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Conjugation Length Effect on TPA-Based Optical Limiting Performance of a Series of Ladder-Type Chromophores.

ABSTRACT: Nonlinear optical properties of a series of newly-synthesized ladder-type chromophores containing oligo-p-phenylene moiety with different π-conjugated lengths were theoretically studied by numerically solving the rate equations and the field intensity equation with an iterative predictor-corrector finite-difference time-domain technique. Ab initio calculation results show that the compounds can be described by the three-level model. Based on the two-photon absorption mechanism, highly efficient optical limiting performances are demonstrated in the chromophores, which strongly depend on the π-conjugated length of the molecule. Special attention has been paid to the dynamical two-photon absorption, indicating that the parameter of the medium can affect the dynamical two-photon absorption cross section. Our numerical results agree well with the experimental measurements. It reveals that the increase in the π-conjugated length of ladder-type oligo-p-phenylene for these chromophores leads to enhanced nonlinear optical absorption. The results also provide a method to modulate the optical limiting and dynamical two-photon absorption of the compounds by changing the molecular density and thickness of the absorber.

SUBMITTER: Zhang Y

PROVIDER: S-EPMC5344550 | biostudies-other | 2017 Jan

REPOSITORIES: biostudies-other

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Project description:Octopolar D2-symmetric chromophores, based on the MPZnM supermolecular motif in which (porphinato)zinc(II) (PZn) and ruthenium(II) polypyridyl (M) structural units are connected via ethyne linkages, were synthesized. These structures take advantage of electron-rich meso-arylporphyrin or electron-poor meso-(perfluoroalkyl)porphyrin macrocycles, unsubstituted terpyridyl and 4'-pyrrolidinyl-2,2';6',2″-terpyridyl ligands, and modulation of metal(II) polypyridyl-to-(porphinato)zinc connectivity, to probe how electronic and geometric factors impact the measured hyperpolarizability. Transient absorption spectra obtained at early time delays (tdelay < 400 fs) demonstrate fast excited-state relaxation, and formation of a highly polarized T1 excited state; the T1 states of these chromophores display expansive, intense T1 → T n absorption manifolds that dominate the 800-1200 nm region of the NIR, long (μs) triplet-state lifetimes, and unusually large NIR excited absorptive extinction coefficients [ε(T1 → T n ) ∼ 105 M-1 cm-1]. Dynamic hyperpolarizability (βλ) values were determined from hyper-Rayleigh light scattering (HRS) measurements, carried out at multiple incident irradiation wavelengths spanning the 800-1500 nm spectral domain. The measured βHRS value (4600 ± 1200 × 10-30 esu) for one of these complexes, RuPZnRu, is the largest yet reported for any chromophore at a 1500 nm irradiation wavelength, highlighting that appropriate engineering of strong electronic coupling between multiple charge-transfer oscillators provides a critical design strategy to realize octopolar NLO chromophores exhibiting large βHRS values at telecom-relevant wavelengths. Generalized Thomas-Kuhn sum (TKS) rules were utilized to compute the effective excited-state-to-excited-state transition dipole moments from experimental linear-absorption spectra; these data were then utilized to compute hyperpolarizabilities as a function of frequency, that include two- and three-state contributions for both β zzz and β xzx tensor components to the RuPZnRu hyperpolarizability spectrum. This analysis predicts that the β zzz and β xzx tensor contributions to the RuPZnRu hyperpolarizability spectrum maximize near 1550 nm, in agreement with experimental data. The TKS analysis suggests that relative to analogous dipolar chromophores, octopolar supermolecules will be likely characterized by more intricate dependences of the measured hyperpolarizability upon irradiation wavelength due to the interactions among multiple different β tensor components.

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Conjugation Length Effect on TPA-Based Optical Limiting Performance of a Series of Ladder-Type Chromophores.

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