The Design and Synthesis of New Organic Molecules with Large Two-Photon Absorption Cross-Sections for Optical Limiting A
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ABSTRACT The molecular structure / nonlinear optical (NLO) property relationship is explored with seven recently synthesized chromophores. Two symmetrical compounds were made using electron withdrawing groups separated by an electron rich core while five asymmetrical molecules were developed using electron donating and withdrawing groups coupled by a it electron bridging group. Pendant chains were added to some of the chromophores to improve processibility. Their syntheses are described and their optical limiting properties discussed. INTRODUCTION Organic molecules with large two-photon absorption (TPA) cross-sections enable a wide variety of new technologies in the areas of fluorescent imaging [1], upconverted lasing [2], threedimensional data storage [3], photodynamic cancer therapy [4], and optical power limiting [5]. Materials which exhibit strong optical power limiting behavior when exposed to high intensity light are critical in the prevention of radiation damage to eyes and sensitive photodetectors. These compounds must have the ability to react to high intensity light sources extremely rapidly (nanosecond response) while maintaining a high transparency at low intensities. Structure / Property Relationships and Materials Issues The major problem that one has to face when designing new molecules with improved two-photon properties is understanding the structure / two-photon property relationships for various combinations of organic structural units. A convenient place to begin is with equation 1, where G2 is the molecular two-photon absorption cross-section, h is Planck's constant, u is the frequency of the incident light, n is the index of refraction, c is the speed of light in a vacuum, N is the number of absorbing molecules and Im(X(3)) is the imaginary part of X(3), the third-order nonlinear susceptibility. -2
-n
822
hu ( c 2 N .Im(X t )()
From the above expression, materials which have been designed to maximize their X(3) values also should be good candidates for optical limiting via two-photon absorption. A potential 3 Mat. Res. Soc. Symp. Proc. Vol. 479 ©1997 Materials Research Society
problem with this assumption is that as structural changes are incorporated to increase X(3), the absorption maximum for TPA shifts so that challenge is to increase 02 while maintaining Xmax approximately constant. Molecular Design It was decided to focus our work on the study of the optical limiting properties on two somewhat different classes of organic molecules which we assumed would have reasonable limiting activity via a two-photon mechanism at 800 nm. The chromophores of the first class are symmetrical in nature and based on an electron-rich thiophene core D with terminal it electron acceptor benzothiazole end groups A (see figure 1). A-D-D-A
A-D-A
Generalized electronic structures for Class I compounds.
Fig. 1
The molecular structure of the second general class of compounds is based on a nt electron bridging core coupling a diphenyl amino or thiophene it electron donor with a pyridine 7t electron accept
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