Large Hyperpolarizabilities at Telecommunication-Relevant Wavelengths in Donor–Acceptor–Donor Nonlinear Optical Chromophores

Octopolar <i>D</i>₂-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 <i>meso</i>-arylporphyrin or electron-poor <i>meso</i>-(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 (<i>t</i>_delay < 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_<i>n</i> 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_<i>n</i>) ∼ 10⁵ M^–1 cm^–1]. Dynamic hyperpolarizability (β_λ) 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 β_HRS 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 β_HRS 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_<i>zzz</i> and β_<i>xzx</i> tensor components to the RuPZnRu hyperpolarizability spectrum. This analysis predicts that the β_<i>zzz</i> and β_<i>xzx</i> 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

Enhanced Intramolecular Charge Transfer in New Type Donor–Acceptor Substituted Perylenes

Here we report the facile synthesis and physical characterization of new type <i>N</i>-(2,6-diisopropylphenyl)-3,4-perylenedicarboximides (PMI) with alkyl-substituted quinoline-4­(1H)-ylidenemethyl or acridine-9­(10H)-ylidenemethyl units as strong donors in the 9-position. When compared to parent PMI, these perylene dyes, 9-((1-methylquinoline-4­(1H)-ylidene)­methyl)-PMI, 9-((1-benzylquinoline-4­(1H)­ylidene)­methyl)-PMI, 9-((1-heptylquinoline-4­(1H)-ylidene)­methyl)-PMI, and 9-((10-methylacridine-9­(10H)-ylidene)­methyl)-PMI, show a pronounced bathochromic shift of their electronic absorption with solvatochromism because of their intramolecular charge transfer. The solvatochromic behavior of these dyes is further confirmed by second-order nonlinear optical experiments. Remarkably high second-order nonlinear optical values (β_HRS up to 1300 ± 50 × 10^–30 esu at 880 nm in dichloromethane) are obtained by femtosecond hyper-Rayleigh scattering. The one-step synthesis together with the spectroscopic, solvatochromic, and nonlinear optical characteristics qualify these new types of perylene dyes as promising candidates for solvent polarity probes, photovoltaics, or nonlinear optical applications