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Title: Final Report: Vibrational Dynamics in Photoinduced Electron Transfer

Abstract

The objective of this grant was to understand how molecular vibrational states (geometry distortions) are involved in photoinduced electron transfer rates of molecules. This subject is an important component of understanding how molecular absorbers of light convert that energy into charge separation. This is important because the absorption usually excites molecular vibrations in a new electronic state prior to electron transfer to other molecules or semiconductor nanoparticles, as in some types of solar cells. The speeds of charge separation and charge recombination are key parameters that require experiments such as those in this work to test the rules governing electron transfer rates. Major progress was made on this goal. Some of the molecular structures selected for developing experimental data were bimolecular charge transfer complexes that contained metals of cobalt or vanadium. The experiments used the absorption of an ultrafast pulse of light to directly separate charges onto the two different molecular parts of the complex. The charge recombination then proceeds naturally, and one goal was to measure the speed of this recombination for different types of molecular vibrations. We used picosecond and femtosecond duration pulses with tunable colors at infrared wavelengths to directly observe vibrational states and their different ratesmore » of charge recombination (also called electron transfer). We discovered that different contact geometries in the complexes had very different electron transfer rates, and that one geometry had a significant dependence on the amount of vibration in the complex. This is the first and only measurement of such rates, and it allowed us to confirm our interpretation with a number of molecular models and test the sensitivity of electron transfer to vibrational states. This led us to develop a general theory, where we point out how molecular distortions can change the electron transfer rates to be much faster than prior theories predict. This provides a new method to predict electron transfer rates for particular conditions, and it will be important in designing new types of solar cells. A related set of studies were also done to understand how much the environment around the active charge transfer molecules can control the speed of charge transfer. We studied different complexes with femtosecond transient absorption spectroscopy to show that solvent or components of a matrix environment can directly control ultrafast electron transfer when the environmental relaxation time response is on a similar time-scale as the natural electron transfer. Understanding such processes in both liquids and in a matrix is essential for designing new types of solar cells.« less

Authors:
Publication Date:
Research Org.:
Northwestern University
Sponsoring Org.:
USDOE - Office of Energy Research (ER)
OSTI Identifier:
881273
Report Number(s):
DOE/ER/14228-FINAL
TRN: US200716%%111
DOE Contract Number:
FG02-91ER14228
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 14 SOLAR ENERGY; ABSORPTION; ABSORPTION SPECTROSCOPY; COBALT; ELECTRON TRANSFER; ELECTRONS; GEOMETRY; MOLECULAR MODELS; MOLECULAR STRUCTURE; RECOMBINATION; RELAXATION TIME; SENSITIVITY; SOLAR CELLS; SOLVENTS; TRANSIENTS; VANADIUM; VELOCITY; VIBRATIONAL STATES; WAVELENGTHS; electron transfer, vibrations, molecules, charge-transfer, inorganic complex, femtosecond, transient, spectroscopy, solvent, adiabaticity, matrix, vibrational-coherence, density-functional theory, Jahn-Teller

Citation Formats

Kenneth G. Spears. Final Report: Vibrational Dynamics in Photoinduced Electron Transfer. United States: N. p., 2006. Web. doi:10.2172/881273.
Kenneth G. Spears. Final Report: Vibrational Dynamics in Photoinduced Electron Transfer. United States. doi:10.2172/881273.
Kenneth G. Spears. Wed . "Final Report: Vibrational Dynamics in Photoinduced Electron Transfer". United States. doi:10.2172/881273. https://www.osti.gov/servlets/purl/881273.
@article{osti_881273,
title = {Final Report: Vibrational Dynamics in Photoinduced Electron Transfer},
author = {Kenneth G. Spears},
abstractNote = {The objective of this grant was to understand how molecular vibrational states (geometry distortions) are involved in photoinduced electron transfer rates of molecules. This subject is an important component of understanding how molecular absorbers of light convert that energy into charge separation. This is important because the absorption usually excites molecular vibrations in a new electronic state prior to electron transfer to other molecules or semiconductor nanoparticles, as in some types of solar cells. The speeds of charge separation and charge recombination are key parameters that require experiments such as those in this work to test the rules governing electron transfer rates. Major progress was made on this goal. Some of the molecular structures selected for developing experimental data were bimolecular charge transfer complexes that contained metals of cobalt or vanadium. The experiments used the absorption of an ultrafast pulse of light to directly separate charges onto the two different molecular parts of the complex. The charge recombination then proceeds naturally, and one goal was to measure the speed of this recombination for different types of molecular vibrations. We used picosecond and femtosecond duration pulses with tunable colors at infrared wavelengths to directly observe vibrational states and their different rates of charge recombination (also called electron transfer). We discovered that different contact geometries in the complexes had very different electron transfer rates, and that one geometry had a significant dependence on the amount of vibration in the complex. This is the first and only measurement of such rates, and it allowed us to confirm our interpretation with a number of molecular models and test the sensitivity of electron transfer to vibrational states. This led us to develop a general theory, where we point out how molecular distortions can change the electron transfer rates to be much faster than prior theories predict. This provides a new method to predict electron transfer rates for particular conditions, and it will be important in designing new types of solar cells. A related set of studies were also done to understand how much the environment around the active charge transfer molecules can control the speed of charge transfer. We studied different complexes with femtosecond transient absorption spectroscopy to show that solvent or components of a matrix environment can directly control ultrafast electron transfer when the environmental relaxation time response is on a similar time-scale as the natural electron transfer. Understanding such processes in both liquids and in a matrix is essential for designing new types of solar cells.},
doi = {10.2172/881273},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Apr 19 00:00:00 EDT 2006},
month = {Wed Apr 19 00:00:00 EDT 2006}
}

