1. Conformational isomerization of bis-(4-hydroxyphenyl)methane in a supersonic jet expansion. Part II: Internal mixing and low barrier potential energy surface in the S 1 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

2. Motivation Excitonic Coupling & Large-Amplitude Vibrations in Bichromophores

3. Motivation Excitonic Coupling & Large-Amplitude Vibrations in Bichromophores

4. Expectations Similarities with Diphenylmethane: Excitonic Splitting Exciton state order: S 1 state of B symmetry, Energy splitting:  E = 123 cm –1 Transition Dipole Moment orientations: S 1 origin: a/c-type (65% : 35%) S 2 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. S 2 state of A symmetry

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. S 1 origin SVLF S 2 origin SVLF

6. 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.

7. 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.

8. 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

9. 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 S 0 state are 20–50 cm –1. What are the barriers to conformational isomerization in the S 1 /S 2 state?

10. Conformation-specific Excitation Spectra UV-UV Holeburning Spectroscopy Three conformers present in the supersonic jet expansion. Conformer B only present at higher vibrational temperatures.

11. S 0 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

12. The Conventional Scheme for Obtaining the S 0 Vibrational Structure Single Vibronic Level Fluorescence Spectroscopy Frequencies of the resonance fluorescence peaks are characteristic for fluorescence from the zero-point levels of conformers A, B and C.

13. Dispersed Fluorescence Spectra at High Collision Frequency Conditions Dispersed Fluorescence from Multiple Vibronic Levels Barriers to conformational isomerization in the S 1 state: E thresh (A→B) < 43 cm –1 E thresh (A→C) = < 74 cm –1

14. 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

15. 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.

16. S 1 /S 2 Transition Dipole Moment Orientations Rotationally Resolved S 1 ←S 0 and S 2 ←S 0 Origins of Conformer A S 1 origin S 2 origin a/c-type b-type S 1 /S 2 states are delocalized symmetric and antisymmetric combinations of the zero-order locally excited states as in diphenylmethane.

17. Outlook Internal Mixing of S 2 Zero-Point Level with S 1 Vibronic Levels S 1 (v) emission S 2 origin SVLF The S 2 origin is less perturbed than in diphenylmethane. The nearby +134 cm –1 vibronic band probably shows conformational mixing.

18. Deutsche Akademie der Naturforscher LEOPOLDINA NASA Planetary Atmospheres Program Computational Chemistry Grid Acknowledgements Present Group Members William H. James III Josh J. Newby Chirantha P. Rodrigo Josh A. Sebree Evan G. Buchanan Zachary Davis James Redwine Ryan Muir Deepali Mehta Prof. David F. Plusquellic (NIST) Prof. Lyudmila Slipchenko (Purdue) Prof. Timothy S. Zwier