CONFORMATION-SPECIFIC ELECTRONIC AND VIBRATIONAL SPECTROSCOPY OF DIBENZO-15-CROWN-5 ETHER IN A SUPERSONIC JET. Evan Buchanan, Chirantha P. Rodrigo, William H. James, Josh J. Newby, and Timothy S. Zwier Department of Chemistry, Purdue University West Lafayette, Indiana

Dibenzo-15-crown-5 (DB15C) 15-Crown-5 Ethers Motivation: Crown ethers provide a chemical means to preferentially sequester substrates, especially cations. Model enzyme-substrate interactions Conformational consequence of adding a second benzene ring. How flexible is the macrocycle? Where is the second excited state? Investigate the electronic energy transfer between the two chromophores.. Dibenzo-15-crown-5 (DB15C) Benzo-15-crown-5 (B15C) J.-M. Lehn. Pure & Appl. Chem., 1979, 51, 979 R. Kusaka, Y. Inokuchi, T. Ebata. Phys. Chem. Phys., 2007, 9, 4452

Supersonic Expansion MCP Large, flexible molecule with many conformational isomers is entrained in a high pressure backing gas. Gas expands through a pinhole into a vacuum. Molecules collisionally cool with the buffer gas Pulsed valve Expansion reduces the number of transitions observed in the spectra Rotational temperature of a few Kelvin Vibrational temperature of 10-20 Kelvin.

Experimental Methods Resonant Two-Photon Ionization UV-UV Holeburning Ionization Continuum Molecule + + e- Ionization Continuum Sn S0 A B Molecule + + e- Spatially overlapped Sn Δt = 200 ns S0 A 10 Hz HB 20 Hz Probe B R2PI: Electronic Spectrum UVHB: Conformation specific electronic spectrum

LIF and UVHB Spectra of B15C Three conformers present in the jet expansion. One conformer blue shifted by over 500 cm-1. S0 – S1 origin serve as a sensitive probe of the local environment about the phenyl ring. DMB DEB LIF A * B * C * B15C5-H2O V. A. Shubert et al. J. Phys. Chem. In press. R. Kusaka, Y. Inokuchi, T. Ebata. Phys. Chem. Chem. Phys., 2007, 9, 4452

Something Simpler: 1,2-diethoxybenzene γ Carbon Position β γ Relative Origin (cm-1)* in/in in/out -480 out/in 343 out/out -800 to -900 38 to 53 1286 β * TDDFT predicted S0 – S1 origins B15C conclusions: Conformers A, and B: in/in; in/in Conformer C: out/in; in/in

B15C Assigned Structures 1.5 KJ/mol 0.0 KJ/mol B15CA): in/in; in/in B15C(C): out/in; in/in B3LYP/6-31+G(d) Can we use the same technique to assign the local environment for DB1C? 7.1 KJ/mol B15C(B): in/in; in/in

R2PI and UVHB Spectra of DB15C * B (S1 vibronic levels) * C * D * B15C Three conformers indicative of in/in; in/in structures for ring 1. Blue shifted conformer indicative of an out/in; in/in structure for ring 1. Position of S2 in A, B, and C is not clear.

Resonant Ion-dip Infrared Spectroscopy Δt = 50-200 ns IR spatially overlapped Ionization Continuum B+ + e- Sn 10 Hz IR 20 Hz UV S0 UV only IR and UV A Active Baseline Subtraction B RIDIRS: Conformation specific IR S0 spectrum

Alkyl C-H Stretch Spectra C(x3) B D Spectral congestion due to 14 C-H stretch fundamentals. Transitions above 2930 cm-1 indicative of CH…O interactions and the degree of buckling of the crown.

Where is the S2 Origin? B 109 S1 DB15C B15C All planar region Unusually large transition occurring at +109. Possible S2 origin?

Relative Origin (cm-1)* DFL Conformer B B origin Carbon Position β γ Relative Origin (cm-1)* in/in in/out -480 out/in 343 out/out -800 to -900 38 to 53 1286 109 Internal Mixing 00 S2 {S1(ν)}

Where is the S2 origin? D 24 72 48 DB15C B15C +500 cm-1 from conf. A,B, and C. Similar vibronic structure to B15C. Extra transitions could be the S2 origin. Out/in; in/in structure for both rings.

DFL Conformer D D origin + 25 + 48 + 72 Two overlapping transitions under holeburn transition Two stable excited state minima for each ring.

Future work Acquire more DFL to identify the S0 – S2 origin and probe electronic coupling between states. Water clusters: B15C distorts the crown to accommodate hydrogen bonding with water. Does DB15C behave in a similar manner? SEP-PT: measure barriers to isomerization. How flexible is the crown? How does it depend on one vs. two rings?

Acknowledgements Prof. Timothy S. Zwier Current Group Members: Past Group Members: Dr. Christian W. Müller Dr. Tracy LeGreve William H. James Dr. Nathan Pillsbury Josh J. Newby Dr. Alvin Shubert Chirantha P. Rodrigo Dr. Esteban Baquero Josh A. Sebree Zachary Davis James Redwine Ryan Muir Deepali Mehta Computational Resources: Information Technology at Purdue (ITaP) Funding: National Science Foundation