infrared spectra of transfer complexes of metal-free...
TRANSCRIPT
Indian lournal of Pure & Applied Physics Vol. 42, FeblUary 2004, pp. 79-88
(
Infrared spectra of .~harge transfer complexes of metal-free phthalocyanine
Mukesh Patel, Rajiv Vaidya, Mehul Dave, S G Patel & A T Oza
Department of Physics, Sardar Patel University, Vallbh Vidyanagar 388 120
Received J 7 Seplelllber 2003, accepled J 3 Novelllber 2003
A spectroscopic study of thc charge transfer complexes of Metal-free phthalocyanine (H,PC) in the infrared range has becn carried out. Six complexes namely H!PC-Chloranil , H,PC-DDQ, H!PC-TCNQ, H, PC-TCNE, H2PC-I2 and H!PCTNF have been prepared and studied. H2PC acts as an organic donor because of two NH groups in the center and four pheny I rings on the outer core of the molecule. H!PC is a P-conjugated ligand. Forbidden direct transition is found in all the charge transfer complexes.
Keywords: Charge transfer complexes , Metal-free phthalocyanine, Infrared spectra, Nature of transition, Optical absorption edge.
I PC Code : GO I N 30/96
1 Introduction
Phthlocyanine (tetraphthalonitrile) is an-conjugated planner ligand . Metal phthalocyanines are being
characterized with the physical properties in details in the present time! -!! . Lead phthalocyanine contains lead chains
and is also studied in detail! 2. !5. The most interesting part of
phthalocyanine is the vibrations of two electrons across the
entire ligand forming a pair of electrons. Most of the dioximes contain one-electron vibrating across the ligand.
Porphin , heamin, prophyrin, phthalocyanines and chlorophylls contain two electrons across the ligand!6. Thus
this paIring has deep implications regarding
superconductivity. Condensation and long-range order of
Cooper pairs should establish superconductivity. Recently charge transfer complexes of lead phthalocyanine have been
studied with infrared spectroscopy and an indirect transition (phonon-assisted) was found in four complexes I? . For a
comparison, we have studied here the charge transfer
complexes of metal-free phthalocyanine. This proves role
of metal ion on the electronic transition from the valence
band to the conduction band .
2 Experimental Procedure
Dark blue metal-free phthalcyanine formed 1: 1 donor
acceptor complexes of black colour with organic acceptors
like iodine, TCNQ (7,7,8,8-Tetracyano-p-quinodimethane),
TCNE (Tetracyanoethylene), TNF (2,4,5,7-Tetranitro-9-
f1uourenone) , DDQ (2,3-dichloro-5-6 dicyan o-p
benzoquinone and ch loranil colours are listed (Tab le I).
Molecular structures of metal-free phthalocyanine and
Table 1- Optical and infrared properties of CT complexes of value of metnl-free phthalocyanine
Name of the Colour Absorption Nature of Value of band gap complex Function Transition Eg (eV)
H!PC-TCNQ Black Ahv = B(hv-E/" Forbidden direct 0.210
H!PC-TCNE Black Ahv = B(hv-E/" Forbidden direct 0.220 H, PC-TNF Blue Ahv = B(hv-E/" Forbidden direct 0.225
Black
H,PC-DDQ Green Ahv = B(hv-E/ " Forbidden direct 0.225 Black
H2PC- Chloranil Blue Ahv = B(hv-E/" Forbidden direct 0.220 Black
H,PC-I! Black Ahv = I3(hv-E/" Forbidden direct 0.220
80 INDIAN J PURE & APPL PHYS, VOL. 42, FEBRUARY 2004
PHTHAlOCYANINE
O2 O2 N N
~ 02N~0~N02 TNF
2,L.,5)-Tetronitro -9 - f luoren one
N=C'cOCCo N
N-C/ - 'C -N = TCNO =
(7,7,8)8-Tetrocyono-pquinodimethone )
N=C C=N \ / C=C
/ \ N=C C=N
TCNE (Te trocyonoethylene)
o
XVCl X=CI X I I Chloronil
Clx=CN ogo 000
(2) -Oichloro-5,6 -dicyono -pbenzoquinone)
Fig. 1- Molecular structure o\" metal-free phthalocyanine (H2PC)
organi c acceptors are show n Fig. 1. Complexes were
prepared by taking starting material s in molecular weight
proportions and then gr inding in a mortar. Complexes
prepared in thi s manner are solven t-free.
