Polymorphism and Structure-Property Relationships
Diversity amidst Similarity
35th Crystallography Course
Erice, Italy
July 9 - 20, 2004
Michael D. Ward
Department of Chemical Engineering and Materials Science and
University of Minnesota Materials Research Science and Engineering Center
University of Minnesota - Minneapolis
“Classical Materials”
• Structural materials
• Electrical conductors
• Ceramics
• Computer chips
• Lasers
Stretto di Messina
amorphous metals
ceramic superconductor
Intel microchip
• Design and Synthesis
• Processing
• Characterization
ferrite core memory
• Applications
Polymorphism in “conventional materials”
Engineering properties through control of crystal phase
structural steels
(“tough”)
hard steels
(brittle)
Polymorphism in conventional materials
Quartz
•
•
•
•
•
Quartz oscillators
Frequency control
Mass measurement
Sensors
Alpha form required
Polymorphism in conventional materials
Carbon
Hard
Electrical insulator
Thermal conductor
Soft
Semi-metal
Lubricant
Soft
Conductive
Exotic
Molecular Crystal!
Properties in Molecular Materials
Metallic conductivity
Superconductivity
Piezoelectricity
Pyroelectricity
Non-linear optical activity
Second harmonic generation
Ferroelectricity
Semiconductivity
Light emission
Photovoltaicity
Photoconductivity
Color
Bioactivity
Molecular Materials: Technologies
• Xerography
• Displays
• Pigments
• Pharmaceuticals
• Light-emitting diodes
• Flexible electronics
Attributes of Molecular Materials
• Low-cost processing
• Molecular design = control of properties
• Properties depend on solid state structure
---------• Challenge: Control of solid state structure
• Polymorphism
• Structure-property relationships only partially
developed
• Survey structures
• Find true causal relationships
Structure-property relationships in polymorphs
different properties
Identify properties that DO
depend on crystal structure
(specific)
polymorphs
same properties
Identify properties that DO NOT
depend on crystal structure
(not so specific)
Polymorphism and Reactivity
Schmidt, G. M. J.
Organic conductors
• First discovered early 1960s; TCNQ salts
“A new class of organic solids has recently been prepared which exhibits properties unusual for
organic systems” [Chestnut and Arthur, J. Chem. Phys. 1962, 36, 2969]
• Exhaustive investigations: crystal engineering
• Targeted properties
- Semiconductors
- Metals
- Superconductors
- Magnets
S
S
S
S
TTF
• TTF-TCNQ: metallic (TTF0.59+TCNQ0.59-)
NC
CN
• Properties depend on solid-state structure
NC
CN
TCNQ
Why examine polymorphs?
S
S
S
S
NC
NC
CN
CN
NC
NC
CN
CN
F
NC
F
CN
NC
CN
NC
CN
NC
CN
F
F
• TTF-TCNQ: uniform stacks; bond-over-ring; metal
• TTF-TCNQF2: dimerized stacks; ring-over-ring; semiconductor
• Difficult to separate effect of structure from effect of substituents
Blessing, Coppens
Kistenmacher, et al.
TTF-TCNQ polymorphism
S
S
S
S
NC
CN
NC
CN
Peierls
transition
Peierl’s
Transition
increasing temperature
Cowan, et al.
metal
semiconductor
High temp
P21/c
Low temp
P21/c
Organic metals
Conductivity
• Interplanar separation
• Uniform spacing
• Ring overlap
• Electronic configuration (band filling)
D+o
+
S
S
S
S
S
S
b
4t
Do+
S
S
Peierls gap
(semiconductor)
BEDT-TTF (ET)
S
S
S
S
S
S
S
2
S
Solution with ET and X-
- e-
[
S
S
S
S
S
S
S
S
]
• Often made by electrocrystallization
• Extensive variety of counterions (X-)
• Polymorphs common
• Metallic, superconducting
• ET2I3 : 14 polymorphs reported
• Distinguished by electrical properties (?)
• Length of X-X-X- and Y-X-Y- anions affect interplanar spacing
• Internal lattice pressure suppresses Peierls transition
• Increase Tc
Williams, et al.
+
2
ET2I3 Polymorphs
• b-form: thermodynamically preferred
S
S
S
S
• a-form, P-1, metal-semiconductor transition
• b-form, P-1, superconducting at 1.4 K
S
S
S
S
• k-form, P21/c, superconducting at 3.6 K
• Selectivity controlled by growth rate, epitaxy
Williams, et al.
Substrate directed polymorph control
b1
b2
10 nm

Monolayer growth
 Coincident epitaxy
 No epitaxy for a-phase
ß-(ET)2I3 (001)
on graphite
(no a polymorph)
Last, et al.
TMTSeF-TCNQ (dimorphic)
segregated stack (metal)
H3C
Se
Se
CH3
H3C
Se
Se
CH3
NC
CN
NC
CN
mixed stack
(semiconductor)
mixed stacks (semiconductor)
Bechgaard, Kistenmacher, Cowan
Mixed Stack Complexes
D+
AD+
AD+
A-
D
A
D
A
D
A
neutral
D2+
A2D2+
A2D2+
A2ionic
doubly ionic
Barochromic polymorphs
S
S
Cool to 81 K
or apply pressure
TTF
S
Cl
S
Cl
DA
P21/n; yellow
O
O
Cl
chloranil
<e2/r> !
D+APn; red; ferroelectric
smaller D…A spacing
Cl
Torrance, et al.
