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
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
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
Framework Structure
Crystalline, well-defined nano-pore structure
Charged framework Spatial arrangement Extreme Thermal & Chemical Stability
Zeolites
Pores of Zeolite Y
7.4×13Å
Zeolite Entrapment
Size Discrimination
Ion Exchange Encapsulation
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.
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
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
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)
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
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
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.
Acknowledgments Dr. Bern Kohler
Dr. Prabir Dutta
Dr. Haoyu Zhang
Jeremy White
Kohler Group
FundingDepartment of Energy