This contribution explores the use of perturbation theory and the computationally efficient PPP π electron model Hamiltonian to relate quadratic molecular optical nonlinearities to architecture and electronic structure. A detailed study of aniline, nitrobenzene, and p-nitroaniline, using all monoexcited configurations, yields β (hyperpolarizability) tensors in good agreement with all-valence-electron CNDO calculations. Moreover, PPP-derived vector (observable) components for frequency doubling (βvec(-2ω;ω,ω)) are in excellent agreement with experiment over a wide frequency range. For a series of para-disubstituted benzenes, there is a linear relationship between calculated βvec values and Hammett a parameter differences for the substituents. For a series of α,ω-N(CH3)2,NO2-disubstituted trans polyenes, there is a linear relationship over a broad frequency range between calculated In βvec and the number of double bonds between the substituents. Multiple N(CH3)2,NO2 substitution at the polyene ends has little additional effect on βvec beyond that of single substitution. Examination of a simple two-level perturbation model reveals that this insensitivity of βvec to multiple donor/acceptor substitution reflects the corresponding insensitivity of the dipole moment as well as of the energy and oscillator strength of the first optical transition. The utility of the PPP model Hamiltonian in designing new, elaborate nonlinear chromophores is illustrated by an examination of several hypothetical molecules of sequentially varied substitution.
ASJC Scopus subject areas
- Colloid and Surface Chemistry