Comparative ESR studies of the polycrystalline alkali metal tetrafluorotetracyanoquinodimethan(TCNQF4) charge transfer salts

Physica 143B (1986) 518-520 North-Holland, Amsterdam

5]8

COMPARATIVE ESR ST[DIES OF THE POLYCRYSTALLINE ~(TCNQF 4) CHAR(~ TRANSFER SALTS

AI/KALI METAL

TETRAFLUORC/I~.'rKACYAND(~INODI-

M. Thomas Jones, Toshio Maruo, Susan Jansen*, James Roble, and Raymond D. Rataiczak'* Department of Chemistry,

University of Missouri-St.

Louis,

St. Louis, Mo. 63121, USA;

The entire series of alkali metal charge transfer salts of tetrafluorotetracyanoquinodimethan(TC~(~,) have been synthesized and studied by electron spin resonance(ESR) techniques. The tenperature 4dependence of the g-tensor, the temperature dependence of the relative magnetic susceptibility and the temperature dependence of the spectral envelope of each salt is discussed. The spectral properties of these salts are discussed in terms of the relationship between the size, electropesitivity, and other physical properties of the alkali metal ion as one moves through the alkali metal series. The lithium salt exhibits a temperature independent g-tensor and the magnetic susceptibility follows Curie Law from 77 to 300 K. The ESR envelope of the sodium salt consists of two overlapping spectra. The magnetic susceptibility of both species is thermally activated. The spectral envelopes of the potassium and rubidium salts are similar in shape and behavior. Both are strongly dependent upon temperature. Their magnetic susceptibilities are thermally activated. The potassium salt has been studied from ca 4 to 300 K. An abrupt change in the spectral lineshapes of the potassium and rubidium salts is observed at ca 150 K. Finally, the spectral envelope of the cesium salt displays a unique monotonic decrease in linewidth with temperature. able to grow single crystals of this salt in our

1. I ~ D t L - T I O N

Our

laboratory

magnetic salts

has

been

investigating

properties of various charge

and complexes of TCNQ and ~ 4

Studies

of

solutions of KTCNQ and

laboratories.

suspect

growing initiated

is

due

to

of

distributed electronegativity of TCN(~ 4 . We have

in

crystal

difficulty

transfer

KTCNQF 4

single

the

by

ESR.

a

We

the studies of the series of

metal

form ion pairs.

cation size and its electropositivity may effect

pairs

However, the structures for the

are

different

due

to

the

difference in the electronegativity of relative

to

hydrogen I .

polycrystalline show

samples

t_hat they

crystal

do

not

structures 2 .

Studies

of KTCNQ possess The

large

fluorine

solid

and

of

since the changes in

the micro-crystalline structure and of

single

obtained

crystals.

However,

The

isomorphous F~R

ESR

studies

of

the

to

whereas

~ 4

display

growth

have

not salts

asymmetric

demonstrate the effect of the differences in

line shapes and show considerable variation with

interaction

temperature.

structures of these salts.

In particular, an abrupt change in

polycrystalline

alkali-metal TCNQF 4 I: I salts 3 are reported here cation size, electropositivity,

of

the

single crystals of any of these

spectra of KTCNQ exhibit symmetrical line shapes those

we

the

yet.

~ 4

state

salts,

alkali-

ether type solvents indicate both anion radicals ion

TCNQF 4

the

energy

upon

the

and spin orbit microcrystalline

linewidth is observed at 150 K. A has

single crystal X-ray structure of not been reported,

yet.

KTCNQF 4

Nor have we

been

2. EXPERIMI~AL

TCNQF 4 was synthesized at the University

*Present address: Department of Chemistry, Cornell University, Ithaca, New York 14853, USA. **Department of Chemistry, Muskingtln College, New Concord, ohio 43762, USA. 0378 - 4 3 6 3 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Y a m a d a Science F o u n d a t i o n

of

M.T. Jones et al. / ESR studies o f the polycrystalline alkali metal Missouri-St.

Louis and Musking~n College 4.

alkali-metal

simple salts were

the

for

methods

preparation

simple salts of TCNQ 5 . ~ 4 '

Its

synthesized of

Thus,

by

This

for Li,

The

room

salts

have

the

> 2M~-~qQP4 - + M+ + 13-

Rb

and

Cs 1:1 salts

were

prepared

from

the

ESR

of polycrystalline been

alkali-metal

order

spectra

studied

Several

and

the

TCNQF 4

there

are

NaTCN(~ 4 and

different

samples

salts were independently

to

of

alkali-metal

differences between

others.

NaTCN~" 4 The

temperature

series

considerable

3M+I - + 2TCSEF 4 . . . . . .

that all

3.2. ESR measurements

Na, and

CH3CN

shows

TCSEF 4 prepared are 1:1 simple salts.

alkali-metal

the reaction shown below was used.

result

519

prepared

confirm the unique spectrmn

of

of in this

salt.

Li%~N~F 4 in H20:

LiTCN~F 4 exhibits

temperature independent g-

values and spectral lineshape. The g-values and, Li+TCN(~4 - + M+CI - -.... > M+~L-%~4 - + Li + + Cl-.

hence,

the

lineshape

are

symmetric.

The

spectral envelope is rather narrow (i.e. ca All

the alkali-metal T C N ~ 4 1:1 salts

were

obtained

navy blue microcrystals with the exception

G).

magnetic susceptibility obeys Curie law. The

of NaTCNQF 4 which exhibits a purple color in the microcrystalline state. UV-visible measurements were done on ACTA MVI. All

the

Varian

ESR spectrometer equipped

cavity

and

a

dual

channel

g-values

temperature

dependence

in g-value,

magnetic

susceptibility, envelope.

spectra

a

isomorphous,

The

Integration of the spectra for NaTCN~F 4 was done

temperature change

UV-visible to

measurements

were

done

investigate the stoichiometry of

phase

differential

scanning

in the

for

Rb

very strong absorbance at 385 nm and

absorbance

at

364 rim.

