Organic Conjugated System Of Two-photon Fluorescence And Excited State Dynamics Study

Two-photon absorption (TPA) can be defined as simultaneously absorption of two photons through a virtual state. In 1931, Maria Goppert-Mayer gave the theoretic prediction of this effect, and it was approved by Kaiser and Garrett in 1961. Due to the ultrafast optical response, excellent 3-D processing capabilities and high spatial resolution, TPA process shows huge advantage in biology, medicine, physics, chemistry, photoelectronics and many other fields. TPA effect holds promising applications in optical limiting, two-photon excited fluorescence (TPF) imaging, two-photon polymerization and 3-D microfabrication, photodynamic therapy, up-conversion lasing and so on. Since mid of last centry, the organic conjugated system with TPA property attracted lots of attention as soon as the TPA dye with large TPA property was firstly synthesized. Some experience and design strategies have been realized, and a series of organic conjugation materials with relative large TPA properties have been synthesized. Because materials with excellent TPA/TPF properties are the basis for wider applications, to get the materials with large TPA cross section as well as strong TPF has become one of the most attractive topics.Due to the fact that the excited state relaxation process of organic conjugated system contains a lot of photophysics processes which would reflect significant information about the excited state and the interaction between light and materials, the study on the nonlinear optical properties and ultrafast dynamics of the organic conjugated system with intense TPA ability, have been one of the frontier in the photophysics and photochemsitry fields.In this thesis, we measured the TPA cross sections of different organic conjugated systems by using Z-scan technique, TPF method, trasient abosrption spectroscopy and transient fluorescence experiments to investigat the energy relaxtion processes of the excited state. Combining with linear absorption spectroscopy and steady state fluorescence spectroscopy, we systematically investigated the effect of the molecular structure, the length ofπ-conjugated bridges, the push/pull ability of terminal electron-donor/accepter group, and the modification of terminal group by alkyl group, on the TPA response and ultrafast dynamics of the excited state. The results we gained would be helpful for design and synthesis of novel organic conjugated materials with more excellent TPA properties.The main innovative results and conclusions are as follows: 1. Multi-branched systems based on 1,3,5-triazine:The nonlinear optical results indicate that building six-branched structure based on tri-branched counterparts can effectively improve TPA properties of organic conjugated system. Obvious fluorescence quenching and aggregation-induced enhancement of two-photon fluorescence emission were observed accompanying with the enhanced TPA properties. The excited state dynamics results reveal an ultrafast charge localization from the excited state to the intramolecular charge transfer (ICT) state in picosecond domain. The steric hindrance of the bulkier chromophore as well as shortened lifetime of ICT state caused by the oxygen family element should be responsible to the fluorescence quenching. The results indicate that building six-branched structure is an extremelly effective method for the improvement of TPA property;The TPA property decreased with the number of dialkylfluorene fluorene bridge. Ultrafast dynamic results indicate that the formation of ICT state is prolonged with the increasing number of the fluorene bridge. The fact may explain that poor electron transmission properties and less planary property are caused by linking fluorene group directly, thus leading to reduced TPA response;Terminal group replaced by more push/pull electron active group, extended conjugated length by effectiveπ-bridge, modification of terminal group by alkyl chain and many other strategy could also enhance the TPA response of organic conjugated system.2. Red-fluorescence probe organic conjugated system:BODIPY derivatives show large TPA cross sections and relative high fluorescence quantum yields. TPA cross section increased with strength of the terminal electron-donor group and D-π-D is superior to A-π-A structure. The results indicate the enhancement of ICT effect. Ultrafast dynamics reveal a broad region of stimulated radiation of the excited states and the different relaxation processes happened in these compounds. The preliminary two-photon fluorescence imaging experiment shows their good cell permeability, nontoxicity and excellent two-photon fluorescence properties;2,1,3-Benzothiadiazole-based organic conjugated system holds promise D-π-A-π-D structure with efficient ICT effect, showing large TPA cross section and relative high fluorescence quantum yields. The ultrafast dynamic results reveal a competition between pure excited state relaxation process and stimulated radiation in red wavelength range.3. Organic conjugated structure with pyridin-2,6-dicarboxylate as electron acceptor:Under excitation of femtosecond laser pulses, these series TPA materials show intense two-photon fluorescence emission. The nonlinear optical measurements reveal that building tri-branched structure is very effective for emprovement of TPA cross section. Ultrafast dynamics measurements indicate that the ground/excited state properties of one-branched and tri-branched structures are obviously different.