Massimo Baroncini, Serena Silvi, Alberto Credi, Margherita Venturi e-mail: [email protected]
Dipartimento di Chimica “G. Ciamician”, via Selmi 2, 40126 Bologna, Italy
Photochemical Nanosciences Laboratory Bologna
Ø Molecular Logic Ø CdSe Quantum Dots Ø Molecular Machines and Devices
Ø Dendrimers Ø Si Quantum Dots Ø Water Splitting
Photochemical Nanosciences Laboratory Bologna
Supervisors…
Ø Molecular Logic Ø CdSe Quantum Dots Ø Molecular Machines and Devices
Ø Dendrimers Ø Si Quantum Dots Ø Water Splitting
Photochemical Nanosciences Laboratory Bologna
Supervisors…
• IMM: Vittorio Morandi, Luca Ortolani (QDs) • ISOF: Giovanna Barbarella, Francesca Di Maria (Supramolecular Systems)
Rotaxane
Rotaxanes: “Molecules in which a ring encloses another, rod-‐like molecule having end groups too large to pass through the ring opening, and thus holds the rod-‐like molecule in posi;on without covalent bonding.” IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book").
Rotaxane
Rotaxanes: “Molecules in which a ring encloses another, rod-‐like molecule having end groups too large to pass through the ring opening, and thus holds the rod-‐like molecule in posi;on without covalent bonding.” IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book").
Pseudorotaxanes: Rotaxane-‐like molecular assembly in which the threading component(s) has(have) ends small enough to permit threading or dethreading of the macrocyclic molecule(s). Andrey Yerin, Edward S. Wilks, Gerad P. Moss and Akira Harada Nomenclature for rotaxanes and pseudorotaxanes (IUPAC Recommenda;ons 2008)
Pseudorotaxane
Rotaxane
Rotaxanes: “Molecules in which a ring encloses another, rod-‐like molecule having end groups too large to pass through the ring opening, and thus holds the rod-‐like molecule in posi;on without covalent bonding.” IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book").
Pseudorotaxanes: Rotaxane-‐like molecular assembly in which the threading component(s) has(have) ends small enough to permit threading or dethreading of the macrocyclic molecule(s). Andrey Yerin, Edward S. Wilks, Gerad P. Moss and Akira Harada Nomenclature for rotaxanes and pseudorotaxanes (IUPAC Recommenda;ons 2008)
Pseudorotaxane
O
O O
O
O
OO
O
+
DBA DB24C8
NH2
O
O
O
O
DB24C8 ·∙ DBA-‐1H+
NH2
The Beginnings…
The Axle (E,E-1H·PF6) Made of two terminal azobenzene units and a central ammonium station, which is a recognition site for DB24C8.
NH2
N NN N
Me MePF6
The Beginnings…
The Axle (E,E-1H·PF6) Made of two terminal azobenzene units and a central ammonium station, which is a recognition site for DB24C8.
NH2
N NN N
Me MePF6
The Ring (DB24C8) Dibenzo[24]crown-8
O O
O
O
OO
O
O
The Pseudorotaxane 1H NMR (400 MHz, CD3CN, 304 K) 5mM in both components.
NH2
N NN N
Me
O
O
O
O
MePF6
The Beginnings…
The Axle (E,E-1H·PF6) Made of two terminal azobenzene units and a central ammonium station, which is a recognition site for DB24C8.
NH2
N NN N
Me MePF6
The Ring (DB24C8) Dibenzo[24]crown-8
O O
O
O
OO
O
O
Photochemistry
NH2
N NN N
HM
H1H2
H3H4 HN
PF6
NH2
N NN N
HM
H1
H2
H3H4
HN PF6hν
hν’ or Δ
Photochemistry
NH2
N NN N
HM
H1H2
H3H4 HN
PF6
NH2
N NN N
HM
H1
H2
H3H4
HN PF6
δ
HM HN H3 H2 H4 H1
HM HN H3 H2 H4 H1
400 MHz, CD3CN, 304 K
The azobenzene units of the axle-molecule can be almost quantitatively photoisomerized from trans to cis by means of UV-light irradiation. The complete cis > trans thermal conversion takes place in ca. 4 weeks at 20°C.
