Two-photon absorption in quadrupolar π-conjugated molecules: Influence of the nature of the conjugated bridge and the donor-acceptor separation

Quadrupolar-type substitution of π-conjugated chromophores with donor and acceptor groups has been shown to increase their two-photon absorption (TPA) response by up to two orders of magnitude. Here, we apply highly correlated quantum-chemical calculations to evaluate the impact of the nature of conjugated bridge and the charge-transfer distance on that enhancement. We compare chromophores with phenylenevinylene-, thienylenevinylene-, polyene-, and indenofluorene-type backbones substituted by dimethylamino and cyano groups. In all compounds, we find a strongly TPA-active Ag state (either 2A_g or 3A_g) in the low-energy region, as well as a higher lying TPA-active state (mAg) at close to twice the energy of the lowest lying one-photon allowed state; the smaller energy detuning in the mA_g states results in very large TPA cross sections δ. We also investigate the influence of the degree of ground-state polarization on TPA. Independent of the nature of the backbone and the donor-acceptor separation, δ displays the same qualitative evolution with a maximum before the cyanine-like limit; the highest TPA cross sections are calculated for distirylbenzene- and polyene-based systems.