Phosphorescence Studies of Reverse Saturable Absorption Materials
- PDF / 273,533 Bytes
- 5 Pages / 420.48 x 639 pts Page_size
- 31 Downloads / 227 Views
ABSTRACT Phosphorescence studies of reverse saturable absorption materials (RSA) are discussed. Absorption and emission measurements of an RSA material, specifically Zinc 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin (ZnOBP), are performed in a proprietary host polymer. Spectra are analyzed to estimate the relative energy level of the first triplet state of ZnOBP with respect to the ground state. INTRODUCTION Zinc 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin (ZnOBP) is a halogenated metalloporphyrin, characterized as a reverse-saturable absorber having a high excited state absorption cross-section.' The electronic structure of this material has been studied for over thirty years yet the relative energy-level assignment of the lowest triplet state with respect to the ground state is still under debate.2 Previous quantum computations have been reported placing the first triplet to ground state transition at 1137 nm (1.09 eV). 2 However, Bhyrappa et. al. report the transition experimentally to be 728 nm (1.71 eV). 3 The blue shifted transition reported by Bhyrappa may be a result of photochemical reactions between ZnOBP and the chlorinated solvents used in the experiment. Such reactions have been observed in other experiments.4 Other effects of solvent-solute interactions have been observed with ZnOBP and similar chromophores that include peak broadening and red-shifting. However, these red-shifts are normally on the order of 10's of nanometers (0.01 eV in near IR region).5 '6 The purpose of this study is to determine the first triplet energy assignment of ZnOBP relative to the ground state and compare it to quantum mechanical models. When the system is at 300 K, this transition is nonradiative due to multi-phonon quenching. However, when the temperature of the system is reduced, as it approaches 0 K, the transition becomes increasingly radiative. If the lifetime of the state is much larger than a millisecond, the emission is considered phosphorescent. The emission spectra can then be used to determine the relative energy level assignment. EXPERIMENT ZnOBP was doped in a proprietary host polymer at a concentration of 0.2 mg/ml, forming an amorphous solid at 300 K. The sample is held under vacuum at liquid
383 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
S
nitrogen temperature within a dewar having Sapphire windows to allow for the infrared emission to be collected. For the experiment a visible linear absorption spectrum was collected using a Perkin-Elmer Lambda 20 UV/VIS spectrometer. The near Infrared Radiation (IR) linear absorption spectra were collected using a Perkin-Elmer Lambda 9 UV/VIS/NIR spectrophotometer. Both were performed at 300 K. Emission spectra were collected at 300K and 77K using the following experimental layout. The probe source is a 90 femtosecond, 390 nm, gaussian beam having a pulse energy of 190 1tJ per pulse. The beam is generated from frequency doubling the output of a 780 nmTi-Sapphire mode-locked laser with a variable repetition
Data Loading...
