metric analysis (TG & DTG) were performed using a Perkin Elmerthermal analyzer Model Pyris 6 TGA under flow of nitrogen gas(20 ml min�1) with a heating rate of 10 �C min�1.

2.2. Photometric titration

Photometric titration measurements were performed for thereactions between the donor 4BP and each of the acceptors I2,DDQ and TBCHD in CHCl3 at 25 �C in order to determine the reac-tion stoichiometries according to a literature method [5,21]. Themeasurements were conducted under the conditions of fixed donor4BP concentrations while those of the acceptors I2, DDQ, andTBCHD were changed over a wide range, to produce reaction solu-tions where the molar ratio of donor:acceptor varies from 1:0.25 to1:4. The peak absorbencies of the formed CT complexes were mea-sured for all solutions and plotted as a function of the acceptor todonor molar ratio.

A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208 201

2.3. Preparation of the solid CT-complexes

The four solid CT-complexes formed in the reaction of 4BP withI2, TCNQ, DDQ and TBCHD were isolated in CHCl3 by the drop-wiseaddition of a saturated solution (70 ml) of each of the donors to asaturated solution (85 ml) of each of the acceptors. The mixturein each case was stirred for 10–15 min. The mixing of reactantswas associated with a strong change in color. The resulting precip-itate was immediately filtered, washed several times with mini-mum amounts of CHCl3, and dried under vacuum. The complexeswere characterized using spectroscopic techniques (FTIR and UVVis) and by elemental analysis (theoretical values are shown inbrackets): [TCBPQDM] yellow adduct (M/W: 352.46 g); C, 78.03%(78.31%); H, 5.59% (5.67%); N, 15.81% (15.89%); [(4BP)I]+I-

3 darkbrown complex (M/W: 682.89 g); C, 20.98 % (21.09%) H, 2.46%(2.49%); N, 1.99% (2.05%); [(4BP)(DDQ)2]dark red complex (M/W:629.27 g); C, 53.53% (53.39%); H, 2.66% (2.70%); N, 11.07%(11.12%) and [(4BP)(TBCHD)] brownish green complex (M/W:584.97 g); C, 36.78% (36.92%); H, 3.28% (3.25%); N, 2.37% (2.39%).

3. Results and discussions

3.1. Electronic absorption spectra

The electronic absorption bands of the CHCl3 solutions of the formed solid CT- complexes ½ð4BPÞI�þI�3 , [(4BP)(DDQ)2] and [(4BP)(TBCHD)]and the adduct[TCBPQDM] are given in Table1. The elemen-tal analysis of these solid products supports the proposed complexstructures and are in good agreement with the stoichiometric reac-tions based on photometric titration methods as indicated below.

3.1.1. Reaction of 4BP with TCNQUpon the addition of the 4-benzylpiperidine (4BP) to a solution

of TCNQ in CHCl3, a dark blue color was formed which changedvery fast forming a stable yellow yield. Fig. 1 shows the electronicspectrum recorded at the 450–950 nm regions of the reactants 4BPand TCNQ of the final yellow product. Interestingly, both reactantsdo not show any absorption in the region of study while the reac-tion product has absorptions at 831 nm, 765 nm, 746 nm, 582 nmand 542 nm. The formation of the short-lived dark blue productwhich gives the stable yellow reaction product is explained by4BP, acting as an electron donor, reacting with p-acceptor TCNQforming the 4BP–TCNQ CT-complex. This short-lived, dark blueproduct undergoes elimination reaction forming the yellow finalproduct which is identified by elemental analysis to be[TCBPQDM], as seen in Formula 1. A general mechanism for thereaction is proposed as follows:

Fig. 1. Electronic absorption spectra of 4-benzylpiperidine (4BP)–TCNQ reaction inCHCl3. (A) [4BP] = 5 � 10�3 M; (B) [TCNQ] = 3 � 10�3 M; (C) 1: 2 4BP-TCNQ mixture,[4BP] = 5 � 10�3 M and [TCNQ] = 3 � 10�3 M.

(i) Formation of CT-complex:

TCNQ þ 4BP! ½ð4BPÞðTCNQÞ� ðdark blue short-livedÞ

(ii) N-substitution by TCNQ:½ð4BPÞ ðTCNQÞ� ! 7;7;8-tricyano-8-benzylpiperidinylquinodimethane

ðTCBPQDMÞ þHCN ðyellow solid; stableÞ

Formula 1. 7,7,8-Tricyano-8-benzylpiperidinylquinodimethane[TCBPQDM].

