Remarkable NLO response and infrared absorption in simple twisted molecular π-chromophores

Computational modeling reveals that, by substitutional modification of interplanar dihedral angles in a series of quinopyrans, both the optical absorption cross section and the first hyperpolarizability can be widely tuned in both frequency and magnitude. This is a promising structural control scheme for modulating molecular nonlinear optical and infrared absorption properties. The maximum nonresonant μβ estimated for a tetraalkyl- substituted 4-quinopyran, the full AM1-optimized interplanar dihedral angle of which is 104°, is ~70000 x 10^-48 esu, at an excitation energy of 0.10 eV. This is 4 orders of magnitude larger than that of the archetypical NLO chromophore, 4-N,N'-(dimethylamino)-4'-nitrostilbene, at the same excitation frequency. The effects of conjugation pathway, strength of the electron donor/acceptor groups, and auxiliary donor and acceptor on the molecular response properties have also been analyzed. The optical absorption at a twist angle of 90% when the molecule can be described as two disjoint π- subfragments, exhibits characteristic absorptions from the individual subfragments and also inter-subfragment excitation; the former occur at high energies and with high oscillator strengths, while the latter occurs at low energy and with low oscillator strength.