hν
hν’ or Δ
Slow down
+
O O
O
O
OO
O
O
NH2
N NN N
Me Me
PF6
Slow down
+
Slow down
+
400 MHz, CD3CN, 304 K
k(in)ZZ 2.9 x 10-3 M-1s-1
k(out)ZZ 7.2 x 10-5 s-1
Slow down
+
Time (h)
Concen
tra;
on (m
M)
400 MHz, CD3CN, 304 K
k(in)ZZ 2.9 x 10-3 M-1s-1
k(out)ZZ 7.2 x 10-5 s-1
Time (h)
Pseudorotaxane and Rotaxane
hν
hν’or Δ
KEE 820 M-1
k(in)EE 37 M-1s-1
k(out)EE 4.5 x 10-2 s-1
Time (h)
Pseudorotaxane and Rotaxane
hν
hν’or Δ
KEE 820 M-1
k(in)EE 37 M-1s-1
k(out)EE 4.5 x 10-2 s-1
Time (h)
Pseudorotaxane and Rotaxane
KZZ 400 M-1
k(in)ZZ 2.9 x 10-3 M-1s-1
k(out)ZZ 7.2 x 10-5 s-1
hν
hν’or Δ
hν
hν’or Δ
KEE 820 M-1
k(in)EE 37 M-1s-1
k(out)EE 4.5 x 10-2 s-1
Time (h)
Pseudorotaxane and Rotaxane
KZZ 400 M-1
k(in)ZZ 2.9 x 10-3 M-1s-1
k(out)ZZ 7.2 x 10-5 s-1
hν
hν’or Δ
hν
hν’or Δ
Photoisomerization of the azobenzene ends of the axle destabilizes the complex and slows down the threading/dethreading processes: both the energy minimum and the energy barriers are raised.
∆G‡threading
E-azoE
∆G
E
Reversible Photoswitching of Rotaxane Character and Interplay ofThermodynamic Stability and Kinetic Lability in a Self-Assembling Ring–
Axle Molecular System
Massimo Baroncini, Serena Silvi, Margherita Venturi, and Alberto Credi*[a]
Dedicated to Professor Luigi Fabbrizzi, recipient of the 2010 International Izatt–Christensen Award in Macrocyclic Chemistry
! 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 11580 – 1158711580
DOI: 10.1002/chem.201001409
Reversible Photoswitching of Rotaxane Character and Interplay ofThermodynamic Stability and Kinetic Lability in a Self-Assembling Ring–
Axle Molecular System
Massimo Baroncini, Serena Silvi, Margherita Venturi, and Alberto Credi*[a]
Dedicated to Professor Luigi Fabbrizzi, recipient of the 2010 International Izatt–Christensen Award in Macrocyclic Chemistry
! 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 11580 – 1158711580
DOI: 10.1002/chem.201001409
Introduction
Pseudorotaxanes are supramolecular complexes minimallycomposed of an axle-like molecule surrounded by a macro-cycle. These complexes can be rendered kinetically inert,that is, transformed into rotaxanes, by attaching bulkygroups at the extremities of the axle to prevent dethread-ing.[1,2] Therefore, the pseudorotaxane or rotaxane behaviorof a given axle–macrocycle pair is determined by the thread-ing–dethreading rate constants that in turn depend on thetemperature and the energy barriers associated with theseprocesses.[3]
Rotaxanes and related species are primarily interestingfor the construction of molecular machines.[4,5] The opera-tion of most rotaxane-type machines is based on classicalswitching processes between thermodynamically stablestates. It has become clear, however, that functional molecu-lar motors will only be realized if the reaction rates betweenstates can also be controlled,[6] thus enabling the implemen-tation of ratchet-type mechanisms.[7,8] In this context, theability of adjusting the threading–dethreading kinetics[9] bymodulating the corresponding energy barriers through exter-nal stimulation is an important goal.
A previously explored approach to convert a pseudoro-taxane into a rotaxane by using light was based on a stilbenederivative as the photoswitchable capping group at one endof the molecular axle.[10,11] Qualitative observations indicatethat the shape change of the stilbene end group broughtabout by E!Z photoisomerization is sufficient to hamperthe dethreading of a dibenzo[24]crown-8 ring from the axle.
