Structure - Property relationships for two-photon absorbing chromophores: Bis-donor diphenylpolyene and bis(styryl)benzene derivatives

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 x 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 B(u) state, as opposed to the case of simple polyenes and diphenylpolyenes, for which it is an A(g) 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.