Molecular magnets
ferromagnetic
c = 1/(T – q)
M = cH
thru-space
spin exchange
c
paramagnetic
O
antiferromagnetic
M
M
1/T
paramagnetism
Ferromagnetism
•
•
•
•
•
•
•
Spin moments tend to align parallel
Permanent moment (in absence of H)
FM < Tc < paramagnet
FM domains grow in presence of field
Cooperative interactions in solid state
Complicated, poorly understood
Necessary, but not sufficient conditions
identified
H
ferromagnetism
antiferromagnetism
ferrimagnetism
H
Molecular magnets
NC
Fe
+
D+ANC
decamethylferrocene
Purple crystals
Paramagnetic DMFc+; S = ½
Antiferromagnetic (TCNQ)22-
Miller, et al.
CN
CN
TCNQ
Green crystals
Staggered Cp* rings
Metamagnetic
dFe…Fe = 9.635 Å
dA…A = 3.478 Å (AF)
Purple crystals
Eclipsed Cp* rings
Ferromagnetic
dFe…Fe = 8.609 Å (F)
dA…A = 3.635 Å
Metamagnetism in DMFc+-TCNQ-
NC
Fe
+
decamethylferrocene
CN
D+ANC
CN
TCNQ
Hydrogen-bonded magnets
H3C
CH3
-O
OH
CH3
+N
N
.
CH3
O
HQNN
OH
a
a-form
• intramolecular and intermolecular H-bonds
• 3-D ferromagnet
b
b-form
• intermolecular H-bonds
• p-p stacking
• ferromagnet with weak AF at low T
Matsushita, et al.
Hydrogen-bonded magnets (a-HQNN)
hyperconjugation
bifurcated H-bond dimer
OH…O H-bond
positive
CH…ON contact
negative
• Odd alternant hydrocarbons (McConnell)
• Spin distribution alternates sign and magnitude at each carbon atom
• Overlap favored for carbon atoms have opposite spin densities
Magnetism and polymorphism
F
S
S
3.4 Å S…N
F
N
CN
N
F
F
3.1 Å S…N
Conformational polymorphs (slight)
a-form
P-1
Antiferromagnetic
Anti-parallel head-to-tail chains
3.0 Å S…N
3.67 Å S…S (AF)
b-form
Fdd2
Ferromagnetic, Tc = 35.5 K
Parallel head-to-tail chains
Banister, et al.
Polar crystals
Polar crystals from acentric molecules
Grand challenge in organic solid state chemistry
Hydrogen bonded aggregates
Metal coordination networks
Antiparallel ionic sheets
Head-to-tail alignment of dipolar guests
Limited de novo design
Properties
Piezoelectricity
Pyroelectricity
Ferroelectricity
Second harmonic generation
-------Structure-property relationships complex
(polar axis, phase matching, absorption…)
• Perhydrotriphenylene
• Thiourea
Polar crystals
Form 1
N
NO2
PAN
H3COHCHN
• Both crystallize in polar space group P21
• Form 1 grows under slow growth conditions;
not SHG-active
• Form 2 grows under rapid growth conditions;
SHG active
• Same trend for FBNH
• Thermodynamically more stable “non-polar”
forms favored under slow growth
NO2
O
N
O
H
Form 2
H
FBNH
Aldoshin, et al.
Polymorphism and Transistors
• Campbell & Trotter, Acta Cryst, 1962, 15, 289
• Minakata, et al., J. Appl. Phys. 1992, 72, 5220
+++++++
--------
• Bouchoms, et al. Synth. Met. 1999, 104, 175
• Holmes, et al., Chem. Eur. 1999, 5, 3399
• Siegrist, et al., Angew. Chem. Int. Ed. Engl. 2001,
40, 1732 (Bell labs)
• Mattheus, et al., Synth. Met. 2003, 138, 475
• Fritz, et al.. J. Am. Chem. Soc., 2004, 126, 4084
• Pentacene films of technological interest
• Confusion about bulk polymorphs
• Only one bulk polymorph unambiguously identified
• Thin-film phases: 15.4 Å, 15.0 Å, 14.4 Å, 14.1 Å (bulk)
• Thickness and substrate affects “polymorph”
• Structure affects transport
• Transport in layers near the gate electrode important
Pentacene monolayer (AFM)
Grazing incidence X-ray diffraction
SSRL (M. Toney)
Bulk pentacene
a = 6.266 Å, b = 7.775 Å, and g = 84.684º
Monolayer/Thin film
a = 5.916 Å, b = 7.588 Å, and g = 89.95º
Polymorphism depends on thickness
Fritz, et al.
Kidney stones: A major health issue
COM - 98%
(protein - 2%)
Kidney stone formation
nucleation – growth – aggregation - attachment to cells
Stages of stone formation
• Calcium oxalate monohydrate (COM) aggregates and adheres to epithelial cells
• Calcium oxalate dihydrate (COD) “protective”
• Calcium oxalate trihydrate (COT) rare
WHY?
• Crystal attachment/aggregation influenced by urinary macromolecules
• Knockout mice show COM attachment to epithelial cells
Summary
• Polymorphs provide an opportunity to isolate solid state structure from other
factors (e.g., substituents)
• Requires in-depth analysis of properties that are not affected by solid state
structure (less obvious) as well as properties that are affected (not obvious)
• Some structure-property relationships apparent (e.g., crystal polarity,
segregated vs. mixed stack)
• Others not so apparent (e.g., ferromagnetism)
• How many structures required?
Shark attacks
• Does correlation imply causation?
Ice cream sales
True then, true today
“…the synthetic chemist is likely to have many opportunities
to encounter unusual phenemona by accident during
everyday chemical work with crystalline solids and without
the proper background will not be prepared to recognize and
take advantage of such chance discoveries. There is the
further misfortune that those workers in areas where the
most dramatic applications of polar materials have occurred
are, in general, uncomfortable when dealing with structures
of complex organic molecules; the result is a serious lack of
communication between groups whose interaction should be
mutually beneficial.”
-Curtin and Paul, Chemical Reviews, 1981