These results agree quite well with that reported by Frey et al.[22] on their reaction between the p-acceptor tetracyanoethylene(TCNE) and electron donor-like aniline where the later undergoesrapid N-substitution by TCNE:

C6H5NH2 þ C6N4 ! C8H5NHðCNÞ3 þHCN

And with the work of Aljaber and Nour [23] on the reaction ofTCNE and o-phenylene diamine.

C6H4ðNH2Þ2 þ C6N4 ! C10H6N4 þ 2HCN

Mass spectral measurements were performed only for the 4BP–TCNQ adduct [TCBPQDM]. Fig. 2 shows the mass spectrum in the re-gion 100–400 m/z. The molecular ion M+ isobserved as a medium peakat m/z 351 in good agreement with the calculated value for the molec-ular weight of the reaction product [TCBPQDM] of 352.46 g/mol. Thedifference of about 1.42 g/mol between the observed and calculatedmolecular weight value is acceptable within the allowed experimen-tal errors. The mass spectrum (Fig. 2) also shows a number of otherpeaks. The peaks at m/z 176 and 177 are related to the formation offragments shown in (A) and (B) respectively.

(A) m/z 176 (B) m/z 177

3.1.2. Reaction of 4BP with I2

Strong absorption bands, due to the formed product, appearedat 388 nm and 264 nm for 4BP–I2with a color change from yellowto dark brown as shown in Fig. 3. Photometric titration

Fig. 2. Mass spectrum for 4-tricyanobenzylpiperidinyl-8-quinodimethane (TCBPQDM).

Fig. 3. Electronic absorption spectra of 4-benzylpiperidine (4BP)–I2 reaction inCHCl3. (A) [4BP] = 5 � 10�3 M; (B) [I2] = 5 � 10�3 M; (C) 1:2 4BP–I2 mixture,[4BP] = [I2] = 5 � 10�3 M; the band at 264 nm of diluted solution of 4BP/I2 CT-complex (inset).

Fig. 4. Photometric titration curves for 4-benzylpiperidine (4BP)–I2 reaction inCHCl3 measured at 388 nm.

202 A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208

measurements for the reaction in CHCl3 were performed. The mo-lar ratio was found to be 1:2, as shown in Fig. 4. This reaction stoi-chiometry was supported by the obtained elemental analysis of theformed solid products. However, two absorption bands of the

formed iodine complex around 388 nm and 264 nm (Fig. 3) areknown to be characteristics of the formation of the polyiodideion of the type I�3 [24]. The reactions should occur in the followingsteps:

Fig. 6. Photometric titration curves for 4-benzylpiperidine (4BP) – DDQ reaction inCHCl3 measured at 727 nm and 491 nm.

Fig. 7. Electronic absorption spectra of 4-benzylpiperidine (4BP)–TBCHD reactionin CHCl3. (A) [4BP] = 5 � 10�3 M; (B) [TBCHD] = 1 � 10�3 M; (C) 1:1 4BP–I2 mixture,[4BP] = 5 � 10�3 M and [TBCHD] = 1 � 10�3 M.

A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208 203

ðiÞ ð4BPÞ þ I2 ! ½ð4BPÞ�I2

� Formation of the inner complex,ðiiÞ ½ð4BPÞ�I2 ! ½ð4BPÞI�þI�

� Formation of the triiodide CT-complexðiiiÞ ½ð4BPÞI�þI� þ I2 ! ½ð4BPÞI�þI�3

The structure of the formed CT-complex should be ½ð4BPÞI�þI�3and the formation of iodide intermediate ½ð4BPÞI�þI� is character-ized by its known absorption [25] at 245 nm.

