conformational isomerization of bis-(4-hydroxyphenyl)methane in a supersonic jet expansion. part ii:...
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Conformational isomerization of bis-(4-hydroxyphenyl)methane in a supersonic jet expansion. Part II: Internal mixing and low
barrier potential energy surface in the S1 state
Department of Chemistry
West Lafayette, Indiana
International Symposium on Molecular Spectroscopy: 64th Meeting – June 22 –26, 2008
Christian W. Müller, Chirantha P. Rodrigo, Josh J. Newby,William H. James III, Nathan R. Pillsbury, Timothy S. Zwier
Motivation
Excitonic Coupling & Large-Amplitude Vibrations in Bichromophores
Motivation
Excitonic Coupling & Large-Amplitude Vibrations in Bichromophores
Expectations
Similarities with Diphenylmethane: Excitonic Splitting
Exciton state order: S1 state of B symmetry,
Energy splitting: E = 123 cm–1
Transition Dipole Moment orientations:S1 origin: a/c-type (65% : 35%)S2 origin: strongly perturbed b-type
N. R. Pillsbury et al., J. Chem. Phys., 2008, 129, 114301.J. A. Stearns et al., J. Chem. Phys., 2008, 129, 224305.
S2 state of A symmetry
0 500 1000 1500
12581
S2 00
0
Relative wavenumbers [cm-1]
"Clump"emission
1447
1204
10351006
822
739
622
610
55422383
63
3819
S1 00
0
Flu
ore
scen
ce in
tensi
ty [a
rb. u
nits
]
Expectations
Similarities with Diphenylmethane: Internal Mixing
N. R. Pillsbury et al., J. Chem. Phys., 2008, 129, 114301.J. A. Stearns et al., J. Chem. Phys., 2008, 129, 224305.
S1 origin SVLF
S2 origin SVLF
Odd quantum number changes in the coupling coordinate: v=+1,+3,+5
Expectations
Similarities with Diphenylmethane: Internal Mixing
N. R. Pillsbury et al., J. Chem. Phys., 2008, 129, 114301.J. A. Stearns et al., J. Chem. Phys., 2008, 129, 224305.
60 80 100 120 140
(a b a)
(410)(230)(050)_
(T T )
125
11811293
88
81
Flu
ore
scence
inte
nsi
ty [a
rb. u
nits
]
Relative wavenumbers [cm-1]
(070) (250) (430)
Expectations
Similarities with Diphenylmethane: Internal Mixing
N. R. Pillsbury et al., J. Chem. Phys., 2008, 129, 114301.J. A. Stearns et al., J. Chem. Phys., 2008, 129, 224305.
Expectations
Similarities with Diphenylmethane: Internal Mixing
Jortner, Faraday Discuss., 1997, 108, 1.Henry & Kasha, Annu. Rev. Phys. Chem., 1968, 19, 161.
Internal Mixing
Internal Conversion
stationary-state picture
time-dependent picture
Expectations
Dissimilarities with Diphenylmethane: Multiple Conformers
Three distinct conformers are anticipated in a supersonic jet expansion:E(dd) = 0 cm–1 E(uu) = 5 cm–1 E(du) = E(ud) = 10 cm–1
The barriers to conformational isomerization in the S0 state are 20–50 cm–1.What are the barriers to conformational isomerization in the S1/S2 state?
Conformation-specific Excitation Spectra
UV-UV Holeburning Spectroscopy
0 50 100 150 200
*
Wavenumber [cm-1]
*
Conformer C
*Inte
nsi
ty [a
rb. u
nits
]
Conformer B
25 cm-1
10 cm-1
Conformer A
35184 cm-1
LIF
Three conformers present in the supersonic jet expansion.Conformer B only present at higher vibrational temperatures.
S0 State 2D Potential Energy Surface
B3LYP/6-311G(d,p)
up-up (uu)5 cm–1
down-down (dd)0 cm–1
up-down (ud)10 cm–1
The Conventional Scheme for Obtaining the S0 Vibrational Structure
Single Vibronic Level Fluorescence Spectroscopy
0 50 100 150 200 250----
Wavenumber [cm-1]
10 cm-1
25 cm-1
-
B0
0
A0
0
SVLF C0
SVLF B0
SVLF A0
Inte
nsi
ty [a
rb. u
nits
]
0 50 100 150 200
LIF
C0
0
Frequencies of the resonance fluorescence peaks are characteristic for fluorescence from the zero-point levels of conformers A, B and C.
Dispersed Fluorescence Spectra at High Collision Frequency Conditions
Dispersed Fluorescence from Multiple Vibronic Levels
-40 -20 0 20 40 60 80-----
Wavenumber [cm-1]
-
0 50 100 150 200
A+74 cm-1
A+43 cm-1
A+31 cm-1
A+74A+43A+31
A0
0
B0
0
C0
0
LIF
Barriers to conformational isomerization in the S1 state:Ethresh(A→B) < 43 cm–1 Ethresh(A→C) = < 74 cm–1
Excited State 2D Potential Energy Surface
Effective 2D-PES calculated at the TD-B3LYP/SV(P) level of theory
up-up (uu)C conformer
down-down (dd)A conformer
up-down (ud)B conformer
Excited State 2D Potential Energy Surface
Effective 2D-PES calculated at the TD-B3LYP/SV(P) level of theory
Lowest energy isomerization pathway lies along the non-totally symmetric torsion coordinate .
S1/S2 Transition Dipole Moment Orientations
Rotationally Resolved S1←S0 and S2←S0 Origins of Conformer A
0 50 100 150 200
Holeburning SpectrumConformer A
Inte
nsi
ty [a
rb. u
nits
]
S1 00
0
Wavenumber [cm-1]
T1
0
31
T2
0
_
43
1
0
56
T2
0
61
S2 00
0
132
T1
0T2
0
_
74
T1
0T5
0
_
134
S1 origin
S2 origin
a/c-type
b-type
S1/S2 states are delocalized symmetric and antisymmetric combinations of the zero-order locally excited states as in diphenylmethane.
Outlook
Internal Mixing of S2 Zero-Point Level with S1 Vibronic Levels
0 500 1000 1500
125
Flu
ore
scen
ce In
tens
ity [a
rb. u
nits
]
Wavenumber [cm-1]
(112)
_(T T )
S1(v) emission
S2 origin SVLF
S0 vibrational
level
125
147 (070)
141 (112)141 (410)
138 (131)
135 (150)
126 (211)
S2 00
0
123 (230)
v=0
S1 vibronic levels
of B symmetry
The S2 origin is less perturbed than in diphenylmethane.The nearby +134 cm–1 vibronic band probably shows conformational mixing.
Deutsche Akademie der NaturforscherLEOPOLDINA
NASA PlanetaryAtmospheres Program
Computational Chemistry Grid
Acknowledgements
Present Group MembersWilliam H. James IIIJosh J. NewbyChirantha P. RodrigoJosh A. SebreeEvan G. BuchananZachary DavisJames RedwineRyan MuirDeepali Mehta
Prof. David F. Plusquellic(NIST)
Prof. Lyudmila Slipchenko(Purdue)
Prof. Timothy S. Zwier