Technical Report:

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  • Objective is to perform a new type of measurement for optically excited electron transfer processes that can provide unique experimental insight into the molecular mechanism of electron transfer. Measurements of optically excited electron transfer are done with picosecond infrared (IR) absorption spectroscopy to monitor the vibrational motions of the molecules immediately after electron transfer. Theory and experiment suggest that molecular vibrations and distortions are important controlling elements for electron transfer, and direct information has yet to be obtained on these elements of electron transfer mechanisms. The second period of funding has been dedicated to finishing technique development and performing studiesmore » of electron transfer in ion pair systems to identify if vibrational dependent electron transfer rates are present in this system. We have succeeded in measuring, for the first time, electron transfer rates as a function of vibrational state in an ion pair complex in solution. In a different area of electron transfer research we have proposed a new mechanism of solvent gated electron transfer.« less
  • The final report describes studies over a 13 year period having to do with photoinduced electron transfer for active chromophores and redox agents, including assembly of the components in water soluble polymers or polypeptides. The findings include observation of long range charge separation and electron transport using laser phototransient spectroscopy. The systems targeted in these studies include peptide assemblies for which helical conformations and aggregation are documented. Oligomeric peptides modified with non-native redox active groups were also selected for investigation. Highly charged polymers or peptides were investigated as host agents that resemble proteins. The overall goal of these investigations focusedmore » on the design and characterization of systems capable of artificial photosynthesis.« less
  • Work which has been conducted under Department of Energy sponsorship over the past ten years at Boston University is described. A general theme for projects which are summarized involves photochemically induced electron transfer reactions for organic compounds. Early studies in the series were directed to the development of new mechanisms for driving isomerization processes which store light energy as latent heat. Other investigations were devoted to an understanding of the dynamics of charge separation for photoexcited (charge-transfer) complexes or ion-pairs. Recent studies focused on the development of charge relays such as dithioethers and viologen or pyridinium ions, the electron transfermore » photochemistry of high potential quinones, and the photophysical properties of organic dyes bound to water-soluble polymers. 9 figs.« less
  • Goal has been to study light-driven electron transfer and hydrogen atom abstraction processes with emphasis on reactions giving rise to net chemical or electrochemical conversion. The work focused on studies using substrates excitable with visible light - ranging from metal complexes, porphyrins and metalloporphyrins to dyes and ketones - and quencher-mediators capable of acting as electron donors or acceptors by virtue of having multiple closely spaced redox levels. The work can be conveniently divided into five major areas: Generation and Reaction of Reducing and Oxidizing Radicals and Radical Ions in Photoelectrochemical Cells; Studies of Weitz-type Quenchers Having Stable One-electron Redoxmore » Products; Two-electron Oxidative and Reductive Quenching Processes with Weitz-type Systems in Solution and Organized Media; Photoredox Reactions of Indigo Dyes; and Modification of Photochemical Reactivity by Formation of Amylose Inclusion Complexes in Aqueous and Partially Aqueous Solutions.« less
  • The focus of this research has been the study and development of useful chemical reactions initiated via photoinduced electron transfer events.