Final Report: Vibrational Dynamics in Photoinduced Electron Transfer
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.
- Research Organization:
- Northwestern Univ., Evanston, IL (United States)
- Sponsoring Organization:
- USDOE - Office of Energy Research (ER)
- DOE Contract Number:
- FG02-91ER14228
- OSTI ID:
- 881273
- Report Number(s):
- DOE/ER/14228-FINAL; TRN: US200716%%111
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
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