Optical Limiting in Solutions of Polycyclic Aromatic Hydrocarbon Diones.
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OF POLYCYCLIC
M. BRANT, D. McLEAN*, R. L. SUTHERLAND*, AND R. L. CRANE.** T. M. COOPER,**
* Science Applications Drive, Dayton, OH 45431.
International
Corporation,
101
Woodman
** Materials Directorate, Wright Laboratory, WPAFB, OH 45433.
ABSTRACT Optical limiting in toluene solutions of substituted benzanthracene3,4 diones and three other quinones, namely, benzpyrene-7,8-dione, was and chrysene-5,6-dione benzphenanthrene-3,4-dione investigated at laser wavelengths 532 nm and 694 nm. Low limiting threshold values of the order of 0.O1J/cm 2 were obtained for some Strong triplet-triplet absorption at the of the quinones at 532nm. may explain the optical limiting excitation laser wavelengths behavior.
INTRODUCTION Organic dyes exhibiting nonlinear absorption are the focus of recent as optical limiters and optical studies for possible applications switches. Nonlinear absorption has been reported as the mechanism for optical limiting in a variety of organic materials which include porphyrins, 3 indanthrones, 2 tetraphenyl C 60,1 metallophthalocyanins, 4 metallonaphthocyanines,5and iron-cobalt metal clusters. 6 These organic materials are characterized by low absorption in the ground state, when exposed to laser light form excited states which have strong absorption at the excitation wavelength and in some cases orders of magnitude higher than the ground state. These absorptions may originate from excited singletsinglet transition or triplet-triplet excitations. This class of materials is also referred to as reverse saturable absorber(RSA) or excited state absorbers. For a RSA material to be an efficient optical limiter,
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Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
it is desirable to have high ratio of excited state to ground state absorption cross sections and also high triplet quantum yields. In the present work, we have investigated a new class of organic materials, namely, polycyclic aromatic hydrocarbon diones, for their optical limiting efficiency(Fig.I). These diones have absorption bands extending from the visible to the near infra red (Tablel). They have a lowest energy absorption band corresponding to n-it* singlet transition. Upon excitation at 532 NM, they exhibit high
triplet
quantum
yields
due
to
efficient
intersystem
crossing.
Phosphorescence can be detected even at room temperatures. 7 also show strong triplet-triplet absorption ,8-
They
Theoretical calculations employing the INDO method for the spin allowed transitions of both n, x* and xt, yt* types were found to be in good agreement with those experimentally observed. In benz(a) anthracene series, there is a reversal of state ordering of triplet energy levels compared to theoretical predictions. This is due to the substitution of methyl groups closer to the diketo ring, causing distortion from planar geometry as a result of steric effects. The photophysical characteristics of these diones are promising for examination as RSA's for optical limiting at laser wavelengths 532 and 694nm. In this communicat
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