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 × 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.