The black polymorph of TTF-CA: TTF polymorphism and solvent effects in

mechanochemical and vapor digestion syntheses, FT-IR, crystal packing and electronic structure

Dr. Silvina PagolaResearch Scientist, College of William and Mary

Adjunct Professor, Hampton University

VAS 92nd Annual Meeting, VCU, RichmondMay 15, 2014

Overview► New synthesis of tetrathiafulvalene chloranil (TTF-CA) by two essentially ‘solvent-free’ methods, liquid assisted grinding (LAG), and vapor digestion (VD).► TTF-CA is a charge transfer salt. Tetrathiafulvalene (TTF) e- donor, Chloranil (CA) e- acceptor.

►TTF and CA molecules pack in two different crystal structures, called polymorphs.

► Syntheses are useful for product polymorph screening and polymorph control.►Crystal structure and physical properties: synchrotron X-ray powder diffraction, FT-IR, band structure calculations, magnetic susceptibility.

Green polymorph Black polymorph TTF-CA TTF-CA

S

S

S

S

O

O

Cl

Cl

Cl

ClTTFCA

TTF(s) + CA(s) TTF-CA(s)O

O

Cl

Cl

Cl

Cl

S

S

S

S

1) Polymorph control: particular solvents selectively lead to the green or the black TTF-CA – But, why ?

2) Does the TTF polymorph determines also the TTF-CA polymorph ?

3) Are the TTF polymorph and solvent effect in LAG the same for VD ?

TTF-CA synthesis

Grinding 30 min.

green polymorph black polymorph2 TTF polymorphs

Orange TTF Brown TTF

Wait 10 days; identify products with X-ray powder diffraction Vapor digestion (VD): TTF and CA powders react in presence of vapors

Liquid assisted grinding (LAG): grinding with small quantities of liquid

+ solvent drops(acetone, water, toluene, DMSO, ethyl acetate, MeCN)

1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 52 theta / degrees

0

20000

40000

60000

X-r

ay c

ount

s

-30000

3000

Diff

eren

ce

acetoneTTF(s) + CA(s) TTF-CA(s) [1]

[1]- S. Benjamin et al. Synthetic Metals 161 (2011) 996-1000; [2]- Mayerle et al, Acta Cryst. (1979) B35, 2988-2995.

Green TTF-CA: Rietveld fit of the laboratory XRPD data

► Crystal structure known (1979). [2]

► Neutral (=0.3e-) to ionic (0.7e-)transition at 81K and 1atm, together with acrystallographic phase transition (P21/n to Pn)

Viewed along [010] (T=300K)

=0.3e-

(pseudo-neutral)

Black TTF-CA: synchrotron X-ray powder diffraction data (298 K)

DMSOTTF(s) + CA(s) TTF-CA(s) ► Crystal structure solved by direct-space methods.

► Synchrotron data from X16C, NSLS, BNL, =0.69993 Å

a=10.7626 Å, b=11.0511 Å, c=6.6122 Å =101.3543°, =93.6626°, =89.2794°, Z=2, P1

Black TTF-CA: crystal structure analysis

►Experimentally DIAMAGNETIC (magnetic susceptibility balance)

Viewed along [001]: eclipsed (TTF+●)2 radical cation dimers; typical packing motif of ionic, diamagnetic and insulating TTF compounds

Viewed along [110]: columns of TTF and (CA-●)2 radical anion dimers

Product polymorph control in LAG: TTF polymorphism combined with solvent effect

Low polarity solvents

High polarity solvents

►Only the most polar solvents, water and DMSO, lead to the black (ionic) TTF-CA from both TTF forms►ACN yields the black form only from the orange TTF.►The quantities of DMSO and water used to obtain pure black TTF-CA are smaller for the orange TTF than for the brown TTF.

= 1 e-

= 0.3 e-

water

DMSO

ACN

acetone

Ethyl acetate

Toluene

Pola

rity

inde

x

Solvents & polarity index

Brown TTF products Orange TTF products

Water, 9 Black TTF-CA, TTF, CA Black TTF-CA, TTF, CA, green TTF-CA

DMSO, 6.5 Black TTF-CA, TTF, CA, TTFCl0.67, green TTF-CA

Black TTF-CA, TTF, CA, green TTF-CA

MeCN, 6.2 Green TTF-CA, TTF, CA Black TTF-CA, green TTF-CA, TTF, CA

Acetone, 5.4 Green TTF-CA, TTFCl0.67 Black TTF-CA Ethyl acetate, 4.3 Green TTF-CA Green TTF-CA, black TTF-

CAToluene, 2.3 Green TTF-CA, TTF, CA Green TTF-CA, black TTF-

CA, TTF

Vapor digestion (VD) syntheses

From the brown TTF the solvent effect is the same than in LAG.

From the orange TTF all vapor solvents can yield the black TTF-CA

Tentative explanation for LAG and VD reactivity of the orange TTF

Brown TTF

The crystal habit and crystal structure of the orange TTF facilitate

the formation of the black TTF-CA

Orange TTF

Black TTF-CA: Rietveld fit of the synchrotron diffraction data at 20K

►Lattice parameters and relevant distances change less than 2%

Isostructural at 20 K

Rwp= 12.95% 2=1.34RI=6.49%

Density 298 K Low T

Green (g/cm3) 1.841 1.932 at 40K

Black (g/cm3) 1.944 1.997 at 20K

The black TTF-CA is thermodynamically more

stable than the green TTF-CA

Band gap by FT-IR spectroscopy►Semiconductor with band gap of 0.198 eV►Absence of crystal phase transitions 10 K – 300 K; remains ionic.

1000 cm-1

I am grateful for synchrotron powder diffraction data collected at the beamline X16C of the National Synchrotron Light Source, BNL.

Use of the National Synchrotron Light Source, Brookhaven National Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

I am grateful to the College of William and Mary for the use of the single crystal diffractometer and to ODU for the use of the laboratory X-ray powder diffractometer.

Acknowledgements

Thanks !