Improving our understanding of nonadiabatic processes is essential for informed development of photoelectric technologies. Nonadiabatic dynamics arise due to nuclear motion on coupled potential energy surfaces. These nonadiabatic couplings are high in the regions of avoided crossings, where potential energy surfaces approach each other, and near infinity at conical intersections, where two potential energy surfaces are degenerate. Regions of high nonadiabatic coupling play an important role in the electronic dynamics of a system because of their ability to create very rapid (on the order of tens of femtoseconds) radiationless transitions between potential energy surfaces. Recent computational work has shown conical intersections to be prevalent in photochemical reactions; however, there are very few experimental investigations of conical intersections in condensed phase reactions. This is due to the extreme difficulty of detecting their subtle signatures experimentally. In biophysical chemistry, the only system that has been investigated is rhodopsin. In this photoisomerization reaction, the conical intersection produces a very rapid photoproduct formation with a quantum yield of approximately 85%. This led to the idea that the mere presence of a conical intersection in a biophysical system results in photoproducts formed with high quantum yields. In contrast, computational work shows conical intersections to be ubiquitous in all types of photoisomerization reactions. In this thesis I provide experimental evidence of conical intersections in a system with a quantum yield of <15%, thereby showing that the topography of the conical intersection has an effect on the quantum yield of the photoisomerization, and does not just by its mere presence result in a high quantum yield.
The system we investigate here is phytochrome Cph1Δ. It is an ideal model system because it is reversibly photoisomerizable and has known single--site mutations that result in altered potential energy surfaces. However, this system is experimentally very challenging to investigate. Conical intersections already have very subtle signatures, and low quantum yield, combined with the potential for overlapping signals due to the forward and reverse reactions, make detecting these signals even more difficult. In this thesis, I first describe our design and construction of a two--dimensional electronic spectroscopy (2D ES) and visible femtosecond transient transmittance spectrometer capable of performing these measurements. We perform a set of experiments on cresyl violet perchlorate, a laser dye, to show the importance of using our balanced detection methods and correct signal averaging schemes to maximize signal--to--noise and to ensure the signals converge to the correct value. We then use our spectrometer to probe for the presence of nonadiabatic dynamics in both the forward and reverse photoisomerization reactions of Cph1Δ. First, we use high sensitivity 2D ES and vibrational coherence spectroscopy to resolve a long--standing controversy about whether the forward reaction proceeds adiabatically. We find no evidence of nonadiabatic dynamics, and our results are consistent with a single ground state population undergoing a purely excited--state photoisomerizaion process. In the reverse reaction we identify and characterize a conical intersection using transient transmittance spectroscopy and 2D and 3D ES. This result expands on the notion that the presence of a conical intersection results in ultrafast dynamics and high photochemical quantum yield, showing that the topography of the conical intersection plays a role in determining the outcome of a photoexcitation. We also perform the same experiments on two single--site phytochrome mutants as control measurements. Finally, I present the theory of nonadiabatic coupling, and through simulations and experiments demonstrate the ability of our methods to identify signatures of non--Condon activity.
|Advisor:||Turner, Daniel B.|
|Commitee:||Buccella, Daniela, Geacintov, Nicholas, Kahr, Bart, Traaseth, Nate|
|School:||New York University|
|School Location:||United States -- New York|
|Source:||DAI-B 80/03(E), Dissertation Abstracts International|
|Subjects:||Physical chemistry, Biophysics|
|Keywords:||Conical intersections, Nonadiabatic dynamics, Nonlinear spectroscopy, Phytochromes, Two dimensional electronic spectroscopy, Ultrafast|
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