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Photoswitchable Acids and
BasesBases
Georgina A. Carballo
November 5, 2008
Why Should We Care about Photogenerated Acids or Bases?
Photoresist
Base insoluble Base soluble
Ito, H.; Willson, C. G.; Frechet, J. M. J.; Farrall, M. J.; Eichler, E. Macromolecules 1983, 16, 510-517.
Can distinguish between irradiated and non-irradiated areas - photolithography
Applications of Photoresists
Light
1. Remove mask
2. Wash exposed
photoresist
Etch oxide layer
Remove
unexposed
photoresist
Silicon Wafer
Oxide Layer
Photoresist
Mask
(Plasma)
photoresist
Microchip Fabrication
http://www.advantivtech.com/wafers/patterned-test-wafers.html
80nm Line and Space Pattern
Photogenerated Acids
1,8-Naphthalimide sulfonate photoacid generators
Malval, J.-P.; Suzuki, S.; Morlet-Savary, F.; Allonas, X.; Fouassier, J.-P.; Takahara, S.; Yamaoka, T. J. Phys. Chem. A 2008, 112,
3879-3885
Ito, H.; Willson, C. G.; Frechet, J. M. J.; Farrall, M. J.; Eichler, E. Macromolecules 1983, 16, 510-517.
Photogenerated Base
• Light induced generation of a free amine• Light induced generation of a free amine
• 3’,5’-dimethoxybenzoin carbamates as the protecting group
• Benzofuran cyclization photo-product is major
• Applications in polymer crosslinking, coatings and
microlithography
Cameron, J. F.; Willson, C. G.; Frechet, J. M. J. J. Am. Chem. Soc. 1996, 118, 12925-12937.
Acid
Switch
Acid
Empty p Orbital
Switch
Externally Regulated Acidity and Basicity
hν
Switch
Switch
hν
Photochromism
Reversible transformations of a single chemical species upon electromagnetic radiation where the two states show distinguishable absorption spectra
(UV)
Photochromism: Molecules and Systems; 1 ed.; Rau, H., Ed.; Elsevier, 1990; Vol. 40.
http://www.athenseyecare.com/EyeGlassLenses.cfm
(vis)
(UV)
Light Dark
Photochromic Molecules
Diarylethenes Spirooxazines
Fulgides
AzobenzeneSpiropyrans
Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev. 2000, 100, 1741-1754.
Matsuda, K.; Irie, M. J. Photochem. Photobiol., C 2004, 5, 169-182.
Yokoyama, Y. Chem. Rev. 2000, 100, 1717-1740.
Photoswitchable Base
• Photoswitching ability given by azobenzene chromophore
• Z isomerization allows access to piperidine group
Peters, M. V.; Stoll, R. S.; Kühn, A.; Hecht, S. Angew. Chem., Int. Ed. 2008, 47, 5968-5972.
Photoisomerization of Azobenzene
Hexanen,π* 432 nm (εmax =4 x 102 M-1 cm-1)π,π* 318 nm (ε =2.2 x 104 M-1 cm1)
Hexanen,π* 430 nm (εmax = 1500 M-1 cm-1)
π,π* 260 nm
0.99nm 0.55 nm
• Reduced conjugation and planarity of cis-isomer
• Trans isomer thermally stable by 12-14 Kcal/mol
• Change in dipole 0� 3.1D
• Change in length of molecule
Cembran, A.; Bernardi, F.; Garavelli, M.; Gagliardi, L.; Orlandi, G. J. Am. Chem. Soc. 2004, 126, 3234-3243.
Yager, K. G.; Barrett, C. J. J. Photochem. Photobiol., A 2006, 182, 250-261.
Crecca, C. R.; Roitberg, A. E. J. Phys. Chem. A 2006, 110, 8188-8203.
Fujino, T.; Arzhantsev, S. Y.; Tahara, T. Bull. Chem. Soc. Jpn. 2002, 75, 1031-1040.
