Excited State Isomerization of a Stilbene

Analog: Phenylvinylacetylene Josh J. Newby, Christian W. Müller, Ching-Ping Liu, Hsiupu D. Lee and Timothy S. Zwier

64th International Symposium on Molecular SpectroscopyMF03

Department of Chemistry, Purdue UniversityWest Lafayette, IN 47907

Interest in Phenylvinylacetylene (PVA)

0 200 400 600 800 1000 1200 1400 1600

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.000

0

Flu

ore

scen

ce in

tensi

ty [a

rb. u

nits

]

Relative wavenumber [cm–1]

480

289

330

1105

470

1480

1133

480

4186

320

1231

310

1373

300

1479

280

1 (6b0

1)

624

280

1480

2714

260

1 (10

1)

851

260

1480

2941

250

1 (120

1)

1011

230

11048

250

1480

21100

190

1 (130

1)

1219

180

11283

-90 -60 -30 0 30 60 90 120 150 180 210 240 270

0

100

200

300

400

500

600

700

800

900

1000

V(

) [c

m–1]

Torsional angle

J. Phys. Chem. A 106 (24): 5789-5796 (2002)

J. Phys. Chem. A, 112(39), 9454 (2008)

E-PVA

Inte

nsi

ty /

(ar

b.

un

its)

12008004000Wavenumbers (cm

-1)

482

0

472

0 321

031

10

291

0

290

1

310

1

480

2 470

2

320

1

240

1

200

1

TB02Excitation and Emission Spectroscopy

Z-PVA

Inte

nsity

/ (a

rb. u

nits

)

12008004000Wavenumbers (cm

-1)

Inte

nsity

/ (a

rb. u

nits

)

2000150010005000Wavenumbers (cm

-1)

Fluorescence and Absorbance

UVD(absorption)

LIF(fluorescence)

LIF(fluorescence)

UVD(absorption)

Indicates the turn on of a fast, non-radiative process

1000 cm-1 600 cm-1

Relative Wavenumbers (cm-1)Relative Wavenumbers (cm-1)

E/Z Isomerization

1A (S0)

1B (S1)

2A (S2)

trans (E) cis (Z)

1A´ (S0)

1A′ (S1)

2A″ (S2)

trans (E) cis (Z)

?

Is the loss in fluorescence due to cis/trans isomerization?

UV PUMP

To diffusion pump

UV Probe

Reaction Channel

Rxn quenched by expansion on s

timescales

Pulsed Valve

Pump Probe

60-80s

hν

hν hν

Ultraviolet Population Transfer (UVPT) in a time-of-flight mass spectrometer

TOF into plane

Determine product distribution via R2PI

spectroscopy

?

Reaction channel constrains the expansion allowing us to remove large amounts of

internal energy from molecules.

Expt’l protocol:1. Cool2. Pump3. Re-cool4. Probe

Ultraviolet Population Transfer (UVPT)

S1

Z-PVA S0

S1

E-PVA S0

kiso

kIC

Pu

mp

Pro

be

Coo

lk´iso

Hole-filling spectroscopy: Fix Pump, Tune Probe: See where the population went

Population transfer spectroscopy: Tune Pump, Probe on E Watch population come into E from Z Z

Z

ZZ

Z

EE

ZZ*Z Z

E* E*ZZ Z

Cool

EXCITE COOL PROBE

Z*

Z

COOL

Infrared-Population Transfer SpectroscopyInfrared-Population Transfer Spectroscopy

WG13

J. Phys. Chem. A., 110 (12), 4133 (2006)

J. Chem. Phys., 120 (1), 133 (2004)

J. Chem. Phys., 120 (19), 9033 (2004)

William H. James III

E-PVA Population Transfer Spectra

Inte

nsity

/ (a

rb. u

nits

)

12001000800600Relative Wavenumber / (cm

-1)

Inte

nsity

/ (a

rb. u

nits

)

12008004000Relative Wavenumbers / (cm

-1)

E → Z

E → E

UVD

E → Z

E → E

Scan Pump, Probe Z Measures population moved into a product well.

Scan Pump, Probe EMeasures how much population escapes the reactant well under collisional conditions

UVDMeasures how much population can be removed from the reactant well under “zero” collisional conditions

Z-PVA Population Transfer Spectra

Inte

nsity

/ (a

rb. u

nits

)

12001000800600Relative Wavenumber / (cm

-1)

Inte

nsity

/ (a

rb. u

nits

)

12008004000Relative Wavenumbers / (cm

-1)

E → Z

E → E

Z → E

hot bands

Scan Pump, Probe E Measures population moved into a product well.

Scan Pump, Probe ZMeasures how much population escapes the reactant well under collisional conditions

UVDMeasures how much population can be removed from the reactant well under “zero” collisional conditions

Z → Z

UVD

Isomerization Quantum Yields

Δν/cm-1 Φrf (E)a Φr

EZ Δν/cm-1 Φr

f (Z)b ΦrZE

0 1.00 1.00 538 0.30 1.00

326 0.67 0.77±0.23 702 0.02 1.46±0.44

815 0.90 0.90±0.27 937 0.02 0.95±0.29

931 1.26 0.86±0.26 967 0.03 1.59±0.48

1214 0.19 0.96±0.29 1060 0.01 1.54±0.46

1279 0.18 0.77±0.23 1152 0.02 1.05±0.32

1292 0.13 0.81±0.24 1223 0.02 1.19±0.36

1434 0.11 0.61±0.18

Take home: Isomerization is not the cause of the loss in fluorescence!

