Excited State Absorption in Dendrimers Incorporating Diphenylaminodiphenylpolyene Moieties Via Photo-Induced Electron Tr
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ABSTRACT During the past decade there has been considerable progress in the design of new organic materials for optical power limiting (OPL) applications. Among the more promising of the new material approaches are reverse saturable absorbers (RSAs) which derive their optical limiting capability from the photogeneration of highly absorbing transient excited states. Most of the previous research in this field has focused on excited state absorption from singlet or triplet states whose cross-sections are greater than the original So - S, transition- However, other transient states, such as polaronic radical ions or bipolaronic diions, formed by photo-induced electron transfer, are attractive alternatives. We have initiated studies to determine if these transient charge states can be photo-generated efficiently, and if their excited states absorption and lifetimes are acceptable for current optical power limiting requirements. INTRODUCTION Rapid advances in laser technology have resulted in new and improved laser systems that are compact, efficient and operate at a variety of wavelengths The increased presence of lasers in "friendly" applications and in potential enemy weapons systems presents significant hazards to the eyesight of ground-based military personnel, optical sensors and both military and commercial aircraft pilots The military potential of laser weaponry has been discussed in detail in a recent publication by Anderberg and Walbarshts, and most recently by Miller, et al_2 With the advent of frequency agile laser sources, there is a need for protection schemes that can respond to a potentially unknown wavelength, and an equally unknown pulse duration. These requirements and potential approaches to the 3 design of such passive "smart" limiters have been recently reviewed , and a case made for materials that can respond by a variety of mechanisms, such as a combination of both RSA and two-photon absorption. However, in this presentation we will limit the discussion to design criteria for a new class of reverse saturable absorbers, and how these new materials can be characterized for potential limiting behavior. GENERATION OF CHARGE STATES In the current study we have limited our studies to materials that could potentially generate highly absorbing excited states in the visible region of the spectrum (400-800 nm)- In general, RSAs have their Xmax for the primary i-7* absorption at 400 nm or less to give reasonable transmission for solution or thin films. Several structural 3 families have been shown to exhibit RSA behavior, such as porphyrins, phthalocyanines and fullerenes . The vast majority of these RSA materials derive their capability from triplet states resulting in fast, efficient intersystem crossing from the S, state initially populated by the incident laser irradiation. For example, the heavy atom effect in metallophthalocyanines and metalloporphyrins, which magnifies the OPL behavior, can be attributed to the enhancement of ISC from S, to TI, thereby giving greater access to the highly absorbing tr
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