Theoretical study of one-photon and two-photon absorption properties for 2,1,3-benzothiadiazole-based red-fluorescent dyes

We have theoretically investigated a series of D-π-A-π-D type 2,1,3-benzothiadiazoles (BTD)-based fluorescent dyes by using DFT(B3LYP/6-31G(d)) and ZINDO/SOS methods. The geometrical and electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties have been studied in detail. The calculated results indicate that conjugation length, π-electron bridges and terminal electron-donating groups (D) are three important factors for influencing OPA and TPA properties. Both oscillator strength (f) of one-photon absorption and two-photon absorption cross-section values increase by degrees with the strength of terminal electron donors from molecule 1a to 1e. When increasing π-conjugation length between the BTD unit and the amino groups, two-photon absorption cross-section values of molecules 2-6 are enhanced about 1.5-2.6 times relative to 1c. Thereinto, the molecule 5 with additional ethyne π-conjugated bridge shows a larger two-photon absorption cross-section (270 GM) in comparison with ethene, benzene, thiophene and phenylethene π-conjugated bridge. The possible reason is discussed in detail by three-level energy model. Finally, we design a molecule 5′ with additional ethyne π-conjugated bridge and asymmetrical electron-donating groups, its two-photon absorption cross-section (336 GM) is larger than any other molecules in this system because the asymmetry makes Δμ0n increased obviously. The calculations also show low-energy transition arising from charge-transfer makes one-photon and two-photon absorption of these BTD derivatives locate in longer wavelength regions (420-550 nm for OPA and 690-880 nm for TPA). So our theoretical findings demonstrate that D-π-A-π-D type BTD derivatives are promising molecules that can be assembled to a series of materials with large TPA cross-section and emitting spectra in the red region.