Pressure Effect on Electronic Structure of Anthracene Single Crystals:
Formation and Modification of Structural Defects
Z. A. Dreger and Y. M. Gupta
EMSL Project ID
3355
Abstract
BackgroundDue to relatively weak intermolecular forces, molecular crystals undergo considerable compression at high-pressures. Thus, the application of high-pressure often leads to a significant modification of molecular and/or crystal structure. In some cases, these modifications can also be accompanied by irreversible changes associated with a formation of structural defects.
In this project, we will examine the role of pressures on the formation of structural defects in anthracene single crystals. Structural defects in this crystal can be examined using electronic spectroscopy. In particular, fluorescence spectoscopies have been proven to be sensitive tools in detecting the defect structures in many organic crystals.
Our recent studies 1,2 have demonstrated that pressure-induced changes in the fluorescence spectra are sensitive to loading conditions in the cell (e.g., hydrostaticity vs. nonhydrostaticity). In particular, under nonhydrostatic conditions there are irreversible microscopic changes on the crystal surface that accompany the transformations in the electronic spectra. The changes in spectra strongly depend on whether the excitation takes place in the singlet or triplet state. In the latter, a broad red shifted band replaces the delayed fluorescence spectrum at high pressures. The origin and mechanism of this transformation is not completely understood at present.
Objective and approach
The broad objective of this project is to examine the origin and nature of electronically accessible structural defects in compressed anthracene single crystals. Specifically, we want to understand the role of nonhydrostatic stresses in formation and modification of structural defects. To pursue this objective, we plan to complement our previous studies with measurements of fluorescence lifetime. These experiments can help in identifying different excited states and, thus, characterize the species contributing to the fluorescence spectra. We propose to do the lifetime measurements using PNNL facilities.
Proposed experiments
Two sets of experiments are planned: one involves 355 nm excitation and the other with 514 nm excitation. In these experiments, the fluorescence lifetime of single crystals of anthracene compressed up to 10 GPa, in a diamond anvil cell, will be measured. Two different pressurizing media: nitrogen and water will be used to introduce, respectively hydrostatic and nonhydrostatic conditions. Brief description of these experiments follows:
1. Fluorescence lifetime – excitation within a singlet state absorption
Measurements can be performed with a mode-looked Nd-YAG laser, using 355 nm wavelength for excitation. The emission (prompt fluorescence) is expected to occur between 400 and 600 nm with a lifetime in the range of nanoseconds. The occurrence of new components in fluorescence decay is anticipated under nonhydrostatic conditions. Time-correlated single-photon counting system with a multichannel plate photomultiplier tube can be used for detection fluorescence decays. Since the crystals are encapsulated in diamond anvil cell both, the excitation and emission should be measured from the same side of the crystal.
2. Fluorescence lifetime – excitation within a triplet state absorption
The triplet absorption can be achieved by using 514 nm wavelength. This could be accomplished using a dye laser pumped by the Nd-YaG laser. Upon excitation of the triplet state, a delayed fluorescence originating from the annihilation of two triplet excitons can be observed. The resulting emission occurs in the same spectral range as the prompt fluorescence, i.e. below 500 nm. The decay of delayed fluorescence, corresponding to the lifetime of triplet excitons, is usually several ms in good quality anthracene crystals. It is anticipated that this lifetime will decrease due to formation of defects. Also, one expects the occurring of new components in lifetime (microseconds or nanoseconds?) from the pressure-induced low energy band located at 550 – 600 nm.
[1] Z. A. Dreger, H. Lucas and Y. M. Gupta, High pressure effect on fluorescence of anthracene crystals, submitted
[2] Z. A. Dreger, E. Balasubramaniam Y. M. Gupta, High pressure fluorescence and optical imaging of anthracene single crystal: Efect of non-hydrostaticity, in preparation
Project Details
Project type
Exploratory Research
Start Date
2003-03-17
End Date
2004-11-15
Status
Closed
Released Data Link
Team
Principal Investigator
Related Publications
Dreger ZA, E Balasubramaniam, YM Gupta, and AG Joly. 2009. "High-Pressure Effects on the Electronic Structure of Anthracene Single Crystals: Role of Nonhydrostaticity." Journal of Physical Chemistry A 113(8):1489-1496.
Paper in preparation. High-pressure fluorescence and optical imaging of anthracene single crystal: The effect of nonhydrostaticity.