3 Results and Discussion
The infrared spectrum of metal-free phthalocyanine is
shown (Fig. 2a) and the nature of transi ti on is also analyzed.
It is found tha t Ahv = B(hv _E/ '2 is the best fit (Fig. 2b)
where A is absorbance and B is a constant. Band assignments
are carried out (Table 2) . Two electronic abso rpti on
envelopes are found. The infrared spectra of charge transfer
complexes of metal-free phthalocyanine are sho wn
(Figs 3-5). There are two electronic absorpti on envelopes
corresponding to electronic transitions for two electrons
vibrating across the li gand-one around 1600 cm-I and other
Table 2- Band assignments in the IR spectrum of metal-free
phthalocyanine
Wave number Band assignment cm·'
3555 VN.II
3300 v c_"
stretching
3033 v c." stretching
1723 V C=N
1609 bN_" (Asymmetry) 1502 bN_" (Symmetry) 1320 bc_"
hending 1125 VC=N
1017 vc.c ring
876 n C_1I
wagging
750 n C.1I rock ing
716 nc." rock ing
615 n c_" rockinf:
around 750 cm-I. These correspond to in-phase osc illations
of two electrons. The band (broad and intense) around 3450
cm-I in the band due to out-of-phase oscilla tions. Thi s band
is neither osc i Ilator or Lorentzian nor Gauss ian distibust ion.
However, it is almost Gaussian band (very broad and intense)
in phthalocyanine-TCNQ.
Metal-free phthalocyanine acts like a purely organic
donor and charge transfer complexes reveal Ahv = Ail (hv -
E/ 12 behaviour of absorpti on in the infrared range (Fig. 6
and Table I) with Eg = 0.225 e V i .e. Peierl s gap. Band
assignments are carried out (Tabs 2-4). All the complexes
show fo rbidden direct transitions as com pared to lead
phthalocyanine complexes, which show indirect transitions.
Here central N-H groups bind the acceptor and conduction
occurs along alternating DADADA----- stack. This also
proves that phonon involved in indirect transition in the
complexes of lead phthalocyanine is the phonons due to metal - ligand vibrations which show bands below 700 cm-I
.
Band assignments of the charge transfer complexes of metal
free phthalocyanine are carried out (Tables:' and 4) . Below
the featureless absorption , i .e. below 1800 cm -I, and
electronic absorpti on envelope also appears as a doublet
PATEL el al. : INFRARED SPECTRA OF METAL-FREE PATHALOCYAMINE 81 ,
I II II
. 17fj .
"' : L 11
! II
I~ I
..... I
OJ , I I
, I II fIo. , I
' .j4"i:' I
,0 .., .,,,.
400:' ~~ ~'C(i.)
, 1!1OO I cn.:l
.,~o(>¥, I un-I I
Fig. 2(a)- Infrared spectrum of metal-free phtalocyanine
'"
1) '1!
{I ~
~65
r· .J -:i
-[Ie
~~
"I ']
\J .n
.....
",/
/ /.
/ '
/ 0''''/ ,/
,//
,/ /'
Fig. 2 (b)- (AflvfJ.l vs flv for H,PC.
around 1600 cm·1 becausc of two-electron problem . Intermolccular vibrations of phthaloeyaninc molcculc are non-degeneratc when coupled with electronic motions. In
some cases like phthalocyanine-TCNE and phthalocyanine
chloranil acceptor bands overlap on this envelope-doublet.