π,π* 318 nm (εmax =2.2 x 104 M-1 cm1)max
π,π* 260 nm
Simplified Energy Level for Azobenzene - π����π*
n
*
n
*
*
weakenedπ bond
Torsion about the N=N bond
Photochromism: Molecules and Systems; 1 ed.; Rau, H., Ed.; Elsevier, 1990; Vol. 40.
S0 S2
Simplified Energy Level for Azobenzene – n����π*
n
*
n
*
n *
π bond intact
In plane inversion
S0 S1
intact
Photochromism: Molecules and Systems; 1 ed.; Rau, H., Ed.; Elsevier, 1990; Vol. 40.
Accepted Mechanisms for Azobenzene Isomerization
Torsion about the N=N bond
In plane inversionIn plane inversion
Photochromism: Molecules and Systems; 1 ed.; Rau, H., Ed.; Elsevier, 1990; Vol. 40.
Rotation about N=N Bond
• S2 accessed by π�π*
• Single bond character
for N-N in S2
• Rotation about the N-N• Rotation about the N-N
bond possible in S2
Lednev, I. K.; Ye, T. Q.; Matousek, P.; Towrie, M.; Foggi, P.; Neuwahl, F. V. R.; Umapathy, S.; Hester, R. E.; Moore, J. N. Chem.
Phys. Lett. 1998, 290, 68-74.
In Plane Inversion
• S1 accessed by n�π*
• Double bond character
of S1
•
• Excitation with visible
light to S1 will result in in light to S1 will result in in
plane inversion
• Relaxation of S2 �S1 will
also result in inversion
Fujino, T.; Arzhantsev, S. Y.; Tahara, T. Bull. Chem. Soc. Jpn. 2002, 75, 1031-1040.
Lednev, I. K.; Ye, T. Q.; Matousek, P.; Towrie, M.; Foggi, P.; Neuwahl, F. V. R.; Umapathy, S.; Hester, R. E.; Moore, J. N. Chem.
Phys. Lett. 1998, 290, 68-74.
Which Mechanism is Preferred?
Torsion about the N=N bond
In plane inversion
• Need to look at the transition states for both mechanisms
• Vibrational spectroscopy
• N-N ~1000 cm-1, -N=N- 1420-1410 cm-1
Photochromism: Molecules and Systems; 1 ed.; Rau, H., Ed.; Elsevier, 1990; Vol. 40.
Lambert, J.; Shurvell, H.; Lightner, D.; Cooks, G. Organic Structural Spectroscopy; 1 ed.; Prentice Hall: New Jersey, 1998.
Raman Spectroscopy-Detecting the Structures of Excited States
• Inelastic collisions of photons with molecules
• Scattering spectroscopy
• Ti:sapphire laser pump 1.8 ps pulses
• 0 ps delay after the pulse
Lambert, J.; Shurvell, H.; Lightner, D.; Cooks, G. Organic Structural Spectroscopy; 1 ed.; Prentice Hall: New Jersey, 1998.
Fujino, T.; Tahara, T. J. Phys. Chem. A 2000, 104, 4203-4210.
Picosecond Raman Spectroscopy- N=N Stretching
• Photoexciting (273 nm)
from S0 � S2 � S1
• transient Raman detected
at 0 ps delay (410 nm)
• Evidence of double bond 15N
15N
14N14N
N=N stretching
• Evidence of double bond
character of NN at S1
supports in-plane
inversion mechanism
Fujino, T.; Arzhantsev, S. Y.; Tahara, T. Bull. Chem. Soc. Jpn. 2002, 75, 1031-1040.
Fujino, T.; Tahara, T. J. Phys. Chem. A 2000, 104, 4203-4210.
14N14N
Quantum Yield - Efficiency of Photochemical Reactions
Measures the efficiency of a particular light induced process
For efficient photochemical process Φ≈1
Anslyn, E.; Dougherty, D. Modern Physical Organic Chemistry; University Science Books: Sausalito, California, 2006.
hν
For efficient photochemical process Φ≈1
Photostationary State
• Analogous to thermal equilibrium
• Particular proportion of isomers that does not change upon further irradiationirradiation
• Importance of the absorbance
Anslyn, E.; Dougherty, D. Modern Physical Organic Chemistry; University Science Books: Sausalito, California, 2006.
