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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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The synthesis and chemistry of nonlinear optical (NLO) porphyrin-based
materials has been examined in some detail. Porphyrins have several
desirable properties for use in optoelectronics: they have greater
thermal stability (compared to typical organic chromophores); their
extended 7r-conjugated macrocyclic ring gives large NLO effects and
subtle variation in their physical properties can be made easily through
chemical modification of their periphery. Here we highlight progress in
the development of NLO porphyrinic materials describing their second- and
third-
54
Chou et al.
order nonlinear optical properties. Due to space limitations, we provide
here only the most cursory introduction to the physics of nonlinear
optical phenomena; several excellent references are available for further
details.30-37
The optical properties of a medium are characterized by the optical
susceptibility, This parameter is closely related to the refractive index
and the dielectric constant. In an isotropic medium at optical
frequencies (), the relationship between the linear susceptibility, the
refractive index (n), and the dielectric constant (e) can be expressed
as;
I +Anx(f'>) = n:(w) = c(w) (1)
The microscopic polarization (p) induced in an isolated molecule under
the applied electric field (E) of an incident electromagnetic wave can be
expressed by the following equation:
p = a E + /i EE = }' EEE (2)
where p and E are related to the tensor quantities a, /?, y, which are
referred to as the polarizability, first hyperpolar-izability and second
hyperpolarizability, respectively. Similarly, the macroscopic
polarization induced in the bulk media can also be expanded in the
external field power series:
P = X,IIE + XI2,EE + X,-'EEE + --- (3)
where /<l) is the linear optical susceptibility, and x(2) and %<3> are
the second- and third-order nonlinear optical susceptibilities; these
physical meanings similar to their microscopic counterparts a, /), and y,
respectively. The even-order tensor %<2) is zero in a centrosymmetric
environment, whereas the odd-order tensor x(3) does not have any symmetry
restrictions and produces a nonvanishing higher-order optical
susceptibility. Therefore, %(n) are (n + 1) rank tensor associated with
the nonlinear optical response of the medium. Both the field E and the
polarization p are vectors, while the nonlinear optical coefficients are
tensors. The x(n> is frequency dependent, and as a result, resonant and
nonresonant parameters can differ significantly depending upon
frequencies used. There are 9 elements of %(1), 27 elements of x {2\ and
81 elements of x(3); the number of independent elements, however, is
usually much smaller. For example, in liquids, gasses and isotropic
solids, there are only three independent elements for y<3b y(3) <3) and
y<3>
IW1 \xxxi A xyyx' A xyxy*
1. Second-Order NLO Systems
Suslick et a/.38 measured the first hyperpolarizabilities (/?)
of porphyrins having electron-donor (amino) and -acceptor
(nitro) groups in the para-position of 5,10,15,20-substituted
tetraphenylporphyrins (Figure 25) using electric field
induced second harmonic generation (EFISH) technique at
1.19 ^m with chloroform solutions. The dipole moment (p.)
and (S values were affected by the position of donor and
acceptor groups. A j8 value of 30 x 10~ 3() esu and dipole
moment of 7 x 10 ~ 18 esu were measured for the porphyrin
having R1 = R2 = N02, R3 = R4 = NH2; and the porphyrin
Figure 25. Structure of Suslick's donor-acceptor "push-pull"
tetraphenylporphyrin, R= N02 or NH2.
with R1 = N02, R2 = R3 = R4 = NH2 exhibited a ji value of 20 x 10 ~10 esu
and dipole moment of 5 x 10~l8esu (1 au = 1.48176 x 10'25 esu = 1.64877 x
10~41 C2m2J-1). In these cases, the amino group "pushes" electron density
into the macrocycle while the nitro groups "pull," increasing the
hyperpolarizability substantially. The value of a porphyrin with R1 = R2
= R3 = N02, R4 = NH2 was found to be >10 x 10~30 esu. The well-aligned
charge transfer gives rise to large fi values.
Since second-order NLO response occurs only in a noncentrosymmetric
environment, the incorporation of porphyrins into a polar (in the sense
of poled or asymmetric) array is mandatory for most optoelectronic
applications. The importance of the organizational structure is shown in
Figure 26. Langmuir-Blodgett (LB) films are an extremely effective means
of providing the necessary asymmetric environment for good NLO response.
Suslick and coworkers incorporated similar "push-pull" porphyrins
derivatives into LB films in order to evaluate bulk response of the
material.39 They found that good films could be created from these push-
pull porphyrins simply by conjugating a
Hemlncyanlnes
Phenylhydrazones
MNA
KTP -POM -
UNb03
Urea
KDP -
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