I ndian Journal of Chemistry
Vol. 44A. February 2005. pp. 271-276
Characterization of charge-transfer complex formation between 3,6-diamino-9H- carbazole and 3,6-dinitro-9H-carbazole
Subhas Chandra Maily. Dipakranjan Mal & Mrinal M Maiti* Department of Chemistry. Indian Institute of Technology. Kharagpur 721 302. India
Email: mrinal_maiti@yailoo.com Received 12 Jllly 2004; re\'ised 30 November 2004
Electron donor-acceptor (EDA) interaction between 3.6-diamino-9f1-carbazole (DAC) as the donor. and 3.6-dinitro- 9H-earbaZtlle (DNC) as the acceptor. has been characterized from IR spectral. differential scanning calorimetric (DSC). and X-ray diffraction analyses. in addition to UV-vis spectral studies. Benesi-Hildebrand plots of the DAC-D C system in acetonitrile at 300 K in the wavelength range of 620-660 nm indicate a strong EDA interaction with both the thennodynamic formation constant K. and the molar absorptivity E.I. of the order of two. lR spectral. DSC. and XRD data also clearly indicate a strong EDA interaction even in their I: I molar mixture in solid state.
IPC Code: Int. Cl7 GOI N 21/00: C07D 209/82
Over the last fifteen years, the carbazole-based polymers have drawn the attention of researchers as materials of proven photo- and electro-activity and non-linear optical properties. Tn a recent publication, Grazulevicius et al. t have reviewed the synthesis, characterization, and applications of carbazole-based polymers underscoring the growing interest in these materials in recent years. A primary requisite of exhibiting electro-activity by an otherwise electro- inactive polymer is to form a moderate to strong charge-transfer complex with a suitable dopant.
Conventional dopants for carbazole based polymers are the common small molecular electron acceptors like tetracyanoethylene (TCE), tetracyanoquinone (TC Q), etc. But, these electron-acceptor dopants suffer from serious drawbacks of having high human toxicity. A new, but somewhat ambitious, concept is to use polymeric dopants. These dopants, in general, have negligibly low vapor pressure, and have limited solubility in or leachability by common organic solvents. These properties ensure very low human toxicity. An example of such a systcm could be poly(3.6-diamino-9-vinylcarbazolc) as the donor and poly(3,6-dinitro-9-vinylcarbazole) as the acceptor.
These polymcric components arc likely to enter into an effective electron donor-acceptor (EOA) interaction, and mutually dope each other. To assess the viability of such a proposition, it is primarily necessary to establish that their monomeric analogs, I.e., 3,6-diamino-9H-carbazole (DAC) and· 3,6-
dinitro-9H-carbazole (ONC) do form strong EOA complex.
Evidence of electron donor-acceptor interaction or charge-transfer complexation between OAC and ONC may be obtained from the UV-vis spectral characteristics of OAC-O C mixture. Additional evidence of charge-transfer complexation, or any non- covalent interaction between OAC and ONC can also be obtained from OSC studies, X-ray diffraction analysis, and IR spectral data of OAC-ONC mixtures. Commonly investigated systems of donor-acceptor or non-covalent type of interaction involve component molecules that generally do not take part in any chemical reaction. Tn contrast, OAC-ONC system is capable of entering into chemical interaction between the nitro- and amine- functionalities, aided by thermal stresses. The results of characterization of the charge transfer complex formed between OAC and ONC by various analytical techniques are reported here.
Materials and Methods
3,6-0initro-9H-carbazole (0 C) and 3,6-diamino- 9H-earbazole (OAC) were synthesized following the procedures reported by Chen el af.
Acetonitrile, spectroscopic grade, was obtained from E. Merck (I nd i a) and was used as such for U V- visible spectroscopic analyses. The concentrations of OAC ancl ONC were kept low at _10-5 M.
272 I DIAN J CHEM, SEC A, FEBRUARY 2005
3.5 Zl8 3.0
2.5
2.0
Q) 0 c 1.5
<U
-e
g~ 1.0
0.5
0.0
-0.5
200 300 400
ONe DAC
DAC-ONe rrixture
500 600 Wavelength(nm)
700 800
Fig. 1- V-I';siblc spectrum of DNC (- ); DAC (. .. ); rind DNC- /)AC (I: I) mixture (- ) in acetonitrile
Shimadzu, Model 3100, double beam UV-visible spectrophotometer, Shimadzu. Model DT 40, heat- flux type differential scanning calorimeter, and Philips. Model 1760, X-ray dilTractometer with Cu-Ka radiation source were used ror the studies.