3.1.3. Reaction of 4BP with DDQ and TBCHDFigs. 5 and 7 show strong absorption bands due to the formed

product at 727 nm and 491 nm for DDQ and at 557 nm and442 nm for TBCHD. The donor–acceptor molar ratio was found tobe different between the DDQ and TBCHD acceptors. The 4BP:acceptor ratios were found to be 1:2 and 1:1 for the DDQ andTBCHD complexes respectively as shown in Figs. 6 and 8. These ob-tained reaction stoichiometries are in very good agreement withthe obtained elemental analysis for the solid CT-complexes. Theformed CT-complexes are formulated as [(4BP)(DDQ)2] and[(4BP)(TBCHD)] with colors of reddish pink and apricot, respec-tively, and strong absorptions at 727 nm and 491 nm for DDQand absorptions at 557 nm and 442 nm for TBCHD. This variationin reaction stoichiometries could be associated with the molecularsteric hindrance of the acceptor, which is expected to be higher inthe case of TBCHD lowering the donor: acceptor ratio. The chargetransfer bands can be identified by their color, solvatochromism,and intensity. The color of CT-complexes is reflective of the relativeenergy balance resulting from the transfer of electronic chargefrom donor to acceptor. For solvatochromism, while in solutionthe transition energy and therefore the complex color varies withdifferences in solvent permittivity, indicating variation in shiftsof electron density as a result of the transition. This distinguishesit from the p ? p* transitions on the ligand. The CT absorptionbands are intense and often lie in the ultraviolet and visible portionof the spectrum.

The formation constant (KCT) and molar extinction coefficient(eCT) values for the formed CT-complexes of the donor4BP withI2, DDQ and TBCHD in CHCl3 at 25 �C were calculated. The 1:1 mod-ified Benesi–Hildebrand equation (1) [26] was used to calculate thevalues of the formation constant, KCT (L mol�1), and the molarextinction coefficient eC (L mol�1 cm�1), for the complex[(4BP)(TBCHD)].

Fig. 5. Electronic absorption spectra of 4-benzylpiperidine (4BP)–DDQ reaction inCHCl3. (A) [4BP] = 5 � 10�3 M; (B) [DDQ] = 5 � 10�3 M; (C) 1:2 4BP–DDQmixture,[4BP] = [DDQ] = 5 � 10�3 M.

Fig. 8. Photometric titration curves for 4-benzylpiperidine (4BP)–TBCHD reactionin CHCl3 measured at 557 nm and 450 nm.

A0D0‘

A¼ 1

keþ A0 þ D0

eð1Þ

The corresponding spectral parameters for the complexes½ð4BPÞI�þI�3 and [(4BP)(DDQ)2] was calculated using the known[27] Eq. (2) of 1:2 complexes:

Table 2Spectrophotometric and free energy change results of CT-complexes ½ð4BPÞI�þI�3 , [(4BP)(DDQ)2] and [(4BP)(TBCHD)] in CHCl3.

Complex KC/L (mol�1) kmax (nm) �DG� (cal mol�1) ECT (eV) eC (L mol�1 cm�1)

½ð4BPÞI�þI�3 0.249 � 105 388 5 � 103 3.21 0.332 � 103

[(4BP)(DDQ)2] 92.64 � 105 727 9 � 103 1.71 0.700 � 103

[(4BP)(TBCHD)] 0.016 � 105 557 4 � 103 2.81 0.750 � 103

204 A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208

ðA0Þ2D0‘

A¼ 1

keþ A0ðA0 þ 4D0Þ

eð2Þ

Fig. 10. Benesi–Hildebrand plots for the CT-complex [(4BP)(TBCHD)] at 557 nm and442 nm.

where A0 and D0 are the initial concentrations of the acceptors anddonors, respectively, while A is the absorbance at the mentioned CTbands and ‘ is the cell path length (1 cm).

The data obtained throughout this calculation are given inTable 2. Plotting the values of A0D0‘/A against (A0 + D0) values ofEq. (1) and plotting values of (A0)2D0‘/A versus A0 (A0 + 4D0) valuesof Eq. (2), linear lines were obtained with a slope of 1/eCT and inter-cept of (1/KCTeCT) as shown in Figs. 9 and 10.

In general these complexes show high values of both the forma-tion constant (KCT) and the molar extinction coefficient (eCT). Thesehigh values of KCT confirm the expected high stabilities of theformed CT-complexes as a result of the expected high donationof 4BP.