However, these experiments were conducted at 0 8C undernitrogen and an external photosensitizer had to be used.Moreover, the E!Z conversion was not complete and re-versibility was not demonstrated; in fact it is known[12] that,besides isomerization, irradiation of stilbene causes cycliza-tion and dimerization. Thus, stilbene derivatives do notappear to be optimally suited for the reversible photochemi-cal control of motion kinetics in threaded and interlockedcompounds. Conversely, E-azobenzene can be converted tothe Z form by direct UVA irradiation with yields exceeding95 % and virtually no side products. The process is fully pho-tochemically or thermally reversible.[13]
Herein we describe a self-assembling system based on theknown[2,3a,14] dialkylammonium–crown ether recognitionmotif, which can be switched between thermodynamicallystable (pseudorotaxane) and kinetically inert (rotaxane)forms by light irradiation owing to the presence of photoiso-merizable azobenzene units at the extremities of the axle.[15]
The structural formulas of the molecular components,namely the bis-azobenzylamine axle EE-1H+ and the diben-zo[24]crown-8 ether ring 2, are shown in Scheme 1. We pres-ent a quantitative investigation into the effect of the isomer-ic state of the azobenzene end groups on both the associa-tion equilibrium constant and the threading–dethreadingrate constants of the molecular components. Furthermore,we report how a change in the ring–axle intercomponent in-teractions affects the kinetic behavior of the complex.
Results and Discussion
The dibenzo[24]crown-8 ring is commercially available. Theaxle was synthesized in good yield by using Mill!s couplingof bis(4-aminobenzyl)amine and 4-nitrosotoluene in aceticacid, followed by anion exchange with NH4PF6 (Scheme 2).The EE-1H·PF6 salt was fully characterized by using 1H(Figure 1c) and 13C NMR, DQF-COSY, ESI-MS, and UV/Vis absorption spectroscopies.[16] The 1H NMR spectrum of
Abstract: We have designed, synthe-sized, and investigated a self-assem-bling system that can be reversibly in-terconverted between thermodynami-cally stable (pseudorotaxane) and ki-netically inert (rotaxane) forms by lightirradiation. The system is composed ofa dibenzo[24]crown-8 ring and an axlecomprised of a dibenzylammonium rec-ognition site and two azobenzene endgroups. The isomeric form of the azo-benzene units of the axle has a little in-fluence on the stability constants of therespective pseudorotaxanes but greatlyaffects the threading–dethreading rate
constants. In fact, equilibration of thering and the axle in its EE isomericform occurs within seconds in acetoni-trile at room temperature, whereas theZZ axle threads–dethreads the ring atleast four orders of magnitude slower.Moreover, we show that a change inthe stability of the complex, achievedby deprotonating the dibenzylammoni-
um recognition site on the axle, affectsits kinetic behavior. We compare theresults of these experiments with thoseobserved upon dethreading the (pseu-do)rotaxane by using a competitiveguest for the ring, an approach whichdoes not inherently destabilize thering–axle interaction. This study out-lines a general strategy for the reversi-ble photochemical control of motionkinetics in threaded and interlockedcompounds and constitutes a startingpoint for the construction of multicom-ponent structures that can behave asphotochemically driven nanomachines.
Keywords: azo compounds · crowncompounds · hydrogen bonds · mo-lecular devices · supramolecularchemistry
[a] Dr. M. Baroncini, Dr. S. Silvi, Prof. M. Venturi, Prof. A. CrediDipartimento di Chimica “G. Ciamician”Universit" di Bologna, Via Selmi 240126 Bologna (Italy)Fax: (+39) 051-2099456E-mail : [email protected]
Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201001409.
