1 3
Theor Chem Acc (2015) 134:147
DOI 10.1007/s00214-015-1750-3
REGULAR ARTICLE
Helical molecular redox actuators with pancake bonds?
Pierre Beaujean
1,2 · Miklos Kertesz
2
Received: 23 July 2015 / Accepted: 22 October 2015 / Published online: 12 November 2015
© Springer-Verlag Berlin Heidelberg 2015
1 Introduction
Molecular actuators present large and reversible structural
changes triggered by an external stimulus [ 1 , 2 ]. The potential applications of such systems range from molecular
electronics to nanotechnology [ 3 , 4 ]. Actuation mechanism
is based on the ability of the system to transform this external stimulus into mechanical work. The signal can take
many forms: optical [ 5 ], chemical, or electrical [ 6 ], which
allows a wide range of systems to be used, such as synthetic foldamers [ 7 – 9 ] or functional molecular rotaxanes
[ 10 , 11 ]. The review by Baughman et al. [ 1 ] summarized
various advantages and limitations of those systems.
In particular, the external stimuli can be a consequence
of chemical or electrochemical doping, for example in the
case of conducting polymers (polypyrroles [ 12 , 13 ], polyanilines [ 14 , 15 ] or polythiophenes [ 16 ]) or single-walled
carbon nanotubes [ 17 ]. Electromechanical actuators, in
which the conformational or confi gurational change is
caused by a redox event, have attracted signifi cant attention [ 18 – 21 ]. This class also includes, for example, redoxcontrolled S···N interactions [ 22 ] and π-dimers formation [ 23 – 25 ]. In these cases, the redox event triggers a
modifi cation of the frontier orbital pattern which is a
quantum mechanical (QM) orbital effect. The overlap of
the singly occupied molecular orbitals (SOMOs) creates
multicenter/2-electron bonds (mc/2e), which are generally
found in π-radical dimers, where they are called pancake
bonds [ 26 – 33 ]. These multicenter π-stacking bonds present a number of unusual characteristics, one being that
the contact distances are shorter than the classical van
der Waals (vdW) distances [ 34 ]. They also present some
characteristics of covalent bonds: These bonds are typically signifi cantly stronger than vdW contacts and have
ESR-silent diamagnetic character providing evidence for
Abstract In an attempt to design molecular electromechanical actuators with large deformation response, we present here three helicene-like compounds, which offer signifi cant strain above 5 % due to two-electron charge transfer
(CT). The shrinking induced by CT is a quantum mechanical orbital effect. A good π–π overlap across the helical
pitch is critical for this type of actuation. The relevant overlap refers to frontier orbitals that are involved in the CT, and
it has some features common with π–π stacking pancake
bonds; however, these molecules do not represent all aspects
of typical pancake bonding. This overlap is accompanied by
a change in the bond length alternation pattern indicating
signifi cant change in π-conjugation. Additionally, two further helicene-like molecules included in this study also indicate large electromechanical actuation, but a simple orbital
interpretation is not available in those cases.
Keywords Molecular actuators · Charge transfer ·
Helical structure · π–π Overlap · Density functional theory
computations
Published as part of the special collection of articles “Festschrift
in honour of P. R. Surjan”.
Electronic supplementary material The online version of this
article (doi: 10.1007/s00214-015-1750-3 ) contains supplementary
material, which is available to authorized users.
* Miklos Kertesz
kertesz@georgetown.edu
1
Laboratoire de Chimie Théorique, Unité de Chimie Physique
Théorique et Structurale , Université de Namur , rue de
Bruxelles 61 , 5000 Namur , Belgium
2
Department of Chemistry and Institute of Soft Matter ,
Georgetown University , 37th and O Streets, NW , Washington ,
DC 20057-1227 , USA
47
Reprinted from the journal
Theor Chem Acc (2015) 134:147
DOI 10.1007/s00214-015-1750-3
REGULAR ARTICLE
Helical molecular redox actuators with pancake bonds?
Pierre Beaujean
1,2 · Miklos Kertesz
2
Received: 23 July 2015 / Accepted: 22 October 2015 / Published online: 12 November 2015
© Springer-Verlag Berlin Heidelberg 2015
1 Introduction
Molecular actuators present large and reversible structural
changes triggered by an external stimulus [ 1 , 2 ]. The potential applications of such systems range from molecular
electronics to nanotechnology [ 3 , 4 ]. Actuation mechanism
is based on the ability of the system to transform this external stimulus into mechanical work. The signal can take
many forms: optical [ 5 ], chemical, or electrical [ 6 ], which
allows a wide range of systems to be used, such as synthetic foldamers [ 7 – 9 ] or functional molecular rotaxanes
[ 10 , 11 ]. The review by Baughman et al. [ 1 ] summarized
various advantages and limitations of those systems.
In particular, the external stimuli can be a consequence
of chemical or electrochemical doping, for example in the
case of conducting polymers (polypyrroles [ 12 , 13 ], polyanilines [ 14 , 15 ] or polythiophenes [ 16 ]) or single-walled
carbon nanotubes [ 17 ]. Electromechanical actuators, in
which the conformational or confi gurational change is
caused by a redox event, have attracted signifi cant attention [ 18 – 21 ]. This class also includes, for example, redoxcontrolled S···N interactions [ 22 ] and π-dimers formation [ 23 – 25 ]. In these cases, the redox event triggers a
modifi cation of the frontier orbital pattern which is a
quantum mechanical (QM) orbital effect. The overlap of
the singly occupied molecular orbitals (SOMOs) creates
multicenter/2-electron bonds (mc/2e), which are generally
found in π-radical dimers, where they are called pancake
bonds [ 26 – 33 ]. These multicenter π-stacking bonds present a number of unusual characteristics, one being that
the contact distances are shorter than the classical van
der Waals (vdW) distances [ 34 ]. They also present some
characteristics of covalent bonds: These bonds are typically signifi cantly stronger than vdW contacts and have
ESR-silent diamagnetic character providing evidence for
Abstract In an attempt to design molecular electromechanical actuators with large deformation response, we present here three helicene-like compounds, which offer signifi cant strain above 5 % due to two-electron charge transfer
(CT). The shrinking induced by CT is a quantum mechanical orbital effect. A good π–π overlap across the helical
pitch is critical for this type of actuation. The relevant overlap refers to frontier orbitals that are involved in the CT, and
it has some features common with π–π stacking pancake
bonds; however, these molecules do not represent all aspects
of typical pancake bonding. This overlap is accompanied by
a change in the bond length alternation pattern indicating
signifi cant change in π-conjugation. Additionally, two further helicene-like molecules included in this study also indicate large electromechanical actuation, but a simple orbital
interpretation is not available in those cases.
Keywords Molecular actuators · Charge transfer ·
Helical structure · π–π Overlap · Density functional theory
computations
Published as part of the special collection of articles “Festschrift
in honour of P. R. Surjan”.
Electronic supplementary material The online version of this
article (doi: 10.1007/s00214-015-1750-3 ) contains supplementary
material, which is available to authorized users.
* Miklos Kertesz
kertesz@georgetown.edu
1
Laboratoire de Chimie Théorique, Unité de Chimie Physique
Théorique et Structurale , Université de Namur , rue de
Bruxelles 61 , 5000 Namur , Belgium
2
Department of Chemistry and Institute of Soft Matter ,
Georgetown University , 37th and O Streets, NW , Washington ,
DC 20057-1227 , USA
47
Reprinted from the journal