1. CONFORMATION-SPECIFIC ELECTRONIC SPECTROSCOPY OF JET-COOLED 5-PHENYL-1-PENTENE NATHAN R. PILLSBURY, TALITHA M. SELBY, AND TIMOTHY S. ZWIER, Department of Chemistry, Purdue University, West Lafayette, IN 47907

2. Motivation for Studying 5-phenyl-1-pentene 5-phenyl-1-pentene5-phenyl-1-pentyne Barriers to exciplex formation from different starting structures could be reflected in different lifetimes as a function of energy above the origin Ho, C. D.; Morrison, H. J. Am. Chem. Soc. 2005, 127, 2114-2124. What differences arise from replacing the ethynyl group with a vinyl group?

3. Schematic Diagram of TOF Mass Spectrometer pulsed valve laser port to roughing pump diffusion pump cryocooler to roughing pump 2 stage ion acceleration pneumatic gate valve Einzel lens steering plates manual gate valve mass gate pulser ground plate microchannel plate detector

4. 5-phenyl-1-pentene * (S 1 ) 5-phenyl-1-pentene (S 0 ) 5-phenyl-1-pentene + + e - Resonant Two-Photon Ionization Spectroscopy (R2PI) Molecules are cooled to zero point vibrational levels in the free jet expansion Mass selection gives confirmation that the spectrum is due to the molecule of interest

5. R2PI of 5-phenyl-1-pentene

6. Records the UV spectrum of a single conformation free from interference from others present in the expansion Laser Timing 50-500 nsec UV Hole-burn UV probe UV-UV Hole-burning spectroscopy UV Hole-burn laser fixed: Provides  selectivity UV probe laser tuned Boltzmann distribution of conformers in the pre-expansion Collisional cooling to zero-point vibrational level B* C A C C B A A A C A A B C C A A B B B B B B B UV C 5-phenyl-1-pentene * (S 1 ) 5-phenyl-1-pentene (S 0 ) 5-phenyl-1-pentene + + e - Hole-burn Probe Conformer A Conformer B Hole-burn Probe

7. 0 0 6060 1 12 0 & 18 0 1 1 UV-UV Hole-burning Spectra

8. 0.77 kcal/mole 1.64 kcal/mole gauche-anti-eCgauche-anti-eH’ 0.99 kcal/mole anti-gauche-eH Dihedral Angle Definitions  2 = C(1)-C α -C β -C γ  3 = C α -C β -C γ -C δ  4 = C β -C γ -C δ -C ε 0.0 kcal/mole 0.41 kcal/mole 0.68 kcal/mole anti-anti-eH CαCα CβCβ CγCγ C(1) CδCδ Calculated Structures and Relative Energies 0.80 kcal/mole anti-anti-eC CεCε  2 (  3) = 180 0 = anti  2 (  3) = ±60 0 = gauche  4 = 0 0 = eC (eclipsed with C β )  4 = 120 0 = eH  4 = -120 0 = eH’ Dihedral Labels gauche-anti-eH H H’ anti-gauche-eH’

9. Origin Region of 5-phenyl-1-pentene gauche (  anti ( 

10. Transition Dipole Moment Sensitivity The TDM in monosubstituted benzenes has been found to be very sensitive to the nature and orientation of the substituent According to Pratt and Simons*, the TDM in gauche conformations swings about 30 degrees from the anti (trans) conformations Surprisingly, CIS calculations correctly predict the transition moment direction in these gauche structures * Kroemer, R. T. L., K. R; Dickinson, J. A.; Robertson, E. G.; Simons, J. P.; Borst, D. R.; Pratt, D.W. J. Am. Chem. Soc. 1998, 120, 12573.

11. Rotational Band Contours of Origins A-E

12. Rotational Band Contour Fits Experimental Best Fit 0:36:64 28:16:56 0:12:88 67:0:33 16:48:36 %A:%B:%C

13. Structural Assignments B C D E Vib A/B Vib C ga(eC) ga(eH’) ag(eH) aa(eH) ga(eH) A ag(eH’)

14. ag0 cm -1 gg-23 ga-63 5-phenyl-1-pentyne (37601 cm -1 ) aa(eH)0 cm -1 ag(eH)(eH’)-3 ga(eH’)-54 ga(eH)-62 ga(eC)-68 5-phenyl-1-pentene (37580 cm -1 ) Comparison of Electronic Frequency Shifts Vinyl group red shifts the spectrum by a about 20 cm -1 to the red and adds two more conformations; however the shifts between ag and ga are held roughly constant

15. 62ns 51ns 75ns 83ns 80ns 83ns 96ns 88ns 87ns 85ns 56ns 73ns 67ns 35ns 76ns 13ns 14ns 11ns 12ns 10ns 14ns 12ns 7ns Lifetime Study of 5-phenyl-1-pentene

16. Possible Reasons for Lack of Conformation Specificity Barrier to exciplex structures is too high and therefore not probed in the FC region Lifetime shortening may be determined by something other than exciplex formation (e.g. internal conversion or intersystem crossing) IVR may be fast relative to isomerization S1 Lifetimes of 5-phenyl-1-pentyne (nsec) ~80 ~50

17. Future Work Dispersed Fluorescence Shows where strong SEP transitions are located May show conformation-specific IVR effects Probe isomerization in the S 1 state (may see a difference between gauche vs. anti) SEP-Population Transfer Spectroscopy Measures the barriers to isomerization experimentally Jasper Clarkson Talitha Selby

18. Acknowledgements The Zwier Group Dr. Timothy Zwier Department of Energy (DOE)

19. Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1980, 72, 5039. Optically active ring modes of mono-substituted alkylbenzenes

20. S0S0 S1S1 Zero-point level C B A IV. UV Probe, 3 II. UV Dump, 2 I. UV Pump, 1 Excited vibrational Level A*A* III. Collisional cooling, isomerization SEP-Population Transfer Spectroscopy

21. Resonant Ion-Dip Infrared Spectroscopy (RIDIRS) Hydrocarbon * (S 1 ) Hydrocarbon + + e - Hydrocarbon (A) (S 0 ) Records IR spectrum of single species free from interference from others present in the expansion Hydrocarbon * (S 1 ) Hydrocarbon + + e - Hydrocarbon (A) (S 0 ) S 0 RIDIRS S 1 RIDIRS

22. S 0 RIDIRS Spectra of A-E

23. S 1 RIDIRS Spectra of A-E