The formation constants are strongly dependent on the natureof the used acceptors including the type of electron withdrawingsubstituent to it such as cyano and halogen groups. The obtaineddata shows that the CT-complex [(4BP)(TBCHD)] has a much lowervalue of KCT compared with that of ½ð4BPÞI�þI�3 or [(4BP)(DDQ)2].This is explained by the differences in the electronic structure ofthe acceptors DDQ and TBCHD. The DDQ acceptor has two strong

Fig. 9. The plots of A0 (A0 + 4D0) values against [(A0)2D0‘/A] values of the charge-transfer complexes [(4BP)I]+ I�3 at 388 nm and [(4BP)(DDQ)2] at 727 nm.

withdrawing cyano groups in conjugation with an aromatic ringwhich causes high delocalization, leading to an increase in the Le-wis acidity of the acceptor, DDQ, and hence the higher value of KCT

for the [(4BP)(DDQ)2] CT-complex compared with that of theacceptor TBCHD. This allows for a stronger electron donation fromthe 4BP base to the acceptors DDQ and I2 compared to TBCHD.Accordingly, the donation from 4BP to the acceptors are in the or-der of DDQ > I2 > TBCHD.

For the charge transfer energy ECT there is a corresponding rela-tionship between the amount of transferred charge and the changeof the energy originating from charge transfer. As the amount oftransferred charge increases, the total energy decreases and theluminescence intensity increases. The charge transfer energy ECT

of the formed solid CT-complexes is calculated using the followingequation [28,29]:

ECT ¼1243:667

kCTð3Þ

where kCT is the wavelength of the band of the studied CT-com-plexes ½ð4BPÞI�þI�3 , [(4BP)(DDQ)2] and [(4BP)(TBCHD)]. The ECT val-ues calculated from Eq. (3) are listed in Table 2.

The free energy change DG� (cal mol�1) values of the complexes½ð4BPÞI�þI�3 , [(4BP)(DDQ)] and [(4BP)(TBCHD)] were calculated fromGibbs free energy of formation according to the following equation[30,31]:

DG� ¼ �RT ln KCT ð4Þ

where DG� is the free energy change of the charge transfercomplexes; R is the gas constant (1.987 cal mol�1 �C); T is the tem-perature in Kelvin degrees; and KCT is the formation constant of do-nor–acceptor complexes (L mol�1). The DG� values of the complexesare given in Table 2. The obtained results of DG� reveal that the CT-complexes formation process is exothermic in nature and spontane-ous. The results of DG� are generally more negative as the formationconstants of the CT-complexes increase. As the bond between thecomponents becomes stronger and thus the components are sub-jected to more physical strain or loss of freedom, the values ofDG� become more negative. The more negative the value for DG�,

A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208 205

the farther to the right the reaction will proceed in order to achieveequilibrium.

The ionization potential is the minimum energy required to re-move an electron from its ground state. The ionization potential ofthe free donor was determined from the CT energies of the CT bandof its complexes. In case of the acceptors I2, DDQ, and TBCHD, therelationship becomes the following equation [32]:

ECT ¼ Ip� 5:2þ 1:5Ip� 5:2

ð5Þ

where Ip is the ionization potential and ECT is the charge transfer en-ergy of the formed solid CT-complexes. The obtained values of Ip are7.3, 5.9 and 7.2 eV for the CT-complexes ½ð4BPÞI�þI�3 , [(4BP)(DDQ)2]and [(4BP)(TBCHD)] respectively. It has been reported that the ioniza-tion potential of the electron donor may be correlated with the chargetransfer transition energy of the complex [32].

These results (KCT, eCT, DG� and ECT) are clarifying that the ob-tained solid CT-complexes formed in the reaction of the donor4BP and the acceptors I2, DDQ and TBCHD have high CT energyand formation constant KCT.

3.2. Infrared spectra

The obtained infrared spectra of the donor 4BP along with thoseof the formed complexes ½ð4BPÞI�þI�3 , [(4BP)(DDQ)2], and[(4BP)(TBCHD)] and the adduct [TCBPQDM] are shown in Fig. 11and the infrared band assignments are given in Table 3. Theseassignments are based on the comparison of the spectra of theformed products with the spectra of the free reactants, the donor4BP and the acceptors TCNQ, DDQ, and TBCHD (I2 has no infrared

