Journal of Directed Energy (Limited Edition)
Volume 4 Fall 2016

The papers listed below constitute Volume 4 of the Journal of Directed Energy, Limited Edition.
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Broadband Nonlinear Optical Dyes for Army Applications (No.1)
The search for a high-performance, broadband nonlinear optical (NLO) absorber is an important component of the research and development portfolios of all three armed services. Several years ago, a particular bipyridyl platinum complex 1 bearing 2-benzothiazolyl-7-ethynyl-fluorenyl ligands was identified as a promising NLO absorber. The complex exhibits reverse saturable absorption (RSA), a nonlinearity that arises when the intrinsic ability of the excited state(s) of a molecule to absorb radiation exceeds that of the ground state. The strength of the nonlinearity can be increased both (1) by increasing the length of time the chromophore molecule can remain in an excited state, as well as (2) by increasing the efficiency with which the excited state(s) are populated. Pursuing the former avenue for improvement, researchers substituted a naphthalimide group for the electron-withdrawing benzothiazolyl moiety in the ligand, achieving an increase in the triplet-excited state lifetime of the complex from ~1 Ás in deoxygenated solution to ~30 Ás without significant alteration in the other NLO properties of the molecule. The ground-state absorption spectrum of this improved chromophore 2 displays a low-energy tail that extends from the green region of the visible spectrum into the red and near infrared. In order to increase the efficiency of excitation at the far red end of the visible spectrum, it is desirable to "flatten" this absorption tail. To this end, a number of octahedral iridium complexes were synthesized. Each of the new iridium complexes exhibits a weak absorption tail between 500 and 750 nm with a weak absorption peak ("bump") in range 600-650 nm. The bump serves to broaden into the red the region in which single-photon absorption is sufficiently strong to populate the excited states and thus extends the spectral range over which the dye exhibits measurable RSA. We review the development of these materials, report their photophysical parameters, and discuss their individual advantages and shortcomings for Army applications

Journal of Directed Energy, Limited Edition, Volume 4

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