Chem. Eur. J. 2010, 16, 11580 – 11587 # 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 11581
FULL PAPER
The End…
hν
hν’or Δ
A New Beginning
∆G‡threading
EEE
∆GEE
E
∆G‡threading
ZZ
∆GZZ
∆G‡threading
Passive
E
∆G‡threading
E E
∆G‡Threading
Passive
Directionally Controlled Threading/Dethreading
Directionally controlled threading
Photochemical gate locking hν
Directionally controlled dethreading
Gate unlocking Reset hν’ or Δ
E
Fast
∆G‡E-azo
∆G‡P
Slow
E
∆G‡Z-azo
Fast
Slow
∆G‡P
E
E
E
∆G‡E-azo
∆G‡P
Directionally Controlled Threading/Dethreading
Directionally controlled threading
Photochemical gate locking hν
Directionally controlled dethreading
Gate unlocking Reset hν’ or Δ
E
∆G‡Z-azo
Fast
Slow
∆G‡P
E
E
Standing On The Shoulders of Giants
NH2
PF6
P. R. Ashton, I. Baxter, M. C. T. Fyfe, F. M. Raymo, N. Spencer, J. F. Stoddart, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120, 2297-2307.
k(in)C5 1.3 x 10-1 M-1s-1 O
OO
O
O
O O
O+ N
H2
O
O
O
O
PF6
Standing On The Shoulders of Giants
NH2
PF6
P. R. Ashton, I. Baxter, M. C. T. Fyfe, F. M. Raymo, N. Spencer, J. F. Stoddart, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120, 2297-2307.
k(in)C5 1.3 x 10-1 M-1s-1 O
OO
O
O
O O
O+ N
H2
O
O
O
O
PF6
hν
hν’or Δ
NH2
NN
PF6
NH2
NN
PF6E-3H Z-3H
Standing On The Shoulders of Giants
NH2
PF6
P. R. Ashton, I. Baxter, M. C. T. Fyfe, F. M. Raymo, N. Spencer, J. F. Stoddart, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120, 2297-2307.
k(in)C5 1.3 x 10-1 M-1s-1 O
OO
O
O
O O
O+ N
H2
O
O
O
O
PF6
hν
hν’or Δ
2H
[2H⊂R][PF6]
R
NH2
NN
PF6
NH2
NN
PF6E-3H Z-3H
Standing On The Shoulders of Giants
NH2
PF6
P. R. Ashton, I. Baxter, M. C. T. Fyfe, F. M. Raymo, N. Spencer, J. F. Stoddart, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120, 2297-2307.
k(in)C5 1.3 x 10-1 M-1s-1 O
OO
O
O
O O
O+ N
H2
O
O
O
O
PF6
hν
hν’or Δ
Kinetic and thermodynamic data for the self-assembly of the investigated complexes in CD3CN at 298 K.
Complex K (M–1)
–ΔG° (kcal mol–1)
kin
(M–1 s–1) –ΔG#
in (kcal mol–1)
kout
(s–1) –ΔG#
out (kcal mol-1) t½
[EE-1H⊂R][PF6] 820 3.9 37 15 4.5×10–2 19.3 15.4 s [ZZ-1H⊂R][PF6] 400 3.5 2.9×10–3 20.9 7.2×10–6 24.5 27 h [2H⊂R][PF6] ≈30 2 1.3×10–1 18.6 4.4×10–3 20.7 2.6 min [E-3H⊂R][PF6] 225 3.2 22 15.6 0.1 18.8 6.3 s [Z-3H⊂R][PF6] 230 3.2 5.1×10–2 19.2 2.6×10–4 22.3 46 min
2H
[2H⊂R][PF6]
R
hν
hν
K+
hν
hν’or Δ K+
hν
hν’or Δ K+
18C6
[K⊂18C6]+
hν
hν’or Δ K+
18C6
[K⊂18C6]+
Directionally Controlled Transit
Angew. Chem. Int. Ed. 2012 (17) 4223-4226.
E
Fast
∆G‡E-azo
∆G‡P
Slow
E
∆G‡Z-azo
Fast
Slow
∆G‡P
Toward a Molecular Pump
Directionally controlled threading
Photochemical gate locking
hν
Directionally controlled dethreading
Gate unlocking Reset hν’ or Δ
IN
E
E
E
∆G‡Z-azo
Fast
Slow
∆G‡P
Toward a Molecular Pump
Directionally controlled threading
Directionally controlled dethreading
Gate unlocking Reset hν’ or Δ
E
Photochemical gate locking
hν
OUT
E
E
The Group of Photochemistry
Thanks to:
Dr. Elisa Bandini, Dr Massimo Benaglia Lab 506
Dr. Roberto Zamboni CNR-ISOF for the kind hospitality!
Photochemical Nanosciences Laboratory Bologna