| Novel molecular and/or polymeric organic architecture with efficient multi-photon absorption (MPA) are extremely desirable because of emerging notably in the fields of optical power limiting, frequency-upconverted fluorescence miscroscopy, photonrefractivity, three-dimensional optical data storage, or two-photon photodynamic therapy. Two-photon absorption (2PA) is a process wherein two photons are absorbed simultaneously. TPA has received considerable attention recently because of a quadratic dependence of excitation on intensity, produce a spatially confined excitation useful for three-dimensional data storage and imaging. Three-photon absorption (3PA) is a process in which an atom or a molecule simultaneously absorbs three photons, which, if processing the same frequency, yield an excited state lying at three times the photon energy above the ground state. Compared to 2PA, longer excitation wavelengths can be used in 3PA-based applications providing deeper penetration depths in scattering or absorbing media; in addition, the cubic dependence of the three-photon process on the input light intensity leads to a stronger spatial confinement, so that a higher imaging contrast can in principle be obtained. In spite of such intrinsic advantages, three-photon absorption has so far obtained limited attention, mostly as a result of the associated very low cross-sections as it corresponds to a fifth-order nonlinear optical process. Recently, experimental observations of 3PA effects have been reported in organic conjugated molecules. However, there have been only few attempts to model the microscopic mechanisms or to propose guidelines for the design of molecular architectures with enhanced SPA cross-sections.In this paper, we have come up with the theory of the SPA cross-section for the first time and provided a practical guideline for the design of molecular architectures with enhanced SPA cross-sections. We note that the experimental determination of SPA coefficient for organic molecules is quite difficult. So far, the reported SPA values for different molecules fall in a broad range of 7 orders of magnitude: from 10" 81 to 10~74 cm6s2. In particular, we have performed theoretical calculations for two typical systems for the sake of comparison, namely, the 6-propionyl-2-dimethylamino-naphthalene (PRODAN) which has been measured by Bhawalkar et aland the 4,4'-bis(diphenylamino) stilbene (BDPAS) measured by Drobizhev et al. The theoretical value for PROD AN molecule is about 2 orders of magnitude less than the experimental value. However, for BDPAS, the theoretical value is about two orders of magnitudes larger than the experimental value. Thus we can say that the theoretical values are in the midway among the different measurement.Anderson and co-workers have recently reported the synthesis of a compound (AtPtA) in which two anthracene moieties are covalently linked to a central porphyrin unit through two acetylenic bridges. Ultrafast nonlinear absorption studies indicate that AtPtA presents sizeable 2PA cross-sections. While modeling the two-photon absorption response of the triad structure, we realized that it also fulfills the requirements for SPA in the infra-red spectral domain. Thus, we explored the origin for the 3PA response in the porphyrin-anthracene triad by means of quantum-chemical calculations performed at the highly correlated Coupled-Cluster Single and Double Equation of Motion level. The SPA cross-section of AtPtA has been found to be two orders of magnitude larger than the individual entities and about one order of magnitude larger than the values reported for donor-acceptor conjugated structures. Such enhancement of the SPA response is associated to the presence of target three-photon states with predominant intramolecular charge-transfer character, which show large transition dipole moments to a dominant two-photon excited state delocalized over the porphyrin and anthracene moieties. The latter state is itself strongly coupled to the excited states leading to the porphyrin Q and B bands that dom... |
PDF Full Text Request