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