All the

and

alkali

metal 415,

C ~

4

the difference in alkali metals.

exhibits

The spectra of

4 are in good agreement

species

of

g-value

suspected

versus

an a

abrupt possible

transition.

However,

calorimetry and magnetic

The magnetic

susceptibility

with activation

suggests

activated. be

and L i ~

energies

0.04

temperature

that this salt is

thermally

eV. a

CSTCNGF 4

spectral

unique

monotonic

above

110

K.

linewidth

decrease Below

with the reported spectra 6'7 . The ratio between A756/A85 $ = 0 . 4 and A415/A85 $ = 1 . 2 f o r a l l

phenomenon is reflected in the g-values.

salts

including both Rb and

Cs

the

salts.

of

The activation energy is measured to

temperature, the linewidth stays constant.

TCN~ 4

are

The relative magnetic susceptibility study of

756, and 858 nm with rather s~all shifts due to ~ 4

the

data suggests both salts are thermally activated

another

TCNGF 4 salts exhibit strong absorbances at

both

We

phase transition.

0.05 eV.

a

they

both salts exhibit

structural

TCN(~ 4 . The spectr~ of neutral TCN~F 4 exhibits

particularly,

for

150 K.

magnetic

salts,

suspect

graphs

at

Cs

TCNQF 4

relative

and linewidth of

We

same

susceptibility studies indicate no evidence of a

3.1. UV-Visible measurements The

the

and powder diffraction studies are

underw~ly.

by I ~ PC-XT.

order

T C N ~ 4 are

They exhibit

a

3. RESULTS

both K and Rb

dependent.

on

recorder.

of

temperature

with

ESR measurements were performed E-12

dual

Beckman

Acetonitrile was used as the solvent.

0.6

The temperature dependence of the relative

with that This

NaTCN~F 4 exhibits a complex spectral envelope

M.T. Jones et al. / ESR studies o f the polycrystalline alkali metal

520

at room t ~ r a t u r e The

spectral

compared to the other salts.

envelope of this salt at

various

metal

TCNQ

and

TCNQF 4

explanations of the are

magnetic species.

This is further supported by

susceptibility of the cesium

the

magnetic

and

study

of the

dence

The study of the

of

the

magnetic

susceptibility

of

temperature depen-

susceptibility reveals

that from room temperature to 231 K,

spectral

t~rature

changes into a much slower

The

authors

measurements.

activation

energy

is dominant, while in the low temperthe

salt is

salts

magnetic activated

increases

with

0.6 G at 110 K to

ACKNOWL~X~'fS

the species with the

ature region,

The

4.5 G at 298 K).

Megh

larger

KTCN~ 4

2.

linewidth

(i.e. from ca

rate. Therefore, in the high temperature region,

(0.13 eV)

reference

Detailed

there is a

very sharp decrease of the intensity, then below 231 K, the decrement

the

in

behavior of

temperatures suggests the presence of two unique

NaTCN(~ 4.

discussed

series.

spectrum is dominated by the

Singh for

would like to

acknowledge

assistance with the Also,

Dr.

UV-visible

Dr. Geoff Ashwell for the

differential

scanning

KTCNF 4 .

of the authors (TM) acknowledges

One

calorimetry

study

of

Dr. Shelly Kumar and the department of Chemistry

lower activation energy species (0.03 eV).

at University of Missouri-St. Louis for the 1985 Summer Fellowship.

4. DISCUSSION AhD CONCLUSIONS There

are

properties

considerable

variations

in

for these alkali metal ~ 4

salts

with the exception of the K and Rb salts. may

be

due

structure

to the difference in

of the salts.

the

crystal

In the case of

one can assume the position of the metal to

be

hand,

near the cyanide groups. because

negative one

of

the

additional

located

at

cation

the various positions

other

distributed

can assume the alkali metal cation

atoms, may

be

relative

to

the Tt-~QP4 anion radical (See reference 1). This fact

and

large

the size of the cation may result

differences

structure differences

of

in

these in

the

the

salts,

in

microcrystalline and

magnetic

consequently,

properties

as

salt exhibits Curie law in

the

demonstrated as above. The

lithium

magnetic susceptibility. which

i.R. D. Rataiczak, M. T. Jones, J. R. Reeder, and D. J. Sandman, Mol. Phys. 56 (1985) 65.

TCNQ,

On the

charge density on the fluorine

REF~ES

This

2. M.

T. Jones, S. Jansen, A. Berndt, S. Puloka, R. D. Rataiczak, and D. J. Sandman, in preparation.

3. For a preliminary report, see M. T. Jones, T.

Maruo, S. Jansen, J. Roble, and R. D. Rataiczak, Mol. Cryst. Liq. Cryst. 134 (1986) 21. 4. a)

E. L. Martin, U.S. Patent 3,558,671, January 26, 1971; b) R. C. Wheland and E. L. Martin, J. Org. Chem. 40 (1975) 310.

5. R.

L. Melby, R. J. Harder, W. R. Hertler, W. R. Mahler, R. E. Benson, and W. E. Mochel, J. Am. Chem. Soc, 84 (1962) 3374.

6. J.

B. Torrance, J. J. Mayerle, K. Bechgaard, B. D. Silverman, and Y. Tomkiewicz, Phys. Rev. B. 22 (1980) 4960.

This is the only salt

is not themrelly activated in the

alkali

7. I. Zanon and C. (1983) 3657.

Pecile,

J.

Phys. Chem. 87