The two-photon absorption properties of a series of bis dialkylamino- or diarylamino-substituted diphenylpolyenes and bis(styryl)benzenes have been investigated. Two-photon absorption cross sections, δ, as large as 1420 × 10-50 cm4 s/photon-molecule have been observed for molecules with this general bis-donor structure. The effect of the type and length of the conjugated chain and of dialkylamino or diarylamino substitution on the position and magnitude of the peak two-photon absorptivity is reported. The transition dipole moments for the transitions between the ground state and the first excited singlet state (M ge) and between the first and second excited singlet states (M ee ‘) have been estimated using experimental data from the one- and two-photon spectra. It was found that increases in chain length result mainly in an increase in M ge, whereas the addition of donor end groups or going from diphenylpolyene- to phenylene-vinylene-type bridges leads primarily to an increase in M ee ‘. The trends in the energy of the lowest excited singlet states and in the transition moments for the diphenylpolyene series as a function of chain length are in agreement with those calculated by quantum mechanical methods. These results furnish a link between structural features in these classes of molecules and the electronic dipole couplings and state energies that control the strength of the two-photon absorption. In bis(aminophenyl)polyenes containing up to four double bonds (m) the lowest excited singlet state is a Bu state, as opposed to the case of simple polyenes and diphenylpolyenes, for which it is an Ag state for m > 2. The relationship of the state ordering in these systems with the observed values of the radiative and nonradiative decay rates is also discussed.

Large two-photon absorptivities are reported for symmetrical bis-donor stilbene derivatives with dialkylamino or diphenylamino groups. These molecules exhibit strong optical limiting of nanosecond pulses over a broad spectral range in the visible. Relative to bis(di-n-butylamino)stilbene, bis(diphenylamino)stilbene exhibits a 90-nm red shift of its optical limiting band but only a minimal shift of ϳ13 nm of its lowest one-photon electronic absorption band. Mixtures of these compounds offer an unprecedented combination of broad optical limiting bandwidth and high linear transparency. © 1997 Optical Society of America Materials that exhibit strong nonlinear absorption are currently of considerable interest for a variety of applications, including optical limiting. 1 -3 Such materials can be used in eye and sensor protection applications, optical pulse shaping and processing, and laser mode locking, for example. Much of the research on materials for optical limiting over the past 10 years has centered on sequential single-photon absorptive processes (reverse saturable absorption) in molecules. However, the linear transmission for a material composed of reverse saturable absorbers is necessarily significantly less than unity. Two-photon absorption offers the advantage of high transmission at low incident intensity for fundamental optical frequencies well below the bandgap frequency. However, most known molecular two-photon absorptivities are too small for use in optical limiting applications involving nanosecond pulses. Large two-photon absorptivities were reported recently by He et al. 4 -6 for several organic chromophores, and they reported optical limiting of nanosecond pulses at 1064 nm for a dialkylamino stilbazolium iodide.As part of our effort to understand relationships between molecular structure and two-photon absorptivity and to identify advanced materials for optical limiting, we examined a series of symmetrically substituted stilbene derivatives bearing two electron-donor groups. We report that 4,4′-bis(di-n-butylamino)stilbene (BD-BAS) and 4,4′-bis(diphenylamino)stilbene (BDPAS) exhibit strong two-photon-induced absorption at visible wavelengths as measured with nanosecond pulses. We demonstrate strong optical limiting of visible, nanosecond pulses with these compounds in an f ͞5 optical system. Moreover, we show that replacement of the di-n-butylamino groups with diphenylamino groups results in a 90-nm redshift of the two-photon absorption but in only a 13-nm shift of the onephoton absorption edge, leading to mixtures capable of broadband nanosecond optical limiting with high transparency.The UV-visible absorption spectra and molecular structures of BDBAS and BDPAS are shown in Fig. 1. BDBAS shows a peak of its one-photon absorption at 374 nm, whereas for BDPAS the peak is redshifted by 13 to 387 nm. Both compounds form solutions that are highly transparent across most of the visible spectrum and exhibit a linear transmission of .90% at wavelengths of $500 nm for a concentration of 0.1 M.Two-photo...