Conformational Study of Benzyl Ether TB09 Conformational Study of Benzyl Ether A. O. Hernandez-Castillo, Daniel M. Hewett, Chamara Abeysekera, and Timothy S. Zwier Department of Chemistry, Purdue University

𝐌𝐨𝐭𝐢𝐯𝐚𝐭𝐢𝐨𝐧 𝜋-stacking in flexible bichromophores Understand the associated physics and chemistry involved in incipient stages of soot formation. Linker group. 𝜋-stacking in flexible bichromophores A B Propyl linkage will be present in smaller abundance than ethyl in combustion environment… once form it will lead to pi-stacked structures. Important applications, including light- harvesting complexes, chromophore arrays, conjugated polymers, and fluorescence resonance energy transfer (FRET). 1,3 diphenylpropane 𝜃 𝑑 1 =58.9° 𝜃 𝑑 2 =−79.8° Benzylether 𝜃 𝑑 1 =62.4° 𝜃 𝑑 2 =−89.3° Several modes involved in the coupling…. Complicated but interesting spectroscopy…. Levels are mixed *make bigger bold Schematic representation of the process of soot formation. (M.R. Kholghy et al. Carbon 100 508 (2016)) 3 atom linkage O atom changes conformational preferences. 1,3-diphenylpropane

Relative Energies 1,3-diphenylpropane 𝜃 𝑑 1 =180° 𝜃 𝑑 2 =180° 𝜃 𝑑 1 =180° 𝜃 𝑑 2 =180° 𝜃 𝑑 1 =69.2° 𝜃 𝑑 2 =124.3° Pi-stacked excimer red shifted emission The structures were calculated at B2PLYP-D3BJ/aug-cc-pVTZ level of theory. The energies were calculated at B3LYP/def2tzvp with Grimme’s D3BJ dispersion. tg: 1.96 kJ/mol sta: 3.1 kJ/mol **Change figure BE 𝜃 𝑑 1 = 𝜃 𝑑 2 =60.9° 𝜃 𝑑 2 𝜃 𝑑 1 𝜃 𝑑 1 =62.4° 𝜃 𝑑 2 =−89.3° 2

Experimental Setup (Fluorescence Chamber) Molecules are interrogated 5mm from nozzle The fluorescence is collected with -20% efficiency 4-inch spherical mirrors Fluorescence imaged onto a UV coated plano-convex focusing lens emission is turned by a UV-coated 3-inch mirror and imaged onto a fast photomultiplier tube photomultiplier tube converts the detected light into an electrical current which is digitized by a fast oscilloscope digitized signal is sent to LabView software. Laser Induced Fluorescence (LIF)

Single conformation IR and UV Spectroscopy Fluorescence-dip infrared spectroscopy (FDIRS) IR-UV Holeburning Sn Sn FDIRS relies on wavelength selectivity IR (10 Hz) and UV (20 Hz) IR-UV useful: spectrum overly congested, minor conformer Difference spectrum: gated integrator operating in active baseline subtraction mode. S0 S0 4

UV and IR spectra: S0-S1 origin region LIF FDIRS IR-UV HB Temperature: 124 C Concentration: 1.3 % Pressure: 40 sccm Pulse Valve: 500 μm orifice (Parker General Valve Series 9) Contaminants: Benzaldehyde, benzyl alcohol, toluene Toluene stretches: 2925 (alkyl), 3099, 3068, 3032 (aromatic) Leakage from A 2 conformers 5

Ground state IR tentative assignments 6

Taking Fermi resonance mixing into account CH2 scissor combination Model is based on a local mode representation. CH oscillators couple to themselves and to nearly resonant states. 𝑯= 2941.0 −18.9 2900.8 1.0 2.7 1.0 1.0 2900.8 −18.9 2941.0 22.0 22.2 0.1 0.2 0.0 0.1 0.1 0.0 0.2 0.1 22.1 22.1 2905.5 1.6 2914.1 0.0 1.6 2904.8 CH stretch states gg CH2 scissor overtones CH2 scissor combination Fermi Coupling NOT symmetric and antisymmetric stretch Parameters of the Hamiltonian are initially calculated using density functional theory methods (model Hamiltonian requires the computation of the Cartesian force constant matrix and linear dipole derivatives) Overtones of CH bends: 1400-1500 cm-1 Anharmonic couplings 20-30 cm-1 (Ned Sibert// UV-Madison) CH2 scissor combination Symmetric stretch (low frequency end) and antisymmetric strech (high frequency) mode Interaction with O atom …. Partially localizes the modes 2941 cm-1 3 1 and 4 modes are relatively uncoupled. 4 1 2 and 3 mix with scissor overtones. 2 Significant Fermi coupling occurs between stretches and scissor overtones of the individual CH2 groups. 7

stacked Taking Fermi resonance mixing into account Large Amplitude Motions A B A B CH stretch torsional progression Twice the bands Splitting: 19/9.5/17/////5.7 8

Excited state IR 33 cm-1 9

Franck Condon quenching Future Work Where is S0-S2? Spectroscopic consequences? Experimental Approach Dispersed Fluorescence Isotopically label benzyl ether Franck Condon quenching Analysis Multimode vibronic coupling model. Mention coclutions Include multiple vibrational modes and can describe vibronic states in more complex molecular aggregates such as bichromophores Vertical splitting/ calc oscillator strength Dispersed Fluprescence: detect S2 experimentally (Nebgen, Emmert, and Slipchenko, J. Chem. Phys. 137, 084112 (2012))   Benzyl Ether S0-S1 (nm) f1 S0-S2 (nm) f2 ∆𝑬 (cm-1) gg 235.96 0.0026 233.15 0.0003 510.78 stacked 243.14 0.0001 236.30 0.0191 1190.52 tg 234.38 0.0007 233.33 0.0006 192.00 tt 234.79 0.0011 233.29 0.0000 273.85 10

The Zwier Research Group: Dr. Sibert (UV-Madison) Acknowledgements The Zwier Research Group: Dr. Zwier Daniel P. Tabor Dr. Sibert (UV-Madison) 11

Large Amplitude Motions

sta tg gg tt