(a) FL Quantum Yields relative to E-PVA 000

(b) FL Quantum Yields relative to Z-PVA 000

0 20 40 60 80 100 120 140 160 180

0

5000

10000

15000

20000

25000 CIS/6-311+G(d,p)

V(

) [c

m–1

]

Torsion angle [ °]

CIS/6-311+G(d,p)

CASSCF(10,10)/6-31G(d)

S1

S2

kink

Double Bond Torsional PES

Inte

nsity

/ (a

rb. u

nits

)

12008004000Wavenumbers (cm

-1)

Inte

nsity

/ (a

rb. u

nits

)

2000150010005000Wavenumbers (cm

-1)

Fluorescence and Absorbance

UVD(absorption)

LIF(fluorescence)

LIF(fluorescence)

UVD(absorption)

What is the non-radiative process?

Relative Wavenumbers (cm-1) Relative Wavenumbers (cm-1)

PVA Excited States

0

1

2

3

4

5

6

7

8

-80 -40 0 40 80

ZPVA

0

1

2

3

4

5

6

7

8

-80 -40 0 40 80

EPVA

Angle (°) Angle (°)

Rel

ativ

e E

nerg

y (e

V)

Rel

ativ

e E

nerg

y (e

V)

1A′

2A′

3A′

2A′

3A′

1A″

2A″2A″

1A″

1A′

π→π*

π→σ*

π→σ*

π→π*

Close lying A″ states in both E and Z-PVA which correspond to the π→σ*excitation

Excited States Cont.

M. Z. Zgierski and E. C. Lim Chem. Phys. Lett. 387 352 2004

T. Fujiwara, M. Z. Zgierski, and E. C. Lim, JPCA 112, 4736 2008

1,4-Bis(phenylethynyl)benzene

Conclusions• UVPT has been developed to elucidate excited state

dynamics

• E/Z Isomerization does NOT dominate in the excited state

• Isomerization most likely occurs on the ground state surface after interconversion

• PVA most likely departs initially excited A´ state to an A″ state via intersection

• No isomerization channel to naphthalene was observed

AcknowledgementsProfessor Timothy S. Zwier

The Zwier GroupWilliam James WG13Chirantha Rodrigo MF05 Josh Sebree TB03Evan Buchanan TB09Zachary DavisJames RedwineRyan MuirDeepali MehtaDr. Jaime StearnsDr. Talitha SelbyDr. Jasper ClarksonDr. V. Alvin ShubertDr. Esteban BaqueroDr. Tracy LeGreveDr. Nathan PillsburyDr. Ching-Ping LiuDr. Christian Müller MF06, TB10Dr. Mike Nix

PVA Excited States

π→σ*

π→π *

A″A′

π→σ*

π→π *

A′A″

1.50 1.50

1.00 1.00

0.50 0.50

0 0

Inte

nsity

/ (a

rb. u

nits

)

2000150010005000Wavenumbers (cm

-1)

Fluorescence Quantum Yields

1.00 1.00

0.75 0.75

0.50 0.50

0.25 0.25

0.00 0.00

Inte

nsity

/ (a

rb. u

nits

)

120010008006004002000Wavenumbers (cm

-1)

Φr f

Φr f

(b)

(a)

Relative Wavenumbers (cm-1)

Relative Wavenumbers (cm-1)

Inte

nsity

/ (a

rb. u

nits

)

12008004000Wavenumber / (cm

-1)

x100

PVA PT compared to LIF

Inte

nsit

y / (

arb.

uni

ts)

12008004000

Wavenumbers / (cm-1

)

x4 x40

Isomerization is not the non-radiative feature that dominates.

Relative Wavenumbers (cm-1) Relative Wavenumbers (cm-1)

PVA fluorescence spectroscopy

Loss in fluorescence but still show Frank-Condon Factors

Isomerization? Internal Conversion? Intersystem Crossing?

Inte

nsity

/ (a

rb. u

nits

)

12008004000Relative Wavenumber / (cm

-1)

Inte

nsity

/ (a

rb. u

nits

)

12008004000Relative Wavenumbers / (cm

-1)

000

000

000

000

2510

2610 191

0

2601

2501

1901

2401

2901

2910

2001

(a)(b)

LIF LIF

SVLF SVLF

000+ 1000 cm-1 00

0+ 600 cm-1

Inte

nsit

y (A

rbit

rary

Uni

ts)

35000348003460034400342003400033800

Wavenumbers (cm-1

)

UVPT of Z-PVA to E-PVA

1184

X 50TUNE Pump

Probe E-PVA 000

LIF of Z-PVA

UVPT of Z to E

‘Gain’ E-PVA population when

resonant with a Z-PVA transition.

Tdnvib~15 K

Many hot bands of Z-PVA observed.

000

0284

2084

2284

1031

3184

(6b) 2810

(12) 2410

Tupvib~50 K

Hole-filling of PVA Z to E and E to Z

Inte

nsit

y (A

rbit

rary

Uni

ts)

345003400033500Wavenumbers (cm

-1)

Inte

nsit

y (A

rbit

rary

Uni

ts)

34100340003390033800337003360033500Wavenumbers (cm

-1)

LIF of E-PVA LIF of Z-PVA Hole-filling Z to E

LIF of E-PVA LIF of Z-PVA Hole-filling E to Z

Pump Z PVA 000 +933 cm-1 TUNE Probe Pump E PVA 00

0 +931 cm-1 TUNE Probe