Investigating excited state dynamics in 7 azaindole. Nathan Erickson, Molly Beernink, and Nathaniel Swenson

Transcription

1 Investigating excited state dynamics in 7 azaindole Nathan Erickson, Molly Beernink, and Nathaniel Swenson 1

2 Background I 7AI Dimer Previous studies have shown that 7 azaindole (7AI) readily forms H bonded dimers in solution 1 The N H N bonds in 7AI dimer are simple models the of adenine thymine base pair interaction of DNA. The 7AI dimer and DNA base pairs have higher than expected Gibbs energies of association (nonnegative). 2 other significant factors that contribute to the stability of these systems. (1) Ingham, K.; El-Bayoumi, C. M. J. Am. Chem. Soc. 1971, 93, (2) Kyogoku, Y.; Lord, R. C.; Rich, A. J. Am. Chem. Soc. 1967, 89, 496. Example of DNA Base pairs H-Bonding 2

3 Excited state double proton (ESDPT) This is a possible mechanism for photo damage of DNA. Gas phase experiments have given insight into time scales. A serial transition of the protons in the excited state. First electron shuttles in 650 fsec step 1 Solvated system experiments have shown evidence of both parallel and serial transition mechanisms. We are further investigating transition mechanisms in various solvent systems through resonance Raman. 1. Douhal, Kim, and Zewail, Nature, 1995, 378,

4 Goals Solvent dependent geometry and energetics Solvent dependent excited state dynamics Resonance Raman and simulations: are we there yet? 4

5 Computational Overview 7AI dimer geometry Implicit, explicit, and mixed model Gibbs energy of association Resonance Raman spectral simulation Compared with experimental spectra Correlated with dynamic modes of prevalent peaks to search for evidence of ESDPT Generated step wise electron transition models 5

6 7 azaindole dimer geometry B3LYP/6 31G(d) CPCM Image: VMD 6

7 Continuum Solvation Energetic comparison B3LYP/6-31G(d) CPCM implicit solvation Gibbs Energy (Hartree) kcal/mole Solvent monomer dimer G water methanol acetonitrile

8 Laser Raman Spectroscopy Raman vs. resonance Raman Raman Resonance Raman UV-Vis spec showing virtual level absorption 300 Quantum Electronic Diagram Virtual Level 400 Ground State Laser Rayleigh Raman Wavelength [nm] Resonance enhancement: 1. ~ Chromophore selective 3. Sensitive to local structure 4. Sensitive to excited state dynamics (100 s fsec) 8

9 or 532nm light from Nd:YAG laser 2. H 2 Raman Shifter 3. Dispersal Prism Experimental Setup A very simple guide to how our setup works: 5. Wavelength Selection 6. Sample 7. Light Collection 8. SPECTRA! 9

10 Nuts and bolts of spectral simulation I k ω L ( ω ω ) Δ ω L k k k Intensity of spectral line associated with kth vibration Frequency of the laser (L) and the kth vibration (k). Change in geometry (reflected in gradient) between ground and excited state along kth vibrational mode. 10

11 Computational Spectral Simulation Theory Resonance Raman Intensity Calculation Short time wave-packet propagation approximation I k Intensity of the k th vibrational band: ω ( ω ω ) Δ ω L L k k k I k ω L ( ω ) Φ( ω ω ) ( ω ω ) Δ Φ L k k L L k Scaled quantum mechanical force constants (SQM) are added to the final calculated frequencies to better correlate with experimental data. ~15 cm -1 vibrational frequency accuracy Baker, Jarzecki, Pulay, J. Phys. Chem. A., 102, (1998) Jarzecki and Spiro, J. Phys. Chem. A., 109 (2005) 11

12 Resonance Raman spectral Simulation: Three Computational Steps: 1.) Ground State: B3LYP/6-31G(d) frequency and optimization. Vibrational modes for subsequent calculations generated. 2.) Excited State (resonant state): CIS/6-31G(d) force (gradient) using the optimized geometry from calculation #1. 3.) HF/6-31G(d) frequency to correct the gradient predicted in calculation #1. The vibrational modes are then scaled by Quantum mechanical force constants based on internal coordinates. 12

13 Web Interface for Spectral Simulation Three steps:

14 Simulated dimer RR spectrum Mode cm -1 Mode cm -1 Mode cm -1 14

15 Mode 29 Largest RR enhancement Large component along ESDPT coordinate Strong experimental RR enhancement at similar wavenumber Ultrafast ESDPT dynamics sensitivity 15

16 Simulation comparison 16

17 Resonance Raman of 7AI: Experiment Meets Theory 223 nm excitation wavelength 7AI solvated in Methanol 17

18 Explicit solvation 18

19 Implicit and Mixed solvation vs Experimental 19

20 Probing excited state dynamics Strategy: Compute excited state gradient on a grid of proton positions for dimer Simulate corresponding spectra Compare to experimental with different solvents What is timescale for dynamics? Time snapshot for experiment? 20

21 Possible Proton Transfer mechanisms Serial Parallel The transfer positions are in a ratio of 0-0 indicating the starting position and 10-n indicating a fully transferred proton(s). * Please wait for the animation to start, no clicks necessary. 21

22 Simulation Grid Created from computations of implicitly positioning the protons between the N s of the 7AI Dimer ( relative proton position on the right side of the figure) 22

23 Conclusions Dimerization of 7AI is unfavorable in aqueous solution Computation: + G values Experiment al spectra do not match dimer simulations Evidence of solvent interactions with 7AI monomers Hydrogen bonding is favorable for the solvents we studied Can correlate simulated RR peaks of monomer and solvent to experimental spectra Mechanism dynamics were investigated in step placement of protons Mixed Solvation and Implicit simulations are very similar 23

24 Future Directions Analyze isotopic RR spectral data Time domain laser induced fluorescence experimentation of system TDDFT calculations on 7AI system 24

25 Acknowledgements Dr. Jonathan Smith Michael Kamrath, Krista Cruse Midwest Undergraduate Computational Chemistry Consortium NSF MRI ACS PRF NSF CCLI Gustavus Adolphus College Chemistry Department Sigma Xi local chapter 25