Ultrafast processes in molecules Introduction Mario Barbatti email@example.com
settling the bases: photochemistry, excited states, and conical intersections
Photochemistry & Photophysics • Stating the problem: • What does happen to a molecule when it is electronically excited? • How does it relax and get rid of the energy excess? • How long does this process take? • What products are formed? • How does the relaxation affect or is affected by the environment? • Is it possible to interfere and to control the outputs?
Pump-probe experiments based on ultra-fast laser pulses have increased the resolution of the chemical measurements to the femtosecond (10-15 s) time scale.
The need for Theory Theory is necessary to map the ground and excited state surfaces and to model the mechanisms taking place upon the photoexcitation. Theory is indispensable to deconvolute the raw time-resolved experimental information and to reveal the nature of the transition species. In particular, excited-state dynamics simulations can shed light on time dependent properties such as lifetimes and reaction yields.
Basic process I: Radiative decay (fluorescence) P ~ |j|m|i|2 t ~ ns
Basic process II: Non-radiative decay j| |i P ~ v N t~ fs
The Static Problem • How are the excited state surfaces? • 2. For which geometries does the molecule have conical intersections? • 3. Can the molecule reach them?
Conical intersections formamide pyridone Antol et al. JCP 127, 234303 (2007) Barbatti et al., Chem. Phys. 349, 278 (2008)
Conical intersections: Twisted-pyramidalized Barbatti et al. PCCP 10, 482 (2008)
The Dynamics Problem At a certain excitation energy: 1. Which reaction path is the most important for the excited-state relaxation? 2. How long does this relaxation take?
From quantum to (semi)classical Wave packet propagation Surfacehoppingpropagation
Cremer-Pople parameters Chair Twisted-chair Envelope Q Screw-boat q f Boat Ex.: 1S6 = Screw-boat withatoms1abovethe plane and6below Cremer and Pople, JACS 97, 1358 (1975)
UV absorption of nucleobases • PCCP 12, 4959 (2010)
Excited-state lifetimes (vapor) Short lifetimes together with the low fluorescence quantum yields indicate internal conversion through conical intersections Purines: singlestep Pyrimidines: multiple steps • Ullrich, Schultz, Zgierski, Stolow, PCCP 6, 2796 (2004)
2-aminopurine 9H-Adenine 1 ps 30 ps
Adenine: conical intersections N9 H psNH*/cs 9 pp*/cs 6 N1 Many conical intersections available. Which of them are used for internalconversion? Why? On which time scale? pp*/cs C2 H 2
Adenine: photodynamics E (eV) np* A2 A1 G1 pp* G2 pp* G2a np* A2a G1a A1a cs cs pp* P1 P2 pp* P1c C1a C1 C1c C1d P1a P2a P1d np* np* P1b C1b C2 cs cs DR (Å.amu1/2)
Adenine: deactivation mechanisms E (eV) np* A2 A1 pp* pp* np* A2a A1a cs cs pp* pp* np* np* cs cs DR (Å.amu1/2) • JACS 130, 6831 (2008)
Guanine E (eV) np* A2 A1 G1 G2 pp* G2a pp* np* A2a G1a A1a cs cs pp* pp* np* np* cs cs DR (Å.amu1/2) • J Chem Phys 134, 014304 (2011)
Thymine and uracil E (eV) np* A2 A1 G1 G2 pp* G2a pp* np* A2a G1a A1a cs cs P1 pp* P2 pp* P1c P1a P2a np* np* P1b cs cs DR (Å.amu1/2) • J Phys Chem A 113, 12686 (2009) • J Phys Chem A 115, 5247 (2011)
Cytosine E (eV) np* A2 A1 G1 G2 pp* G2a pp* np* A2a G1a A1a cs cs pp* P1 P2 pp* P1c C1a C1 C1c C1d P1a P2a P1d np* np* P1b C1b C2 cs cs DR (Å.amu1/2) • PCCP 13, 6145 (2011)
E (eV) np* A2 A1 G1 pp* G2 pp* G2a np* A2a Single step G1a A1a cs cs pp* P1 P2 pp* P1c C1a C1 C1c C1d P1a P2a P1d np* np* P1b C1b C2 Multiple steps cs cs DR (Å.amu1/2) • PNAS 107, 21453 (2010)
PHOTOINDUCED PHENOMENA IN NUCLEIC ACIDS • Mario Barbatti, Antonio C. Borin, Susanne Ullrich (Eds.) • Coming soon 1. Photoinduced processes in nucleic acids Mario Barbatti, Antonio Borin, Susanne Ullrich 2. UV-excitation I: frequency resolved Mattanjah S. de Vries 3. UV-excitation II: time resolved Thomas Schultz 4. Excitation of nucleobases I: reaction paths Manuela Merchán 5. Excitation of nucleobases II: dynamics Letícia Gonzalez 6. Excitation of paired and stacked nucleobases Dana Nachtigallova, Hans Lischka 7. Modified nucleobases SpiridoulaMatsika 8. UV-excitation of solvated nucleobases I Carlos E. Crespo-Hernandez 9. UV-excitation of solvated nucleobases II Roberto Improta 10. Excitation of single and double strands I Bern Kohler 11. Excitation of single and double strands II ZhenggangLan, Walter Thiel 12. Synchrotron irradiation of DNA fragments Martin Schwell 13. Physiological aspects of excitation of DNA Donat-P. Häder 14. Photoynthesis in prebiotic environments Scott Sandford 15. Photoinduced charge-transfer in DNA and applications in nano-electronics Kiyohiko Kawai, TetsuroMajima 16. Electronic energy transfer in nucleic acids DimitraMarkovitsi
Excited state dynamics: what have we learned? 2-pyridone • Chem. Phys. 349, 278 (2008) 9H-adenine
Adenine is trapped close to 2E conformation and because of this it has time enough to tune the coordinates of the conical intersection. Adenine is a non-fluorescent species. Pyridone does not stay close to any specific conformation long enough in order to have time to tune the coordinates of the conical intersections. Pyridone is a fluorescent species.
MQCD simulationsare not a substitutefortheconventionalquantum-chemistrycalculations, but a complementarytooltobeusedcarefullygiventheirhighcomputationalcosts • Theycanbespeciallyusefultotestspecifichypothesisraisedeitherby experimental analysisorconventionalcalculations
Next lecture • Transient spectrum • Excited state surfaces Contact firstname.lastname@example.org