DSC measurements were carried our at a heating rate of 15°Clrnin from room temperature to 50{)OC in an inert atmosphere.
Results and Discussion
V-\'isible spectral analysis uf DAC-DNC systl.'l11
Figure I represents the UV-yis spectra or pure DNe, pure DAC, and a I: I molar mixture of D!\C and D IC respectivcly from their homogencous solution in acetonitrile at low concentration (_10-5 M).
III the range of wavelength 200-500 nm. DAC shows clear absorption maxima at 238.322, and 371 nl11, in a dccreasi ng order of molar absorpti vi ty. Addi tionally.
onc wcak absorption can be identified at 455 11m at moderately highcr concentration,> (-I(r~ M) of DAC.
DI C shows absorption maxima a 215,251,265,315, and 365 11m with virtually no ab<;orption bcyond -J.50 nin. 1:1 mobr mixturc or DAC-D C in its UV-vis spcctrum retains some of thc individual absorption maxima or the components without any shi fts. Thus, DAC retains its ),""" at 238 nm, but
\"01'
at 371 nmremained virtually obscure. DNC. 011 the other hand, rctain. ),"'," at 36-J. nm (marginal bluc-shirt fro 11 365 nm) and a ,;ubducd/over!apped imprcssion or )'",.1\ at
Table I-Thcrmodynamic formation constant and the molar absorptivity of DAC-DNC charge transfcr complex at differcnt wavelcngths at room tempcrature
Wavelcngth, nm fi20 630 640 650 660 500 423 970 726 549
143 142 121 117 113
265 nm, but thc Amax at 251 nm could not be identified. The mixture, however exhibits a new, distinct and sharp absorption maximL1m at 319 nm which is red-shifted from 315 nm of DNC, but blue- shifted from 322 nm of DAC. The~c shifts, though small, are clear indication of a donor-acceptor (or weak charge-transfer) interaction l·,etween DAC (donor) and DNC (acceptor).
The formation constant of the complex can be determined from I3enesi-Hildebrand plot" with due adhcrence to precautions (on the concentration range of the donor and acceptor) as suggestcd by SCOlt~ and Person ('/ (t15 The UV-vis spectra of DAC-D C mixturc at a constant donor (DAC) concentration of 2.Sx 10-3 M, and at varying acceptor (D C) concentrations of 0.0, 2.5x
I(r-\
5.0x I (f4, 7 .5x I O-~, and lOx I O'~ M show appreciablc absorption in the wavclcngth rangc of 550-800 nm which increases with donor concentration but decreascs with increasing wavclength. 0 absorption can bc recorded, however, at and beyond 800 nm.For the Bencsi-Hildebrand plot, the absorbance due to the complex has bcen c timated at G20, 630, 640.
650 and 660 nm, and the ' E' and ' K' values of thc complcx havc becn calculatcd at room tcmperature (JOO K) using linearized Scott cquation:
where K, and t\ are respectively the thermodynamic formation Cll1stant and molar ab.orptivity of [he complcx, and IDjl) and [Aln ar thc initial C()nccntralions or the donor and a 'ceptor rcspectively, I i<; the path Icngth (which is I cm and constanl for all the spcctra rccorded) and D" is the optical density (or absorbance) cxclusively duc to the complex. D; is g.ivcn by:
D;. = !);.IlIl\IUll· -D"tdollor - {J;"II.:t.'c..'!)"lr
MAlTY el al.: CHARACTERIZATION OF CHARGE·TRANSFER COMPLEX FORMATION 273
For ONC as acceptor, DA acceptor is zero in the wavelength range. K and £)< of the complex at various wavelengths are listed in Table 1. The £). values are consistent with the decreasing absorbance of the mixture as wavelength increases. The numerical values of K at different wavelengths are also consistent in order (-10\ and are fairly constant (except for a high value of 970 at 640 nm) given that a wide scattering in 'K' values are very commonly encountered in Scott plot which is derived on some basic assumptions.
