Investigating excited state dynamics in 7-azaindole

1. Investigating excited state dynamics in 7-azaindole Nathan Erickson,Molly Beernink, andNathaniel Swenson

2. Background I 7AI Dimer • Previous studies have shown that 7-azaindole (7AI) readily forms H‑bonded dimers in solution1 • 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 (non-negative).2 • other significant factors that contribute to the stability of these systems. Example of DNA Base pairs H-Bonding (1) Ingham, K.; El-Bayoumi, C. M. J. Am. Chem. Soc.1971, 93, 5023. (2) Kyogoku, Y.; Lord, R. C.; Rich, A. J. Am. Chem. Soc.1967, 89, 496.

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 step1 • 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, 260.

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

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

6. 7-azaindole dimer geometryB3LYP/6-31G(d) CPCM Image: VMD

7. Continuum Solvation Energetic comparison B3LYP/6-31G(d) CPCM implicit solvation

8. Laser Raman Spectroscopy Raman vs. resonance Raman Raman Resonance Raman UV-Vis spec showing virtual level absorption Quantum Electronic Diagram Virtual Level • Resonance enhancement: • ~105 • Chromophore selective • Sensitive to local structure • Sensitive to excited statedynamics (100’s fsec) Raman Laser Rayleigh Ground State

9. Experimental Setup A very simple guide to how our setup works: 355 or 532nm light from Nd:YAG laser H2 Raman Shifter Dispersal Prism • Wavelength Selection • Sample • Light Collection • SPECTRA!

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

11. Computational Spectral Simulation Theory Resonance Raman Intensity CalculationShort time wave-packet propagation approximation Intensity of the kth vibrational band: 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)

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.

13. Web Interface for Spectral Simulation Three steps: 1 2 3

14. Simulated dimer RR spectrum Mode 29 808 cm-1 Mode 48 1145 cm-1 Mode 62 1469 cm-1

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

16. Simulation comparison

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

18. Explicit solvation

19. Implicit and Mixed solvationvs Experimental

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?

21. Possible Proton Transfer mechanisms Parallel Serial 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.

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)

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

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

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