Chapter 9
Design and Synthesis of a Polar Dipyrromethene Dye 1
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Downloaded by STANFORD UNIV GREEN LIBR on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch009
M . B. Meinhardt , P. A. Cahill , T. C. Kowalczyk , and K. D. Singer
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Chemistry of Materials Department, Mail Stop 0368, Sandia National Laboratories, Albuquerque, N M 87185-0368 Department of Physics, Case Western Reserve University, Cleveland, OH 44106-7079 2
Semiempirical methods were applied to the design of a new second order nonlinear optical (NLO) dye through polar (noncentrosymmetric) modifications to the symmetric dipyrromethene boron difluoride chromophore. Computational evaluations of candidate structures suggested that a synthetically accessible methoxyindole modification would have second order NLO properties. This new dye consists of 4 fused rings, is soluble in polar organic solvents and has a large molar extinction coefficient (86 x 10 ). Its measured hyperpolarizability, β, is -44 x 10 esu at 1367 nm. The methoxyindole therefore induces moderate asymmetry to the chromophore. 3
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The large potential market for electrooptic materials and devices for high speed data transfer, either as part of telecommunications or CATV networks or within computer backplanes, has prompted efforts towards the synthesis, processing and evaluation of new organic nonlinear optical (NLO) materials. Such devices would operate through the linear electrooptic effect that requires noncentrosymmetry on both molecular and macroscopic scales. Stability of the poled state is one of many requirements placed on these materials; other requirements include suitable and stable refractive indices for core and cladding, thermal stability as high as 350 °C, electrooptic coefficients greater than 30 pm/V, suitable electrical resistivity in both the core and cladding for efficient poling, and excellent optical transparency (losses < 0.3 dB/cm) at operating wavelengths. Simultaneous attainment of all these parameters has proved extremely difficult. Because organic NLO dyes are often the limiting factor in the ultimate thermal stability of a NLO material, a promising approach to improved materials is through the synthesis of new dyes. Dye Design Our approach is based on Marder et al.'s observations that a maximum in p, the first hyperpolarizability, occurs near the zero-bond alternation, or cyanine limit, in polar (noncentrosymmetric) chromophores (7). Therefore the nonlinearity of organic dyes 3
Corresponding author
0097-6156/95/0601-0120$12.00/0 © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
9. MEINHARDT ET AL.
Design & Synthesis of Polar Dipyrromethene Dye
Downloaded by STANFORD UNIV GREEN LIBR on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch009
may be maximized through asymmetric modifications to known, centrosymmetric cyanine or cyanine-like dyes (Figure 1). This approach might also lead to dyes with narrow electronic absorption spectra and correspondingly lower absorption losses to the red of the principal charge-transfer absorption which gives rise to the nonlinear optical effect. Poling of cationic dyes such as the cyanines is problematic; therefore, charge neutralization must first be addressed.
Figure 1. Cyanine dye structure. An internally charge compensated class of cyanine dyes are the dipyrromethene difluoroborates shown in Figure 2. (2) Highly fluorescent symmetric dipyrromethenes are commercially available as biological probes (J) and laser dyes (4). Derivatives that are soluble in either organic (R2 = alkyl) or aqueous media (R2 = sulfonate) are known. In addition, the dipyrromethenes are among the most photochemically stable dyes. (J) High fluorescence quantum yields have been reported over a wide spectral range. (3) Therefore, this chromophore is a good starting point for the synthesis of second order NLO dyes by asymmetric modification.
Figure 2. Dipyrromethene difluoroborate dye structure. The type and location of donor and acceptor groups on this chromophore that will maximize second order NLO properties is not obvious, however, because the ends of the cyanine chromophore are coordinated to the boron atom and therefore not available for direct modification. A polar substitution pattern must provide for both a charge transfer transition that is related to the strong absorption in the symmetric molecule and a ground state dipole moment which is substantially parallel to this transition. Candidate structures were therefore evaluated computationally with MOPAC using the AMI basis for geometry optimization. Spectroscopic INDO/S methods with configuration interaction (ZINDO) were used for electronic spectra estimation. (6) Direct calculation of the first hyperpolarizability of the candidate molecules with these semiempirical methods was considered, but a lack of closely related model compounds that could be used to verify such methods were not available. The twostate model, however, provides a means of relating molecular hyperpolarizability to information readily obtainablefromspectral calculations and was used to estimate the magnitudes of the hyperpolarizability of candidate molecules.(7) 2
/J(-2fi>;fi),fi)) =
3e
