Theoretical Investigations On Organic Two-photon Absorption Materials

Now the investigation for two-photon absorption materials is one of most active fields in the world because two-photon absorption was found many applications in advanced scientific fields, such as 3D-microfabrication, photodynamic therapy, two-photon microscopy, optical power limiting, optical data storage, and two-photon laser scanning fluorescence imaging etc. Therefore, more and more researchers begin to develop and study two-photon absorption materials. The relationship between molecular structures and two-photon absorption properties and theoretical designing and finding good two-photon absorption materials are the basis of this investigation field, and will give support and evidence for experimental exploiting novel organic two-photon absorption materials.A series of novel organic two-photon absorption materials have been investigated using DFT and semiexperiential ZINDO quantum chemistry methods. Theoretical studies and molecular design for multiply N-confused porphyrins and their metal complexes, D-π-A-π-D benzothiadiazoles (BTD)-based fluorescent dyes and star-brust-typed pseudo octupolar molecules, spiro-linked oligofluorenes and derivatives and platinum acetylide complexes have been completed in detail. The geometrical and electronic structures, one-(OPA) and two-photon absorption(TPA) properties have been calculated. The structure-property relationship has been revealed. The investigation in this paper will provide some useful information and valuable basis on theoretical designing and experimental synthesizing new two-photon absorption materials with large TPA cross-section values.1. The investigations for multiply N-confused porphyrins and their metal complexes indicate that when N atoms one by one are inverted from core toβposition to form multiply N-confused porphyrins,OPA spectra are red-shifted and the oscillator strength decrease compared with normal porphyrin(N0CP). The maximum two photon absorption wavelengths are blue-shifted and TPA cross sections are increased 22.7-112.1nm when N atoms one by one are inverted from core toβposition to form multiply N-confused porphyrins. Especially the ?max of N3CP reaches 164.7GM because of the small difference between transition energies E0k - E0n/2 and a more obvious intermolecular charge transfer from HOMO - 2 to LUMO. The electron donors at meso-position make the TPA cross sectionδmax increase, and the stronger the electron donors are, the more the TPA cross sectionδmax increases. After forming metal complexes with Cu or Zn, the TPA properties are further increased.2. BTD-based D-π-A-π-D fluorescent dyes and star-burst-typed pseudo octupolar molecules have been investigated. The results indicate that conjugation length,π-electron bridges and terminal electron-donating groups (D) are three important factors for influencing OPA and TPA properties. Oscillator strength and TPA cross-sections increase with the strength of terminal electron donors. When increasingπ-conjugation length between BTD unit and the triphenylamino groups, TPA cross-section values are enhanced 1.5–2.6 times. The molecule 5 with ethyneπ-conjugated bridge shows a larger TPA cross-section comparing with ethene, benzene, thiophene and phenyletheneπ-conjugated bridge. Systemically study for OPA and TPA absorption properties of two series of star-burst-typed pseudo octupolar molecules have been completed (a series are benzene-centered and b series are triphenylamino-centered). The results show that b series have larger TPA cross-sections and longer OPA and TPA maximal wavelength. The TPA cross-sections of three-branches pseudo octupolar molecules increase over 3 times relative to single branch because of dipolar interaction between branches. The calculations show theTPA of BTD derivatives locate in IR or near-IR regions.3. bis ((4-phenylethynyl) phenyl)ethynyl) bis-(tributylphosphine) platinum (II) (simplified PE2) and its three analogs (1-3) that feature highlyπ-conjugated ligands have been investigated at the first time using ZINDO/SOS. The resonance integrals parameters of Pt are obtained to make one-photon absorption spectra calculated by ZINDO closest to the experimental data and TDDFT results. The results indicate that both trans and cis-isomers of molecules 1-3 take on two TPA peaks in 600-800nm region. The peak in 700-750nm can not been attribute to the appearance of noncentrosymmetric cis-isomers in solution although trans and cis-isomers adhere to different selection rule. The molecules 1-3 show greater two-photon absorption strength comparing with PE2, and remain good transparency. 4. The investigation for spiro-linked oligofluorenes and derivatives indicates theδTmax values of spiro-linked oligofluorenes increase with the increasing of chain length. The effects of spiro center and interconnection pattern of fluorene rings are also studied. The results show silicon center and interconnection in para-position can increase TPA cross-sections. Ethyne and ethene conjugated-bridges are introduced to design the molecules with larger TPA cross-sections in longer wavelength range. All calculations show the maximum OPA wavelengths of studied molecules are located in UV region, which demonstrate spiro-linked oligofluorenes and derivatives exhibit at the same time strong two-photon absorption and good transparency.