Substitution, environment, and excitation wavelength effects on the optical nonlinearities of some novel cis -/trans-π-conjugated azobenzenes

The transient nonlinear optical response of a series of highly π-conjugated 4,4′-diethynylazobenzene derivatives, symmetrically substituted with different electron-rich aromatic moieties (i.e., a N,N′-dihexylaniline, 4-ethynyl-(N,N′,dihexylaniline), and a Zn(II)-porphyrin fragment, respectively) is investigated by means of the Z-scan technique. The systematic study of their nonlinear optical properties, using 4 ns laser excitation, revealed that all molecules possess very large third-order nonlinearity excited at 532 nm, whereas only the aniline derivatives showed nonlinear optical (NLO) response under 1064 nm excitation. In-depth investigation, both in solutions and in thin films, established that the NLO response of these azobenzene (AB) derivatives is totally attributed to their high refractive nonlinearity, presenting negligible nonlinear absorption. These findings strongly suggest that these molecules can be exploited for the development of new materials suitable for photonic/optoelectronic devices, since their strong nonlinear refraction combined with the absence of any nonlinear absorption ensures low losses, very low heating of the organic material, and eventually longer operative lifetimes.