Chromophores, two-photon

Our collaborators, Kim, Lee and coworkers have also synthesized a series of novel two-photon chromophores by utilizing dithienothiophene (DTT) as the n-center [15]. The experimental result has shown that the DTT moiety leads to an enhancement of molecular two-photon absorptivity. We hypothesized that the dramatic improvement of 03 values is due to this rigid, planar and polarizable fused-terthiophene structure, which provides a significant reduction of the band gap and extension of Tt-electron delocalization. The chemical structures and the measured 02 values of these chromophores are depicted in Fig. 5. 

Fig. 15. Chemical structures of two-photon chromophores, ASPT and APSS, for up-converted lasing application...

Fig. 20. Process of the conventional photodynamic therapy (right) and the newly proposed approach based on two-photon process (left). PS photosensitizer TPC two-photon chromophore...

From a materials point of view, synthesis of two-photon chromophores with even higher two-photon cross-sections is required, so that photopolymerization may be induced by less expensive picosecond, nanosecond and even CW lasers for commercial applications. Also the high 5 materials would enable polymerization simultaneously from multibeam split out of one output of a laser for batch production. Also desired is the use of copolymerization of functional molecules and resins, or doping of other functional components like nanocrystals into resins, so that particular functions could be imparted to polymerized devices. Uniform dispersion of the alien components into the matrix polymer is an important problem to solve. Finally, we need to synthesize functional polymers in other words, instead of doping, we directly induce functional chromophores as a bonded component of monomers or oligomers, so that functions such as light-emission, polarization control, refractive index tuning and so forth become more pronounced. 

Generally two-photon chromophores constitute three distinct stmctural motifs namely acceptor groups (A), donor groups (D), and conjugate n-btidge centers (ti). All the chromophores are a combination of any two or all three of these stmctural motifs. They are coimected through suitable linkages to control the stmcmre of the molecules and their optical properties. 

For two-photon memories, a number of media types and reading mechanisms have been used (165). Generally, media comprise two photon-absorbing chromophores dissolved within a soHd polymer matrix. Suitable reversible photochromic dyes are, for example, spiropyrans. Although photochromic materials often suffer from photobleaching, as well as from instability leading to self-erasure, new materials and host environments are under development (172). Bacteriorhodopsin (BR) also has been proposed as a two-photon memory material. 

Birge RR, Pierce BM (1979) A theoretical analysis of two-photon properties of linear polyenes and the visual chromophores. J Chem Phys 70 165-178... 

Sutherland RL, Brant MC, Heinrichs J, Rogers JE, Slagle JE, McLean DG, Fleitz PA (2005) Excited-state characterization and effective three-photon absorption model of two-photon-induced excited-state absorption in organic push-pull charge-transfer chromophores. J Opt... 

Beverina L, Fu J, Leclercq A, Zojer E, Pacher P, Barlow S, Van Stryland EW, Hagan DJ, Bredas JL, Marder SR (2005) Two-photon absorption at telecommunications wavelengths in a dipolar chromophore with a pyrrole auxiliary donor and thiazole auxiliary acceptor. J Am Chem Soc 127 7282-7283... 

Terenziani F, Katan C, Badaeva E, Tretiak S, Blanchard-Desce M (2008) Enhanced two-photon absorption of organic chromophores theoretical and experimental assessments. Adv Mater 20 4641 1678... 

Like Raman scattering, fluorescence spectroscopy involves a two-photon process so that it can be used to determine the second and the fourth rank order parameters. In this technique, a chromophore, either covalently linked to the polymer chain or a probe incorporated at small concentrations, absorbs incident light and emits fluorescence. If the incident electric field is linearly polarized in the e direction and the fluorescent light is collected through an analyzer in the es direction, the fluorescence intensity is given by... 

Push-push/pull-pull chromophores 118 and 119 have exhibited material properties to show their prospect for several applications particularly in the fields of two-photon microscopy and optical power limitation <1999CC2055, 2002SM17>. 

In TPM, fluorescence emission of a fluorophore (either intrinsic to the sample or an exogenously applied chromophore) is detected. A fluorophore is used to obtain specific chemical information in the skin, as discussed in more detail below. To excite the fluorophore, two-photon excitation is used, which is achieved by the simultaneous... 

In xanthenes, even if all one-photon allowed transitions are also two-photon allowed, the shape of the bands and their relative intensities are very different in the IPA and 2PA spectra [76,78]. This is not the case for other laser dyes and chromophores, for which the two spectra are almost identical (if represented as a function of the total transition energy), showing peaks in the same position and with very similar band shapes. Some example of chromophores in this category are coumarin 307 [78], coumarin 102 [80], 7-hydroxycoumarin [81], lucifer yellow [78], and cascade blue [78]. 

The class of compounds that has been most extensively investigated from the point of view of two-photon absorption is that of so-called quadrupolar chromophores. hi essence, these molecifles are linear conjugated chains with electron donating or withdrawing substituents arranged symmetrically with respect to the center of the molecifle (Fig. 8, classes I-IV). With the inversion center being preserved, the lowest order moment supported by these molecules is the quadrupole moment. 

For this three-state model the values of 52pa can be determined by the Eqs. 10 and 11. In this simple model the chromophore system of the molecule can be modeled by two arbitrarily oriented linear oscillators, (igg and Afigg (for excitation into the first excited electronic state Si), or by the figg and ligg/ (for excitation into the final electronic state Sy), which simultaneously absorb two photons and transfer their energy to the emission oscillator, fi. It has been shown that the limiting value of fluorescence anisotropy T2pa can be written as [23] ... 

---