The present donor-acceptor system is similar to the one reported by Yamamoto et al.6 who have used monomeric donor-acceptor system of 9- ethylcarbazole (ECz) and ethyl 3,6-dinitrobenzoate (EtONB). They have characterized the CT complexation between 4S0-470 nm, and report higher '£' values in the range 729-428 M-I cm-I, but appreciably low values of K. Values of K (Table I), obtained with OAC-ONC system are about three order of magnitude higher than those reported for Ecz-EtONB system (-0.80 M-I).
The apparently low values of £A in OAC-ONC system compared to those of the Ecz-EtONB system should be assessed in terms of two modes of interaction opposing each other: a donor-acceptor interaction involving n-electrons between carbazole moieties of OAC and ONC, and the possibility of an effective H-bonding between the -NH2 and -N02 groups in OAC and ONC respectively. While the first interaction leads to decreased electron density in OAC, some degree of electronic charge transfer from -N02 in DNC to OAC can take place through H- bonding (H-N-H ... O-N=O). The net effect would be a small shift (blue or red) in the relevant Amax values of OAC and ONe.
The analysis presented so far unambiguously establishes a possible sttong CT complexation between OAC and ONe.
OSC analysis of OAC-ONC mixture
The DSC traces of OAC, ONC and a 1: 1 molar mixture of OAC and ONC at a heating rate of lSoCimin are shown in Fig. 2. It is seen from the figure that the first exothermic change of DAC rcmains virtually unaffected in the presence of ONe.
The onset temperature of -190°C remained unchanged, but the peak temperature suffered a small decrease from 2l8-21S°e.
218
0 )(
(0) 439
w
~
I
-
VI...,
E
j
1:) 0
w
c20 140 260 380 500 Temperature (OC)
Fig. 2-0SC traces of (a) OAC, (b) ONC and (c) OAC-ONC (I: I molar mixture) at a heating rate of 15°C/min
Tn contrast to OAC, DNC is found to exhibit different thermal response in presence of OAe. The first endothermic event of pure ONC at -300°C, which has been interpreted as the melting process, is missing in the OSC trace of the mixture. Additionally, a new exothermic event is found to occur in the temperature range 228-274°C with a peak temperature of -2S4°e. Strong exothermic peak temperatures of ONC at 403 and 439°C also become subdued, and a weaker exothermic event with a peak temperature of -364°C appears in the presence of OAe.
The features of the OSC traces indicate that ONC might have entered into some chemical reaction with OAC and/or its self-condensation product above -230°e.
IR spectra of (1: 1) mixture of OAC and ONC
In case of any charge-transfer complex formation, some of the individual characteristic IR absorptions of DAC and ONC are likely to suffer blue or red shifts in their 1: 1 molar mixture. In solution phase, these shifts may not be detectable because of dilution effect, and of the observed weak to moderate charge-transfer interaction. However, there is a possibility that the shifts could be detectable in solid phase (KBr pellet).
Figs 3-S show the IR spectra of ONC, OAC, and a I: I
274 INDIAN J CHEM, SEC A, FEBRUARY 2005
n 1609
~ 3421 3345 1097
~
4000
GJ u C
o
-
:t:
E
III C
o
L.
~
1520\
1215
1479
1337
2000 1400 800
Wavenumber (cm-1 ) Fig. 3- 1R spectrum of D C in Kill'
3396
4000 3200 2400
Wavenumber (cm-1)
I:ig. ~-I R spectrulll 01' DNC in K 111'
200
3600 2800
Wavenumber (cm-1)
Fig. 5- 1R spectruill
or
D C-DI\C (I: I 11101' r) mixtlll'l': in KBrmolar mixture of ONC and 0 C. respectively. The close doublet of almost equal intensity at 3421 and 3345 cm-I in the spectrum or 0 C in Fig. 3 is an indication of a strong intermolecular H-bonding of the type, -ONO ... H ... N9<, "hereby the absorption of
> I)-H is red-shifted to 3345 CIll·1 from -3418 cm-I
of carbazole, and a new red-shifted O-H stretching frequency appears at 3421 cm·l
. For 0 C-DAC mixture, the spectrum in the aromatic stretching region (Fig. 5), shows the most intense and sharp ab.orption at 3393 em-I together with ones of relatively much less intensity at 3220, 3093, 2923.
and 2850 cm'l. There i' no absorption at 3421 cm'l detectable anymore, nor is the one at ::'345 cm'l. The absorption of DAC at 3285 cllfl
(Fig . ...j.), also ha:
suffered a large red shift to 3220 cm,l, while the one at 3193 is a bit blue-.hifted to 320() cm'l. rhe absorptions al 3082 em l
(or DAC) and at 3093 em'l
MAlTY el af.: CHARACTERIZATION OF CHARGE-TRANSFER COMPLEX FORMATION 275
2200 2000 1800 1600 .-c
:J
~ 1400
~ 1200 :0 ~ 1000 i?:' 'iii c 800
Q)
C 600
400 200
1200
1000
'§ 800
~
:0 ~ 600
;n
~ 'iii 400 c 2 c
200
o
".t-
O f'..
