The meta-stable molecular configuration that gives rise to the chemical transition state often defines the salient properties of a reaction mechanism. In photochemical systems, this transition state decays rapidly and has no clear correlation to the reactants and products of the reaction, making characterization difficult except under extreme circumstances. The ability to generate pulsed laser light on the order of femto-seconds (fs, 10−15 s), however, unlocked new ways of tracing reaction coordinates and deciphering energy transfer, leading to an explosion of research devoted to the mechanistic understanding of photochemical systems.
Almost immediately after femtosecond pulse generation became a widely accessible research tool, researchers observed vibrational coherence across two distinct electronic states and quickly assigned this spectroscopic signature to a conical intersection. These special types of photochemical transition states, long predicted by theoretical and computational models but rarely observed experimentally, became the mechanistic driver of a photosynthetic bacteria and the retinal chromophore in rhodopsin.
These early observations of vibrational coherence proved immensely influential on the early field of ultrafast spectroscopy, setting a new "paradigm of photochemistry" that explicitly invoked conical intersections and femtosecond dynamics as primary drivers in photochemical reaction mechanisms. This success prompted certain diagnostic properties such as low fluorescence quantum yield, high photostability, and ultrafast dynamics, to be considered highly indicative of containing a conical intersection. These diagnostic properties, however, obscure one of the main theoretical predictions of conical intersections. Despite having special topology, conical intersections should be a common, even unremarkable, feature of photochemical systems.
This thesis describes work performed in the Turner Lab devoted to testing the theoretical prediction that conical intersections exist as common photochemical transition states across disparate chemical systems. We developed two ultrafast spectroscopy techniques in the Turner Lab, coherent wavepacket evolution analysis and two-dimensional electronic spectroscopy, to measure systems outside the these properties. For cresyl violet perchlorate, a highly fluorescent, photostable laser dye, the identification of a conical intersection mediating 5–20% of the total excited-state energy proved unexpected. Nevertheless, these types of persistent but low-efficiency conical intersections may be a general feature of oxazine dyes and other systems with high fluorescence quantum yields nevertheless below 100%. In iridium(IV) hexabromide, the spin-orbit splitting of the electronic energy levels was thought to preclude the existence of a conical intersection, instead favoring intersystem crossing as the excited-state energy relaxation mechanism. The observation of ultrafast wavepacket dynamics in iridium(IV) hexabromide, however, along with the fully coupled and vibrationally coherent set of excited-states revealed by two-dimensional electronic spectroscopy, suggested that the definition of conical intersection needed to include the total angular momentum of the interacting electronic states. Lastly, for the nitrogen vacancy center in diamond, the Jahn-Teller distortion that occurs in the excited state represents one of the oldest predicted conical intersections, and a strong LO phonon vibration was found to effectively scramble the polarization dependence of the excited state transition because of the Jahn-Teller distortion's high efficiency.
By expanding the library of molecules investigated by ultrafast spectroscopy to include those systems without the physical properties understood by conventional wisdom to predict conical intersections, the effects of these transition states on photochemical reaction mechanisms can be appreciated on the merit of their unique topographies. Indeed, ultrafast spectroscopy techniques prove to be remarkably direct probes of photochemical transition states.
|Advisor:||Turner, Daniel B.|
|Commitee:||Diao, Tianning, Kahr, Bart, Woerpel, Keith, Jerschow, Alexej|
|School:||New York University|
|School Location:||United States -- New York|
|Source:||DAI-B 81/5(E), Dissertation Abstracts International|
|Subjects:||Physical chemistry, Chemistry|
|Keywords:||Conical intersection, Jahn-Teller distortion, Two-dimensional electronic spectroscopy, Ultrafast spectroscopy, Wavepacket analysis|
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