Synthesis And Two-photon Optical Properties Investigation Of Fluorescent Dyes Bearing Benzophenone Parts

Recently, extensive researches have been paid on the photochemical and photophysical processes induced by two-photon absorption Due to the fact that the two-photon absorption (TPA) probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of orderλ3 (whereλis the laser wavelength). The certain photophsical and photochemical reaction can be induced in any point of three-dimension space. Thus, two-photon matetials have some potential application aspect such as in optical power limiting, two-photon upconversion lasing, two-photon fluorescence excitation microscopy, three-dimensional optical data storage and microfabrication. It has been the hot studying field in material science.For two-photon free radical photo-polymerization, the polymerization rate and photo-initiation threshold is closely related to the absorption behavior of the initiator at the excitation wavelength (generally twice its single-photon absorption wavelength). Although 3D microfabrication has been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, the TPA cross sections of these initiators are typically very small, and as a result they exhibit low two-photon sensitivity. At present, effective initiators are still rare because the photopolymerization mechanism and relationships of structure/property for the initiator remained unclear. Some approaches to improve the photosensitivity of photoinitiator molecules that have been reported are based on the idea of increasing their TPA cross-sections. According to Albota and Reinhardt et al., the conjugation length, donor and acceptor strength, and planarity of theπcenter are the important parameters for enhancing the TPA. Based on this design strategy, some one-dimensional two-photon polymerization initiators with charge-transfer conjugated character were synthesized. Recently, ab initio calculations have been carried out for increased dimensionality of the charge-transfer networks, showing that the TPA cross sections of mufti-branched molecules can be strongly enhanced compared with the one-branched counterparts. Experimental measurements have also confirmed this conclusion.The concrete work is listed in the following paragraphs.1. Synthesis and characteristic of D-π-A和D(-π-A)n(n=1-3) type compounds. The newly designed stilbenes, through varying the donor strengths, and changing the dimensionality of the charge-transfer networks with the one-branched counterparts were synthesized and they have been characterized by 1H NMR spectra, 13C NMR spectra, elemental analyses and thermal analyses. Two single crystals were prepared and their X-ray crystallography was analyzed and discussed.2. Study of the single-photon optical properties.The UV/vis absorption, fluorescence and two-photon induced fluorescence as well as the fluorescence quantum yields for the compounds were measured in various solvents; fluorescence lifetimes were measured,The conclusions were listed as following:○1 the absorption at long-wavelength is induced from intramolecular charge transfer (ICT). increasing of the donor strength, compounds have a stronger intramolecular charge transfer nature.○2 the maximum fluorescence peaks clearly show a slight red-shift and the fluorescence intensity show a slight decrease with the increase of the polarity of the solvent , Except compound 4.○3 increasing the dimensionality of the charge-transfer networks with the one-branched counterparts, the absorption and emission red-shift and the fluorescence intensity show a gradually enhancement.3 The study of two-photon optical properties.The two-photon up-conrersed fluorescences were determined. By two photon induced fluorescence technique two-photon absorption cross-section could be measured. The experimental results indicated that the two-photon absorption cross-section of three new initiators with one-, two-, and three-branched structures displayed enhancement with the increasing the dimensionality of molecule, which was accord with Macak's result. It was evident that the increase ment of molecular branch number played a major role in enhancement the molecular nonlinear absorption. Multibranched chomophores exhibited greater TPA cross-sections.