N
o 0
M<o MN NM
~ ~
0
LO N
ci <q-
10 20 30 40 50 60 70 80 90
20 (degree)
Fig. 6- X-Ray diffraction pattern of DNC
o 10 20 30 40 50 60 70 80 90 20 (degree)
Fig. 7 -X-Ray diffraction pattern of DAC
(of 0 C), however, merge to a single peak at 3093 cm·l. The absorptions of DNC at 2920 and 2845 cm-I remain almost unaltered in the mixture at 2923 and 2850 cm-I. The~e IR spectral features of the OAC- ONC system are clear indication of a possible strong intermolecular interaction in solid state involving
- I-h
and - O2 groups of OAC and ONe.X-ray diffraction pattern of DAC-Di\C mixture
X-ray diffraction patterns of 0 C, OAC, and their I: I molar mixture are shown in Figs 6-8, respectively. In Fig. 6, the diffraction lines of 0 C are broad and fewer in number. The base peak appe<irs at a 20-value of -27.04°. The other lines of lI1uch lesser intensity appear at 12.33. 13.2R, 17.55,24.85, and 40.25°. For
1600
0
<0
N '<2;
1400 f'..
<0 N
'0 a,
1200 ~ M
.-c M
:J
~ 1000 :0 ~ 800
~ i?:' 'iii 600
c
Q)
E 400
200
10 20 30 40 50 60 70 80 90
20 (degree)
Fig. 8 -X-Ray diffraction pattern of DAC-DNC (I: t) mixture
DAC (Fig. 7), the lines are sharper. Many low- intensity lines are also present. The most intense lines appear at 19.54, 20.80, and 27.09°. Prominent low- intensity lines are at 12.78, 14.29, and 35.56°.
]n Fig. 8, the diffraction pattern of OAC-D C mixture shows that some of the lines of the individual components are retained, some lines are suppressed/subdued, and some new lines appear. Two most intense lines, one of D IC at 27.04° and the other of DAC at 27.0 I ° are almost obscured, and a new line of highest intensity appears at 27.48° in Fig.
8. Two of the most intense lines of OAC at 19.54 and 20.80° get mueh subdued in intensity, and appear almost unchanged at 19.53 and 20.76°, respectively. Further, three new I ines, which are not characteristic of either of the two components, appear with appreciable intensities at 16.21, 18.12, and 24.31°.
The results clearly show that a new crystalline order is present in the I: I molar mixture of ONC and DAC, and conform to a possible strong intermolecular interaction between the components in solid state.
Conclusion
3,6-0iamino-9H-carbazole and 3,6-c1initro-9H- carbazole, the monomeric base carbazole molecules of poly (3,6-diamino-9-vinylcarbazole) and poly 0,6- dinitro-9-vinyJcarbazole) respectively, do form strong ED complex. The thermodynamic formation constalll, K, of the complex in acetonitrile at roum temperature (300 K) is as high as 970, as measured from Benesi-Hilclebrand plot at 640 nm. Results or IR spectral, OSC, and XRD analyses are con.,istent with
276
INDIAN J CHEM, SEC A, FEBRUARY 2005a possible strong EDA interaction even in the solid state of the I: 1 molar mixture of the components.
References
Grazulevicius J V, Strohriegl P, Pielichowski J &
Pielichowski K, Prog PO/YIII Sci, 28 (2003) 1297-1353.
2 Chen Jiang Ping & Natansohn A, Macromolecllles, 32( I 0) (1999) 3171-3177.
3 Bcnesi H A & Hildebrand J H, J Alii Chelll Soc, 71 (1949) 2703.
4 Scott R L, Rec Trav Chilli, 75 (1956) 787. 5 Person W I3, J Alii Chelll Soc, 87 ( 1965) I (j7.
6 Yamamoto M. Shimazaki Y, Mitsuishi M & Ito S, ulIIglllllir.
13 (1997) 1385.
