# The porphyrin handbook - Kadish K.M.

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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 (ñî), 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) ó<3) and

y<3>

IW1 Ë • Ë \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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