Fig. 11. Infrared absorption spectra of: (A) 4-Benzylpiperidine (4BP), (B) [T

activity). Interestingly, the spectra of the reaction products containthe main infrared bands for both the reactants in each case. Thisstrongly supports the formation of the donor–acceptor CT-com-plexes. However, the absorptions of 4BP and the acceptors in theformed products show some changes in band intensities and insome cases small shifts in the frequency wavenumber values.These changes can be understood based on the expected symmetryand electronic structure modifications in both donor and acceptorunits in the formed products compared with those of the free mol-ecules. The m(NAH) vibrations of the free 4BPare 3360 cm�1and3284 cm�1, for [(4BP)(DDQ)2] at 3247 cm�1, 3164 cm�1 and3016 cm�1, for the ½ð4BPÞI�þI�3 at 3164 cm�1, 3104 cm�1, and3029 cm�1, and for[(4BP)(TBCHD)] two absorption are observedat 3227 cm�1 and 3082 cm�1. The outlined changes in m(NAH)upon complexation clearly support the involvement of the nitro-gen atom of the NH group in the donor 4BP through the CT-inter-action process. It might also to indicate that m(C„N) vibrations ofthe acceptors TCNQ and DDQ show some changes in terms of bandwavenumber values upon complexation. The m(C„N) vibration forfree TCNQ is observed at 2223 cm�1, for free DDQ at 2230 cm�1,and for free DDQ a doublet is observed at 2196 cm�1and2182 cm�1. Vibrations occur at 2142 cm�1 and 2176 cm�1 for thespectrum of [TCBPQDM], and at 2215 cm�1 and 2252 cm-1 for[(4BP)(DDQ)2].

3.3. Thermal measurements

Thermogravimetric (TG) and differential thermogravimetric(DTG) were carried out under a nitrogen gas flow (20 ml min�1)

CBPQDM], (C) [(4BP)(DDQ)2], (D) [(4BP)(TBCHD)] and (E) ½ð4BPÞI�þI�3 .

Table 3Infrared wavenumbers (cm�1)and tentative band assignments for 4BP, [TCBPQDM], [(4BP)(DDQ)2], [(4BP)(TBCHD)] and ½ð4BPÞI�þI�3 .

4BP [(TCBPQDM) [(4BP)(DDQ)2] [(4BP)(TBCHD)] ½ð4BPÞI�þI�3 Assignments

3401s,br 3434m,br 3434s 3434s,br 3428s,br m(H2O); KBr3084m 3077w 3022w 3077w 3100sm m(CH); aromatic,4BP3026sm 3022m 2961w 3022w 3023sm

2142s 2252m m(C„N); TCNQ, and DDQ2176s 2215sm

1632sm 1632sm m(C@O); DDQ, and TBCHD1464s 1471sm 1495w 1470sm 1448m m(CH2); 4BP1451s 1428sm 1453sm 1446sm 1405m1585w 1595sm 1566s 1523m 1524sm m(C@C);TCNQ, DDQ

4BP and TBCHD1351s 1374w 1357sm 1369w 1344w Free and complexed1293sm 1336m 1295m 11303w 1300w 4BP1217m 1256sm 1207w 1285sm 1211w m(CAN); 4BP1196m 1182sm 1192w 1199m 1198w1131s 1152w 1131m 1135w 1128m m(CAC); 4BP1079sm 1053m 1060w 1081m 1101m

600sm m (CABr); TBCHD759m m(CACl); DDQ

m, medium; s, strong; w, weak; br, broad; m, stretching.

(A) (B)

(C) (D)

Fig. 12. Thermograms of CT-complexes: (A) ½ð4BPÞI�þI�3 , (B) [TCBPQDM], (C) [(4BP)(DDQ)2] and (D) [(4BP)(TBCHD)].

206 A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208

Table 4Thermal decomposition dataa for the [(4BP) I] +I3

- , [4BP)(DDQ)2], [TCBPQDM] and [(4BP)(TBCHD)] CT- complexes.

Complex Reaction stoichiometry DTG max. (�C) TG% mass loss found/cal. Lost species

½ð4BPÞI�þI�3 1:2 163 62.78/62.8 4BP + I2

364 37.2/37.2 I+. I�

[(4BP)(DDQ)2] 1:2 264 28.1/27.85 2DDQ600 71.6/72.15 4BP

[(4BP)(TBCHD)] 1:1 201 76.1/75.8 (4BP + 3Br + CO)309 20.9/24.2 [Br] + [C5H2]345

(adduct) 1:1 198 64.1/64.23 4BP + 2CN[TCBPQDM] 348 35.80/35.75 (C9H4N)

385

a Thermal measurements were carried out under N2 flow rate at 20 ml min�1.

