Journal of Directed Energy (Limited Edition)
The papers listed below constitute Volume 4 of the Journal of Directed Energy, Limited Edition.
Persons with the necessary credentials can obtain copies of the individual papers by contacting
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