The envelope doublet extends up to 1200 cm·1 and then
various other bands due to acceptor molec ules and
resonance, anti-resonance spikes are observed. The spikes indicate that materials are transmitting between 800 cm·1
and 1200 em' I wavenumbers details of high frequency
82
;-/
'-
INDIAN J PURE & APPL PHYS, VOL. 42, FEBRUARY 2004
)~
") r. " trrJ 1:\1.< It.; r fl. i" 1 NF ' I( '1 m'·J!: }.
:,111
29 ,
:111 Ii , .. l!:, !
~~~I~ ;;,-,
,~
4~
: 1hll, '<Ii~~ .,
" , \
....
:- . • ~ I .. I
1. 1· . ..
,,,-(XI 1'.. 11.1 :Jlxr, . , :i: "".I""I'U- t. •• 1 ': :-Jtl 1/
Fig, 3(a)- Infrared spectrum of phthalocyani ne
• • j - , '
~ I ~ ' !r : I
. r" " .... '. ' , II
J .. .. "
" -
", ,
.. .. \
. ":{'YJ 7t:O:-, \.ilrttM..m;..:e< ~ iOll 1.,
Ij '.
II .. ·.; .:
I ~ . '"
Fig. 3(b)- complex, Inrrared spectrum or phthalocyani ne-TNF complex,
1
;
~I I . -. :. :
envelope at 3400 cm,l and low frequency envelope at 1600
cm' l are summari zed (Tab le 5) . A s the full w idth at half
max imum increases, elec tron-phonon coupling constant
increases, There is one more Lorentzian envelope around
700 cm,l in all the spec tra o f six complexes and can be
attributed to v ibrati o ns ( roc kin g and w agg in g) of
phthal ocya nine. Ro tati onal l eve l s also lead to such a
loca li zati o n o f charge carri ers and ' stat ic di sto rti on
PATEL eT al.: INFRARED SPECTRA OF METAL-FREE PATHALOCYAMINE 83
Table 3 - Band assignments in the IR spectra of charge transfer complexes of metal-free phthalocyanine
H 2PC - TCNQ H2PC -TCNE H2PC - TNF
Wave number Band Wave number
cm assignment cm-'
3443 VN_H 3449
stretching
3059 VC_H 3281
2234 VC=N 3059
2200 VC= N 2234
1643 vc=c 2200
ring
1602 8 N-H 1609
bending
1515 8 C-H 1508
bending
1441 8 C-H 1454
bending
1334 8c=c 1334
1125 8c=c 11 32
bending
1132 8c=c 1005
1004 (C-H 884
wagging)
884 (C-H 850
wagging)
749 (C-H 756
rocking)
727 (C-H 722
rocking)
622 (C-H 614
rocking)
494 (C-H 554
rocking)
434 (C-H 487
rocking)
con-esponds to orientational Peierls distortion 18.
In O'",ax VS II where O'",ax is maximum optical
conductivity and /I is the number o f bands in the envelope
is also plotted (Fig. 7 and Table 6).
Band Wave number Band
assignment cm-' assignment
VN_H 3416 VN_H
stretching stretching
VC_H 3093 VC_H
VC_H 2918 VC_H
vc= c 2858 New bend
VC= N 1750 vc=o
8 N-H 1635 vc=o
ring
vc=c 1609 8 N-H
ring bending
VC= N 1555 8 N-H
bending
VN_H 1441 8 C-H
bending
8c= c 1300 8c= c
bending bending
11:C_H 1213 vC-N
wagging
(C-H 1125 (C=C
wagging) ring)
(C-H 1105 (C-H
rocking) wagging)
(C-H 1004 (C-H
rocking) wagging)
(C-H 884 (C-H
rocking) wagging)
(C-H 736 (C-H
rocking) rocking)
(C-H 628 (C-H
rocking) rocking)
(C- H
rocking)
4 Conclusion
However, all complexes studied here are Peierl s
semiconductors with Peierls gap of the order of 0.225 eV .