A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208 207

within a temperature range of 30–950 �C and heating rate10 �C ml�1 to confirm the proposed formula and structure for theobtained CT-complexes. Figs. 12(A), (B), (C) and (D) show the ther-mograms of ½ðA4BPÞI�þI�3 , [TCBPQDM], [(4BP)(DD)2] and[(4BP)(TBCHD)], respectively. The thermogravimetric data forthese complexes are shown in Table 4. The obtained data supportthe calculated formulas and structures of the formed CT-com-plexes. The degradations steps and their associated temperaturesvary from one complex to another depending on the type of con-stituents as well as on the stoichiometry in each case. These twofactors have pronounced effects on the type of bonding, relativecomplex stabilities and geometries.

It is interesting to see that the triiodide complex [(4BP)I]I3

shown in Fig. 12(A) decomposes in two degradation steps: at163 �C corresponding to the decomposition of [(4BP)I2] with amass loss of 62.78% very close to the calculated value of 62.8%,and followed by another degradation step at 364 �C correspondingto the loss of [I2] species with a total mass loss of 37.2% with nodeviation from the calculated value (37.2%). This is an evidencethat the donor 4BP represents 25.67% of the complex, as shownin Table 4.

Accordingly, a proposed mechanism for the thermal decomposi-tion of ½ð4BPÞI�þI�3 is as follows:

ðiÞ ½ð4BPÞI�þI�3 ����!163�C ½ð4BPÞI2� þ ½Iþ � I��

ðiiÞ ½Iþ:I�� ����!364�C Iþ � I�

The second product [TCBPQDM] is shown in Fig. 12 (B), whereat198 �C it corresponds to the loss of [(4BP) + 2CN] with total massloss of 64.1% (64.23% calculated). The remaining TCNQ acceptorhas been decomposed at 348 �C and 385 �C with mass loss of35.80% (35.75% calculated) as shown below:

ðiÞ ½TCBPQDM�����!163�C ½ð4BPÞðCNÞ2� þ ½C9H4N�

ðiiÞ ½C9H4N� ����!348�C;385�CC9H4NðdecomposedÞ

This thermal decomposition mechanism shows that a moleculeof HCN was lost during the formation of [TCBPQDM] (Formula 1).

The third complex [(4BP)(DDQ)2] is shown in Fig. 12 (C),with the peak at 264 �C corresponding to the loss of the donor4BP with a mass loss of 28.1%, very close to the calculated valueof 27.85%. The DDQ acceptor decomposed at 600 �C with mass lossof 71.6% (72.15% calculated)equivalent to (2DDQ). A proposedmechanism for the thermal decomposition of [(4BP)(DDQ)2] is asfollows:

ðiÞ ½ð4BPÞðDDQÞ2� ����!264�C ð4BPÞ þ ½2DDQ �

ðiiÞ ½2DDQ � ����!600�C decomposition

Fig. 12(D) shows the fourth complex [(4BP)(TBCHD)] decom-posing in three temperatures at 201 �C, 309 �C and 345 �C. Thetemperature 201 �C shows a total mass loss of 76.1%, correspond-ing to the loss of [4BP + 3 Br + CO], very close to the calculatedvalue of 75.8%.

The decomposition temperatures at 309 �C and 345 �C are asso-ciated with a total mass loss of 20.9%, corresponding to the loss ofthe [C5H2Br] equivalent to the acceptor after losing three bromineatoms and a carbonyl group from the decomposition step at 201 �C,in agreement with the calculated value of 24.2%. Accordingly, aproposed mechanism for the thermal decomposition of[(4BP)(TBCHD)] is as follows:

ðiÞ ½ð4BPÞðC6H2Br4OÞ� ����!201�C ½ð4BPÞðBrÞ3 þ CO� þ ½C5H2Br�

ðiiÞ ½C5H2Br� ����!309�C;345�C½Br� þ C5H2

4. Conclusion

Charge transfer interactions between the donor (4BP) and ther- acceptor I2 and the p-acceptors TCNQ, DDQ and TBCHD werestudied in CHCl3 at 25 �C. We were able to show that the reactionstoichiometry is 1:1 for the acceptor TBCHD and is 1:2 for DDQ andI2. The reaction of the donor 4BP with the acceptor TCNQ has beenundergone N-substitution producing adduct [TCBPQDM]. Theresulting CT-complexes were shown to have the formulas:½ð4BPÞI�þI�3 , [(4BP)(DDQ)2] and [(4BP)(TBCHD)]. The obtained datashow that the CT- complex [(4BP)(TBCHD)] has much lower valueof KCT compared with that of ½ð4BPÞI�þI�3 , and [(4BP)(DDQ)2]. Fur-ther studies will focus on using different acceptors and donors tofurther investigate the nature of such complexation.

