Liquid Crystals: self-assembly and dynamics

Discotic liquid crystals (DLC), consisting of rigid disk-shaped aromatic cores and disordered alkyl substituents tend to organize into columnar supramolecular structures. Their self-assembly is driven by noncovalent intermolecular interactions favoring the π-stacking of aromatic cores and the unfavorable interactions between the cores and the alkyl chains leading to nanophase separation. Applications of DLC as electronic devices rely on the optimal stacking of the aromatic cores that allow for charge carrier mobility along the columnar axis (i.e., molecular wires). Recently the controlled synthesis of DLC bearing large aromatic cores, such as hexa-peri-hexabenzocoronenes (HBC) (Group of Prof. K. Müllen, MPI-P), allowed extensive investi­ga­tions of the self-assembly and electronic properties. Recently, with the aid of pressure, the complete phase diagrams of two HBCs were constructed that in addition to the two equilibrium phases contain also a kinetically arrested (i.e., glassy) phase. Other studies emphasized the anisotropy in the thermal expansion of DLCs. The latter reflects the anisotropy in the molecular interactions; intra vs. inter-columnar, originating from π-π stacking and van der Waals interactions, respectively. Recently, HBCs were shown to have large thermal expansion within the Colh phase but thermal contraction within the Cr phase. The latter originates from the increasing disk tilt with respect to the columnar axis and reflects the tendency towards increasing the packing density within the Cr phase. With respect to the molecular dynamics, earlier investigations identified the main α-process as reflecting the axial motion of disks around the columnar axis as well as regions of high and low packing density along the columns, first described by de Gennes as pincements. Recent concerted efforts by site-specific NMR (Group of Prof. H. Spiess, MPI-P) and dielectric spectroscopy (DS) in addition to the “fast” axial process associated with in- and out-of plane motions, identified slower dynamics reflecting a collective re-organization of disks within the columns that result in the complete relaxation of the dipole moment.

Collaborators: K. Müllen (MPI-P), H.W. Spiess (MPI-P), Y.H. Geerts (ULB, Belgium)

Recent publications (2011-2016)

[1] N. Tasios, C. Grigoriadis, M.R. Hansen, H. Wonneberger, C. Li, H.W. Spiess, K. Müllen, G. Floudas, 

“Self-assembly, dynamics and phase transformation kinetics of donor-acceptor substituted perylene derivatives”, 

J. Am. Chem. Soc.  132, 7478, 2010

[2]  C. Grigoriadis, N. Haase, H.-J. Butt, K. Müllen, G. Floudas, 

“To tilt or not to tilt? Kinetics of structure formation in a discotic liquid crystal”,

 Soft Matter, 7, 4680, 2011.

[3] N. Haase, C. Grigoriadis, H.-J. Butt, K. Müllen, G. Floudas,

 “Effect of dipole functionalization on the thermodynamics and dynamics of discotic liquid crystals”,

 J. Phys. Chem. B  115, 5807, 2011.

[4] C. Grigoriadis, H. Duran, M. Steinhart, M. Kappl, H.-J. Butt, G. Floudas, 

“Suppression of phase transitions in a confined liquid crystal”,

 ACS Nano 11, 9208, 2011.

[5] M. R. Hansen, X. Feng, V. Macho, K. Müllen, H.W. Spiess, and G. Floudas, 

“Fast and Slow Dynamics in a Discotic Liquid Crystal with Regions of Columnar Order and Disorder”,

 Phys. Rev. Lett.,  107, 257801, 2011.

[6] P. Papadopoulos, C. Grigoriadis, N. Haase, H.-J. Butt, K. Müllen and G. Floudas, “

Dynamics of structure formation in a Discotic Liquid Crystal by infrared spectroscopy and related techniques”,

 J. Phys. Chem. B,   115, 14919, 2011.

[7] L. Chen, X. Dou, W. Pisula, X. Yang, D. Wu, G. Floudas, X. Feng, K. Müllen,

 “Discotic hexa-peri-hexabenzocoronenes with strong dipole: synthesis, self-assembly and dynamic studies”,

 Chem. Commun. 48, 702, 2012.

[8] C. Grigoriadis, C. Niebel, C. Ruzié, Y. H. Geerts and G. Floudas

“Order, viscoelastic and dielectric properties of symmetric and asymmetric alkyl[1]benzothieno[3,2-b][1]benzothiophenes”

 J. Phys. Chem. B 118, 1443-1451, 2014.