Ultrafast Dynamics Of Several Two-Photon Organic Oligomers

Because of femtosecond laser with a narrow pulse width, high time resolution and high instantaneous power, it has been widely applied to study ultrafast processes in physics, biology, chemistry, optoelectronics, medical and other fields. Since the inception of ultrafast pulse laser, the rapid development of femtosecond experimental techniques, people have achieved the femtosecond resolution about the measurement of the excitation process. And the time scale corresponds to the beginning of the relaxation of the excited state, such as vibrational relaxation, internal conversion, intersystem crossing. Therefore, the study of the ultrafast dynamics of materials becomes one of the frontier topics in the optical science.Along with the development of the molecular engineering and the organic synthesis, the organic materials with excellent nonlinear optical properties have been designed and synthetized. Two-photon absorbing organic materials has shown great potential applications in optical limiting, optical microfabrication, three-dimentional data storage, up-converstion lasering and photobiology and so on. Many scientists have synthetized a lot of two-photon absorbing organic materials, studied the methods of extending two-photon absorption cross sections and the structure-property relationshiop. In order to understand deeply such materials, we need to study the information of its excited state.Under the excitation of the laser pulse, the organic materials will produce some photophysical and photochemical processes, however, these processes are from the first excited singlet state and the first excited triplet state. Molecules in the excited state are unstable and, easy to release the energy which was absorbed when they transit from the ground state to the excited state, and then transit to the ground state. The deactivation processes of the excited state contain rediative transition (fluorescence and phosphorescence, etc.), nonrediative transition(internal conversion and intersystem crossing), energy transfer, electron transfer and chemical reactions. In this paper, the ultrafast dynamics in several novel two-photon organic oligomers were studied by using femtosecond pump-probe technique, and we got some conclusion.Anthracene derivatives An-OFVn (n=1,2,3,4) with two-photon absorption properties:by the UV-visible absorption spectra, the absorption peaks became more powerful and exhibited red-shift significantly with the extending ofπ-conjugation. By the transient absorption spectra we knew that the excited state relaxation processes of the four two-photon oligomers completed within a few hundred picoseconds, and found that the relaxation processes included vibrational relaxation and internal conversion by comparing with the characteristic time. In addition, the conjugation length of the oligomers increases, the energy gap decreases and the excimers form difficultly, the time of relaxation decreases.Two-photon oligomers with fluorenone as acceptor and different donor:known by the UV-visible absorption spectra, as the donor intensity in the oligomers increased, the intra-molecular charge symmetric transfer enhanced, the maximum absorption peak red-shifted, and the weakest peak was the absorption one of the charge transfer state. From the transient absorption spectra we found the capacity of the electron donor group increased, the effect of intra-molecular charge transfer increased, the time of relaxation gradually reduced, the whole time of relaxation processes was 660ps,590ps and 510ps for F-FO-F, TPA-FO-TPA and P-FO-P respectively. In addition, the relaxation processes of the excited state contained vibrational relaxation, internal conversion and intersystem crossing.Two-photon oligomers (P-FO-P,P-F-FO-F-P) with fluorenone as acceptor and different conjugation length:by the UV-visible absorption spectra, the absorption peaks became more powerful and exhibited red-shift with the extending of conjugation length. The fluorene group in the P-F-FO-F-P prevented the charge symmetric transfer between the phenothiazine group and the fluorenone group, compared with P-FO-P, the the absorption peak of the CT state was weaker. Known by the transient absorption spectra, because the fluorene group in the P-F-FO-F-P prevented the charge transfer between the donor phenothiazine group and the acceptor fluorenone group, the relaxation time was similar for P-F-FO-F-P and P-FO-P, about 510ps. In addition, the relaxation processes of the excited state included vibrational relaxation, internal conversion and intersystem crossing.In addition, the light intensity has the different degrees of dip in the equal optical path point for these samples. Although the degree of dip decreased with the enhancement of the two-photon absorption capacity of oligomers, whether it was related to the monodisperse linear structure of the molecules, this needed to further verify.