[7]. In organic molecules, 3PA typically occurs at long wavelength in the near infrared region. Therefore, minimization of the scattered light losses is expected and a reduction of the undesirable linear absorption in organic compounds that present semitransparent windows at near infrared region is anticipated. As a result, radiation penetration depth can be maximized, and so light-activated therapy can be realized, which will afford an unprecedented penetration depth avoiding complicated and high risk surgery [8]. Also, the cubic dependence of the three-photon absorption process on the input light intensity provides a stronger spatial confinement, so that a higher contrast in imaging can be obtained [9].
Several theoretical and experimental studies have addressed the effect of group symmetry and conjugation of the donor and acceptor groups, and the length of π-electron delocalization on the 3PA cross-section of organic materials [8]. These studies have lead to development of new molecules with high 3PA cross section. However, there is a lack of systematic studies on the 3PA induced frequency upconversion fluorescence process for 3PA materials. The relationship between the structure and the 3PA-induced fluorescence property hasn’t been seen in previous literature.