[T]
[C]=
c c
t t
Where ε = efficiency of absorptionΦ= quantum yield
Rotationally Locked Azobenzenes
trans,trans trans,cis cis,cis
Testing the photisomerization mechanism via rotationally restricted
azobenzene derivatives
• Cyclic array of
Azobenzenophanes (ABP)
restricts rotation about the N=N
bond
• Isomerization more plausible via
inversion
• Increase in Φπ,π* might indicate
prior relaxation to n,π*
Rau, H.; Lueddecke, E. J. Am. Chem. Soc. 1982, 104, 1616-1620.
trans,trans trans,cis cis,cis
Irradiation at 366 nm to the
photostationary state
PhotoReactionAzobenzene ABP
Φ t�c Φt,t�t,c
S1 � S0 0.23 0.24
S2 � S0 0.1 0.21
Structural Basis for Photoswitchable Base
E-isomer Z-isomer
Peters, M. V.; Stoll, R. S.; Kühn, A.; Hecht, S. Angew. Chem., Int. Ed. 2008, 47, 5968-5972.
Blocked
Features of Photoswitchable Bases
4a R= Me, R'= t-Bu
Accessible basic Site
Blocked basic site
• Conformationaly restricted
• Steric shielding of active site
• Orthogonal positioning of the chromophore
4b R= t-Bu, R'= t-Bu
4c R= t-Bu, R'= 2,6-Me2C6H3
Peters, M. V.; Stoll, R. S.; Kühn, A.; Hecht, S. Angew. Chem., Int. Ed. 2008, 47, 5968-5972.
OFF ON
Photoisomerization of base vs. time
• Isomers or base have different absorption spectra.
• E�Z photoisomerization can be followed overtime with UV-visspectroscopy.
N
O
O
Me
N
N
tBu
tBu
E
Z
UV-Vis spectrum of E and Z Isomers
Z
E���� Z isomerization365 nm for 13 min
Z���� E isomerization400 nm for 4 min
Peters, M. V.; Stoll, R. S.; Kühn, A.; Hecht, S. Angew. Chem., Int. Ed. 2008, 47, 5968-5972.
Parameters Measured for Photoswitchable Bases
4a R= Me, R'= t-Bu
4b R= t-Bu, R'= t-Bu
4c R= t-Bu, R'= 2,6-Me2C6H3
PSS (Z/E) t 1/2 (h) pKa E pKa Z
Peters, M. V.; Stoll, R. S.; Kühn, A.; Hecht, S. Angew. Chem., Int. Ed. 2008, 47, 5968-5972.
Hall, H. K. J. Am. Chem. Soc. 1957, 79, 5441-5444.
PSS (Photostationary state) irradiation at 365 nm
Half life of Z isomer at 20°C in the dark
PSS (Z/E) t 1/2 (h) pKa E pKa Z
4a 90:10 268 NR NR
4b 90:10 286 15.9 ± 0.1 16.7 ± 0.1
4c >90:10 466 16.0 ± 0.1 16.7 ± 0.1
R NO
R2
NO2
O
R1R
Henry Reaction
CTA-Cl72%
R NO2
R NO2
B
B H
R2
NO2
OHR1
R
O
R2R1
LA
LA
R1 R2
O
Ballini, R.; Bosica, G. J. Org. Chem. 1997, 62, 425-427.
Luzzio, F. A. Tetrahedron 2001, 57, 915-945.
Henry Reaction Catalyzed by a Smart Base – NMR Kinetics
Catalytic Activity with 10 mol% base
k [10 s ] k rel
Base
k [10-5 s-1 ] k relexp. 100% Z isomer 1.4 35.9
Z����E back rxn. 1.2 30.8Z-isomer at PSS 1.1 28.2
E-isomer 0.039 1E-azobenzene 0.0024 0.062
no catalyst 0.039 1
Peters, M. V.; Stoll, R. S.; Kühn, A.; Hecht, S. Angew. Chem., Int. Ed. 2008, 47, 5968-5972.
