ultrafast spectroscopy of methyl viologen: effects of zeolite entrapment joseph henrich, haoyu...
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Ultrafast Spectroscopy Of Methyl Viologen: Effects Of Zeolite Entrapment
Joseph Henrich, Haoyu Zhang, Jeremy White, Prabir Dutta and Bern Kohler
The Ohio State University Department of Chemistry
Molecular Spectroscopy SymposiumJune 17, 2008
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zeolite cavities
Electron acceptor
N
N
N
N
N
N
NN
Ru
2+
solid zeolite membrane
h
photosensitizer molecule
fluid phase
fluid phasecharge transport
initial CS state
N N+ +
N
N
+
+
Efficient and long-lived charge separated state of a Ru polypyridyl complex
N N+ +
N N+ +
e-
fs pump-probe spectroscopy of MV2+ in zeolite cavities
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Methyl Viologen
• Good electron acceptor E° (MV2+/MV•+) = -0.45 V
• Highly efficient excited-state electron acceptor E° (MV2+*/MV•+) = 3.65 V
• Photoreduction in methanol takes less than 250 fs (Peon, JPCA 2001)
• Excellent probe molecule for electron transfer from surrounding environment
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Framework Structure
Crystalline, well-defined nano-pore structure
Charged framework Spatial arrangement Extreme Thermal & Chemical Stability
Zeolites
Pores of Zeolite Y
7.4×13Å
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Zeolite Entrapment
Size Discrimination
Ion Exchange Encapsulation
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16
14
12
10
8
6
4
2
0
A
/ 1
0-3
500025000
Time / fs
CH3OH 430 fs
-5
-4
-3
-2
-1
0
A
/ 1
0-3
400020000
Time / fs
Ultrafast ET in a H-bonding donor solvent
• Forward ET is instrument-limited (t 250 fs) from femtosecond fluorescence upconversion
• Ultrafast decay due to back ET (t = 400 fs)
• Approximately 12% MV radial cation formation
Probe 600 nm
Probe 270 nm
Pump 265 nm
J. Peon et al., J. Phys. Chem. A, 2001, 105, 5768.
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MV2+ Fluoresces in Nitrile Solvents
• fluorescence quantum yield, fF = 0.03
f = 1.0 ns
A
bs
6004002000
Time / psx103
= 1 ns
solid line=fit to TCSPC data
10x103
8
6
4
2
0
TC
SP
C s
ign
al
6004002000
Time / ps
emission excited state absorption
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Water is Important
100
80
60
40
20
0
A
/ a
rb.
un
its
3020100Time / ps
H2O 3.1 ps
D2O 5.3 ps
ps
Pump 266 nm
Probe 600 nm
No radical formation
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MV in Zeolites
• Reports of methyl viologen radical formation when entrapped inside of zeolites supercages
• In Zeolite Y (Scaiano, JCPB 1997)
• In Zeolite X (Ranjit, JPCB 2002)
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0.6
0.4
0.2
0.0
403020100time delay (ps)
MV2+
in Zeolite in ACN
MV2+
in ACN
0 10 20 30 40ps0
0.2r(t)
zeolite/acetonitrile acetonitrile
zeolite/waterwater
No
rma
lize
d A
bs
orp
tio
n a
nd
Em
iss
ion
500450400350300250wavelength (nm)250 350 450nm
absorption emission
Effects of Zeolite Entrapment in Solvated Zeolites
absorption emission
No
rmal
ize
d A
bso
rptio
n a
nd
Em
issi
on
500450400350300250Wavelength (nm)250 350 450 nm
anisotropy
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5x10-3
4
3
2
1
0
O
D
200150100500time delay (ps)
Pump 266 nmProbe 600 nm
MV2+
in Zeolite in H2O
MV2+
in H2O
Time delay (ps)
MV2+ in H2O MV2+ in Z/H2O
Pump 266 nm
Probe 600 nm
t1 = 3.3 ± 0.1 ps t1 = 4.6 ± 1.3 ps
t2 = 37.7 ± 6.8 ps
0 50 100 150 200ps
• Transient absorption of entrapped methyl viologen shows a bulk solvent like decay as well as a 25-40 ps decay component
Photodynamics of MV2+ in a NaY Zeolite
4x10-3
3
2
1
0
OD
2000150010005000time delay (ps)
Pump 266 nmProbe 600 nm
MV2+
in Zeolite in ACN
MV2+
in ACN
MV2+ in ACN MV2+ in Z/ACNt1 = 24.8 ± 2.35 ps
t2 = 414 ± 59 pst1 = 817 ± 18 ps
0 500 1000 1500 2000ps
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Conclusions• The methyl viologen excited state has distinct decay
pathways that are solvent dependent.
• Nano-zeolites make it feasible to make traditional pump/probe transient absorption measurements on encapsulated molecules with high signal-to-niose.
• When entrapped in zeolites methyl viologen is rigidly held within the supercage.
• A new decay process is created due to electron transfer from the zeolite framework to excited-state methyl viologen.
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Acknowledgments Dr. Bern Kohler
Dr. Prabir Dutta
Dr. Haoyu Zhang
Jeremy White
Kohler Group
FundingDepartment of Energy