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