NLO properties

Light-induced activation of second-order NLO properties 

Interestingly, the disorientation-induced distortion of the matrix can be avoided if the photo switching is performed under an external electric field. This was demonstrated in the case of the polyimide of the structure shown in Chart 5.16 [60]. 

the SHG signal decays under irradiation due to trans-cis isomerization and recovers almost completely in the dark after the light is switched off The influence of the external electric field is thought to allow a compensation of the photo-induced distortion through photo-assisted poling. 

Chart 5.14 Chemical structure of 6-nitro-l, 3, 3 -trimethylspiro[2H-l-benzopyran-2,2 -indoline] [59]. 

Upper part Second harmonic generation (SHG). Lower part Optical absorption of the merocyanine isomer at 7=532 nm. Adapted from Atassi et al. [59] with permission from the American Chemical Society. 

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There is a growing interest in the non-linear optical (NLO) properties of organic materials. Organic and polymeric materials with large non-linear optical coefficients can be used in principle in optoelectronic and photonic devices, and a great deal of research effort has been expended in efforts to design new compounds with optimal NLO properties. 

Such efforts have met with limited success, and the reason usually advanced is our lack of understanding of the frequency dependence of molecular NLO properties. In classical electromagnetism, we refer to properties that depend on the frequency of radiation as dispersive and we say that (for example) dispersion is responsible for a rainbow. The blue colour of the sky is a dispersion effect, as is the red sky at night and morning. There is more to it than that, and you might like to read a more advanced text (Hinchliffe and Munn, 1985). 

In recent years research in the field of transition-metal thiocyanates and selenocyanates received a new impetus, because of the partly interesting physical properties of such crystalline species. A review on Cd and Hg thiocyanate systems collects and sorts results of this endeavor.371 The nonlinear optical (NLO) properties of Cd thiocyanate and selenocyanate systems and criteria for the design of NLO crystals (crystal engineering), especially, have been discussed afterwards.372 Further contributions to the field have also been described.37, 374 The structure of mercury chlorothiocyanate has been re-determined.375... 

COORDINATION COMPLEXES WITH NLO PROPERTIES 9.14.2.1 Complexes of Nonchelating Pyridyl Ligands... 

The cubic NLO properties of Ru11 tris-(a-diimine) complexes have been investigated only relatively recently by Ji and co-workers using 540 nm Z-scan all of the complexes studied exhibit both NLO absorption and SDF behavior.135-138 The large and 7 values of complexes such as (73) depend on the position of the -N02 group in the order 4- > 2- > 3-.135 Also, the NLO responses decrease on deprotonation of the imidazolyl nitrogens (although this effect is only... 

On account of their particularly extensive delocalized 7r-systems, MPs have received comparatively more attention for their cubic, as opposed to quadratic, NLO properties. From the viewpoint of practical applications, such complexes (and also metallophthalocyanines and other closely related compounds) are of major interest for OL due to their tendency to exhibit RSA behavior. These materials are particularly well suited in this regard because they often exhibit strongly absorbing, long-lived triplet excited states as well as reasonably wide transparency windows over the visible region of interest between the intense B- and Q-bands. 

Blau and co-workers first reported observations of RSA in toluene solutions of Zn11 or Co11 meso-tetraphenylporphyrins with 80 ps pulses at 532 nm,147 and also proposed a theoretical mechanistic description for the cubic NLO properties of these complexes.148 RSA in Fe11 meso-tetraphenylporphyrin was later described by Fei et al.149 Other early 532 nm DFWM studies150-152 involve complexes of tetrabenzoporphyrins which were found in THF to display (3) values... 

Torres and co-workers have investigated the cubic NLO properties of various metallotriazo-lehemiporphyrazines.190-194 Early THG studies at 1,340 nm and 1,907 nm with CHCI3 solutions show that metal ions with partially filled d-shells enhance 7 when compared to the uncomplexed macrocycles.190,191 However, later 1,064 nm THG studies with LB films of some Co11 complexes indicate that metallation has no significant effects on the NLO responses.192 More recently,... 

The NLO properties of metallophthalocyanines (MPcs) and metallo-2,3-naphthalocyanines (MNPcs) have been extensively studied, primarily due to their OL potential. This area has been reviewed,197-200 and although citations are near-comprehensive, discussion here will be limited to selected highlights and more recent developments. 

Many studies dealing with the cubic NLO properties of MPcs have been reported since 1990, and these will be discussed only briefly here, focusing on the research of the following leaders in this field Agullo-Lopez,220,225-232 Nalwa,196,233-239 Perry,240-248 Shirk,249-265 Hanack,239,264-271 Wada,272-284 Heflin,285-290 Yamashita,291-297 and Fang.298-302... 

NIR region (see Chapter 9.13), complexes of 1,2-dithiolene (DT) and related ligands have attracted considerable attention for their (largely cubic) NLO properties. The complex (156) (a.k.a. BDN) is a highly photochemically stable, saturable absorber and has hence found extensive applications in laser Q-switching. The cubic NLO properties of (156) have been studied by DFWM148,403-407 and more recently, Z-scan.408 Time-resolved DFWM has been applied to square planar Co, Ni, Cu, or Pt complexes of 1,2-benzenedithiolate (BDT) or 1,2-aminobenzenethiolate ligands by Lindle and co-workers.409,410... 

Winter, Underhill, and co-workers have published extensively on the cubic NLO properties of complexes of DT and related ligands,411 22 particularly those containing formally Ni11 centers. For example, time-resolved 1,064 nm DFWM was used to obtain resonantly enhanced values for group 10 complexes such as (157).411 15 The smaller of (157) compared with (156) is largely due to resonance effects since the absorption maximum of (157) is somewhat removed from the laser fundamental. However, figures of merit derived from measurements of 2 and linear and two-photon absorption (TPA) coefficients show that low optical losses render complexes such as (157) superior to (156)413 for potential all-optical switching applications.411 14... 

Several reports have also considered the quadratic NLO properties of DT complexes.458-461 Chen et al. have used NIR absorption data and SC A//12 values to estimate f30 according to the TSM for various Ni11 or Pt11 mixed a-diimine/DT complexes,458,459 and also for several asymmetrically substituted bis(DT)s (e.g., (175) and (176)).460 Dipole analyses indicate that the NIR transition in (175) is primarily LLCT in nature, whereas that of (176) has only limited CT character.460 The complex (177) has a red-shifted LLCT absorption and a considerably larger fa value than (175).461 The // 2 for the NIR band of (177) is also somewhat larger than that of (175), but the two complexes have very similar A//12 values.461... 

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