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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