Only applied field can lead to depending of charge density
84 IND IAN J PURE & APPL PHYS, VOL 42, FEBRUARY 2004
Table 4- Band assignments in the IR spectra of charge transfer complexes of metal-free phthalocyanine
Wave no. em - I
3173
1676
1575
1454
1286
1179
IIII
10 17
749
635
H2PC-DDQ H2PC-Chlorani l
Band Wave no, Band Wave no. assignment cm_1 assignment em - I
V N.11 3503 starching
vc;o 3274 ring
V C =N 3052
8 (.11 1696 bending
vc.c 1588
V (.N 1508
vec 1448 ring V C•N 1334
11C•11 1132 rock ing
11C•11 10 18 rocking
877
850
749
722
1 ,~ I PI',11 1.'>.!. :c:,y.:"t': liS"'c, ;CNI J
l~ •
p,
~I / r " .-.' '''''1 • • ./~.J (r'
...... .. "'. "I' --,' ,-/
::,J. ...
r' .~
,./ '" /
V N•11 3416 starching
VC•H 2932 stretching
V C. H 2865 stretchin
v(.c 1635 ring V C;( 1608 ring
V (;N 1447
8 C. 11 1334 bending
VC;( 1125 ring
V (;N 1018
11(. 11 877 wagg ing
11C•H 756 wagging
11( ' 11 729 wagging
11C. H 626 rocking
11C. 11
rocking
1,}o .. 1
,II ,'I,t "J:-
"""'.11
, ill I '1 I " , ," ,
~il ~ ... : 1 _ I
j I .... j
I I
c)r J .' ........
·l· ,-~. if I i ;3 I • •
1:- 11 ,"-U '~' 1 1 1 , 1 _~ ""~ ...... ' .-..r
~~----, T'------------------·----·----~----------~-;.~) ~'\U.I l UX' 'fM....-..n:...n jrn- I ,
Fig, 4(a)- Infrared spectrum of phthalocyanine-TCNQ complex
H2PC- 12
Band assign ment
V N•11
starching
V C. 11
stretching New band
V C;N
vc=c ring
8 C. 11
bending VC•N
V ( . N
8 C.11
bending 11c.11
wagging
11( . 11
rocking
11c.11
rocking
11( .11
rocki ng
PATEL el al. : INFRARED SPECTRA OF METAL-FREE PATHALOCYAMINE
14
'-' ,~ i~
"
' (I j
PI-rIlIALOC,,{ ANlNI·: - Cj I (:kA t-.' I:.
1: : '- ,'
l ..
.r''''
t~.) ~nJ
I ~ ....
·tlil'IIr>.ITb;!~ lID, I ,
.: ....
Fig. 4(b)- Infrarcd spcctrum of phthalocyaninc-chloranil complcx.
Namc of thc complex
Table 5 - Detai Is of e lectronic absorpti on envelopes
High frcqucncy envc lopc Low frequency cnvelopc
Abs . Max. Arb. Units
H ~PC-TCNQ 96
H,PC-TCNE 92
H~PC-TNF 73
H ~PC-DDQ 22
H1PC-Chloranil 95
H~PC-I ~ 83
G means Gaussian and L mcans Lorentzian
K,Ul'X
14.+1
1609
15)5
12)0
1696
1420
Fu ll width at half max. (cnf ')
500 (G)
700 (G)
400 (G)
750 (G)
750 (G)
300 (G)
Abs. Max. Arb. Units
94
86
60
18
94
75
KUH, X
(cm" )
750
756
736
750
750
730
Full width at half max .
(cm" )
200 (L)
250 (L)
150 (L)
200 (L)
250 (L)
300 (L)
Table 6- In (J"" " vs n where (J",,,, is the maximulll optical conducti vity and n is the no. of bands in the envelope
amc of the complex
H~PC-TNF
H1PC-I ,
H1PC-TCNE
H1PC-Chloranil
\H 1PC-TCN Q
No. of bands in the envelope n
4
7
8
9
10
Absorption maximum
(Arb. Units)
73
83
92
95
97
36.5
39.2
44.17
45 .5
46.31
In (Ju ,
28.930
290657
29.12 17
29. 1)14
29.1690
85
86 INDIAN J PURE & APPL PHYS, VOL. 42, FEBRUARY 2004
3to
::>4
3~ PI fTHALOCYANIN F. . 'I CNT.
