K.L. Sebastian IPC Department, IISc Chennai, September 14, 2005 Molecular Devices

Motivation – examples from biology Molecular Rollers and Rocker Molecular Wheel Molecular Rattle. Fluxionality for Rotational Motion Nature does it very well! (Biological Molecular Motors) Synthetic Molecular Motors Light driven molecular motor Outline

Height ~ 8 nm Width ~ 10 n Is that a flower? It is a motor

Can one design molecules that would prefer to roll on a surface? Seems rather difficult, perhaps we can try to use fluxionality! What do you mean? ROLLERMOLECULAR

To explain, let us start with Pentaprismane Consider Hypostrophene-it is fluxional -perhaps we can use this property!   !!

Pentaprismane (C 10 H 10 ) D 5h Hypostrophene C 2v Symmetry is broken! It can be broken in FIVE different ways! hypostrophene pentaprismane H

Five degenerate minima! It should be possible to jump from one to the other It does! Known as Degenerate Cope Rearrangement

Rate constant Degenerate Cope Rearrangement for Hypostrophene GSTS Activation energy Q and Q* are the partition functions of GS and TS Rate constant ~ 1.8 X sec -1 Activation energy B3LYP/6-31G** ( kcal/mol)

Think of Hypostrophene adsorbed on Al(100)

Rolling-TS E act ~ 18 kcal/mol Translation-TS: E act ~ 65.5 kcal/mol Rolling Motion

Same thing can happen with syn-TOD! Molecular Roller

GS TS Activation energy B3LYP/6-31G** (kcal/mol) Syn-TOD C 2v Cubane TdTd

Activation energy 13.6 kcal/mol (B3LYP/6-31G** C,H and 3- 21G for Al) TS

We conclude that: Hypostrophene and tricyclooctadiene when chemisorbed on Al(100) surface should behave as ‘Molecular roller’ Bidisa Das, K.L Sebastian, Chemical Physics Letters, 330, 433 (2000). MOLECULAR ROLLERS

MOLECULAR ROCKER

GS Cope Rearrangement of Semibullvalene Activation energy 5.5 B3LYP/6-31G** (kcal/mol) TS

Semibullvalene on Al(100)

Metal surface: cluster of 14 or 32 Al atoms in two layers Hydrogen atoms at the edges. B3LYP/Al:3-21G, C,H:6-31G** E a = 21.8 kcal/mol MOLECULAR ROCKER

Fe(CO) 3 Fe(CO) 3 moving around hypostrophene Fluxionality for Rotational Motion

Hypostrophene 33.6 kcal/mol 4.3 kcal/mol 203i cm -1

Molecular wheel

C 5 H 5 Ge(CH 3 ) 3 is known to be fluxional! E act ~ 16.0 kcal/mol MOLECULAR WHEEL

The activation barriers and rate constants 3.7X Sn (C,H: 6-31G** & Sn: 3-21G) 1.6X Ge (C,H: 6-31G** & Ge: 6-31G**) 3.2X Si (C,H: 6-31G** & Si: 6-31G**) rate constant at K (sec -1 ) E act (kcal/mol) M atom bonded to Cp and basis-sets used

Cyclopentadienyl adsorbed on Ge surface, should move like a wheel! E act ~ 11.9 kcal/mol Adsorbed to the same site! Hopping onto adjacent Ge atoms

2.5X X X Rate constant (sec -1 ) K E act (kcal/mol) Sn (C,H: 6- 31G** & Sn: 3-21G) Ge (C,H: 6- 31G** & Ge: 6-31G**) Si (C,H: 6- 31G** & Si: 6-31G**) M atom bonded to Cp and basis-sets used

Molecular Wheel Sn E a = 5.97 kcal/mol B. Das and K.L. Sebastian: CPL 357, 25 ( 2002) That is not bad! Why don’t you call it a molecular “seal”?

We conclude that : Bidisa Das, K.L Sebastian, Chemical Physics Letters, 357, 25 (2002). The cyclopentadienyl co-adsorbed with hydrogen on Si/Ge/Sn (111) surfaces would form a system where the five membered ring can undergo spinning motion with low activation energies.

Molecular Rattle A B BB A A

Does not happen! Ionization potential of H too large! Perhaps, in an excited state, this might happen H+H+ - H+H+ -

Replace H with Li! Li + - Ring too small Li + -

These are the molecules that we studied but activation energies for the ‘umbrella inversion’ kind of motion was found to be high. E act ~42.4 kcal/mol E act a:315 kcal/mol b:36.6kcal/mol c:33.6kcal/mol M. Oda, Pure & Appl. Chem. 58, 7 (1986), T.Z. Ktaz, P. A. Garratt, J. Am. Chem. Soc. 85, 2852 (1963).

C9H9-C9H9-

E a = 11.7 kcal/mol B. Das and K.L. Sebastian: CPL, 365, 320 (2002) Molecular Rattle Proton going through benzene (C 6 H 7 + ) Mahapatra, Sathyamurthy, Current Science, 1995

Cyclononatetraenyl- lithium The activation barrier for the ‘umbrella inversion’ in this case is ~11.5 kcal/mol Normal mode analysis: 276 cm -1 (GS), 274i cm -1 (TS) 1D through ring motion calc.: 277 cm -1 (GS), 267i cm -1 (TS)

Nature does it well!

Biological Molecular Motors We know of several, efficient molecular motors! All of them occur in BIOLOGICAL systems

Figure from:

Energy from photosynthesis Figures and animation from:

ATP Synthase Synthesizes ATP. Rotates while it does this! Most powerful known motor ATP Synthase (Rotary)

Kinesin (Walker) Proteins that WALK! Works like a PORTER at the railway station See animation at

Myosin Proteins that PUSH! Myosin For an animation, see the CD of the book: Molecular Biology of the Cell by B. Alberts et. Al. See also: em/MBWeb/mb2/part1/myosin.htm:// em/MBWeb/mb2/part1/myosin.htm

Synthetic Molecular Motors NO WAY near the natural ones!

Rotaxane shuttle station stopper station

-e+e Electron Removal

+ H + - H + Proton Addition

Either electron removal or proton addition

Catenanes 2-catenane3-catenane

Switching by Oxidation-Reduction Reations -e +e

Catenanes – how to have light driven motor? Leigh et. al. Nature, 424, 174 (2003) Excitation of the station leads to unbinding! station shuttle

Light induced excitation of the shuttle is better!

K.L. Sebastian: Current Science, 87, 232 (2004)

Light induced conformational change driving translation! Cis-trans isomerization as in Azobenzene could be useful Can one have……?

Brownian motion would drive the ratchet! But it can rotate only in one direction! So thermal, random motion can be used to drive the ratchet in one direction! Molecular Ratchets, Second Law and Detailed Balance

[4]helicene triptycene Axle It can undergo ratchet-like internal rotation! A molecular ratchet was Synthesized and studied by Kelly et. al. Angewandte Chemie 109, 1969 (1997).

To probe rotational motion use NMR That is clever! Second Law verified!

But wait! Let us think about this! There he goes! As mad as a coot! The fact that you can see rotation means that your initial state is a non-equilibrium one! So even if you had seen net rotation, that would not violate the second law!

But you do not see any net rotation! What does that mean? Because of Detailed Balance! K.L. Sebastian, Physical Review E61, 937 (2000)

CONCLUSIONS Molecular Roller, Wheel, Rocker, Rattle! Fluxional behavior can be used to get interesting mechanical motion Ratchets, Light driven motor……

Acknowledgements Dr. Bidisa Das Prof. Ashoka Samuelson