PSS= Photostationary State 90:10
A Photoswitchable Lewis Acid
• Photoswitchable• Photoswitchable
• Lewis acidity “turned OFF” by
electron donation from N to B
• Lewis acid “ON” when donor is
away from B
N N
B
O
O
Yoshino, J.; Kano, N.; Kawashima, T. Tetrahedron 2008, 64, 7774-7781.
Dative Bond
Substituent Effects on the Strength of the B-N bond
Stronger B-N Weaker B-N
1a 1b 1c
N N
B
O
O
F
N N
B
O
O
OMe Electron Donating
ElectronWithdrawing
Yoshino, J.; Kano, N.; Kawashima, T. Tetrahedron 2008, 64, 7774-7781.
1.8247 Å
Stronger B-N1.773 Å
Weaker B-N1.8947 Å
Photoisomerization of 2-(phenylazo)-phenylboranes
N N
B
O
O
F
N N
B
O
O
OMe
Substituent at 4' position
Bond Length (Å)
Photoisomerization Irradiation at 360 nm or
431 nm to the
Yoshino, J.; Kano, N.; Kawashima, T. Tetrahedron 2008, 64, 7774-7781.
position Length (Å) 360 nm E/Z 431 nm Z/E
H 1.8247 65:35 98:2
OMe (EDG) 1.773 100:0 --
F (EWG) 1.8947 54:46 94:6
431 nm to the
photostationary state
Electron donating groups
strengthen the B-N
interaction
Electron withdrawing
groups weaken the B-N
interaction
Measured with NMR in C6D6
EDG=Electron donating group
EWG = Electron withdrawing group
Isomerization 2-(Phenylazo)-catecholboranes at 360 nm
I II III
Isomerizes No isomerization Isomerizes
Kano, N.; Yoshino, J.; Kawashima, T. Org. Lett. 2005, 7, 3909-3911.
Compound E:Z ratioBond
Length Å
I 65:35 1.8247
II 100:0 1.721
III 69:31 --
Trend is reversed
EWG strengthens B-N interaction
EDG should weaken B-N
interaction
Testing Lewis Acidity Followed by 11B NMR
11B δ 21.8 ppm 11B δ 30.8 ppm
11B δ 18.0 ppm 11B δ 13.8 ppm
Kano, N.; Yoshino, J.; Kawashima, T. Org. Lett. 2005, 7, 3909-3911.
E-isomer does not act as a
Lewis acid
Z-isomer shows Lewis acidity
Attempts to Diversify 2-(Phenylazo)-Phenylboranes
(HO)2B N N
HO OH
B N N
O
O
R1
R2
R3
HO
HON N
(E)-6 R1=R3=Me, R2=H
(E)-7 R2=R3=Cl, R1=H
HOHO
HO OH
H2N
NH2
(E)-12(E)-5a
B
O
O R1
R2
R3
N N
R' R' O
N N
B
(E)-9, R'=H(E)-10, R'=PH
N NBHN
NH
(E)-8
(E)-11
B
O
O
O
OO
R'R'
Yoshino, J.; Kano, N.; Kawashima, T. Tetrahedron 2008, 64, 7774-7781.
Photoswitchable Lewis Acid
BO O
Resonance stabilization associated
with dioxaborole ring system
Claimed to be a 4n+2 system
ONOFF
Letsinger, R. L.; Hamilton, S. B. J. Org. Chem. 1960, 25, 592-595.
Lemieux, V.; Spantulescu, M. D.; Baldridge, Kim K.; Branda, Neil R. Angew. Chem., Int. Ed. 2008, 47, 5034-5037.
Prevention of Irreversible Oxidation (aromatization)
Aromatic
Kellogg, R. M.; Groen, M. B.; Wynberg, H. J. Org. Chem. 1967, 32, 3093-3100.
Oxidation is prevented by the addition of substituents at the 2-positions
Aromatic
π-Orbital Symmetry
Woodward-Hoffmann Rules
Thermally Photochemicaly
4n conrotatory disrotatory
4n+2 disrotatory conrotatory
ConrotatoryDisrotatory
Woodward, R. B.; Hoffmann, R. J. Am. Chem. Soc. 1965, 87, 395-397.
