Polymethine and squarylium molecules with large excited-state absorption

    Authors

    J. H. Lim; O. V. Przhonska; S. Khodja; S. Yang; T. S. Ross; D. J. Hagan; E. W. Van Stryland; M. V. Bondar;Y. L. Slominsky

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    Abbreviated Journal Title

    Chem. Phys.

    Keywords

    NONLINEAR ABSORPTION; RELAXATION DYNAMICS; CYANINE DYES; Z-SCAN; PHOTOISOMERIZATION; PHOTOPHYSICS; PICOSECOND; Chemistry, Physical; Physics, Atomic, Molecular & Chemical

    Abstract

    We study nonlinear absorption in a series of ten polymethine dyes and two squarylium dyes using Z-scan, pump-probe and optical limiting experiments. Both picosecond and nanosecond characterization were performed at 532 nm, while picosecond measurements were performed using an optical parametric oscillator (OPO) from 440 to 650 nm. The photophysical parameters of these dyes including cross sections and excited-state lifetimes are presented both in solution in ethanol and in an elastopolymeric material, polyurethane acrylate (PUA). We determine that the dominant nonlinearity in all these dyes is large excited-state absorption (ESA), i.e. reverse saturable absorption. For several of the dyes we measure a relatively large ground-state absorption cross section, sigma(01), which effectively populates an excited state that possesses an extremely large ESA cross section, sigma(12). The ratios of sigma(12)/sigma(01) are the largest we know of, up to 200 at 532 nm, and lead to very low thresholds for optical limiting. However, the lifetimes of the excited state are of the order of 1 ns in ethanol, which is increased to up to 3 ns in PUA. This lifetime is less than optimum for sensor protection applications for Q-switched inputs, and intersystem crossing times for these molecules are extremely long, so that triplet states are not populated. These parameters show a significant improvement over those of the first set of this class of dyes studied and indicate that further improvement of the photophysical parameters may be possible. From these measurements, correlations between molecular structure and nonlinear properties are made. We propose a five-level, all-singlet state model, which includes reorientation processes in the first excited state. This includes a trans-cis conformational change that leads to the formation of a new state with a new molecular configuration which is also absorbing but can undergo a light-induced degradation at high inputs. (C) 1999 Elsevier Science B.V. All rights reserved.

    Journal Title

    Chemical Physics

    Volume

    245

    Issue/Number

    1-3

    Publication Date

    1-1-1999

    Document Type

    Article

    Language

    English

    First Page

    79

    Last Page

    97

    WOS Identifier

    WOS:000082219000008

    ISSN

    0301-0104

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