The reverse saturable absorption and the optical-limiting response of metal phthalocyanines can be enhanced by use of the heavy-atom effect. Phthalocyanines containing heavy-metal atoms, such as In, Sn, and Pb, show a nearly factor-of-2 enhancement in the ratio of effective excited-state to ground-state absorption cross sections compared with those containing lighter atoms, such as Al and Si. In an f/8 optical geometry, homogeneous solutions of heavy-metal phthalocyanines, at 30% linear transmission, limit 8-ns 532-nm laser pulses to ' 3 ,uJ (the energy for 50% probability of eye damage) for incident energies as high as 800 ,IJ.Materials that exhibit reverse saturable absorption (RSA) are currently of interest for use in opticallimiting devices for protection of sensors and eyes from energetic light pulses. RSA can occur when states with an absorption cross section (ire) in excess of the ground-state cross section (ag) are produced. RSA in the visible spectrum that is due to electronic excitation was reported for several classes of organic chromophores.'1 7 The effectiveness of RSA molecules for optical limiting is determined mainly by the ratio of cross sections (oaelaJg), which is function of wavelength, and the populations of the states that evolve in time during the pulse. There is considerable interest in approaches to enhancing the performance of RSA chromophores.RSA and optical limiting in metallophthalocyanines (MPc's) such as chloroaluminum phthalocyanine (CAP) and bis [tri-(n-hexyl) (Ref. 9) indicate that at/ag values (t refers to the triplet state) are -20 to 50. However, the triplet populations achievable during nanosecond pulses are limited by the small intersystem crossing rate, kje,, for these molecules. 9 ' 10 These observations suggested to us the use of the heavy-atom effect as an approach to enhancing the optical-limiting performance of phthalocyanines. When the atomic number of the central metal atom is increased, ki,, for the ir-electronic states can be increased,' 0 permitting more efficient population of the triplet state.In this Letter we examine the RSA performance of MPc's containing metals (and metalloids) from groups IIIA (Al, Ga, In) and IVA (Si, Ge, Sn, and Pb). For Al, Ga, and In phthalocyanines (Pc's) the molecular structures were of the form [tri-(n-hexyl)siloxy] MPc, whereas Si, Ge, and Sn Pc's were of the form bis[tri-(n-hexyl)siloxy] MPc and Pb Pc was of the form Pb tetra(t-butyl)Pc. These molecules were synthesized according to literature methods." The electronic absorption spectrum of Sn Pc is shown in Fig. 1, which reveals a strong Q-band absorption at 678 nm and a region of weak absorption from -425 to 600 nm. Also shown is the transient triplet-triplet absorption spectrum, which shows a maximum at 510 nm and a bandwidth of -130 nm. The ground-state and triplet-triplet spectra for the other molecules are quite similar, with a slight red shift of the Q band as the metal becomes heavier.The values of the triplet quantum yield (Fet) and the first excited-singlet lifetime ...

A variety of hexacyclen derivatives with aliphatic substituents have been prepared and characterized via X-ray diffraction, calorimetry, thermogravimetric analysis, and computer simulations. A subset of these compounds (2a and 2e) are seen to display a liquid crystalline mesophase, while others transform directly from the crystalline to the isotropic liquid phase. The merits of tubular and smectic liquid crystalline models for the mesophase structure are discussed.

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