Alignment of Orbitals Prior Cyclization
Antiparallel
Prior E����Z isomerization
Mallory, F. B.; Wood, C. S.; Gordon, J. T. J. Am. Chem. Soc. 1964, 86, 3094-3102.
Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990, 63, 1311-1315.
AntiparallelParallel
Ring Closed Isomer
Features of Dithienylethene Photoswitches
• Prevention of irreversible oxidation to aromatic compound
• Thermal irreversibility
• Sensitivity to irradiation
• Fatigue resistance
Matsuda, K.; Irie, M. J. Photochem. Photobiol., C 2004, 5, 169-182.
Thermal Stabilization of Diarylethenes of Ring-Closed Isomer
Thermal Reversion
Lifetime 3 min at 30°C Thermal Reversion
Lifetime 12h at 80°C
Thermal irreversibility is
achieved by using
substituents with low
aromatic stabilization
energies
Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985-996.
Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803-808.
Aromatic stabilization Energies in Kcal/mol
Features of Lewis Acid
OFF ON
• Lewis acidity is “OFF” due to delocalization of π electrons that partially
occupy the Boron p orbital.
• Lewis acidity is “ON” when electrons are crossconjugated to the polyene
system.
OFF ON
Lemieux, V.; Spantulescu, M. D.; Baldridge, Kim K.; Branda, Neil R. Angew. Chem., Int. Ed. 2008, 47, 5034-5037.
Photocyclization of Lewis Acid
OFF
Photostationary state
285 nm
Photostationary state
reached in 12s with 81% of
ring closed isomer
Reverts to open ring isomer
after irradiation with visible
light for 5 min
535 nm
Lemieux, V.; Spantulescu, M. D.; Baldridge, Kim K.; Branda, Neil R. Angew. Chem., Int. Ed. 2008, 47, 5034-5037.
370 nm
Calculated LUMO of Simplified Model
O OB
H
S SH H
Lower in energy by
19 Kcal/mol
Node at the B
Node
Lemieux, V.; Spantulescu, M. D.; Baldridge, Kim K.; Branda, Neil R. Angew. Chem., Int. Ed. 2008, 47, 5034-5037.
Available p-orbital
Lewis acidic
Probing Lewis Acidity with Pyridine Followed by 1H NMR
1H NMR δ will not change
δ= 8.1 ppm
1H NMR δ will shift
ONOFF
• Open ring isomer will not bind to pyridine
• Closed ring isomer has Lewis acid reactivity in the presence of
pyridine (Lewis base)
Lemieux, V.; Spantulescu, M. D.; Baldridge, Kim K.; Branda, Neil R. Angew. Chem., Int. Ed. 2008, 47, 5034-5037.
Binding of Closed Ring Isomer to Pyridine
• Lewis Acidity : “ON”8.05
8.1
8.15
8.2
1H NMR • Lewis Acidity : “ON”
• Ring Closed Isomer Binds
with pyridine
• Chemical shift decreases
upon binding
•1H δ consistent with 1:1
stoichiometry
Lemieux, V.; Spantulescu, M. D.; Baldridge, Kim K.; Branda, Neil R. Angew. Chem., Int. Ed. 2008, 47, 5034-5037.
7.85
7.9
7.95
8
8.05
0 1 2 3 4
1H NMR Chemical
Shift (ppm)
Equivalents of Pyridine
Summary
• Acids and bases were given the ability to be “turned on” and
“turned off” by attaching a molecular switch.
• Azobenzene and dithienylethene were shown to be effective
at photoswitching acidity and basicity.
• The applications of photoswitchable acids and bases are yet
to be explored.
Acknowledgements
• Dr. Baker
• Qin, Sampa, Hui, Tom
• Dr. Jackson
• Dr. Hecth
• Dr. Blanchard
• Dr. Wulff• Dr. Wulff
• Babak
• Karrie, Luis S., Wynter, Heyi, Yiding, Wen, QuanXuan, Scott