References

[1] R.S. Mulliken, W.B. Person, Molecular Complexes, A Lecture and ReprintVolume, Wiley, New York, 1969.

[2] K.K. Lahiri, K.K. Mazumdar, Spectrochim. Acta 46A (1990) 1137.[3] R. Foster, Organic Charge Transfer Complexes, Academic Press, London, 1969.

pp. 51, 387.[4] R. Foster, J. Chem. Soc. 1075 (1960).[5] H.S. Bazzi, A. Mostafa, S.Y. AlQaradawi, E.M. Nour, J. Mol. Struct. 842 (2007) 1.[6] H.S. Bazzi, S.Y. AlQaradawi, A. Mostafa, E.M. Nour, J. Mol. Struct. 879 (2008) 60.[7] A. Mostafa, H.S. Bazzi, J. Mol. Struct. 983 (2010) 126.[8] A. Mostafa, H.S. Bazzi, Spectrochim. Acta Part A 79 (2011) 1613.[9] S.Y. AlQaradawi, H.S. Bazzi, A. Mostafa, E.M. Nour, Spectrochim. Acta Part A 71

(2008) 1594.[10] J. Casaszar, Acta Phys. Chim. 29 (1998) 34.[11] E.M. Nour, Spectrochim. Acta Part A 167 (2000) 56.[12] A.S.N. Murthy, A.P. Bhardwaj, Spectrochim. Acta 39A (1983) 415.[13] S.Y. AlQaradawi, E.M. Nour, Spectrosc. Lett. 37 (2004) 337.[14] E.M. Nour, L.H. Chen, J. Laane, J. Raman Spectrosc. 467 (1986) 17.[15] P.J. Trotter, P.A. White, Appl. Spectrosc. 32 (1978) 323.[16] E.M. Nour, L.A. Shahada, Spectrochim. Acta 44A (1988) 1277.[17] W. Kiefer, H.J. Bernstein, Chem. Phys. Lett. 16 (1972) 5.[18] G.A. Bowmaker, R.J. Knappstein, J. Chem. Soc. Dalton (1977) 1928.

208 A. Mostafa et al. / Journal of Molecular Structure 1054–1055 (2013) 199–208

[19] S. Licht, Sol. Energy Mater. Sol. Cells 35 (1995) 305.[20] H. Salem, J. Pharm. Biomed. Anal. 29 (2002) 527.[21] D.A. Skoog, F.J. Holler, T.A. Nieman, Principle of Instrumental Analysis, vol. 347,

fifth ed., Saunders College Publishing, New York, 1992.[22] J.E. Frey, R.D. Cole, E.C. Kitchen, L.M. Suprenant, M.S. Sylwestzak, J. Am. Chem.

Soc. 748 (1985) 107.[23] A.S. Aljaber, E.M. Nour, Spectrochim. Acta A 70 (2008) 997.[24] E.M. Nour, Spectrochim. Acta 47A (1991) 743.

[25] M. Mizuno, J. Tanaka, I. Harda, J. Phys. Chem. 85 (1981) 1789.[26] R. Abu-Eittah, F. Al-Sugeir, Can. J. Chem. 54 (1976) 3705.[27] A. El-Kourashy, Spectrochim. Acta A 37 (1981) 399.[28] G. Briegleb, Z. Angew. Chem. 72 (1960) 401.[29] G. Briegleb, Z. Angew. Chem. 76 (1964) 326.[30] M. Arslan, H. Duymus, Spectrochim. Acta A 67 (2007) 573–577.[31] A.A.A. Boraei, Spectrochim. Acta A 58 (9) (2002) 1895.[32] M. Pandeeswaran, K.P. Elango, Spectrochim. Acta A 65 (2006) 1148.