Theoretical study of two-photon absorption in donor-acceptor chromophores tetraalkylammonium halide/carbon tetrabromide

Two-photon absorption properties of a series of donor-acceptor chromophores of tetraalkylammonium halide/ carbon tetrabromide ([NR₄h· CBr₄], h = Cl, Br, I; R = Me, Et, Pr) complexes are investigated in terms of the calculated results by the time-dependent density functional theory (TDDFT) technique combined with the sum-over-states (SOS) method. The modeling two-photon absorption spectra show that these charge-transfer complexes have large two-photon absorption (TPA) cross sections and the [NEt ₄I·CBr₄] has the largest TPA cross section δ with the value of 5.0 × 10^-45 cm⁴ s photon ^-1. The maximum values of δ increase with increasing separations between the donor/acceptor in the order Cl⋯Br < Br⋯-Br < I⋯Br for [NEt₄h·CBr₄] complexes; however, the TPA cross sections δ vary slightly as the size of the alkyl group increases from methyl to propyl for the bromide as a donor, and the maximum wavelength of the TPA peak λ_max indicates a bathochromic shift. The charge transfers from the halide anion to the carbon tetrabromide make a significant contribution to the excited states, and the donor-acceptor charge transfer plays an important role in the TPA activity, whereas changes in size of alkyl group do not make a substantial contribution to TPA.