First order hyperpolarizability-structure relationships in donor-acceptor substituted ethylenes and 1,3-butadienes

Abstract

he progress in photonic devices requires the development of novel materials exhibiting strong optical nonlinearities (at both the molecular and bulk levels). The overall objective of this work is to arrive at structure-property relationships in substituted ethylenes and 1,3-butadienes (conjugated organic systems) leading to large second-order nonlinearities.

Chapter 1 starts with a brief introduction to the second-order nonlinear optical processes, followed by a discussion on various inorganic and organic materials known to exhibit second-order nonlinearities. Based on earlier reports, the microscopic and macroscopic characteristics of materials having large nonlinearities are discussed, with an emphasis on the existing synthetic strategies to develop new materials. A discussion on the experimental methods to evaluate the second-order nonlinear coefficients is also given.

Chapter 2 deals with the measurement of powder second harmonic generation (SHG) efficiency and charge transfer contribution to the first-order hyperpolarizability, ?CT ? CT ?

, of a series of nitroenamines using the two-level model:

?CT=3e3i2 ? CT ?

= i 2 3e 3 ?

where the change in dipole moment, ?? ??, was obtained from known values or theoretical estimates of ?eg ? eg ?

through semi-empirical methods and the oscillator strength obtained from absorption spectra of the compound.

The effect of a second electron-donating group on the ? ? values of nitroenamines is discussed in Chapter 3. As expected, the results indicate an altered electronic charge distribution and an increased steric crowding, resulting in a decrease in the magnitude of ?CT ? CT ?

. The 1,1-bispiperidino-2-nitroethylene was found to have the maximum first hyperpolarizability value in the series, and it also showed SHG. The other compounds did not exhibit any measurable SHG intensity. The crystal structure of the compound that exhibited SHG was determined, and the compound crystallizes in a noncentrosymmetric structure. One interesting aspect of the structure is the elongated C=C bond distance (1.40 Å), which indicates extensive charge delocalization in the molecule.

Chapter 4 deals with molecular and crystal engineering approaches to produce noncentrosymmetric structures. Introduction of chiral and hydrogen-bonding groups is expected to play a role in orienting the molecules and hence improve the packing characteristics. Although the ?CT ? CT ?

values obtained were not very different from the compounds discussed earlier, the compound having the electron donors methylamino- and ? ?-methylbenzylamino groups (with the nitro group as the acceptor) exhibited a hyperpolarizability. The same compound also generated measurable second harmonic intensity. The crystal structure of the compound has been determined. Both intra- and intermolecular H-bonding were present in the compound, with extensive charge transfer between the donor and acceptor groups. Effects of variation of the acceptor from nitro to dicyanovinyl group have also been studied.

In Chapter 5, the effect of increasing the chain length of the ? ?-system on ? ? is discussed. Only in 1-N,N-dimethylamino-4,4-dicyano-1,3-butadiene was there an enhancement in ? ? compared to that of the similarly substituted ethylene. However, none of the butadiene compounds produced detectable SHG. In order to understand the second-order nonlinearities in these systems, the crystal structure of 1-N,N-dimethylamino-4,4-bis(trifluoroacetyl)-1,3-butadiene was determined, and it crystallizes in an acentric space group.

In conclusion, substitution of hydrogens in ethylene by donor and acceptor groups leads to a large ground-state dipole moment, because of which the molecules tend to form pseudo-inversion dimers in the crystal lattice. Orientation and packing of molecules in the unit cell are very crucial for second-order nonlinearities. The donor strength in enhancing ?CT ? CT ?

is proportional to their basicity, and the acceptor strength is proportional to their substituent constants. The scaling of ?CT ? CT ?

in N,N-dimethylamino- and cyano-substituted polyenes follows an L?3 L ?3 (where L L is the length of the molecule) behavior, which agrees well with a ? ?-electron exact model for calculating ? ?.