It" l ..
.J;;
21)
~
24 -~
»
:.. n .. ~
]0 .. -- It! -E ,~
1+3 c " . .
-4 . ! . P!
12
to II l~ : , .. ' l
I IJJ I., fl 'j
. .. - . - . : .. . 2$()O ;!(iOO
, ....... Wl\JfT,O'. ('CTI, - I ;.
Fig. 5- (a) Infrared spectrum of phthalocyanine-TCNE complex.
I;) om:I "9ti,.,U 'r II 1 10 ;.~ 11
S~I. ~ Ir .. 1 r.. I ~l 10 :!i~
~''';
!; DO :)
!l :;e -.. -- ;;6 <Il ("' ,,, .-,.1 '-
g~
_0
1::'; ,
!: I", l i
1(10 )
Fig. 5- (b) Infrared spectrum of phthalocyanine-DDQ complex .
C'l
N
PATEL el al.: INFRARED SPECTRA OF METAL-FREE PATHALOCYAMI NE
Pbtbaiocyaninc - U·
121
10
~
...... .......
;> (,
.r. «
.. .. ......
(a)
........ .. N~ 61 ~ 5, « • i , 1
2 .; , 1 ·1
Pbtbalocyaninc - TNF
. ... . .......... . ...
.... .. (b)
.... . .
"L oL---~----~---0+.)----U+. )-~---0+ .• ~-:O~.4~~--o 0 .1 (125
0.2 0.2S ltv
0.3 0.)5
hV (eV I
0.4 0 .• 5
('")
N ~
;> .r. «
hv hV leV)
Pbthalocyaninc - TCNQ (e)
"1 10
~ , (. ! . " i 21
........ .' ....
.... ...........
......
PbUaaiocyaniDc - Cbloranil
121
10
, ;> 6 .r. «
.... .. ......
.' .... .. . ...
(d)
.. ........ ..
01 o~----~--~---+--------+---~
" 0.2 0 .25 (I. ) 0 .35 0 .4
h\' leV I
Phtbalocyanine - TCNE 12
Iv .. (e)
.... .. .... ..
0 .45 o
10
0.2 025 OJ 0.35
hV (eV )
Pbtllalocyaaille -DDQ.
04
(I)
121 .. .. ,. . .. . .... . .. .....
<""l •
N~
;> (; .r. «
u
U.2
.' .. .....
025
.. ..
0 .) 0)5
h\' leV) 0.4 0 .45
,.,
" -> .r: «
8
G . .. ...
0.2 (I.B
...... .....
O.l o.H
hV (eV)
Fig. 6-(a) (Ahv)"!}3 Vs hv for phthalocyaninc
(b) (A hv)"!}3 Vs hv for phthalocyaninc-TNF.
(c) (Ahv2/3 Vs hv for phthalocyanine-TCNQ.
(d) (Ahv2/3 Vs hv for phthalocyanine-Chloranil. (e) (Allv"3 Vs hv for phthalocyan inc-TCNE. (f) (Ahv"!}3 Vs hi for phthalocyani ne-DDQ.
0.4
O.H
0 .• 5
87
88 INDIAN J PURE & APPL PHYS, VOL. 42, FEBRUARY 2004
~2r-----------------------------------------,
29.15
29 1
E b
29.05 E
~
28.95
28.9 ______ '--____ '--____ L-. ____ !:-____ '--____ '=--____ '-'
10
v
Fig. 7-lna",", vs II where a"", is the maximum optical conductivity and 11 is the number of bands in the envelop.
waves. The internal field o f acceptor is not strong enough
to suppress Peierls transition. The envelope-doublet remains in the charge transfer complexes, which reveal that the
transfelTed charge is not the same, which for two e lectron
pairs on the ligand.
Acknowledgement
The authors are thankful to the University Grants
Commission. New Delhi for DRS/SAP funding by the he lp
of which this research work as carried out.
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