Transcript MolSpec2009 - The Ohio State University

ULTRAFAST STUDIES OF
PHOTOISOMERIZATION
REACTIONS
Nicole Dickson
Jessica Donehue, Terry Gustafson
The Ohio State University
Photoisomerization of Retinal
 Photoisomerization takes
place in 200 fs in protein
environment
 In solution,
photoisomerization takes
place in 3 ps
Ha
Rhodopsin
Hb
Ha
H
N
hv
NH
Hb
11-cis-retinal
all-trans-retinal
Model Molecules: Linear Polyenes
 1,4-diphenyl-1,3-
butadiene (DPB)
 1,1,4,4-tetraphenyl-
1,3-butadiene (TPB)
Photoisomerization Reactions
 One-bond Flip
 Most accepted, involves a
torsional relaxation
*
Ha
Hb
Hb
*
Ha
Hb
Hb
Hb
Hb
Ha
Hb
 Hula-Twist
 Simultaneous rotation about
double bond and an adjacent
single bond
Ha
Ha
Ha
 Bicycle Pedal
 Simultaneous rotation in S1
about two polyene double
bonds
Ha
Ha
Hb
Fate of the Excited State of DPB
 Bu state
experiences bond
order reversal,
photoisomerization
 Ag state
experiences a
reduced bond order
photoisomerization
1Bu
2Ag
Rotation of
phenyl and
olefin to
form planar
DPB
1Ag
Experimental Methods and Goals
 Single wavelength transient absorption
 Broadband transient absorption
 How do the excited state processes of TPB
compare to DPB?
 Can we unveil the mechanism of
photoisomerization?
Absorption, Emission, Transient Absorption
of DPB and TPB
Absorption
Emission
DPB
Transient Absorption
TPB
Broadband TA Spectra of TPB in Ethanol
300 nm pump
60
A (mOD)
40
30
20
-1 ps
1 ps
3 ps
5 ps
7 ps
9 ps
10 ps
20 ps
45
A (mOD)
50
30
15
10
-1 ps
30 ps
40 ps
50 ps
100 ps
200 ps
500 ps
2000 ps
0
0
-10
400 450 500 550 600 650 700
Wavelength (nm)
400 450 500 550 600 650 700
Wavelength (nm)
Components of the TA Spectra of DPB in
Ethanol
 Absorption with maximum around 660 nm
 Decays to ground state in less than 2 ns
 Bleach with maximum around 500 nm
 Peak shifts on ps timescale
 Returns to ground state in less than 2 ns
 Absorption with maximum


Peak shifts on ps timescale
Does not decay back to ground
state in 2 ns
A (mOD)
around 400 nm
45
30
15
-1 ps
30 ps
40 ps
50 ps
100 ps
200 ps
500 ps
2000 ps
0
400 450 500 550 600 650 700
Wavelength (nm)
TA Decays of TPB in alcohols
385 nm pump, 650 nm probe
1.0
0.8
0.6
1.0
0.4
Normalized Intensity
0.8
0.2
0.0
0.6
0
0.4
5
10
Time (ps)
0.2
0.0
0
500
Methanol
1000
1500
Time (ps)
Ethanol
Hexanol
2000
Decanol
15
20
Comparison of TA of TPB and DPB in
Ethanol 0-0 pump, max. of TA probe
ultrafast component
(τ1) not present in
DPB
 τ2 is about twice as
fast in TPB as DPB
0.8
Normalized Intensity
 TPB has an
1.0
TPB/Ethanol
DPB/Ethanol
0.6
0.4
0.2
0.0
0
10
20
30
Time (ps)
Molecule
1 (ps)
2 (ps)
3 (ps)
TPB
0.21 ± 0.03
4.6 ± 0.3
34 ± 1
DPB
-
9±3
54 ± 13
40
50
TA Decays of TPB in alkanes
385 nm pump, 650 nm probe
1.0
0.8
0.6
1.0
0.4
0.2
Normalized Intensity
0.8
0.0
0
0.6
5
10
Time (ps)
0.4
0.2
0.0
0
500
Hexane
1000
1500
Time (ps)
Heptane
Octane
2000
Decane
15
20
Comparison of TA of TPB and DPB in
Hexane 0-0 pump, max. of TA probe
 τ2 is about 10
times longer in
DPB than TPB
 τ3 is about 3 times
longer in TPB
than in DPB
Normalized Intensity
1.0
TPB/Hexane
DPB/Hexane
0.8
0.6
0.4
0.2
0.0
0
100
200
300
Time (ps)
Molecule
τ1 (ps)
τ2 (ps)
τ3 (ps)
TPB
--
2.7±0.1
1190±13
DPB
--
25±3
405±40
400
500
600
TA Decays of TPB Probed at 400 nm
300 nm pump
Ethanol
Hexane
1.0
A (mOD)
A (mOD)
1.0
0.5
0.5
0.0
-0.5
0.0
0
100
200
300
400
Time (ps)
-1.0
0
40
80
120
Time (ps)
 Rise time corresponds to τ2
 Decay time in alkanes corresponds to τ3, does not correspond
in alcohols (new time constant, τ4)
Comparison of TA Decay Kinetics at 650
nm and 400 nm
650
nm
400 nm
Solvent
τ1 (ps)
τ2 (ps)
τ3 (ps)
τ1 (ps)
τ2 (ps)
τ4 (ps)
Ethanol
0.21 ±
0.03
4.6 ±
0.3
34 ± 1
--
8.7 ± 5,
rise
42 ± 20
Hexane
--
2.7±0.1
1190±13
--
1.2 ± 0.3,
rise
1295 ±
280
Summary of Time Constants
 τ1 is only present in TPB and corresponds to a phenyl
torsional motion
 τ2 in DPB and TPB corresponds to a conformational
change


τ2 in TPB is shorter than in DPB
rise at 400 nm corresponds to τ2
 τ3 in TPB and DPB corresponds to the lifetime of the
B state
 τ4 is only present in TPB in alcohols and corresponds
to the lifetime of the A state
 In TPB, alcohols and alkanes show different
dynamics
Fate of the Excited State of TPB in
Alcohols
 τ1 is due to phenyl
torsional motion (PT)
 τ2 arises from a
conformational change
(isomerization)
 τ3 represents the
lifetime of the B state
 τ4 represents the
lifetime of the A state
PT
1
B
3
385
nm
2
A
4
Fate of the Excited State of TPB in
Alkanes
τ2 arises from a
conformational
change
(isomerization)
 τ3 represents
the lifetime of
the A / B state
 No τ4 present
B
A
2
3
385
nm
Conclusions
 First observation of viscosity-dependent
dynamics in TPB
 First observation of fast (fs and ps)
components in TPB
 Can observe both the A and B states in TPB
 Fast photoisomerization?
 Different dynamics than DPB
 Mechanism for photoisomerization remains
unclear
Acknowledgments
 Jessica Donehue
 Terry Gustafson
 Gustafson Group
 Kohler Group