Abstract
Organic semiconductors as active materials in thin-film electronic devices such as alkynes, heterocycles, dyes, ferrocenes, spiranes, or porphyrins, with special geometries and certain electronic molecular parameters, which possess nonlinear optical (NLO) properties and offer several major advantages over their inorganic counterparts, are presented in this chapter. There are a number of simple and versatile techniques that can be employed for the deposition of these important classes of materials. The matrix-assisted pulsed laser evaporation (MAPLE) technique provides advantages with regard to making organic films of different morphologies on different types of substrates. New insights into the crystallization growth mechanisms in MAPLE-deposited conjugated polymer films, which realize the connection between the structure and the carrier transport properties, are discussed herein. Second harmonic generation (SHG) capabilities of the thin films were also investigated.
Highlights
During the last decades, the nonlinear optical (NLO) materials have gained significant role because of their various applications in medicine, molecular switches, luminescent materials, laser technology, spectroscopic and electrochemical sensors, data storage, microfabrication and imaging, modulation of optical signals, and telecommunication [1–3]
Other classes of heterocyclic compounds were reported to have nonlinear optical properties, and some of them are presented below. 1,2,3,4,5,6,7,8Octahydroacridine (OHA) 47 thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) showed the second harmonic generation (SHG) signal of the conformational asymmetry of OHA and the nonlinear optical applications of it [114]
Nonlinear optical properties of ferrocene carboxaldehyde 49 thin films grown by matrix-assisted pulsed laser evaporation (MAPLE) were studied by Constantinescu et al [116] using two-photon absorption investigations
Summary
The nonlinear optical (NLO) materials have gained significant role because of their various applications in medicine, molecular switches, luminescent materials, laser technology, spectroscopic and electrochemical sensors, data storage, microfabrication and imaging, modulation of optical signals, and telecommunication [1–3]. Recent advances in chromophore design report some features for classic dipolar organic structures with good nonlinear optic properties [89]: (1) presence of a π-conjugated systems with π electron delocalization, (2) a “push-pull” system, which is a couple donor-acceptor or connected to a system that contributes to the delocalization of the π electrons; (3) presence of a strong electron donor groups (e.g., ─NR2, ─NHR, ─OR, ─OH), and strong electron withdrawing groups (e.g., ─CF3, SO2CF3, ─SO3H, ─NO2, ─CN), positioned at opposite ends of a conjugated molecule in case of dipolar molecules; (4) great values of dipole moment and polarizability; (5) small HOMO-LUMO energy gap; (6) planarity of the molecule for neutral, polar, and zwitterionic resonance structures. Alternative NLO materials, present more advantages compared with dipole molecules [90]: (1) the second harmonic response (SHG) does not depend on the polarization of the incident light because they are more isotropic than the dipolar molecules; (2) β values of the octupoles can be increased by increasing of intramolecular charge transfer; (3) octupoles form noncentrosymmetric crystals; and (4) they are less likely to undergo relaxation due to the lack of ground-state dipole moment
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